Collision-induced broadening, shifting, and narrowing

in the CN A-X (1-0) band by Ar

Quantitative pressure-dependent line shape studies are of importance both for converting optical spectra into usable concentration data, and for the insights they may provide into collision dynamics and intermolecular potentials.  Deviations from Voigt line shapes (an uncorrelated convolution of Gaussian Doppler width with a pressure-dependent Lorentzian line shape) are typically observed in high-precision line shape measurements, and may be attributed to several second-order effects.  The speed-dependence of relaxation rates increases the effective Lorentzian broadening for those velocity groups sampled at larger Doppler shifts (and thus higher average collision velocities).  Elastic velocity changing collisions can also cause phenomenologically similar narrowing effects through a totally different mechanism: averaging the Doppler shift of individual molecules during multiple elastic velocity-changing collisions prior to the next inelastic collision that terminates the coherent interaction with the optical field.  Pressure-induced frequency shifts can be considered the consequence of a differential interaction of the perturber with upper and lower states of the transition, causing a transient shift in the vertical transition energy during the perturbation, a process that may also vary with collision velocity.

 
Fits to pressure dependent FM line shapes at increasing Ar pressure.  Blue shift and broadening are clearly evident; a narrowing parameter is also well-determined from data like this.

We have measured accurate room temperature line shapes of selected rotational transitions in the CN A 2Π X  2Σ+ (1-0) band near 900 nm in the presence of argon.  Measurements were performed with FM spectroscopy, recording two channels in quadrature, sampling a phased combination of absorption and dispersion signals.  The figure shows an example of pressure-dependent line shapes and fits. The linear pressure dependence of all fit parameters gives accurate experimental values for collision-induced broadening, shifting, and narrowing.  No systematic asymmetry in the line shapes was observed.  Of particular interest is the distinctive and qualitatively different rotational-state dependence of the pressure-dependent broadening, shifting, and narrowing parameters.  Interesting contrasts can be drawn between the present results and previous measurements by the Hanson group on pressure broadening of CN (B 2Σ+ X 2Σ+), OH (A 2Σ+ X 2Π) and NO (B 2Σ+ X 2Π) by rare gases, and related pressure broadening and shifting in vibrational spectra of the same systems.  Understanding trends and relative contributions of rotational energy transfer collisions on ground and excited state surfaces, and the competition between elastic velocity-changing, elastic depolarizing and inelastic collisions in such atom-radical systems is a feasible target for experimental and theoretical comparison.  Features of rare-gas-CN collisions probed by our recent double-resonance studies of rotationally inelastic collisions, speed-dependent inelastic collision rates, elastic depolarization and elastic velocity-changing collisions are the same ones that play a central role in the interpretation and modeling of pressure-broadening of isolated spectroscopic lines.  

D. Forthomme, C.P. McRaven, T.J. Sears, G.E. Hall, J. Phys. Chem. A ASAP (2013).

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Last Modified: September 6, 2013
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