T Koizumi

Momentum transfer cross sections for low-energy electrons in krypton and xenon from characteristic energies

Characteristic energies of low-energy electrons in krypton and xenon gases have been measured at room temperature by the Townsend method as a function of reduced electric field. For both gases, observed characteristic energies were twice or even three times as large as the early estimates by Frost and Phelps (1964) based on the momentum transfer cross sections for electrons in these gases derived from drift velocity data. The cross sections were determined from the present experimental data over an energy range from 0.01 to 6.0 eV for krypton and from 0.01 to 5.0 eV for xenon. The minimum values and positions of Ramsauer minima in the derived cross sections were 1.0*10-17 cm2 at 0.5 eV for krypton and 4.0*10-17 cm2 at 0.6 eV for xenon. These minimum values are both definitely smaller than the previous experimental results of Frost and Phelps, while being rather close to, but still somewhat smaller than, those of Hoffmann and Skarsgard (1969). Theoretical cross sections calculated for krypton and xenon by Sin Fai Lam (1982) and by McEachran and Stauffer (1984) and those for krypton by Fon et al. (1984) all agree with the present experimental ones within +or-30% for energies below about 5 eV in both gases.

Drift Tube Study of Symmetric Resonance Single- and Double-Charge Transfer in Kr++Kr, Xe++Xe, Kr+++Kr, and Xe+++Xe Systems

An injected-ion drift tube mass spectrometer is used to determine the cross sections of processes Kr + +Kr→Kr+Kr + , Xe + +Xe→Xe+Xe + , Kr ++ +Kr→Kr+Kr ++ , and Xe ++ +Xe→Xe+Xe ++ . The drift tube was operated with He buffer gas and cooled with liquid nitrogen so that the energy range studied was widened as from 10 meV to 10 eV. It was found that, at the lowest energy end, the energy dependence of cross section for single charge transfer deviates from that expected by linear trajectory model. For double charge transfer, the cross section below 1 eV almost coincide with the orbiting cross section with a charge transfer probability 1/2. Above 1 eV, the energy dependence of the cross section is like that for single charge transfer. Mobilities of Kr + , Xe + , Kr ++ , and Xe ++ in He at 296 K and 82 K are presented also.

Photoionization of Ba+ ions due to creation of 4d hole states

Photoion spectrometry has been applied to determine the relative photoionization yields of Ba+ ions around the 4d ionization threshold. A ground state Ba+ ion beam was merged with monochromatized synchrotron radiation over a length of 15 cm. Counting the number of doubly and triply charged ions produced in the interaction region, we have obtained partial photoion yields as a function of photon energy. Prominent peaks due to resonance Auger processes have been observed below the threshold. A giant resonance structure above 4d threshold was seen only for the Ba3+ spectrum.

Dissociative photoionization of SiH4 in the 12–19 eV region

Abstract Relative photoexcitation spectra of ionic fragments SiH+3, SiH+2, SiH+ and Si+ originating from photofragmentation of molecular silane (SiH4) have been measured in the 12–19 eV region using a time-of-flight mass-analytic method combined with synchrotron radiation as a light source. No SiH+4 could be detected in a measurement time of a few microseconds. The excitation spectra show a maximum photoionization efficiency around 13.3 eV for SiH+2 and around 14.8 eV for SiH+3. Correlations between photoionization and dissociation are discussed. The assignment of the autoionization structure between 15 and 18 eV by Borlin et al. has been positively examined and extended to include two vibrational progressions composed of the ν1 mode with a frequency of 1690 ± 60 cm−1 and the ν2 mode with 730 ± 80 cm−1.

Ionic fragmentation of SiH4 following the L-shell excitation

Ionic photofragmentation of SiH4 has been studied in the vicinity of the Si: L-edge using synchrotron radiation and time-of-flight mass spectrometry. Relative ionization efficiencies for more then 10 ionic fragment species and pair-ionization efficiencies for the pairs H+ + SiHn+ (n=0, 1 and 2) have been obtained as a function of the photon energy from 100 to 200 eV from the photoelectron-photion and the photoion-photoion coincidence measurements. Ionic dissociation of the continuum Si: 2p hole states may occur mostly via double ionization of the parent molecule giving rise to two types of dissociation channels; (type A) SiH4++ → H+ + H+ + neutrals and (type B) SiH4++ → H+ + Si+ (or SiH+) + neutrals. The average kinetic-energy release given to H+ is much higher in the dissociation channel B than in the dissociation channel A suggesting temporal existence of localized and delocalized valence-two-hole states.

Photoionization of 4d-electrons in singly charged Xe ions

Using a photon - ion merged-beam technique, we measured the relative yield spectra of and ions created by photoionization of ions as a function of the photon energy in the 4d ionization region. The ion production, i.e. the double-electron ionization process is found to be dominant throughout the energy range investigated. A broad structure of 4d excitation ionization around 100 eV photon energy was observed in the yield spectrum. A spectral shoulder was observed for at around 85 eV, which was absent in the photoionization of neutral Xe. Some discrete lines were observed for both the - and -yield spectra below 75 eV. The experimental spectra were analysed by a multiconfiguration Dirac - Fock calculation. The largest structure at around 100 eV is attributable to (n=6,7) two-electron transitions. The discrete lines were found to be due to , n f transitions. A serious 4f-orbital collapse is suspected in the 4 fn p two-electron excited states, whereas the collapse is moderate in the singly excited 4f-orbitals.

4d-shell photoionization of Xe, Cs, Ba, Sm, Eu and Yb atoms studied by photoion-yield spectra

A comparative study of multiple photoionization in the 4d ionization region (78-270eV) of M = Xe, Cs, Ba, Sm, Eu and Yb atoms is reported. Partial photion-yield spectra for Mn+ (n = 1-4) ions have been measured by using synchrotron radiation as a light source. Main products in Xe, Cs and Ba are Mn2+ and M3+ ions due to the creation of a 4d vacancy. In Sm and Eu, a large fraction of M+ ions are populated via the 4d-4f resonant excitation and subsequent autoionization to the 4f ionization continuum, which is squeezing the population of multiply-charged ions to about 50%. In Yb, a large amount of M+ ions are also populated, but are due largely to the direct 4f ionization. In all of the atoms studied, an appreciable or considerable amount of M4+ ions are observed, indicating the existence of photoelectron and/or Auger shake-off processes.

Charge distribution of Xe ions as a result of multiple photoionisation of Xe atoms between 4.1 and 8.0 keV

The charge distributions of Xe ions as a result of photoionisation have been studied experimentally and theoretically. The initial vacancies are produced by 4.1-8.0 keV synchrotron orbital radiation and the charge distribution of ions was measured with a time-of-flight mass spectrometer at various photon energies. The theoretical estimation of the charge distribution was made by the use of the Monte Carlo simulation. The calculated charge distributions as a function of incident photon energy are in good agreement with the measured results.

Absolute photoionization cross section measurements of Xe + ions in the 4d threshold energy region

Using the photon–ion merged-beam technique, we have measured the absolute photoionization cross sections for Xe 2+ and Xe 3+ formation from Xe + at selected energies between 80 and 140 eV, and utilized the absolute data to normalize our previously reported relative cross sections. The structure and energy dependence of the cross sections for Xe + deviate from those of neutral Xe; however, the maximum total cross section for Xe + is, within the experimental error, the same as for Xe.

Production of multiply charged Xei+ ions via photoionization and excitation in the L-edge region

Fractional charge distributions of multiply charge Xei+ ions produced under (4.1-8.0 keV) X-ray impact on Xe atoms have been measured and the corresponding cross sections for their production have been determined after normalizing the known total photoionization cross sections. Both of them show stepwise variations near the L edges, indicating that the inner-shell ionization followed by a series of vacancy cascades play a significant role in the production of these multiply charged ions. In addition, sharp enhancement of the production cross sections for ions with some charge states has been observed over very narrow energy regions just below the L edges, in particular the L3 edge, and is qualitatively understood to be due to the photoexcitation of an inner-shell electron into the unfilled outer levels.