Femtosecond electron diffraction studies of ultrafast structural dynamics.
Contributors: University of Toronto.  Dept. of Chemistry.
Published: 2005
Language: eng
Number of pages: 132
Description: Homogeneous nucleation clearly plays a strong role in the global melting dynamics of these superheated metal films. This mechanism is limited by the physical fact that the atoms must collisionally disorder in order for the liquid to form. The speed at which this can occur---subject to the statistical requirement of several collisions for disordering--is the key parameter that limits the melting rate. This work records the critical atomic displacements throughout the course of the phase transition and sets the stage for future investigations of even irreversible chemical reaction dynamics in the condensed phase.This thesis work presents the development and implementation of the new technique of femtosecond electron diffraction and its application to uncovering ultrafast structural dynamics during strongly-driven melting.We employed intense femtosecond laser excitation to create superheated films of aluminum. Using 600 fs electron pulses, we recorded the transient diffraction patterns from aluminum excited with 70 mJ/cm2, 775 nm pulses and observed lattice heating and subsequent disordering in only 3.5 ps. The formation of the liquid was confirmed by comparing the reduced pair distribution function, G (r), before and after melting to the equilibrium solid and liquid G ( r). The loss of lattice order, as distinguished from lattice heating, occured in only 2 ps. This rapid liquid formation can be explained by heterogeneous (surface) nucleation only if the melt front propagates at its limit of the speed of sound. In contrast, this timescale is natural for homogeneous nucleation of liquid throughout the superheated bulk. We subsequently extended our investigation of strongly-driven melting to gold. The weaker electron-phonon coupling constant---by an order of magnitude---in gold versus aluminum leads to lower rates of lattice heating. Use of this noble metal furthermore permits the investigation of the material-dependence of strongly-driven melting. Under equivalent excitation conditions, we observed the same hallmarks of simple lattice heating as in aluminum, albeit on a longer timescale. A new algorithm was developed and permitted the extraction of G (r)  at each time delay throughout the heating and eventual lattice collapse. We were thus able to access the real-space structural dynamics.
Notes: Source: Dissertation Abstracts International, Volume: 66-10, Section: B, page: 5420.

Advisor: Dwayne Miller

Thesis (Ph.D.)--University of Toronto, 2005.

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