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Journal of Physics B Atomic Molecular and Optical Physics

Theoretical study of a LICET-like process in singlet helium systems

Ricardo Joseph Simeoni, G. Peach, Ian B. Whittingham

Published Nov 28, 1996• 3 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/29/22/028

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Chinese Physics B•Aug 1, 2012•1 citation
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.
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.
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.
The theory of light-induced collisional energy transfer (LICET) in the Eu-Sr system is extended to include all the degenerate M states that occur in the separated-atom basis states. Previous treatments were restricted to only the M=0 substates. Numerical solutions are obtained for the resulting nine-state dynamic coupled system to determine transitions probabilities on a given straight-line path. Intense laser fields of linear and circular polarization are treated. It is found that the full M-state treatment results in dependences on the trajectory orientation angles which are much more complex than in the M=0 treatment. The exact and quasisymmetries involving these orientation angles are described in simple group-theoretic terms. The resulting spectral line shapes (LICET transition cross sections as a function of laser detuning) are seen to have structures that are understood in terms of the coupling matrix elements, as well as appreciable Stark shifts. These shifts, which appear to be an essential outcome of the theory, are not yet seen in measurements of the LICET line shape.
A laser induced quadrupole–quadrupole collisional energy transfer system of Xe–Kr is proposed, and the three-level laser induced collisional energy transfer (LICET) model is presented. Calculating results show that the peak of the LICET profile moves to the red and the FWHM becomes narrower, obviously with the laser field intensity increasing, while the resulting spectra is shifted in frequency toward the blue and the full width at half peak of the profile becomes larger as the relative velocity increases. The spectrum of the quadrupole–quadrupole LICET in the Xe–Kr system has a wider tunable range in an order of magnitude than the dipole–dipole LICET spectra. A cross section of 3.36×10−15 cm2 is obtained, which indicates that the quadrupole–quadrupole LICET process also can be an effective way to transfer energy selectively from a storage state to a target state.
A four-state model considering the relative velocity distribution function for calculating the cross section of laser-induced collisional energy transfer in a Sr—Li system is presented and profiles of laser-induced collision cross section are obtained. The resulting spectra obtained from different intermediate states are strongly asymmetrical in an opposite asymmetry. Both of the two intermediate states have contributions to the final state, and none of the intermediate states should be neglected. The peak of the laser-induced collisional energy transfer (LICET) profile shifts toward the red and the FWHM becomes narrower obviously with laser field intensity increasing. A cross section of 1.2 × 10−12 cm2 at a laser field intensity of 2.17 × 107 V/m is obtained, which indicates that this collision process can be an effective way to transfer energy selectively from a storage state to a target state. The existence of saturation for cross section with the increase of the laser intensity shows that the high-intensity redistribution of transition probabilities is an important feature of this process, which is not accounted for in a two-state treatment.