Tunable soft ionization coupled to mass spectroscopy is a powerful method to
investigate isolated molecules, complexes and clusters and their spectroscopy and
dynamics.[1-4] Fundamental studies of photoionization processes of biomolecules
provide information about electronic structure of these systems. Furthermore
determinations of ionization energies and other properties of biomolecules in the gas
phase are not trivial, and these experiments provide a platform to generate these data.
We have developed a thermal vaporization technique coupled with supersonic
molecular beams that provides a gentle way to transport these species into the gas
phase. Judicious combination of source gas and temperature allows for formation of
dimers and higher clusters of the DNA bases. The focus of this particular work is on the
effects of non-covalent interactions, i.e., hydrogen bonding, stacking, and electrostatic
interactions, on the ionization energies and proton transfer of individual biomolecules,
their complexes and upon micro-hydration by water.[1, 5-9]
We have performed experimental and theoretical characterization of the
photoionization dynamics of gas-phase uracil and 1,3-methyluracil dimers using
molecular beams coupled with synchrotron radiation at the Chemical Dynamics
Beamline[10] located at the Advanced Light Source and the experimental details are
visualized here. This allowed us to observe the proton transfer in 1,3-dimethyluracil
dimers, a system with pi stacking geometry and with no hydrogen bonds[1]. Molecular
beams provide a very convenient and efficient way to isolate the sample of interest
from environmental perturbations which in return allows accurate comparison with
electronic structure calculations[11, 12]. By tuning the photon energy from the
synchrotron, a photoionization efficiency (PIE) curve can be plotted which informs us
about the cationic electronic states. These values can then be compared to theoretical
models and calculations and in turn, explain in detail the electronic structure and
dynamics of the investigated species [1, 3].