Reflection of clusters of helium, hydrogen, and nitrogen as function of the reflector temperature

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Abstract
Clusters of helium, hydrogen, and nitrogen are reflected at a polished stainless steel plate at temperatures ranging from 80 to 550 K. The incident clusters contain on the average about 1.5×105 atoms of helium or molecules of hydrogen or 104 molecules of nitrogen, as measured by time‐of‐flight mass spectrometry. The angular distributions of the average sizes, the velocities, and the molecular intensities of the reflected cluster beams show that in the investigated range of reflector temperatures the reflection of the helium clusters corresponds to the hydrogen cluster reflection at higher reflector temperatures while the nitrogen cluster reflection corresponds to the hydrogen cluster reflection at lower reflector temperatures. The transition between the two regimes of reflection as observed with hydrogen clusters is marked by an optimum reflector temperature leading to a maximum intensity of the reflected beam, a minimum loss of clustered material, and a distinct angular separation of incident cluster sizes. At a grazing incidence angle of 84.3° the measured optimum reflector temperature for the hydrogen cluster reflection is 215 K and increases with decreasing angle of incidence. The two regimes of high‐temperature and low‐temperature reflection of clusters exhibit close phenomenological relationship to the regimes of thermal and structure dominated scattering of atoms from single crystal planes. In both cases the transition region is characterized by a maximum angle of reflection and a minimum divergence of the reflected beam. The features of high‐temperature cluster reflection are explained by a semiempirical model based on the evaporation recoil of the cluster molecules ablating after contact with the comparatively hot reflector surface.