Identification of Stable Perfluorocarbons Formed by Hyperthermal Decomposition of Graphite Fluoride Using Anion Photoelectron Spectroscopy

Abstract

Anion photoelectron (PE) spectroscopy was used to characterize several perfluorocarbon (PFC) species generated from pulsed-laser ablation of graphite fluoride (GF) and compare them to PFCs introduced into the gas phase and negatively charged by using a gentle photoemission source. The PE spectra of C6F6 –, C7F8 –, and C5F8 – produced by ablation of GF are nearly identical with the PE spectra of the anions of hexafluoro­benzene, perfluoro­toluene, and perfluoro­cyclopentene, respectively, generated by electron attachment to the neutral perfluorocarbon molecules. This result suggests that laser ablation of GF, which is a hyperthermal decomposition event, produces species larger than CF2 and with stable molecular structures. In addition, anion PE spectra were obtained for several larger PFC molecular anions, some of which have been the subject of past computational studies. The electron affinities of these species cannot be determined unambiguously from the broad PE spectra, though a systematic approach to identifying the onset of detachment signal was used to approximate the electron affinities. The vertical detachment energy of perfluoro­ethylcyclohexene was determined to be 2.98 ± 0.05 eV. The vertical detachment energies of additional perfluorocarbon radical anions, including perfluoroheptene, perfluoro­methylcyclohexane, perfluoro-1,3-dimethyl­cyclohexane, and perfluorodecalin, all exceed 3.495 eV. We also demonstrated that photoemission from gadolinia (Gd2O3) using a pulsed laser is an efficient and effective method of generating radical anions of larger volatile molecules in the gas phase.

Anion Photoelectron Imaging Spectroscopy of C6HF5–, C6F6–, and the Absence of C6H2F4–

“…This value is close to the 1493 cm –1 a 1g C–F stretch of C 6 F 6 . This partially resolved feature was not observed in previously reported spectra. − …”

“…A similar effect was observed in other reported PE spectra of C 6 F 6 − . 31,34 The PE spectrum of C 6 F 6 − shown in Figure 1b was measured previously in our group using a different instru- ment 33 and by others, 30 −32,34 and is shown here for direct comparison with the spectrum of C 6 HF 5 − obtained using the same instrument and under the same ion source conditions. The onset of electron signal in band X of our spectrum, which again approximates the EA, is 0.76 ± 0.05 eV, and the VDE is at 1.60 ± 0.05 eV.…”

“…Anions could only be generated with higher laser fluences in the photoemission source, which leads to molecular fragmentation. 33 Mass spectra of anions generated from all three C 6 H 2 F 4 isomers, along with C 6 HF 5 and C 6 F 6 are shown in Figure 4. Figure 4a−c specifically shows the mass distributions from (a) 1,2,3,4-, (b) 1,2,3,5-, and (c) 1,2,4,5tetrafluorobenzene.…”

“…A previous density functional theory (DFT) study on F-substituted (and BO 2 -substituted) benzene reported by Driver and Jena predicted a crossover from negative to positive EA in the C 6 H 6– x F x (0 ≤ x ≤ 6) series, where penta- and hexafluorobenzene were predicted to have definitively positive EAs; the EA of tetrafluorobenzene was predicted to be near zero; and the less fluorinated species were predicted to have definitively negative EAs. Experimentally, the hexafluorobenzene molecule has been shown to form both valence − and nonvalence-bound anions, ,, both studied by anion photoelectron (PE) spectroscopy, but to our knowledge, evidence of the pentafluorobenzene anion has only been reported in past thermodynamic studies …”

Trend in the Electron Affinities of Fluorophenyl Radicals ·C6H5-xFx (1 ≤ x ≤ 4)

“…The substituted phenide anions were produced and spectroscopically probed using an anion PE imaging using an apparatus that has been described in detail elsewhere. , Fluorophenide ions were generated using a photoemission ion source in which a combination of low-energy electron production and photodissociation of a gas mixture composed of fluorobenzenes (seeded in an ultrahigh-purity helium carrier gas) generated ions ranging from small carbon clusters to species with seven carbon atoms. As noted above, fluorobenzene (Sigma-Aldrich, 99% purity), 1,4-difluorobenzene (Sigma-Aldrich, 99% purity), 1,3,5-trifluorobenzene (97% purity), and 1,2,4,5-tetrafluorobenzene (Santa Cruz Biotechnology, 98% purity) were selected for this study.…”