Collisional broadening and shifting parameters of the Raman Q branch of H2 perturbed by N2 determined from speed-dependent line profiles at high temperatures

Femtosecond time resolved coherent anti-Stokes Raman spectroscopy: experiment and modelization of speed memory effects on H2-N2 mixtures in the collision regime

With the aim of temperature diagnostic, femtosecond time-resolved CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in H2-N2 mixtures. In a first part, a Lorentzian profile is used to model the femtosecond CARS response. A difference between the experimental broadening and the expected one is observed in the collision regime. The observed broadening increases strongly in an inhomogeneous way with respect to the perturber concentration. This is of considerable importance for temperature measurements. In a second part, we show that in the collision regime, this inhomogeneous broadening is due to the speed dependence of the collisional parameters and the memory effects of the radiator speed. A new modelization of the time-resolved CARS response taking into account the speed memory effects is presented and applied to the temperature diagnostic in H2-N2 mixtures. The numerical results are in good agreement with experiments.

Self and Polar Foreign Gas Line Broadening and Frequency Shifting of CH3F: Effect of the Speed Dependence Observed by Millimeter-Wave Coherent Transients

The shape of the (J, K: 2, 1 <-- 1, 1) millimeter line of CH3F in collision with polar buffer molecules has been investigated in the temperature range 140-300 K. The experiments exploit a Stark switching coherent transient technique, namely the optical free precession phenomenon, the Fourier transform of which is the usual steady state absorption lineshape. Using various buffer gases (CH3Br, CH3F, and NH3), the observed time domain signals provide the first experimental evidence in the millimeter range that line broadenings as well as frequency shiftings depend on the relative speed of collision partners; that is, lineshapes can become narrowed and asymmetric according to the molecular mass ratio and the type of collisional interaction involved. The experimental signals are analyzed with a time domain speed-dependent Voigt profile: for the polar buffer molecules considered, it is shown that a simultaneous interpretation of the broadening and narrowing parameters as well as of their temperature dependence can be satisfactorily obtained only with a realistic collision theory; in contrast with atomic buffer gases, velocity changing collisions play a negligible role. Copyright 1997 Academic Press. Copyright 1997Academic Press