Quantum evaporation of 4He atoms by phonons and rotons in liquid 4He is measured for a wide range of conditions. The results confirm that an atom is evaporated in a single-excitation to single-atom process and that the boundary conditions of conservation of energy and parallel momentum are obeyed. Excitations in the liquid are generated by pulse heating a thin metal film heater, and the evaporated atoms are detected by the energy yielded on condensation at the surface of a superconducting transition edge bolometer. The heater and bolometer can be rotated about a common axis in the liquid surface, and the collimation of the excitations and atoms into beams allows the angles of incidence and evaporation to be defined. It is first shown that the input power and pulse length must be carefully chosen to produce beams of phonons and rotons that are fully ballistic. The time of flight from the heater to the bolometer is measured, and the angular distribution of the evaporated atoms is determined for different angles of incidence. In particular, the wavevector dependence of the roton to atom pulse shape can be seen. The authors see no sign of Pitaevskii roton decay over the long liquid path lengths involved ( approximately 6 mm), and there is no indication that ripplons are created in the evaporation process.

We have measured the contact angle 0 of He on cesium-coated tungsten plates as a function of temperature.We find that 0 decreases to zero at T = 2.0 K in agreement with the wetting temperature found on bulk Cs. At T = 0 K the contact angle is 48~1, significantly larger than the predicted value of approximately 30 . The energy of the interface between Cs and liquid He has a large temperature dependence. This suggests that there are low-lying excitations on the liquid helium surface at this interface. Indeed it appears that liquid "He at this interface is similar to that at a free surface. PACS numbers: 67.70.+n, 68.10.Cr, 68.45.Gd Liquid He wets most materials and was thought until recently to be a universal wetting liquid. However, theoretical calculations [1,2] have predicted that He will not wet some alkali metals below a certain temperature called the wetting temperature T . Subsequent experiments have indeed shown that He does not wet Cs [3-5] and Rb [6]. This has been demonstrated by showing that the adsorbed helium film on Cs or Rb for T ( T is atomically thin and only becomes macroscopically thick for T ) T . This arises from the fact that the substrate surface is always attractive to He, but, if it is less attractive than the liquid-liquid attraction, then only a few atomic layers of He at most can form at T ( T . This thin film can disappear as T~0 as found for Cs [5].Another important property of a liquid that does not wet a substrate is that it forms drops of macroscopic size with a finite contact angle O. This angle is in general temperature dependent for T ( T and goes to zero at T and remains zero at all higher temperatures where the liquid wets the substrate. In fact, a nonzero contact angle is the usual indication of nonwetting, for example, water drops on solid hydrocarbons and mercury on glass.Besides this visual indication of wetting and nonwetting, the contact angle has an important connection to the free energies of the three interfaces involved.These are oi, o, , and o,I, which are, respectively, between liquid-vapor, substrate-vapor, and substrate-liquid. Young's equation connects these to 0' as sv O sl cosH = While oI can be readily measured and is indeed well known for He [7], the other interface free energies are not known, and so a measurement of O(T) allows a. , -o. ,~= Ao. to be determined and deductions to be made for a. , and o. ,~. The first theoretical estimate for O(0) for He on Cs was =95 [1]. This was revised to =30 [8] when T was measured to be =2 K [3,4], which was half that estimated theoretically. These estimates were based on the assumption that Ao. was temperature independent so that O(0) is given by cos8(0) = Ao. /crt (0) with Ao. = o. t (T ). Since then there have been calculations of the temperature dependence of Atr [9], especially o. ,t, which show that it is rather small compared to that of a. t, (T), and so the assumption above appears to be a good first approximation. However, there is recent experimental evidence that o,I is a strong function of temperature and t...

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We have created two sheets of ∼1 K phonons in liquid 4 He at ∼55 mK such that they intersect each other as they move towards a common point. If the two sheets have a small angle between them, they interact strongly and create a hot line in the liquid helium. This line is continuously fed with energy from the two sheets and loses energy by creating high energy phonons. If the angle between the sheets is larger than ∼ 30 • they do not interact but pass through each other. These results give direct evidence for the composition of the sheets: they comprise strongly interacting low-energy phonons which occupy a narrow cone in momentum space.Phonon pulses have been created in cold liquid 4 He over many years since the initial work of Gernsey and Luszczynski in 1971 [1]. The pulses were found to travel at the velocity of sound and so it was believed, that to first order, the phonons were independent, non-interacting ballistic wave packets. The liquid 4 He was sufficiently cold that the ambient thermal phonons could be ignored. However it was recognised that phonons could spontaneously decay by the three phonon process (3pp) [2, 3], which is allowed by the upward curvature of the dispersion curve [4,5]. This picture was challenged recently by Adamenko et al. [6,7] who argued that a number of observations, such as the creation of high energy phonons in the liquid, could be explained if the phonons in the pulse were treated as strongly interacting and were confined to a narrow cone in momentum space. This has prompted us to look for evidence of the proposed phonon momentum distribution.A short electrical pulse in a planar thin-film heater is usually used to create a pulse of phonons. It was proposed [6,7], that the phonons in the 1

Experimental evidence is presented that shows that the momentum of a R- roton (a particle-like excitation in liquid helium-4) is antiparallel to its velocity. Although this is anticipated from the negative slope of the dispersion curve for these excitations, it has only been possible to test since the development of a source of ballistic R- rotons. The backward refraction of the quantum evaporation process, which is the signature of antiparallel momentum and velocity, is observed.