Effect of CH$_{4}$ addition on excess electron mobility in liquid Kr

The excess electrons mobility µ has been measured recently in liquid mixtures of Kr and CH 4 as a function of the electric field up to E ≈ 10 4 V/cm and of the CH 4 concentration x up to x ≈ 10% , at temperatures T ≈ 130 K, fairly close to the normal boiling point of Kr ( T b ≈ 120 K ) 1 . We present here new data which extend the previous set in the region of low electric field. The experimental results are interpreted in terms of a kinetic model previously proposed to explain the concentration dependent behavior of µ in liquid Ar–Kr and Ar–Xe mixtures. The main result is that CH 4 is more effective in enhancing energy–transfer rather than momentum–transfer in comparison with mixtures of liquified noble gases. The field dependence of µ is quite complicate. In particular, at intermediate values of the field, there appears to be a crossover between two different electric– field dependent behaviors of µ. The electric field strength at crossover is well correlated with the concentration of CH 4 . This fact suggests that different excitations of the molecular solute might be involved in the momentum– and energy–transfer processes for different values of the mean electron energy.