Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density–dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator–based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

The physical behavior of condensed matter can be drastically altered in the presence of interfaces. Using a high-energy x-ray transmission-reflection scheme, we have studied ice-SiO2 model interfaces. We observed the formation of a quasiliquid layer below the bulk melting temperature and determined its thickness and density as a function of temperature. The quasiliquid layer has stronger correlations than water and a large density close to rho(HDA)=1.17 g/cm(3) of high-density amorphous ice suggesting a structural relationship with the postulated high-density liquid phase of water.

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Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process. DOI: 10.1103/PhysRevLett.100.135502 PACS numbers: 79.20.Ds, 61.05.cp, 61.20.Lc, 64.60.ÿi The liquid state of matter can be both supercooled below its freezing point and superheated above its boiling point. Although these are nonequilibrium states, their lifetime is longer than typical laboratory observation times and they are thus classified as metastable, stuck in a local minimum of the free energy. Despite their apparent stability, these systems exhibit localized fluctuations, corresponding to nuclei of the lower energy phase which rapidly form and dissolve, a process for which there exists a critical size for stable growth of the equilibrium phase, typically of nanoscale dimensions [1-3] (depending on the degree of superheating). When the transition does occur, it occurs rapidly, as is familiar to anyone who has observed the catastrophic boiling that suddenly occurs in superheated liquid water [4]. The development of ultrafast, atomic-scale sensitive techniques enables one to capture these transient, intermediate states, which under current laboratory methods, may spontaneously transform to lower energy stable states on inaccessible time scales. Techniques like time-resolved x-ray or electron scattering record these intermediate states in the form of a diffuse scattering pattern whose shape reflects the local atomic-scale structure that characterizes the transforming material. As a result of the orientational isotropy in the system and the short correlation lengths, these are typically broad diffraction rings, in contrast to the well-defined diffracted beams one obtains from a crystalline system. The radially averaged diffracted intensity as a function of scattering angle (normalized by atomic scattering factors) is a direct measure of the liquid structure factor S Q , where Q 4 sin = is the momentum transfer, 2 is the scattering angle, and is the wavelength, which directly encodes the short-range correlations in disordered materials.Recent experiments probing atomic-scale dynamics in disordered systems using electron diffraction techniques have focused on gas-phase photochemical reactions [5],