Absorption spectroscopy of adenine, 9-methyladenine, and 2-aminopurine in helium nanodroplets

High-resolution absorption spectra of adenine, 9-methyladenine and 2-aminopurine in helium nanodroplets have been recorded. In contrast to molecular beam experiments, large variations in linewidths are observed for adenine and 9-methyladenine. At the same time, the spectrum of 2-aminopurine remains sharp upon solvation in helium droplets. The line broadening observed for adenine and 9-methyladenine is attributed to a significant decrease of the lifetime of the ¹L_b(ππ*) state and of ¹nπ* levels vibronically coupled to this state. The origin of the lifetime reduction is argued to be related to the increased accessibility of the ¹nπ*/¹L_b(ππ*) conical intersection upon solvation of these molecules in liquid helium.