Excitation and ionic fragmentation of the carvone molecule (C₁₀H₁₄O) around the O 1s edge

Highlights

• Interaction of the carvone molecule with high energy photons results in an extensive fragmentation of the molecular skeleton.

• The occurrence of a site selective mechanism is suggested based on a production of atomic oxygen single and doubly charged, notably around the O 1s edge.

• The photoabsorption spectra around O 1s edge present four resonances and the cross section estimation shows a maximum on the first one.

Abstract

The electronic excitation and associated ionic dissociation of the carvone molecule have been studied around the oxygen 1s edge, using synchrotron radiation and time-of-flight techniques. Photoabsorption spectrum (total ion yield) and mass spectra have been obtained in the range between 520 and 545 eV. For the sake of comparison, carvone mass spectra have also been obtained following valence (21.21 eV) and core (carbon 1s) ionization. Fragmentation of the molecule is seen to be greatly enhanced following core excitation. Around the oxygen 1s edge, we observe an extensive fragmentation of the molecular skeleton, as exemplified by the appearance of several previously unreported ions: H⁺, H₂⁺, CH⁺, CH₂⁺ and CH₃⁺, which are not formed at low energies. A maximum is observed at 536 eV photon energy in the relative intensity of the oxygen-containing ions O⁺, O²⁺ and OH⁺, as an evidence for the existence of site-selective fragmentation of the carvone molecule excited around the O 1s edge. Absolute values for the photoionization and photodissociation cross sections were estimated using the molecular additive rule.

Introduction

The interaction of a molecule with soft X-ray photons (arbitrarily defined here as photons with energy greater than 100 eV) basically results in the ejection of one or more electrons. In fact, while in most molecules the first ionization potential (IP) occurs at less than 15 eV, the direct double ionization potential is usually of the order of 30–40 eV and the triple ionization potential is of the order of 50–80 eV [1], [2]. Although strictly forbidden in an independent particle model, double and triple ionization become possible through electron correlation and shake-off type processes. Around the edges of core–shell electrons, additional photoabsorption processes come into play, namely the excitation and ionization of inner-shell electrons. For molecules built with light atoms, the main relaxation mechanism, following photon absorption, is Auger decay. Below the ionization edge, transitions to unoccupied electronic states will be followed by autoionization-type processes and one or more electrons will be emitted by the molecule. In these so-called resonant Auger processes, the excited electron may remain either as spectator or participant in the relaxation decay mechanism. In the first case (spectator resonant Auger), and assuming the ejection of a single valence electron, the final state will be that of an excited ion. When the excited electron participates in the electronic decay (participating resonant Auger), the final state will be a singly charged ion, indistinguishable in principle from the final state associated with a simple valence shell ionization of the molecule. Above the core ionization edge, Auger decay involves the ejection of one (or more) valence-shell electrons and the final state will be a doubly, triply, etc., cation. The stability of the remaining ion, in all cases, will very much dependent on the nature (bonding, antibonding, Rydberg) of the depleted valence orbital and electronic state. Doubly or multiply charged ions will in general have a more unstable nature, due to coulomb repulsion and to the possible loss of bonding electrons, resulting in ionic states highly dissociative. The large energy gap between the ionization edges of different atoms and the availability of tunable synchrotron radiation allows for the excitation of different sites in a given molecule. As a consequence, site-selective fragmentation may be sometimes observed [3], [4]. With ionic dissociation processes prevailing after the interaction of molecules with high energy photons, mass spectrometric techniques have become very useful toward the understanding of the relaxation mechanisms associated with high energy photon absorption.

In this respect, time-of-flight techniques (mass spectrometry, electron-ion and electron-ion-ion techniques) are known as powerful tools for the elucidation of the dissociation mechanisms of highly excited molecules. Excellent reviews about the photoionization of molecules and the use of time-of-flight techniques may be found in the literature [5], [6], [7].

We have recently started a systematic study of the core excitation and ionic dissociation of molecules of biological interest, in the gas phase, focusing mainly in aminoacids [8], [9], nucleic acid bases [10], [11] and volatile natural products [12], [13]. In this paper, we present and discuss new experimental results related to a volatile natural product, carvone (C₁₀H₁₄O), excited at valence and around the C 1s and O 1s edges. Carvone is an enantiomeric substance found in plants like caraway, dill and spearmint (Fig. 1). The antimicrobial activity of the carvone isomers has been demonstrated against a wide spectrum of human pathogenic fungi and bacteria [14]. The amount of mentha essential oil, which contains appreciable quantities of carvone, is suppose to be associated with the incidence of UV-B radiation, reflecting, probably, a defense mechanism [15]. The carvone molecule is chemically classified as a monoterpene (C10), consisting of an unsaturated cyclic ketone with methyl and isopropenyl substitution in opposite (equatorial) positions. The carvone present in essentials oils is a mixture of three conformers, differing in respect to the bond connecting the ring and the isopropenyl moiety [16]. The first systematical study of monoterpenes ketones and aldehydes, employing gas chromatography and mass spectrometry [using 20 eV electrons], was published in 1964 [17]. Recently, the mass spectrum and threshold photoelectron spectrum of carvone has been studied in the 8–11 eV photon energy range. It was pointed out that the HOMO orbital in carvone is associated with the oxygen atom lone pair, essentially localized around the carbonyl group, with an ionization potential of 8.7 eV [18]. The circular dichroism of the carvone enantiomers, randomly oriented in gas-phase, has also been investigated both theoretically and using circularly polarized synchrotron radiation at the C 1s edge [19], [20]. In our present study, we have recorded electron–ion coincidence (PEPICO) and total ion yield (TIY) spectra for the carvone molecule. The PEPICO spectra were obtained below (520 eV), around (530–540 eV) and above (545 eV) the O 1s edge, which is arbitrarily taken to occur at 540 eV. For the sake of comparison, PEPICO spectra were also obtained following valence ionization (21.21 eV) and before (275 eV) and above (310 eV) the C 1s edge, which is arbitrarily taken to occur at 290 eV. Compared to valence excitation, it is well known that core excitation usually induces a much larger degree of fragmentation of the molecule. In particular, fragments such as O⁺ and O²⁺ are observed only around the O 1s edge, pointing out to possible site-selective fragmentation mechanisms.