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Journal of Physics B Atomic and Molecular Physics

Collisional redistribution of near-resonant scattered light in Sr vapour

John L. Carlsten, A. Szöke

Published Jun 21, 1976• 39 citations
Publisher

Abstract

Strontium vapour was irradiated by a dye laser tuned near the 460.73 nm ( 1 P 1 0 - 1 S 0 ) resonance transition. Collision-induced fluorescence at the resonance frequency and Rayleigh scattering at the laser frequency were studied as a function of detuning Delta over a range of about 30 nm (1500 cm -1 ). The Rayleigh component was found to vary as Delta -2 , as expected, whereas the collision-induced fluorescence was markedly asymmetric. This experiment constitutes a direct measurement of the redistribution function.

DOI
10.1088/0022-3700/9/9/005

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Journal of Physics B Atomic and Molecular Physics•Feb 28, 1979•24 citations
Strontium vapour was irradiated by a dye laser tuned near the 460.73 nm ( 1 P 1 0 - 1 S 0 ) resonance transition. Collision-induced fluorescence at the resonance frequency and Rayleigh scattering at the laser frequency were studied as a function of detuning Delta over a range of about 30 nm (1500 cm -1 ). The Rayleigh component was found to vary as Delta -2 , as expected, whereas the collision-induced fluorescence was markedly asymmetric. This experiment constitutes a direct measurement of the redistribution function.
This paper reports on our studies of near-resonant scattering of laser light in a collisional environment. A pulsed dye laser with a peak power of 55 MW/cm2{\mathrm{cm}}^{2}cm2 was tuned near the 460.73-nm resonance line of strontium and the side emission was observed from an oven containing both strontium vapor and argon buffer gas. The emission was composed of three spectral components: Rayleigh scattering at the frequency of the laser ${\ensuremath{\omega}}_{L}$, fluorescence at the resonance frequency ${\ensuremath{\omega}}_{0}$ of strontium, and a third component at $2{\ensuremath{\omega}}_{L}\ensuremath{-}{\ensuremath{\omega}}_{0}$. These three components have been studied as a function of the frequency and intensity of the laser and also as a function of the argon buffer gas pressure. While the Rayleigh emission was found to vary as ${\ensuremath{\Delta}}^{\ensuremath{-}2}$ ($\ensuremath{\Delta}={\ensuremath{\omega}}_{0}\ensuremath{-}{\ensuremath{\omega}}_{L}$), the fluorescence component, which was produced by Sr-Ar collisions, was found to be asymmetric with the sign of $\ensuremath{\Delta}$ as predicted by line-broadening theory. By measuring the ratio of the intensities of the fluorescence and Rayleigh components, we were able to measure directly the collisional redistribution function, important in the study of radiative transfer in stellar and planetary atmospheres. At high laser intensities all three components were found to saturate. The results were compared with the theoretical predictions of Mollow's steady-state theory. Theoretical fits for the high-intensity results were obtained when the collisional cross sections were taken to be considerably smaller than in our low-intensity measurements. We believe the discrepancy lies in the use of a steady-state theory for a transient experiment. Effects of radiative trapping and spatial averaging are also discussed.
By tuning a pulsed dye laser through the resonance line of calcium at 422.7 nm the authors have investigated the spectral redistribution of laser radiation by calcium vapour. Two main components were resolved in the side emission from an oven containing calcium vapour and argon buffer gas. These are produced by Rayleigh scattering and by collision-induced fluorescence. The intensity of the Rayleigh component was found to vary as Delta -2 where Delta = omega 21 - omega L is the laser detuning from resonance. In contrast the intensity of the collision-induced fluorescence showed a markedly asymmetric dependence on detuning. Detailed studies of the collision-induced fluorescence have enabled the complete line shape of the calcium resonance line perturbed by argon to be determined. The interpretation of this data in terms of a frequency-dependent collision rate, gamma E ( Delta ), is discussed. This paper also reports calculations of theoretical line profiles using the unified Franck-Condon theory and trial interatomic difference potentials. The comparison of the experimental and theoretical line profiles enables the range of validity of the unified Franck-Condon theory to be assessed and the form of the optimum Ca-Ar difference potential to be selected.
Light scattered from strontium vapor near its 460.73-nm ($^{1}{P}_{1}^{\ensuremath{\circ}}\ensuremath{-}^{1}S_{0}$) resonance transition was resolved into its three spectral components: Rayleigh scattering, collision-induced fluorescence, and three-photon scattering. The saturation behavior of the central (Rayleigh) component and the intensity and ac Stark shift of the three-photon component were studied as a function of detuning and laser intensity. Good agreement is obtained with theoretical predictions.
Light scattered from strontium vapor near its 460.73-nm ($^{1}{P}_{1}^{\ensuremath{\circ}}\ensuremath{-}^{1}S_{0}$) resonance transition was resolved into its three spectral components: Rayleigh scattering, collision-induced fluorescence, and three-photon scattering. The saturation behavior of the central (Rayleigh) component and the intensity and ac Stark shift of the three-photon component were studied as a function of detuning and laser intensity. Good agreement is obtained with theoretical predictions.
Redistribution and depolarization of near-resonant radiation is studied for the He 2(1P)-3(1D) line (668 nm) and some data for H-alpha (656 nm) are also presented. A Nd:YAG pumped dye laser, with a bandwidth of 0.1/cm and a power density of 100 kW/sq cm, is used in the experiment. The transitions have lower levels with natural line widths that are large compared with their upper-level natural widths, and, under experimental conditions, their collisional widths are also small. The ratio of fluorescence to Rayleigh intensity is measured and the results indicate that while the Rayleigh emission has the same temporal development as the laser pulse (5 ns FWHM), the fluorescent intensity has a decay time determined by the radiative decay lifetime and by collisional quenching. The prediction that redistribution of radiation occurs in the absence of collisions for transitions having significant lower-level radiative widths is confirmed. Depolarization rates by collisions with helium are also inferred from the measurements.
The authors have studied collisional redistribution of near-resonance radiation in thallium vapour caused by collisions with argon buffer gas. Electronic Raman scattering and collision-induced fluorescence were spectrally resolved, both to the weakly populated metastable 6 2 P 3/2 level in thallium. Thus the problem with radiative trapping encountered in previous experiments is partially avoided. By measuring the total collision-induced fluorescence as a function of the incident laser frequency, the absolute collisional redistribution line shape is obtained.