| Abstract
| - We report studies on several methylated uracils and thymines and thymine−water complexes in the gas phaseusing resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy.Results from two different REMPI experiments provided strong evidence that, after photoexcitation to the S1state, bare molecules were funneled into and trapped in a dark state via fast internal conversion. Lifetimes ofthis dark state were determined to be tens to hundreds of nanoseconds, depending on the internal energy andthe degree of methyl substitution. The mass spectra of hydrated thymine clusters demonstrated dependenceon the excitation wavelength, and the gradual loss of the ion signal with increasing water content across theabsorption region of the S1 state indicated a reduced lifetime of the S1 state by the water solvent. In addition,the lifetime of the dark state also decreased gradually as thymine became more hydrated. On the basis ofthese results, we conclude that, in water solutions, the decay from the S1 state should be essentially fastinternal conversion to the ground state, in agreement with studies from the liquid phase. Our work revealsthat the photostability is not an intrinsic property of these pyrimidine bases. Rather, it is the water solventthat stabilizes the photophysical and photochemical behavior of these bases under UV irradiation.
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