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arXiv:physics/0401075

Nonlinear Variation of Light-Pressure-Induced Multilevel Atomic Line Shifts with Vapor Pressure in Helium Spectroscopy

Tanya Zelevinsky

Published Jan 15, 2004• 0 citations
arXivHTMLPDF

Abstract

This paper has been temporarily withdrawn. Highly nonlinear dependences of 4He 2^3S - 2^3P lines on pressure were observed in vapor cell saturation spectroscopy. Line shifts agree with those expected from light-pressure-induced perturbation of atomic velocity distribution. In the lin-perp-lin pump and probe polarization configuration, multilevel atomic structure must be taken into account. For a two-level atom, the shift magnitude is determined by the collisional time constant, but when the system has dark states there is an additional time constant due to optical pumping. The collisional and optical time constants compete, and become equal at a certain pressure p_0. Saturation signal vanishes at p_0 whereas signal due to light pressure remains. This leads to a dependence of line centers on pressure that exhibits apparently singular behavior at p_0. The 2S - 2P(J=1) line shift reaches the natural half width. Vapor cells may be more suitable for investigation of this effect than atomic beams since the shifts vanish at sufficiently high pressures. Two ways of controlling the shifts for the helium 2P fine structure measurement are discussed.

DOI
10.48550/arxiv.physics/0401075

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This paper has been temporarily withdrawn. Highly nonlinear dependences of 4He 2^3S - 2^3P lines on pressure were observed in vapor cell saturation spectroscopy. Line shifts agree with those expected from light-pressure-induced perturbation of atomic velocity distribution. In the lin-perp-lin pump and probe polarization configuration, multilevel atomic structure must be taken into account. For a two-level atom, the shift magnitude is determined by the collisional time constant, but when the system has dark states there is an additional time constant due to optical pumping. The collisional and optical time constants compete, and become equal at a certain pressure p_0. Saturation signal vanishes at p_0 whereas signal due to light pressure remains. This leads to a dependence of line centers on pressure that exhibits apparently singular behavior at p_0. The 2S - 2P(J=1) line shift reaches the natural half width. Vapor cells may be more suitable for investigation of this effect than atomic beams since the shifts vanish at sufficiently high pressures. Two ways of controlling the shifts for the helium 2P fine structure measurement are discussed.
We calculate the effect of light-induced forces on the fluorescence line shape of a two-level atom crossing at right angles two counterpropagating light beams of parallel linear polarizations $(\mathrm{lin}\ensuremath{\Vert}\mathrm{lin})$ in a common configuration for ultrahigh-precision optical spectroscopy. For an incident atomic beam with a narrow spread of transverse velocities the dipole force induces a redshift of the fluorescence maximum, while in the reverse case of a wide spread of transverse velocities the radiation-pressure force induces a blueshift of the saturation dip minimum. We then use our theory to explain the blueshift of the saturation line-center dip occurring for the closed transition $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ of a 4He{}^{4}\mathrm{He}4He beam. The observed shift, which is in quite good agreement with the theory, can be of the order of 1/10 of the transition natural linewidth and hence quite important for ultrahigh-precision spectroscopy measurements.
We report on detailed studies of the effect of resonant light pressure on the optical response of an atomic gas to a single monochromatic laser field. In this very elementary situation of laser spectroscopy, the redistribution of atomic velocities that is induced by spontaneous light pressure leads to a novel contribution to the optical dispersion curve of the medium. This light-pressure-induced dispersion phenomenon displays a pronounced nonlinear dependence on the laser intensity. Moreover, for a given intensity, its strength is closely related to the laser beam diameter. As most important feature, this light-pressure-induced dispersion displays an even symmetry with respect to the optical detuning from line center. As a result, the total Doppler-broadened dispersion curve of the gas can become asymmetric, and a significant shift of the dispersion line center can occur. In addition to a detailed theoretical description of the phenomenon, we report on its experimental investigation on the \ensuremath{\lambda}=555.6 nm 1^{1}1S0{\mathit{S}}_{0}S0​${\mathrm{\ensuremath{-}}}^{3}$P1{\mathit{P}}_{1}P1​ transition in atomic ytterbium vapor with the use of frequency-modulation spectroscopy. The experimental findings are in good quantitative agreement with theoretical predictions.
We report the results of studying the effect of the resonant light pressure force on the amplitude and frequency properties of the nonlinear susceptibility and well known absorption resonance dip occurring in optical saturation spectroscopy of two level atomic gas. On the basis of numerical solutions of the Liouville and Fokker-Plank equations the behaviour pattern of the velocity distribution function of a two-level atomic gas in relation to the saturation intensity and time of interaction with a strong resonant optical field was obtained. Time course of the velocity distribution function due to the action of spontaneous light pressure force leads to modification of the nonlinear susceptibility, absorption saturation resonant line form and to change of frequency dip relative to the atomic resonant frequency transition. The represented numerical modeling results explained qualitatively a number ofexperimental facts, that could not be explained before.
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We report on the observation of a light-pressure-induced asymmetry of the well-known absorption dip occurring in optical saturation spectroscopy. This asymmetry is due to the small modification of the atomic velocity distribution that is caused by the spontaneous scattering force of the saturating light field. Our measurements were performed on the \ensuremath{\lambda}=555.65 nm 1S0^{1}\mathrm{S}_{0}1S0​${\mathrm{\ensuremath{-}}}^{3}$P1{\mathrm{P}}_{1}P1​ transition in atomic ytterbium vapor by the use of a frequency-modulated test field propagating collinearly with the scattering field. The experimental results clearly confirm theoretical predictions and demonstrate the significance of the phenomenon for high-resolution laser spectroscopy.
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.