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Physical Review A

Quenching of Li low- n Rydberg atoms by ground-state He atoms: A low-energy molecular-state calculation

Bidhan C. Saha, Anil Kumar

Published Feb 11, 2000• 2 citations
Publisher

Abstract

Total cross sections for quenching of low-n Rydberg states of Li $(n<10,$ $l=0)$ in collision with the ground state He have been calculated in the low-energy region $(v<~0.1\mathrm{a}.\mathrm{u}.)$ by employing the molecular orbital approximation. Results for the total depopulation of the parent states, along with those for excitation and de-excitation mechanisms have been presented. Present cross sections exhibit the low-energy Stueckelberg oscillations. Using a model two-state calculation we present a detailed comparative study between the quantum-mechanical and semiclassical findings. Except at very low energies, the semiclassical cross sections agree nicely with their quantum counterparts. The reaction rates at thermal energies are also reported.

DOI
10.1103/physreva.61.032709

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Total cross sections for quenching of low-n Rydberg states of Li (n<10,(n<10,(n<10, l=0)l=0)l=0) in collision with the ground state He have been calculated in the low-energy region (v< 0.1a.u.)(v<~0.1\mathrm{a}.\mathrm{u}.)(v< 0.1a.u.) by employing the molecular orbital approximation. Results for the total depopulation of the parent states, along with those for excitation and de-excitation mechanisms have been presented. Present cross sections exhibit the low-energy Stueckelberg oscillations. Using a model two-state calculation we present a detailed comparative study between the quantum-mechanical and semiclassical findings. Except at very low energies, the semiclassical cross sections agree nicely with their quantum counterparts. The reaction rates at thermal energies are also reported.
The molecular expansion method within the framework of the semiclassical approximation is applied to quenching of low-lying excited states of Rydberg atoms colliding with ground-state He at thermal energies. Interactions between the colliding atoms are accounted for in terms of pseudopotentials, and their relative motion is described by a classical linear trajectory. A fairly large basis set of Slater-type orbitals is used to obtain molecular eigenstates, and subsequently a 14-state close-coupling calculation is performed to evaluate the total quenching cross sections and the contributions of individual transitions. The energy dependence of the calculated cross sections is investigated, and possible mechanisms responsible for individual transitions are explained. Finally, the collision rates are calculated and compared with experimental results.
A theoretical model for the collision of atoms in high Rydberg states (nl) with neutral atoms or molecules is proposed, based on the assumption that the electron-perturber interaction is of very short range compared with the mean radius of the Rydberg atom. A semiclassical method is used to derive a compact expression for the collision probability. The calculations need total cross sections for the scattering of a free and low-energy electron by the perturber. The model is applied to the calculation of a free and low-energy electron by the perturber. The model is applied to the calculation of cross sections for the quenching of n 2 D or n 2 F Rydberg states of Na and Rb by rare-gas atoms of N 2 . On the whole the calculated cross sections are in reasonable agreement with experimental data for n>10 approximately. The model and the results of the calculations are discussed and compared with other theoretical works.
l-mixing collisions Li(n2{n}^{2}n2D)+X\ensuremath{\rightarrow}Li(n2{n}^{2}n2L,L>2)+X induced in Rydberg states of lithium (n=4,5,...,13) by rare-gas atoms or Li ground-state atoms are studied in a heat-pipe arrangement. The l-mixing cross sections deduced from the time analysis of the n2{n}^{2}n2D laser-induced fluorescence agree with experimental values obtained with the heavier alkali-metal atoms and with different theoretical models developed for quasielastic Rydberg-electron--target collisions. Quenching of the n2{n}^{2}n2D fluorescence at higher rare-gas pressures is interpreted as inelastic collisional transfer of the L\ensuremath{\ge}1 mixed states to the nonhydrogenic S sublevel. Finally the radiative lifetimes of the n2{n}^{2}n2D states and n2{n}^{2}n2L mixed states with L\ensuremath{\ge}1 are deduced, respectively, from the low-pressure limit and intermediate-pressure study of the n2{n}^{2}n2D fluorescence relaxation rate.
We report the results of comparative studies of ion-pair formation and quenching processes in collisions of Rydberg Li( nl ) and Cs( nl ) atoms with Ca(4s 2 ), Sr(5s 2 ) and Ba(6s 2 ) atoms possessing small electron affinities. Our consideration includes both the cases of selectively excited Rydberg nl -states with small orbital angular momentum ( l ≪ n ) and nearly circular states with l ∼ n − 1. Calculations of the electron-transfer processes are based on the semiclassical theory of nonadiabatic transitions and exact expression for the Rydberg-covalent–ionic coupling terms. Calculations of nonresonant quenching processes are carried out within the framework of the available theory of inelastic and quasielastic transitions between Rydberg-covalent states. The ion-pair formation and resonant quenching cross sections are shown to be significantly dependent not only on the principal quantum number n but also on the orbital angular momentum l and the binding energy of the alkaline-earth anion. For each system under study we find the regions of n in which either the resonant quenching or the ion-pair formation processes are predominant. The relative role of the resonant and nonresonant mechanisms of depopulation of Rydberg states is investigated.
The total quenching cross sections for $\mathrm{He}({n}^{1,3}S)(2\ensuremath{\le}n\ensuremath{\le}11)$ states by thermal collisions [600(+ 200, - 100) K] with He, Ne, Ar, Kr, and Xe have been measured. The main quenching mechanism is attributed to Penning ionization for lower nnn, and collisional angular-momentum transfer for highly excited states. Measured cross sections are of the order of ${10}^{\ensuremath{-}15}$ to ${10}^{\ensuremath{-}14}$, cm2{\mathrm{cm}}^{2}cm2. The values for higher nnn states are much smaller than the geometrical cross section; this is explained by a relatively large energy spread between the SSS state and $l\ensuremath{\ge}1$ states.
The authors have measured the cross sections for the collisional quenching of highly excited (34<or=n<or=43) S rubidium atoms by ground-state rubidium atoms. The comparison between their data and the values calculated using a theoretical approach that only takes into account the valence electron-ground-state rubidium interaction provides the first experimental estimation of the cross section for the elastic scattering of ultra-low-energy electrons (E approximately=0.01 eV) by rubidium atoms. Their results for the quenching are connected to the available measurements of the self-broadening of the rubidium (5S-nS) lines by using a recent theoretical model: for n approximately 40 the self-broadcasting is essentially due to inelastic collisions. An upper limit is given for the cross section for the isotopic transfer between 85 Rb(nS) and 87 Rb(nS) states due to collisions with ground-state rubidium atoms.