The impact theory of spectral line shapes: a paradigm shift

Abstract

An overview of the binary collision impact theory of spectral line shapes has been given to provide a unified statistical mechanical approach to line-shape theory, laser theory, nonlinear optics, and transport phenomena in dilute gases. The computation of spectral line profiles corresponding to those obtained from ultra-high-resolution spectral line-shape measurements requires numerical ab initio calculation of scattering amplitudes directly from the underlying dynamics of collisions between radiatively active molecules and their perturbers. The Wigner distribution function–density matrix is utilized to describe the kinetic theory of spectral line shapes and to discuss the various collisional processes that contribute to the kernel of kinetic equations. The influence of features of the potential energy surface on spectral parameters is also discussed, and the importance of comparing experimental line profiles directly with numerically computed line shapes obtained from reliable interaction potentials is emphasized. This contrasts sharply with the universal practice of comparing experimental line widths and shifts using some average or approximate theoretical scattering cross-sections and it contrasts sharply with fitting experimental profiles to some convenient analytical line-shape model; hence the phrase “a paradigm shift” in the title of this work.

Line shapes, positions and intensities of water transitions near 1.28μm

“…Recent advances in laboratory measurements that yield extremely high signal-to-noise-ratios (SNRs) for absorption spectra [44] now enable increasingly stringent tests of fundamental line shape theories [45,46]. For water vapor, the identification and validation of accurate line shape models to reproduce both the pressure-and compositiondependence of isolated transitions, has been the focus of several recent studies [41][42][43].…”

Velocity-changing collisions in pure H2 and H2-Ar mixture

“…19,21,53,64 We use here the formalism presented by Shapiro et al 53 In this formalism the spectral line shape is given by…”

“…calculations based on quantum mechanical scattering and solving transport/relaxation equation [19][20][21] or based on classical molecular dynamic simulations (CMDS) 22,23 can significantly reduce the number of approximations for spectra evaluation.…”

Recent advances in collisional effects on spectra of molecular gases and their practical consequences

“…II.5 of [1]). A general review on this topic can be found in [130]. In a purely theoretical study, using the 1/r n potential form, [131] solved these kinetic equations and showed the influence of the interaction potential (ie of n) on the calculated spectral shape.…”