Helium Atom Scattering

Helium Atom Scattering (HAS) is one of the most powerful and versatile surface science techniques. Thermal energy atoms (including He) differ from other diffractive probes (such as electrons, photons and neutrons) in the sense that they are completely surface sensitive and non-destructive. This unique behaviour results from the fact that the interaction of a He atom with a surface is dominated by a long range, Van-der-Waals like attraction and a shorter range repulsive interaction due to electronic overlap between the He and the surface species. Consequently, all thermal energy He atoms are reflected from the surface typically at a distance of 3 Å to 5 Å from the local surface plane of atoms. Helium atoms are chemically inert and their translational energies are much smaller than typical binding energies of adsorbates. Helium atom scattering can be used to study metallic, semiconducting and insulating surfaces as well as adsorbate systems and growth phenomena.

Thermal energy helium atoms have de Broglie wavelengths of between 0.5 Å and 1.5 Å, which are comparable with atomic dimensions. By the use of a nozzle expansion source, we are able to produce a very intense and essentially monochromatic He beam. Analysis of the angular distribution of He atoms backscattered from a surface yields the corrugation function of the surface which in turn reveals the surface structure. The excitation spectra of the surface is obtained from the time of flight analysis of the backscattered He atoms. We are particularly interested in using HAS to study surface defects and low concentrations of isolated adsorbate species. We have developed new concepts behind the elastic diffraction of He from step edge defects and now plan to extend these concepts to include inelastic diffuse intensity contributions.

The new HAS apparatus is shown below.

The apparatus is illustrated schematically;

Schematic of the He scattering apparatus. Circles represent the location and pumping speeds of differential pumping stages. (The largest pump, between nozzle and source has a pumping speed of 18,000 liters per sec.) The UHV chamber is pumped by a turbo pump and cryo pump. Aperture diameters, between pumping stages are indicated. Those marked with an asterisk may be changed freely without breaking vacuum.

An intense He atom beam emerges from a 10µm nozzle into the first differential pumping stage (18,000 l/s.) Beam intensities as high as 3x1019 atoms/sterad/sec are produced. There are 3 more differential pumping stages before the sample UHV chamber. One of these contains the chopper with a gating fraction between 0.5% and 2%. The sample can be cooled with liquid nitrogen and is held in a UHV chamber which contains other surface science tools such as a SPALEED, a double pass CMA, an ion sputter gun, deposition sources and gas dosing facilities.

The scattering angle (source-sample-detector) is fixed at 100°. The time of flight path contains 4 differential pumping stages and the detector chamber. He atoms are ionized and detected with a quadrupole mass spectrometer. The energy resolution of the apparatus is dictated by several factors including beam energy resolution (<1.5%) and time-of-flight path length.

In looking at the diffusely scattered intensities from defects (or single adsorbate molecules) and dispersion-less modes, we can afford to intentionally degrade our angular resolution to get maximal signal levels. By varying aperture sizes we can choose to illuminate the entire sample and degrade the angular acceptance of the detector to approximately 0.5°. The angular acceptance can be increased further to 1° at the expense of a simple rebuild.

Much of this work is motivated by the development of a deeper understanding of the dynamics of small chemisorbed molecules. This includes studies of frustrated translations and frustrated rotations. Some molecules may be free rotors at a surface, although the axis of rotation can be confined. Frustrated and free rotational modes are often extremely low in energy and, in practice, have scarcely been studied despite a growing theoretical interest concerning their roles in many physical and chemical surface phenomena. For example, low lying excitations can dominate partition functions and site occupation probabilities of adsorbates. They can also determine line shapes of higher lying modes. In addition, rotational excitation of a molecule in surface-scattering/adsorption can be an important channel for the uptake of translational energy and a strong influence on adsorbate trapping and sticking. Surface transport phenomena such as thin film resistivities can be strongly influenced by adsorbates through the coupling of low frequency vibrational modes to surface electronic excitations. Frustrated translational modes of small molecular adsorbates have been observed for many years, but the underlying torsional barriers and angular potential surfaces are not well known. This work uses the excitation spectra of these frustrated modes to study these interactions. Initial experiments include the study of locking in of the angular orientation of PF3 on Ni and Pt surfaces.

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