Non-Adiabatic Dynamics in Liquid Jets Studied by Time-Resolved XUV Photoelectronic Spectroscopy

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Daniel Neumark and his research group at the University of California-Berkeley are carrying out experiments that probe the interaction of nucleic acid constituents with ultraviolet light. All nucleic acids have strong absorption bands in the ultraviolet at photon energies that are sufficient to dissociate these species into reactive free radicals, a process that would have significant negative biological consequences. However, electronic relaxation is known to compete with dissociation, rapidly channeling the electronically excited nucleic acids to their ground electronic states and releasing the excess energy harmlessly as heat. The team led by Prof. Neumark will perform novel time-resolved experiments to map out the pathways for electronic relaxation of nucleic acids in aqueous solution from the instant of photoexcitation to the ultimate production of ground state molecules, thus providing new insights into the electronic relaxation of these species. The results of these studies are not only of interest in fundamental chemical physics but also have potential impact in radiation chemistry and biology. The broader impacts of the project also include advanced training opportunities for students, including mentoring efforts that seek to enhance the advancement of female scientists and the recruitment of research students from groups that are underrepresented in science. Time-resolved photoelectron spectroscopy (TRPES) in liquid water microjets will be carried out in which solute molecules are excited with ultraviolet pump pulses and ionized using femtosecond extreme ultraviolet (XUV) pulses at 22 eV. These experiments probe the non-adiabatic relaxation dynamics of mono- and di-nucleotides subsequent to photoexcitation of their pi-pi* transitions around 4.7 eV. The energy of the 22 eV probe photons is sufficiently high to ionize these species not only in the initially excited states formed by UV excitation, but also in any electronic states that serve as intermediates in the overall relaxation mechanism as well as in the ground state. Hence, these experiments have the potential to provide a complete dynamical mapping of the electronic states that play a role in the relaxation of photoexcited mono- and di-nucleotides. Specifically, the research team will examine whether photoexcited nucleobases relax through optically dark states and also will probe the formation of transient exciplexes in photoexcited dinucleotides. A secondary goal of this project is to generate femtosecond pulses around 100 eV, a photon energy sufficient to eject core electrons from halogens and transition metals. Initial steps toward this capability will be undertaken during the current grant period toward the longer-term goal of enabling transformative new measurements of the excited-state dynamics of solvated molecules using core ionization. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.