Experimental investigation of large nitrogen cluster scattering from graphite: Translational and rotational distributions of evaporated N2 molecules

Abstract

We present experimental results on the translational and rotational energy transfers due to the collisions of large (N2)n clusters with a highly oriented pyrolytic graphite sample at 750 ms−1 incident velocity, for two surface temperatures Ts (440 and 580 K), for average cluster sizes n from 200 to 1000 monomers, and for incidence angles θi from 30° to 70°. Angular distributions of flux and angularly resolved time-of-flight profiles of scattered particles are measured with a quadrupole mass spectrometer while rotational state distributions are determined by resonantly enhanced multiphoton ionization. For all incidences but the largest (70°) the translational distributions of scattered molecules recorded at detection angles θd from 20° to 70° are compatible with a simple thermokinetic model assuming that monomer evaporation occurs at a well-defined temperature Tloc from the cluster gliding on the solid surface. Fits of the translational data yield Tloc values between 250 and 500 K, increasing with Ts and decreasing with n and θi. On the other hand, the rotational distributions are always non-Boltzmann. Actually, they are well described as sums of two contributions, at 75 and 375 K, respectively. The relative weight of the cold contribution is found to increase with θd, with a slope increasing with θi. Possible interpretations of the observed differences between translational and rotational degrees of freedom of evaporated molecules are discussed.