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Soviet Journal of Quantum Electronics

Influence of collisions on the saturation of a Doppler line profile

В. А. Алексеев, A. V. Malyugin

Published Nov 30, 1977• 1 citation
Publisher

Abstract

An analysis is made of the influence of the changes in the velocity of atoms in collisions on the saturation of a Doppler line profile. It is shown that when the homogeneous line width is sufficiently small the saturation effect cannot be described by Bloch-type phenomenological relaxation constants.
DOI
10.1070/qe1977v007n11abeh008253

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A theory of the influence of binary collisions on the line shapes associated with Doppler-free spectroscopy is presented. The specific calculation is for two-photon absorption through a real or virtual intermediate state, but the extension of the results to alternative level schemes is included. Collisions are treated quite generally at first, but, ultimately, a simple but reasonable collision model is adopted to properly account for both the "phase-interrupting" and "velocity-changing" aspects of collisions. It is shown that collisional processes can be used to distinguish between the "two-quantum" and "stepwise" contributions to the two-photon excitation rate. Moreover, it is demonstrated that systematic experimental line-shape investigations, in addition to providing tests of current collision theories, can lead to values for collision broadening and shift parameters, excitation transfer rates, magnetic substate relaxation rates, velocity-changing collision rates, and collision kernels. Consequently, Doppler-free spectroscopy can provide a new and important probe of collision effects in atomic and molecular systems.
The effect of collisions on the resonance fluorescence spectrum of three-level atoms with a V-configuration of levels when excited by two monochromatic resonance fields has been theoretically investigated. The analysis has been carried out for the systems with a small Doppler broadening compared to the collision frequency (high buffer gas pressures) and for the general case of an arbitrary change (from complete failure to complete preservation) of the phase memory at any of the three transitions in the V-system. It has been shown that in the three-level V-type atoms excited by two monochromatic fields, the resonant fluorescence spectrum (the number of spectral lines and their widths) at one of the transitions strongly depends on the presence of resonant radiation at the adjacent transition. Under certain conditions, the line widths will be completely determined by the relaxation characteristics of the adjacent transition and, in particular, can be repeatedly narrowed with an increase in the radiation intensity at the adjacent transition. It has been shown that despite the homogeneous nature of the broadening of the absorption lines, the resonance fluorescence spectrum has a pronounced anisotropy with respect to the mutual orientation of the wave vectors of spontaneous and exciting radiation. It has been established that the line widths in the resonance fluorescence spectrum are proportional to the diffusion coefficient of the atoms interacting with radiation under certain conditions. This fact can serve as the basis for a spectroscopic method for measuring the transport frequencies of the collisions of the radiation-absorbing particles with the buffer ones. It has been shown that the effects of phase memory in the collisions lead to a strong qualitative and quantitative modification of the resonance fluorescence spectrum.
It is found that allowance for the quadratic Doppler effect broadens and changes the resonance absorption line profile of particles trapped in an oscillator potential comprising a periodic lattice of resonances having an envelope in the form of a conventional Doppler profile, if the particle oscillation frequency is greater than the homogeneous line width. When the quadratic Doppler shift is small compared with the homogeneous line width, its influence is reduced to a shift of the resonances. A new possibility for eliminating thermal broadening of individual resonances is discussed.
A theory of saturation spectroscopy in three-level gas-vapor systems, including collisional effects, is presented. Using a model of collisions in which they are phase interrupting in their effect on level coherences and velocity changing in their effect on level population densities, the authors calculate the absorption profile of a weak probe field on a given transition when an arbitrarily strong pump field acts on a coupled transition. Line shapes are derived for an arbitrary collision kernel describing the velocity-changing collisions, and these line shapes are evaluated for the collision kernel proposed by Keilson and Storer. Various limiting forms for the line shapes are derived and representative line shapes are displayed. Several new features of the line shapes, including collisionally induced increased probe absorption are discussed. The line shapes are seen to reflect the various collisional processes that may occur in atomic or molecular systems.
We report experiments which show that saturation of the inhomogeneously broadened 6328-\AA{} neon-laser line is strongly affected by cross relaxation which tends to redistribute the excited atoms over the Maxwellian velocity distribution and thus leads to partially homogeneous line saturation. Our results for Ne20{\mathrm{Ne}}^{20}Ne20 discharges agree well with theoretical calculations based on a "strong-collision" model. Results for He3{\mathrm{He}}^{3}He3:Ne20{\mathrm{Ne}}^{20}Ne20x mixtures show deviations from this model that we attribute to the "weak" cross relaxation caused by He-Ne collisions.