Friday October 6, 2017
Proteomics Rm 120 12:00 pm
"Pressure Broadening, Sub-Doppler Hole-Burning, and Collosion Dynamics of Free Radicals"
The pressure-dependent broadening of sharp spectral lines has been known since before the days of quantum mechanics to be associated with a reduction in the time between collisions, during which the resonant interaction of light and matter can proceed unperturbed. The corresponding pressure-dependent Lorentzian line shape, when combined with the Doppler shifts associated with a Gaussian thermal distribution of gas phase velocities, yields the well-known Voigt profile, which provides a good first approximation to pressure-dependent line shapes. With higher precision modern measurements, however, observed line shapes typically deviate from best-fit Voigt profiles at the 1-5% level, with systematically sharper peaks observed than the best global Voigt fits. Distinct physical mechanisms have been invoked to explain this difference related to a speed-dependence to the collisional broadening, or to the effects of velocity changing collisions. Line shape measurements alone do not discriminate effectively between these mechanisms.
Our prior work with energy transfer collision dynamics of free radicals, originally motivated by combustion chemistry applications, provided tools that could be applied to study the underlying collision dynamics of pressure broadening. Speed-dependent collision rates and cross sections for inelastic and elastic velocity-changing collisions can be assessed with both time and frequency domain experiments with transient hole burning and saturation recovery kinetic spectroscopy in a prototype system of ground-state CN radicals, colliding with Ar. High precision measurements in this system can be compared and combined with accurate quantum scattering calculations on a validated ab initio potential energy surface. One conclusion of these studies is that speed-dependent effects dominate velocity diffusion in the deviations from Voigt line shapes, at least for pressure broadening of CN radicals by Ar.