Franck–Condon approach to collisional dephasing of spectral lines in liquids

We consider the broadening of spectral lines in liquids at low temperatures. To lowest order in density, the relevant microscopic information is given by the spectral function J(ω) which is related to the single perturber spectrum. The line shape may then be calculated using J(ω). We have calculated J(ω) both classically and quantum mechanically for a collinear model and show quantitatively how different they are for typical experiments in cryogenic liquids. The absorption line shape I(Δ) is then evaluated using J(ω) and we discuss how sensitive it is to the details of J(ω). This gives us an estimate of the microscopic information content of collisionally broadened line shapes.