M Nakamura

Photoionisation mass spectrometric study of acetylene in the VUV region

Photoionisation mass measurements on acetylene have been made between 1100 and 400 AA by using synchrotron radiation as a light source. Photoionisation efficiency curves are obtained for the molecular ion C2H2+ and also for fragment ions C2H+, C2+, CH2+ and CH+. To assign the structures of C2H2+, ab initio calculations have been performed for the excitation from the second (3 sigma g) and third (2 sigma u) occupied molecular orbitals. Two broad maxima at 930 and 810 AA between the first and second ionisation thresholds are assigned to the autoionisation from valence-type excited states. New Rydberg series (np pi u) converging to the second ionisation threshold were found. The appearance potentials of fragment ions by photoexcitation are given and discussed in terms of the heats of formation of molecular ions. For each of C2+ and CH+, two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C2+ is estimated from the appearance potential to be 1980 kJ mole-1.

Photoionisation mass spectrometry of O2 in the VUV region

Photoproduction cross sections of molecular oxygen for O2+ and O+ ions in the wavelength region from 1030 to 400 AA have been obtained using a quadrupole mass spectrometer and synchrotron radiation as a continuum light source. Photoionisation and dissociative photoionisation of O2 in the wavelength region from 750 to 400 AA are discussed. In the yield spectra of O2+ and O+, vibrational structures of the b 4 Sigma u- and B 2 Sigma u- states of O2+ are observed and the features are considered to be due to autoionisation and predissociation. In the Rydberg series converging to the c4 Sigma u- (O2+) state, characteristic features of the window-type and asymmetric profiles appear in both the spectra of O2+ and O+.

Dissociative photoionisation of N2 from threshold to 29 eV

Photodissociation of N2 has been studied using synchrotron radiation as a light source. The ratio of the yield of N+ to that of N2+ is measured by the time-of-flight method using the pulse characteristics of synchrotron radiation and the ratios thus obtained are 7 to 8.5% at 450, 470 and 490 AA. Using these ratios and the N+ yield curve measured by a mass filter, and the partial cross sections for the fragment ion production are determined in the region from threshold to 450 AA. Many peaks due to the autoionising states converging to the C 2 Sigma u+ state are observed in the N+ yield curve obtained with the mass filter. The kinetic energies of the fragment ion N+ are also measured at several wavelengths with the threshold electron-ion coincidence method. It is suggested from the analysis of the kinetic energy of the N+ ion that a potential curve which directly dissociates or predissociates of N(4S)+N+(1D)(26.2 eV) crosses the Franck-Condon region at the vicinity of 471 AA (26.3 eV).

Multiple photoionization from 3p excitation of Kr and 4p excitation of Xe

The photoionization cross sections for multiply charged ions produced by 3p excitation of Kr and 4p excitation of Xe have been obtained by means of a time-of-flight mass spectrometer and synchrotron radiation. It is found that the main formation of doubly to quadruply charged ions in both Kr and Xe is caused from the each initialp-hole state through a Coster-Kronig transition followed by Auger or double Auger processes. The formation of singly charged ions in these excitation energy regions is caused by direct photoionization from outermost shell electrons in both Kr and Xe. Triply charged ions are prominently produced among the multiply charged ions. The quadruple photoionization cross sections show clearly the structures due to the Rydberg series, 3p−1nl of Kr and 4p−1nl of Xe. Their main structures were assigned to the 3p−1nd series in Kr and the 4p−1nd series in Xe.

Angular distributions of photoelectrons from the autoionising states between the 2P3/2 and 2P1/2 ionisation thresholds of Kr and Xe

Angular distribution of photoelectrons of Kr and Xe were measured within the autoionising structure between the 2P3/2 and 2P1/2 ionisation thresholds, with higher energy resolution (5-7 meV) than previous measurements, using a steradiancy electron energy analyser and polarised radiation from the synchrotron at the Photon Factory at the National Laboratory for High Energy Physics in Tsukuba. The results showed a periodic variation in the asymmetric parameter beta as Samson and Gardner previously showed (1973) in the photoelectrons for Xe.

Determination of the photoionisation asymmetry parameter

It is shown that, from the measurements of the relative intensities of two independent photoionisation processes at more than three angles, the individual asymmetry parameters beta for the two photoionisation processes are determined. This method of analysis is applied to the measurement of the intensity ratios of the Q and S rotational branches of H2 carried out by Ruf et al. (1983). Using the beta values obtained for the Q and S branches, the asymmetry parameter for the (0-0) rotational transition is deduced for the first time from the experimental data. The value is close to 2.0, as expected from theory.