The dissolution of acids is one of the most fundamental solvation processes, and an important issue is the nature of the hydration complex resulting in ion pair formation. We used femtosecond pump-probe spectroscopy to show that five water molecules are necessary for complete dissolution of a hydrogen bromide molecule to form the contact ion pair H+.Br-(H2O)n in the electronic ground state. In smaller mixed clusters (n < 5), the ion pair formation can be photoinduced by electronic excitation.

The ultrafast dynamics of HBr–water clusters have been investigated using pump–probe spectroscopy coupled with reflectron time-of-flight mass spectrometry. HBr clusters, mixed HBr–water clusters, and protonated water clusters are observed in the mass spectra. Dynamic studies reveal that when an HBr chromophore of a cluster with less than five solvent molecules is excited electronically, solvent reorganization occurs to form the solvent separated ion-pair [S. M. Hurley et al., Science 298, 202 (2002)]. The present paper focuses on the influence of clustering on the dynamics of the C and D states of HBr. In addition, further evidence is presented which confirms that complete dissolution of HBr requires five solvent molecules in the isolated species found in complexes comprised of pure water or HBr/H2O mixtures.

Clusters of SO 2 have been interrogated using pump-probe spectroscopy employing a femtosecond laser system coupled to a reflectron time-of-flight mass spectrometer. Upon excitation of the C state of SO 2 with 397.5 nm light and 399.0 nm light, it was found that the photodissociation of SO 2 clusters involves multiple reaction pathways. A mechanism is proposed where the excited cluster either follows a simple exponential decay corresponding to the dissociation of the excited moiety directly from the C state or, alternatively, the excited cluster first fragments and then the excited molecule proceeds through the dissociation process associated with the C state.

The temporal behavior of the photoinduced ion-pair formation process in the (HI)m(H2O)n (n=1-6 for m=1 and n=1-4 for m=2) cluster system has been studied via the coupling between the g 3Sigma- Rydberg and V 1Sigma+ valence states. Comparison of the time constants obtained to those measured in previous experiments for the analogous process in HBr-water clusters, along with a detailed analysis of the signal intensity as a function of laser-pulse power, provides new insight into and confirmation of the previously proposed ion-pair formation mechanism.

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