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Journal of Physics B Atomic and Molecular Physics

Level-crossing expansion of laser-induced collisional energy transfer

Michele Crance, Stig Stenholm

Published Apr 28, 1980• 6 citations
Publisher

Abstract

The authors introduce a model for radiative collisions which allows one to obtain a simple analytical expression for the transition probability. This model basically consists of an improvement to the Landau-Zener model, in order to obtain a more realistic representation of the Hamiltonian of the system throughout the collision. They apply the results to the problem of radiative collisions when the dipole-dipole interaction is responsible for the energy transfer and obtain an analytical expression for the cross section as a function of the frequency and intensity of the laser field. Their results are compared with experimental results and reasonable agreement is obtained.

DOI
10.1088/0022-3700/13/8/012

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The authors introduce a model for radiative collisions which allows one to obtain a simple analytical expression for the transition probability. This model basically consists of an improvement to the Landau-Zener model, in order to obtain a more realistic representation of the Hamiltonian of the system throughout the collision. They apply the results to the problem of radiative collisions when the dipole-dipole interaction is responsible for the energy transfer and obtain an analytical expression for the cross section as a function of the frequency and intensity of the laser field. Their results are compared with experimental results and reasonable agreement is obtained.
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The authors have studied resonance collisions amongst atoms dressed by a strong laser field. The equations of motion have been solved numerically to yield collisional transition rates for various values of the Rabi frequency (V) due to the laser and its detuning ( Delta ) from the atomic transition frequency ( omega 0 ). The atoms are assumed to follow classical rectilinear paths and only undergo binary collisions. The resonance interaction is taken to be the first-order dipole-dipole interaction acting within the ground and resonance levels of a pair of atoms. A qualitative explanation is offered for the results obtained by splitting the Hamiltonian into parts that represent the collision-free evolution of the system at the Rabi frequency, the 'normal' resonance interaction, and the 'dressed interaction' responsible for opening new scattering channels. The latter interaction depends upon the relative strengths of the resonance interaction (R), the generalized Rabi frequency ( Omega ) and the ratio V/ Delta .
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.
A three-state model for calculating the cross section of laser-induced dipole-quadrupole collisional energy transfer in Sr-Ca system is presented. The motion equations for the probability amplitudes of the three intermediate states are obtained. The expression of the cross section is derived. Various factors including field intensity, relative speed, system temperature which influence the collisional cross section are discussed to illustrate the features of the dipole-quadrupole laser-induced collisional energy transfer (LICET)process. Calculating results show that the peak of the LICET profiles moves to the red, the tuning range of the profiles obviously becomes narrower with the laser field intensity increasing and a cross section of 1.25 × 10(-13)cm(2) at a laser intensity of 8.29 × 10(9)W/cm(2) is obtained. Our results indicate that dipole-quadrupole LICET process can be the effective way to transfer energy selectively from a storage state of arbitrary parity to a target state of arbitrary parity.