K F Dunn

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.

Negative photoion spectroscopy of SF6 in the inner valence and S 2p energy regions

Negative fragment ion formation from SF6 has been studied using synchrotron radiation within the energy range 20-205 eV. The fragment ions F-, F2-, S- and SF- have been observed. It is shown that shape resonance and unoccupied valence orbitals play an important role in negative ion formation after photoexcitation of inner valence and S 2p electrons. The negative ion yields after photoexcitation of a S 2p core electron show evidence of localization of the shape resonance orbitals around the central S atom. The yields of F- and F2- in the outer valence and inner valence energy regions not only confirm that ion pair formation occurs via predissociating Rydberg states but also demonstrate, for the first time, the importance of intravalence transitions.

Photoionization of hydrogen and deuterium

Cross sections for the formation of slow molecular and energetic fragment ions in the photoionization of hydrogen and deuterium have been determined using synchrotron radiation within the energy range 25–40 eV. Superexcited states have been shown to play an important role in the photoionization process but recently predicted resonance effects have not been observed.

The dissociative photoionization of methane in the VUV

The dissociative photoionization of CH4 and CD4 has been studied using synchrotron radiation within the energy range of 12-60 eV. The mass spectra of secondary CHn+/CDn+ ions have been found to display significant isotope effects and to depend on the target gas temperature. The kinetic energy spectra of the fragment protons and deuterons have also been determined and it is shown that, in addition to direct photoionization, intermediate, superexcited states play an important role in the photodissociative ionization process.

The molecular and dissociative photoionization of ethane in the inner and outer valence energy regions

The molecular and dissociative photoionization of C2H6 has been studied using synchrotron radiation within the energy range 11-40 eV. Positive and negative photoion mass spectra and threshold photoelectron spectra have been obtained. These show that superexcited states, which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant and hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

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.

Fragment anion spectroscopy of water in the inner and outer valence regions

Photoion-pair formation in H 2 O and D 2 O has been studied using synchrotron radiation within the energy range 15-50 eV. Appearance potentials for the formation of O - , OH - and H - have been determined and coincide with their thermochemical thresholds. Above threshold it is shown that superexcited states play an important role in the polar photodissociation process. Substantial isotope effects have been observed.

Ionization and charge transfer in fast H+-He+ collisions using an intersecting beam technique

A fast intersecting beam technique has been used to obtain total cross sections sigma (He2+) for He2+ formation from the combined processes H++He+ to H++He2++e (ionization) and H++He+ to H+He2+ (charge transfer) in the centre-of-mass energy range 72-402 keV. At these energies, ionization is believed to provide the main contribution to the measured cross sections which are shown to be in good agreement with cross sections for ionization predicted theoretically by scaling data for H+-H collisions. Values of sigma (He2+) also show the expected tendency to converge to corresponding cross sections for ionization by electron impact at high equivalent velocities.

The photodissociative ionization of hydrogen and deuterium in the VUV via states

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of and by 29.5 - 60 eV photons have been measured perpendicular to the E vector of the radiation. It is shown that the superexcited state and the repulsive state play a dominant role in the photoionization process and autoionization state widths have been determined.

Isotope effects in the autoionization of superexcited 1Sigma+u(Q1) states in hydrogen

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of H2 and D2 by 27.5-32.0 eV photons have been measured. It is shown that the results can provide an experimental estimate of the autoionizing state width of the 1Sigmau(1) superexcited state as a function of internuclear separation R. The experimentally determined state widths are significantly larger than theoretical predictions especially at large R. Furthermore there is an unexpectedly large isotope effect.