Effect of CH4 addition on excess electron mobility in liquid Kr

The mobility μ of excess electrons has been measured in liquid mixtures of Kr and CH4 as a function of the electric field up to E~104 V/cm and of the CH4 concentration x up to x~10%, at a temperature T~125 K, fairly close to the normal boiling point of Kr (Tb~120 K) [Nucl. Instrum. Methods Phys. Res. A 430, 277 (1997)]. We present here new data extending the previous set in the low E region. At small E, μ appears to be quite independent of x. CH4 impurities prove to be less efficient in enhancing momentum transfer than liquified rare gas impurities Kr and Xe in liquid Ar. The dependence of μ on E at higher strengths is complicated. On the one hand, the addition of CH4 extends the range of E in which μ is field independent, by efficiently thermalizing the electrons. On the other hand, at the highest field, the presence of the impurities accomplishes a large increase of the electron drift velocity with respect to the pure liquid (up to a factor of 7 for the highest x). Moreover, at intermediate values of E, where electrons are epithermal, there appears to be a crossover between two different behaviors of μ as a function of E. The electric field strength at the crossover, E*, is well correlated with x. The behavior of μ can be rationalized in terms of a gas-kinetic model proposed to explain its concentration dependence in the liquified rare gas mixtures Ar-Kr and Ar-Xe. This analysis suggests that the observed crossover is related to the excitation of the first vibrational level of CH4.