UC Berkeley

Femtosecond time-resolved photoelectron spectroscopy (TRPES) in liquid microjets is a powerful tool for elucidating the ultrafast photoinduced dynamics of species in the condensed phase. A pump pulse first excites the molecule of interest and is then followed by a probe pulse, which detaches the nascent electron distribution to vacuum at varying time-delays. The transient lifetimes, solvation timescales, and binding energies of the molecule can then be elucidated by tracking the time-evolving photoelectron distribution with a magnetic bottle time-of-flight spectrometer.

Of particular interest to many fields of chemistry and biology is the process by which DNA and its nucleic acid (NA) constituents shed excess energy imparted by UV radiation. Beyond their intrinsic interest, study of UV-photoexcited NA constituents can also provide fundamental insights into the non-adiabatic processes of organic molecules generally. DNA components are known to undergo rapid de-excitation through a slew of conical intersections that involve ring-puckering modes of the nitrogenous base. Importantly, the local environment and small structural changes in the NA constituent are known to drastically affect these dynamics. For this reason, a bottom up approach to DNA photophysics is necessary.

This dissertation explores the photodeactivation of the NA constituents adenosine and adenosine monophosphate in aqueous solution. In a series of pseudo-degenerate experiments, 4.69-4.97eV and 6.20eV photons were used as both the pump and probe pulses. The lowest ππ* excited state was populated by photons ranging in energy from 4.69-4.97eV, and this state was found to decay via internal conversion to vibrationally hot ground state on a sub-ps timescale. Another non-adiabatic channel was seen at an excitation energy of 6.20eV, and was tentatively assigned to the decay of a higher-lying ππ* excited state.

These experiments mark the first reported use of a 6.20eV pulse to study the photoinduced dynamics in NA constituents with TRPES in liquid microjets. As a probe pulse, the 6.20eV pulse provides a fuller picture of the excited state relaxation dynamics. When used as a pump, this pulse is posited to interrogate different excited states than previously studied by others. Regardless, in both cases, information about the ground electronic state is energetically inaccessible.

The lack of information regarding the ground state dynamics of NA constituents, and, in fact, many solvated species, remains an outstanding issue for TRPES in liquid microjets. The second half of this dissertation focuses on remedying this through the implementation of a new XUV source. High harmonic generation in a semi-infinite gas cell will be used to produce a femtosecond probe pulse ranging in energy from 20-100eV. With a more energetic probe, new regimes of fundamental physical chemistry in the condensed phase will be accessible.