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Physical Review A

Strong-field effects in bichromatic laser-induced collisional energy transfer

Roberto Buffa, Stefano Cavalieri, R. Eramo, Lorenzo Fini, M. Matera

Published Aug 1, 1995• 8 citations
Publisher

Abstract

We discuss a scheme suitable for the study of strong-field effects on the excitation spectrum of the laser-induced collisional energy-transfer process. The use of a weak radiation field, to induce the interatomic transition, and a strong radiation field, to modify the energy-level position, is found to give rise to a splitting of the resonance peak. The position and the relative intensity of the two spectral components are strongly dependent on the intensity and detuning of the strong field. Line-shape calculations performed for the Eu-Sr system show that the effect is expected to be detectable even for moderate intensities of the strong field.

DOI
10.1103/physreva.52.1382

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We discuss a scheme suitable for the study of strong-field effects on the excitation spectrum of the laser-induced collisional energy-transfer process. The use of a weak radiation field, to induce the interatomic transition, and a strong radiation field, to modify the energy-level position, is found to give rise to a splitting of the resonance peak. The position and the relative intensity of the two spectral components are strongly dependent on the intensity and detuning of the strong field. Line-shape calculations performed for the Eu-Sr system show that the effect is expected to be detectable even for moderate intensities of the strong field.
We describe a new scheme suitable for the study of strong-field effects on the laser-induced collisional energytransfer process. The use of two independently tuned radiation fields, to induce the interatomic transition and to modify the energy levels, is found to give rise to a splitting of the excitation spectrum peak, similar to the Autler-Townes doublet. Owing to the resonant coupling, an observable splitting of a few inverse centimeters can be induced by a dressing laser intensity of the order of 10 MW/cm(2).
We consider the response of a laser-induced quadrupole-dipole collisional system driven by a strong dressing laser field with the aim of calculating the collisional cross section of a weak inducing laser probe. The addition of a second driving field to the traditional arrangement will cause magnitude changes of the spectra and modify the profile. The calculation results show that the bichromatic laser-induced collisional energy-transfer process can be an efficient way to probe Stark splitting of both the final state and intermediate state. The magnitude and position of the splitting spectral lines are strongly dependent on the intensity of the dressing laser field. The peak cross section almost reduces by a factor of 2 with the presence of the dressing laser. Also, in the antistatic wing, bright and dark lines are periodic, appearing with the increasing of the dressing laser intensity.
Based on the existing four-level model of laser-induced collisional energy transfer, the collision process in Ba-Sr system in both weak and strong fields is calculated by immediate integration of the equation of motion, and profiles of laser-induced transition probability and collision cross section under both conditions are obtained. Numerical results for the weak field are in fair to good agreement with former analytical results, while the ones for the strong field show that the effect of laser-induced collision is enhanced when the transfer laser intensity increases. Moreover, the peak of the laser-induced collisional energy transfer spectrum shows an evident shift from resonant frequency to the violet side in strong field, and full width at half maximum (i.e. tuning range) of the spectrum becomes obviously narrower compared with that in weak field.
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.
We report a measurement of the excitation spectrum of the laser-induced collisional energy transfer process for the Eu-Sr system in the weak-field regime. The measurement has been performed in a detuning range extending up to 85 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the static wing, providing a wider check of existing theoretical models. The frequency resolution of the measurement (0.1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) was suitable for a quantitative study of the core of the line spectrum, where a comparison with the results of numerical calculations based on an effective three-level model gives complete evidence for a cooperative decay from the Sr(5p2{\mathit{p}}^{2}p2)1{)}^{1})1D2{\mathit{D}}_{2}D2​ final level.
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.