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

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.

Ionization of water clusters by collisions with graphite surfaces

“…We have recently used molecular dynamics simulations to show that this mechanism makes large cluster fragments survive surface scattering of Ar n from Pt(111) [27]. The mechanism is also supported by surface scattering experiments with van der Waals clusters [12][13][14] and water clusters [9][10][11]. Experimental angular distributions show that the scattered flux of large fragments is forced towards the tangential direction, indicating that energy is taken from the normal component while the parallel component is essentially conserved.…”

“…They measured the neutral flux from the surface and observed a peak at large scattering angles if the incident angle was not too steep. The appearance of a peak close to the tangential direction has been shown for other systems to be due to the survival of large cluster fragments [12][13][14]. In related work, surface scattering of clusters with impact velocities of 1-10 km/s has been shown to result in microshock wave propagation in the cluster which may induce intracluster reactions [15][16][17].…”

Normal to tangential velocity conversion in cluster-surface collisions: ArN on graphite

“…At such moderate energies the main effect observed both in experiments and simulations is extensive cluster fragmentation, essentially into monomers, with however, possibility of large fragment survival. 6,8,9 In our previous experimental studies on the Ar N /graphite system, 16,[19][20][21][22] carried out at incidences of 20°or larger, we observed that in the angular distributions of scattered particles the large surviving fragments give rise to very narrow features, localized at grazing exit angles. The broader superspecular lobes centered at less grazing angles were assigned to monomers ͑with some other very small fragments͒ evaporated from the clusters without direct interaction with the surface.…”

“…Collisions of van der Waals clusters with surfaces have been studied both theoretically by molecular dynamics ͑MD͒ calculations, 1-8 and experimentally, [9][10][11][12][13][14][15][16][19][20][21][22] by supersonic beam scattering techniques. MD simulations indicate that for high collision energies extreme temperatures and pressures should be attainable as a result of the intracluster shock wave.…”

The Interaction of Monatomic and Diatomic Molecules with Solid Surfaces: The Structure of Water Cluster Beams and Their Scattering from Surfaces.

“…Gspann and Krieg [44] have studied the scattering of helium, hydrogen, and nitrogen clusters from a stainless steel surface. The surface was carefully polished (the measured roughness depth was less than 500 A and cleaned by baking to 4500 K. Their measurements included mass spectrometry and crude velocity analysis.…”