Excitation of the helium autoionizing states in He/sup +/+He collisions, between 3 and 140 keV
The autoionization of the helium atom has been experimentally studied in He/sup +/+He collisions between 3 and 140 keV by electron spectrometry. The excitation of the two collision partners has been considered. Above 10 keV, the shapes and excitation cross sections of the 2s/sup 2/ /sup 1/S, 2s2p /sup 3/P, 2p/sup 2/ /sup 1/D, and 2s2p /sup 1/P are determined by a numerical fitting procedure which is reported in detail; below 10 keV the (/sup 1/D+/sup 1/P), and 2p/sup 2/ /sup 1/S line intensities are obtained by planimetry since important post-collision effects are observed. From the angular distributions measured below 15 keV, the relative sublevel populations are deduced for the 2p/sup 2/ /sup 1/D and 2s2p/sup 1/ P levels and are compared with those obtained by other authors in a coincidence experiment; the excitation processes are then discussed within the quasimolecular-excitation model. For the highest collision energies, the asymmetry of the angular distributions with respect to 90 /sup 0/ as well as the line shapes above 100 keV are interpreted by the occurrence of sudden electronic transitions to the continuum. A comparison of the differential cross sections for emission of electrons by autoionization of the fast and slow particles permits us to show that the quasimolecule model cannot explain what is observed above a collision velocity of about 0.5 a.u. The dependence of the total cross sections against the collision energy is also discussed in terms of an evolution of the excitation mechanism from a quasimolecular to an atomic one; the specific variation of the 2s2p /sup 3/P cross section strengthens this interpretation. These total cross sections are compared with those estimated from earlier H/sup +/+He data published by us; similar autoionization cross-section values are expected for the two systems at high collision velocity.
- Research Organization:
- Centre de Physique Atomique, Equipe de Recherche Associee au Centre National de la Recherche Scientifique n 598, Universite Paul Sabatier, 118, Route de Narbonne, 31062 Toulouse Cedex, France
- OSTI ID:
- 5541814
- Journal Information:
- Phys. Rev. A; (United States), Vol. 25:1
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
HELIUM
AUTOIONIZATION
ION-ATOM COLLISIONS
HELIUM IONS
ANGULAR DISTRIBUTION
ASYMMETRY
CROSS SECTIONS
ENERGY DEPENDENCE
EXCITATION
KEV RANGE
ATOM COLLISIONS
CHARGED PARTICLES
COLLISIONS
DISTRIBUTION
ELEMENTS
ENERGY RANGE
ENERGY-LEVEL TRANSITIONS
FLUIDS
GASES
ION COLLISIONS
IONIZATION
IONS
NONMETALS
RARE GASES
640304* - Atomic
Molecular & Chemical Physics- Collision Phenomena