Phonon-Induced Desorption of Helium

Abstract

Phonons from a solid substrate can interact with the atoms in an adsorbed film and cause them to be desorbed; understanding the microscopic mechanism of this process is one of the basic goals of surface science. Desorption of helium at low temperatures is a particularly simple and interesting system for testing our understanding of the phonon-adsorbate interaction. The binding energy of helium to a substrate is smaller than any other adatom, and the magnitude and shape of the binding potential is well known for many surfaces. Because helium is so weakly bound to a surface, it also has the unique property that it is energetically possible for a single substrate phonon to desorb an atom in a photo-electric effect type of process. The helium system also offers experimental advantages over more traditional desorption investigations using heavier adsorbates. The low-temperature techniques of ballistic phonon scattering which provide a means of controlling the frequency, intensity, duration, polarization and wave vector of the desorbing phonons provide a much more detailed probe of the desorption process than high-temperature experiments can possibly achieve. Using these techniques, one can generate both equilibrium and non-equilibrium substrate phonon distributions and detect the phonon-induced desorption of sub-monolayer helium films. The experimental apparatus and a typical desorption signal are illustrated in Figs. 1 and 2. Previous experimental work is described in references [1–4].