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Physical Review A

l- andn-changing collisions during interaction of a pulsed beam of Li Rydberg atoms with CO2

Bernard Dubreuil, Mimoun Harnafi

Published Jul 1, 1989• 1 citation
Publisher

Abstract

The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric ${\mathrm{CO}}_{2}$ laser--induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in ${\mathrm{CO}}_{2}$ 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 ${\mathrm{CO}}_{2}$ 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 ${10}^{4}$ A${\r{}}^{2}$ are measured; they increase with n as opposed to the plateau observed for ${\mathrm{Li}}^{\mathrm{*}}$ 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${\r{}}^{2}$) are also observed even in the case of large electronic energy change (\ensuremath{\Delta}${E}_{\mathrm{nn}\mathcal{'}}$${>10}^{3}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and ${\mathrm{CO}}_{2}$ vibrational modes.

DOI
10.1103/physreva.40.69

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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}{E}_{\mathrm{nn}\mathcal{'}}$${>10}^{3} cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\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.
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.
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.
Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.
Lithium atoms in high Rydberg states, excited in an atomic beam by pulsed electron impact, are passed through a target gas and detected following electric-field state selection. Cross sections obtained from time-of-flight spectra are virtually independent of the principal quantum number and in numerical agreement with calculations for deflection of the core ion alone. This deflection has also been observed directly. These results suggest a new method for very-low-energy ion-atom scattering experiments.
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${\mathrm{s}}^{\ensuremath{-}1}$.