Coherent effects in solution photochemistry

We have in recent years studied the photolysis of the triiodide ion in solution, using femtosecond laser flash photolysis. The impulsive nature of the excitation sets the dissociation in motion in the form of a coherent wavepacket, and induces coherent vibration in the symmetric stretching vibration on the ground potential of the I₃^-. Using photoselective transient transmission as a tool, all of the important modes of motion in the diatomic ionic product are directly followed in real time after dissociation, including vibrational coherence induced by the kinematics of the process of bond fission. Similarly, primary and secondary geminat recombination is recorded as well. We have now completed a comprehensive solvent dependence of this process, and recorded all said degrees of freedom of triiodide ions dissolved in a series of alcohols and in water. While the heavy alcohols prove to be strong caging solvents, capable of inducing primary geminate recombination with high probability, they are not as effective as the light alcohols in facilitating strong coherence in the vibration of the nascent diiodide. Water is outstanding in that the coherence of diiodide vibration is large in aqueous solution, yet it also induces strong primary coherence in the products. Using both classical and quantum models for this process we will interpret these results. The ground state coherence induced by resonance impulse photoexcitation, which has now been observed in many molecular systems, has been used in our experiments as a novel transient vibrational spectroscopy in order to follow the vibrational relaxation of nascent I₂^- ions following triiodide photodissociation. Transient resonance impulsive stimulated Raman scattering (TRISRS) has been collected at various delays. We will present quantum simulations to demonstrate the utility of this novel method, and interpret our data. Finally, application of the above methods to larger molecules, such as transition metal carbonyls will be discussed.