Some of the material in is restricted to members of the community. By logging in, you may be able to gain additional access to certain collections or items. If you have questions about access or logging in, please use the form on the Contact Page.
The evolution of material structures is governed by the making and breaking of chemical bonds and the rearrangement of atoms, which occurs on the time scale of an atomic vibrational period, hundreds of femtoseconds. Atomic motion on this time scale ultimately determines the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the function of biological processes. Direct observation and understanding these ultrafast structural dynamics at the time and length scales of atomic motions represent an important frontier in scientific research and applications. We have developed a femtosecond electron diffraction system (FED) capable of directly measuring the atomic motions in sub-picosecond temporal resolution and sub-milli-angstrom spatial resolution. In the path of the development of FED various technical challenges have been overcome and an unprecedented capability has been achieved. These advancements allow us to study a range of ultrafast structural dynamics directly on the fundamental level of atomic motions for the first time. With FED we measured laser-induced ultrafast structural dynamics in a 20-nm Al film by taking real-time snapshots of transmission electron patterns. The damped single-mode breathing motion of the Al film along the surface normal was recorded as coherent and in-phase oscillations of all the Bragg peak positions. The concurrent lattice heating was measured by tracking the associated Bragg peak intensity attenuation. This acoustic phonon can be well fitted with a classical harmonic oscillator model using a driving force which includes both electronic and lattice contribution. The pressure of the free electrons contributes significantly in driving the coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. In addition, by using a pair of optical excitation pulses and varying their time delay and relative pulse intensities, we demonstrated successful control of coherent lattice motions.
Femtosecond Electron Diffraction, Time-Resolved Diffraction, Ultrafast Structural Dynamics, Femtochemistry
Date of Defense
December 7, 2005.
A Dissertation Submitted to the Department of Physics in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.
Includes bibliographical references.
Jianming Cao, Professor Directing Dissertation; Sanford A. Safron, Outside Committee Member; Nicholas Bonesteel, Committee Member; Jorge Piekarewicz, Committee Member; Peng Xiong, Committee Member.
Florida State University
Use and Reproduction
This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them.