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1. Hot-Band Absorption Can Mimic Entangled Two-Photon Absorption

   https://doi.org/10.1021/acs.jpclett.1c03751  

   Mikhaylov, Alexander; Wilson, Ryan N.; Parzuchowski, Kristen M.; Mazurek, Michael D.; Camp, Charles H.; Stevens, Martin J.; Jimenez, Ralph (February 2022, The Journal of Physical Chemistry Letters)
2. Direct absorption spectroscopy baseline fitting for blended absorption features

   https://doi.org/10.1364/AO.57.009086  

   Weisberger, Joshua M.; Richter, Joseph P.; Parker, Ronald A.; DesJardin, Paul E. (October 2018, Applied Optics)
3. The absorption indicatrix as an empirical model to describe anisotropy in X-ray absorption spectra of pyroxenes

   https://doi.org/10.2138/am-2021-7950  

   Steven, Cody J.; Dyar, M Darby; McCanta, Molly; Newville, Matthew; Lanzirotti, Antonio (April 2022, American Mineralogist)

   Abstract Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectrosco-pies, anisotropy in Fe K-edge X-ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X-ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single-crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation is energy-dependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X-ray wavelengths are modeled with a three-dimensional (3D) pedal surface, which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3D lemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals. 

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4. Optical attenuation without absorption

   https://doi.org/10.1103/PhysRevA.100.013850  

   Nodurft, I. C.; Brewster, R. A.; Pittman, T. B.; Franson, J. D. (July 2019, Physical Review A)
5. Can Formaldehyde Absorption Measurements Resolve the Kinematic Distance Ambiguity?

   Luisi, Matteo; Anderson, Loren; Liu, Bin; Balser, Dana; Bania, Thomas; Wenger, Trey; Haffner, L. (June 2022, American Astronomical Society Meeting #240)

   Kinematic distance determinations are complicated by a kinematic distance ambiguity (KDA) within the Solar orbit. For an axisymmetric Galactic rotation model, two distances, a "near" and "far" distance, have the same radial velocity. Formaldehyde (H2CO) absorption measurements have been used to resolve the KDA toward Galactic HII regions. This method relies on the detection of H2CO absorption against the broadband radio continuum emission from HII regions. H2CO absorption at velocities between the HII region velocity and the maximum velocity along the line of sight (the tangent point velocity) implies that the HII region lies at the far kinematic distance whereas a lack of absorption implies that it lies at the near kinematic distance. The reliability of KDA resolutions using H2CO is unclear, however, as disagreements between distances derived using H2CO absorption and those derived using other methods are common. Here we use new H2CO and radio recombination line data from the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) to test whether H2CO absorption measurements can accurately resolve the KDA for 44 Galactic HII regions that have known distances from maser parallax measurements. For each of the 44 HII regions we determine whether the parallax distance is consistent with either the near or the far kinematic distance. We find that the Galactic distribution of H2CO is too sparse to reliably determine whether an HII region is at its near kinematic distance. The H2CO method also incorrectly resolves the KDA for 80% of HII regions that it places at the far kinematic distance; in such cases H2CO absorption may be caused by other sources of radio continuum emission (possibly the CMB, diffuse free-free, or synchrotron). Our results indicate that the H2CO method is unsuitable to resolve the KDA toward Galactic HII regions. 

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