The photophysics of DNA and its constituents has attracted considerable interest for many years, with the central aim to understand electronic excitation and relaxation pathways that can initiate reactivity and lesions. Further interest relates to the mechanisms underpinning the remarkable photo-stability of DNA bases and its possible evolutionary implications. Experimental studies of isolated biomolecules can precipitate relatively clear photophysical interpretations, while equivalent measurements on hydrogen-bonded complexes enable closer analogies to be drawn with biological environments where different isomeric forms, intermolecular energy transfer processes, and reactivity can be significant.

This experiment combines a compact supersonic jet apparatus with nanosecond-timescale UV multi-photon ionization (MPI) time-of-flight mass spectrometry to probe the effects of clustering on the electronic excited state dynamics and ionization pathways of biomolecules. Photon orders (indicating the number of photons absorbed in the MPI process) can be determined efficiently by analysing the production of specific ions and cluster ions as a function of laser fluence on a pulse-by-pulse basis. The formation of hydrated clusters has been observed to alter photon orders and the fragmentation patterns of multi-photon ionized DNA and RNA bases. In particular, we interpret these effects in terms of excited state isomeric transitions in hydrated complexes and the quenching of intra-molecular vibrational excitation in clusters via conversion to inter-molecular vibrational modes.

Left: Bartlomiej Barc in front of the multi-photon ionization - time-of-flight mass spectrometry experiment. Right: Michal Ryszka optimizing the UV laser beam.