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Physical Review A

Theory of laser-induced excitation transfer and atomic association. II. Resonances

Dumont M. Jones, John S. Dahler

Published Jan 1, 1989• 2 citations
Publisher

Abstract

We have extended our previous analysis of diatom laser-induced excitation transfer, illustrated by several examples, to include modifications due to laser-induced resonances. Compact and rigorous final results have been obtained through the use of a projection-operator scheme in combination with the ``symmetric-top'' basis set and a full quantal treatment of the radiation field. The principal limitations on our analysis are a neglect of fine-structure coupling and a simplifying assumption that the ground electronic manifold supports no bound nuclear states. Simple order-of-magnitude estimates of the predicted resonant cross sections also are provided.

DOI
10.1103/physreva.39.78

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We have extended our previous analysis of diatom laser-induced excitation transfer, illustrated by several examples, to include modifications due to laser-induced resonances. Compact and rigorous final results have been obtained through the use of a projection-operator scheme in combination with the ``symmetric-top'' basis set and a full quantal treatment of the radiation field. The principal limitations on our analysis are a neglect of fine-structure coupling and a simplifying assumption that the ground electronic manifold supports no bound nuclear states. Simple order-of-magnitude estimates of the predicted resonant cross sections also are provided.
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.
The weak-field soft-photon theory for free - free transitions is generalized to include atomic potentials which support resonances. The present formulation is not restricted by geometrical constraints. In particular, it may be applied in situations where the laser polarization is perpendicular to the electron momentum transfer. In this special geometry, former theories have predicted that laser-assisted electron - atom scattering resonances should appear as pure Breit - Wigner peaks on a vanishingly small background. The present theory includes higher-order terms and shows that the background amplitude may compete with the resonance part to produce characteristic Fano-type resonance signals. Considering tunable radiation fields it is shown that the laser may induce resonance-interference effects. The shapes of overlapping resonances are in general determined by an interplay between resonance scattering amplitudes and a laser-controlled contrast parameter. Characteristic spectral features in the laser-induced overlap between fine-structure components are uniquely related to the distinctly different interference properties of direct and spin-flip amplitudes.
In the first paper of this series a formal theory of atomic scattering of electrons in the presence of an intense electromagnetic field was given. Cross sections, near the forward direction, between atomic states (modified by the laser) were obtained. However, it was assumed that the atom could not emit spontaneous radiation. In this paper the effect of spontaneous radiation is included, and it is shown that in most cases the measured cross section will be a weighted average of the two different cross sections starting from the two different initial states which are those atomic states resonantly linked by the laser. The Born approximation for the scattering is obtained, and it is shown that for the simplest description of the spontaneous radiation field the effect of the laser on the cross sections is simply to multiply them by a factor which depends upon the laser detuning.
Cross sections for inelastic transfer between the 82D3/2 and 82D5/2 fine-structure states in rubidium, induced in resonant collisions with ground-state Rb atoms, have been determined using an experimental method involving two-photon excitation of atomic fluorescence. Rubidium vapor in a fluorescence cell was irradiated with pulses of 641 nm radiation from a N2 laser-pumped dye-laser tuned to excite one of the 82D states. The resulting fluorescence included the direct component originating from the optically excited state and a sensitized component arising from the other fine-structure state populated by collisions. Relative intensities of the fluorescent components, determined by photon-counting techniques, yielded the cross sections for excitation transfer: Q(2D3/2 → 2D5/2) = 8.1 × 10−13 cm2; Q(2D3/2 ← 2D5/2) = 5.5 × 1013 cm2; as well as [Formula: see text], the effective quenching cross section. The excitation transfer cross sections which are considered accurate to within ±20% are in the ratio predicted by the principle of detailed balancing.
The cross sections for excitation transfer between the 62P fine structure levels of rubidium, induced in collisions with ground state rubidium atoms, have been measured using a nitrogen laser pumped dye laser as the optical excitation source in a fluorescence experiment. Rubidium vapor was irradiated with each component of the 2P rubidium doublet in turn, and measurements of the relative intensities of fluorescence yielded the following cross sections: [Formula: see text] These results are consistent with the empirical relationship between the magnitude of the cross sections and the fine structure splitting that has previously been established for the alkalis.