SCATTERING OF A POTASSIUM ATOM BEAM FROM POTASSIUM-PROMOTED CATALYST SURFACES VIA ELECTRONICALLY EXCITED CLUSTERS

Surface scattering of potassium atom beams is observed from surfaces o f a potassium promoted catalyst, which is known to emit Rydberg K spe cies and clusters K-n. The surfaces studied are cut flat from pellets of an industrial catalyst, the promoted iron oxide catalyst for styre ne production. The scattering is studied in the temperature range 500- 1000 K in an UHV apparatus with a K atom beam at 45 degrees towards th e normal, with surface ionization and ion detection over an angular ra nge of -90 degrees to +90 degrees with respect to the surface normal. Bilobular scattering patterns are observed, which are mainly back-scat tering at low temperatures, below 750 K. A large signal due to ions em itted in the backwards direction is also found with a voltage on the s ample. This back-scattering indicates that the scatterers are heavy cl usters outside the surface. The ion formation in the backwards directi on is proposed to be due to collisions with electronically excited clu sters K-n of the type recently observed by field ionization detection (Kotarba et al. 1994). The bilobular scattering transforms into asymm etric patterns with a larger forward (specular) lobe at higher tempera tures, above 800 K. Only a small fraction of the beam molecules is sca ttered off the surface. The scattering is well described by inelastic surface scattering theory. This shows that the actual scattering surfa ce is rather flat, which is proposed to be due to an antibonding Rydbe rg type interaction, of long range (hundreds of Angstrom), between the impinging excited K atom and the surface. The temperature dependence of the neutral scattering gives a barrier of 0.96 eV, close to what is generally found for Rydberg species emission from such surfaces. At l arger K surface densities, the contributions to the peaks from the bea m flux is shown to agree with this picture involving collisions with e xcited clusters outside the surface.