A photoemission study of the metals Ga, Tl, Pb and Bi in the liquid state.

Low energy (7.0eV < ?w) < 21.2eV) photoelectron spectra are presented for the simple metals Ga, T1, Pb and Bi in the liquid and solid (frozen) states. The results for the solid phase are consistent with other published data and are broadly similar to the results for the liquid phase. Little change is found in the spectra with increasing temperature for Ga and Bi above the melting point. The three step model of photoemission described by Bergland and Spicer is extended to allow for higher order inelastic scattering mechanisms. It is applied to the results for the liquid specimens and optical density of valence states functions are derived. More structure is observed than would be expected from a weak scattering description of these systems; in particular, the separation of the 6s and 6p derived bands in T1, Pb and Bi is clearly evident. The optical density of states function in these cases reflects the prominent features of the calculated density of states curves for the solid crystalline phases. This is attributed to the strength of the scattering potential and the relatively small change in the local ionic arrangement that takes place on melting. By fitting the absolute photoelectron yields with a Monte Carlo simulation of the photoemission process, the elastic electron-ion mean free path has been determined. In all cases it is found to be much larger than the value obtained from transport measurements. The apparent inconsistency between the solid state characteristics of the optical density of states functions and the free electron behavior observed in optical measurements is attributed to an increase in the Drude-like absorption and the influence of the f-sum rule. The calculated interband contribution to the optical conductivity, although much reduced, is similar to that observed for the solid state.