E. S. Wisniewski

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...

A new time-of-flight gating method for analyzing kinetic energy release in Coulomb exploded clusters: application to water clusters

Abstract A new method is presented for the analysis of high energy multicharged cations produced by Coulomb explosion in a time-of-flight mass spectrometer. The technique employs energy gating through a manipulation of electric potentials in the Wiley-McLaren lens assembly, and the use of a reflecting electric field. Water clusters of sizes up to about 20 molecules were irradiated with femtosecond laser pulses to generate protons and multicharged oxygen atoms. Protons with energies in excess of 3 kV are reported as well as O4+ ions with energies in excess of 13 kV.

Solvation effects on electronic excited states of methanol: A study of neat and mixed methanol/water clusters

Femtosecond pump-probe spectroscopic studies on mixed methanol/water clusters are presented. Two-photon absorption of light was employed to generate methanol molecules in the 3p Rydberg excited state, with transfer to the 3s Rydberg state during the ensuing dynamics. The experiments provide evidence that excited state hydrogen transfer occurs between a methanol molecule, which undergoes hydroxyl dissociation following electronic excitation, and a water molecule, leading to the involvement of a metastable H3O species.

Comparison of methyl and hydroxyl protons generated in a Coulomb explosion event: application of a time-of-flight gating technique to methanol clusters

Abstract A time-of-flight (TOF) mass spectrometry gating technique is applied to a study of methanol clusters subjected to ionizations via intense femtosecond laser pulses. The resulting high charged species (C2+, C3+/O4+) acquire large amounts of kinetic energy resulting from Coulomb repulsion of multicharged atomic ions that reside in close proximity to one another. Protons which are of two kinds, methyl and hydroxyl, also acquire large amounts of kinetic energy. When compared with protons generated from the Coulomb explosion of water clusters ((H2O)n, n≤20), protons from methanol clusters ((CH3OH)n, n≤10) acquire less overall average kinetic energy, which is in agreement with earlier findings that suggest greater clustering yields higher energy. Interestingly, despite the lower average kinetic energy released, the methanol protons peak at a higher value of energy than those generated in the water cluster system, an effect attributed to the presence of both methyl and hydroxyl groups.

Calculation to determine the mass of daughter ions in metastable decay

Abstract A new method is presented for determining the mass of products resulting from metastable decay in a reflectron time-of-flight mass-spectrometer. The validity of the calculation was confirmed through comparison to experimental values obtained while studying a water cluster and a methanol cluster system.