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

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.