Journal of Chemical Physics, Vol.111, No.15, 7038-7046, 1999
Experimental investigation of large nitrogen cluster scattering from graphite: Translational and rotational distributions of evaporated N-2 molecules
We present experimental results on the translational and rotational energy transfers due to the collisions of large (N-2)(n) clusters with a highly oriented pyrolytic graphite sample at 750 ms(-1) incident velocity, for two surface temperatures T-s (440 and 580 K), for average cluster sizes n from 200 to 1000 monomers, and for incidence angles theta(i) from 30 degrees to 70 degrees. 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 degrees) the translational distributions of scattered molecules recorded at detection angles theta(d) from 20 degrees to 70 degrees are compatible with a simple thermokinetic model assuming that monomer evaporation occurs at a well-defined temperature T-loc from the cluster gliding on the solid surface. Fits of the translational data yield T-loc values between 250 and 500 K, increasing with T-s and decreasing with n and theta(i). 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 theta(d), with a slope increasing with theta(i). Possible interpretations of the observed differences between translational and rotational degrees of freedom of evaporated molecules are discussed.