An important technique for discovering and characterizing planets beyond our solar system relies upon measurement of weak Doppler shifts in the spectra of host stars induced by the influence of orbiting planets. A recent advance has been the introduction of optical frequency combs as frequency references. Frequency combs produce a series of equally spaced reference frequencies and they offer extreme accuracy and spectral grasp that can potentially revolutionize exoplanet detection. Here we demonstrate a laser frequency comb using an alternate comb generation method based on electro-optical modulation, with the comb centre wavelength stabilized to a molecular or atomic reference. In contrast to mode-locked combs, the line spacing is readily resolvable using typical astronomical grating spectrographs. Built using commercial off-the-shelf components, the instrument is relatively simple and reliable. Proof of concept experiments operated at near-infrared wavelengths were carried out at the NASA Infrared Telescope Facility and the Keck-II telescope.
Experimental evidencein the form of a specific-heat anomalyfor instability-induced amorphization of ErFez by hydrogenation was recently reported by Fecht, Fu, and Johnson [Phys. Rev. Lett. 64, 1753 (1990)]. We have attempted to study this anomaly by in situ elastic neutron diffraction and differential-scanning-calorimetry (DSC) measurements of deuterated ErFe2 below and above the reversible, endothermic, A, -shaped enthalpy signal that they found at -200'C. Our combined diffraction and DSC results reveal that the amorphization transition is irreversible, strongly exothermic and occurs only at a significantly higher temperature than that of the specific-heat anomaly. Rather than resulting from an underlying instability of the crystalline phase, amorphization occurs as a by-product of short-range clustering of the Er and Fe atoms, which is driven by the creation of energetically more favorable sites for the deuterium atoms.
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