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# Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br and CF3I

Simpson, M. J. and Tuckett, R. P. and Dunn, K. and Hunniford, A. and Latimer, C. J. (2009) Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br and CF3I. The Journal of Chemical Physics, 130. pp. 194302-1. ISSN 0021-9606

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URL of Published Version: http://dx.doi.org/10.1063/1.3137103

Identification Number/DOI: doi:10.1063/1.3137103

Using synchrotron radiation negative ions have been detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF$$_3$$Cl), trifluorobromomethane (CF$$_3$$Br) and trifluoroiodomethane (CF$$_3$$I). The anions F$$^-$$, X$$^-$$, F$$_2^-$$, FX$$^-$$, CF$$^-$$, CF$$_2^-$$ and CF$$_3^-$$ were observed from all three molecules, where X = Cl, Br or I, and their ion yields recorded in the range 8-35 eV. With the exception of Br$$^-$$ and I$$^-$$, the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF$$_3$$Br and CF$$_3$$I as the source of low-energy electrons, is shown to dominate the observed Br$$^-$$ and I$$^-$$ signals, respectively. Cross sections for ion-pair formation are put on to an absolute scale by calibrating the signal strengths with those of F$$^-$$ from both SF$$_6$$ and CF$$_4$$. These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for $$D^0$$(CF$$_3$$-X) which are consistent with literature values. We report new data for $$D^0$$(CF$$_2$$I$$^-$$-F) ≤ 2.7 ± 0.2 eV and $$\Delta_fH^0_{298}$$ (CF$$_2$$I$$^+$$) ≤ (598 ± 22) kJ mol$$^{-1}$$. No ion-pair formation is observed below the ionization energy of the parent molecule for CF$$_3$$Cl and CF$$_3$$Br, and only weak signals (in both I$$^-$$ and F$$^-$$) are detected for CF$$_3$$I. These observations suggest neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.

Type of Work: Article 2009 (Publication) Colleges (2008 onwards) > College of Engineering & Physical Sciences School of Chemistry QD ChemistryQC Physics University of Birmingham American Institute of Physics 740 YES

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