Femtosecond Time-Resolved Dynamics of Geminate and Nongeminate Recombination: Iodine Enclosed in the Nanocavities of a Microporous SiO₂ Modification

Abstract

In this study, we report the ultrafast dissociation and recombination dynamics of I2 enclosed in well-defined cages formed by the electroneutral host framework of the crystalline microporous SiO2 modification decadodecasil 3R (DDR). In this composite, each cage is occupied by one guest molecule, corresponding to a high density of I2. The dynamics are investigated by femtosecond pump−probe spectroscopy with variable probe wavelengths. After excitation into the bound B state, collision-induced predissociation is observed followed by recombination onto the A and A‘ states, where the molecules undergo vibrational relaxation. A kinetic model is applied to simulate the experimental data. The results are compared with results from a former study [Materny et al. J. Phys. Chem. 1996, 100, 18650] where I2 predissociation and caging dynamics in compressed argon were probed along the reaction coordinate. Geminate primary recombination (within the cage) is characterized by rate constants which are nearly identical to those found for I2 in Ar at 200−400 bar. No geminate secondary recombination (diffusive, outside the cage) is observed. However, approximately 10% of the recombinations are assumed to be nongeminate with a rate that corresponds to the motion of iodine atoms into the adjacent cages.