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Journal of Physics B Atomic and Molecular Physics

Depolarisation in Na-Ar optical collisions

Wolfgang Behmenburg, V. Kroop, Frank Rebentrost

Published Jul 14, 1985• 19 citations
Publisher

Abstract

The depolarisation of the redistributed Na D1and D2line fluorescence caused by Na-Ar collisions after far-wing excitation was investigated. In the limit of zero Ar density the J=1/2orientation and J=3/2alignment reflect the distribution among Zeeman sublevels of Na 32PJproduced after absorption of a photon into one or several excited Na-Ar molecular states at small internuclear separation, followed by a mixing between these states during dissociation. Measurements of these quantities as functions of the detuning up to Delta omegaJ=50 cm-1in the red wings are compared with quantum-mechanical coupled-channel (CC) calculations using realistic molecular potentials and thermal averaging, and with predictions from the factorisation approximation (FA) of the theory of redistribution. Fair agreement with CC calculations and FA predictions was obtained within the impact region, proving the validity of the factorisation assumption there. Outside this region the overall depolarisation is primarily due to reorientation of the Na 3P electronic angular momentum and is reproduced qualitatively by the CC calculations but not by the FA theory. Additional depolarisation connected with the fine-structure coupling was observed.

DOI
10.1088/0022-3700/18/13/018

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The depolarisation of the redistributed Na D1and D2line fluorescence caused by Na-Ar collisions after far-wing excitation was investigated. In the limit of zero Ar density the J=1/2orientation and J=3/2alignment reflect the distribution among Zeeman sublevels of Na 32PJproduced after absorption of a photon into one or several excited Na-Ar molecular states at small internuclear separation, followed by a mixing between these states during dissociation. Measurements of these quantities as functions of the detuning up to Delta omegaJ=50 cm-1in the red wings are compared with quantum-mechanical coupled-channel (CC) calculations using realistic molecular potentials and thermal averaging, and with predictions from the factorisation approximation (FA) of the theory of redistribution. Fair agreement with CC calculations and FA predictions was obtained within the impact region, proving the validity of the factorisation assumption there. Outside this region the overall depolarisation is primarily due to reorientation of the Na 3P electronic angular momentum and is reproduced qualitatively by the CC calculations but not by the FA theory. Additional depolarisation connected with the fine-structure coupling was observed.
Redistribution and depolarisation of light scattered by atoms undergoing collisions outside the impact region of the line profile has been studied experimentally and theoretically. In a cell experiment intensity and linear degree of polarisation of the fluorescence components 32S1/2 to or from 32P1/2,3/2 of Na atoms induced by collisions with Ar buffer gas atoms were measured as functions of the laser frequency omega L. The decrease of polarisation of the D2 component observed when omega L is shifted away from the D2 toward D1 resonance disagrees with calculations on the basis of the factorisation approximation of the theory of redistribution. Using Baylis (1975) potentials for the Na/Ar system, the effect is interpreted in terms of adiabatic coupling of the 32P3/2 electronic angular momentum to the collision axis during 'strong' collisions, giving rise to a greater than average depolarisation.
The spectra of fluorescence depolarisation (orientation, alignment) applying to single collision conditions are analysed for the alkali-helium series. These are derived from a quantum calculation of the optical collision cross sections for non-resonant collision pair excitation and include non-adiabatic couplings and thermal averaging over the collision energy. The authors find strong spectral variations of the polarisation which reflect the potential characteristics of these systems. A numerical test is performed for the classical Lewis model using realistic trajectories and a reasonable choice of the decoupling radius and fair quantitative agreement with the quantum calculation is shown. Large residual alignments in the far wings demonstrate a lack of complete reorientation of the transition dipole in a strong collision.
The close-coupled theory of collisions in a radiation field is used to calculate the absorption profile for the Sr $^{1}\ensuremath{\leftarrow}^{1}$S resonance line broadened by collisions with Ar. The calculations predict the polarization ratios of Sr 1P^{1}\mathrm{P}1P fluorescence following line wing excitation by either linear or circular polarized light. Ab initio calculations were used to obtain the ground and excited SrAr molecular-potential curves, which were adjusted to give improved agreement with experiment. The radiative-scattering theory gives a unified description of the absorption coefficient and polarization redistribution from the small detuning impact limit region to the far spectral wings. The cross sections for elastic and inelastic depolarizing collisions of Sr 1P1^{1}\mathrm{P}_{1}1P1​+Ar were also calculated. The calculated absorption coefficient, impact-broadening rate, linear- and circular-polarization ratios, and depolarization rate coefficients are for the most part in good agreement with experiment.
Measurements of polarization of the Na 3p2{\mathit{p}}^{2}p2P3/2{\mathit{P}}_{3/2}P3/2​ level produced in Na-Ar, Na-Kr, and Na-Xe optical collisions are reported. The measurements extend over a range of detunings from 150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ red to 350 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ blue of resonant excitation of the Na 3p levels. The short-pulse, pump-probe technique employed in the experiments eliminates large systematic effects due to the hyperfine interaction, collisions, and radiative trapping. For far-blue-wing excitation, nearly constant polarizations of 21(2)%, 13(2)%, and 8(2)% are obtained for Na-Ar, Na-Kr, and Na-Xe, respectively.
The polarisation of atomic line fluorescence resulting from the excitation of a collisional complex is calculated on the basis of a simple molecular picture for the pair of colliding atoms. The red-wing region is treated in detail as arising from excitation of both Pi and Sigma molecular states at different internuclear distances, and the blue wing is identified as predominantly due to the Sigma state. A calculation for the strontium 4607 AA line gives good qualitative agreement with the experiment.
We have measured the polarization of collisionally redistributed light as a function of detuning from line center for the Hg3P1–1S0 transition in the presence of Ar. We present a polarization profile for the red wing between 5 and 18 cm−1 and polarizations for two detunings in the blue wing. We compare this type of experiment to the complementary method of photodissociation and discuss the issue of coherence between excitation path-ways. The data are analyzed using a simple molecular reorientation model suggested by others which includes coherent excitation. The model is shown to be unable to completely account for the high polarizations measured in this experiment. We suggest reasons for the failure of this model.