title: Internal conversion and intersystem crossing pathways in UV excited, isolated uracils and their implications in prebiotic chemistry.
abstract: The photodynamic properties of molecules determine their ability to survive in harsh radiation environments. As such, the photostability of heterocyclic aromatic compounds to electromagnetic radiation is expected to have been one of the selection pressures influencing the prebiotic chemistry on early Earth. In the present study, the gas-phase photodynamics of uracil, 5-methyluracil (thymine) and 2-thiouracil-three heterocyclic compounds thought to be present during this era-are assessed in the context of their recently proposed intersystem crossing pathways that compete with internal conversion to the ground state. Specifically, time-resolved photoelectron spectroscopy measurements evidence femtosecond to picosecond timescales for relaxation of the singlet (1)ππ* and (1)nπ* states as well as for intersystem crossing to the triplet manifold. Trapping in the excited triplet state and intersystem crossing back to the ground state are investigated as potential factors contributing to the susceptibility of these molecules to ultraviolet photodamage.
authors.affiliation: Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA. ullrich@physast.uga.edu.
authors.forename: Hui
authors.initials: H
authors.lastname: Yu
authors.affiliation: Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA. ullrich@physast.uga.edu.
authors.forename: Jose A
authors.initials: JA
authors.lastname: Sanchez-Rodriguez
authors.affiliation: Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, Cleveland, OH 44106, USA.
authors.forename: Marvin
authors.initials: M
authors.lastname: Pollum
authors.affiliation: Department of Chemistry and Center for Chemical Dynamics, Case Western Reserve University, Cleveland, OH 44106, USA.
authors.forename: Carlos E
authors.initials: CE
authors.lastname: Crespo-Hernández
authors.affiliation: Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Mai
authors.affiliation: Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Philipp
authors.initials: P
authors.lastname: Marquetand
authors.affiliation: Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Leticia
authors.initials: L
authors.lastname: González
authors.affiliation: Department of Physics and Astronomy, University of Georgia, Athens, GA 30602, USA. ullrich@physast.uga.edu.
authors.forename: Susanne
authors.initials: S
authors.lastname: Ullrich