Search

The Jetset experiment (PS 202) at LEAR will search for gluonic hadrons and other exotics in the interaction of in‐flight antiprotons with protons at rest. The mass range to be covered extends from 1.96 to 2.43 GeV. Our experiment uses a molecular hydrogen cluster jet target which is inserted in the LEAR ring and is surrounded by a general‐purpose detetor of advanced design. In ‘‘Phase I’’ the detector has been constructed to provide selection at the trigger level of four kaon events, allowing a search for resonances in the OZI‐forbidden, gluon‐rich reaction pp→φφ→4K. The combination of LEAR with our detector is unique in that it allows scanning over a large momentum range with excellent momentum resolution, implying corresponding excellent mass resolution. The physics interest in this process is outlined briefly, and the design of the detector elments is described in some detail. A brief discussion of possible upgrade paths (‘‘Phase II’’) for the detecor is given at the end.

Molecular beams, microcalorimetry, and mass spectrometry are used to study the physisorption kinetics of H2 and D2 molecules on a low temperature substrate. The energy transferred to the surface of a liquid‐helium cooled bolometer by an incident room temperature beam of H2 and D2, and the number density of the scattered molecules are measured. The sticking coefficient S and the accommodation coefficient α are obtained as a function of N, the surface coverage by H2 or D2. For both isotopes, S and α depend strongly on N. S(H2) changes from 0.07±0.05 to 0.80±0.02 and α(H2) from 0.31±0.07 to 0.97±0.01 as N increases from ?0 to 3×1015 molecule cm−2. In the same coverage range, S(D2) changes from 0.25±0.05 to 0.92±0.04 and α(D2) from 0.3±0.1 to 0.975±0.025.

The diffractive scattering of nearly monochromatic helium and neon beams from a (001) cleaved LiF surface at low temperatures was experimentally studied. A quantum theory of surface rainbow based on a corrugated hard surface model was developed. The calculated relative intensities of diffraction peaks agree with the measured values.

The growth of ultra thin iron films (up to a thickness of 5–6 ML) on O(1×1)–Fe/Ag(0 0 1) has been investigated by means of He reflectivity ( R He ) and metastable de-excitation spectroscopy. The presence of oxygen induces a quasi-ideal layer-by-layer growth at variance with the case of the homoepitaxial growth of iron on Fe(0 0 1). The surface electronic density of states suddenly changes upon the deposition of the first half of a monolayer. After the sudden change in the first stages of the growth, the surface density of states of both O 2p and Fe 3d states remains essentially unchanged, irrespectively of the thickness of the deposited film. This provides a clear indication that oxygen floats at the surface acting as a surfactant for the growth of iron on O(1×1)–Fe/Ag(0 0 1). The stationary fraction of oxygen that remains on the topmost layer as growth proceeds depends on the substrate temperature. Post-growth annealing up to 650 K restores the initial coverage of oxygen and the ordered O(1×1) phase.

A highly flexible empirical potential Vp(z) suitable to describe the main features of the laterally averaged atom-surface interaction is presented and an accurate expression for the vibrational spectrum of the atom in Vp(z) is given. This establishes a simple method of inversion of the atom-surface vibrational spectra particularly suitable for levels located in the deepest 9/10 of the potential well. The present method is complementary to the one introduced by Le Roy [5].

In scattering measurements of atoms and molecules from surfaces the intensities of the diffraction and specular peaks contain contributions from quasi-elastic scattering events, in addition to the coherent elastic scattering. These may occur because of inelastic collisions with very small changes in energy and momentum and because of elastic collisions with random defects of the surface. In the present article it is shown how to separate the quasi-elastic contribution from the elastic one by measuring both angular and energy distributions of the scattered particles. As an example the procedure is used to study the scattering of Ne from LiF in order to measure the Debye-Waller Factor (DWF) at low surface temperatures. The analysis of the DWF temperature dependence yields detailed information on the nature of the interaction, and shows good agreement with the Levi-Suhl model for the DWF.