Femtosecond electron diffraction studies of strongly driven structural phase transitions

Abstract

The new technique of femtosecond electron diffraction is applied to the study of a strongly driven, laser induced solid-to-liquid phase transition in polycrystalline aluminum. This method provides an unprecedented view into the microscopic details of this process by providing a direct measurement of the atomic configuration of the system. We employ 600 fs electron pulses to see, in real time, the loss of the long-range order present in the crystalline phase and the emergence of the liquid structure in which only short-range atomic correlations are present; this transition occurs in 3.5 ps for thin-film aluminum with an excitation fluence of 70 mJ/cm2. The sensitivity and temporal resolution were sufficient to capture the time-dependent radial distribution function as the material evolved from the solid to the liquid state. These observations provide an atomic level description of the melting process in which the dynamics are best understood as a thermal phase transition under strongly driven conditions. In this paper, we also give a detailed description of the technical challenges associated with the production of femtosecond electron pulses and their application to the study of ultrafast structural dynamics in solid state samples and how these difficulties can be overcome.