THE BREAKING AND REMAKING OF A BOND - CAGING OF I-2 IN SOLID KR

The caging of I-2 in solid Kr is followed in real-time following its dissociative excitation on the A((3) Pi(1u)) surface. The experiments involve pump-probe measurements with a time resolution of greater than or equal to 150 fs. The experimental signals are reproduced using classical molecular dynamics simulations, and the classical Franck approximation. The comparison between experiment and simulation allows an unambiguous interpretation of features in the observed signal as being due to the initial impulsive stretch of the I-I bond, collision of the atoms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I-2 molecule. These detailed observations are possible due to retention of coherence along the I-I coordinate throughout the caging process. The extent of coherence is dictated mainly by the initial impact parameters of the molecule-cage collision, which in turn is controlled by the thermal and zero-point amplitudes of lattice vibrations. The caging is well-described as a sudden process, involving a binary collision between I and Kr atoms with nearly complete energy loss of the I atom upon completion of the first collision. Vibrational relaxation of the bound molecule proceeds on the time scale of 12 ps. The nontrivial relation between this relaxation time and decay rates that may be extracted from experimental transients is discussed. Although the interplay between the nested A and A' potentials is not detectable, it is clear that in the studied range of initial excess energies, 1000-1700 cm(-1), the recombination remains effectively adiabatic, and does not involve the ground state.