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Physical Review A

Redistributed spectrum of scattered light

E L Lewis, Daryl Szebesta

Published Aug 1, 1990• 1 citation
Publisher

Abstract

The spectrum of redistributed light that results from excitation by monochromatic near-resonant incident light has been observed in calcium perturbed by argon. The two components were found to be well described by the astrophysical redistribution functions ${\mathit{R}}_{\mathrm{II}}$ and ${\mathit{R}}_{\mathrm{III}}$ for the Rayleigh and fluorescent peaks, respectively. The Doppler width of the Rayleigh component was observed to be \ensuremath{\surd}2 larger than that of the fluorescent component, for our observation at right angle to the incident beam, clearly demonstrating the coherence of this part of the scattering. The ratio of fluorescent to Rayleigh intensity was independent of detuning within our range, which was well within the impact region, and increased linearly with argon pressure in a manner that is consistent with an identification of the cross section for redistributing collisions with that for line broadening.

DOI
10.1103/physreva.42.1424

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The spectrum of redistributed light that results from excitation by monochromatic near-resonant incident light has been observed in calcium perturbed by argon. The two components were found to be well described by the astrophysical redistribution functions RII{\mathit{R}}_{\mathrm{II}}RII​ and RIII{\mathit{R}}_{\mathrm{III}}RIII​ for the Rayleigh and fluorescent peaks, respectively. The Doppler width of the Rayleigh component was observed to be \ensuremath{\surd}2 larger than that of the fluorescent component, for our observation at right angle to the incident beam, clearly demonstrating the coherence of this part of the scattering. The ratio of fluorescent to Rayleigh intensity was independent of detuning within our range, which was well within the impact region, and increased linearly with argon pressure in a manner that is consistent with an identification of the cross section for redistributing collisions with that for line broadening.
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 resonant scattering of monochromatic light in gases is analyzed. Explicit expressions for the differential cross section are obtained in the limits (Doppler width) \ensuremath{\gg} (natural width) \ensuremath{\gg} (collision width) and (collision width) \ensuremath{\gg} (natural width) \ensuremath{\gg} (Doppler width). In the former limit, the frequency distribution of scattered light is narrowed in the forward and backward directions, and has the full absorption width for right-angle scattering. In the collision-dominated limit, the cross section separates into a coherent and a fluorescent component. The motion of the atoms broadens the $\ensuremath{\delta}$ function distribution characterizing coherent scattering from a stationary target. The interference between light scattered by different atoms is assessed.
By tuning a pulsed dye laser through the resonance line of calcium at 422.7 nm the authors have investigated the spectral redistribution of laser radiation by calcium vapour. Two main components were resolved in the side emission from an oven containing calcium vapour and argon buffer gas. These are produced by Rayleigh scattering and by collision-induced fluorescence. The intensity of the Rayleigh component was found to vary as Delta -2 where Delta = omega 21 - omega L is the laser detuning from resonance. In contrast the intensity of the collision-induced fluorescence showed a markedly asymmetric dependence on detuning. Detailed studies of the collision-induced fluorescence have enabled the complete line shape of the calcium resonance line perturbed by argon to be determined. The interpretation of this data in terms of a frequency-dependent collision rate, gamma E ( Delta ), is discussed. This paper also reports calculations of theoretical line profiles using the unified Franck-Condon theory and trial interatomic difference potentials. The comparison of the experimental and theoretical line profiles enables the range of validity of the unified Franck-Condon theory to be assessed and the form of the optimum Ca-Ar difference potential to be selected.
The study of spectral line shapes has traditionally been mainly concerned with the measurement and interpretation of absorption or emission profiles. Often only the line widths are studied. The present investigation has the objective to evaluate the additional information which can be obtained by scattering light (usually from a laser) from an atomic system which is being perturbed by collisions. A scattering experiment is discussed. The scattered light consists of two components, a (coherent) Rayleigh component and a redistributed (fluorescent) component. In order to obtain the absorption spectrum, questions regarding the probability of photon absorption are considered. By observing the fluorescence subsequent to absorption during a collision it is found possible to obtain information on the evolution of the system from the point of absorption to the completion of the collision. The information on the intracollisional evolution is the justification for the title of the study, namely 'Why half a collision is better than a whole one'.
Resonant scattering (redistribution) of weak monochromatic light by atoms in the presence of collisions has been considered by Omont et al., who allowed for the broadening of both upper and lower levels of the transition. Mollow (who also has considered this problem for intense radiation) has shown that their results are applicable only if collisional repopulation of the levels (in contrast to collisional depopulation) is ignored. We here consider in detail the effect of repopulation. It leads, in the weak-field limit, to a true-coherent component ($\ensuremath{\delta}$-function spectrum) at the driving frequency, as well as a component at this frequency whose width is essentially the inelastic collisional rate of the lower state. An additional redistributed component due to repopulation of the upper level is also obtained.
We have observed collisional redistribution of laser light scattered by a three-level atom, thallium, in the presence of argon. The two observed spectral components correspond to Raman scattering and collision-induced fluorescence. We have seen the growth of both components from spontaneous to stimulated scattering as the laser intensity is increased. While the gain for the Raman component agrees with theory, the observed gain of the collision-induced fluorescence is over an order of magnitude lower than predicted.