On Brownian Motion of Helium Ions in the Ballistic Regime

Abstract: Discussed in the paper is the possibility of introducing the concept of Brownian motion of various mesoparticles in the ballistic regime. The case in point is the effect of collisions between thermal excitations in the liquid and the test mesoparticle (allowing to trace its position) on the thermal motion of the latter. The standard criterion for the brownian nature of the particle motion assumes that the inequality $l_s<R_i$ is satisfied and, consequently, a large number of collisions with the averaged response of the mesoparticle to such a bombardment (here $l_s$ is the mean free path of helium excitations, $R_i$ is the effective mesoparticle radius). However, the opposite limit $l_s>R_i$, which we refer to as the ballistic regime, is also possible. It is characterized with the specific features in the behavior of mesoparticles. The emphasis is made on the already available evidences indicating this behavior in the thermal motion of helium ions.