REMPI Spectroscopy of Jet-Cooled Guanine

There have been many theoretical studies of vibrational and electronic states of DNA bases. These are complicated by multiple lone pair electrons, limited symmetry, and possible tautomerism. Experimentally there have been many reports of absorption spectroscopy, IR data, and Raman spectroscopy. Samples have been in the form of vapor, solutions, polycrystalline material, single crystals, and cold matrices. Gas-phase studies can offer the advantage of eliminating intermolecular interactions. However, if gas-phase spectroscopy is to provide vibrational resolution, it requires entrainment of the molecules in a supersonic expansion in order to reduce their internal temperature. Brady et al. studied the pyrimidine bases, uracil and thymine, in a supersonic jet and obtained only broad spectra. Brown et al. have reported microwave spectra of all bases except guanine and assigned tautomers. Kim et al. have formed clusters of nucleotides and water molecules in a jet and measured their ionization potentials. Caminati et al. have recorded the millimeter wave spectrum of jet-cooled purine. Viant et al. have used a slit nozzle to obtain high-resolution IR spectra of jet-cooled uracil. While the latter work reveals details of the molecular ground state, we employ REMPI spectroscopy of jet-cooled DNA base molecules to probe the first excited electronic state. After exploring the basic building blocks we aim to extend these studies to oligonucleotides and to their hydrogen-bonded clusters. Since those cannot be vaporized intact by heating, we have based our approach on laser desorption.