Using a high-resolution difference-frequency spectrometer, we have studied the shape of four infrared lines of CO, highly diluted in N~. We have approached the analysis from two perspectives. In the first, the spectral profile of an isolated line is empirically represented by a variation from the "standard" model in which the mass-diffusion constant is replaced with a pressureand line-dependent "optical" diffusion constant. In the second, we abandon the concept of an "optical" diffusion constant and replace it with a physically more tenable hypothesis that the departures from the standard model are due to a slight deviation from the usual exponential decay used to describe the collision broadening of spectral lines. This shifts the focus of the discussion from an analysis of the translational dynamics to an analysis of the dynamics of the internal degrees of freedom.PACS number(s): 33.70. Jg, 35.20.Yh, 34.10.+x
High-resolutioninverse Raman spectroscopy has been used to obtain the line shifting and line broadening coefficients of H2 perturbed by He. Measurements have been made for the Q-branch transitions {J =0 to 5) in a density range of 10 to 20 amagat and from 296 to 995 K. Up to 795 K we have directly deduced from the experimental broadening coeKcients the inelastic rotational state-to-state and vibrational dephasing rates. At higher temperatures, owing to the larger number of channels of relaxation which occur, the results have been analyzed using a scaling law. The line shift and broadening coefficients exhibit a square root and a linear dependence on temperature, respectively, and a significant J dependence. Semiclassical calculations based on an accurate ab initio potential lead to line-shape parameters consistent with experiment. They allow a clear understanding of their observed temperature dependence.PACS number(s): 33.70.Jg
Precise line-shape measurements of the Q(0) to Q {4)lines in Dz have been made using Raman gain spectroscopy. At the densities studied {2. 5 -30 amagat), the Q-branch lines are well separated but slightly asymmetric due to quantum-mechanical line mixing. We have measured both the symmetric and the asymmetric contributions to the line shape as a function of the density. From the broadening coeKcients and the line-mixing parameters, we have deduced the dephasing and the state-to-state rotational relaxation rates. Semiquantitatively, the results support the empirical modified-exponential-gap law.PACS number (s): 33.70.Jg, 34.50.Ez, 34.50.Pi INTRO)DUCTI(ON
The details and performance of a shot-noise-limited high-resolution (≤ 2 MHz) Raman gain spectrometer are discussed and compared to a "standard" continuous wave (cw) spectrometer. For a single pass cell and 1 s integration time, a signal-to-noise ratio greater than 1000 is obtained for the Q(2) line of D2 at pressures above a few atmospheres. The quality of the spectrometer for the determination of spectral profiles is demonstrated by examining the Dicke narrowing of the line.
With high quality spectral data, we have observed departures from the soft collision model for translational motion, in the transition region from Doppler broadening to Dicke narrowing. The departures are in agreement with theoretical calculations based on the Boltzmann equation. The implications of the results concerning the dynamics of fluids are discussed. In addition we show that the mass diffusion constant describes the translational diffusion of the optical coherence and we give precise measurements of the broadening coefficients of the Q(O) to Q( 6) lines.
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