A SIMPLE DESIGN FOR A HELIUM SCATTERING APPARATUS

We have designed a simple helium scattering apparatus for gas-surface scattering experiments. The effusive source is a glass multicapillary array, operated at large Knudsen numbers which products a highly colli mated thermal He beam (FWHM approximate to 3 degrees) with a Maxwellia n velocity distribution. The detector is a Bayard Alpert ionization ga uge, mounted in a tube that is separated from the main UHV chamber via a second multicapillary array. This setup allows one to select only t hose He atoms that are reflected by the sample in the specular directi on and thus to measure the reflectivity of the surface. Examples of it s performance are presented for the adsorption of CO on Ru(001), and f or thermal annealing of a Ru(001) surface sputtered with argon ions at different energies.