Implications for Extraterrestrial Hydrocarbon Chemistry: Analysis of Ethylene (C₂H₄) and D4-Ethylene (C₂D₄) Ices Exposed to Ionizing Radiation via Combined Infrared Spectroscopy and Reflectron Time-of-flight Mass Spectrometry

The radiation exposure of small hydrocarbons like methane (CH₄), ethane (C₂H₆), and acetylene (C₂H₂) has received considerable attention during the past decade. However, the closely related C₂ hydrocarbon, ethylene (C₂H₄), has been rather disregarded, as only a few experiments investigating its role in extraterrestrial ices and their mixtures have been conducted. Ethylene was detected in the interstellar environments CRL 618 (Cernicharo et al. 2001a) and IRC +10216 (Betz 1981; Hinkle et al. 2008) and in the outer solar system, in places like Pluto (Merlin 2015; Gladstone et al. 2016), Titan (Hanel et al. 1981; Niemann et al. 2005; Shemansky et al. 2005), Neptune (Schulz et al. 1999), Saturn (Encrenaz et al. 1975), and—in its condensed form as an ice—on Makemake (Brown et al. 2015). Also, laboratory experiments simulating the processing of methane ice in interstellar clouds (Boogert et al. 2015) via energetic particles produces ethylene from methane ice (Gerakines et al. 1996; Kaiser & Roessler 1998; Bennett et al. 2006; Jones & Kaiser 2013; Paardekooper et al. 2014).

Ausloos & Gorden (1962) reported the first processing of pure solid ethylene (77 K) to study the formation of molecular hydrogen from γ-radiation; no other products were determined. Wagner (1962) showed that the processing of solid ethylene (77 K) with 3 MeV bremsstrahlung produced hydrogen, methane, acetylene, and ethane, as well as larger hydrocarbons in additional units of C₂ from C₂–C₁₈. The C₂ compounds were analyzed via mass spectrometry, while the C₄–C₁₈ molecules were determined via gas chromatography. Specific molecules, larger than C₂, included butadiene (C₄H₆), 2-butene (C₄H₈), 1-butene (C₄H₈), n-butane (C₄H₁₀), 3-hexene (C₆H₁₂), cis-2-hexene (C₆H₁₂), trans-2-hexene (C₆H₁₂), 1-hexene (C₆H₁₂), 2-ethyl-1-butene (C₆H₁₂), 3-methyl-2-pentene (C₆H₁₂), 3-methyl-1-pentene (C₆H₁₂), 3-methylpentane (C₆H₁₄), n-hexane (C₆H₁₄), 3,4-dimethylhexane (C₈H₁₈), 3-methyl-3ethylpentane (C₈H₁₈), 3-ethylhexane (C₈H₁₈), 3-methylheptane (C₈H₁₈), n-octane (C₈H₁₈), and decene (C₁₀H₂₀). The C₁₀ and C₁₂ compounds were determined to be branched; the major product shown was 1-butene (C₄H₈) (Wagner 1962). Tschuikow-Roux et al. (1967) studied solid ethylene (36 K) processed by ultraviolet photolysis (8.4 eV) and collected products that were condensable with liquid nitrogen and analyzed off-line via gas chromatography. The products identified from the ultraviolet photolysis of pure ethylene ice were acetylene (C₂H₂), ethane (C₂H₆), allene (C₃H₄), cyclopropene (c-C₃H₄), propene (C₃H₆), cyclopropane (c-C₃H₆), propane (C₃H₈), methylcyclopropene (c-C₄H₆), 1-butene (C₄H₈), isobutene (C₄H₈), trans-2-butene (C₄H₈), cis-2-butene (C₄H₈), methyl cyclopropane (c-C₄H₈), n-butane (C₄H₁₀), and isobutane (C₄H₁₀); the primary product was determined to be 1-butene (Tschuikow-Roux et al. 1967). Gorden & Ausloos (1971) added to the literature the ultraviolet photolysis (8.4 eV, 10.0 eV, 11.6–11.8 eV, 21.2 eV) and γ-radiolysis of pure ethylene ice at 20 and 77 K, respectively, in order to determine the mechanism that ethylene undergoes to form larger hydrocarbons. The products analyzed with gas chromatography–mass spectrometry (GC-MS) were determined to be molecular hydrogen (H₂), acetylene (C₂H₂), propene (C₃H₆), cyclopropane (c-C₃H₆), 1-butene (C₄H₈), 2-butene (C₄H₈), cyclobutene (c-C₄H₆), n-butane (C₄H₁₀), cyclobutane (c-C₄H₈), and at least eight hexene (C₆H₁₂) isomers; no attempt was made to analyze molecules larger than C₆ products.

Kaiser & Roessler (1998) provided the first astrochemistry-based study of irradiated pure ethylene ice at 10 K with 9 MeV α-particles to investigate the interaction of cosmic-ray particles with this hydrocarbon ice as a simple model of interstellar ices. Fourier transform infrared (FTIR) spectroscopy showed vibrational modes attributed to ethane (C₂H₆), acetylene (C₂H₂), and ethyl radicals (C₂H₅), as well as acetylinic (H-CC-R), olefinic (=CH₂), and aliphatic (R-CH₂-R') structures. Meanwhile, temperature programmed desorption (TPD) studies using a quadrupole mass spectrometer (QMS) detected acetylene (C₂H₂), ethane (C₂H₆), allene (C₃H₄), methylacetylene (C₃H₄), propene (C₃H₆), cyclopropane (C₃H₆), propane (C₃H₈), n-butane (C₄H₁₀), isobutane (C₄H₁₀), dimethylpropane (C₅H₁₂), n-pentane (C₅H₁₂), and isopentane (C₅H₁₂), as well as larger alkanes (C_nH_2n+2; n = 6–14). Bennett & Mile (1973) studied the reaction of solid ethylene with hydrogen atoms using electron spin resonance and found that the addition of hydrogen atoms to the ethylene molecule occurred readily to form ethyl radicals (C₂H₅) and—via reaction with a neighboring ethylene molecule—n-butyl radicals (C₄H₉). To better understand the nondetection of ethylene in comets, the reaction of hydrogen atoms with solid ethylene was investigated again later using FTIR and QMS analytical techniques, and it was found that ethane was the primary product while butane was produced at about 5% of ethane (Hiraoka et al. 2000, 1999). These experiments suggest that ethylene present in interstellar ices at 10 K would be easily converted to ethane; these findings are supported by the detection of acetylene and ethane and nondetection of ethylene in comets, which are the least altered records from planetary formation. Also, Strazzulla et al. (2002) studied the ion irradiation of pure ethylene ice while monitoring the changes in situ with FTIR. After irradiation with 30 keV He⁺ ions, the most abundant products were methane, ethane, and acetylene, and a "polymer-like" refractory residue was formed. Irradiation with 15 keV N⁺ ions resulted in the production of HCN and mononitriles (R-CN). Compagnini et al. (2009) studied the irradiation of solid ethylene with 200 keV H⁺ ions and detected, via Raman spectroscopy, acetylene and features assigned to polyynes. Ennis et al. (2011) processed solid ethylene with 5 keV oxygen ions to investigate solar system hydrocarbon ices exposed to solar wind and planetary magnetosphere type ions. Products detected online and in situ via FTIR spectroscopy and QMS were acetylene and vibrational modes attributed to larger aliphatic hydrocarbon species. Recently Zhou et al. (2014) used energetic keV electrons to process solid ethylene to explore hydrocarbon chemistry pathways on Titan and on methane-bearing interstellar grains. Using FTIR and QMS, products were determined to be methane, acetylene, ethane, the ethyl radical, 1-butene, and n-butane.

These previous studies utilized a broad assortment of techniques to analyze the samples ranging from online and in situ FTIR and QMS using electron impact ionization to off-line and ex-situ gas chromatography–mass spectrometry. FTIR has been a very common tool in assessing the consequences of irradiating astrophysical ice analogs (Khare et al. 1989; Moore et al. 1996; Caro & Schutte 2003; Abplanalp et al. 2016), but this technique has several limitations. First, the detection of only individual molecules via FTIR is possible if they are small, such as carbon monoxide (CO), water (H₂O), carbon dioxide (CO₂), methane (CH₄), and ammonia (NH₃), all of which have been detected as constituents in interstellar ices. Second, FTIR fails at giving unique identification to larger complex organic molecules (COMs) and can instead only provide information of what functional groups are present in the sample that was analyzed (Socrates 2004). Third, this often leads to overlapping of infrared vibrations from multiple different molecules (Bennett et al. 2005b; Zhou et al. 2008) and incomplete assignments of products. Therefore, product assignments of infrared bands should be supported by complimentary analytical techniques.

Gas phase analysis, via mass spectrometry, during TPD can provide the supporting data to product assignments that are complimentary to FTIR. Typically gas phase products are analyzed, upon sublimation, using an electron impact quadrupole mass spectrometer (EI-QMS; Kaiser et al. 1995a, 1995b; Fraser et al. 2002; Ioppolo et al. 2011; Jiménez-Escobar & Caro 2011; Duvernay et al. 2014). This technique is often operated at an ionization energy where organic molecules have a maximum cross section (70–100 eV), to easily ionize most molecules. However, the trade-off to this approach is that this type of ionization often causes complex fragmentation patterns and, possibly, the nondetection of the molecular ions of molecules. Additionally, these fragment ions overlap from multiple parent ions, especially between structural isomers, making the assignment of a specific isomer extremely challenging (Kaiser et al. 1997a, 2010; Kaiser & Roessler 1997; Bennett et al. 2005a; Bennett & Kaiser 2007).

The third type of analytical technique employed by several of the previous studies was off-line GC-MS, which has been used to analyze nonvolatile residues produced from analog ices (Meinert et al. 2012; Callahan et al. 2013; Abou Mrad et al. 2014; de Marcellus et al. 2015). The product molecules, especially residues, need some type of further processing like acid hydrolyses and trimethylsilyl (–Si(CH₃)₃) derivatization in order to perform this analysis, and it is possible that these techniques cause modification and/or degradation of the initial residue formed (Fang et al. 2015).

Although multiple attempts have been made to investigate the hydrocarbon chemistry of processed solid ethylene ice, these studies have not utilized sensitive analytical techniques online and in situ, and these traditional methods that were employed are not able to identify specific new products. To solve this problem, we have incorporated the method of tunable photoionization coupled with reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). The capabilities of soft photoionization via vacuum ultraviolet (VUV) light, with the application to analyzing the subliming products of astrophysically relevant ice analogs, have previously been reported (Jones & Kaiser 2013; Kaiser et al. 2014, 2015; Maity et al. 2014a, 2014b, 2015; Förstel et al. 2015; Maksyutenko et al. 2015; Turner et al. 2015, 2016; Abplanalp et al. 2016) and show that this is an extremely useful tool. This soft photoionization method results in minimal, if any, fragmentation of the molecular ion close to the ionization energy and thus avoids the drawback that occurs when using EI-QMS. Furthermore, this superior alternative method allows for the discrimination of structural isomers based on their individual ionization energy. For example, if EI-QMS was used to analyze a simple system such as a mixed carbon monoxide (CO)—methane ice (CH₄) for acetaldehyde (CH₃CHO; m/z = 44), there would be overlapping signals at this mass-to-charge ratio from propane (C₃H₈), carbon dioxide (CO₂; m/z = 44), ethylene oxide (c-C₂H₄O; m/z = 44), and vinyl alcohol (H₂CCHOH; m/z = 44), all of which are expected products from this ice mixture (Kaiser et al. 2014). However, using PI-ReTOF-MS bypasses this problem by tuning the photon energy to selectively photoionize only one of these molecules/isomers and uniquely identify the isomers formed and relative yields (Abplanalp et al. 2015, 2016; Forstel et al. 2016). By employing PI-ReTOF-MS and FTIR concurrently, the monitoring of gas and ice phase products, respectively, can further constrain FTIR assignments, as a decrease in an infrared peak during the appearance of an ion in the PI-ReTOF-MS data shows their relationship.

In this study we present the detection of complex hydrocarbon molecules from C₃–C₁₆ via PI-ReTOF-MS produced from the interaction of ionizing radiation, in the form of energetic electrons, with pure ethylene and D4-ethylene ices. The previous investigations (FTIR, QMS, GC-MS) detected several products that would differ from one investigation to the next using analytical methods that are not very useful in untangling the complex chemistry taking place in the ethylene ice. Therefore, PI-ReTOF-MS data complimenting FTIR allow the extraction of the chemical pathways that are present in ethylene ice and allow for a better understanding of ethylene pathways in more complex ices. The detection of ethylene ice as a constituent on Makemake (Brown et al. 2015) and the product of methane ice irradiation (Gerakines et al. 1996; Kaiser & Roessler 1998; Bennett et al. 2006; Jones & Kaiser 2013; Paardekooper et al. 2014) show that a better understanding of the ethylene chemistry available in astrophysical ice analogs is desired.

The experimental setup consisted of an ultrahigh vacuum (UHV) chamber evacuated to about 3 × 10⁻¹¹ torr via magnetically suspended turbo molecular pumps backed by dry oil-free scroll pumps. Within the UHV chamber the substrate, a polished silver mirror, is interfaced via indium foil to promote thermal conductivity to an oxygen-free high-conductivity copper target that is cooled to 5.5 ± 0.1 K using a UHV compatible closed-cycle helium compressor (Sumitomo Heavy Industries, RDK-415E). The target is translatable in the vertical axis using a UHV compatible bellows (McAllister, BLT106) and rotatable in the horizontal plane via a differentially pumped rotary feedthrough (Thermoionics Vacuum Products, RNN-600/FA/MCO).

Pure ethylene gas (C₂H₄, Linde, 99.999%) was deposited onto the cooled silver substrate via a glass capillary array positioned 30 mm from the target at main chamber background pressures of about 5 × 10⁻⁸ torr held for up to 7 minutes until the desired ice thickness is achieved. The thickness of the ice was monitored online and in situ via laser interferometry during the gas deposition from an HeNe laser (λ = 632.8 nm; CVI Melles-Griot; 25-LHP-230) reflecting off of the silver substrate into a photodiode (Groner et al. 1973; Maity et al. 2014a; Turner et al. 2015). By using a refractive index (n) of n = 1.35 (Hudson et al. 2014a), the thickness of the ethylene ice was determined to be 470 ± 25 nm. Alternatively, by using a modified Lambert–Beer relationship with ethylene absorption coefficients of 1.03 × 10⁻¹⁸, 1.51 × 10⁻¹⁸, 1.04 × 10⁻¹⁹, 2.77 × 10⁻¹⁹, and 1.03 × 10⁻¹⁹ cm molecule⁻¹ (Hudson et al. 2014a) for the integrated areas of the infrared peaks at 2974 (ν₁₁), 3085 (ν₉), 4188 (ν₆ + ν₁₁), 4495 (ν₅  + ν₁₂), and 4703 (ν₂ + ν₉) (Table 1), we calculated an average thickness of 540 ± 150 nm, which is very similar to the laser interferometry method used. This procedure was repeated with isotopic ices of D4-ethylene (C₂D₄, CDN Isotopes, 99.8% D) and then irradiated to confirm FTIR assignments and subliming molecules assignments via their isotopic shifts induced by the added deuterium.

An area of 1.0 ± 0.1 cm² of the ethylene ice was then irradiated with 5 keV electrons for 1 hr with a current of 30 nA. The irradiation was done at an angle of incidence of 70° relative to the surface normal. Next, utilizing CASINO 2.42 software (Drouin et al. 2007), the average penetration depth of the energetic electrons into the ice was determined to be 320 ± 20 nm, with an average dose deposited of 5.0 ± 0.8 eV molecule⁻¹ in the ethylene ice utilizing a density of 0.75 g cm⁻³ (Hudson et al. 2014a; van Nes 1978) (Table 2). It should be noted that this calculated penetration depth (320 ± 20 nm) is much lower than the measured total thickness of the ethylene ice (470 ± 25 nm), so that no processing of the substrate occurs. The chemical evolution of the ethylene ice was monitored online and in situ via FTIR (Nicolet 6700) using a reflection angle of 45° for absorption-reflection-absorption mode. The FTIR data were recorded from 6000 to 500 cm⁻¹ at a resolution of 4 cm⁻¹ continuously throughout the experiment from before irradiation through TPD to 300 K to monitor the products formed within the sample. Once the irradiation phase of the experiment was completed, the ice was held at 5.5 K for an additional hour before starting TPD studies of heating the substrate from 5.5 to 300 K at a rate of 0.5 K min⁻¹.

Table 2.  Data Applied to Calculate the Irradiation Dose per Molecule in the C₂H₄ and C₂D₄ Ice

Initial kinetic energy of the electrons, Einit	5 keV
Irradiation current, I	30 ± 2 nA
Total number of electrons	(6.7 ± 0.5) × 1014
Average kinetic energy of backscattered electrons, Ebsa	3.1 ± 0.3 keV
Fraction of backscattered electrons, fbsa	0.30 ± 0.03
Average kinetic energy of transmitted electrons, Etransa	1.8 ± 0.3 keV
Fraction of transmitted electrons, ftransa	0.14 ± 0.01
Average penetration depth, la	320 ± 20 nm
Density of the ice, ρ	0.75 ± 0.05 g cm−3
Irradiated area, A	1.0 ± 0.1 cm2
Total # molecules processed	(5.1 ± 1.4) × 1017
Dose per 28 amu, ${D}_{{\rm{C}}2{\rm{H}}6}$	5.0 ± 0.8 eV
Dose per 36 amu, ${D}_{{\rm{C}}2{\rm{D}}6}$	5.7 ± 0.9 eV

Note.

^aCASINO output values.

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Throughout the TPD process the ice was monitored via FTIR, while the subliming molecules were detected using a QMS (Extrel, Model 5221) operating in residual gas analyzer (RGA) mode and the PI-ReTOF-MS procedure. The RGA operates with an electron impact ionization source (100 eV), an emission current of 1 mA, and a mass range from 1 to 300 amu. A complete description of the PI-ReTOF-MS procedure employed has previously been discussed (Jones & Kaiser 2013; Abplanalp et al. 2015), and only a brief summary of the procedure will be discussed here. First, the generated pulsed coherent VUV light with an energy of 10.49 eV (λ = 118.2 nm) and flux of 10¹⁰ photons pulse⁻¹ (Förstel et al. 2016) was used to ionize subliming molecules from the substrate during TPD. These ions are then detected via a modified reflectron time-of-flight mass spectrometer (ReTOF; Jordan TOF products, Inc.) using a dual chevron configured multichannel plate (MCP). Next, a fast pre-amplifier (Ortec 9305) was used to amplify the MCP signals, which are then shaped by a 100 MHz discriminator. Finally, the spectra are recorded using a personal-computer-based multichannel scaler (FAST ComTec, P7888-1 E) with 4 ns bin widths triggered at 30 Hz using a pulse delay generator (Quantum Composers, 9518) and 3600 sweeps per mass spectrum per 1 K increase in temperature during TPD.

3.1. Infrared Spectroscopy

Several discrete irradiation products were detected in situ via FTIR along with the broadening of ethylene stretches, the latter of which is possibly due to overlapping of vibrational modes of products with those of the reactant (Abplanalp & Kaiser 2016). Product assignments consisting of methane [CH₄ (CD₄)], acetylene [C₂H₂ (C₂D₂)], the ethyl radical [C₂H₅ (C₂D₅)], ethane [C₂H₆ (C₂D₆)], 1-butene [C₄H₈ (C₄D₈)], and n-butane [C₄H₁₀ (C₄D₁₀)] were assigned in both the ethylene and D4-ethylene ices, respectively (Table 1, Figure 1). There was a single radical identified, the ethyl radical, which was observed in the D4-ethylene ice experiments and was most likely a contributor to the broadening of the large ethylene fundamental stretch (ν₉). The FTIR spectrum from 6000 to 3350 cm⁻¹ and from 2800 to 500 cm⁻¹ showed a small number of newly emerged absorptions that were easily assigned to the previously mentioned products (Figure 2). Figure 3 shows that this was not the case in the range of 3350–2800 cm⁻¹, though, and that multiple new infrared stretches and several overlapping reactant positions developed from the processing of the ethylene ice with the impinging electrons. Therefore, the infrared spectrum was deconvoluted (Abplanalp et al. 2015) to identify possible constituents. This technique allowed the identification of 11 new infrared peaks that could be assigned to five different product molecules: methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆), 1-butene (C₄H₈), and n-butane (C₄H₁₀) (Table 1; Figure 3). These infrared identifications agree with the only two other studies that incorporated in situ infrared analysis of the sample (Kaiser & Roessler 1998; Zhou et al. 2014).

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Further FTIR analysis during TPD shows that molecules of higher molecular weight than those identified in Table 1 are formed, as infrared stretches are still visible in the temperature range of 90–200 K (Figures 4(a)–(f)). No new infrared bands were detected during heating, and no increase in any infrared band signal detected at 5.5 K was observed during the TPD, showing that these molecules were formed at 5.5 K from the electron irradiation. The deconvoluted spectrum from 3000 to 2800 cm⁻¹ is presented at 90 K, which is the temperature just prior to the sublimation event correlated with n-butane via mass spectrometry (Figure 4(a)). Although n-butane was the largest molecule identified via FTIR, there were still seven infrared stretches visible at 105 K (Figure 4(b)), which is above the sublimation temperature of n-butane of 90 K. These infrared bands are observed to decrease in intensity during TPD (Figures 4(c)–(d)), and the simultaneous monitoring using the RGA and PI-ReTOF-MS provides further information of the contributing molecules to the observed infrared stretches observed at higher temperatures.

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3.2. Mass Spectrometry—RGA

3.3. Mass Spectrometry—PI-ReTOF-MS

During TPD, the subliming molecules were detected via PI-ReTOF-MS utilizing 10.49 eV photons as an ionizing source (Figure 5, Table 3). Figure 5 depicts the intensities of the mass-to-charge ratio signals as a function of temperature detected during TPD of irradiated ethylene and D4-ethylene ices with signals approaching m/z = 275. It should be noted that the difference in intensity of signals between these two ices is due to the photon flux rather than the abundance produced within each ice. Seven groups of hydrocarbons with the following general molecular formulae were detected: C_nH_2n+2 (n = 4–10), C_nH_2n (n = 2–12, 14, 16), C_nH_2n−2 (n = 3–12, 14, 16), C_nH_2n−4 (n = 4–12, 14, 16), C_nH_2n−6 (n = 4–10, 12), C_nH_2n−8 (n = 6–10), and C_nH_2n−10 (n = 6–10). The majority of the ionized products detected within these groups, as well as the entire C_nH_2n−4, C_nH_2n−6, C_nH_2n−8, and C_nH_2n−10 species, represent molecules that have been hitherto unidentified as products of irradiated ethylene ice.

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Table 3.  Masses Correlated to Molecules Detected in Both Experiments

m/z	C2H4 (30 nA; 10.49 eV)	C2D4 (30 nA; 10.49 eV)	m/z
28	C2H4	C2D4	32
40	C3H4	C3D4	44
42	C3H6	C3D6	48
50	C4H2	C4D2	52
	N/A	C4D3H	55
52	C4H4	C4D4	56
53	13CC3H4	13CC3D4	57
	N/A	C4D5H	59
54	C4H6	C4D6	60
55	13CC3H6	13CC3D6	61
	N/A	C4D7H	63
56	C4H8	C4D8/aC5D2	64
57	13CC3H8	13CC3D8	65
58	C4H10	n.d.	68
59	13CC3H10	n.d.	69
62	aC5H2	C4D8/aC5D2	64
64	C5H4	n.d.	68
65	aC5H5	n.d.	70
66	C5H6	C5D6	72
67	aC5H7	aC5D7	74
68	a13CC4H7	a13CC4D7	75
68	C5H8	C5D8/C6D2	76
69	13CC4H8	n.d.	77
69	aC5H9	aC5D9	78
70	C5H10	C5D10/C6D4	80
71	13CC4H10	n.d.	81
71	aC5H11	aC5D11/aC6D5	82
72	C5H12	C5D12/C6D6	84
73	13CC4H12	n.d.	85
74	C6H2	C5D8/C6D2	76
76	C6H4	C5D10/C6D4	80
77	aC6H5	aC5D11/aC6D5	82
78	C6H6	C5D12/C6D6	84
79	aC6H7	aC6D7/aC7D1	86
80	13CC5H7	13CC5D7/C6D7H	87
80	C6H8	C6D8	88
81	13CC5H8	13CC5D8	89
81	aC6H9	aC6D9/aC7D3	90
82	a13CC5H9	13CC5D9/C6D9H	91
82	C6H10	C6D10/C7D4	92
83	13CC5H10	13CC5D10	93
83	aC6H11	aC6D11/aC7D5	94
84	a13CC5H11	C6D11H	95
84	C6H12	C6D12/C7D6	96
85	13CC5H12	13CC5D12	97
85	aC7H1	aC6D7/aC7D1	86
	N/A	C6D13H/13CC6D7	99
86	C6H14	C6D14/C7D8	100
87	13CC5H14	n.d.	101
87	aC7H3	aC6D9/aC7D3	90
88	C7H4	C6D10/C7D4	92
89	aC7H5	aC6D11/aC7D5	94
90	C7H6	C6D12/C7D6	96
91	aC7H7	aC7D7	98
92	13CC6H7	13CC6D7/C6D13H	99
92	C7H8	C6D14/C7D8	100
93	aC7H9	aC7D9	102
94	C7H10	C7D10/C8D4	104
95	aC7H11	aC7D11/aC8D5	106
96	C7H12	C7D12	108
97	13CC6H12	n.d.	109
97	aC7H13	aC7D13/aC8D7	110
98	C7H14	C7D14/C8D8	112
99	13CC6H14	13CC6D14/13CC7D8	113
99	aC7H15	aC7D15/aC8D9	114
100	C7H16	C7D16/C8D10/C9D4	116
101	13CC6H16	n.d.	117
101	aC8H5	aC7D11/aC8D5	106
102	C8H6	C7D12/C8D6	108
103	aC8H7	aC7D13/aC8D7	110
104	C8H8	C7D14/C8D8	112
105	13CC7H8	13CC6D14/13CC7D8	113
105	aC8H9	aC7D15/aC8D9	114
106	C8H10	C7D16/C8D10/C9D4	116
107	aC8H11	aC8D11/aC9D5	118
108	C8H12	C8D12/C9D6	120
109	13CC7H12	n.d.	121
109	aC8H13	aC8D13/aC9D7	122
110	C8H14	C8D14/C9D8/C10D2	124
111	13CC7H14	13CC7D14/13CC8D8/13CC9D2	125
111	aC8H15	aC8D15/aC9D9	126
	N/A	C8D15H /C9D9H	127
112	C8H16	C8D16/C9D10	128
113	13CC7H16	13CC7D16/13CC8D10/13CC9D4	129
113	aC8H17	aC8D17/aC9D11/aC10D5	130
113	aC9H5	aC8D11/aC9D5	118
114	C8H18	C8D18/C9D12	132
115	13CC7H18	n.d.	133
115	aC9H7	aC8D13/aC9D7	122
116	C9H8	C8D14/C9D8	124
117	aC9H9	aC8D15/aC9D9	126
118	C9H10	C8D16/C9D10	128
119	13CC8H10	13CC7D16/13CC8D10	129
119	aC9H11	aC8D17/aC9D11/aC10D5	130
120	C9H12	C8D18/C9D12	132
121	aC9H13	aC9D13	134
122	C9H14	C9D14	136
123	aC9H15	aC9D15/aC10D9	138
124	C9H16	C9D16/C10H10	140
125	13CC8H16	n.d.	141
125	aC9H17	aC9D17/aC10D11/aC11D5	142
125	aC10H5	aC8D17/aC9D11/aC10D5	130
126	C9H18	C9D18/C10D12	144
127	13CC8H18	n.d.	145
127	aC9H19	aC9D19/aC10D13/aC11D7	146
128	C9H20	C9D20/C10D14	148
129	aC10H9	aC9D15/aC10D9	138
130	C10H10	C9D16/C10H10	140
131	aC10H11	aC9D17/aC10D11/aC11D5	142
132	C10H12	C9D18/C10D12	144
133	aC10H13	aC9D19/aC10D13/aC11D7	146
134	C10H14	C9D20/C10D14	148
135	aC10H15	aC10D15/aC11D9/aC12D3	150
136	C10H16	C10D16	152
137	aC10H17	aC10D17/aC11D11/aC12D5	154
137	aC11H5	aC9D17/aC10D11/aC11D5	142
138	C10H18	C10D18	156
139	13CC9H18	n.d.	157
139	aC10H19	aC10D19/aC11D13/aC12D7	158
139	aC11H7	aC9D19/aC10D13/aC11D7	146
140	C10H20	C10D20	160
141	13CC9H20	n.d.	161
141	aC10H21	aC10D21/aC11D15/aC12D9	162
141	aC11H9	aC10D15/aC11D9/aC12D3	150
142	C10H22	C10D22	164
143	13CC9H22	n.d.	165
143	aC11H11	aC10D17/aC11D11/aC12D5	154
145	aC11H13	aC10D19/aC11D13/aC12D7	158
147	aC11H15	aC10D21/aC11D15/aC12D9	162
147	aC12H3	aC10D15/aC11D9/aC12D3	150
149	aC11H17	aC11D17/aC13D5	166
149	aC12H5	aC10D17/aC11D11/aC12D5	154
150	C11H18	C11D18	168
151	aC11H19	aC11D19/aC13D7	170
151	aC12H7	aC10D19/aC11D13/aC12D7	158
152	C11H20	C11D20	172
153	aC12H9	aC10D21/aC11D15/aC12D9	162
154	C11H22	C11D22	176
161	aC12H17	aC12D17/aC13D11	178
161	aC13H5	aC11D17/aC13D5	166
162	C12H18	C12D18	180
163	aC12H19	aC12D19/aC13D13	182
163	aC13H7	aC11D19/aC13D7	170
164	C12H20	C12D20	184
165	aC12H21/aC13H9	n.d.	186/174
166	C12H22	C12D22	188
167	13CC11H22	n.d.	189
167	aC13H11	aC13D11	178
168	C12H24	C12D24	192
169	13CC11H24	n.d.	193
169	aC13H13	aC12D19/aC13D13	182
192	C14H24	C14D24	216
194	C14H26	C14D26	220
196	C14H28	C14D28	224
220	C16H28	C16D28	248
222	C16H30	C16D30	252
224	C16H32	C16D32	256

Note. Italics represent a minor possible contributor to an ion signal; N/A designates that there is no respective non-isotopic form of the observed deuterated molecule; n.d. denotes that no signal was detected corresponding to this ion.

^aDesignates the assigned formula as a fragment.

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3.3.1. C_nH_2n+2

Figure 6 shows seven TPD profiles of the ion signals corresponding to the most saturated hydrocarbon group, alkanes (C_nH_2n+2; n = 4–10), that were detected with PI-ReTOF-MS during TPD. The lack of signal for methane (CH₄) and ethane (C₂H₆) was expected, as they have ionization energies of 12.61 and 11.52 eV (Lias et al. 2016), respectively, which are both larger than the 10.49 eV photoionization energy used in the experiment. The FTIR detection of n-butane was confirmed via the detection of m/z = 58 (${{\rm{C}}}_{4}{{{\rm{H}}}_{10}}^{+}$), which began to sublime at 90 K. Although FTIR determined the largest molecule to n-butane, several larger alkanes were also detected at m/z = 72 (${{\rm{C}}}_{5}{{{\rm{H}}}_{12}}^{+}$, 107 K), m/z = 86 (${{\rm{C}}}_{6}{{{\rm{H}}}_{14}}^{+}$, 111 K), m/z = 100 (${{\rm{C}}}_{7}{{{\rm{H}}}_{16}}^{+}$, 125 K), m/z = 114 (${{\rm{C}}}_{8}{{{\rm{H}}}_{18}}^{+}$, 129 K), m/z = 128 (${{\rm{C}}}_{19}{{{\rm{H}}}_{20}}^{+}$, 148 K), and m/z = 142 (${{\rm{C}}}_{10}{{{\rm{H}}}_{22}}^{+}$, 151 K). An increase in sublimation temperature from 3 to 19 K is observed for each additional CH₂ unit and from 18 to 23 K for each C₂H₄ unit added. These sublimation onset temperatures correlate well with several of these recently detected alkanes using the same apparatus (Abplanalp & Kaiser 2016). The ion signal for n-butane, m/z = 58 (${{\rm{C}}}_{4}{{{\rm{H}}}_{10}}^{+}$), was determined to be very low, due to n-butane (CH₃CH₂CH₂CH₃) having an ionization energy of 10.5 ± 0.1 eV (Lias 1982, p. 409), which is at the limit of the 10.49 eV photoionization energy used in the present experiment.

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A trend displayed in this hydrocarbon group is that signal intensity decreases with an increase in the molecular size. However, this trend is divided by even or odd numbers of carbon units contained in the ions. This trend is detected starting after m/z = 58, due to n-butane having a photoionization energy near the threshold of the experimental photoionization energy used. For example, when an odd carbon ion signal (${{\rm{C}}}_{5}{{{\rm{H}}}_{12}}^{+}$) is compared to the next largest odd carbon ion signal (${{\rm{C}}}_{7}{{{\rm{H}}}_{16}}^{+}$), a decrease in intensity is observed, and the same trend is detected when comparing an even carbon unit (${{\rm{C}}}_{6}{{{\rm{H}}}_{14}}^{+}$) to the next largest even carbon alkane (${{\rm{C}}}_{8}{{{\rm{H}}}_{18}}^{+}$). However, this trend does not occur when comparing a smaller odd carbon unit ion signal (${{\rm{C}}}_{5}{{{\rm{H}}}_{12}}^{+}$) to the next largest even carbon unit ion signal (${{\rm{C}}}_{6}{{{\rm{H}}}_{14}}^{+}$), and rather this trend is reversed as the signal of even carbon unit alkanes is higher than the smaller odd carbon unit ion they are compared to. This later observation also shows a second trend that all intensities of odd carbon ion signals (${{\rm{C}}}_{5}{{{\rm{H}}}_{12}}^{+}$, ${{\rm{C}}}_{7}{{{\rm{H}}}_{16}}^{+}$, ${{\rm{C}}}_{9}{{{\rm{H}}}_{20}}^{+}$) have lower intensities than even carbon ion signals (${{\rm{C}}}_{6}{{{\rm{H}}}_{14}}^{+}$, ${{\rm{C}}}_{8}{{{\rm{H}}}_{18}}^{+}$, ${{\rm{C}}}_{10}{{{\rm{H}}}_{22}}^{+}$) of similar size. These trends may have implications for the formation mechanism discussed in Section 4.9; however, these ion signals have not been normalized with their respective photoionization cross sections, as each isomer would need to be discriminated to determine relative quantities produced.

3.3.2. C_nH_2n

Although the previous group, alkanes C_nH_2n+2, can only belong to this one type of hydrocarbon arrangement involving single carbon–carbon bonds, the next group, C_nH_2n(n = 2–12, 14, 16), can be described as alkenes or the double-bond equivalent (DBE) (cycloalkanes). Figure 7 shows the PI-ReTOF-MS data for this group of hydrocarbons, with the top panel displaying the ion signal for the reactant ethylene (C₂H₄), but all other panels shown represent detected products from the irradiation of ethylene. Many unsaturated hydrocarbons relevant to this group were detected, including m/z = 28 (${{\rm{C}}}_{2}{{{\rm{H}}}_{4}}^{+}$, 60 K), m/z = 42 (${{\rm{C}}}_{3}{{{\rm{H}}}_{6}}^{+}$, 73 K), m/z = 56 (${{\rm{C}}}_{4}{{{\rm{H}}}_{8}}^{+}$, 86 K), m/z = 70 (${{\rm{C}}}_{5}{{{\rm{H}}}_{10}}^{+}$, 101 K), m/z = 84 (${{\rm{C}}}_{6}{{{\rm{H}}}_{12}}^{+}$, 111 K), m/z = 98 (${{\rm{C}}}_{7}{{{\rm{H}}}_{14}}^{+}$, 123 K), m/z = 112 (${{\rm{C}}}_{8}{{{\rm{H}}}_{16}}^{+}$, 130 K), m/z = 126 (${{\rm{C}}}_{9}{{{\rm{H}}}_{18}}^{+}$, 140 K), m/z = 140 (${{\rm{C}}}_{10}{{{\rm{H}}}_{20}}^{+}$, 147 K), m/z = 154 (${{\rm{C}}}_{11}{{{\rm{H}}}_{22}}^{+}$, 158 K), m/z = 168 (${{\rm{C}}}_{12}{{{\rm{H}}}_{24}}^{+}$, 163 K), m/z = 196 (${{\rm{C}}}_{14}{{{\rm{H}}}_{28}}^{+}$, 179 K), and m/z = 224 (${{\rm{C}}}_{16}{{{\rm{H}}}_{32}}^{+}$, 193 K). Similar to the alkanes, an increase in sublimation temperature of 5–15 K per CH₂ unit added was observed. The detection of m/z = 56 (${{\rm{C}}}_{4}{{{\rm{H}}}_{8}}^{+}$) via PI-ReTOF-MS, which may correspond to 1-butene, substantiated the FTIR detection of this alkene.

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Similar to the alkanes, this group depicts the trend that, typically, a decrease in intensity of the ion signal is observed as the molecule increases in size based on the division of even and odd carbon unit molecules. The signal produced for m/z = 28 should be excluded from this trend, as it was the reactant. However, this trend fails for m/z = 84 (${{\rm{C}}}_{6}{{{\rm{H}}}_{12}}^{+}$) in this group of hydrocarbons, which may suggest that this is a major product of the ethylene irradiation, but without the untangling of this ion signal's contributors, and application of the respective photoionization cross sections does not allow for this speculation to be confirmed.

3.3.3. C_nH_2n−2

In situ analysis using FTIR showed that acetylene was a product of ethylene irradiation; however, this was the only alkyne detected with this method. Figure 8 shows several other ion signals, detected via PI-ReTOF-MS, corresponding to alkynes (C_nH_2n−2) or their DBEs (dienes, cycloalkenes, bicycloalkanes) of n = 3–12, 14, 16. These ion signals were detected at m/z = 40 (${{\rm{C}}}_{3}{{{\rm{H}}}_{4}}^{+}$, 78 K), m/z = 54 (${{\rm{C}}}_{4}{{{\rm{H}}}_{6}}^{+}$, 90), m/z = 68 (${{\rm{C}}}_{5}{{{\rm{H}}}_{8}}^{+}$, 104 K), m/z = 82 (${{\rm{C}}}_{6}{{{\rm{H}}}_{10}}^{+}$, 114 K), m/z = 96 (${{\rm{C}}}_{7}{{{\rm{H}}}_{12}}^{+}$, 126 K), m/z = 110 (${{\rm{C}}}_{8}{{{\rm{H}}}_{14}}^{+}$, 133 K), m/z = 124 (${{\rm{C}}}_{9}{{{\rm{H}}}_{16}}^{+}$, 144 K), m/z = 138 (${{\rm{C}}}_{10}{{{\rm{H}}}_{18}}^{+}$, 149 K), m/z = 152 (${{\rm{C}}}_{11}{{{\rm{H}}}_{20}}^{+}$, 160 K), m/z = 166 (${{\rm{C}}}_{12}{{{\rm{H}}}_{22}}^{+}$, 165 K), m/z = 194 (${{\rm{C}}}_{14}{{{\rm{H}}}_{24}}^{+}$, 176 K), and m/z = 222 (${{\rm{C}}}_{16}{{{\rm{H}}}_{30}}^{+}$, 189 K). Once again, each additional CH₂ unit resulted in the increase of the sublimation onset temperature of 5–14 K. This group of hydrocarbons follows the similar trend seen in both the C_nH_2n+2 and C_nH_2n ion signals with a decrease in intensity based on even or odd units of carbon contained in the molecules.

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3.3.4. C_nH_2n−4

The next most highly unsaturated group detected in these experiments, based on PI-ReTOF-MS but undetected by FTIR, had the general formula C_nH_2n−4 (n = 4–12, 14, 16) (Figure 9) and can correspond to multiple different structures (yne-ene, trienes, cyclodialkenes, bicycloalkenes). The ion signals related to this group were detected at m/z = 52 (${{\rm{C}}}_{4}{{{\rm{H}}}_{4}}^{+}$, 92 K), m/z = 66 (${{\rm{C}}}_{5}{{{\rm{H}}}_{6}}^{+}$, 108 K), m/z = 80 (${{\rm{C}}}_{6}{{{\rm{H}}}_{8}}^{+}$, 115 K), m/z = 94 (${{\rm{C}}}_{7}{{{\rm{H}}}_{10}}^{+}$, 128 K), m/z = 108 (${{\rm{C}}}_{8}{{{\rm{H}}}_{12}}^{+}$, 134 K), m/z = 122 (${{\rm{C}}}_{9}{{{\rm{H}}}_{14}}^{+}$, 149 K), m/z = 136 (${{\rm{C}}}_{10}{{{\rm{H}}}_{16}}^{+}$, 152 K), m/z = 150 (${{\rm{C}}}_{11}{{{\rm{H}}}_{18}}^{+}$, 163 K), m/z = 164 (${{\rm{C}}}_{12}{{{\rm{H}}}_{20}}^{+}$, 168 K), m/z = 192 (${{\rm{C}}}_{14}{{{\rm{H}}}_{24}}^{+}$, 179 K), and m/z = 220 (${{\rm{C}}}_{16}{{{\rm{H}}}_{28}}^{+}$, 192 K). An increase in the onset sublimation temperature of 3–16 K was observed for each additional CH₂ unit. As observed in the previous hydrocarbon groups, a decrease in ion signal intensity was observed as the molecular size increase and was based on the incorporation of even or odd units of carbon incorporated into the molecule.

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3.3.5. C_nH_2n−6

The PI-ReTOF-MS also detected the hydrocarbon group C_nH_2n−6 (n = 4–10, 12) (Figure 10). Ion signals at m/z = 50 (${{\rm{C}}}_{4}{{{\rm{H}}}_{2}}^{+}$, 99 K), m/z = 64 (${{\rm{C}}}_{5}{{{\rm{H}}}_{4}}^{+}$, 114 K), m/z = 78 (${{\rm{C}}}_{6}{{{\rm{H}}}_{6}}^{+}$, 118 K), m/z = 92 (${{\rm{C}}}_{7}{{{\rm{H}}}_{8}}^{+}$, 127 K), m/z = 106 (${{\rm{C}}}_{8}{{{\rm{H}}}_{10}}^{+}$, 136 K), m/z = 120 (${{\rm{C}}}_{9}{{{\rm{H}}}_{12}}^{+}$, 150 K), m/z = 134 (${{\rm{C}}}_{10}{{{\rm{H}}}_{14}}^{+}$, 156 K), and m/z = 162 (${{\rm{C}}}_{12}{{{\rm{H}}}_{18}}^{+}$, 169 K) were detected. Although the previous trend does not appear to continue for this group of hydrocarbons, this could be due to the much lower signals, as these are most likely minor products, and therefore photoionization cross sections may play a larger role in determining the relative ratios. Although this group is highly unsaturated, the mass-to-charge ratios are not able to be assigned to other molecules for n = 4–9, and the sublimation events for n = 10 and 12 occur at the anticipated temperatures for this group. Also, it should be clarified that the large peaks observed in the deuterated ion signals for n = 7 and 9 correspond to earlier identified hydrocarbon groups; however, these signals also show a smaller second peak that matches the sublimation event of their unambiguous nondeuterated partners.

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3.3.6. C_nH_2n−8

Also, the previously unidentified hydrocarbon group C_nH_2n−4 (n = 6–10) (Figure 11) was detected via signals at m/z = 76 (${{\rm{C}}}_{6}{{{\rm{H}}}_{4}}^{+}$, 111 K), m/z = 90 (${{\rm{C}}}_{7}{{{\rm{H}}}_{6}}^{+}$, 117 K), m/z = 104 (${{\rm{C}}}_{8}{{{\rm{H}}}_{8}}^{+}$, 133 K), m/z = 118 (${{\rm{C}}}_{9}{{{\rm{H}}}_{10}}^{+}$, 138 K), and m/z = 132 (${{\rm{C}}}_{10}{{{\rm{H}}}_{12}}^{+}$, 162 K). Again the trend previously observed of a distinct change in signal intensity based on even or odd carbon units incorporated into the molecule is no longer detected. Similarly to the C_nH_2n−6 hydrocarbon group, the deuterated compounds have multiple possible molecules associated with these ion signals, but a match between the deuterated peaks and the unambiguous nondeuterated sample shows that these signals in fact belong to multiple ions.

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3.3.7. C_nH_2n−10

Finally, the most highly unsaturated hydrocarbon group detected belonged to the general formula C_nH_2n−4 (n = 6–10) (Figure 12). The ions corresponding to this group were detected at m/z = 74 (${{\rm{C}}}_{6}{{{\rm{H}}}_{2}}^{+}$, 114 K), m/z = 88 (${{\rm{C}}}_{7}{{{\rm{H}}}_{4}}^{+}$, 118 K), m/z = 102 (${{\rm{C}}}_{8}{{{\rm{H}}}_{6}}^{+}$, 122 K), m/z = 116 (${{\rm{C}}}_{9}{{{\rm{H}}}_{8}}^{+}$, 129 K), and m/z = 130 (${{\rm{C}}}_{10}{{{\rm{H}}}_{10}}^{+}$, 159 K). No trend was observed for odd or even carbon unit molecules. Also, the observed peaks have multiple peaks or are broad, which may be due, in part, to these ions being produced as fragments of larger hydrocarbon groups (see Section 3.3.8). Also, just as for the previous two groups, the deuterated ion signals have multiple ions associated with these signals.

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3.3.8. Isotopes and Fragments

There were also several ion signals detected, which are able to be explained by either natural isotopic substitution or the fragmentation of larger hydrocarbons. First, these ion signals were overlaid with their possible parent isotopologues to confirm an isotopic assignment (Figures 13–15). The analysis technique employed the method of comparing sublimation onset temperatures to deduce whether ion signals that could belong to multiple molecules had similar profiles when compared to their isotopically shifted analog, as this shift separates the overlapping signals in question. For the ethylene (C₂H₄) ice the most abundant isotopologues would be a carbon-13 (+1 amu) substitution; therefore, only the next highest mass was analyzed for relative abundance to determine whether this was a correct assignment (Figures 13–14, Table 3). The D4-ethylene (C₂D₄) ice has the possibility of producing naturally substituted isotopologues containing carbon-13 (+1 amu) or hydrogen (−1 amu); therefore, both the next highest and lowest masses were investigated (Figure 15), if detected, to determine the validity of the assignment (Table 3). This technique reveals that some of these irregular ion traces are due to isotopic differences; however, there are several ions that are not able to be assigned as isotopologues.

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Therefore, it was necessary to investigate whether these remaining ions could be due to fragmentation of larger molecules. Typically PI-ReTOF-MS utilizing 10.49 eV photons is a "soft" ionization technique that results in no fragmentation; however, many of the larger hydrocarbons detected in this experiment are known to fragment at or below 10.49 eV (Bell et al. 2013; Lias et al. 2016; Urness et al. 2013). Ions that were suspected to be isotopologues but that had stronger ion signals than their prospective parent were thus not able to be assigned as an isotopologue, but it is possible that the isotopic component may contribute to the signal. To determine the assignment of these ion signals, the ethylene (C₂H₄) products and their respective ions from the D4-ethylene ice were compared (Figures 16–17) and then assigned accordingly (Table 3). It should be pointed out that if an assignment does not appear in Table 3, this is not due to a lack of analysis, but rather a lack of agreement with the previously defined analysis method. Also, the parent molecules that produced these fragment ions are able to be determined by comparing their sublimation profiles to larger hydrocarbons' sublimation profiles that could possibly produce these ions (Figures 18–21). Since both C₂H₄ and C₂D₄ ices were analyzed, each questionable fragment or isotope was also cross-analyzed between systems (Figures 13–21). This technique has been shown to be a useful tool in assigning certain ion signals that do not directly correspond to an expected molecular ion (Kaiser et al. 2014; Turner et al. 2015, 2016; Abplanalp & Kaiser 2016).

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4.1. Summary of Results

4.2. C_nH_2n+2

4.3. C_nH_2n

4.4. C_nH_2n−2

4.5. C_nH_2n−4

4.6. C_nH_2n−6

4.7. C_nH_2n−8

4.8. C_nH_2n−10

4.9. Reaction Mechanism

Previously, Zhou et al. (2014) irradiated solid ethylene and showed that multiple competitive pathways were available for the decomposition of ethylene. This was accomplished by taking into account the FTIR detection of methane (CH₄), acetylene (C₂H₂), ethylene (C₂H₄), the ethyl radical (C₂H₅), 1-butene (C₄H₈), and n-butane (C₄H₁₀) and kinetically fitting the coupled differential equations with the column density profiles of these products. The decomposition of ethylene (C₂H₄) via molecular hydrogen or two hydrogen atom loss to form acetylene (C₂H₂) (reaction (1)) is possible, as well as the competing reaction of the addition of a hydrogen atom to the ethylene molecule to form the ethyl radical (C₂H₅) (reaction (2)). A minor pathway available to ethylene decomposition was determined to be the carbon retro-insertion from methylcarbene (HCCH₃) (reaction (3)). Finally, ethylene was also found to be able to dimerize and produce 1-butene (reaction (4)). Also, the newly formed ethyl radicals can add another hydrogen atom to form ethane (reaction (5)), or two of the ethyl radicals may recombine barrierlessly to form n-butane (reaction (6)). The newly produced ethane is able to decompose into methane and carbene via retro-insertion (reaction (7)). Finally, the n-butane was also observed to decompose into 1-butene via hydrogen loss (reaction (8)). Most of these proposed reaction pathways, other than radical recombination, are endoergic up to a few eV, but this energy is supplied by the 5 keV electrons. Recall that these electrons deposited on average 5.0 ± 0.8 eV in the C₂H₄ ice and processed (5.1 ± 1.2) × 10¹⁷ ethylene molecules (Table 2).

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{4}({X}^{1}{A}_{1g})\to {{\rm{C}}}_{2}{{\rm{H}}}_{2}({X}^{1}{A}_{g})+2{\rm{H}}{(}^{2}{S}_{1/2})/{{\rm{H}}}_{2}({X}^{1}{{{\rm{\Sigma }}}_{g}}^{+})\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{4}({X}^{1}{A}_{1g})+{\rm{H}}{(}^{2}{S}_{1/2})\to {{\rm{C}}}_{2}{{\rm{H}}}_{5}({{XA}}^{\prime })\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{\mathrm{CH}}_{3}\mathrm{CH}({X}^{1}{A}^{\prime })\to {\mathrm{CH}}_{4}({X}^{1}{A}_{1})+{\rm{C}}{(}^{3}P)\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&2{{\rm{C}}}_{2}{{\rm{H}}}_{4}({{XA}}^{\prime })\to 1\mbox{--}{{\rm{C}}}_{4}{{\rm{H}}}_{8}({X}^{1}A)+{\rm{H}}{(}^{2}{S}_{1/2})\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{5}({{XA}}^{\prime })+{\rm{H}}{(}^{2}{S}_{1/2})\to {{\rm{C}}}_{2}{{\rm{H}}}_{6}({X}^{1}{A}_{1g})\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&2{{\rm{C}}}_{2}{{\rm{H}}}_{5}({{XA}}^{\prime })\to n\mbox{--}{{\rm{C}}}_{4}{{\rm{H}}}_{10}({X}^{1}{A}_{g})\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{6}({X}^{1}{A}_{1g})\to {\mathrm{CH}}_{4}({X}^{1}{A}_{1})+{\mathrm{CH}}_{2}({a}^{1}{A}_{1})\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&n\mbox{--}{{\rm{C}}}_{4}{{\rm{H}}}_{10}({X}^{1}{A}_{g})\to 1\mbox{--}{{\rm{C}}}_{4}{{\rm{H}}}_{8}({X}^{1}A)+{{\rm{H}}}_{2}({X}^{1}{{\rm{\Sigma }}}_{g}^{+}).\end{eqnarray} \tag{ 8 }$$

These reaction mechanisms can thus be used to help propose and to explain the formation routes of the complex hydrocarbons that have been detected here. Starting with the alkanes, C_nH_2n+2 (n = 4–10), multiple reaction pathways can be involved. For instance, ethylene (C₂H₄) can be inserted into a carbon–hydrogen bond of the alkane and increase the alkane's size by two carbon atoms (reaction (10)). Alternatively, carbene (CH₂) can be inserted into the carbon–hydrogen bond of an alkane, which leads to the carbon chain growth by one carbon atom (reaction (9)). Also, an alkane can lose a hydrogen atom (reaction (11)) via radiolysis, such as the ethane to ethyl transformation. This type of reaction creates an alkyl radical, which is then able to combine barrierlessly with a neighboring radical such as the methyl (CH₃) (reaction (12)) or ethyl radical (reaction (13)) (C₂H₅), which then increases the size of the alkane by one or two carbon atoms, respectively. The pathways utilizing an ethyl radical or ethylene molecule will therefore grow the carbon chain by two units (reactions (10) and (13)), while the carbene insertion and methyl radical pathways lead to an increase by one carbon atom (reactions (9) and (12)).

Although both even- and odd-carbon-containing molecules were detected, there was a much greater production of even carbon unit molecules. This suggests that the pathways that are able to increase the molecular size by one carbon unit are a minor pathway. Furthermore, since only methane, and not the methyl radical, was observed in this study, this suggests that these odd-carbon-containing molecules are formed via the carbene mechanism (reaction (9)) (Kaiser & Maksyutenko 2015a, 2015b; Maksyutenko et al. 2015; Tsegaw et al. 2016). For the even alkanes the growth can be accounted for by the addition of ethylene and/or the ethyl radical. However, there is no known reaction that allows the insertion of an ethylene molecule into the proposed carbon–hydrogen bond as suggested by reaction (10). Therefore, the ethyl radicals, which have been determined to produce n-butane, are likely the source of the even alkanes (reaction (13)), as successive steps of reaction (11) followed by reaction (13) can repeat to account for the production of even alkanes as large as the decanes (C₁₀H₂₂) detected in this experiment.

$$\begin{eqnarray}&&{{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+2}+{\mathrm{CH}}_{2}\to {{\rm{C}}}_{n+1}{{\rm{H}}}_{2{\rm{n}}+4}\end{eqnarray} \tag{ 9 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+2}+{{\rm{C}}}_{2}{{\rm{H}}}_{4}\to {{\rm{C}}}_{n+2}{{\rm{H}}}_{2{\rm{n}}+6}\end{eqnarray} \tag{ 10 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+2}\to {{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+1}+{\rm{H}}\end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+1}+{\mathrm{CH}}_{3}\to {{\rm{C}}}_{{\rm{n}}+1}{{\rm{H}}}_{2{\rm{n}}+4}\end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{{\rm{n}}}{{\rm{H}}}_{2{\rm{n}}+1}+{{\rm{C}}}_{2}{{\rm{H}}}_{5}\to {{\rm{C}}}_{{\rm{n}}+2}{{\rm{H}}}_{2{\rm{n}}+6}.\end{eqnarray} \tag{ 13 }$$

Abplanalp & Kaiser (2016) suggested that the detected unsaturated hydrocarbon groups could be accounted for by successive dehydrogenation of the previous more saturated group, for example, alkanes produce the alkenes, and alkenes produce alkynes just as suggested in reaction (8). This was supported by the detection of the largest molecule containing the same number of carbons as the previous more saturated group. However, in the present experiment this theory is not as well supported, as a larger alkene was detected than the proposed alkane parent. Therefore, either the parent alkane has completely decomposed, or alkenes are formed primarily via another mechanism. It should also be reiterated that without the knowledge of the exact isomers detected here, the following is a speculative mechanism for the case of a greater production of alkenes over alkanes. The obvious difference between ethane and ethylene is that ethylene contains a carbon–carbon double bond, and when ethylene's double bond is added to via a radical, another radical is formed. Although this addition is not barrierless, it was determined that alkene type molecules have a low activation energy with the ethyl radical addition to ethylene to be 23 kJ mol⁻¹ (0.24 eV) (Pinder & Roy 1957; Lampe & Field 1959), which can easily be supplied by the energetic electrons processing the ice. This reaction can be extrapolated to multiple neighboring ethylene molecules (reaction (14)), causing a cascade of radicals that are able to be terminated by another ethyl radical or a hydrogen atom (reaction (15)).

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{5}+{({{\rm{C}}}_{2}{{\rm{H}}}_{4})}_{{\rm{n}}}\to {{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{4{\rm{n}}+5}\end{eqnarray} \tag{ 14 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{4{\rm{n}}+5}+{{\rm{C}}}_{2}{{\rm{H}}}_{5}/{\rm{H}}\to {{\rm{C}}}_{2{\rm{n}}+4}{{\rm{H}}}_{4{\rm{n}}+10}/{{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{4{\rm{n}}+6}.\end{eqnarray} \tag{ 15 }$$

However, again these reactions lead to alkane production rather than alkene production. Therefore, an alternate mechanism involving the highly reactive product acetylene is involved as an alternate partner in the ethyl radical addition (reaction (16)), which is then followed by the same termination step as before with another ethyl radical or hydrogen atom (reaction (17)), which results in the production of alkenes.

$$\begin{eqnarray}&&{{\rm{C}}}_{2}{{\rm{H}}}_{5}+{({{\rm{C}}}_{2}{{\rm{H}}}_{2})}_{n}\to {{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{2{\rm{n}}+5}\end{eqnarray} \tag{ 16 }$$

$$\begin{eqnarray}&&{{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{2{\rm{n}}+5}+{{\rm{C}}}_{2}{{\rm{H}}}_{5}/{\rm{H}}\to {{\rm{C}}}_{2{\rm{n}}+4}{{\rm{H}}}_{2{\rm{n}}+10}/{{\rm{C}}}_{2{\rm{n}}+2}{{\rm{H}}}_{2{\rm{n}}+6}.\end{eqnarray} \tag{ 17 }$$

However, detailed mechanisms are not able to be currently isolated to support these speculations, but the acknowledgment that a previously studied simple system has more to offer with new analytical techniques is obvious from these conjectures.

Although the exact isomer for each molecular formula is not currently known at this stage, and therefore the exact photoionization cross section to apply is unknown, an approximation that the alkanes correspond only to the n-alkane structure will allow for a general determination of their relative abundance. This can also be done for the alkenes, assuming that all have the 1-alkene structure up to 1-hexene, as all signals of larger alkenes are significantly less intense and their cross sections are not well known. To accomplish this relative comparison, each ion signal was integrated starting at the onset sublimation temperature until the signal returned to the baseline signal. This integrated signal was then corrected utilizing the photoionization cross sections at 10.5 eV for n-hexane (2.4 Mb), n-heptane (3.5 Mb), n-octane (3.1 Mb), n-nonane (2.0 Mb), and n-decane (3.7 Mb) (Adam & Zimmermann 2007). These photoionization cross sections determined the ratio to be 20.6 ± 5.2:1.9 ± 0.5:7.3 ± 1.8:1.0 ± 0.3:2.1 ± 0.5 (n-pentane:n-hexane:n-heptane:n-octane:n-nonane:n-decane) and 10.6 ± 2.7:2.0 ± 0.5:4.0 ± 1:2.6 ± 0.6:1.0 ± 0.3 for the deuterated species. These data show reasonable agreement between the isotopologue relative abundances for these assumed n-alkanes, with the same trend observed when taking the error into account. It is clear that the even carbon alkane abundance varies greatly from the odd carbon alkanes, but both display a decrease in relative abundance with size, which further reinforces the suggested reaction mechanism that large alkanes were produced from smaller alkanes. The even carbon alkanes n-hexane, n-octane, and n-decane had a relative ratio of 9.9 ± 2.5:3.5 ± 0.9:1.0 ± 0.3 and 10.6 ± 2.7:4.0 ± 1.0:1.0 ± 0.3 for the non-isotopic and isotopic ion signals, respectively, which shows excellent agreement between the two sets of data.

In a broader context, low-temperature interstellar medium (ISM) ices have been shown to contain water (H₂O), ammonia (NH₃), carbon dioxide (CO₂), carbon monoxide (CO), methanol (CH₃OH), and even a small percentage of methane (CH₄) (Boogert et al. 2015). Also, small hydrocarbons (methane and ethane) have been observed on Titan (Griffith et al. 2006), Pluto (Holler et al. 2014), Makemake (Brown et al. 2015), and Quaoar (Dalle Ore et al. 2009). To date there have been no observations of a pure ethylene ice in the ISM or in any solar system body, but ethylene has been observed in several environments (Encrenaz et al. 1975; Betz 1981; Hanel et al. 1981; Schulz et al. 1999; Cernicharo et al. 2001a; Niemann et al. 2005; Shemansky et al. 2005; Hinkle et al. 2008; Brown et al. 2015; Merlin 2015; Gladstone et al. 2016). However, the chemical evolution of laboratory methane ices shows production of the C2 hydrocarbons acetylene, ethylene, and ethane (Bennett et al. 2006). Furthermore, these small hydrocarbons (C1 and C2) have been shown to be used in the construction of important aromatic molecules from benzene (Zhou et al. 2010) up to PAHs (Kaiser & Roessler 1997; Jones & Kaiser 2013). Therefore, the investigation and understanding of the chemistry that is available to these small hydrocarbons are necessary to fully realize the formation routes of the hydrocarbons present in both interstellar and planetary ices.

In detail, the irradiation of ethylene ices resulted in the detection of six molecules after FTIR analysis: methane [CH₄ (CD₄)], acetylene [C₂H₂ (C₂D₂)], the ethyl radical [C₂H₅ (C₂D₅)], ethane [C₂H₆ (C₂D₆)], 1-butene [C₄H₈ (C₄D₈)], and n-butane [C₄H₁₀ (C₄D₁₀)], which agrees with previous FTIR analysis (Zhou et al. 2014). During TPD, which simulates the transition of a cold molecular cloud into a star-forming region, the sensitive PI-ReTOF-MS study was able to detect a much more complex array of product groups: C_nH_2n+2 (n = 4–10), C_nH_2n (n = 2–12, 14, 16), C_nH_2n−2 (n = 3–12, 14, 16), C_nH_2n−4 (n = 4–12, 14, 16), C_nH_2n−6 (n = 4–10, 12), C_nH_2n−8 (n = 6–10), C_nH_2n−10 (n = 6–10). The detection of these hydrocarbon groups of varying degrees of saturation shows that very complex chemistry is taking place in a relatively simple starting material. There are many individual molecules represented by these groups that hold important astrophysical relevance.

Recently Abplanalp & Kaiser (2016) showed that several of the hydrocarbon groups identified here (C_nH_2n+2, C_nH_2n, C_nH_2n−2, C_nH_2n−4) were also synthesized in pure ethane irradiated ices and presented details on a few specific molecules within these groups, showing their astrophysical importance. Within the alkane group (C_nH_2n+2) several experiments investigating the processing of methane ice were also able to detect multiple signals belonging to alkanes from heptane (C₇H₁₆) (Jones & Kaiser 2013) to dodecane (C₁₂H₂₆) (Kaiser et al. 1992). Alkanes have also been a common group detected from meteorite analysis (Gelpi & Oró 1970; Sephton et al. 2001). The smallest signal detected belonging to the alkene group (C_nH_2n) can be assigned to C₃H₆. The propylene isomer (CH₂CHCH₃; X¹A'), which has been observed in the ISM toward TMC-1 (Marcelino et al. 2007; Rawlings et al. 2013; Zhou et al. 2013), has been shown to react with carbon atoms (C; ³P_j) (Kaiser et al. 1997b) and dicarbon molecules (C₂; X¹Σ_g⁺/a³${{\rm{\Pi }}}_{u}$) (Dangi et al. 2013) via crossed molecular beams to form the PAH precursors methylpropargyl radical (C₄H₅) and 1- and 3-vinylpropargyl, respectively. Meanwhile, its isomer, cyclopropane (c-C₃H₆), is the simplest of all possible cycloalkanes. Also, alkenes (n = 2–6) have been incorporated into theoretical models in order to better understand Titan's atmosphere (Woon & Park 2009). With respect to the alkyne group (C_nH_2n−2), the C₃H₄ isomers methylacetylene (CH₃CCH; IE = 10.36 eV), which has been detected by several astronomical surveys toward SgrB2, PKS 1830-211, L1544, and tentatively in NCG 4418 (Belloche et al. 2013; Muller et al. 2014; Vastel et al. 2014; Costagliola et al. 2015), and allene (H₂CCCH₂; IE = 9.69 eV) have been used to model Titan's atmospheric chemistry (Vakhtin et al. 2001; Goulay et al. 2007; Zhang et al. 2009a). Also, these two isomers have been shown to be very important for interstellar chemistry PAH formation, as they both have been proposed to take part in reactions with the phenyl radical (C₆H₅) to form indene (C₉H₈) (Zhang et al. 2011a; Parker et al. 2011, 2015; Yang et al. 2015). Similarly, from this hydrocarbon group the C₄H₆ isomer 1, 3-butadiene (H₂CCHCHCH₂) was determined to react with dicarbon (C₂), ethynyl radical (CCH), and tolyl radicals (C₆H₄CH₃) in the gas phase to form the important PAH molecules: the phenyl radical (C₆H₅) (Zhang et al. 2010), benzene (C₆H₆) (Jones et al. 2011), and 6-methyl-1, 4-dihydronaphthalene (Parker et al. 2014). Our results show that these small hydrocarbons can be produced in ices, and it is possible that after their formation in the ice phase they can sublime from the ice mantle and take part in gas phase reactions that are producing PAH-like molecules. For example, methylacetylene or allene can react with the phenyl radical to produce indene in the gas phase, or 1, 3-butadiene with the tolyl radical to from 6-methyl-1, 4-dihydronaphthalene (Figure 22). Dangi et al. (2014) also revealed that another C_nH_2n−2 type molecule, isoprene (CH₂C(CH₃)CHCH₂), was a key hydrocarbon in understanding how methyl-substituted PAHs form. In the solid phase it was shown that UV photolysis of 1, 5–hexadiene (C₆H₁₀) produced a carbonaceous polymer (Dartois et al. 2005). Next, the C_nH_2n−4 group contains the interesting vinylacetylene (H₂CCHCCH) molecule, which has been studied in relation to hydrocarbons on Titan (Vuitton et al. 2012). Also, vinylacetylene (H₂CCHCCH) has been studied in the gas phase reacting with the phenyl radical (C₆H₅) to produce naphthalene (C₁₀H₈) (Parker et al. 2012).

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While all of these very important molecules belong to groups detected from pure ethane or ethylene ice irradiation experiments, the following groups have been determined to belong solely from the processing of ethylene ice and suggest a different type of hydrocarbon chemistry available where this reactant is available. First, the C_nH_2n−6 group has many very interesting molecules related to it, including diacetylene (C₄H₂) and benzene (c-C₆H₆), which have both been identified in Titan's atmosphere (Coustenis et al. 2003, 2007), as well as in other sources (Bézard et al. 2001; Cernicharo et al. 2001b; Burgdorf et al. 2006; Guerlet et al. 2010). Cuylle et al. (2014) showed that the processing of solid acetylene was able to produce diacetylene by UV photolysis, but not able to produce benzene. However, Zhou et al. (2010) determined that the irradiation of acetylene with energetic electrons was able to produce benzene, but not diacetylene. The gas phase reaction of diacetylene with dicarbon was shown to produce the 1, 3, 5-hexatriynyl radical, a PAH precursor (Zhang et al. 2009b). Recently, Sivaraman et al. (2015) showed that the irradiation of propargyl alcohol produced benzene as the major product, which further supports that this molecule can be synthesized in the ice. Also, Jones et al. (2011) showed that benzene is able to be synthesized in the gas phase via the reaction of the ethynyl radical and 1, 3-butadiene. The reactions that benzene can undergo have also been studied in the gas phase with phenyl radicals (C₆H₅) (Zhang et al. 2008), tricarbon molecules (Gu et al. 2007a), dicarbon (Gu et al. 2007b), and carbon atoms (Bettinger et al. 2000; Kaiser et al. 2003) producing diphenyl (C₆H₅C₆H₅), phenyltricarbon (C₆H₅CCC), phenylethynyl radical (C₆H₅CC), and 1, 2-didehydrocycloheptatrienyl radical (C₇H₅), respectively. Another molecule in this group that has been detected in the ISM is methyl diacetylene in TMC-1 (Loren et al. 1984; MacLeod et al. 1984; Walmsley et al. 1984). Gudipati & Yang (2012) showed that hydrocarbons such as toluene (C₇H₈) undergo hydroxylation when present in astrophysical ice analogs.

In the C_nH_2n−8 group ortho-benzyne (o-C₆H₄) was a product of the gas phase reaction of the ethynyl radical with vinylacetylene (Zhang et al. 2011b). Computational studies to determine possible formation pathways of C₆H₄ isomers from acetylene have also been reported (Bera et al. 2015). Also, the reaction of fulvenallene (C₇H₆) with the hydroxyl radical (OH) (Thapa et al. 2015) and atomic carbon (da Silva 2014) has been studied to investigate the formation of the fulvenallenyl radical, as it is an important PAH molecule. Another gas phase study reacted ethynyl radicals (C₂H) with styrene (C₈H₈) to study PAH growth mechanisms. In the C_nH_2n−10 group the 1, 3, 5-hexatriene (C₆H₂) molecule was detected in CRL 618 (Fonfría et al. 2011), and the methyltriacetylene (C₇H₄) molecule has been observed toward TMC-1 (Remijan et al. 2006). The previously mentioned computational study of C₆H₄ isomers also undertook the study of C₆H₂ isomers that may be formed from acetylene and its fragments (Bera et al. 2015). The aromatization reaction of acetylene over forsterite and olivine samples has been shown to produce triacetylene (C₆H₂) and phenylacetylene (C₈H₆) (Tian et al. 2012). Bouwman et al. (2016) recently showed that there is evidence that pentalene (C₈H₆) may be produced from dissociative ionization of naphthalene.

With the penetration of galactic cosmic rays into Titan's atmosphere, even more complex chemistry can take place with these newly formed hydrocarbons, such as the inclusion of oxygen and possibly producing prebiotic chemicals (Sittler et al. 2009). Several trans-Neptunian objects (TNOs), such as Pluto, can be irradiated with the solar wind (Cravens & Strobel 2015), as well as UV photons (Gladstone et al. 2015), resulting in an average dosage of 22 eV for small hydrocarbons (Hudson et al. 2009). The current experiment supplied a much lower dose to the target ice of about 5 eV and detected a very diverse suite of molecules, which suggests that TNOs containing ethylene can produce similar hydrocarbons. According to the literature, there has only been one attempt to study the chemistry at play and in situ that takes place within ethylene ices, and no additional atoms, utilizing energetic 5 keV electrons to simulate the track of a galactic cosmic ray in ices (Zhou et al. 2014). However, this study relied on the traditional techniques, which were far less sensitive than the current results presented here, and an overall understanding of this small molecule's role in hydrocarbon ices has been lacking until now. Using the traditional methods of FTIR coupled with PI-ReTOF-MS has provided a new insight into the hydrocarbon chemistry available in simple ethylene ices processed by energetic electrons detecting complex hydrocarbons associated with the general formulae C_nH_2n+2, C_nH_2n, C_nH_2n−2, C_nH_2n−4, C_nH_2n−6, C_nH_2n−8, and C_nH_2n−10. Although no pure ethylene ice has been observed in an extraterrestrial body or in the ISM, the results presented here represent a starting point necessary to understand the full network of chemical reactions available during the chemical evolution of binary or even more complex ices. There are numerous interesting isomers of astrophysical importance, and currently experiments are being designed to untangle which of the C3 and C4 hydrocarbon isomers were produced from processed ethylene ices by utilizing tunable PI-ReTOF-MS (Abplanalp et al. 2015). The detection and nondetection of certain isomers will shed light onto the correctness of proposed reaction mechanisms as well and help lead to a more complete understanding of complex hydrocarbon chemistry of astrophysical relevance.

R.I.K. and M.J.A. thank the US National Science Foundation (AST-1505502) for support to conduct the experiments and data analysis. The experimental setup was financed by the W. M. Keck Foundation through an equipment grant.