Electronic relaxation dynamics in DNA and RNA bases studied by time-resolved photoelectron spectroscopy

We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA and RNA bases adenine, cytosine, thymine, and uracil in a molecular beam. We discuss in detail the analysis of our adenine TRPES spectra. A global two-dimensional fit of the time and energy-resolved spectra allows for reliable separation of photoelectron spectra from several channels, even for overlapping bands. Ab initio calculations of Koopmans' ionization correlations and He(I) photoelectron spectra aid the assignment of electronically excited states involved in the relaxation dynamics. Based upon our results, we propose the following mechanism for electronic relaxation dynamics in adenine: Pump wavelengths of 250, 267 and 277 nm lead to initial excitation of the bright S2(ππ*) state. Close to the band origin (277 nm), the lifetime is several picoseconds. At higher vibronic levels, i.e. 250 and 267 nm excitation, rapid internal conversion (τ < 50 fs) populates the lower lying S1(nπ*) 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(πσ*) state, previously proposed as an ultrafast relaxation pathway from S2(ππ*). We present preliminary results from TRPES measurements of the other DNA bases at 250 nm excitation.