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Acta Physica Sinica

INELASTIC COLLISION PROCESS OF Rb ATOM BETWEEN INITIAL AND FINAL CHANNELS

Qu Yi-Zhi, Jianguo Wang, Jiaming Li, 中国科学院物理研究所

Published Jan 1, 1997• 4 citations
Publisher

Abstract

Based on the quantum defect theory, the infinite Rydberg states and its adjacent continue states with the same angular moment property will form a channel. Previous studies have considered the collision-induced transitions from a single initial state to a final channel. Here we extend these thoery to include all possible transitions between two channels. By defining a renormalized generalized oscillator strength density (GOSD) which varies smoothly across threshold, the inelastic collisions from an initial channel to a final channel can be treated in a unified manner. As an example, we present the renormalized GOSD of the Rb atom from s-initial to p-final channel, and also from s-initial to s-final channel.

DOI
10.7498/aps.46.249

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Based on the quantum defect theory, the infinite Rydberg states and its adjacent continue states with the same angular moment property will form a channel. Previous studies have considered the collision-induced transitions from a single initial state to a final channel. Here we extend these thoery to include all possible transitions between two channels. By defining a renormalized generalized oscillator strength density (GOSD) which varies smoothly across threshold, the inelastic collisions from an initial channel to a final channel can be treated in a unified manner. As an example, we present the renormalized GOSD of the Rb atom from s-initial to p-final channel, and also from s-initial to s-final channel.
Based on multichannel quantum-defect theory, a method to calculate and compile radiative transition processes for atoms (including highly ionized high-ZZZ atoms) is proposed. Through defining the renormalized transition matrix elements from initial eigenchannels to final eigenchannels, the transition processes between infinitely many initial states and infinitely many final states can be treated conveniently. As illustrative examples, we study the alkali-metal atoms in detail.
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For $20\ensuremath{\le}n\ensuremath{\le}45$, two types of quasielastic state-changing-collisional (QESCC) processes, i.e., (i) lll-changing and (ii) nnn-and lll-changing processes, have been theoretically investigated based on the free-electron model in which an excited Rydberg electron behaves as if it were "free" and slow with its interaction with a rare-gas atom playing a decisive role. It is shown that a parameter ${u}_{min}[=\frac{\ensuremath{\nu}{\ensuremath{\nu}}^{\ensuremath{'}}\ensuremath{\Delta}E{a}_{0}}{(\ensuremath{\hbar}V)}]$ is quite useful for characterizing both of these QESCC processes in a unified way where $\ensuremath{\nu}(\ensuremath{\simeq}{\ensuremath{\nu}}^{\ensuremath{'}})$ is an effective principal quantum number of the initial (final) state, $\ensuremath{\Delta}E$ is an energy defect, and VVV is a relative velocity. It is theoretically found that higher-angular-momentum states are dominantly populated in the final channel of these QESCC processes with $1<{u}_{min}\ensuremath{\lesssim}\ensuremath{\nu}$ such as in the nnn- and lll-changing collision of Rb∗∗(ns)+He{\mathrm{Rb}}^{**}(\mathrm{ns})+\mathrm{He}Rb∗∗(ns)+He. This is in contrast with the uniform distribution over the final angular-momentum states for the usual lll-changing processes, i.e., ${u}_{min}\ensuremath{\lesssim}1$. The calculated cross sections of these QESCC processes with $1<{u}_{min}\ensuremath{\lesssim}\ensuremath{\nu}$ are found to be in reasonable agreement with experimental findings available at present.
The mechanisms of nonresonant excitation transfer process in thermal collisions of two rubidium atoms Rb(7S) + Rb(5S) → Rb(5D) + Rb(5S) was studied theoretically, under conditions when the initial and final states of a system of two colliding atoms are not connected with each other by electric multipole transitions. Population processes of Rb 5D states were considered using calculated adiabatic potential curves of the Rb 2 quasimolecule in asymptotic approximation. The calculated cross-sections (σ theor = 5.0 × 10 −15 cm 2 ) are in good agreement with experimental results [σ exp = (8.0 ± 4.0) × 10 −15 cm 2 ] at thermal collision energies ( T ∼ 500 K).
The problem of impact ionization of two Rb atoms, one of which belongs to the Rydberg P series and the other of which is in the ground state, is studied and solved with the use of the methods of stochastic dynamics for description of the path instability of a Rydberg electron in a collision diatomic complex. The actual potential of the atomic framework was taken into account in terms of the single-channel quantum-defect method, whose accuracy was demonstrated on the example of determination of the photoionization cross sections in the quasi-classical approximation. A representation of the effective average lifetime of an external electron in the bound state that takes into account both the diffusion migration within the Coulomb crowding of the potential energy curves for the quasi-molecular complex and the direct quantum autoionization is obtained in analytical form. It is shown that the rate constants of diffusion collisional ionization calculated by us are in good agreement with the experimental data.