QUANTUM SIMULATIONS OF ENERGY-TRANSFER AND STATE-TO-STATE TRANSITIONSIN COLLISION OF AN ATOM WITH A LARGE ANHARMONIC CLUSTER - HE-13(AR)

A time-dependent self-consistent field approach is used to simulate a He atom colliding with an Ar-13 cluster. Direct energy transfer during the collision, and energy redistribution among the vibrational degree s of freedom of the anharmonic cluster following the collision, are st udied. An important advantage of the method used is that quantum state -to-state transition cross sections can be computed for large systems. The following main results are obtained: (1) The process can be inter preted in terms of a direct collision, followed by post-collision ener gy redistribution in Ar-13, a description that appears only when the c luster vibrations are not described by the eigenstates of this system. A time scale of one picosecond is found for the post-collision intrac luster energy distribution. (2) The long-time final state distribution of Ar-13 is less state selective than the distribution immediately af ter the impact, but it is also not completely statistical. (3) There a re state-to-state transitions having cross sections of observable magn itude. (4) The dominant transitions are those involving zero, one, and two ''phonon'' excitations. Some of the ''two phonon'' excitations ha ve cross sections comparable to strong ''single phonon'' transitions. (5) Different types of modes show different propensities for excitatio ns in the collision, in close relation to the geometric character of t he modes. The results show that the TDSCF approximation is a powerful tool for treating both direct collision dynamics and collision-induced dynamics in scattering of large anharmonic systems. (C) 1997 American Institute of Physics.