Pump-probe techniques are used in conjunction with a tandem time-of-flight mass spectrometer to investigate the I* * *I-cage recombination dynamics following IF photodissociation in sizeselected 11 (CO,), cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated IT, exhibits a strong cluster size dependence in the range of n= 13-15. Over this limited cluster size range, the absorption recovery time decreases from-40 ps (n<12) to-10 ps (n) 15). In addition, a recurrence is observed at ~2 ps in the absorption recovery of the larger clusters (II = 14-17). This feature results from coherent 1. * *Imotion following photodissociation. Measurement of the absorption recovery with both parallel and perpendicular pump-probe polarizations demonstrates that the pump and probe transition dipoles lie in the same direction. Analysis of the 1, transition dipole directions shows that the coherent motion takes place on the first two repulsive excited potential surfaces. The two-photon photofragment distribution reflects the solvent cage structure as a function of pump-probe delay time.

A 193 nm laser photofragmentation timeofflight mass spectrometric study of CS2 and CS2 clusters

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.Coherent vibrational motion extending over many periods has been observed in isolated diatomic molecules such as I2 (Refs. 8 and 9) and Na,." Here the nuclear dephasing is due predominantly to anharmonicity in the potential surface, and thus the nuclear phase coherence can persist for many vibrational periods. However in a liquid environment, collisions with solvent molecules dominate vibrational dephasing, greatly reducing the time over which coherent nuclear motion can be maintained. For example, Ruhman and co-workers" have observed a few oscillations in the transient absorption of 11 produced by the photodissociation of I; in ethanol. The authors attribute these oscillations, which last for =: 1 ps, to coherent IF vibrational motion in a low vibrational energy level. Scherer et al. l2 report multiple I2 vibrational recurrences lasting a few picoseconds in n-hexane solvent. All of these studies represent nuclear coherences taking place on a single potential energy surface. One possible example of a recurrence following dissociation is the report by Nelson and co-workers13 of what may be coherent Mn-Mn vibrations following excitation of Mn,( CO) i. in ethanol. Otherwise, we believe our results represent the f...