FIELD-IONIZATION OF RYDBERG ALKALI STATES OUTSIDE IRON-OXIDE CATALYST SURFACES - PEAKED ANGULAR-DISTRIBUTIONS OF IONS

The angular dependence of alkali-ion emission is studied from samples of an industrial potassium-promoted iron-oxide catalyst, used for styrene production. In this system, the desorption and emission of Rydberg-state atoms of the potassium promoter are well documented. The temperature of the catalyst sample is 770-1020 K, thus covering the operating temperature range for styrene production at around 900 K. The ion angular distributions often show very sharp peaks, with widths as small as 5-degrees. With external field strengths of the order of 4-40 V cm-1 at the surface, the largest ion peaks are found at +/- 70-degrees and +/- 30-degrees-40-degrees. In the most probable direction, along the surface normal, the signal is almost zero. A retarding potential measurement shows that the ions are formed by field ionization outside the sample. Several possible peak formation mechanisms are discussed. The two types of peaks have different voltage and temperature dependencies, which agrees with the previous studies of the solid-state energetics of the catalyst. It is concluded that the ions leave the surface with a few eV of energy along the surface tangent. Collective phenomena due to long-range forces like cluster formation and possibly interference effects may be important for the shape of the angular distributions.