Multiphoton ionization with a tunable UV laser allows measuring excited state adiabatic energies of DNA bases
The group of Samuel Eden at the Open University Milton Keynes is expert in the field of irradiation of biomolecules and clusters by lasers and electrons. In particular, they probe the influence of nanohydration and clustering on the fragmentation of DNA and RNA bases in the gas phase. Recently, they discovered a microsecond-timescale dissociation channel from isolated uracil and thymine after UV-multiphoton ionization, with an unexplained wavelength threshold. The aim of this STSM was to investigate further this intriguing observation.
We measured the mass spectrum of thymine after UV multiphoton ionization as a function of wavelength, and obtained a threshold of 224 ± 0.5 nm (5.53 ± 0.02 eV) for the metastable dissociation channel (HCNO loss). Our hypothesis is that this threshold corresponds to accessing the S1 state with vibrational excitation matching the energy difference between the ionic ground state (8.82 ± 0.03 eV) and the dissociative ionic state leading to HNCO loss (10.70 ± 0.05 eV) 1, yielding an S1 adiabatic energy of 3.65 ± 0.07 eV. This value agrees with the most recent DFT calculation: 3.72 eV 2. Preliminary results on Adenine-Thymine clusters also suggest a stabilizing effect on the S1 state due to clustering.
Furthermore, we also carried out experiments on cytosine, and observed metastable dissociation, thus demonstrating that measurement of metastable channel wavelength threshold is a possible tool to measure S1 adiabatic energies. We will write a journal article in early 2015 including data from the STSM as well as the host group’s follow-up measurements. We will also arrange further collaborative experiments in the near future with the aim of probing electronic excitation and ionization induced processes in isolated and clustered amino acids.
(1) Jochims, H. W.; Schwell, M.; Baumgärtel, H.; Leach, S. Chemical Physics 2005, 314, 263.
(2) Etinski, M.; Marian, C. M. Physical Chemistry Chemical Physics 2010, 12, 4915.