11 photofragmentation / recombination dynamics in size - selected Ir ( COs ) , cluster ions : Observation of coherent I - I - vibrational motion

Molecular clusters offer a unique environment in which the size of a solvent cage surrounding a chromophore can potentially be controlled, allowing one to study the effects of increasing solvation on reaction dynamics.‘” If charged species are employed, size selection of the initial cluster is easily accomplished with standard mass spectrometric techniques.1*2 Studies of the 720 nm IF ( C02), photofragmentation and the subsequent recombination of 11 showed6 no caging for n<5, increasing steadily from n = 6 to complete caging at n = 16, where the first solvent shell is complete. Preliminary picosecond pump-probe experiments on 11 (CO,), demonstrated7 that the absorption recovery following 11 photodissociation depended upon cluster size. The recovery time, 10 ps for n=16 and 30 ps for n =9, was attributed to the time required for the recombination and subsequent vibrational relaxation of the photodissociated IT. The time resolution was only 6 ps, and scatter in the data allowed only a singleexponential fit to the data.7 The experiments reported here represent substantial improvements in both aspects, and exhibit a clear recurrence at 2 ps in the photodissociated 1, (CO,) 16 absorption recovery. long-lasting coherence in this cluster ion lies in the strong ion-solvent bondI ( 150 meV/CO1) and the low internal temperature ( 40 K) . These two conditions produce a much more restricted set of initial geometries than is the case in the liquid studies, and would appear to allow the initial nuclear motion coherence to survive for 2-3 ps in spite of the ensemble averaging inherent in the observations. Another important feature is the strong C02-C02 bonding interaction, giving the first solvent shell some of the character of a soft “net.” In any event, the motion rapidly loses coherence as the cluster disintegrates.