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Optics and Spectroscopy

Effect of strong collisions of atoms with gas-cell walls on the shape of the double radiooptical resonance

Igor M. Sokolov

Published Aug 1, 1984• 0 citations
OpenAlex

Abstract

An analytic expression for the double radiooptical resonance signal is obtained in the approximation of three-level atoms for an optically thin 1-Di cell with strong collisions of atoms with the surface taken into account. It is discovered that the effect of these collisions does not lead to an additive correction to the linewidth that agrees with the rate of boundary relaxation. Interference effects also take place on the joint consideration of the disorientation effect of the surface and the nonuniformity of the rf field, which can both neutralize as well as intensify the effect of the wall. A narrowing of the transition line is possible in individual cases.

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The influence of phase memory effects on the spectrum of resonance Raman scattering by three-level atoms with the Λ configuration of levels experiencing collisions with buffer gas atoms in a strong monochromatic radiation field is studied theoretically. Systems with a small Doppler broadening compared to the collision frequency (large buffer gas pressures) are analysed in the general case of an arbitrary change (from complete change to complete preservation) in the phase memory at any of three transitions in the Λ system. It is shown that in the absence of the collision relaxation of the low-frequency coherence at the transitions between two lower levels of the Λ system, the radiation scattering spectrum has a spectrally narrow component at the Raman frequency, which, despite the homogeneous broadening of the absorption line, exhibits a strongly pronounced anisotropy. In the direction, close to the propagation direction of exciting radiation, this line maximally narrows down. It is significant that upon optical pumping to the level unaffected by a strong field the resonance Raman spectrum noticeably differs from the spectrum in the case of the probe field. A simple expression is proposed for calculating the degree of the phase memory preservation in collisions from the relative amplitude of the Raman resonance.

Quantum Electronics•Feb 28, 2009•0 citations
An analytic expression for the double radiooptical resonance signal is obtained in the approximation of three-level atoms for an optically thin 1-Di cell with strong collisions of atoms with the surface taken into account. It is discovered that the effect of these collisions does not lead to an additive correction to the linewidth that agrees with the rate of boundary relaxation. Interference effects also take place on the joint consideration of the disorientation effect of the surface and the nonuniformity of the rf field, which can both neutralize as well as intensify the effect of the wall. A narrowing of the transition line is possible in individual cases.
Based on the semiclassical theory of the interaction of two-frequency laser radiation with a resonant atomic medium, an analytical expression is obtained that describes the form of resonances of electromagnetically induced transparency detected in a cell with a finite direction in the longitudinal laser beam direction with an anti-relaxation coating of walls, taking into account the motion of atoms and collisions with walls. The following types of reflection from walls are considered: diffuse, specular, and specular-incoherent. It is found that in a number of cases the diffuse type of reflection exhibits the properties of a mirror-incoherent one. It is also shown that the nondegeneracy of the excited state in a number of cases significantly changes the shape of the transparency resonance. Keywords: Electromagnetically induced transparency, Gas cell, anti-relaxation coating, diffuse reflection, specular reflection, specular-incoherent reflection.
Microwave double resonances were measured in a wall-coated Rb vapor cell as a function of additional buffer-gas pressure. These data were compared with similar measurements in an uncoated cell. It was found that the linewidth in the coated spherical cell of diameter 1.6 cm displays a distinct maximum around 0.2 kPa. This agrees well with theoretical solutions of the diffusion equation, assuming a complex reflection coefficient at the wall. It was also found that at intermediate pressures the lineshapes of the microwave resonances become asymmetric with a low-frequency tail. This agrees with the explanation that where the alkali mean free path is substantially smaller than the average distance between wall collisions at zero pressure, there exist two classes of atoms in the cell. The atoms that get trapped near the walls accumulate much larger phase shifts than those toward the center of the cell. This effect is not seen in the longitudinal relaxation rate, indicating that it is related to a phase-shift effect.
The shape of the coherent population trapping resonance in optically thin cells with antirelaxation coating of the walls has been determined by solving the quantum kinetic equation for the single-atom density matrix. It has been found that motion and collisions with walls in the case of nondegenerate ground and excited states result in a significant change in the shape of the spectrum, in the appearance of a comb of additional resonances, and in shifts of the main resonance of coherent population trapping, which nonmonotonically depend on the size of the cell.
Lineshapes of three-level resonance Raman scattering and two-level resonance fluorescence, homogeneously broadened by collisions, in the presence of arbitrarily strong coherent radiation, are studied by means of a tetradic (density-matrix) scattering theory. In the case of medium coupling strength V L to the radiation field (where T 2 -1 <or=V L << tau c -1 , with T 2 and tau c being the coherence damping and collision times, respectively) the Mollow lineshape (1977) for resonance fluorescence is rederived, with proper T 2 and population cross-relaxation effects included. In particular it is shown that relaxation through intermediate non-resonant levels can considerably alter the lineshape in resonance fluorescence. In the case of strong coupling (V L >or approximately= tau c -1 ) the collision rates are modified by the applied field (optical collisions). It is shown that, whereas in self attenuation spectra optical collision rates can be obtained only from line intensity or line wing measurements, in resonance scattering these rates are directly obtained from linewidth measurements.
The influence of phase memory effects on the spectrum of resonance Raman scattering by three-level atoms with the Λ configuration of levels experiencing collisions with buffer gas atoms in a strong monochromatic radiation field is studied theoretically. Systems with a small Doppler broadening compared to the collision frequency (large buffer gas pressures) are analysed in the general case of an arbitrary change (from complete change to complete preservation) in the phase memory at any of three transitions in the Λ system. It is shown that in the absence of the collision relaxation of the low-frequency coherence at the transitions between two lower levels of the Λ system, the radiation scattering spectrum has a spectrally narrow component at the Raman frequency, which, despite the homogeneous broadening of the absorption line, exhibits a strongly pronounced anisotropy. In the direction, close to the propagation direction of exciting radiation, this line maximally narrows down. It is significant that upon optical pumping to the level unaffected by a strong field the resonance Raman spectrum noticeably differs from the spectrum in the case of the probe field. A simple expression is proposed for calculating the degree of the phase memory preservation in collisions from the relative amplitude of the Raman resonance.