to the study of adsorbates on surfaces (and, in some instances, atoms within the surface itself) for nearly two decades. However, the information that could be extracted from the data has been limited, and compromised by the limited resolution of probes used for this purpose.

In the past five years qualitative advances in experimental method have led to an explosive expansion in the data available, and we are in a position to explore many new questions that simply could not be addressed in the recent past. The near future looks exciting indeed.

For the first time, it has proved possible to perform detailed studies of the dispersion curves of surface phonons on clean and adsorbate-covered surfaces. This is done through the use of particle probes in energy-loss experiments that are a surface analogue of the neutron-scattering experiments that have proved so powerful in studying crystals. Two complementary methods may be used to study surface phonons. One method makes use of highly monoenergetic, well-collimated beams of neutral helium atoms directed at the surface. Their kinetic energy is in the thermal range, and energy resolutions of a few tenths of a millivolt (three or four wave numbers) can be realized. Surface phonons have already been studied for a range of materials, from insulators to metals.

Shortly after helium beams were used in the first studies of surface phonons, a second method was demonstrated. Electron energy-loss spectroscopy as a function of scattering angle was used to study clean and adsorbate-covered nickel surfaces. The key experimental development was the use of much higher energies (50 eV-300 eV) than are used in the more traditional spectrometer. One must use such high energies to probe well out into the Brillouin zone, and the experimental challenge is to produce high-energy beams, sufficiently monoenergetic to allow resolution of the low-energy losses associated with scattering from surface phonons.

The two methods are complementary in that the helium beams may be used to study rather low-frequency surface phonons whose excitation energy is far too small to be resolved by the electron energy-loss method. One such example is the study of vibrational modes of rare-gas monolayers, bilayers, and trilayers adsorbed on Ag(111) by using helium beams. Here dispersion curves are studied in the frequency range from 10 cm–1 (1.2 meV) to 30 cm–1 (3.7 meV). The electron energy-loss method can detect modes as “soft” as 30 cm–1 (3.7 meV), but is best suited to the study of the spectral range above 100 cm–1 (12 meV). There is no upper bound to the frequency range that may be explored with electrons, whereas multiphonon scattering obscures single phonon features in spectra taken with helium beams above roughly 250 cm–1 (30 meV). Electrons can also resolve modes in which the atomic motion is predominantly parallel to the surface, and the extraction of useful information from the spectra will be assisted by the appearance of a quantitative theory of the excitation process, which can be used to predict energy regimes within which excitation cross sections are enhanced.



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