(e, 2e) spectroscopy

Using the technique of laser ionization mass spectrometry, studies have been carried out of multiphoton ionization (MPI) and fragmentation of molecules under collision‐free conditions. MPI spectra and mass fragmentation patterns are obtained via irradiation of isolated molecules by a pulsed, tunable dye laser focused down on a molecular beam traversing the ion‐source region of a quadrupole mass filter. At each resonance, corresponding to m‐photon ionization of an n‐photon‐excited intermediate state, the fragmentation pattern of the ions is measured. Resonance‐enhanced MPI/mass spectra provide detailed information on the identity and relative abundances of the ionic fragments formed in the overall n+m photoionization process. From a knowledge of the appearance potentials of the various fragment ions, the minimum number of photons absorbed per molecule can be deduced. Increasing the laser pulse peak energy (or fluence) increases the average number of photons absorbed and thus the extent of fragmentation. Ho...

Resonance‐enhanced multiphoton ionization and fragmentation of molecular beams: NO, I2, benzene, and butadiene

Recommended values are presented for qm, qt and QT (momentum transfer cross-sections for elastic collisions, total cross-sections for elastic collisions and total collision cross-sections, respectively) of electrons in xenon for electron energies from 0 to 104 eV. Recommended values of ionisation cross-sections qi are also presented. Using these values of qm together with values of qi and Townsend ionisation coefficients alpha , recommended values of total excitation cross-sections qe are determined. The values of qm for electron energies from 5 to 20 eV are smaller than those of Frost and Phelps (1964) by a considerable amount. The values of qe for energies from about 20 to 100 eV are much larger than those of de Heer et al. (1979).

https://iopscience.iop.org/article/10.1088/0022-3727/16/4/018/pdf

Determination of electron-xenon total excitation cross-sections, from threshold to 100 eV, from experimental values of Townsend's α

Electron-photon angular correlations were measured by delayed coincidences between electrons scattered inelastically from helium and 58.4 nm photons emitted in the decay of 2 1P state. The ratio of the differential cross sections for exciting the degenerate magnetic sublevels and the relative phase of the corresponding excitation amplitudes for electron scattering angles between 16 degrees and 40 degrees and for incident electron energies in the range 40 to 200 eV is deduced. The data show that a high degree of atomic orientation is produced by collisions in this range of energy and angle. The orientation increases rapidly towards its theoretical limit as the scattering angle is increased, and higher incident energy produces, in general, greater orientation at a fixed scattering angle. The data also show that the alignment of the atomic dipole radiation pattern can depart significantly from the direction of linear momentum transfer at the larger scattering angles. The results are compared with the predictions of the first Born and Glauber approximations, and good agreement is obtained with a recent distorted wave calculation.

Electron-photon angular correlations in electron-helium collisions: measurements of complex excitation amplitudes, atomic orientation and alignment

/pdf/one-century-of-experiments-on-electron-atom-and-molecule-53rvvfyzak.pdf

One century of experiments on electron-atom and molecule scattering: A critical review of integral cross-sections

Absorption by Cs2, Rb2 and RbCs of radiation at the nine wavelengths of an Ar-ion laser leads to production of molecular and atomic ions via two-photon processes. The authors report absolute probabilities for these processes in Cs2 and Rb2 (and relative numbers for RbCs) and discuss the spectral behaviour in the light of spectroscopic information on the potential energy diagrams. For Cs2, values for the photoionization cross sections of the E state, the intermediate state in the two-step process, are derived.

http://iopscience.iop.org/article/10.1088/0022-3700/9/5/029/pdf

Two-photon ionization of Cs2, Rb2 and RbCs using an Ar-ion

Measurements are reported of the spin polarisation of electrons elastically scattered from a xenon atomic beam for scattering angles between 30 degrees and 120 degrees at collision energies between 5.5 and 300 eV. Good agreement is found with the experimental work of Schackert (50 and 100 eV, 1968) and of Kessler et al. (150-300 eV, 1977). In the 150-300 eV range, the best agreement is found with the exchange calculations of Walker (150-300 eV, 1971) whereas at energies below 150 eV all existing calculations fail to predict the correct polarisation.

The polarisation of electrons elastically scattered from xenon at energies between 5 and 300 eV

In a cross-beam experiment electrons are scattered elastically from xenon atoms The dependence of the relative cross section on collision energy over the range 2-300 eV and on scattering angle over the range 30 degrees -120 degrees is measured Very satisfactory overall agreement is found with the experimental work of Mehr (1967), Jost and Heindorff et al (1976) At angles below 50 degrees there are serious discrepancies between several experiments The optical-model calculation of McCarthy et al (1977) predicts the angular behaviour for energies of 100 eV and above The comparison with calculations using a real potential shows that the calculation of Walker (1971), which includes exchange, is to be preferred over that of Fink and Yates (1970), which neglects exchange effects

Angular distributions of electrons elastically scattered from xenon at energies between 2 and 300 eV

“Photofluorescence” by electron impact: determination of the relative oscillator strength for formation of the B2σ State of CO+

Measurements are reported on the photoelectron spin polarization and ratio of the ion count rates for linearly and circularly polarized light obtained in two-photon ionization of caesium atoms via the 72P1/2 and 72P3/2 intermediate resonance states. Depending on the pulse length of the radiation used the transitions have to be described either in the fine-structure or in the hyperfine-structure coupling scheme. A simple model is used to describe intermediate cases. From these measurements ratios are obtained for the various bound-free matrix elements.

M Klewer

Papers

Two-photon ionization of Cs2, Rb2 and RbCs using an Ar-ion

The polarisation of electrons elastically scattered from xenon at energies between 5 and 300 eV

Angular distributions of electrons elastically scattered from xenon at energies between 2 and 300 eV

“Photofluorescence” by electron impact: determination of the relative oscillator strength for formation of the B2σ State of CO+

Two-photon ionization of caesium via the 72P1/2, 3/2 intermediate states