No abstract

Using mass spectrometry and electron spectrometry, we have studied ionization of Ar and Kr atoms in thermal energy collisions with laser-excited short-lived Ne(2p5 3p J=1,2,3) atoms. The cross sections show a strong statedependence, which is reproduced in calculations based on theoretical potential energy curves and a single exponential autoionization width. Significant polarization effects have been observed in the total and partial ion yields and in the electron spectra; they can only be explained with R-dependent autoionization probabilities favouring o + o electron transfer.

No abstract

Using transverse and longitudinal excitation of a collimated metastable Ne(3s 3P2.o) beam with average velocities of 500, 800, and 1,200 m/s by means of a single mode dye laser on the 2~ 3P2~3 p 3D3) transition, we have investigated ionizing collisions of polarized Ne(3s 3p2) and Ne(3p 3D3) atoms with Ar atoms. The product electrons were energy analyzed with high resolution (9-25meV). The resulting Ne(3p 3D3) electron spectra exhibit a strong dependence on the three types of laser polarization (7~11,7~• chosen to prepare the excited atoms. In contrast, the Ne(3s 3P2) spectra are only weakly dependent on polarization. Detailed model calculations have been carried out for the Ne(3p)+ Ar cross sections, using computed excited-state potential curves, semi-empirical ionic potentials, and local autoionization width functions. A semiclassical closecoupling method is applied to describe the evolution of the polarized collision system in the coupled entrance channels. It is found that a single autoionization width F(R) is not sufficient to describe the measured polarization effects properly. The dependence of F on the initial and final state is expressed in terms of few reduced electronic transition matrix elements, which are determined by comparison of measured and calculated total cross sections and Ar+(2p3/2)/Ar+(2P1/2) branching ratios for ionizing collisions of the various Ne(3p J = 1, 2, 3) multiplet states with Ar. The matrix elements corresponding to Ar(3p a)~Ne(2pa) electron transfer during autoionization are found to dominate, but Ar(3pr0-*Ne(2prc) transfer has also to be included. The resulting calculated electron spectra reproduce the measured polarization effects in a semi-quantitative way.

The energy spectra of electrons released in thermal energy (~ 50 meV) ionizing collisions of He* (2 iS, 23S) with H 2 have been measured with high resolution and low background. Based on a detailed data analysis, we report accurate H~ (v') vibrational populations P(v') for both He*(21S)+Hz (v'=O-lO) and He*(23S)+H2(v'=0-15) and the spectral shape S(e) for the individual vibrational peaks. The vibrational populations P(v') are quite similar to the Franck-Condon factors fv, 0 for unperturbed H 2 (v" = 0) "-~ H + (v') transitions, but, more in detail, the ratios P(v')/f~,o show a characteristically different v'-dependence for He*(23S), He*(21S), and HeI~(58.4 nm) ionization. The vibrational level separations in the He*(21S, 23S) + H 2 spectra agree with those in the HeI photoelectron spectrum to within 1-2 meV. The spectral shapes S(e) are characteristically different for He* (2 tS)+ H 2 and He* (23S) + H2, reflecting the respective differences in the entrance channel potentials, as determined previously in ab initio calculations and from scattering experiments.