Ultrafast reaction dynamics in different environments

We review recent work done on the femtosecond time-resolved investigation o f the influence of solvation on elementary molecular and reaction dynamics of solvated molecules. Using the photo dissociation reaction of I-2 and NaI molecules as models, the solvation effect is studied for varying environme nts. By adding inert gases (He, Ar, Nz) at pressures ranging from 0 to 3000 bar, the dynamics is studied starting from isolated molecules, going stead ily to solvent-like conditions. Clearly different reaction behaviour can be found when comparing I-2 and NaI. While I-2 molecules predissociate due to the collisions with the surrounding gas particles and afterwards recombine due to the caging effect, NaI is stabilized in its adiabatic potential wel l formed by the first excited electronic state and the ionic ground state. I-2 is also embedded in a solid matrix, formed by a crystalline porosil. Fo r I-2 in both cases the recombination dynamics is studied by varying the pr obe wavelength. The experimental data are discussed using a theoretical mod el. For I-2 in a high pressure gas environment, geminate primary and second ary recombination can be detected; in the porosils also nongeminate process es contribute on a very short time scale.