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Physical Review Letters

Fine-Structure Transitions Occurring in Collisional Redistribution of Light

Mark D. Havey, Gary E. Copeland, W. J. Wang

Published May 30, 1983• 22 citations
Publisher

Abstract

Experimental results on fine-structure transitions occurring in collisional redistribution of far---off-resonant radiation are presented. Measurements on Na-Ar atomic collisions illustrate that electronically nonadiabatic transitions occurring in the final state, following absorption, are important for describing the redistribution process. The results also provide details of the fine-structure transition process that are not obtainable from the usual resonant scattering experiments.

DOI
10.1103/physrevlett.50.1767

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Experimental results on fine-structure transitions occurring in collisional redistribution of far---off-resonant radiation are presented. Measurements on Na-Ar atomic collisions illustrate that electronically nonadiabatic transitions occurring in the final state, following absorption, are important for describing the redistribution process. The results also provide details of the fine-structure transition process that are not obtainable from the usual resonant scattering experiments.
Recent experimental results of Havey, Copeland, and Wang on the collisional redistribution of light by Na in Ar showed a dependence of the relative intensities of the Na D1{D}_{1}D1​ and D2{D}_{2}D2​ resonance lines on laser detuning. Calculations of these intensity ratios are reported here. The agreement between theory and experiment obtained here shows the importance of fine-structure transitions occurring during the dissociation of the excited-state molecule.
Theory shows that to explain the polarization of light collisionally redistributed from the far line wings of an atomic transition, one must consider correlated events in which absorption during a collision, and propagation to the final Zeeman-state superposition at the end of the collision is important. Polarizations of up to about 40% have been measured in the far line wings, substantially confirming this prediction, and showing that scattering experiments cannot just be characterized by simple absorption or emission profiles.
The close-coupled theory of atomic collisions in the presence of a radiation field may be used to calculate the distribution of final atomic states which results from absorption of polarized light during a collision. The theory applies equally well to optical collisions (line broadening) and to radiative collisions (laser-induced collisional energy transfer). For an optical collision the detuning $\ensuremath{\omega}\ensuremath{-}{\ensuremath{\omega}}_{\ensuremath{\infty}}$ is restricted to be larger than either the Rabi frequency or the widths due to natural, Doppler, or pressure broadening. The radiation field is assumed to be weak enough that the transition probabilities are linear in field intensity. The molecular picture is emphasized in which the wave function is expanded in a basis of field-free molecular states and the Hamiltonian is blocked in accordance with molecular quantum numbers. The quantities needed to predict experimental observables are reduced radiative scattering SSS-matrix elements ${s}_{\ensuremath{\omega}}$ in the asymptotic Hund's case-(e) representation. The theory of product orientation and alignment is equivalent to that which has been developed for molecular photodissociation. It requires coherent sums of ${s}_{\ensuremath{\omega}}$ matrix elements from different transition branches, that is, for PPP-, QQQ-, and RRR-type transitions corresponding to changes of -1, 0, and +1 in total molecular angular momentum. On the other hand, the total absorption coefficient and the branching ratio to different final fine-structure states irrespective of orientation or alignment depend on incoherent sums of the same matrix elements.
Resonant scattering (redistribution) of weak monochromatic light by atoms in the presence of collisions has been considered by Omont et al., who allowed for the broadening of both upper and lower levels of the transition. Mollow (who also has considered this problem for intense radiation) has shown that their results are applicable only if collisional repopulation of the levels (in contrast to collisional depopulation) is ignored. We here consider in detail the effect of repopulation. It leads, in the weak-field limit, to a true-coherent component ($\ensuremath{\delta}$-function spectrum) at the driving frequency, as well as a component at this frequency whose width is essentially the inelastic collisional rate of the lower state. An additional redistributed component due to repopulation of the upper level is also obtained.
Assume that collisional redistribution occurs in a collision of two atoms in scattering state i by absorption of a photon through an isolated molecular state a to an isolated molecular state b, followed by separation to excited product scattering state f. If the separation velocity of the two atoms is sufficiently large, the final state can be approximated by the same recoil limit which has been developed for photofragmentation.1 Our close coupling calculations on Na + He systems show how the recoil limit for fine-structure branching rations is approached for both red and blue detuning as product separation velocity increases. Comparisons are made between the lighter Na-He system at thermal energies and the heavier Na-He system.