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Canadian Journal of Physics

Equations de bilan dépendentes du temps appliquées aux collisions inélastiques

A. Lupaşcu, St. Tudorache, I. M. Popescu

Published May 1, 1982• 0 citations
Publisher

Abstract

A general matrix method for the time dependent rate equations applied to inelastic collisions is presented. The minimal information needed to compute the collisional cross-sections is deduced and used to study the particular case of two excited levels.
DOI
10.1139/p82-090

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A two-level model of an atomic system is investigated in order to study the cross section for certain inelastic collision processes. An impact-parameter method is employed. The time-dependent Schr\"odinger equation has been integrated numerically for some simple interaction potentials. Cross sections for excitation have been determined and are presented as functions of the parameters describing the model. The results are compared with those obtained by certain approximate methods. The effects of inclusion of diagonal matrix elements in the interaction Hamiltonian are found to be large. For a fixed form of the interaction potential, two cross sections, which may differ quite substantially, are obtained, depending on the algebraic sign of the coupling parameters or the energy difference between the states.
A general approximate method for computing the cross sections for certain inelastic collision processes is developed. It is based on the possibility of obtaining, with the use of the impact-parameter method, an exact expression for the transition amplitude in a special case. The dependence of the cross section on the parameters of coupling constant, velocity, and energy difference between states is discussed for a two-level model. Detailed calculations are made of cross section for collisions of the type $A(^{2}P_{\frac{3}{2}})+B\ensuremath{\rightleftharpoons}A(^{2}P_{\frac{1}{2}})+B$, where AAA is an excited atom of an alkali metal and BBB is an atom of a noble gas. An Appendix contains a review of the effective long-range interaction of atoms.
Les niveaux 6 1D2 ou 6 3D2 de la configuration 6s 6d de Hg sont peuplés sélectivement par un laser à colorant à partir du niveau de résonance 6s 6p 1P1 peuplé optiquement. On mesure les sections efficaces de transfert de population vers le niveau 6 3D3 sous l'effet de collisions inélastiques avec des atomes de gaz rares ou des molécules d'azote. L'excitation sélective du niveau 61P1 permet d'améliorer considérablement la précision des résultats déjà obtenus et publiés récemment [1].
Collisional transfers induced in the n=6--13 Li Rydberg states by H2{\mathrm{H}}_{2}H2​, N2{\mathrm{N}}_{2}N2​, and CO molecules have been experimentally studied in a heat-pipe oven. The l-mixing cross sections have been deduced from the fast component of the population relaxations following pulsed laser excitation. The order of magnitude of these cross sections (103{10}^{3}103 A˚2{\r{}}^{2}˚2) is well predicted by the existing theories. For large n, the cross sections stay approximately constant. This can be partly explained by the influence of inelastic processes such as n changing. In the particular case of H2{\mathrm{H}}_{2}H2​, n-changing collisions have been identified by the fluorescence starting from the n'=n\ifmmode\pm\else\textpm\fi{}1, n-2 levels. Cross sections of the order of 10 A˚2{\r{}}^{2}˚2 have been measured. The systematic study of the n-changing cross sections as a function of the transferred energy shows some evidence of resonance phenomena corresponding to an energy exchange between the Rydberg electron and the molecular rotation. This effect is also observed when H2{\mathrm{H}}_{2}H2​ is replaced by D2{\mathrm{D}}_{2}D2​. These resonances are well described by the theory of Petitjean and Gounand based on the e-quadrupolar interaction.