Photoionizaion as a Probe of Ultrafast Molecular Dynamics

• Main Author: Horton, Spencer Lourdes
• Language: EN
• Published: State University of New York at Stony Brook 2018
• Subjects: Physics
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=10932704

<p> Ionization can serve as a universal probe of excited state dynamics in molecules, such as internal conversion, dissociation, and isomerization. These processes are of fundamental importance to a wide array of dynamics in biology, chemistry and physics. In this thesis the topic of how to best construct a &ldquo;molecular movie&rdquo; of these photoinduced exited state molecular dynamics is addressed. Directly measuring a molecular structure or wave function amplitude as a function of time in order to construct a &ldquo;molecular movie&rdquo; is arguably impossible. Rather, the most insight is typically gained by comparing experiment with theoretical calculations of observables in order to verify the calculations, and then generating the &ldquo;molecular movie&rdquo; from calculations. Thus, an important criterion in evaluating different measurement approaches is how easily they can be compared with theoretical calculations of the measured observable. </p><p> Arguments are presented for why time-resolved ionization spectroscopy with a weak-field ionization probe is ideally suited for this goal. For the work conducted in this thesis, an ultrafast weak-field ionization pump-probe time-resolved ionization spectrometer is constructed utilizing UV (260 nm) and Vacuum-UV (156 nm) light to measure neutral excited state dynamics. Time-resolved VUV-pump UV-probe measurements were conducted to study of the highly excited states of pyrrole, where rapid internal conversion to the ground state appears to be the dominate relaxation channel. Time-resolved UV-pump and VUV-probe measurements in internal conversion of 1,3-cyclohexadiene are also performed. The measurements reveal a substantial ionization of the &ldquo;hot&rdquo; ground state following internal conversion despite the fact that our probe photon energy is below the ionization potential of the molecule. With the aid of electronic structure calculations, the results are interpreted in terms of vibrationally assisted below threshold ionization, where vibrational energy is converted to electronic energy. A comparison of time-resolved ionization spectroscopy is done using weak and strong field ionization as probes of these dynamics. It is found that though the two probes capture the same general dynamics only the weak-field ionization probe can be quantitatively compared to dynamics calculations. Lastly, weak-field ionization time-resolved photoelectron spectroscopy experiments are conducted on halogenated methanes. The high degree of agreement between the experiment and calculation enables us to construct &ldquo;molecular movies&rdquo; of the dynamics.</p><p>