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1. Using crystallographic models to experimentally measure partial atomic charges with comparison to quantum calculations

   Taylor M. Keller, Michael J. (August 2020, American Crystallographic Association Annual Meeting 2020)

   Modern crystallographic refinement methods treat each atom in a molecule as neutral with spherical electron density. Atoms, however, exhibit partial atomic charges arising from intramolecular forces via bonding. These partial charges are crucial for understanding electronic structure and bulk physical properties of molecules. Typically the polarity and polarizability of molecules are calculated using IR and Raman spectroscopy, respectively. While these techniques can be used on small molecules, fine elucidation of partial charges on individual atoms is still unrealized. Here we present crystallographic refinement developments that allow us to refine electron density around individual atoms to experimentally calculate partial atomic charges. Comparison between these experimentally calculated charges to theoretical quantum calculated charges will also be presented. 

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2. Calorimetric wire detector for measurement of atomic hydrogen beams

   https://doi.org/10.1140/epjd/s10053-025-00987-y  

   Astaschov, M.; Bhagvati, S.; Böser, S.; Brandsema, M_J; Cabral, R.; Claessens, C.; de_Viveiros, L.; Enomoto, S.; Fenner, D.; Fertl, M.; et al (May 2025, The European Physical Journal D)

   AbstractA calorimetric detector for minimally disruptive measurements of atomic hydrogen beams is described. The calorimeter measures heat released by the recombination of hydrogen atoms into molecules on a thin wire. As a demonstration, the angular distribution of a beam with a peak intensity of$$\approx 10^{16} \,{\textrm{atoms}}/{(\textrm{cm}^2 \textrm{s})}$$ $\approx {10}^{16}\text{atoms}/\left({\text{cm}}^2\text{s}\right)$is measured by translating the wire across the beam. The data agree well with an analytic model of the beam from the thermal hydrogen atom source. Using the beam shape model, the relative intensity of the beam can be determined to 5% precision or better at any angle. Graphical abstract 

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3. Measurement of partial atomic charges by least-squares refinement of variable electron density crystallographic models.

   Michael J. Zdilla, Taylor M. (August 2020, American Crystallographic Association Annual Meeting 2020)

   Of interest in understanding electronic structure, bulk physical properties, enthalpies of phase changes, dipole moments, and numerous other properties of molecules, is the determination of realistic partial atomic charges on atoms. Atoms may take on partial positive or negative charges due to polar covalent bonds, coordinate covalent bonds, or due to formal charges imposed by Lewis structure constraints. Traditional crystallographic refinement treats each atom as a neutral, spherical atom however. We present a ongoing developments of a mode of crystallographic model refinement that permits refinement of electron density at individual atoms in order to arrive at partial atomic charges of atoms in a crystallographic model. Comparison to calculated partial charges (CHELPG, NBO, Mulliken) from quantum calculations (DFT, MP2) in both the gas phase and crystalline state will be presented. 

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4. Long-Range Interactions for Hydrogen Atoms in Excited D States

   https://doi.org/10.3390/atoms10010006  

   Adhikari, Chandra M.; Jentschura, Ulrich D. (March 2022, Atoms)

   Pressure shifts inside an atomic beam are among the more theoretically challenging effects in high-precision measurements of atomic transitions. A crucial element in their theoretical analysis is the understanding of long-range interatomic interactions inside the beam. For excited reference states, the presence of quasi-degenerate states leads to additional challenges, due to the necessity to diagonalize large matrices in the quasi-degenerate hyperfine manifolds. Here, we focus on the interactions of hydrogen atoms in reference states composed of an excited nD state (atom A), and in the metastable 2S state (atom B). We devote special attention to the cases n=3 and n=8. For n=3, the main effect is generated by quasi-degenerate virtual P states from both atoms A and B and leads to experimentally relevant second-order long-range (van-der-Waals) interactions proportional to the sixth inverse power of the interatomic distance. For n=8, in addition to virtual states with two states of P symmetry, one needs to take into account combined virtual P and F states from atoms A and B. The numerical value of the so-called C6 coefficients multiplying the interaction energy was found to grow with the principal quantum number of the reference D state; it was found to be of the order of 1011 in atomic units. The result allows for the calculation of the pressure shift inside atomic beams while driving transitions to nD states. 

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5. Enhanced luminescence of oxygen atoms and nitrogen molecules in nitrogen–krypton nanoclusters

   https://doi.org/10.1063/10.0036201  

   Korostyshevskyi, O; Wetzel, C K; Lee, D M; Khmelenko, V V (April 2025, Low Temperature Physics)

   We studied luminescence accompanied by an injection of nitrogen–krypton–helium gas mixtures after passing radiofrequency discharge into dense cold helium gas. In the cold helium gas N2–Kr nanoclusters were formed, with a core of Kr atoms and N2 molecules on the surface. Atomic nitrogen and oxygen resided in the N2 surface layers. When the temperature in the observation zone was in the range of 20–36 K, we observed enhanced emission of oxygen atom β-group and molecular nitrogen Vegard–Kaplan bands from N2–Kr nanoclusters. At these temperatures, nitrogen atoms efficiently recombine on the surface of nanoclusters with the formation of exited nitrogen molecules, leading to enhanced emission of Vegard–Kaplan bands. Simultaneously, the energy transfer from exited nitrogen molecules to the oxygen atoms enhanced O atom β-group emission. 

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