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The studies presented herein elucidate details of the Coulomb explosion event initiated through the interaction of heterocyclic clusters with an intense femtosecond laser beam (⩾1 PW/cm2). Clusters studied include 7-azaindole and pyridine. Covariance analysis verifies that the fragmentation channels are competitive. Kinetic-energy analyses, from experiment and simulation, suggest that Coulomb exploded fragments are created with varying amounts of energy and have a strong mass-to-charge relationship. Backward-ejected protons are found to impact the repeller and eject adsorbed protons from the surface. Moreover, delayed fragmentation is suggested by fast-Fourier transformation of a proton time-of-flight mass spectrum and confirmed by deconvoluting the aforementioned signal through intensity decrements. Voltage gradient, laser power, and microchannel plate detector studies yield insight into the solvation effect of clusters in the Coulomb explosion event. Conceptually, the dynamic charge resonance enhanced ionization (Dynamic CREI) model best explains these results of heterocyclic Coulomb explosion.

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.

A review of the Coulomb explosion imaging method is presented along with a discussion regarding its use in determining the mechanism of the excited state double proton transfer (ESDPT) reaction of 7-azaindole dimer. A comprehensive discussion of the tautomerization process is outlined to fully explain the stepwise mechanism exhibited in our studies. Finally, recent comments are addressed relating to our technique and the interpretation of our results obtained by the method. Our findings are shown to be in excellent agreement with those of Zewail and co-workers who employed a different method of following the evolution of the stepwise ESDPT.

Presented here are femtosecond pump-probe studies on the water-solvated 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.

A new concept to study the dynamics of chemical reactions in real time is presented using the phenomenon of Coulomb explosion to arrest and directly interrogate the intermediates in a chemical reaction. This technique is employed to study the tautomerization reaction of the model DNA base pair 7-azaindole, providing direct mass spectral evidence of a stepwise versus a concerted mechanism for the proton transfer upon excitation into the S1 state.