Collision broadening and shift of excited s states in thallium

The collision broadening by xenon of the 6p-ns absorption transitions in thallium has been studied with a high resolution spectrometer and a charge-integrating photodiode array detector. The spectra have excellent signal-to-noise ratio, and can be analysed directly with profiles calculated from unified line broadening theory and a Fermi interaction potential. For the first time, the calculations are made with a numerical potential, rather than an approximate analytical form. This allows meaningful comparisons without the adjustment of potential parameters. The agreement between experimental profiles and these exact unified theory calculations improves with increasing n* and is quite good for values of n* greater than 6.25, corresponding to the 11s state. Analysis of line shifts follows the simpler Fermi shift expression at large n*, as expected. Deviations of the profiles and shifts at smaller n* from those theoretically expected seem related to the increased size of the effective range theory corrections to the scattering length of the valence electron. Evidence is also presented for the nonlinear dependence of line width on density for large n* when the impact approximation is not valid.