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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...

Ultrafast pump-probe spectroscopic studies on the hydrated 7-azaindole monomer are presented. The experiments provide evidence that excited-state proton transfer in the gas-phase hydrated 7-azaindole monomer may be possible, as predicted by theory, and can occur by means of a water proton bridge. As explained in the text, full tautomerization may not be occurring, but rather the transfer of a proton from the 7-azaindole monomer to the solvating water molecules. A calculated geometry for the structure of 7-azaindole clustered with four waters is presented and used to explain the rate increase observed in this species compared to the 7-azaindole monomer clustered with two and three waters. Experimental SectionHydrated clusters of 7Aza were formed by means of supersonic expansion into a vacuum. Vapor from 7Aza (Sigma), † Part of the special issue "C. Bradley Moore Festschrift".

Direct evidence is reported of the connectiveness of charged clusters with highly charged species (N4+, N3+, and N2+) produced upon the interaction of molecular ammonia clusters with an intense femtosecond laser beam (∼1015 W/cm2 at 120 fs). The value of covariance analysis as a general technique for studying dynamical processes in clusters is demonstrated through elucidating the details of various Coulomb explosion events. Positive covariance determinations identify concerted processes such as the concomitant explosion of protonated cluster ions of unsymmetrical size, while anticovariance mapping is exploited to distinguish competitive reaction channels such as the production of highly charged nitrogen atoms formed at the expense of the protonated members of the cluster ion ensemble. The present study demonstrates the great potential which covariance analysis offers in identifying the precursors and products of dynamical events in clusters and in the present case provides further support to the ignition model as a mechanism contributing to the initial ionization events in clusters leading to highly charged atomic species.