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.

Presented here are femtosecond pump-probe studies on the watersolvated 7-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392-398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer. C luster science studies have long been utilized to yield unique perspectives of microscopic properties related to the bulk condensed phase (1). This approach involves the investigation of a broad spectrum of clusters ranging from isolated species to the study of fully solvated species, thus illustrating the progression from the gas phase to the condensed phase. In reference to our specific experiments as an example, we have been able to study the excited state double-proton transfer (ESDPT) of the 7-azaindole (7-Aza) dimer under conditions ranging from an isolated dimer to a state of solvation where the hydrogen-bonded dimer is clustered with as many as nine water molecules. As the number of water molecules on the nonreactive dimer increases, we begin to see evidence that the dimer molecule is behaving more as it would in a fully solvated condensed-phase environment. In support of these findings, it has been reported that clusters with as little as six waters begin to show liquid-like properties (2-4).The model DNA base pair 7-Aza has proven to be an interesting and enlightening species for study in both the gas and condensed phases. Of utmost interest is the double-proton transfer that the 7-Aza dimer undergoes on excitation to the S 1 state. This ESDPT was first observed in solution by Kasha and coworkers (5). Later, Kaya and coworkers performed extensive supersonic jet spectroscopic studies on the 7-Aza monomer, dimer, and solvated forms of these species (6-9). The first direct determination of the rates of the double-proton transfer was made in the gas phase by Zewail and c...

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.

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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.