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Physical Review Letters

Collisional Narrowing of a Fine-Structure Raman Transition

S. Saoudi, Ch. Lerminiaux, M. Dumont

Published May 19, 1986• 6 citations
Publisher

Abstract

The line shape of the Raman transition between metastable levels of neon ($^{3}p_{0}\mathrm{and}^{3}p_{2}$) is studied as a function of the pressure of a helium buffer gas. Dicke narrowing of the Doppler-broadened nonresonant Raman signal (for large optical detunings) gives the first evidence of conservation of a fine-structure coherence by velocity-changing collisions. Pressure broadening of the resonant Raman signal gives a measurement of the velocity-changing-collision rate.

DOI
10.1103/physrevlett.56.2164

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The line shape of the Raman transition between metastable levels of neon (3p0and3p2^{3}p_{0}\mathrm{and}^{3}p_{2}3p0​and3p2​) is studied as a function of the pressure of a helium buffer gas. Dicke narrowing of the Doppler-broadened nonresonant Raman signal (for large optical detunings) gives the first evidence of conservation of a fine-structure coherence by velocity-changing collisions. Pressure broadening of the resonant Raman signal gives a measurement of the velocity-changing-collision rate.
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.
Measurements of resonant, two-step excitation of sodium in the presence of very-low-pressure buffer gases (He, Ar, Xe, and N2{\mathrm{N}}_{2}N2​) are reported. These data are analyzed to isolate the effects of line broadening and velocity-changing (VC) collisions in the single-collision limit. Velocity dependence of the several line broadenings involved is obtained from this data analysis. The inelastic VC kernels are also obtained as a function of initial velocity.
A general method for analysing line broadening caused by Doppler, phaseshift, inelastic and velocity-changing collisional effects is proposed for transitions at optical wavelengths. In a unified approach Poisson statistics are used to describe collision probabilities randomly distributed in a time interval which can terminate the optical coherence in a combination of the above effects; markoffian independence is assumed for each collision. Then the dipole moment correlation function is obtained in a form which is virtually exact in accounting for all effects of isotropic forces, and which is correct to second-order terms in the anisotropic interaction potentials. Phenomena as diverse as standard optical pressure broadening, broadening due to velocity-changing collisions in nonlinear spectroscopies, and Dicke narrowing of the Doppler contour are described. The implications of the theory are discussed in detail and applied to the case of the NaD lines perturbed by He buffer gas. Numerical calculations reveal that the substantially different lineshift and broadening parameters in nonlinear and in standard optical spectroscopies can be reasonably well understood within the limitations posed by present uncertainties in the Na 3 2 P-He interaction potentials. However, the large differences in shift and width between the D 2 and D 1 lines in saturation spectroscopy cannot be explained within the present theory.
We present a theoretical and experimental study of the time evolution of the saturation-spectroscopy line shape when the saturating beam is suddenly switched on. We consider two metastable levels coupled by two Doppler-broadened transitions with a common excited level. In this special case, the fast transients from the upper level are rapidly damped, and the slow transients from populations and from Raman coherence in the metastable levels are easy to observe and are used for studying the collisional relaxation. For the sake of clarity, the theoretical analysis is developed according to a perturbation calculation. Nevertheless, saturation of the metastable levels must be taken into account in a more realistic theory (velocity-selective optical pumping): We give only a few results that are in excellent agreement with the experiments performed on neon metastable levels 3P0 and 3P2.
Dark resonances with widths below 30 Hz have been observed in a rubidium cell filled with neon as buffer gas at room temperature. We compare an approximate analytic solution of a \ensuremath{\Lambda} system to our data and show that under our experimental conditions the presence of the buffer gas reduces the power broadening of the dark resonances by two orders of magnitude. We also present numerical calculations that take into account the thermal motion and velocity-changing collisions with the buffer-gas atoms. The resulting dark-resonance features exhibit strong Dicke-type narrowing effects and thereby explain the elimination of Doppler shifts and Doppler broadening, leading to observation of a single ultranarrow dark line.
Accurate values for the density shift and broadening of five isolated ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure transitions have been measured. Doppler-free polarization and double-quantum saturation spectroscopy with a cw dye laser are used in a He dc discharge with 1% H2{\mathrm{H}}_{2}H2​. The negative shifts and the huge broadening values are correctly predicted by an impact-approximation theory taking into account the large cross sections for fine-structure transfer (quenching) collisions and only Van der Waals interaction between H∗{\mathrm{H}}^{*}H∗ and He. Consequences of a comparison with the positive shift of the unresolved ${\mathrm{H}}_{\ensuremath{\alpha}}$ line and of the alkali-metal DDD lines in He buffer gas are discussed. For pressures below 0.1 Torr hfs decoupling collisions give rise to a positive shift of single ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance.