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Physics Today

Laser assists excitation transfer in collisions

H. Richard Leuchtag

Published Oct 1, 1976• 1 citation
Publisher

Abstract

In an experiment that introduces a completely new approach to inelastic collisions, the transfer of excitation from one atom to another has been enormously enhanced by resonant optical irradiation. Incident laser light was tuned to a wavelength corresponding to the energy difference between the final and initial states of the colliding atoms. The experiments, in which the 5p strontium atoms transferred their excitation energy to 6s calcium atoms, was performed at Stanford University by Stephen Harris, Roger Falcone, William Green, Derek Lidow, Jonathan White and James Young.

DOI
10.1063/1.3024404

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In an experiment that introduces a completely new approach to inelastic collisions, the transfer of excitation from one atom to another has been enormously enhanced by resonant optical irradiation. Incident laser light was tuned to a wavelength corresponding to the energy difference between the final and initial states of the colliding atoms. The experiments, in which the 5p strontium atoms transferred their excitation energy to 6s calcium atoms, was performed at Stanford University by Stephen Harris, Roger Falcone, William Green, Derek Lidow, Jonathan White and James Young.
We report the observation of a laser-induced charge-transfer collision. In the presence of an intense laser beam, ground-state calcium ions collide with ground-state strontium atoms, selectively producing excited strontium ions and calcium neutrals. The laser-induced collision cross section has a linewidth of about 50 ${\mathrm{cm}}^{\ensuremath{-}1}$ and peaks \ensuremath{\sim} 70 ${\mathrm{cm}}^{\ensuremath{-}1}$ from that wavelength which satisfies the energy defect of the separated atoms.
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.
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.
Resonant energy transfer collisions, those in which one atom or molecule transfers only internal energy, as oppposed to translational energy, to its collision partner require a precise match of the energy intervals in the two collision partners. Because of this energy specificity, resonant collisional energy transfer plays an important role in many laser applications, the He–Ne and CO 2 lasers being perhaps the best known examples. It is interesting to imagine an experiment in which we can tune the energy of the excited state of atom B through the energy of the excited state of atom A, as shown in Fig. 14.1. At resonance we would expect the cross section for collisionally transferring the energy from an excited A atom to a ground state B atom to increase sharply as shown in Fig. 14.1. In general, atomic and molecular energy levels are fixed, and the situation of Fig. 14.1 is impossible to realize. Nonetheless systematic studies of resonant energy transfer have been carried out by altering the collision partner, showing the importance of resonance in collisional energy transfer. The use of atomic Rydberg states, which have series of closely spaced levels, presents a natural opportunity for the study of resonant collisional energy transfer. One of the earliest experiments was the observation of resonant rotational to electronic energy transfer from NH 3 to Xe Rydberg atoms by Smith et al .
We describe two new experiments which demonstrate laser-induced inelastic collisions. Energy was stored in the Sr 5p1Po5p^{1}P^{o}5p1Po state and selectively transferred both to the Ca $4{p}^{2}^{1}S$ and 5d1D5d^{1}D5d1D states. The collision cross sections maximized at the interatomic wavelengths of 4977 and 4711 \AA{}, respectively, and had a half-power width of about 14 ${\mathrm{cm}}^{\ensuremath{-}1}$.