The spectroscopy and dynamics of isolated biological molecules

The isolation of building blocks for a biological system in the gas phase or within a theoretical model for the electronic structure allows experimental and theoretical techniques in spectroscopy and reaction dynamics to be used to study important molecules in nature. In these examples, the absence of solvent or a host has given insight into the intrinsic properties of the molecular subunit.

Recent experiments in our group have used meso-tetraphenylporhyrin as a model system to study the photochemistry of isolated porphyrins in a vacuum. The large organic molecules are seeded into an inert carrier gas by evaporation from an oven located in the expansion region of a molecular beam. The mass spectrum for the prophyrin can then be obtained by laser ionization followed by time-of-flight detection of the ion-signal intensity.

Resonance enhanced two-photon ionization (R2PI) is currently being used to obtain information on the photoexcited states of porphyrins under isolated conditions. We have a range of different pulsed valves that have been designed to generate cold molecular beams containing various biological molecules. The low temperature of these molecules will reveal structure in the R2PI spectrum corresponding the vibrational levels in the excited state. In addition, these gas-phase measurements provide an insight into the initial evolution of the electronic states involved in the photoexcitation of biological molecules and an improved understanding of the radiative and non-radiative decay pathways. Eventually, we will extend this work to examine the pathways for energy transfer between these excited states in biological molecules to small molecules such as oxygen.