Dynamics in Anion Clusters and Liquid Jets Using Ultraviolet and Extreme-Ultraviolet Time-Resolved Photoelectron Spectroscopy

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In this project, funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Daniel Neumark from the University of California, Berkeley uses ultrafast laser pulses to examine chemical reactions in the gas phase and in aqueous solution. The gas phase experiments are aimed at understanding the interactions between electrons excited by light and nucleic acid (NA) constituents; this interaction is important in radiation biology and plays a key role in cell mutation, cancer, and cell death. Complementary experiments in aqueous solution are focusing on the changes of nucleic acid components subsequent to exposure to ultraviolet light. In these experiments, ultraviolet light excites the electrons in the molecules, and the stability of the molecules depends on how rapidly they can convert the light energy into vibrational energy (which can then be dissipated by the surrounding solvent molecules before reactions occur). The experiments are providing new insights into these relaxation dynamics. In addition to its implications for radiation biology and other areas of medical technology, this research project provides training for the next generation of scientists. Professor Neumark promotes the training of female scientists, with the ultimate goal of increasing the representation of women scientists in academia.This project focuses on complementary studies of chemical reaction dynamics in gas phase ion complexes and liquid water microjets by means of femtosecond time-resolved photoelectron spectroscopy. The goal of the gas phase experiments is to investigate electron-induced chemistry in nucleobases, nucleotides, and nucleosides. Starting with binary complexes of iodide anions (I-) bound to a nucleic acid constituent, an electron is transferred from the I- to the nucleic acid (NA) constituent through excitation by an ultraviolet (UV) femtosecond pump pulse. The subsequent dynamics are followed by photodetaching the excess electron with a femtosecond probe pulse and measuring the resulting photoelectron kinetic energy distribution. In the liquid water microjet experiments, femtosecond high harmonic generation (HHG)experiments are being developed as a very general probe of reaction dynamics in solution via photoemission at photon energies as high as 100 eV. Here, for example, a nucleic acid constituent is excited by a UV pump pulse, and time-resolved photoelectron spectra obtained by ionization with the HHG probe pulse tracks the relaxation dynamics of the photoexcited species all the way back to its ground state. One can also perform time-resolved core level photoemission experiments that rely on the 4d electron binding energy of atomic iodine exhibiting a measureable chemical shift that depends on its overall charge and chemical bonding environment. This method is being used to follow electron recombination with iodine atoms in solution subsequent to charge-transfer-to-solvent excitation.