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Physical Review A

Theory of laser-induced excitation transfer and atomic association

H. P. Saha, John S. Dahler, Dumont M. Jones

Published Sep 1, 1984• 7 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.30.1345

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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.
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.
Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.
Excitation transfer process between atoms is investigated theoretically in the framework of the two-state approximation. Two methods are proposed with regard to the treatment of the non-local potential which results from the projection-operator formalism. One is a local potential approximation and the other is an extension of the method of Chadan. A new expression for the cross section is found. This formula is more accurate than the usual distorted wave one. The matrix elements of a coupling potential with respect to the wave functions of the relative motion are estimated by making use of the JWKB approximation. The new cross section formula obtained will be useful for a wide class of collision processes in which initial and final states are mutually orthogonal.
We calculate the total and differential cross sections for radiative-collisional excitation of the metastable 2 1 S state of He atoms at collisions with Ne atoms in external radiation fields of various frequencies and polarizations. The calculations are performed for a thermal collision energy of E = 10 −3 atomic units and light intensity of I = 1 MW cm −2 , which corresponds to a single photon absorption by a quasi-molecule during the collision. Both the differential and total cross sections are shown to depend strongly on the relative orientation of the radiation polarization vector and the initial relative velocity vector of the colliding atoms. We analyze the azimuthal scattering asymmetry related to the orientation of the angular momentum of the absorbed photon.
An approach to the calculation of cross sections for laser-assisted excitation transfer processes is outlined for the dipole–dipole case. The internal atomic variables are treated in the Heisenberg picture, while the simplest form of semiclassical collision theory is used for the atomic trajectories. A simple physical picture in the spirit of the Weizsäcker–Williams method emerges: The laser-assisted excitation transfer may be treated approximately as a single-atom multiphoton process in an applied field.
We consider collisions between (quasi-) one- or two-electron atoms, each of which has been prepared in a fixed distribution of hyperfine magnetic substates. The resulting ``polarization-dependent'' reaction rates are functions of (1) the (experimentally adjustable) orientation of the quantization axis associated with the hyperfine substates, (2) the hyperfine terms of the colliding atoms, and (3) the detailed reaction dynamics as characterized by preparation- and fine-structure-independent cross sections. These effects are deconvoluted and analyzed, and their use in elucidating reaction mechanisms is discussed. Particular emphasis is placed on the associative ionization of two Na(3p) atoms, each prepared by adsorption of resonant laser photons. The quasimolecular states of Na...Na that might react in this case are limited by selection rules. More information on the reaction mechanism is then obtained by using experimental data, in combination with the formal analysis, to draw further conclusions about the active quasimolecular states.