C J Latimer

Charge transfer between H2+ and AR: the effect of molecular vibration studied using photoelectron-photoion coincidence spectroscopy

Charge transfer between H2+ ions in specific vibrational states and argon atoms has been investigated. H2+ ions were produced by photoionisation and state analysis was achieved by the registration of charge transfer events in coincidence with energy-analysed photoelectrons. Results at 20 eV are interpreted with the aid of a simple theory which incorporates the effects of changes in different vibrational levels and the 2P3/2,1/2 states of Ar+. The implications of these results for the variation of vibrational distribution in molecular beam experiments are discussed.

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 effect of molecular vibration on charge transfer between H2+ and H2

Charge transfer between H2+ ions in specific vibrational states and H2 molecules has been investigated within the energy range 8-1000 eV using photoelectron-photoion coincidence spectroscopy. Results up to the sixth vibrational level show that cross sections vary significantly with vibrational energy in a complicated manner which is not in accord with multistate impact parameter theory.

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 effect of vibrational and electronic energy on near-resonant charge-transfer processes involving the rare gases and simple molecules

Charge transfer between H2+ and Ar, O2+ and Xe has been investigated as a function of the vibrational quantum number of the molecular ion within the energy range 20-1000 eV. The reverse reactions have also been studied and the ratio of the cross sections for the two spin-orbit levels of the ground-state rare-gas ion, sigma (2P32/)/ sigma (2P12/) determined. The results are interpreted in terms of a simple theory which incorporates the appropriate energy defects and Franck-Condon factors.

Some state-selected charge transfer processes involving 10-1500 eV rare-gas ions and simple molecules

Near-resonant charge transfer collisions involving Ar+ ions in H2, N2 and CO and Kr+ ions in CO have been measured as a function of the internal energy state of the primary ions within the translational energy range 10 eV to 1.5 keV. The ratios of the cross sections for the two spin-orbit levels of the ground-state rare-gas ions sigma (2P32/)/ sigma (2P12/) have been determined. The results are in accord with recent semiclassical multistate computations using ab initio potential energy surfaces. The validity of simple models incorporating appropriate energy defects and the Franck-Condon principle is discussed.

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.

The double ionisation of hydrogen by 5-30 keV protons

The energy and angular distribution of pairs of fragment protons produced in H+-H2 collisions has been investigated in the energy range 5-30 keV. It is shown that the energy distribution is a simple transformation of the ground-state wavefunction of H2 and an experimental determination of the ground-state probability density distribution is presented. Total and differential double ionisation cross sections are found to be in accord with earlier measurements.

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.