Effect of the cluster angular momentum J and the projectile orbital momentum L on capture probability and postcollision dynamics

In this work, collisions between rotating atomic clusters composed of Lennard-Jones (LJ(n)) particles and an identical projectile have been investigated by means of trajectory simulations as a function of the cluster angular momentum J and internal energy E, and for different values of the projectile impact parameter b and relative velocity v(p). As expected, the collision (P(c)(b)) and capture [or sticking P(s)(b)] probabilities are found to decay below unity for values of b larger than the average surface radius of the cluster, with dP/db being strongly dependent on v(p). Both P(c)(b) and P(s)(b), however, appear to be largely insensitive to the modulus of the cluster angular momentum vertical bar J vertical bar and only weakly dependent on E for collisions involving target clusters with a lifetime tau > 100 ps. The latter findings are interpreted as indicating the absence of strong changes in the structure of the target as a function of vertical bar J vertical bar and E. The comparison between the dissociation lifetime (tau(dyn)) of the postcapture complexes (LJ*(n+1))obtained continuing trajectories after monomer capture and the one computed from the fragmentation of statistically prepared clusters (tau(stat)) supports the validity of a two-step capture-dissociation model; similarly, the comparison between the average amount of energy exchanged during trajectories (Delta E(dyn)) in the process LJ(n) + LJ --> LJ*(n+ 1) --> LJ(n) + LJ and the one predicted by statistical simulations (Delta E(stat)) suggests a fast statistical energy redistribution in the collisional complex even for very short tau(dyn) (e.g., 40 ps). In the case of projectiles aimed at the edge of the cluster [(grazing collisions, P(c)(b) < 1]; however, the time elapsed between formal collision and dissociation, tau(coll), is such that tau(coll) < tau(stat) and the trajectories indicate the presence of ballistic dynamics and of a weak energy exchange (Delta E(coll) < Delta E(dyn), with Delta E(coll) being the average energy exchanged during collisions). The relevance of these results to the study of gas phase nucleation and to the possibility of building a fully microcanonical framework for its description is discussed. (C) 2009 American Institute of Physics. [doi:10.1063/1.3239476]