Ultrafast solvation dynamics in a Stockmayer fluid

Dynamics of charge (or charge distribution) solvation is a subject of considerable interest.’ Recent molecular dynamics (MD) computer simulation studies reveal that solvent relaxation occurs in two time regimes.2-7 During the initial regime, which can be described by a Gaussian function, the relaxation occurs very fast (25-150 fs). The fast regime contributes about 60%-80% towards the total relaxation and the motion of the solvent molecules in this regime has an inertial character. The short time regime is followed by a long time regime, which can be described by an exponential function. The motion of solvent molecules in this regime has a diffusional character. The two phase behavior of the solvent relaxation was observed in MD simulations performed in solvents such as water,2 acetonitrile,3 methanol,4 and methyl chloride.5 Very recently the two regime behavior of solvent relaxation was also observed in MD simulations performed in simple polar fluids described by a Stockmayer potential.6*7 The presence of two time regimes in the solvation dynamics was also confirmed by the experimenL8 While the existence of the two regimes in the relaxation is firmly established, the nature of the solvent dynamics in these regimes is not completely clear. Thus, for example, using the results of his simulations in acetonitrile, Maroncelli has assigned the fast regime to a small amplitude independent, inertial (mostly rotational) motion of molecules in the first solvation shelL3 Contrary to this assignment Bagchi and Chandra recently proposed that the short time decay of the relaxation in simple Stockmayer solvents could be described in terms of collective excitation.’ To separate between these two different proposed mechanisms we performed one extra set of nonequilibrium computer simulations in addition to our previous simulation@ where we studied the dynamics of the Stockmayer solvent following a sudden change in the solute charge. In our previous study, we performed three nonequilibrium simulation sets NESI, NESII, and NESIII, in which we followed the time development of the normalized response function S(t) defined as