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

Using synchrotron radiation, negative ions are 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)(-) are observed from all three molecules, where X = Cl, Br, or I, and their ion yields recorded in the range of 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 onto 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(o)(CF(3)-X), which are consistent with literature values. We report new data for D(o)(CF(2)I(+)-F) < or = 2.7+/-0.2 eV and Delta(f)H(o)(298)(CF(2)I(+)) < or = (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 that neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.

Vacuum-UV negative photoion spectroscopy of SF5CF3.

Ion pair formation, generically described as AB-->A(+)+B(-), from vacuum-UV photoexcitation of trifluoromethyl sulfur pentafluoride, SF(5)CF(3), has been studied by anion mass spectrometry using synchrotron radiation in the photon energy range of 10-35 eV. The anions F(-), F(2)(-), and SF(x)(-) (x=1-5) are observed. With the exception of SF(5)(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from ion pair formation. SF(5)(-) arises from dissociative electron attachment, following photoionization of SF(5)CF(3) as the source of low-energy electrons. Cross sections for anion production are put on to an absolute scale by calibration of the signal strengths with those of F(-) from both SF(6) and CF(4). Quantum yields for anion production from SF(5)CF(3), spanning the range of 10(-7)-10(-4), are obtained using vacuum-UV absorption cross sections. Unlike SF(6) and CF(4), the quantum yield for F(-) production from SF(5)CF(3) increases above the onset of photoionization.

A search for circular dichroism in the VUV photofragmentation mass spectra of 2-amino-1-butanol.

The dissociative photoionization of a single-enantiomer chiral molecule by circularly polarized synchrotron radiation was investigated, for the first time, in the gas phase. Photoion mass spectra were produced by the interaction of (+)-(S)-, (-)-(R)- and rac-2-amino-l-butanol with circularly polarized light. Comparison of these spectra places an upper bound of approximately 2% on circular dichroism in the dissociative photoionization of 2-amino-l-butanol at 21 eV, which may have consequences for the theory that the origin of biological homochirality was predominantly enantioselective photofragmentation by circularly polarized light. We have also identified and elucidated many of the difficulties of performing gas phase CD measurements in crossed beam experiments.