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Physical Review A

Quenching and excitation transfers in n=3 lithium sublevels

Catherine Chaleard, Bernard Dubreuil, A. Catherinot

Published Sep 1, 1982• 21 citations
Publisher

Abstract

Quenching and excitation-transfer processes involving the $3^{2}P$ and $3^{2}D$ states of the Li atom are studied in a temperature-controlled heat-pipe oven with the use of two-step laser excitation of the $4^{2}S$ state followed by radiative cascade towards the $3^{2}P$ state. Numerical identification of the collision-induced $3^{2}D\ensuremath{\rightarrow}2^{2}P$ fluorescence with a three-level model allows the determination of radiative coefficients and collisional rate coefficients. $4^{2}S$, $3^{2}D$, and $3^{2}P$ radiative lifetimes are found to be in agreement with accepted values. Quenching and excitation transfer strongly depend on the colliding atoms. Rate coefficients for the $3^{2}D\ensuremath{\rightarrow}3^{2}P$ excitation processes induced by Li, He, and Ne ground-state atoms are measured in the ratio 1.05: 0.23: 0.02 expressed in ${10}^{\ensuremath{-}9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{3}$.

DOI
10.1103/physreva.26.1431

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Collisional transfers induced in the n=6--13 Li Rydberg states by ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, 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 (${10}^{3}$ A${\r{}}^{2}$) 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 ${\mathrm{H}}_{2}$, 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${\r{}}^{2}$ 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 ${\mathrm{H}}_{2}$ is replaced by ${\mathrm{D}}_{2}$. These resonances are well described by the theory of Petitjean and Gounand based on the e-quadrupolar interaction.

Physical Review A•Aug 1, 1986•3 citations
Quenching and excitation-transfer processes involving the 32P3^{2}P32P and 32D3^{2}D32D states of the Li atom are studied in a temperature-controlled heat-pipe oven with the use of two-step laser excitation of the 42S4^{2}S42S state followed by radiative cascade towards the 32P3^{2}P32P state. Numerical identification of the collision-induced 32D\ensuremath→22P3^{2}D\ensuremath{\rightarrow}2^{2}P32D\ensuremath→22P fluorescence with a three-level model allows the determination of radiative coefficients and collisional rate coefficients. 42S4^{2}S42S, 32D3^{2}D32D, and 32P3^{2}P32P radiative lifetimes are found to be in agreement with accepted values. Quenching and excitation transfer strongly depend on the colliding atoms. Rate coefficients for the 32D\ensuremath→32P3^{2}D\ensuremath{\rightarrow}3^{2}P32D\ensuremath→32P excitation processes induced by Li, He, and Ne ground-state atoms are measured in the ratio 1.05: 0.23: 0.02 expressed in 10\ensuremath−9{10}^{\ensuremath{-}9}10\ensuremath−9 sec\ensuremath−1{\mathrm{sec}}^{\ensuremath{-}1}sec\ensuremath−1 cm3{\mathrm{cm}}^{3}cm3.
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.
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${\mathrm{s}}^{\ensuremath{-}1}$.
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.
The cross sections for the excitation energy transfer induced by collisions with He, Ar and Na ground-state atoms have been measured. The sodium atoms in the 4D state have been produced by two-photon excitation using cw laser radiation at 578.89 nm. Within a model which takes into account collisional population and depopulation of the 4F level as well as radiative relaxation (direct or cascade) of the 3D, 4P, 4D and 4F states ending in the 3P level, the rate for the collisional transfer turns out to be related to the ratio of the steady-state populations created in 3D and 4D states. They have been determined by measuring the fluorescence intensities of the and transitions. The measurements yield the following cross section values for those processes investigated: , and , at .
We report the results of a series of pulsed and cw laser experiments which measure spontaneous and effective lifetimes of the Cs(5DJ) levels and investigate excitation-transfer collisions involving Cs(5DJ) atoms and ground-state-cesium perturbers. With cw excitation of the dipole-forbidden but electric-quadrupole-allowed 6S1/2\ensuremath{\rightarrow}5D5/2 transition, we monitor the ratio of sensitized to direct fluorescence, i.e., I5D3/2\ensuremath{\rightarrow}6P1/2/I5D5/2\ensuremath{\rightarrow}6P3/2. A rate-equation analysis of these data yields values for the Cs(5D5/2)+Cs(6S)\ensuremath{\rightarrow}Cs(5D3/2)+Cs(6S) excitation-transfer rate, and for the rate of quenching of Cs(5D) by ground-state perturbers. The role of out-of-multiplet quenching is discussed at length, and we have demonstrated that quenching by ground-state atoms dominates over that by cesium dimers under these conditions. In the pulsed-laser experiments, the temporal evolution of the 5DJ\ensuremath{\rightarrow}6S and cascade 6PJ\ensuremath{'}\ensuremath{\rightarrow}6S fluorescence was observed following either direct forbidden-line pumping of one fine-structure level, or indirect excitation of both fine-structure levels (i.e., molecular excitation followed by predissociation). Analysis of the buildup and decay rates of the various levels as a function of cesium density yields values for the natural lifetimes of the Cs(5DJ) levels, as well as for the excitation transfer and quenching rates. Best values for the lifetimes and cross sections for Cs-Cs collisions obtained from these combined experiments are \ensuremath{\tau}5D=1250\ifmmode\pm\else\textpm\fi{}115 ns, \ensuremath{\sigma}5/2\ensuremath{\rightarrow}3/2=36\ifmmode\pm\else\textpm\fi{}8 \AA{}2, and \ensuremath{\sigma}5D=30\ifmmode\pm\else\textpm\fi{}3 \AA{}2. In addition, numbers for the 6P level-quenching cross sections due to collisions with Cs atoms and Cs2 molecules, respectively, were obtained: \ensuremath{\sigma}6P(Cs)=6.6\ifmmode\pm\else\textpm\fi{}3.0 \AA{}2 and \ensuremath{\sigma}6P(Cs2)=863\ifmmode\pm\else\textpm\fi{}260 \AA{}2. In the discussion, we show how these new values can be used to reconcile the seemingly discrepant results of several previous studies.
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.