Origin of the isostructural electronic states of the topological insulator Bi[subscript 2]Te[subscript 3]
Abstract not provided
Hong, Xinguo↗
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The Reaction of HO2 and CH3O2: CH3OOH Formed from the Singlet Electronic State Surface
High-level coupled-cluster calculations in combination with two-dimensional master equation simulations were used to study the HO2 + CH3O2 reaction, which plays an important role in the oxidation of methane and hydrocarbons in the Earth’s atmosphere and low-temperature combustion. The main reaction pathways taking place on the lowest-lying triplet and singlet potential energy surfaces (PES) were characterized. Interestingly, methyl hydroperoxide (CH3OOH), the sole product, could be produced from both the triplet and singlet PESs, with a ratio of roughly 9:1. Formaldehyde is not made as a primary product, but can be formed via secondary chemistry. The formation of methyl tetraoxide (MTO) from the singlet PES is unimportant. The calculated reaction rate coefficients were found to be practically pressure-independent for p ≤ 760 Torr and can be given by k(T)=2.75×10−13×e+1.75 kcal mol−1/RT (in cm3/s), an expression useful for kinetics modeling over the range T = 200–800 K. The rate constant has a slight negative Arrhenius energy dependence of about −1.75 kcal mol–1, falling about a factor of 30 from 200 K to 800 K.
2DME↗
Strong Quantization of Current-carrying Electron States in δ-layer Systems.
Abstract not provided.
Mamaluy, Denis↗
Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States
Ground-state electron spin polarization (ESP) is generated in radical elaborated (bpy)Pt(CAT-NN) and (bpy)Pt(CAT- p -Me 2 PhMe 2 -NN) (bpy = 5,5'-di- tert -butyl-2,2'-bipyridine, CAT = 3- tert -butylcatecholate, p -Ph = para -phenylene, NN = nitronylnitroxide). Photoexcitation produces an exchange-coupled, three-spin, charge-separated doublet 2 S 1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to a 2 T 1 (T = chromophore excited spin triplet configuration) excited state. The SQ-bridge-NN bond torsions affect the magnitude of the excited state exchange interaction ( J SQ-NN ), which determines the 2 T 1 – 4 T 1 energy gap. Ground state ESP is dependent on the magnitude of J SQ-NN , and we postulate that this results from differences in 2 T 1 and 4 T 1 state mixing. Mechanisms that lead to the rapid transfer of the excited state ESP to the ground state are discussed. Although subnanosecond 2 T 1 state lifetimes are measured optically in solution, the ground state ESP decays very slowly at 20 K and is observable for more than a millisecond.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Observation of the molecular response to light upon photoexcitation
Abstract When a molecule interacts with light, its electrons can absorb energy from the electromagnetic field by rapidly rearranging their positions. This constitutes the first step of photochemical and photophysical processes that include primary events in human vision and photosynthesis. Here, we report the direct measurement of the initial redistribution of electron density when the molecule 1,3-cyclohexadiene (CHD) is optically excited. Our experiments exploit the intense, ultrashort hard x-ray pulses of the Linac Coherent Light Source (LCLS) to map the change in electron density using ultrafast x-ray scattering. The nature of the excited electronic state is identified with excellent spatial resolution and in good agreement with theoretical predictions. The excited state electron density distributions are thus amenable to direct experimental observation.
74 ATOMIC AND MOLECULAR PHYSICS↗
Cascade of vestigial orders in two-component superconductors: Nematic, ferromagnetic, s -wave charge- 4 e , and d -wave charge- 4 e states
Electronically ordered states that break multiple symmetries can melt in multiple stages, similarly to liquid crystals. In a partially melted phase, known as vestigial phase, a bilinear made out of combinations of the multiple components of the primary order parameter condenses. Multicomponent superconductors are thus natural candidates for vestigial order since they break both the U(1)-gauge and also time-reversal or lattice symmetries. Here, we use group theory to classify all possible real-valued and complex-valued bilinears of a generic two-component superconductor on a tetragonal or hexagonal lattice. While the more widely investigated real-valued bilinears correspond to vestigial nematic or ferromagnetic order, the little explored complex-valued bilinears correspond to a vestigial charge-4e condensate, which itself can have an underlying s-wave, d x 2 –y 2 -wave, or d xy -wave symmetry. To properly describe the fluctuating regime of the superconducting Ginzburg-Landau action and thus access these competing vestigial phases, we employ both a large-N and a variational method. We show that while vestigial order can be understood as a weak-coupling effect in the large-N approach, it is akin to a moderate-coupling effect in the variational method. Despite these distinctions, both methods yield similar results in wide regions of the parameter space spanned by the quartic Landau coefficients. Specifically, we find that the nematic and ferromagnetic phases are the leading vestigial instabilities, whereas the various types of charge-4e order are attractive albeit subleading vestigial channels. The only exception is for the hexagonal case, in which the nematic and s-wave charge-4e vestigial states are degenerate. Finally, we discuss the limitations of our approach, as well as the implications of our results for the realization of exotic charge-4e states in material candidates.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Characterizing the Excited States and Electronic Absorption Spectra of Small Alkylperoxy (RO 2 •) and Hydroperoxy (•QOOH) Radicals
Organic peroxy (ROO•) and hydroperoxy (•QOOH) radicals are key reactive intermediates that are formed via the oxidation of volatile organic compounds during combustion or in the Earth’s atmosphere. Their primary fate is continued unimolecular decay or bimolecular chemistry, the relative branching for which is heavily structure- and temperature-dependent. This article outlines a combined single- and multi-reference quantum chemical study to characterize the near-UV accessible electronically excited states of the prototypical ROO• and •QOOH intermediates, tert-butyl peroxy and hydroperoxy-tert-butyl radicals—the ground-state chemistries of which have been well studied both experimentally and computationally. Additionally, we simulate the electronic absorption profiles of these ROO• and •QOOH intermediates with a variety of multi- and single-reference methods. The results show an interesting conformer dependence on the electronically excited-state character and electronic absorption maxima of •QOOH. The results show promise for electronic absorption spectroscopy to be used as a selected probe for determining •QOOH conformers. Additionally, electronic absorption may contribute to the daytime removal of long-lived •QOOH intermediates formed in the troposphere. We expect that our studies will motivate experiments on the electronic absorption spectra of experimentally achievable ROO• and •QOOH.
excited states↗
Photoinduced Magnetic Exchange-Jump Promotes Ground State Biradical Electron Spin Polarization
Photoinduced electron spin polarization (ESP) is reported in the electronic ground states of three Pt(II) complexes that are comprised of two S = ½ nitronyl nitroxide (NN) radicals attached through different length para-phenylethynyl bridges to the 3,6 positions of a catecholate (CAT, donor) and 4,4’-di-tert-butyl-2,2’-bipyridine (bpy, acceptor). Complexes 1-3 have from 17 to 41 bonds separating NN radicals, and display cw-EPR spectra consistent with |J NN-NN | >> |a N |, |J NN-NN | ≥ |a N |, and |J NN-NN | < |a N |, respectively, where J NN-NN is the magnetic exchange coupling between NN radicals in the electronic ground state, and a N is the isotropic 14 N hyperfine coupling constant. Light-induced transient EPR spectra characterized as enhanced ground-state absorption were observed for all three complexes using 532 nm pulsed laser excitation into the ligand-to-ligand charge transfer (LL’CT) band of the (CAT)Pt(bpy) chromophore. The magnitude of the observed ESP increases in the order 1 < 2 < 3 and is inversely correlated with the magnitude of ground-state J NN-NN . In addition to the experimental observation net absorptive polarization in 1-3, light excitation also produces multiplet polarization in 2. Since the weak dipolar coupling leads to strong spectral overlap of the absorptive and emissive components, the multiplet polarization is not observed in 1 and 3 and is very weak in 2. Here, the ability to spin polarize multiple radical spins with a single photon is anticipated to advance new photoinduced multi qubit/qudit ESP protocols for quantum information science applications.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Different Kinetic Reactivities of Electrons in Distinct TiO 2 Nanoparticle Trap States
Electrons added to TiO 2 and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO 2 nanoparticles have two distinct classes of thermally equilibrated trapped electrons, termed Red/e – and Blue/e – . Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring the reactions of TiO 2 /e – with substoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO 2 /e – absorbance followed by a secondary, slow (minutes) increase in the broad TiO 2 /e – optical feature. The analysis revealed that the fast decrease is due to the preferential oxidation of the Red/e – trap states and the slow increase results from the re-equilibration of electrons from Blue/e - to Red/e - states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/e – react ~5 times faster than Blue/e – with the nitroxyl radical oxidant 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-MeO-TEMPO). Similar reactivity patterns were also observed in oxidations of TiO 2 /e – by O 2 , which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant potassium triiodide (KI 3 ). Furthermore, this suggests that the faster intrinsic reactivity of one trap state over another on the seconds–minutes time scale is likely a general feature of reduced TiO 2 reactivity. This differential trap-state reactivity is likely to influence the performance of TiO 2 in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Revealing electronic state-switching at conical intersections in alkyl iodides by ultrafast XUV transient absorption spectroscopy
Conical intersections between electronic states often dictate the chemistry of photoexcited molecules. Recently developed sources of ultrashort extreme ultraviolet (XUV) pulses tuned to element-specific transitions in molecules allow for the unambiguous detection of electronic state-switching at a conical intersection. Here, the fragmentation of photoexcited iso-propyl iodide and tert-butyl iodide molecules (i-C 3 H 7 I and t-C 4 H 9 I) through a conical intersection between 3 Q 0 / 1 Q 1 spin-orbit states is revealed by ultrafast XUV transient absorption measuring iodine 4d core-to-valence transitions. The electronic state-sensitivity of the technique allows for a complete mapping of molecular dissociation from photoexcitation to photoproducts. In both molecules, the sub-100 fs transfer of a photoexcited wave packet from the 3 Q 0 state into the 1 Q 1 state at the conical intersection is captured. The results show how differences in the electronic state-switching of the wave packet in i-C 3 H 7 I and t-C 4 H 9 I directly lead to differences in the photoproduct branching ratio of the two systems.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Influence of spin state and electron configuration on the active site and mechanism for catalytic hydrogenation on metal cation catalysts supported on NU-1000: insights from experiments and microkinetic modeling
Spin state is found to determine the mechanism and active site of catalytic hydrogenation on metal cation catalysts.
Shabbir, Hafeera↗
Calibration of the Falcon Solid-state Energetic Electron Detector (SEED)
The Falcon Solid-state Energetic Electron Detector (FalconSEED) is an energetic charged particle sensor that has been developed and tested at the United States Air Force Academy in an effort to monitor electron flux across the energy range of 14 to 145 keV in geosynchronous orbit. This sensor has been developed to complement ongoing efforts by the Air Force Research Laboratory to advance a comprehensive space environment sensor suite, Compact Environmental Anomaly Sensor (CEASE3), for anomaly resolution. In addition, FalconSEED is intended to demonstrate the ability to operate a radiation sensor based on predominantly commercial off the shelf components in the geosynchronous environment. This paper describes the design, development, and calibration of FalconSEED. The electron energy spectrum of interest for the FalconSEED sensor is 14 to 145 keV, and is intended to supplement data acquired from the CEASE3 instrument for anomaly resolution. The calibrations at the Space Atmospheric Research Center at the United States Air Force Academy and Kirtland Air Force Base were used to first establish the conversion between histogram bin number and deposited energy in the sensor, and second have been used to determine the geometric factor as a function of energy. The FalconSEED will be integrated as a science payload on the Space Test Program Satellite-6 which also includes the first flight of the CEASE3 instrument and is scheduled to launch in 2021.
47 OTHER INSTRUMENTATION↗
High-Energy Hybridized States Enable Long-Lived Hot Electrons in Cobaloxime-Silicon Nanocrystal System
Strong electronic coupling is achieved between the molecular catalyst cobaloxime ([Co]) and silicon nanocrystals (Si NCs) bridged by an ethylenepyridine group derived from vinylpyridine (vpy) covalently bound to the Si NC surface (Si-vpy-[Co]). The ethylenepyridine tether in Si-vpy-[Co] is key to dramatic changes to the system’s physical properties which are not observed in the corresponding formylpyridine (fpy) system (Si-fpy-[Co]) consistent with strong electronic coupling previously observed only in dark electrochemical systems. UV−vis absorption spectroscopy reveals new [Co]-centered electronic states in Si-vpy-[Co], and transient absorption spectroscopy finds a strong absorption feature appearing within 250 fs and persisting for at least 5 ns. Astoundingly, spectroelectrochemical measurements reveal that this absorption feature is consistent with both the singly reduced [Co] − and doubly reduced [Co] 2− complexes, leading to the conclusion that these long-lived charges are derived from high-energy “hot” electrons residing in [Co]-centered states. Detailed analysis using cyclic voltammetry, spectroelectrochemistry, electron paramagnetic resonance spectroscopy, and density functional theory (DFT) calculations provides insight into the unique electronic structure created in Si-vpy-[Co]. DFT reveals that the new electronic states arise from hybridization between deep Si NC band states and high-energy molecular orbitals of the ethylenepyridine tether and the [Co] catalyst and are facilitated by σ-bonding character at the ethylenepyridine linkage. This study demonstrates that strong electronic coupling achieved through precise molecular chemistry can change the paradigm of otherwise fixed energy levels in hybrid photoelectrochemical systems for artificial photosynthesis and related applications.
14 SOLAR ENERGY↗
Physically Informed Machine Learning Prediction of Electronic Density of States
The electronic structure of a material, such as its density of states (DOS), provides key insights into its physical and functional properties and serves as a valuable source of high-quality features for many materials screening and discovery workflows. Still, the computational cost of calculating the DOS, most commonly with density functional theory (DFT), becomes prohibitive for meeting high-fidelity or high-throughput requirements, necessitating a cheaper but sufficiently accurate surrogate. To fulfill this demand, we develop a general machine learning method based on graph neural networks for predicting the DOS purely from atomic positions, six orders of magnitude faster than DFT. This approach can effectively use large materials databases and be applied generally across the entire periodic table to materials classes of arbitrary compositional and structural diversity. We furthermore devise a highly adaptable scheme for physically informed learning which encourages the DOS prediction to favor physically reasonable solutions defined by any set of desired constraints. This functionality provides a means for ensuring that the predicted DOS is reliable enough to be used as an input to downstream materials screening workflows to predict more complex functional properties, which rely on accurate physical features.
36 MATERIALS SCIENCE↗
Electronic excited states from a variance-based contracted quantum eigensolver
Electronic excited states of molecules are central to many physical and chemical processes, and yet they are typically more difficult to compute than ground states. Here, in this paper we leverage the advantages of quantum computers to develop an algorithm for the highly accurate calculation of excited states. We solve a contracted Schrödinger equation (CSE)—a contraction (projection) of the Schrödinger equation onto the space of two electrons—whose solutions correspond identically to the ground and excited states of the Schrödinger equation. While recent quantum algorithms for solving the CSE, known as contracted quantum eigensolvers (CQE), have focused on ground states, we develop a CQE based on the variance that is designed to optimize rapidly to a ground or excited state. We apply the algorithm to compute the ground and excited states of H 2 , H 4 , and BH.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Average-atom model with Siegert states
In plasmas, electronic states can be well-localized bound states or itinerant free states, or something in between. In self-consistent treatments of plasma electronic structure such as the average-atom model, all states must be accurately resolved in order to achieve a converged numerical solution. Furthermore, this is a challenging numerical and algorithmic problem in large part due to the continuum of free states which is relatively expensive and difficult to resolve accurately. Siegert states are an appealing alternative. They form a complete eigenbasis with a purely discrete spectrum while still being equivalent to a representation in terms of the usual bound states and free states. However, many of their properties are unintuitive, and it is not obvious that they are suitable for self-consistent plasma electronic structure calculations. Here it is demonstrated that Siegert states can be used to accurately solve an average-atom model and offer advantages over the traditional finite-difference approach, including a concrete physical picture of pressure ionization and continuum resonances.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Metal Ion Control of Photoinduced Electron Spin Polarization in Electronic Ground States
The sign and intensity of photoinduced electron spin polarization (ESP) in the electronic ground doublet states ( 2 S o /D o ) of chromophore-radical complexes can be controlled by changing the nature of the metal ion. The complexes consist of an organic radical (nitronylnitroxide, NN) covalently attached to a donor acceptor chromophore via a meta-phenylene bridge, (bpy)M(CAT-m-Ph-NN) (1) (bpy = 4,4’-di-tert-butyl-2,2’-bipyridine, M = Pd II (1-Pd) or Pt II (1-Pt), CAT = 3-tert-butylcatecholate, m-Ph = meta-phenylene). In both complexes, photoexcitation with visible light pro-duces an initial exchange-coupled, 3-spin (bpy •- , CAT +• = semiquinone (SQ) and NN • ), charge-separated doublet 2 S 1 (S = chromophore excited spin singlet configuration) excited state that rapidly decays to the ground state via a 2 T 1 (T = chromophore excited spin triplet configuration) state. This process is not expected to be spin selective and only very weak emissive ESP is found for 1-Pd. In contrast, strong absorptive ESP is generated in 1-Pt. Furthermore, it is postulated that zero-field splitting induced transitions between the chromophoric 2 T 1 and 4 T 1 states (1-Pd and 1-Pt) and spin-orbit induced transitions between 2 T 1 and NN-based quartet states (1-Pt) ac-count for the differences in polarization.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗