Diffractive Imaging of Laser Induced Molecular Reactions with Kiloelectron-Volt Ultrafast Electron Diffraction

Capturing the structural changes during a molecular reaction with ultrafast electron diffraction (UED) requires a high spatiotemporal resolution and sufficiently high signal-to-noise to record the signals with high fidelity. In this dissertation, I have focused on the development of a tabletop gas phase keV-UED setup with a femtosecond temporal resolution. A DC electron gun was employed to generate electron pulses with a high repetition rate of 5 kHz. The space charge effect in the electron pulse was ameliorated by compressing the 90 keV electron pulse longitudinally with a time varying electric field in an RF cavity. The velocity mismatch between electron and laser pulses was mitigated using a tilted laser pulse with an incident angle such that longitudinal component of the laser velocity is matched to the speed of electron pulse. The combination of these two techniques enabled the setup to reach a temporal resolution of 240 fs, more recently ~200 fs, and a timing drift of 50 fs rms over several hours. The UED was used to capture the laser induced alignment of linear and nonlinear molecules. The high beam current and femtosecond resolution allowed us to extract the molecular orientation distribution (MOD) of the molecular ensemble with high fidelity as it evolved from the prompt alignment to the past multiple revivals. To retrieve the MOD of nonlinear molecules, I developed a theory that maps the MOD to the atom-pair angular distributions. The retrieval method does not require solving Schrödinger equation and works for any alignment methods. We also investigated ionization, fragmentation and isomerization of toluene generated by an IR strong laser field. Combined with the time-of-flight mass spectrometry, UED can determine the structure and yield of cations. A comparison of measurements to scattering calculations shows that scattering computation with independent atom model is inadequate to describe electron scattering from cations, and ab-initio calculation is required. Finally, the molecular photodissociation experiments with CF3I and iodobenzene induced by a UV pulse were demonstrated with the keV-UED. Advisor: Martin Centurio

High-resolution movies of molecular rotational dynamics captured with ultrafast electron diffraction

Imaging the structure of molecules during a photoinduced reaction is essential for elucidating reaction mechanisms. This requires high spatiotemporal resolution to capture nuclear motions on the femtosecond and subangstrom scale, and a sufficiently high signal level to sample their continuous evolution with high fidelity. Here we show that, using high-repetition-rate ultrafast electron diffraction, we can accurately reconstruct a movie of the coherent rotational motion of laser-aligned nitrogen molecules. We have used a tabletop 90-keV photoelectron gun to simultaneously achieve high temporal resolution of 240 fs full width at half maximum and an electron beam current that is more than an order of magnitude above the previous state of the art in gas-phase ultrafast electron diffraction. With this, we have made an essentially continuous real-space experimental movie of the rotational motion of the molecular wave packet as it evolves from initial alignment and past multiple revivals. Supplemental materials and 2 videos attached

Ultrafast electron diffraction from transiently aligned asymmetric top molecules: Rotational dynamics and structure retrieval

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

A New Trigonometrically Fitted Two-Derivative Runge-Kutta Method for the Numerical Solution of the Schrödinger Equation and Related Problems

A new trigonometrically fitted fifth-order two-derivative Runge-Kutta method with variable nodes is developed for the numerical solution of the radial Schrödinger equation and related oscillatory problems. Linear stability and phase properties of the new method are examined. Numerical results are reported to show the robustness and competence of the new method compared with some highly efficient methods in the recent literature

Retrieval of the molecular orientation distribution from atom-pair angular distributions

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

Chinese Herbal Medicine for Acute Mountain Sickness: A Systematic Review of Randomized Controlled Trials

Objectives. We aimed to assess the current clinical evidence of Chinese herbal medicine for AMS. Methods. Seven electronic databases were searched until January 2013. We included randomized clinical trials testing Chinese herbal medicine against placebo, no drugs, Western drugs, or a combination of routine treatment drugs against routine treatment drugs. Study selection, data extraction, quality assessment, and data analyses were conducted according to Cochrane standards. Results. Nine randomized trials were included. The methodological quality of the included trials was evaluated as low. Two trials compared prescriptions of Chinese formula used alone with Western drugs. A meta-analysis showed a beneficial effect in decreasing the score of AMS (MD: −2.23 [−3.98, −0.49], = 0.01). Only one trial compared prescriptions of Chinese formula used alone with no drugs. A meta-analysis showed a significant beneficial effect in decreasing the score of AMS (MD: −6.00 [−6.45, −5.55], < 0.00001). Four trials compared Chinese formula used alone with placebo. A meta-analysis also showed a significant beneficial effect in decreasing the score of AMS (MD: −1.10 [−1.64, −0.55], < 0.0001). Two trials compared the combination of Chinese formula plus routine treatment drugs with routine treatment drugs. A meta-analysis showed a beneficial effect in decreasing the score of AMS (MD: −5.99 [−11.11, −0.86], = 0.02). Conclusions. No firm conclusion on the effectiveness and safety of Chinese herbal medicine for AMS can be made. More rigorous high-quality trials are required to generate a high level of evidence and to confirm the results

Ultrafast electron diffraction instrument for gas and condensed matter samples

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

Quantum state tomography of molecules by ultrafast diffraction

Ultrafast electron diffraction and time-resolved serial crystallography are the basis of the ongoing revolution in capturing at the atomic level of detail the structural dynamics of molecules. However, most experiments capture only the probability density of the nuclear wavepackets to determine the time-dependent molecular structures, while the full quantum state has not been accessed. Here, we introduce a framework for the preparation and ultrafast coherent diffraction from rotational wave packets of molecules, and we establish a new variant of quantum state tomography for ultrafast electron diffraction to characterize the molecular quantum states. The ability to reconstruct the density matrix, which encodes the amplitude and phase of the wavepacket, for molecules of arbitrary degrees of freedom, will enable the reconstruction of a quantum molecular movie from experimental x-ray or electron diffraction data