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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. Balmer Absorption in Iron Low-ionization Broad Absorption Line Quasars

   https://doi.org/10.3847/1538-4357/ae04df  

   Leighly, Karen M; Gallagher, Sarah C; Choi_최, Hyunseop 현섭; Terndrup, Donald M; Voelker, Julianna R; Richards, Gordon T; Morabito, Leah K (October 2025, The Astrophysical Journal)

   Abstract While Civis the most common absorption line in broad absorption line quasar spectra, Balmer absorption lines (BALs) are among the rarest. We present analysis of Balmer absorption in a sample of 14 iron low-ionization BAL quasars (FeLoBALQs); eight are new identifications. We measured velocity offset, width, and apparent optical depth. The partial covering that is ubiquitous in BAL quasar spectra alters the measured Balmer optical depth ratios; accounting for this, we estimated the true H(n= 2) column density. We found the anticipated correlation between Eddington ratio and outflow speed, but it is weak in this sample because nearly all of the objects have the low outflow speeds characterizing loitering outflow FeLoBAL quasars, objects that are also found to have low accretion rates. Measurements ofdN/dv, the differential column density with respect to the outflow speed, are anticorrelated with the luminosity and Eddington ratio: the strongest absorption is observed at the lowest speeds in the lowest-luminosity objects. The absorption line width is correlated withα_oi, theF_λpoint-to-point slope between 5100 Å and 3μm. This parameter is strongly correlated with the Eddington ratio among low-redshift quasars. BALs have been recently found in the spectra of little red dots (LRDs), a class of high-redshift objects discovered by JWST. We note suggestive similarities between LRDs and FeLoBAL quasars in the emission-line shape, the presence of steep reddening and a scattered blue continuum, the lack of hot dust emission, and X-ray weakness. 

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4. 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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5. 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)