EXCITED STATES OF NUCLEIC ACID BASES

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The location, identity, and lifetime of low-lying excited states of nucleic acid bases will be investigated by combining laser photoionization and laser photoelectron spectroscopy with a spray-jet, which allows supersonic expansion of nonvolatile and thermally labile compounds. Both one-color and two-color ionization schemes, involving various time delay between pump and probe lasers, will be used to measure the spectra and lifetimes of the excited states. The question of the nature (nPi or PiPi) of the lowest excited singlet state, and the number of low-lying PiPi states, etc. can be probed by analyzing, and comparing, the photoionization and photoelectron spectra, as demonstrated by our recent experiments on isoquinoline. The state assignments (in bare and unsubstituted bases) can be aided by studying spectral consequences of methyl substitution and hydrogen bonding (with alcohol and water), which are known to blue shift nPI* relative to PiPi* states. The effects of hydrogen bonding on the spectra are also important for the understanding of electronic structure of nucleic acid bases in biologically more relevant conditions of aqueous solutions. Finally, the study of the variation of the lowest-excited-singlet-state (S1) lifetimes with energylevel dispositions in different bases, and with methyl substitution and hydrogen bonding, will be carried out to obtain information concerning the possible origin of the ultrafast internal conversion in these molecules. The results of these experiments can lead to better understanding of spectroscopy and photophysics of nucleic acid bases, which remain incomplete and fragmented.