Electronic structure of adenine and thymine base pairs studied by femtosecond electron-ion coincidence spectroscopy

Abstract

Femtosecond pump - probe spectroscopy was combined with photoelectron - photoion coincidence detection to investigate the electronic structure and dynamics of isolated adenine (A) and thymine (T) dimers and the adenine-thymine (AT) base pair. The photoelectron spectra show that ππ* and ππ* states are only weakly perturbed in the hydrogen-bound dimers as compared to the monomers. For cationic base pairs with internal energies greater than 1 eV, we observed considerable cluster fragmentation into protonated monomers. This process selectively removed signals from the ππ* → n-1 ionization channel in all dimers. The photoelectron spectra are compared to time-resolved mass spectra and confirm the assignment of short-lived ππ* and nπ* populations in the adenine, thymine, and mixed AT dimers.