The Energy Dependence of Roton Quantum Evaporation from Superfluid 4He

Abstract

We have measured the energy dependence of roton quantum evaporation. A transiently heated cavity filled with superfluid ⁴He generates R⁺ and R⁻ rotons. The emitted rotons are collimated and arrive at the free liquid surface with a narrow range of incident angles. We detect two beams of evaporated atoms, one due to R⁺ rotons and the other due to R⁻ rotons. Our numerical simulation of roton and atom trajectories, using an evaporation probability of 1, yields two angular distributions of atom flux which are similar to our experimental results, but do have systematic differences which we attribute to the evaporation probability. The ratio of the observed signal to the computed value at each bolometer position gives the relative roton quantum evaporation probability as a function of roton energy. We find that this probability increases with roton energy, except perhaps for low energy R⁻ rotons. We compare these results with theoretical predictions.