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We report the modification of a gas phase ultrafast electron diffraction (UED) instrument that enables experiments with both gas and condensed matter targets, where a time-resolved experiment with sub-picosecond resolution is demonstrated with solid state samples. The instrument relies on a hybrid DC-RF acceleration structure to deliver femtosecond electron pulses on the target, which is synchronized with femtosecond laser pulses. The laser pulses and electron pulses are used to excite the sample and to probe the structural dynamics, respectively. The new system is added with capabilities to perform transmission UED on thin solid samples. It allows for cooling samples to cryogenic temperatures and to carry out time-resolved measurements. We tested the cooling capability by recording diffraction patterns of temperature dependent charge density waves in 1T-TaS2. The time-resolved capability is experimentally verified by capturing the dynamics in photoexcited single-crystal gold.

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 (FC6H4CF3) 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 C2F4I following dissociation of an iodine atom from C2F4I2 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 CF2 group opposite the remaining iodine atom in the radical.

Ultrafast electron diffraction (UED) from aligned molecules in the gas phase has successfully retrieved structures of both linear and symmetric top molecules. Alignment of asymmetric tops has been recorded with UED but no structural information was retrieved. We present here the extraction of two-dimensional structural information from simple transformations of experimental diffraction patterns of aligned molecules as a proof-of-principle for the recovery of the full structure. We align 4-fluorobenzotrifluoride with a linearly polarized laser and show that we can distinguish between atomic pairs with equal distances that are parallel and perpendicular to the aligned axis. We additionally show with numerical simulations that by cooling the molecules to a rotational temperature of 1 K, more distances and angles can be resolved through direct transformations.

Imaging laser-induced rotational dynamics is an important and active field due to its applications in capturing reactions in the molecular frame and in molecular imaging. Experimental measurement of the molecular orientation distribution, as a function of the Euler angles, has been demonstrated for special cases when the detectable signal is generated along the molecular symmetry axis. Here we developed the general theory that maps the probability density distribution of the molecular orientation to the atom-pair angular distributions for nonlinear molecules. With the theory, the molecular orientation distribution can be retrieved from the measured atom-pair angular distribution, which we demonstrate experimentally using ultrafast electron diffractive imaging of impulsively aligned trifluoro-iodomethane molecules. The retrieved molecular orientation distribution is in good agreement with direct numerical simulations of the time-dependent Schrödinger equation using the experimental conditions. Unlike the existing retrieval methods, the retrieval method does not require solving the Schrödinger equation, works for any alignment method, and is in principle applicable to asymmetric top molecules.

We resolve the structural dynamics of the ultrafast photoinduced ring opening reaction of 1,3-cyclohexadiene in space and time employing megaelectronvolt gas phase ultrafast electron diffraction. We, furthermore, observe coherent large amplitude motions of the photoproduct.