Abstract
We have measured sets of mass spectra for positive ions produced by low-energy electron impact on phenanthrene. Ions have been mass resolved using a reflectron time-of-flight mass spectrometer, and the electron impact energy has been varied from 0 to 100 eV in steps of 0.5 eV. Ion yield curves of most of the fragment ions have been determined by fitting groups of adjacent peaks in the mass spectra with sequences of normalized Gaussians. The aim of this paper is to provide a detailed comparison of phenanthrene with its isomer anthracene, for which we have published results in a previous paper [1]. Appearance energies for a selection of fragment ions of phenanthrene have been determined, and are compared with anthracene. The most significant differences are observed in the ion yield curves of the ions containing 12 carbon atoms. The ion yield curves of phenanthrene have higher maximum yields and lower appearance energies compared to anthracene. For the fragments containing 9 and 10 carbon atoms the phenanthrene yields are slightly lower, but the appearance energies are the same as for anthracene. Small differences in yields are also observed for the fragments with 6 and 7 carbon atoms. The double and triple ionization energies of phenanthrene have been determined and are in agreement with anthracene.
Highlights
Polycyclic aromatic hydrocarbons (PAHs) are relevant in astrophysical processes and in environmental chemistry, and these molecules have been the focus of much scientific research
We identify groups of peaks in the mass spectra by the number of carbon atoms contained in the fragments
The purpose of the present paper is to examine the differences between phenanthrene and anthracene in more detail, and on close examination we do find small differences in the fragmentation patterns of these isomers
Summary
Polycyclic aromatic hydrocarbons (PAHs) are relevant in astrophysical processes and in environmental chemistry, and these molecules have been the focus of much scientific research. Emission bands in the infrared spectra of many interstellar objects are commonly attributed to PAH molecules [2]. PAHs are susceptible to hydrogen attachment and are considered to play a key role as catalysts in the formation of molecular hydrogen in the interstellar medium [3, 4]. PAHs are considered as essential components in the pathway to the origin of life [5]. In the Earth’s environment, PAHs are widespread pollutants generated by the combustion of organic materials, and are of concern because many have toxic, mutagenic and/or carcinogenic properties [6, 7]. Electron induced processes are important in the interstellar medium were irradiation of molecules in interstellar dust grains by ultraviolet light and cosmic rays releases many secondary electrons which may themselves induce chemical reactions [8]
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