X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution

Abstract

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],

Atomic/Molecular-Level Simulations of Laser–Materials Interactions

“…The extremely high heating rates achievable with short-pulse laser irradiation, however, create the conditions for competition between the heterogeneous and homogeneous melting mechanisms and provide unique opportunities for the investigation of the kinetic limits of achievable superheating. Moreover, the emerging timeresolved electron and X-ray diffraction experimental techniques are capable of probing the transient atomic dynamics in laser melting with subpicosecond resolution [14][15][16][17][18][19][20][21][22]. The complexity of the fast nonequilibrium phase transformation, however, hinders the direct translation of the diffraction profiles to the transient atomic structures.…”

“…At the fundamental science level, short-pulse laser irradiation has the ability to bring material into a highly nonequilibrium state and provides a unique opportunity to probe the material behavior under extreme conditions. In particular, optical pump-probe experiments have been used to investigate transient changes in the electronic structure of the irradiated surface with high (often subpicosecond) temporal resolution [9][10][11][12][13], whereas recent advances in time-resolved X-ray and electron diffraction techniques [14][15][16][17][18][19][20][21][22] provide an opportunity to directly probe the ultrafast atomic dynamics in laser-induced structural transformations. Further optimization of experimental parameters in current applications, the emergence of new techniques, and interpretation of the results of probing the transient atomic dynamics in materials and at surfaces can be facilitated by computational modeling of laser-materials interactions.…”

The liquid-liquid phase transition in silicon revealed by snapshots of valence electrons

“…Experiments with high laser excitation already hinted on different phases that can be reached in silicon (40) and suggested the transient occurrence of a liquid phase at solid density in another system of the tetrahedral class of matter, InSb (41). Evolved models have been developed to describe the physical mechanisms in silicon after nonthermal melting (42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52).…”

Time-resolved imaging using x-ray free electron lasers

“…Designing diagnostics to clearly distinguish between one theory and the other is difficult. Many experiments have been performed to probe these states as best as possible [26,27,43,44,67], however direct experimental evidence in support of one model or another is still missing. The comparatively wide variation in results depending on experimental conditions and individual material properties only complicates matters further.…”