Mobility of excess electrons in fluid argon: Single- and double-phonon theory

A theory of the zero-field mobility of excess electrons in fluid argon is presented. The mobility is considered to be limited by scattering from longitudinal-acoustic phonons. Single-phonon as well as double-phonon processes are considered. Contrary to single-phonon scattering, the double-phonon process is found to be inelastic, and therefore the theory is not a simple extension of the regular one-phonon theory. For densities far enough from the well-known mobility peak, we find that only single-phonon scattering is important. The calculations give excellent agreement with experiment in the dense gas (0.1nc<n<nc) and dilute liquid (nc<n<1.4nc) regimes (n is the number density of the fluid, nc is its critical density). For greater densities, the calculated mobility is significantly higher than the experimental one. Even so, we show that static density fluctuations are not likely to play an important role in limiting the mobility. The authors suggest that additional phonon modes (i.e., transverse and optical modes) should be considered in order to explain the electron mobility in the dense liquid regime.