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Physical Review A

Cross sections of laser-induced collisional processes: Excitation transfer and charge transfer

S. Geltman

Published May 1, 1987• 19 citations
Publisher

Abstract

The standard theory for laser- (or light-) induced collisional processes is applied to excitation transfer and charge transfer. Detailed calculations are carried out for specific processes which have been studied experimentally, with good understanding resulting for most of the observed features. We study the variation in cross-section magnitude and line shape with transfer laser intensity, and find characteristic differences between the excitation-transfer and charge-transfer processes.

DOI
10.1103/physreva.35.3775

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The standard theory for laser- (or light-) induced collisional processes is applied to excitation transfer and charge transfer. Detailed calculations are carried out for specific processes which have been studied experimentally, with good understanding resulting for most of the observed features. We study the variation in cross-section magnitude and line shape with transfer laser intensity, and find characteristic differences between the excitation-transfer and charge-transfer processes.
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.
Experimental studies of two laser-induced charge-transfer reactions in Ca and Sr are presented. In both reactions, a Ca ion collides with an Sr neutral in the presence of a laser field to produce a Ca neutral and an excited Sr ion. The two reactions differ in the final level of the Sr+{\mathrm{Sr}}^{+}Sr+ species. The collision cross sections associated with these reactions are studied as functions of applied laser frequency and power density. The largest cross section induced is equal to ${10}^{\ensuremath{-}16}$ cm2{\mathrm{cm}}^{2}cm2, obtained at a power density of 2\ifmmode\times\else\texttimes\fi{}109{10}^{9}109 W/cm2{\mathrm{cm}}^{2}cm2. Differences in experimental results associated with the two reactions are explained qualitatively by an examination of the potential curves of the CaSr+{\mathrm{Sr}}^{+}Sr+ quasimolecule.
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.
Cesium molecules are pumped by a CW dye laser. Some of the excited molecules then pre-dissociate or collisionally transfer excitation to produce excited atoms at the 5D and 6P levels. The cross sections (in units of 10-14cm2) for the excitation transfer process Cs(6P)+Cs(5D) →Cs(7DJ)+Cs(6S)for J being 5/2 and 3/2 are 5. 4 ± 50% and 4. 9 ± 50%, respectively. The cross sections (in units of 10-14cm2) for the excitation transfer out of the 7D doublet [i. e. , Cs(7DJ)+Cs(6S) →states other than Cs(7D)] for J being 5/2 and 3/2 are 1. 6 ±50% and 0. 34±50%, respectively. To obtain these cross sections, the intensities of 7D and 5D fluorescence emissions and the 6P atom density are measured by using the absorption equivalent width technique. The results are discussed in eomparison with those of other experiments.
Formulas are derived for the differential and integral cross sections associated with laser-induced excitation transfer. The theory is patterned closely after our recent treatment of laser-induced chemi-ionization. It is found that cross sections specific to single-photon absorption are proportional to the square of the cosine of the angle between the laser polarization and the initial relative velocity of the two colliding atoms. The distortion by a laser of the cross section for an elastic scattering event also is treated. Finally, a cross-section formula is derived for a collision-induced two-photon absorption, mediated by a single intermediate electronic state.