Hydrogenated Benzene in Circumstellar Environments: Insights into the Photostability of Super-hydrogenated PAHs

Figure 4, top panel, shows the production of ${{\rm{C}}}_{6}{{\rm{H}}}_{12}^{+\cdot }$ as a function of the photon energy in the UV range (10.8 eV to 200.0 eV). For energies close to the first Ionization Potential (IP) of cyclohexane (9.8 eV, as determined by Dewar & Worley 1969), the photodissociation process is poorly activated. The PIY value of the parent ion decreases by a factor of almost 9 in the UV range, remaining at only 4.5% at 200.0 eV.

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A significant production of ${{\rm{C}}}_{4}{{\rm{H}}}_{8}^{+\cdot }$ is observed when the production of ${{\rm{C}}}_{6}{{\rm{H}}}_{12}^{+\cdot }$ starts to drop fast at around 12.4 eV (see Figure 3). That is, the original molecular ion breaks up through the loss of a neutral ethylene (C₂H₄) unit. The loss of only hydrogen atoms from the parent ion (${{\rm{C}}}_{6}{{\rm{H}}}_{n}^{+}$, n = 1–8) as well the loss of one carbon atom (${{\rm{C}}}_{5}{{\rm{H}}}_{n}^{+}$, n = 1–8) are inhibited, as observed by their low relative amounts (see Table 1). The production of ${{\rm{C}}}_{4}{{\rm{H}}}_{8}^{+\cdot }$ reaches a maximum value (34.1%) at 14.0 eV and then it starts to decrease, reaching 26.8% at 20.0 eV. In this energy range, the production of ${{\rm{C}}}_{4}{{\rm{H}}}_{7}^{+}$ more than doubles, from 4% to 9%. This indicates that part of the closed-shell ${{\rm{C}}}_{4}{{\rm{H}}}_{7}^{+}$ production comes through the loss of a hydrogen atom from the open-shell ${{\rm{C}}}_{4}{{\rm{H}}}_{8}^{+\cdot }$ radical cation system. Other significant ions produced in the 14.0 eV to 20.0 eV energy range are the ${{\rm{C}}}_{5}{{\rm{H}}}_{9}^{+}$ (M⁺–${\mathrm{CH}}_{3}^{\cdot }$), ${{\rm{C}}}_{3}{{\rm{H}}}_{6}^{+\cdot }$ (M⁺–C₃H₆), and ${{\rm{C}}}_{3}{{\rm{H}}}_{5}^{+}$ species.

At 100.0 eV, the production of ions from the ethyl (${{\rm{C}}}_{2}{{\rm{H}}}_{n}^{+}$), methyl (${\mathrm{CH}}_{n}^{+}$), and hydrogen (${{\rm{H}}}_{n}^{+}$) groups begins to increase. The most produced ion is the ${{\rm{C}}}_{3}{{\rm{H}}}_{5}^{+}$ species, followed by ${{\rm{C}}}_{2}{{\rm{H}}}_{3}^{+}$ (M⁺–${{\rm{C}}}_{4}{{\rm{H}}}_{9}^{\cdot }$) and ${{\rm{C}}}_{4}{{\rm{H}}}_{8}^{+\cdot }$. The production of the cyclopropenyl molecule ${{\rm{C}}}_{3}{{\rm{H}}}_{3}^{+}$ becomes significant at 100.0 eV, and at 200.0 eV, this species is the most produced one.

By comparing the present results with the PIY of the literature data of the National Institute of Standards and Technology (NIST; Johnson 2013), we verified that the fragmentation pattern at 70 eV electron energy already resembles the one induced by 16 eV photons. It is also interesting to note that at the electron impact of 70 eV (or photon impact of 16 eV), the abundance of the aromatic ring parent ion is twice that of the cyclohexane parent ion.

The fragmentation pattern observed in the soft X-ray region around the C1s resonance energy is significantly different from that measured in the UV region (see Figure 3). From 284.8 eV to 307.0 eV, more than 90% of the ion yield comes from the light fragment ion contribution, from the ${\mathrm{CH}}_{n}^{+}$ (and ${{\rm{H}}}_{n}^{+}$) to ${{\rm{C}}}_{3}{{\rm{H}}}_{n}^{+}$ groups. The parent ion production (see Figure 4, bottom panel) ranges from merely 0.6% (307.0 eV) to 1.4% at the C1s resonance energy (287.7 eV). This profile contrasts reasonably with the one presented by the parent ion of benzene, for which a production of 5.4% was observed at 282.4 eV, a value more than five times larger than the average production of the parent ion of cyclohexane at the C1s edge. These values reveal a higher propensity of cyclohexane to photodissociate after absorbing a soft X-ray photon in the C1s edge. This tendency is related to the higher molecular rigidity of benzene and its parent ion, in comparison to cyclohexane and its ionization product. By far, the most relevant fragments produced by the photodissociation of cyclohexane are ${{\rm{C}}}_{3}{{\rm{H}}}_{3}^{+}$ and ${{\rm{C}}}_{2}{{\rm{H}}}_{3}^{+}$, which are related to cyclopropenyl cation (Zhao et al. 2014) and protonated acetylene (Glassgold et al. 1992), respectively.

Figure 5 shows the comparison of the different ions resulting from the fragmentation of benzene and cyclohexane at the C1s resonance energy. By analyzing the production of the parent ion and other ${{\rm{C}}}_{6}{{\rm{H}}}_{n}^{+}$ species, it is possible to see that the main difference between benzene and cyclohexane is that the backbone fragmentation is significantly more pronounced in the latter. This result gives evidence that the hydrogenated benzene molecule shows, after X-ray photoionization, a higher tendency for dissociation than its aromatic counterpart. Consequently, a high production of ions pertaining to the ${\mathrm{CH}}_{n}^{+}$, ${{\rm{C}}}_{2}{{\rm{H}}}_{n}^{+}$, and ${{\rm{C}}}_{3}{{\rm{H}}}_{n}^{+}$ groups is observed for cyclohexane. In fact, the formation of ${\mathrm{CH}}_{3}^{+}$, ${{\rm{C}}}_{2}{{\rm{H}}}_{3}^{+}$, and ${{\rm{C}}}_{3}{{\rm{H}}}_{3}^{+}$ is particularly high for C₆H₁₂ compared to aromatic systems, such as benzene (Boechat-Roberty et al. 2009) and toluene (C₇H₈; Monfredini et al. 2016). On the other hand, the fragmentation of benzene into the ionic species ${{\rm{C}}}_{4}{{\rm{H}}}_{n}^{+}$ to ${{\rm{C}}}_{6}{{\rm{H}}}_{n}^{+}$ is significantly more pronounced. Concerning the H_n series, the H⁺ production is relatively the same for both species. In turn, ${{\rm{H}}}_{2}^{+\cdot }$ and ${{\rm{H}}}_{3}^{+}$ are much more efficiently produced from the break up of the aliphatic structure than from benzene.

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A comparison between the fragmentation pattern of benzene and cyclohexane around the parent ion m/q value at 301 eV photon impact is shown in Figure 5, bottom panel. For benzene, all C₆H_n cations were observed, and the most abundant ions are C₆H⁺, ${{\rm{C}}}_{6}{{\rm{H}}}_{2}^{+\cdot }$, ${{\rm{C}}}_{6}{{\rm{H}}}_{5}^{+}$, and ${{\rm{C}}}_{6}{{\rm{H}}}_{6}^{+\cdot }$. For cyclohexane, the results are quite different. Several of the C₆H_n cations were absent, such as ${{\rm{C}}}_{6}{{\rm{H}}}_{4}^{+\cdot }$, ${{\rm{C}}}_{6}{{\rm{H}}}_{6}^{+\cdot }$, ${{\rm{C}}}_{6}{{\rm{H}}}_{8}^{+\cdot }$, ${{\rm{C}}}_{6}{{\rm{H}}}_{9}^{+}$, and ${{\rm{C}}}_{6}{{\rm{H}}}_{10}^{+\cdot }$. These results indicate that stable C₆H_n ionic structures are not directly achievable from the molecular rearrangement of cyclohexane after ionization. The most relevant C₆H_n ions produced by the photodissociation of C₆H₁₂ are ${{\rm{C}}}_{6}{{\rm{H}}}_{2}^{+\cdot }$ and ${{\rm{C}}}_{6}{{\rm{H}}}_{5}^{+}$ and ${{\rm{C}}}_{6}{{\rm{H}}}_{11}^{+}$.

NGC 7027 is a young carbon-rich planetary nebula located in the Cygnus constellation at a distance of 880 pc (Masson 1986; Latter et al. 2000; Bernard-Salas et al. 2001; Hasegawa & Kwok 2001, 2003; Kastner et al. 2001; Wesson et al. 2010). This nebula has a distinct structure, with an ionized elliptical envelope lying at the center of an extended molecular envelope. Intense lines of highly ionized Ne atom were observed in infrared spectra obtained by ISO-SWS of NGC 7027 (Bernard-Salas et al. 2001) and in X-ray spectra taken by the Chandra X-ray Observatory (Kastner et al. 2001). At the interface between the cold molecular region and the ionized front, there is the PDR where the chemistry is controlled by penetrating UV and X-ray photons from the central star (Lau et al. 2016; Latter et al. 2000). The morphology of NGC 7027 presents three outflows in the H ii region interacting with the outermost regions (Cox et al. 1997; Santander-García et al. 2012; Lau et al. 2016). The variations in the radiative flux of the object provide a non-equivalent mixture of chemical species within the outer layers, thus providing environments with different characteristics within the same nebula (Arnoult et al. 2000). Consequently, it stimulates the formation and destruction of a wide variety of molecular species detected in both neutral and ionized states, such as H₂, CO, ${\mathrm{CH}}_{3}^{+}$, ${\mathrm{CH}}_{2}^{+}$, CH⁺, c-C₃H₂, PAHs, and aliphatic hydrocarbons (Hasegawa et al. 2000; Hasegawa & Kwok 2001; Lau et al. 2016). The physical properties of the central star, a very hot white dwarf with a temperature around 2 × 10⁵ K and an estimated luminosity of 7700 L_⊙ (Latter et al. 2000), promote a chemically rich medium. This implies that different reaction mechanisms could be taking place, both in the gas phase and on the surface of grains. Ultimately, such an environment could uphold the formation of complex ions and organic molecules, among them PAHs (Herbst & van Dishoeck 2009).

The determination of molecular σ_phd values is of significant importance for estimating the molecular abundance in both interstellar and circumstellar environments. The decreasing abundance of a given molecule subjected to a radiation field in the photon energy range E₂ − E₁ inside a gaseous dusty cloud can be written as (Cottin et al. 2003; Boechat-Roberty et al. 2009)

$$\begin{eqnarray}&&-\displaystyle \frac{{dN}}{{dt}}={{Nk}}_{\mathrm{phd}},\end{eqnarray} \tag{ 3 }$$

where N is the column density (cm⁻²) and k_phd is the photodissociation rate (s⁻¹), given by

$$\begin{eqnarray}&&{k}_{\mathrm{phd}}={\int }_{{E}_{1}}^{{E}_{2}}{\sigma }_{\mathrm{phd}}(E){F}_{X}(E){dE},\end{eqnarray} \tag{ 4 }$$

where ${\sigma }_{\mathrm{phd}}(E)$ is the photodissociation cross-section (cm²) and F_X(E) is the photon flux (photons cm⁻² eV⁻¹ s⁻¹), both as a function of the photon energy E = hν.

Therefore, it is possible to determine the half-life of a given molecule with the following equation (Andrade et al. 2010):

$$\begin{eqnarray}&&{t}_{1/2}=\displaystyle \frac{{ln}2}{{k}_{\mathrm{phd}}}.\end{eqnarray} \tag{ 5 }$$

In addition, we can also determine the photoionization rate, k_phi, given by

$$\begin{eqnarray}&&{k}_{\mathrm{phi}}={\int }_{{E}_{1}}^{{E}_{2}}{\sigma }_{\mathrm{phi}}(E){F}_{x}(E){dE},\end{eqnarray} \tag{ 6 }$$

where σ_phi(E) (cm²) is the photoionization cross-section.

In order to know the X-ray photon flux (F_X) values in the PDR of NGC 7027, we used the equation

$$\begin{eqnarray}&&{F}_{X}=\displaystyle \frac{{L}_{X}}{4\pi {r}^{2}h\nu }{e}^{-{\tau }_{x}},\end{eqnarray} \tag{ 7 }$$

where L_x = 1.3 × 10³² ergs s⁻¹ is the integrated X-ray luminosity from 0.2 to 2.5 keV, reported by Kastner et al. (2001), r = 5.21 × 10¹⁶ cm is the distance from the central star to a position inside the PDR (Agúndez et al. 2010), and τ_X is the X-ray optical depth, given by Deguchi et al. (1990):

$$\begin{eqnarray}&&{\tau }_{X}=\alpha {\tau }_{d}{\left(\displaystyle \frac{E}{0.6\mathrm{keV}}\right)}^{-2.67},\end{eqnarray} \tag{ 8a }$$

$$\begin{eqnarray}&&{\tau }_{d}=4.6x{10}^{-21}{N}_{{{\rm{H}}}_{2}},\end{eqnarray} \tag{ 8b }$$

which accounts for the absorption due to materials on the envelope along a radius. In Equations (8(a)) and (8(b)), τ_d is the UV optical depth of dust grains at 1000 Å, proposed by Morris & Jura (1983), the factor α = 0.054 for energies below 0.6 keV, and ${N}_{{{\rm{H}}}_{2}}$ is the column density of H₂, for which we adopted the value of 1.3 × 10²¹ cm⁻² obtained by Agúndez et al. (2010). The energy 0.6 keV corresponds to the oxygen O1s absorption edge (Deguchi et al. 1990).

Figure 9 shows the X-ray optical depth in the PDR of NGC 7027, its photon flux with and without the attenuation, and the estimated half-lives of cyclohexane and benzene in this object. The X-ray optical depth ranges from 2.4 (282.0 eV) to 0.3 (600.0 eV), which shows that the object is essentially transparent for energies above the O1s resonance energy. In the vicinity of the C1s resonance energies, the attenuated photon flux values range from 7.4 × 10⁵ cm⁻² s⁻¹ eV⁻¹ to 1.1 × 10⁶ cm⁻² s⁻¹ eV⁻¹. Although the maximum X-ray attenuated photon flux is not located on the C1s edge, the photoabsorption cross-sections significantly decay after the inner shell excitation energies (Sakamoto et al. 2010). Therefore, the half-lives obtained in this work are a good estimation of the X-ray survival rates of the molecules in NGC 7027.

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The photoionization and photodissociation rates, as well as the estimated t_1/2 values of C₆H₁₂ and C₆H₆, are shown in Table 3. We estimate that the half-life of cyclohexane is 3.5 × 10³ yr in the C1s resonance energy (287.7 eV), more than three times the half-life of benzene at 285.2 eV (1.1 × 10³ years). This large distinction is directly related to differences in the photoabsorption cross-sections of both molecules at the mentioned energies (7.7 × 10⁻¹⁸ cm² for cyclohexane and 2.6 × 10⁻¹⁷ cm² for benzene). On the other hand, in the vicinity of the C1s resonance energy, the survival of both molecules is practically the same. By integrating the half-life curves of both molecules and comparing the areas, it is possible to see that the survival of cyclohexane on the C1s edge is ∼20% higher than that of benzene. Since the abundances of interstellar molecules depend on both their formation and destruction rates, we suggest that the richness of interstellar benzene in comparison to its fully hydrogenated counterpart comes from more effective mechanisms of formation, such as the one described by Jones et al. (2011). This result indicates, therefore, that the addition of peripheral atoms in the basic PAH unit could impart greater X-ray stability in photodissociation regions.

At this point, it is important to make a comparative analysis of the findings discussed in the last paragraph with the ones presented in Figure 5, as they could be naïvely interpreted as contradictory. From Figure 5, it is shown that the backbone fragmentation of cyclohexane is significantly more pronounced than that of benzene. This means that once photoabsorption has taken place, the fully hydrogenated molecule is more likely to photodetach its carbon skeleton. In other words, the survival probability of cyclohexane is smaller than that of benzene after the photoabsorption process. However, the benzene molecule has a strong absorption feature at the C1s edge as shown in Figure 7. This feature disappears entirely when additional hydrogen atoms are inserted into the carbon backbone. Consequently, the photoabsorption cross-section of benzene is larger than that of cyclohexane, which ultimately leads to a higher efficiency of dissociation for the aromatic molecule. The protective effect of additional hydrogen atoms is, therefore, related to a damping out process of the strong absorption feature of benzene. The extrapolation of such a finding to PAHs of high molecular mass will be discussed in the next subsection.

A final remark should be made with regard to the survival of cyclohexane in PDR regions, such as the one in NGC 7027. As shown in Section 3.2, the global minimum of the C₆H₁₂ radical cation (²1^+·) is a highly branched structure. The first alkyl branched molecule detected in the interstellar medium was the isopropyl-cyanide (i-C₃H₇CN) species, in a recent study developed by Belloche et al. (2014). The present results suggest that, in a PDR region, the photoionization of C₆H₁₂ could induce the formation of the mentioned highly branched radical cation. Since methyl substituents help in the stabilization of carbocations and radical centers, it is expected that the molecular rearrangement that follows the photoionization process of both cyclic and open-chain hydrocarbons could lead to an enhancement of branched molecules in PDR regions.

In this last subsection, the consequences of the results presented herein on the stability of PAHs and H_n–PAHs in X-ray photon-rich environments are examined and discussed in the context of previous studies (Reitsma et al. 2014; Gatchell et al. 2015; Wolf et al. 2016).

In Section 3.1, we have shown that, after an X-ray photoabsorption process, the cyclohexane molecule is almost entirely dissociated, as the average partial ion yield of ${{\rm{C}}}_{6}{{\rm{H}}}_{12}^{+\cdot }$ is around 1%. The small production of cyclohexane's parent ion is due to the weak carbon backbone rigidity of the fully hydrogenated six-membered ring, whose high energetic chair-like structure of the C₆H₁₂ monocation (²5^+·, see Section 3.2) will break away to form more stable acyclic ions, such as ²1^+·, or dissociate into smaller fragments. The carbon backbone rigidity of benzene, on the other hand, is responsible for keeping the PIY values around 5% at the C1s resonance energy. In fact, only after a double ionization process is the six-membered ring surpassed by a different structure as the global minimum. In this case, it acquires a pentagonal–pyramidal carbon arrangement (Jašík et al. 2014; Fantuzzi et al. 2017a).

As the carbon backbone size increases, the resistance of super-hydrogenated PAHs toward dissociation is also expected to increase. The fragmentation of perhydropyrene (C₁₆H₂₆) will still be greater than that of pyrene (C₁₆H₁₀), as the collision experiments developed by Gatchell et al. (2015) suggest, but not so pronounced as that of C₆H₁₂. However, by increasing even more the number of carbon atoms, the fragmentation of the carbon backbone in a H_n-PAH could be reduced to a secondary process, being exceeded by hydrogen elimination to ultimately form a PAH ion, as suggested by Reitsma et al. (2014) in experiments with coronene (C₂₄H₁₂). In this scenario, the excess of peripheral H atoms acts as a protection mechanism for the PAH structure, as already described earlier in this paper.

Parallel to the mechanism proposed by Reitsma et al. (2014), an auxiliary photostabilization mechanism could be present in super-hydrogenated PAHs. As shown in Section 3.3, the strong photoabsorption cross-section of the C1s $\,\to \,\pi$* resonance energy of benzene (2.6 × 10⁻¹⁷ cm²) is more than three times higher than the one of cyclohexane (7.7 × 10⁻¹⁸ cm²). This is a common feature of aromatic molecules, as shown in Figure 10, and is expected to be present in the X-ray photoabsorption spectra of PAHs. In comparison, adamantane (C₁₀H₁₆), a polycyclic fully hydrogenated non-aromatic hydrocarbon, does not have such a feature. In fact, the spectral profile of adamantane shows several resemblances to that of cyclohexane, and as such its dissociative ionization (Candian et al. 2018). In this perspective, we suggest that an auxiliary protection mechanism could exist in super-hydrogenated PAHs. By decreasing the X-ray photoabsorption cross-section, a H_n-PAH molecule will also have a smaller photodissociation cross-section, which ultimately enhances its photostability in PDR regions. This mechanism, to the best of our knowledge, proposed for the first time in the literature, should contribute to explaining the existence of hydrogenated PAHs in interstellar and circumstellar media. The importance of such a mechanism in comparison to the one described by Reitsma et al. (2014) will be the focus of a future study.

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