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Physical Review A

Quasistatic wing behavior of collisional-radiative line profiles

Arturo Bambini, P. R. Berman

Published May 1, 1987• 38 citations
Publisher

Abstract

It is shown that the conventional two-state theory of laser-induced collisional energy transfer (LICET) cannot be expected to provide an adequate description of the quasistatic wing behavior observed in a number of high-resolution LICET experiments. The theory breaks down whenever the detuning parameter \ensuremath{\Delta} of the LICET reaction is comparable to some frequency mismatch \ensuremath{\delta}\ensuremath{\omega} between either the initial or final state and a (virtual) intermediate state in the reaction. A simple extension of the conventional two-state theory is presented which leads to good agreement with experiment. For a van der Waals collisional interaction, the quasistatic wing is shown to fall off as \ensuremath{\Vert}\ensuremath{\Delta}${\ensuremath{\Vert}}^{\mathrm{\ensuremath{-}}1/2}$(\ensuremath{\delta}\ensuremath{\omega}+\ensuremath{\Vert}\ensuremath{\Delta}\ensuremath{\Vert}${)}^{\mathrm{\ensuremath{-}}3/2}$ instead of \ensuremath{\Vert}\ensuremath{\Delta}${\ensuremath{\Vert}}^{\mathrm{\ensuremath{-}}1/2}$ predicted by the two-state theory.

DOI
10.1103/physreva.35.3753

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Physical Review A•Aug 1, 1987•33 citations
It is shown that the conventional two-state theory of laser-induced collisional energy transfer (LICET) cannot be expected to provide an adequate description of the quasistatic wing behavior observed in a number of high-resolution LICET experiments. The theory breaks down whenever the detuning parameter \ensuremath{\Delta} of the LICET reaction is comparable to some frequency mismatch \ensuremath{\delta}\ensuremath{\omega} between either the initial or final state and a (virtual) intermediate state in the reaction. A simple extension of the conventional two-state theory is presented which leads to good agreement with experiment. For a van der Waals collisional interaction, the quasistatic wing is shown to fall off as \ensuremath{\Vert}\ensuremath{\Delta}\ensuremath∥\ensuremath−1/2{\ensuremath{\Vert}}^{\mathrm{\ensuremath{-}}1/2}\ensuremath∥\ensuremath−1/2(\ensuremath{\delta}\ensuremath{\omega}+\ensuremath{\Vert}\ensuremath{\Delta}\ensuremath{\Vert})\ensuremath−3/2{)}^{\mathrm{\ensuremath{-}}3/2})\ensuremath−3/2 instead of \ensuremath{\Vert}\ensuremath{\Delta}\ensuremath∥\ensuremath−1/2{\ensuremath{\Vert}}^{\mathrm{\ensuremath{-}}1/2}\ensuremath∥\ensuremath−1/2 predicted by the two-state theory.
The behavior of the quasistatic wing of the light-induced collisional-energy-transfer (LICET) line shape is analyzed. The reaction can be written as A∗{A}^{\mathrm{*}}A∗+B+\ensuremath{\Elzxh}\ensuremath{\Omega}\ensuremath{\rightarrow}A+B∗∗{B}^{\mathrm{*}\mathrm{*}}B∗∗. The two-level model is found to be inadequate to describe the falloff of the spectral line shape in the quasistatic wing. The three-level model, on the other hand, leads to the prediction of a cross section that depends on the detuning in various ways, according to the strength of the collisional coupling and the influence of the van der Waals shifts on the levels under consideration. Good agreement with existing experimental results is found. To check the approximations made in the derivation of these results, numerical evaluation of the transition probability has been performed for several cases. A Monte Carlo simulation of the full LICET spectrum is also presented.
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 reaction of the form Ai+Ai\ensuremath{'}\ensuremath{'}+\ensuremath{\Elzxh}\ensuremath{\Omega}\ensuremath{\rightarrow}Af+Af\ensuremath{'}\ensuremath{'} has been referred to as light-induced collisional energy transfer (LICET). In such a reaction, atoms A and A\ensuremath{'} collide in the presence of a radiation field and absorb a photon during the collision, taking them from initial states (Ai,Ai\ensuremath{'}\ensuremath{'}) to different final states (Af,Af\ensuremath{'}\ensuremath{'}). We calculate the final-state magnetic polarization produced in LICET for detunings \ensuremath{\Delta}=[(Ef+Ef\ensuremath{'})-(Ei+Ei\ensuremath{'}+\ensuremath{\Elzxh}\ensuremath{\Omega})]/\ensuremath{\Elzxh} in the quasistatic wing: that is, for |\ensuremath{\Delta}|\ensuremath{\tau}c\ensuremath{\gg}1, where \ensuremath{\tau}c is the collision duration. Our results are compared with analogous depolarization rates for so-called ``optical'' collisions (reactions of the form Ai+Ai\ensuremath{'}\ensuremath{'}+\ensuremath{\Elzxh}\ensuremath{\Omega}\ensuremath{\rightarrow}Ai+Af\ensuremath{'}\ensuremath{'}). Similarities and differences between the LICET and ``optical'' collision theories are noted. Final-state LICET magnetic-state polarization is evaluated for a number of different A-A\ensuremath{'} level schemes, assuming a dipole-dipole collisional interaction. Our results are shown to be in good agreement with experiment [A. D\'ebarre, J. Phys. B 15, 1693 (1982)].
A fully quantum treatment of the laser induced collisional energy transfer (LICET) processes, , where n = 3,4, is presented, which allows for non-adiabatic mixing between molecular states during the collision. LICET profiles have been calculated for both Lennard-Jones 12 - 6 and model adiabatic molecular potentials. The weak field LICET profiles obtained differ from those of conventional mixed metal vapour systems in that they are very broad, significantly shifted from zero detuning and exhibit novel complex resonant structure in the quasistatic wing. The profiles are very sensitive to the nature of the molecular potential at internuclear separations of less than and consequently differ greatly for the two sets of potentials used.
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.