Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

Martin Centurion

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Bradley Shadwick, Craig Zuhlke, Daniel Claes, Kees Uiterwaal

Department

Physics & Astronomy

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Physics

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

Ultrafast electron diffraction (UED) has emerged since the late 20th century as a powerful technique for resolving molecular dynamics. With its high spatial and temporal resolution, UED can now measure rotational motion induced by linearly polarized, nonresonant, nonadiabatic laser pulses—one of the fundamental molecular degrees of freedom with broad applications. By combining laser-induced alignment with UED, new capabilities such as three-dimensional structural retrieval and isotope detection have been demonstrated, as discussed in Chapter 2.

In Chapter 3, three symmetry relations in the coefficients of rotational wave packets are identified and derived, reducing the computational cost of simulating asymmetric-top molecular alignment by a factor of eight in both time and memory. Several numerical techniques are also introduced to further improve computational efficiency, enabling simulations of rotational dynamics at room temperature.

Vibrational dynamics induced by nonresonant laser pulses in the impulsive regime share similarities with nonadiabatic rotational excitation, as both involve ultrashort laser “kicks” that coherently excite molecules into superpositions of eigenstates. However, vibrational motion induced by impulsive stimulated Raman scattering (ISRS) has not yet been detected by UED due to the extremely small displacements, where the harmonic oscillator approximation remains valid. In Chapter 4, time-resolved UED signals are simulated using both classical and quantum harmonic oscillator models and compared in detail. Because current UED temporal resolution (~100 fs) is comparable to or longer than the vibrational period (~100 fs), time-averaged diffraction patterns are analyzed. The classical model agrees well with the quantum results at time delays corresponding to the largest signal amplitudes but deviates for time-averaged signals. Quantum simulations further predict that UED with temporal resolution of about half the vibrational period can still capture vibrational dynamics induced by ISRS, yielding similar but weaker signal amplitudes.

Advisor: Martin Centurion

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