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The UV chromophores in DNA are the nucleic bases themselves, and it is their photophysics and photochemistry that govern the intrinsic photostability of DNA. Because stability is related to the conversion of dangerous electronic to less-dangerous vibrational energy, we study ultrafast electronic relaxation processes in the DNA base adenine. We excite adenine, isolated in a molecular beam, to its pipi* state and follow its relaxation dynamics using femtosecond time-resolved photoelectron spectroscopy. To discern which processes are important on which timescales, we compare adenine with 9-methyl adenine. Methylation blocks the site of the much-discussed pisigma* state that had been thought, until now, minor. Time-resolved photoelectron spectroscopy reveals that, although adenine and 9-methyl adenine show almost identical timescales for the processes involved, the decay pathways are quite different. Importantly, we confirm that in adenine at 267-nm excitation, the pisigma* state plays a major role. We discuss these results in the context of recent experimental and theoretical studies on adenine, proposing a model that accounts for all known results, and consider the relationship between these studies and electron-induced damage in DNA.

We present femtosecond time-resolved photoelectron spectra of adenine in a molecular beam, recorded at pump wavelengths of 250, 267, and 277 nm. This leads to initial excitation of the bright S2(pipi*). Close to the band origin (277 nm), the lifetime is several picoseconds. Higher vibronic levels (267 and 250 nm excitation) show much shorter lifetimes of t < 50 fs, and we observe strong coupling between S2(pipi*) and S1(npi*). Rapid internal conversion (t < 50 fs) populates the lower lying S1(npi*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(pisigma*) state, previously proposed as an ultrafast relaxation pathway from S2(pipi*).