Dynamics of Electron-Bubble Formation in Helium

A theoretical interpretation of recent experiments on ruby-laser-induced breakdown in liquid helium is presented. The dynamics of unbound electrons initially at nonthermal energies in helium proceed until the electrons are finally and stably trapped in a fully thermalized electron bubble. Arguments are presented indicating that intermediate steps in the dynamical process are collisions between the unbound electron and helium atoms, followed by a low-mobility state which may be thermally destroyed, and ending with a thermally stable electron bubble which grows to its equilibrium configuration and thermalizes. Several experimental results are correlated to deduce numerical estimates for the lifetime of the low-mobility state against thermal destruction and against development into a thermally stable electron bubble. An experiment is suggested to unambiguously measure the photoionization threshold energy of an electron bubble.