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Physical Review A

l-changing collisions in the intermediate Rydberg states of lithium

Mimoun Harnafi, Bernard Dubreuil

Published Mar 1, 1985• 9 citations
Publisher

Abstract

l-mixing collisions Li(${n}^{2}$D)+X\ensuremath{\rightarrow}Li(${n}^{2}$L,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 ${n}^{2}$D 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 ${n}^{2}$D 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 ${n}^{2}$D states and ${n}^{2}$L mixed states with L\ensuremath{\ge}1 are deduced, respectively, from the low-pressure limit and intermediate-pressure study of the ${n}^{2}$D fluorescence relaxation rate.

DOI
10.1103/physreva.31.1375

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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.
Two-step laser-induced-fluorescence techniques are used to investigate nnn-changing collisional processes $\mathrm{Li}(n=9)+\mathrm{Li}(2^{2}S)\ensuremath{\rightarrow}\mathrm{Li}({n}^{\ensuremath{'}}\ensuremath{\ne}n) +\mathrm{Li}(2^{2}S)+\ensuremath{\Delta}E_{\mathrm{nn}}^{\ensuremath{'}}$ in a temperature-controlled lithium heat-pipe oven. Numerical identification of the resonance- and collision-induced fluorescences with the hydrogenic rate equations describing the time variation of the n=7,8,9,10,11n=7,8,9,10,11n=7,8,9,10,11 populations allows the determination of collisional transfer and quenching rate coefficients. nnn-changing collisions with $\ensuremath{\Delta}n=|n\ensuremath{-}{n}^{\ensuremath{'}}|=1$ predominate, and the quenching rate coefficients are found nearly independent of $n: {k}_{n}\ensuremath{\sim}1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ cm3{\mathrm{cm}}^{3}cm3${\mathrm{s}}^{\ensuremath{-}1}$.
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.
The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric CO2{\mathrm{CO}}_{2}CO2​ laser--induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in CO2{\mathrm{CO}}_{2}CO2​ gas at low pressure. Atoms in the beam are probed by laser-induced fluorescence spectroscopy. This allows the determination of time-of-flight and velocity distributions. Li Rydberg states (n=5--13) are populated in the beam by two-step pulsed-laser excitation. The excited atoms interact with CO2{\mathrm{CO}}_{2}CO2​ molecules. l- and n-changing cross sections are deduced from the time evolution of the resonant or collision-induced fluorescence following this selective excitation. l-changing cross sections of the order of 104{10}^{4}104 A˚2{\r{}}^{2}˚2 are measured; they increase with n as opposed to the plateau observed for Li∗{\mathrm{Li}}^{\mathrm{*}}Li∗ colliding with a diatomic molecule. This behavior is qualitatively well explained in the framework of the free-electron model. n\ensuremath{\rightarrow}n' changing processes with large cross sections (10--100 A˚2{\r{}}^{2}˚2) are also observed even in the case of large electronic energy change (\ensuremath{\Delta}Enn′{E}_{\mathrm{nn}\mathcal{'}}Enn′​>103{>10}^{3}>103 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and CO2{\mathrm{CO}}_{2}CO2​ vibrational modes.
Excitation transfers between the 42D4^{2}D42D and 42F4^{2}F42F states of lithium are studied in a temperature-controlled heat-pipe oven. These transfers are induced by collisions with Li, He, and Ar ground-state atoms. Quenching cross section and radiative rates are also measured. For the excitation transfer process, the thermally averaged cross sections are, respectively, ${\ensuremath{\sigma}}^{\mathrm{Li}}=400$ A˚2{\mathrm{\AA{}}}^{2}A˚2, ${\ensuremath{\sigma}}^{\mathrm{He}}=120$ A˚2{\mathrm{\AA{}}}^{2}A˚2, and ${\ensuremath{\sigma}}^{\mathrm{Ar}}=190$ A˚2{\mathrm{\AA{}}}^{2}A˚2. These values agree with the experimental results of Gallagher in Na and the calculations of Gersten. Finally, the behavior of the $4^{2}D\ensuremath{-}4^{2}F$ two-level system in the lll-mixing regime is investigated in the case of Ar collisions.
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