Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous CS_2

摘要

The photolysis of aqueous CS_2 has been studied using subpicosecond transient absorption spectroscopy. CS_2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS_2 dissociates into CS+S. However, 93% +- 2% of the fragments geminately recombine on the electronic ground state potential of CS_2 within a few picosecond leaving only 7% +- 2% of the CS + S fragment separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS_2. The vibrationally excited CS_2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4 +- 1 and 33 +- 7 ps. The rotational reorientation time of ground state CS_2 is 6 +- 1 ps, suggesting a surprisingly strong interaction between the neutral, nonpolar CS_2 and the surrounding water molecules.