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Laser Induced Molecular Dynamics Imaged with Ultrafast Electron Diffraction

Thesis Defense

Date: Time: 9:00 am–11:00 am
Jorgensen Hall Room: 245
Contact: Physics Department, (402) 472-2770, paoffice2@unl.edu
Kyle Wilkin will defend his thesis, “Laser Induced Molecular Dynamics Imaged with Ultrafast Electron Diffraction” via Zoom and in-person.

Zoom Link: https://unl.zoom.us/j/97903120178

Abstract: Gas-phase ultrafast electron diffraction (UED) is used to extract information from molecular sources on their natural length and time scale. Understanding physical phenomenon on the molecular scale leads to better control of those processes. Dynamics are induced through interaction with a laser and the subsequent motion of the molecule is studied through recorded diffraction patterns. In this dissertation, I describe how improvements to a table-top keV apparatus allow for the study of a wider range of samples, modify the manner in which the samples are injected into a vacuum chamber, increase the stability of the setup, widen the accessible angle of diffraction, and allow for the control of experiments through a single interface. These changes lead to the capture of diffraction images of aligned complex top molecules with a factor of two improvement over previous experiments in the maximum scattering vector. 4-fluorobenzotrifluoride ($ FC_6H_4CF_3 $) was aligned using a linearly polarized IR laser pulse. The anisotropy in the diffraction patterns was followed through the first 1.5 ps and show good agreement with simulations. The images are transformed to real space using a two-dimensional Fourier transform followed by an Abel inversion revealing angularly resolved atomic distances. Simulations show that lowering the initial rotational temperature to 1K significantly improves the angular resolution. Experiments performed with the UED machine at SLAC National Laboratory using MeV electrons reveal the nature of transient $ C_2F_4I $ following dissociation of an iodine atom from $ C_2F_4I_2 $ after absorption of a UV photon. The dynamics are followed for the first 800 fs and compared to calculated trajectories to understand oscillations observed in real space transformations of the diffraction signal. Analysis of the trajectories reveals coherent motion in the $ CF_2 $ group opposite the remaining iodine atom in the radical.

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This event originated in Physics.