Quantum Evaporation from LiquidHe4by Rotons

Abstract

We have shown that rotons as well as phonons can evaporate 4 He atoms in a singlequantum process. Measurements of the time of flight and the angular distribution of the evaporated atoms clearly distinguish between evaporation by phonons and rotons. The results indicate that energy and the parallel component of momentum are conserved at the free liquid surface.PACS numbers: 67.20. + k, 43.35.+d, 62.60. + v We have recently shown that He atoms can be evaporated by phonons in liquid 4 He in a singlequantum process in which one phonon evaporates one He atom. 1 In this Letter we report measurements of the angular distribution of 4 He atoms evaporated by a beam of phonons and rotons which are incident at an angle of 13° to the normal to the free surface. This distribution can be understood in terms of the boundary conditions at the surface. This, together with the good agreement between the measured and calculated times of flight, allows us to conclude that 4 He atoms are evaporated by rotons as well as phonons in a quantum interaction in which an excitation in the liquid is annihilated and an atom in a free state is created.The evaporation of liquid 4 He has been discussed for many years. 2,3 Interest in it was rekindled by the work of Johnston and King 4 who attempted to measure the velocity distribution of atoms evaporating from liquid 4 He at 0.6 K, although their results turned out to be incorrect. 5 They apparently saw a spectrum of atoms with characteristic temperature (1.5 K) well above that corresponding to the liquid temperature (0.6 K). This stimulated 6,7 explanations in terms of a roton, at the roton minimum energy A, giving its energy to an atom in the liquid which after evaporation would have kinetic energy equal to A -E B = 1.5 K, where E B is the latent heat per atom. The large number of rotons at this energy (8.65 K) where the density of states is infinite was thought to give rise to a dominant number of atoms with energy 1.5 K. However, Cole 8 pointed out that these rotons, although numerous, have a low attack frequency on the interface because their group velocity tends to zero. In fact, the effect of the density of states and group velocity cancel each other. More recent measurements 5,9,10 have shown that evaporated 4 He atoms do have energies commensurate with the temperature of the liquid.The important suggestion made by Anderson 6 and Hyman, Scully, and Widom 7 that a single excitation could evaporate a single atom was tested by Balibar et al.However, at the low temperatures T < 0.3 K which are needed for long phonon mean free paths, they were unable to detect atoms evaporated by phonons. The signals which they ascribed to roton-atom desorption lead to a maximum velocity for these rotons of 160 msec -1 . This is a mysterious result for which no convincing reason could be given. We now believe that they used heater powers which were too high, as we find that the detected atom-signal changes shape for powers above 10 mW mm -2 and it appears slower than those for lower powers. We can reproduce t...

Quasiparticle scattering at helium surfaces: A microscopic theory

“…Further, since the liquid and the vapor are comprised of the same material, the distinction between liquid and vapor operators made in these tunnelling Hamiltonian approaches is not appropriate. Maris 23 used an ''adiabatic'' model and found that quasiparticles ͑phonons and rotons͒ with energies between the roton minimum energy ⌬ ϳ8.7 K and the maxon energy ⌬ m ϳ13.9 K do not evaporate atoms, whereas experiments 6,24 show that R ϩ rotons with these energies do contribute to quantum evaporation.…”

Quantized conductance through the quantum evaporation of bosonic atoms

“…They are also expected to exhibit the phenomenon of quantum evaporation, whereby an elementary excitation of a superfluid reaches its surface and causes the evaporation of a single atom. This phenomenon had so far been studied experimentally [17,18] and theoretically [19,20] in superfluid 4 He, and we consider it for the first time in the context of superfluid atomic gases.In this Letter, we show that conductance quantization occurs with bosonic atoms as well, and that the Bose statistics strongly enhances the value of the conductance step compared to fermions. Unlike for fermions, this value explicitly depends on temperature, and the effect occurs with bosons up to temperatures higher than with fermions.…”

“…They are also expected to exhibit the phenomenon of quantum evaporation, whereby an elementary excitation of a superfluid reaches its surface and causes the evaporation of a single atom. This phenomenon had so far been studied experimentally [17,18] and theoretically [19,20] in superfluid 4 He, and we consider it for the first time in the context of superfluid atomic gases.…”

Horst Meyer and Quantum Evaporation

“…Wyatt found no measurable energy loss in the evaporation by rotons [32][33][34]. However, Edwards [15,46] found evidence of inelastic processes due to the emission of ripplons during the approach of the surface by incoming atoms, as found also in a calculation by Echenique and Pendry [26].…”

“…Wyatt and his group made a long series of new experiments, some of which are described in a review article published in 1992 [32]. In 1983, they discovered the evaporation by phonons [33]. A year later, they showed that the parallel momentum is conserved in the evaporation by both ballistic phonons and ballistic rotons [34].…”