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 d issociative excitation on the A((3) Pi(1u)) surface. The experiments i nvolve pump-probe measurements with a time resolution of greater than or equal to 150 fs. The experimental signals are reproduced using clas sical molecular dynamics simulations, and the classical Franck approxi mation. The comparison between experiment and simulation allows an una mbiguous interpretation of features in the observed signal as being du e to the initial impulsive stretch of the I-I bond, collision of the a toms with the cage wall, recoil and recombination, and the subsequent coherent oscillations of the nascent I-2 molecule. These detailed obse rvations are possible due to retention of coherence along the I-I coor dinate throughout the caging process. The extent of coherence is dicta ted mainly by the initial impact parameters of the molecule-cage colli sion, which in turn is controlled by the thermal and zero-point amplit udes of lattice vibrations. The caging is well-described as a sudden p rocess, involving a binary collision between I and Kr atoms with nearl y complete energy loss of the I atom upon completion of the first coll ision. Vibrational relaxation of the bound molecule proceeds on the ti me scale of 12 ps. The nontrivial relation between this relaxation tim e and decay rates that may be extracted from experimental transients i s discussed. Although the interplay between the nested A and A' potent ials is not detectable, it is clear that in the studied range of initi al excess energies, 1000-1700 cm(-1), the recombination remains effect ively adiabatic, and does not involve the ground state.