Development of ultrafast spectroscopic techniques to study rapid chemical and physical changes in materials under extrem
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Development of ultrafast spectroscopic techniques to study rapid chemical and physical changes in materials under extreme pressure and temperature conditions Alexander F. Goncharov,1 D. Allen Dalton,1 R. Stewart McWilliams,1,2 and Mohammad F. Mahmood1,2 1 Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015, U.S.A. 2 Howard University, Washington, DC, 20059, U.S.A. ABSTRACT In the study of materials at extreme pressures and temperatures, there is an enduring need to extend the range of experiments to previously inaccessible regimes. To accomplish this, improvements in diagnostics for in situ material characterization at extremes must proceed in parallel with techniques used to generate extreme states. Simultaneously, there is a need to study material phenomena – e.g. phase transformations and chemical reactions triggered by the application of extreme conditions – on their natural timescales. Here we report on recent developments in the application of ultrafast laser spectroscopic techniques to high-pressure hightemperature experiments on materials confined in a diamond-anvil cell. Using a bright broadband source coupled to ultrafast detection to discriminate signal from high thermal and fluorescent backgrounds, we conducted broadband optical spectroscopy up to 60 GPa and 1560 K. By coupling the broadband source to a monochromatic pulse, nonlinear Coherent AntiStokes Raman Spectroscopy (CARS) with high signal brightness was achieved. Optical absorption data in hot compressed O2 and CARS data in N2 at extreme pressures are reported. INTRODUCTION Knowledge of the behavior of materials under extreme pressures and temperatures is fundamental for many fields of science. This information is also important for technology and for national security as it allows predictions of material behavior under relevant conditions. In spite of ongoing technical advances, the development of the required in situ measurements of materials properties under extreme conditions remains a challenging problem. For example, classical optical spectroscopy methods such as absorption (e.g. FTIR) or Raman spectroscopy at high temperatures are affected by thermal radiation. Thermal sources provide incoherent radiation, whereas lasers are coherent, making spatial and temporal discrimination feasible for laser-based spectroscopic probes. Thermal radiation may be substantially suppressed through the use of short-pulse, high-brightness sources and time-resolved detectors or through signal modulation and synchronous acquisition [1]. Suppression of background is increasingly important at very high temperatures (approaching 1 eV) and in systems exhibiting strong fluorescence. In previous technical developments, our research group has coupled pulsed laser heating of the high-pressure diamond-anvil cell (DAC) with time-resolved temperature measurements (using spectroradiometry) [2], spontaneous Raman scattering [3], and x-ray diffraction [4]. The purpose of these studies was many-fold: to achieve higher temp
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