DESORPTION AND EMISSION OF POTASSIUM RYDBERG ATOMS AND CLUSTERS FROM IRON-OXIDE CATALYST SURFACES

The angular dependence of alkali promoter desorption emission is studied by molecular beam interaction and surface ionization detection from samples of an industrial potassium-promoted iron oxide catalyst, used for styrene production. In this type of system, highly excited Rydberg state atoms are formed at the surface, which has been observed previously by field ionization at a distance from the surface. A molecular beam of potassium may be directed towards the surface at a fixed angle of 45-degrees to the surface normal. The temperature range used in the studies is 550-1000 K, thus covering the operating temperature range for styrene production. Several different types of angular distributions of potassium are found: flat-topped cosine, tilted cosine, bilobular angular distributions and peaks in the normal direction. The tilted cosine distribution is probably due to back-desorption from the porous surface structure. The bilobular distributions are due to the formation of highly excited Rydberg states of potassium. It is argued that Rydberg atoms can easily diffuse over macroscopic distances, due to their long-range interaction with the surface. The observed peaks in the normal direction are due to clusters of potassium formed by the strong interaction between the Rydberg states, which agrees with previous results.