Quantum simulations of energy transfer and state-to-state transitions in collision of an atom with a large anharmonic cluster: He+Ar₁₃

A time-dependent self-consistent field approach is used to simulate a He atom colliding with an Ar₁₃ cluster. Direct energy transfer during the collision, and energy redistribution among the vibrational degrees of freedom of the anharmonic cluster following the collision, are studied. 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 interpreted in terms of a direct collision, followed by post-collision energy redistribution in Ar₁₃, a description that appears only when the cluster vibrations are not described by the eigenstates of this system. A time scale of one picosecond is found for the post-collision intracluster energy distribution. (2) The long-time final state distribution of Ar₁₃ is less state selective than the distribution immediately after the impact, but it is also not completely statistical. (3) There are state-to-state transitions having cross sections of observable magnitude. (4) The dominant transitions are those involving zero, one, and two "phonon" excitations. Some of the "two phonon" excitations have cross sections comparable to strong "single phonon" transitions. (5) Different types of modes show different propensities for excitations in the collision, in close relation to the geometric character of the 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.