Evaporation Probability of R⁻ Rotons Relative to R⁺ Rotons in Superfluid ⁴He

Abstract

The evaporation of superfluid ⁴ He by rotons is investigated using a recently developed pulsed source of both positive (R ⁺ ) and negative (R ⁻ ) group velocity rotons. The R ⁺ and R ⁻ rotons have very different momenta parallel to the free liquid surface and this causes angular dispersion of the two beams of evaporated atoms in the vacuum. On moving a bolometer horizontally through these beams, we find that the maximum flux of atoms from R ⁻ rotons occurs at an angle corresponding to an average R ⁻ roton energy of ħω/k _B ≃ 10.5 K. The signal at this angle is compared with the evaporation signal at the maximum flux caused by R ⁺ rotons. These R ⁺ rotons have an average energy of ≃ 10.7K. The relative sizes of these two signals enables an estimate to be made of the probability of evaporation by R ⁻ rotons relative to that for R ⁺ rotons. We find that «P _−a »/«P _+a » ≃ 4 × 10⁻³ where the brackets signify averages over the angles and energies allowed by the geometry of the experiment.