4 research outputs found
Near-infrared absorption properties of oxygen-rich stardust analogues: The influence of coloring metal ions
Several astrophysically relevant solid oxides and silicates have extremely
small opacities in the visual and near-infrared in their pure forms. Datasets
for the opacities and for the imaginary part k of their complex indices of
refraction are hardly available in these wavelength ranges. We aimed at
determining k for spinel, rutile, anatase, and olivine, especially in the
near-infrared region. Our measurements were made with impurity-containing,
natural, and synthetic stardust analogs. Two experimental methods were used:
preparing small sections of natural minerals and synthesizing melt droplets
under the electric arc furnace. In both cases, the aborption properties of the
samples were measured by transmission spectroscopy. For spinel (MgAl2O4),
anatase, rutile (both TiO2), and olivine ((Mg,Fe)2SiO4), the optical constants
have been extended to the visual and near-infrared. We highlight that the
individual values of k and the absorption cross section depend strongly on the
content in transition metals like iron. Based on our measurements, we infer
that k values below 10^(-5) are very rare in natural minerals including
stardust grains, if they occur at all. Data for k and the absorption cross
section are important for various physical properties of stardust grains such
as temperature and radiation pressure. With increasing absorption cross section
due to impurities, the equilibrium temperature of small grains in circumstellar
shells increases as well. We discuss why and to what extent this is the case
Bent crystal spectrometer for both frequency and wavenumber resolved x-ray scattering at a seeded free-electron laser
We present a cylindrically curved GaAs x-ray spectrometer with energy
resolution and wave-number resolution of
, allowing plasmon scattering at the resolution
limits of the Linac Coherent Light Source (LCLS) x-ray free-electron laser. It
spans scattering wavenumbers of 3.6 to \AA\ in 100 separate bins, with
only 0.34\% wavenumber blurring. The dispersion of 0.418~eV/m agrees
with predictions within 1.3\%. The reflection homogeneity over the entire
wavenumber range was measured and used to normalize the amplitude of scattering
spectra. The proposed spectrometer is superior to a mosaic HAPG spectrometer
when the energy resolution needs to be comparable to the LCLS seeded bandwidth
of 1~eV and a significant range of wavenumbers must be covered in one exposure