Excited state spectroscopy and dynamics of isolated adenine and 9-methyladenine

We report electronic spectra of the DNA base adenine and its derivative 9-methyladenine in a molecular beam, using [1+1] resonantly enhanced multiphoton ionisation. In addition an excited state lifetime of adenine was determined by picosecond time-resolved [1+1'] photoionisation. The spectrum of adenine is dominated by a band at 36 105 cm(-1), with several low-frequency bands nearby. A comparison with the spectrum of 9-methyladenine with its origin at 36 136 cm(-1) confirms that the electronic spectrum of adenine is dominated by the pi --> pi* transition of the N-9-H tautomer. Several bands observed before and assigned to a n --> pi* transition were not observed. For the most intense band of adenine at 36 105 cm(-1) we determined an excited state lifetime of 9 ps.