Bestimmung der mittleren Agglomeratgröße und des Restgasanteils kondensierter Molekularstrahlen durch Streuung eines K-Atomstrahls

The mean size of the clusters in a condensed nitrogen molecular beam is evaluated from the attenuation of a potassium atomic beam crossing the nitrogen beam at right angles. The method is based on the fact that molecules in a cluster will shield each other and therefore contribute the less in the scattering processes the larger the cluster is. The percentage of unclustered molecules in the condensed beam is determined by measuring the decrease of the potassium beam attenuation resulting from filtering the condensed nitrogen beam with a scattering chamber. Assuming a spherical drop as a model for the clusters the average number of molecules per cluster can be estimated from the measured effective cross sections. It increases from 160 to 6500 when the nitrogen source pressure is increased from 100 to 700 mm Hg. At the same time the percentage of single molecules in the original condensed beams decreases from 30% to 0.15% of the total beam intensity.