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Journal de physique

Broadening, shift and depolarization of broad fine structure alkali spectral lines by helium

E. Roueff, Annick Giusti-Suzor

Published Jan 1, 1974• 26 citations
Publisher

Abstract

Due to the large values of the fine structure interval for the first resonance lines of Rb and Cs, we may treat separately the 2P1/2 and 2P3/2 atomic levels when perturbed by Helium atoms. Within the framework of semi-classical and impact approximations, and assuming an exchange interaction, the diffusion matrices S1/2 and S3/2 are numerically computed ; firstly by neglecting all 1/2 - 3/2 coupling, and secondly, by a more accurate treatment which makes use of the molecular potentials and shows that the D1-lines depolarization is entirely due to the short range nonadiabatic coupling. The validity of analytical approximate resolutions for asymptotic regions is studied, and also the trajectory for the P1/2 levels, correlated with a single molecular curve; the rectilinear assumption appears to be insufficient for very short range phenomena. The agreement with experimental results confirms that the exchange interaction predominates for intermediate distances.

DOI
10.1051/jphys:019740035010072700

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Canadian Journal of Physics•Jan 1, 1986•0 citations
Due to the large values of the fine structure interval for the first resonance lines of Rb and Cs, we may treat separately the 2P1/2 and 2P3/2 atomic levels when perturbed by Helium atoms. Within the framework of semi-classical and impact approximations, and assuming an exchange interaction, the diffusion matrices S1/2 and S3/2 are numerically computed ; firstly by neglecting all 1/2 - 3/2 coupling, and secondly, by a more accurate treatment which makes use of the molecular potentials and shows that the D1-lines depolarization is entirely due to the short range nonadiabatic coupling. The validity of analytical approximate resolutions for asymptotic regions is studied, and also the trajectory for the P1/2 levels, correlated with a single molecular curve; the rectilinear assumption appears to be insufficient for very short range phenomena. The agreement with experimental results confirms that the exchange interaction predominates for intermediate distances.
The fluorescence intensities for the D lines in non-resonant excitation of the alkali-helium pairs under single-collision conditions were obtained from calculations of the optical collision spectra by the quantum coupled-channels method. The close relation of the non-adiabatic transition probability from a molecular level to an atomic state during the final stage of an optical collision and the spectral dependence of the relative fluorescence intensities is investigated. An analysis of the impact region in terms of line-broadening and fine-structure cross sections calculated separately for all systems confirms the factorisation approximation.
Accurate values for the density shift and broadening of five isolated ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure transitions have been measured. Doppler-free polarization and double-quantum saturation spectroscopy with a cw dye laser are used in a He dc discharge with 1% H2{\mathrm{H}}_{2}H2​. The negative shifts and the huge broadening values are correctly predicted by an impact-approximation theory taking into account the large cross sections for fine-structure transfer (quenching) collisions and only Van der Waals interaction between H∗{\mathrm{H}}^{*}H∗ and He. Consequences of a comparison with the positive shift of the unresolved ${\mathrm{H}}_{\ensuremath{\alpha}}$ line and of the alkali-metal DDD lines in He buffer gas are discussed. For pressures below 0.1 Torr hfs decoupling collisions give rise to a positive shift of single ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance.
Calculated potentials for the NaHe molecule are used to obtain collision-induced widths and shifts for the sodium DDD lines together with cross sections for fine-structure transitions and the relaxation of the multipole polarizations of the resonance levels. The atomic collision is treated in an adiabatic approximation in which the colliding atoms form a molecular system which rotates during the collision. We find the resonance lines to be broadened equally and to be approximately 70% wider, and with associated shifts of an order of magnitude smaller, than expected on the basis of van der Waals forces. The dependence of the cross sections for relaxation of the multipole polarizations of the resonance levels on the molecular-coupling conditions and the rotation of the system during collision is discussed and indicates a limitation on the sum rule for these cross sections. Good over-all agreement with the existing experimental data is obtained.
Résumé. 2014 On calcule les constantes de dépolarisation, d’élargissement et de déplacement de la raie de résonance du rubidium 5 2P1/2 ~ 5 2S1/2 perturbée par tous les gaz rares dans l’approximation semi-classique en négligeant le transfert d’excitation vers le niveau 5 2P3/2 dans la gamme de tem-pérature 100-1 000 K. Les effets de trajectoire sont discutés. Les calculs ont été effectués en utilisant les potentiels interatomiques récemment publiés par Baylis [1] (1) et Pascale et Vandeplanque [2]. La comparaison des résultats théoriques et expéri-mentaux, quand ils existent, permet un test des potentiels et du traitement de la collision. Abstract. 2014 The depolarization, broadening and shift constants of the 5 2P1/2 ~ 5 2S1/2 resonance line perturbed by rare gases are calculated in the semi-classical approximation when neglecting the excitation transfer to the 2P3/2 state in the energy range 100-1 000 K. Trajectory effects are discussed. The calculations have been performed with the interatomic potential recently published by Baylis [1] (1) and Pascale and Vandeplanque [2] for all rare gases. Comparison with experiments, when available, permits a test of the potentials and the collision treatment.
A theoretical model is presented for the interaction between an alkali atom in the first excited 2 P state and a ground state rare gas atom, which is effective in causing transitions among the alkali fine structure states. The main difference from previously used models is a more detailed treatment of short range effects including deviations from straight line paths in the semiclassical treatment. An application to the case of Rb interacting with He, Ne, Ar, and Kr gives results for the depolarization and inelastic cross sections which are in reasonable accord with experiment.
The dependence of the circular polarization of the fluorescence of alkali atoms on the intensity and spectral composition of the σ+-polarized exciting D1 line has been investigated by considering the stationary solutions of rate equations for populations of the Zeeman sublevels. The calculations were performed for pumping-light intensities corresponding to those of classical light sources, e.g., radio frequency or microwave discharge lamps, and commonly accepted values of cross sections for collisional relaxation of ground-state and excited atoms. The systematic errors of cross sections for collisional disorientation of 2P1/2 alkali atoms due to the neglect of intensity and spectral composition of the exciting light are estimated.