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Physical Review A

Explanation of high-foreign-gas-pressure effects on resonant light scattering in I2 vapor

Seth D. Silverstein, R. L. St. Peters

Published Jun 1, 1974• 12 citations
Publisher

Abstract

We demonstrate that at all pressures, the resonant scattering emission in ${\mathrm{I}}_{2}$ vapor as excited by a 501.7-nm Ar-ion laser line is dominated by the combined effects of discrete-state resonant fluorescence and dissociative-state continuum resonant Raman scattering (RRS). The changes in the emission upon increased pressures, such as relative and absolute intensities of Stokes overtones, and the variation in depolarization ratios are demonstrated to be due to the gradual quenching of the resonance fluorescence at high pressure, while the dissociative-state RRS contribution remains unaltered.

DOI
10.1103/physreva.9.2720

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Soviet Journal of Quantum Electronics•Apr 30, 1976•2 citations
We demonstrate that at all pressures, the resonant scattering emission in I2{\mathrm{I}}_{2}I2​ vapor as excited by a 501.7-nm Ar-ion laser line is dominated by the combined effects of discrete-state resonant fluorescence and dissociative-state continuum resonant Raman scattering (RRS). The changes in the emission upon increased pressures, such as relative and absolute intensities of Stokes overtones, and the variation in depolarization ratios are demonstrated to be due to the gradual quenching of the resonance fluorescence at high pressure, while the dissociative-state RRS contribution remains unaltered.
An examination of the foreign-gas pressure dependence of the intensity and spectrum of the signal from I2{\mathrm{I}}_{2}I2​ vapor under off-resonance monochromatic excitation indicates that the re-emission process is not resonance Raman scattering as recently reported by Fouche and Chang.
The spontaneous resonance emission of CH3I in high pressures (800–1600 psi) of CH4 and Ar excited in the region of the Rydberg B-state origin (∼201 nm) are reported. These emission spectra consist of narrow Raman-like (RL) and broad fluorescence-like (FL) spectral features. The observed ratio of the Raman/fluorescence intensity in these high pressure solutions is a function of the excitation wavelength as the incident radiation is tuned through the pressure broadened electronic origin band. Molecular dynamics simulations are implemented for the analysis of the observed emission spectral shapes and their excitation frequency dependence. The four-time dipole correlation functions required for the calculation of this nonlinear polarization derived signal are successfully approximated by a product of two two-time dipole correlation functions for these chromophore-bath systems (factorization approximation). The complex emission band shapes and their excitation frequency dependence are captured by this approach. The dispersion in the RL/FL emission redistribution is due to the multiple time scales inherent to the decay of the resonant optical coherence of these pressure-broadened absorptions. The wavelength dependent pure-dephasing rate is determined by the nonlinear shape of the solute–solvent difference potential. The observational time scale dependence of the spectroscopic homogeneous and inhomogeneous line broadening labels is clearly demonstrated and contrasted here for absorption and Raman scattering.
This paper reports on our studies of near-resonant scattering of laser light in a collisional environment. A pulsed dye laser with a peak power of 55 MW/cm2{\mathrm{cm}}^{2}cm2 was tuned near the 460.73-nm resonance line of strontium and the side emission was observed from an oven containing both strontium vapor and argon buffer gas. The emission was composed of three spectral components: Rayleigh scattering at the frequency of the laser ${\ensuremath{\omega}}_{L}$, fluorescence at the resonance frequency ${\ensuremath{\omega}}_{0}$ of strontium, and a third component at $2{\ensuremath{\omega}}_{L}\ensuremath{-}{\ensuremath{\omega}}_{0}$. These three components have been studied as a function of the frequency and intensity of the laser and also as a function of the argon buffer gas pressure. While the Rayleigh emission was found to vary as ${\ensuremath{\Delta}}^{\ensuremath{-}2}$ ($\ensuremath{\Delta}={\ensuremath{\omega}}_{0}\ensuremath{-}{\ensuremath{\omega}}_{L}$), the fluorescence component, which was produced by Sr-Ar collisions, was found to be asymmetric with the sign of $\ensuremath{\Delta}$ as predicted by line-broadening theory. By measuring the ratio of the intensities of the fluorescence and Rayleigh components, we were able to measure directly the collisional redistribution function, important in the study of radiative transfer in stellar and planetary atmospheres. At high laser intensities all three components were found to saturate. The results were compared with the theoretical predictions of Mollow's steady-state theory. Theoretical fits for the high-intensity results were obtained when the collisional cross sections were taken to be considerably smaller than in our low-intensity measurements. We believe the discrepancy lies in the use of a steady-state theory for a transient experiment. Effects of radiative trapping and spatial averaging are also discussed.
The general method developed in a previous paper (see ibid., vol.10, p.345 (1977)) is applied to the problem of resonance Raman scattering in very intense laser fields. At very low intensities the spectrum consists of three monochromatic lines (Rayleigh, Raman Stokes and anti-Stokes) which split into a triplet and two doublets when the laser light, tuned on one of the two transitions connecting the upper state to the two lower sublevels, has a sufficiently high intensity to saturate this transition but not the second one. The Raman and Rayleigh lines are completely mixed when the intensity becomes so high that both transitions are saturated. The corresponding evolution of the spectrum is determined. Optical-pumping effects leading to asymmetries in the fluorescence spectrum are taken into account.
When a principal atomic transition (dipole allowed from the ground state) is driven by odd- multi-photon excitation or by stimulated hyper-Raman scattering, a nonlinear multi-wave mixing field is generated at the transition frequency. A number of dramatic effects on such odd-photon mediated processes can ensue as a result of the internally generated wave-mixing fields. Excellent agreement between theory and experimental studies in Xe and in metal vapors is illustrated for a number of predicted effects including: suppression and strong shifting of resonance lines and hyper-Raman emissions under single and multi-laser excitation. We illustrate the dependence of the interference-related effects on pressure, oscillator strength, wavelength combination and relative propagation directions of pump laser beams. Contrary to most other nonlinear effects, we show that none of the features (in the semi-classical regime) depends on the intensity of the driving laser field.
A report is given of the first quantitative study of the spontaneous and stimulated electronic Raman scattering by a resonance doublet of the sodium atom excited resonantly at the D 1 and D 2 lines by a tunable dye laser. Theoretical and experimental analyses of the differential scattering cross section and polarization properties at the Stokes and anti-Stokes frequencies show that the observed scattering is due to a nearly resonant process involving the ground state 3 S 1/2 as the dominant intermediate state. The stimulated Stokes and anti-Stokes scattering lines, which are shifted from the D 1 and D 2 lines by an amount corresponding to the frequency separation between lowest-lying excited doublets 3 P 1/2 and 3 P 3/2 , are observed by exciting a sodium vapor cell with a passively mode-locked dye laser.