Femtosecond Photoelectron Imaging of Anions

Abstract

Several recent results of a time-resolved photoelectron imaging experiment are presented. Following a broad introduction into the area of femtochemistry and time-resolved photoelectron spectroscopy, a detailed description of the spec- trometer is given. This utilises an electrospray ionisation source, coupled to an electrostatic ion trap. Ions are mass selected using time-of-flight methods and investigated using photoelectron imaging in a velocity-mapping geometry. Ultrafast dynamics are investigated by pump-probe spectroscopy with femtosec- ond laser pulses.
Recent results are separated into three distinct projects:
(i) The investigation of electron acceptor radical anions based on the quinone backbone. These commonly exhibit electron transfer rates exceeding those pre- dicted by Marcus theory by orders of magnitude. We show that an alternative pathway to electron transfer could involve the participation of electronic excited states, as these couple strongly to the anion ground state. Specifically, for p- Benzoquinone we show that electronic resonances located in the detachment continuum primarily undergo internal conversion via a number of conical inter- sections.
(ii) Several polyanions have been investigated in the gas-phase. These systems exhibit unusual electronic properties, due to the presence of multiple excess charges, leading to the formation of a repulsive Coulomb barrier to photode- tachment. We investigate the effect of excess internal energy on this barrier and how it affects outgoing photoelectrons. We show that the trajectories of electrons are strongly influenced by this potential and demonstrate its use as a probe for large amplitude structural dynamics in polyanions.
(iii) The isolated chromophore of the green fluorescent protein (GFP) has been studied, and the vertical and adiabatic detachment energies determined for the first time. Using time-resolved spectroscopy the excited state dynamics are in- vestigated. We show that the first singlet excited state of the anion primarily decays through internal conversion, explaining the absence of fluorescence in the gas-phase. Using high level quantum chemistry calculations we show the specific motion involved and hence confirm the function of the protein back- bone in GFP.
This thesis is concluded with a few suggested experimental improvements and ideas for future studies of anions using the presented spectrometer.