Mechanism for DNA base photodamage
We explore the excited state dynamics of DNA building blocks subsequent to UV excitation using both spectrally resolved broadband TA (Transient Absorption) and TR-PES (Time Resolved PhotoElectron Spectroscopy) in aqueous solution. Our main focus is to establish ultrafast relaxation pathways and disentangle competing decay channels by varying excitation wavelength Presently, we are looking at the role of different states (e.g., ππ*, nπ*) and the effect of solvent on the excited state dynamics of adenine and guanine.
TR-PES provides greater mechanistic insight as to the electronic character of intermediate states. With a future EUV (Extreme UV) source available for probing, the rearrangement of the occupied valence orbitals will also be possible.
Following the evolution of orbitals during transition metal charge transfer and spin flipping
Electronically excited aqueous TM complexes relax through several intermediate configurations each with different orbital configurations, formal oxidation states and spin states. Determination of the energies and lifetimes of these intermediates is crucial for understanding chemical reactivity. The advantage of PE spectroscopy of solvated transition metal complexes is that its valence spectra are characteristic of the ligands, charge and especially of the spin state of the central ion. In general, the orbital energy is a new way to probe changes associated with excited state charge transfer, electronic relaxation and overall solvation dynamics. For example, after exciting to LMCT (ligand to metal charge transfer) state with UV light , we can follow the ultrafast photoreduction of metal center of ferricyanide (FeIII) to FeII and the subsequent relaxation back to its ground state.