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Journal of Physics B Atomic and Molecular Physics

Cross sections for sodium fine-structure transitions induced by collision with caesium atoms

Martin A. Harris, E L Lewis

Published Sep 4, 1982• 6 citations
Publisher

Abstract

Cross sections for the process Na(3 2 P 1 2/)+ Delta E to or from Na(3 2 P 3 2/) induced by inelastic collisions with ground-state caesium atoms have been measured and the results are discussed with reference to resonant and rare-gas-induced cross sections and non-adiabatic effects.
DOI
10.1088/0022-3700/15/17/008

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3 2P1/2 ↔ 3 2P3/2 excitation transfer in sodium, induced in inelastic collisions with ground-state N2, H2, HD, and D2 molecules, has been investigated in a series of sensitized fluorescence experiments. Mixtures of sodium vapor at a pressure of 5 × 10−7 Torr, and the gases, were irradiated with each NaD component in turn, and the fluorescence which contained both D components was monitored at right angles to the direction of the exciting beam. Measurements of the relative intensities of the NaD fluorescent components yielded the following collision cross sections for excitation transfer. For Na–N2 collisions: Q12(2P1/2 → P3/2) = 144 Å2, Q21(2P1,2 ← 2P3/2) = 76 Å2 for Na–H2 collisions: Q12 = 80 Å2, Q21 = 42 Å2. For Na–HD collisions: Q12 = 84 Å2, Q21 = 44 Å2. For Na–D2 collisions: Q12 = 98 Å2, Q21 = 52 Å2. The cross sections Q21 exhibit a slight resonance effect between the atomic and molecular rotational transitions.

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The cross sections for the excitation energy transfer in the collisional processes Na*(3P 1 2/)+M(nS 1 2/) to or from Na*(3p 3 2/)+m(Ns 3 2/), m being ground-state Rb (n=5) or Cs (n=6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T=530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma ( 1 / 2 to 3 / 2 )=170 AA 2 , with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma ( 1 / 2 to 3 / 2 )=140 AA 2 , sigma ( 3 / 2 to 1 / 2 )=83 AA 2 . The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.
A diode-laser fluorescence experiment was performed in order to study fine-structure transitions between 5 2 P states of rubidium atoms colliding with ground-state potassium or caesium atoms. The Rb (5 2 P 3 2/) state was optically excited and the intensity ratio of sensitized to direct fluorescence was measured. The obtained cross sections for the excitation energy transfer Rb( 2 P 3 2/) to Rb( 2 P 1 2/) induced by collisions with K and Cs (T=573 K) are sigma =54 AA 2 +or-30% and sigma =15 AA 2 +or-30%, respectively. Assuming detailed balancing, the cross sections for the reverse process Rb( 2 P 1 2/) to Rb( 2 P 3 2/) are sigma =60 AA 2 +or-30%, and sigma =16.5 AA 2 +or-30% for K and Cs, respectively.
P^2P3/2 m ^-x^n8 *n sodium vapour, induced by inelastic collisions with the diatomic molecules HD, and N^, was studied using techniques of sensitized fluorescence.The mixtures of sodium vapour with the various molecular gases were irradiated with one component of the appropriate resonance doublet.The fluorescence which contained both components of the doublet, was monitored at right angles to the direction of excitation.The measurement of the relative intensities of the fluorescent components 2 yielded the total inelastic cross sections for P mixing, Ql2(2Pi/2---> 2p3/2^ and ^21^2pi/2^---' 2p3/2^* The sodium vaPour *n 5. Cross Sections for 3 P. ---3 P. Mixing Induced by Sodium Molecule Collisions.38 6. Cross Sections for Excitation Transfer in Na-Molecule Collisions, together with the Corresponding Relative Velocity of the Colliding Partners 42 2 2 7. Comparison of the Cross Sections for P. ---^-s/o Mixing Induced by Alkali Atom-Molecule Collisions 43 vi
Cross sections for the transfer of excitation between fine-structure sublevels of the 9D state of caesium due to collisions with noble-gas atoms are measured. The experimental method employs stepwise excitation with a pulsed dye laser in the second step (6 2 P 3/2 to 9 2 D 5/2 ) and time-resolved detection of sensitised fluorescence 9 2 D 3/2 to 6 2 P 1/2 . The cross sections for the process Cs(9 2 D 3/2 )+A to Cs(9 2 D 5/2 )+A are (in units of 10 -14 cm 2 ) 7.6(1.5), 2.5(5), 2.7(5), 3.0(6) and 3.2(6) for A=He, Ne, Ar, Kr and Xe, respectively. For all the noble gases except helium the experimental cross sections are a few times smaller than the ones obtained in a recent semiclassical calculation.
Applying the cw laser fluorescence method, the cross sections for the fine structure mixing and quenching of the Cs 6P state, induced by collisions with ground-state caesium atoms and molecules, have been measured. Caesium atoms were optically excited to the 5D J states via quadrupole-allowed 6S 1/2 5D J transitions, while the resonance states were populated by the radiative and collisional 5D J 6P J transitions. The relative populations of the Cs 6P J sublevels, as well as ratio of the 5D 3/2 to 6P 3/2 populations, were measured as a function of the caesium ground-state number density. From these measurements we obtained the cross section of (14±5) × 10 -16 cm 2 at T = 585 K for the process Cs(6P 1/2 ) Cs(6P 3/2 ) induced by collisions with ground-state caesium atoms. The applied experimental approach enabled the determination of the effective spontaneous rates for the 6P J states which are in agreement with the predictions of Holstein's theory. The cross sections for quenching of 6P J by caesium atoms and molecules were measured at T = 635 K and the obtained values are (1.6±1.4) × 10 -16 cm 2 and (1210±260) × 10 -16 cm 2 , respectively. Using recently calculated Cs*+Cs potentials we performed an analysis which shows a good agreement between the measured values and the theoretical estimates.
3 2P1/2 ↔ 3 2P3/2 excitation transfer in sodium, induced in inelastic collisions with ground-state N2, H2, HD, and D2 molecules, has been investigated in a series of sensitized fluorescence experiments. Mixtures of sodium vapor at a pressure of 5 × 10−7 Torr, and the gases, were irradiated with each NaD component in turn, and the fluorescence which contained both D components was monitored at right angles to the direction of the exciting beam. Measurements of the relative intensities of the NaD fluorescent components yielded the following collision cross sections for excitation transfer. For Na–N2 collisions: Q12(2P1/2 → P3/2) = 144 Å2, Q21(2P1,2 ← 2P3/2) = 76 Å2 for Na–H2 collisions: Q12 = 80 Å2, Q21 = 42 Å2. For Na–HD collisions: Q12 = 84 Å2, Q21 = 44 Å2. For Na–D2 collisions: Q12 = 98 Å2, Q21 = 52 Å2. The cross sections Q21 exhibit a slight resonance effect between the atomic and molecular rotational transitions.