D. E. Folmer

Arresting intermediate states in a chemical reaction on a femtosecond time scale: proton transfer in model base pairs

Abstract 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.

Excited state double proton transfer in the 7-azaindole dimer revisited

Abstract 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.

Dynamics of Coulomb explosion and kinetic energy release in clusters of heterocyclic compounds

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 i...

Femtosecond excitation dynamics of acetone: Dissociation, ionization, and the evolution of multiply charged elemental species

Recent femtosecond pump–probe experiments have suggested that a stepwise dissociative mechanism is operative for acetone excited to Rydberg states and upper regions of the mixed singlet/triplet state. The present work focuses on the excitation of acetone and acetone clusters to the 3d (or perhaps 4s) electronic intermediate state in order to further explore the operative dissociation mechanisms and the effects of solvation (clustering). As reported herein, results from femtosecond pump–probe experiments suggest that the availability of additional vibrational modes in clusters, where internal energy may be dispersed, increases the fraction of acetyl intermediates which remain behind the barrier to dissociation into methyl and CO fragments. At progressively higher laser fluences, multiply charged elemental carbon and oxygen ions abruptly appear. Interestingly, the extent of their formation is observed to depend on both laser intensity and the relative time delay between the pump and probe laser beams respon...

Clusters In Intense Laser Fields: Multiple Ionization and Coulomb Explosion

Findings of the production of highly charged atomic species (e.g. Xe20+, Kr17+, I17+, Ar8+, N5+, O5+, and C4+) resulting from the interaction of intense laser fields (up to ∼1015 W/cm2) with atomic and molecular clusters, are reported herein. The processes are also investigated using ultrafast pump‐probe techniques, showing distinct beating patterns for the ionization structure in the molecular systems. A comparison of our results with predictions of several different theoretical models provides strong support for the ionization ignition mechanism.