Short-time charge motion in He-n(+) clusters

The mechanism of short-time charge delocalization in He-n(+) clusters, n = 3, 14, 40, and 112, is studied using time-dependent wave packets for the el ectronic states for fixed nuclear positions chosen randomly from quantum Mo nte Carlo distributions for the neutral clusters. A DIM (diatomics-in-molec ules) potential is used to calculate electronic eigenvalues and eigenvector s for the positively charged clusters, and a coherent superposition of stat es is created to initially localize the charge on a single atom. The charge distribution as a function of time is then calculated by propagating the i nitial wave function for 200 fs, during which little nuclear motion will oc cur. The charge motion is separated into three categories. For initial ioni zation of relatively isolated atoms in the cluster, the charge is only weak ly coupled to other atoms, and there is little charge motion in 200 fs. For initial positions at which two or three atoms are coupled, but isolated fr om the rest of the cluster, the charge resonates among these atoms. For ini tial ionization positions at which more than three atoms are coupled, the c harge can effectively delocalize over that part of the cluster. For the fir st two types of positions, which tend to occur near the surface of the clus ter, He-2(+) is likely to form and the cluster will fragment before signifi cant charge migration can occur. Charge delocalization tends to occur for i nitial ionization at the third type of location in the core of the cluster. For the initial positions that lead to charge delocalization, the distance and direction of the delocalization was characterized (C) 1998 American In stitute of Physics. [S0021-9606(98)00848-4].