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Physical Review A

Laser-induced collisional-energy transfer in thermal collisions of lithium and strontium

Friedhelm Dorsch, S. Geltman, P. E. Toschek

Published Apr 1, 1988• 35 citations
Publisher

Abstract

Fluorescence of Li excited by laser light and simultaneous thermal ${\mathrm{Sr}}^{\mathrm{*}}$ collisions in the reaction Sr(5p)+Li(2s)+\ensuremath{\Elzxh}\ensuremath{\omega} (\ensuremath{\lambda}=669.8 nm)\ensuremath{\rightarrow}Sr(5${s}^{2}$)+Li(4d) shows an asymmetric excitation spectrum of the energy transfer versus \ensuremath{\lambda}. The peak variation with light intensity demonstrates the strong-field regime and verifies current models. A change in the slope of the quasistatic wing agrees with the result of a recent three-level model.

DOI
10.1103/physreva.37.2441

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Fluorescence of Li excited by laser light and simultaneous thermal Sr∗{\mathrm{Sr}}^{\mathrm{*}}Sr∗ collisions in the reaction Sr(5p)+Li(2s)+\ensuremath{\Elzxh}\ensuremath{\omega} (\ensuremath{\lambda}=669.8 nm)\ensuremath{\rightarrow}Sr(5s2{s}^{2}s2)+Li(4d) shows an asymmetric excitation spectrum of the energy transfer versus \ensuremath{\lambda}. The peak variation with light intensity demonstrates the strong-field regime and verifies current models. A change in the slope of the quasistatic wing agrees with the result of a recent three-level model.
We examine theoretically the behavior of cross sections for laser-induced collisional energy transfer (LICET) over a wide range of detunings and laser intensities for the two-atom pairs Li-Sr and Eu-Sr which have been previously studied experimentally. These two cases are complementary in that the laser and collisional interactions enter in opposite orders, and their level structures are such as to give weak-field line shapes of opposite asymmetry and intensity shift. A three-state model is used and the Bambini-Berman formula for the weak-field quasistatic wing is verified. At high intensities the peak cross section is found to go through a maximum and the LICET profile is found to become symmetric about a shifted center with notable departures from a uniform falloff in the far quasistatic wing, apparently arising from radiative collisional interference effects.
A four-state model considering the relative velocity distribution function for calculating the cross section of laser-induced collisional energy transfer in a Sr—Li system is presented and profiles of laser-induced collision cross section are obtained. The resulting spectra obtained from different intermediate states are strongly asymmetrical in an opposite asymmetry. Both of the two intermediate states have contributions to the final state, and none of the intermediate states should be neglected. The peak of the laser-induced collisional energy transfer (LICET) profile shifts toward the red and the FWHM becomes narrower obviously with laser field intensity increasing. A cross section of 1.2 × 10−12 cm2 at a laser field intensity of 2.17 × 107 V/m is obtained, which indicates that this collision process can be an effective way to transfer energy selectively from a storage state to a target state. The existence of saturation for cross section with the increase of the laser intensity shows that the high-intensity redistribution of transition probabilities is an important feature of this process, which is not accounted for in a two-state treatment.
We report a measurement of the excitation spectrum of the laser-induced collisional energy transfer process for the Eu-Sr system in the weak-field regime. The measurement has been performed in a detuning range extending up to 85 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the static wing, providing a wider check of existing theoretical models. The frequency resolution of the measurement (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) was suitable for a quantitative study of the core of the line spectrum, where a comparison with the results of numerical calculations based on an effective three-level model gives complete evidence for a cooperative decay from the Sr(5p2{\mathit{p}}^{2}p2)1{)}^{1})1D2{\mathit{D}}_{2}D2​ final level.
An experimental study of laser-induced collisional energy transfer in a mixture of europium and strontium vapors is reported. The profile of the excitation spectrum in the static wing has been measured with high accuracy and compared with available calculations, demonstrating that simple two-level models are inadequate to describe the transfer process. A comparison with the predictions of a three-level model assuming dipole-dipole interaction, recently developed by Bambini and Berman, shows for the first time agreement between theory and experiment.
Based on the existing four-level model of laser-induced collisional energy transfer, the collision process in Ba-Sr system in both weak and strong fields is calculated by immediate integration of the equation of motion, and profiles of laser-induced transition probability and collision cross section under both conditions are obtained. Numerical results for the weak field are in fair to good agreement with former analytical results, while the ones for the strong field show that the effect of laser-induced collision is enhanced when the transfer laser intensity increases. Moreover, the peak of the laser-induced collisional energy transfer spectrum shows an evident shift from resonant frequency to the violet side in strong field, and full width at half maximum (i.e. tuning range) of the spectrum becomes obviously narrower compared with that in weak field.