Toward a unified impact theory for everything concerning neutral species line shapes

A classical path theory for spectral lines that incorporates all impact-type phenomena (that is, no duration of collision or ternary interaction effects) is presented. Limiting cases will be explored, including: Standard collisional broadening and line mixing; speed dependent Voigt profiles; nonlinear spectroscopic ('Doppler-free') broadening; Doppler broadening/narrowing in the presence of large internal state perturbations; and Doppler broadening/narrowing for small internal state perturbations, including a full range of velocity persistence effects. As examples, the velocity changing effects in the presence of weak internal state perturbation has a high importance in the description of hydrogen molecular quadruple and Raman spectra, and of the vector intercollisional interference features in the Q1(J) collision induced absorption by H2 at low pressures. Velocity changing effects in the presence of large internal state disruption represents a useful limit for describing the highly accurate data on the vibration-rotation spectra of HCl and HF perturbed by N2, as developed in the Ph.D. thesis of Looney