UC Berkeley

The interaction of ultraviolet (UV) light with cellular media is a problem of fundamentalinterest in a wide variety of fields. Specifically, this thesis focuses on the elucidation of the electronic dynamics of nucleobases (NBs), the building blocks of DNA, after they have absorbed UV light. Understanding the different photochemical and photophysical pathways that lead to DNA damage, and those that prevent it, is the primary focus of this thesis work. These processes are monitored via time-resolved photoelectron spectroscopy (TRPES) applied to a water microjet. In these experiments, a UV pump pulse is used to excite valence electrons in the NBs, and a time-delayed probe pulse is used to photoionize the excited species. The arrival time of the ejected electrons is detected by a magnetic bottle time-of-flight spectrometer. By varying the time delay between the pump and probe pulse the electronic dynamics occurring on the excited state surface(s) are mapped out.

The NB thymine (T) and its derivatives thymidine (Thd) and thymidine-5’-monophosphate(TMP) are studied by TRPES using two UV pulses, one tuned over 4.74 – 5.17 eV and the other at 6.2 eV. The tunable UV pulse excites valence electrons into the lowest lying ππ* state which is found to decay back into the ground state in ∼400 fs in both T and Thd independent of pump photon energy, and in 670 – 840 fs in TMP with a small amount of pump energy dependence. The longer lifetime of TMP compared to T and Thd is found to be a result of the conformational differences between the molecules in solution by QM/MM calculations done at the XMS-CASPT2//CASSCF/AMBER level. Notably, no signal in any of the three molecules is found to persist for longer than a few ps, contradicting previous experiments that claimed a portion of the initially excited electron population is trapped in an intermediate state for tens of ps before decaying. When using the 6.2 eV pulse as the pump pulse, a band of multiple ππ* states is populated and found to decay into the lowest lying ππ* state within the cross correlation of our laser pulses before decaying back into the ground state.

A major drawback of the previous study is the relatively low energy of the probe pulse. Whilephotoionization signal is observed from the excited ππ* states, no signal is observed from the ground state as it is bound in excess of the probe photon energy. To remedy this problem a new light source was designed and built to generate probe photons in the XUV regime with sufficient energy to ionize both the ground and any excited states in these molecules. XUV pulses are generated at 21.7 eV with very high flux (up to 100 nJ/pulse). This XUV light has been applied to a wide variety of gaseous, liquid, and solvated species as discussed in Chapter 4 of this thesis. The laser-assisted photoelectric effect (LAPE) has been utilized to characterize the temporal profile of the XUV beam. This new source is now being applied to NBs, and a variety of other systems, to further elucidate the dynamics described above and to explore other ultrafast phenomena in solution.