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Journal of Physics B Atomic and Molecular Physics

Time-dependent effects in near-resonant scattering of laser light on the three-level atom. II. Collisional relaxation in the presence of a laser beam

Janusz Czub, Jan Fiutak

Published Jul 14, 1985• 3 citations
Publisher

Abstract

For pt.I see ibid., vol.15, p.4127 (1982). The master equation has been rederived with particular emphasis on the influence of the frequency and strength of the field on the relaxation process. A modification of the master equation is also proposed and the results are discussed for the three-level atom. The time dependence of the spectrum is analysed.

DOI
10.1088/0022-3700/18/13/016

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For pt.I see ibid., vol.15, p.4127 (1982). The master equation has been rederived with particular emphasis on the influence of the frequency and strength of the field on the relaxation process. A modification of the master equation is also proposed and the results are discussed for the three-level atom. The time dependence of the spectrum is analysed.
The density-matrix approach is used to describe the dynamics of thallium-type dressed atoms in a gaseous environment. The effects of spatial degeneracy are accounted for. It is shown that the stationary contribution to the spectrum is not always dominant and may even be negligible. Thus, a consistent description requires the use of the time-dependent spectrum such as that of Eberly and Wodkiewicz (1977). In this connection a new interpretation of the experiment on near resonant scattering of light on thallium-type atoms is proposed.
We present a theoretical study of strong laser-atom interactions, when the laser field parameters are subjected to random processes. The atom is modelled by a two- and a three-level system, while the statistical fluctuations of the laser field are described by a pre-Gaussian model. The interaction of the laser-target is treated nonperturbatively by using the calculation method based on the Hermitian Floquet theory. Our aim consists in studying the kinetics of atomic relaxation induced by laser noise. In the resonant case with electric field strengths small with respect to the atomic unit electric strength, the present nonperturbative results are in agreement with those obtained within the rotating wave approximation of Eberly et al and Wodkiewicz et al for an atom modelled by a two-level system. We discuss some examples which demonstrate the destruction of atomic coherence by the noise, the regime of relaxation to the equilibrium state and the optical analogue of motional narrowing. We also give new results for two- and three-level systems, and for a strong laser field at exact resonance, in the case of phase, amplitude noises. The case of fluctuation due to collisions is also discussed. Our numerical results indicate that ionization effects, in the presence of laser noise, can lead to important modifications of the populations for strong laser-atom interactions. The changes in the ionization rates generated by the noise are also investigated.
A set of coupled equations for the particle scattering by a two level atom in a strong near resonant radiation field is derived from the time dependent Schrodinger equation. This set of equations is discussed in a first and second order approximation. The result can be visualized in terms of a scheme in which the two well separated (nondegenerate) atomic states split, each into two energy levels. The splitting depends on the strength of the field and on the difference between the resonance frequency and the frequency of the incident light and is typically a few meV. In this scheme all possible elastic, quasielastic, inelastic and superelastic transitions between the four levels can be induced by collision processes. In addition, multiphoton processes may occur if the scattered particle remains long enough in the interaction region. Estimates of the two photon gain process for electron and heavy particle scattering by sodium atoms in a dye laser field are given. It is suggested that the superelastic processes may play an important role in laser induced gas breakdown, even if the laser frequency does not coincide with any atomic transition. Spontaneous emission of atoms in laser fields is also treated. The results are similar to the particle scattering processes.
The interaction of simple, two- and three-level atomic systems with monochromatic and nonmonochromatic laser radiation near resonance transitions is discussed based on the Schroedinger equation with radiation decay and no collisional relaxation, outside the framework of the usual perturbation theory. The mathematical schemes employed include the resonance approximation, exact methods, the quasi-energy method, the adiabatic approximation, etc. A large number of physical phenomena, such as oscillating and inverted populations, self-induced resonances, processes of first order in the weak field, Raman scattering, coherent population transfer, etc. are considered. The review is intended for a broad readership.
We describe strong-field effects in a class of collisional energy-transfer processes occurring among atoms in a gas irradiated by a laser beam. The time evolution of the atomic system is evaluated by solving a system of differential equations, and by using the Magnus-Light expansion of the scattering matrix. Results from the two methods are then compared. The resulting spectrum is shifted in frequency towards the antistatic side, and the line shape becomes less asymmetric. All these theoretical predictions are based on the assumption that the motion of the atoms during the collision can be described by classical mechanics, with no appreciable change of the kinetic energy of the atoms.
Attention is focused on the dynamic evolution of the emission spectrum following abrupt excitation of a near-resonant two-level absorber by a strong laser field. The multiphoton character of the absorption-emission process is evident in the AC Stark splitting of the spectrum, which begins approximately half an atomic lifetime after excitation, if the laser field is strong enough. Analytic expressions for the spectra are given of arbitrary laser field strength, atomic lifetime, laser-atom detuning, and the bandwidth of the spectrometer used to analyse the spectrum. Selected graphs are included to illustrate various time-dependent features of the spectrum. The influence on the transient spectrum of three different states of atomic excitation prior to laser irradiation and the effect on the spectrum of irradiation by a phase-noisy laser are also considered. Comparison of the transient spectra from atoms initially prepared in ' pi /2' and '3 pi /2' states show clearly that phase relationships between the irradiating field and the emitting dipole are important even in fluorescence.