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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Understanding 2p core-level excitons of late transition metals by analysis of mixed-valence copper in a metal–organic framework

The L 2,3 -edge X-ray absorption spectra of late transition metals such as Cu, Ag, and Au exhibit absorption onsets lower in energy for higher oxidation states, which is at odds with the measured spectra of earlier transition metals. Time-dependent density functional theory calculations for Cu 2+ /Cu + reveal a larger 2p core-exciton binding energy for Cu 2+ , overshadowing shifts in single-particle excitation energies with respect to Cu + . Finally, we explore this phenomenon in a Cu + metal–organic framework with ~12% Cu 2+ defects and find that corrections with self-consistent excited-state total energy differences provide accurate XAS peak alignment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Pairing preferences of the model mono-valence mono-atomic ions investigated by molecular simulation

We carried out a series of potential of mean force calculations to study the pairing preferences of a series of model mono-atomic 1:1 ions with evenly varied sizes. The probabilities of forming the contact ion pair (CIP) and the single water separate ion pair (SIP) were presented in the two-dimensional plots with respect to the ion sizes. The pairing preferences reflected in these plots largely agree with the empirical rule of matching ion sizes in the small and big size regions. In the region that the ion sizes are close to the size of the water molecule; however, a significant deviation from this conventional rule is observed. Our further analysis indicated that this deviation originates from the competition between CIP and the water bridging SIP state. The competition is mainly an enthalpy modulated phenomenon in which the existing of the water bridging plays a significant role.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Low valency in lanthanides: A theoretical study of NdF and LuF

The ground and low-lying excited state potential energy curves of neodymium monofluoride were calculated using multireference (CASSCF) and single reference (EOM-CR-CCSD(T)) methods. Optimized bond lengths were obtained and accurate bond dissociation energies were computed. The EOM-CR-CCSD(T) method was used to determine the bond dissociation energy of lutetium monofluoride, and it is shown that core correlation is required to produce bond dissociation energies in agreement with experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Investigating vibrational relaxation in cyanide-bridged transition metal mixed-valence complexes using two-dimensional infrared and infrared pump-probe spectroscopies

Using polarization-selective two-dimensional infrared (2D IR) and infrared pumpprobe spectroscopies, we study vibrational relaxation of the four cyanide stretching (CN) vibrations found in [(NH3)5RuIIINCFeII(CN)5] (FeRu) dissolved in D2O or formamide and [(NC)5FeIICNPtIV(NH3)4NCFeII(CN)5] 4 (FePtFe) dissolved in D2O. These cyanide-bridged transition metal complexes serve as models for understanding the role high frequency vibrational modes play in metal-to-metal charge transfers over a bridging ligand. However, there is currently little information about vibrational relaxation and dephasing dynamics of the anharmonically coupled CN modes in the electronic ground state of these complexes. IR pump-probe experiments reveal that the vibrational lifetimes of the CN modes are 2 times faster when FeRu is dissolved in D2O versus formamide. They also reveal that the vibrational lifetimes of the CN modes of FePtFe in D2O are almost four times as long as for FeRu in D2O. Combined with mode-specific relaxation dynamics measured from the 2D IR experiments, the IR pump-probe experiments also reveal that intramolecular vibrational relaxation is occurring in all three systems on 1 ps timescale. Center line slope dynamics, which have been shown to be a measure of the frequency-frequency correlation function, reveal that the radial, axial, and trans CN modes exhibit a 3 ps timescale for frequency fluctuations. This timescale is attributed to the forming and breaking of hydrogen bonds between each mode and the solvent. The results presented here along with our previous work on FeRu and FePtFe reveal a picture of coupled anharmonic CN modes where the spectral diffusion and vibrational relaxation dynamics depend on the spatial localization of the mode on the molecular complex and its specific interaction with the solvent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Combined theoretical and experimental study of the valence, Rydberg, and ionic states of chlorobenzene

New photoelectron (PE) and ultra violet (UV) and vacuum UV (VUV) spectra have been obtained for chlorobenzene by synchrotron study with higher sensitivity and resolution than previous work and are subjected to detailed analysis. In addition, we report on the mass-resolved (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectra of a jet-cooled sample. Both the VUV and REMPI spectra have enabled identification of a considerable number of Rydberg states for the first time. The use of ab initio calculations, which include both multi-reference multi-root doubles and singles configuration interaction (MRD-CI) and time dependent density functional theoretical (TDDFT) methods, has led to major advances in interpretation of the vibrational structure of the ionic and electronically excited states. Franck-Condon (FC) analyses of the PE spectra, including both hot and cold bands, indicate much more complex envelopes than previously thought. The sequence of ionic states can be best interpreted by our multi-configuration self-consistent field computations and also by comparison of the calculated vibrational structure of the B and C ionic states with experiment; these conclusions suggest that the leading sequence is the same as that of iodobenzene and bromobenzene, namely: X{sup 2}B{sub 1}(3b{sub 1}{sup −1}) < A{sup 2}A{sub 2}(1a{sub 2}{sup −1}) < B{sup 2}B{sub 2}(6b{sub 2}{sup −1}) < C{sup 2}B{sub 1}(2b{sub 1}{sup −1}). The absorption onset near 4.6 eV has been investigated using MRD-CI and TDDFT calculations; the principal component of this band is {sup 1}B{sub 2} and an interpretation based on the superposition of FC and Herzberg-Teller contributions has been performed. The other low-lying absorption band near 5.8 eV is dominated by a {sup 1}A{sub 1} state, but an underlying weak {sup 1}B{sub 1} state (πσ{sup ∗}) is also found. The strongest band in the VUV spectrum near 6.7 eV is poorly resolved and is analyzed in terms of two ππ{sup ∗} states of {sup 1}A{sub 1} (higher oscillator strength) and {sup 1}B{sub 2} (lower oscillator strength) symmetries, respectively. The calculated vertical excitation energies of these two states are critically dependent upon the presence of Rydberg functions in the basis set, since both manifolds are strongly perturbed by the Rydberg states in this energy range. A number of equilibrium structures of the ionic and singlet excited states show that the molecular structure is less subject to variation than corresponding studies for iodobenzene and bromobenzene.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Communication: Protonation process of formic acid from the ionization and fragmentation of dimers induced by synchrotron radiation in the valence region

The ionization and fragmentation of monomers of organic molecules have been extensively studied in the gas phase using mass spectroscopy. In the spectra of these molecules it is possible to identify the presence of protonated cations, which have a mass-to-charge ratio one unit larger than the parent ion. In this work, we investigate this protonation process as a result of dimers photofragmentation. Experimental photoionization and photofragmentation results of doubly deuterated formic acid (DCOOD) in the gas phase by photons in the vacuum ultraviolet region are presented. The experiment was performed by using a time-of-flight mass spectrometer installed at the Brazilian Synchrotron Light Laboratory and spectra for different pressure values in the experimental chamber were obtained. The coupled cluster approach with single and double substitutions was employed to assist the experimental analysis. Results indicate that protonated formic acid ions are originated from dimer dissociation, and the threshold photoionization of (DCOOD)⋅D{sup +} is also determined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene

New photoelectron spectra (PES) and ultra violet (UV) and vacuum UV (VUV) absorption spectra of fluorobenzene recorded at higher resolution than previously, have been combined with mass-resolved (2 + 1) and (3 + 1) resonance enhanced multiphoton ionization (REMPI) spectra; this has led to the identification of numerous Rydberg states. The PES have been compared with earlier mass-analyzed threshold ionization and photoinduced Rydberg ionization (PIRI) spectra to give an overall picture of the ionic state sequence. The analysis of these spectra using both equations of motion with coupled cluster singles and doubles (EOM-CCSD) configuration interaction and time dependent density functional theory (TDDFT) calculations have been combined with vibrational analysis of both the hot and cold bands of the spectra, in considerable detail. The results extend several earlier studies on the vibronic coupling leading to conical intersections between the X{sup 2}B{sub 1} and A{sup 2}A{sub 2} states, and a further trio (B, C, and D) of states. The conical intersection of the X and A states has been explicitly identified, and its structure and energetics evaluated. The energy sequence of the last group is only acceptable to the present study if given as B{sup 2}B{sub 2}<C{sup 2}B{sub 1}<D{sup 2}A{sub 1}, a conclusion which is in agreement with most previous EOM-CCSD and other calculations. However, this symmetry ordering of the B and C states forces reconsideration of the nature of the PIRI spectrum. The coupling between these two states is induced by the a{sub 2} modes, ν{sub 12} and ν{sub 14} and we propose that the 14{sup 1} band is observed in the B{sup 2}B{sub 2} band in the PES for the first time, because of the improved resolution. This same assignment is given to the lowest energy band in the PIRI spectrum which was previously assigned as the origin band and further conclude that the entire PIRI spectrum is induced by ν{sub 12} and ν{sub 14}. The relative intensities of the various Rydberg state peaks in the VUV absorption and REMPI spectra of fluorobenzene are very similar to those observed in the equivalent spectra of benzene.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission

We use time-resolved X-ray photoelectron spectroscopy to probe the electronic and magnetization dynamics in FeRh films after ultrafast laser excitations. We present experimental and theoretical results which investigate the electronic structure of FeRh during the first-order phase transition, identifying a clear signature of the magnetic phase. We find that a spin polarized feature at the Fermi edge is a fingerprint of the magnetic status of the system that is independent of the long-range ferromagnetic alignment of the magnetic domains. We use this feature to follow the phase transition induced by a laser pulse in a pump-probe experiment and find that the magnetic transition occurs in less than 50 ps and reaches its maximum in 100 ps.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ultrafast x-ray pump x-ray probe transient absorption spectroscopy: A computational study and proposed experiment probing core-valence electronic correlations in solvated complexes

Femtosecond X-ray pump X-ray probe experiments are currently possible at free electron lasers such as the linac coherent light source (LCLS), which opens new opportunities for studying solvated transition metal-ligand complexes. In order to make the most effective use of these kinds of experiments, it is necessary to determine which chemical properties an X-ray pump pulse will measure. In this paper, we have combined electron cascade calculations and excited-state time-dependent density functional theory calculations to predict the initial state prepared by an X-ray pump and the subsequent X-ray probe spectra at the Fe K-edge in the solvated model transition metal complex, K 4 Fe II (CN) 6 . We find several key spectral features that report on the ligand-field splitting, solvent-solute interactions, and the 3p and 3d electron interactions. We then show how these features could be measured in an experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Thermal transport and mixed valence in ZrTe3 doped with Hf and Se

Two-dimensional transition metal trichalcogenides (TMTCs) feature covalently bonded metal-chalcogen layers separated by the van der Waals (vdW) gap. Similar to transition metal dichalcogenides (TMDCs), TMTCs often host charge density waves (CDWs) and superconductivity, but unlike TMDCs, atomic chains in the crystal structure give rise to quasi one-dimensional (quasi 1D) conduction. ZrTe3 features the CDW below TCDW = 63 K and filamentary superconductivity below 2 K that can be enhanced by pressure or chemical substitution. Here, we report the presence of mixed valent Zr2+ and Zr4+ atoms in ZrTe3 crystals that are reduced by doping in ZrTe3−xSex and Zr1−yHfyTe3. Superconductivity is enhanced via disorder in Te2-Te3 atomic chains that are associated with CDW formation. Hf substitution on the Zr atomic site enhances TCDW due to unperturbed Te2-Te3 chain periodicity and enhanced electron-phonon coupling. Weak electronic correlations in ZrTe3−xSex are likely governed by the lattice contraction effects.

Liu, Yu (ORCID:0000000188862876)↗
DOE OSTI2015

Photoionization of the valence shells of the neutral tungsten atom

Findings from large-scale theoretical cross section calculations for the total photoionization of the 4f, 5s, 5p and 6s orbitals of the neutral tungsten atom using the Dirac Coulomb R-matrix approximation (DARC: Dirac-Atomic R-matrix codes) are introduced. Comparisons are drawn with previous theoretical methods and prior experimental measurements. In previous experiments a time-resolved dual laser approach was employed for the photo-absorption of metal vapours and photo-absorption measurements on tungsten in a solid, using synchrotron radiation. The lowest ground state level of neutral tungsten is 5p 6 5d 4 6s 2 5 D J , with J=0, and requires only a single dipole matrix for photoionization. To make a meaningful comparison with existing experimental measurements, we statistically average the large-scale theoretical PI cross sections from the levels associated with the ground state 5p 6 5d 4 6s 2 5 D J [J = 0, 1, 2, 3, 4] levels and the 5d 5 6s 7 S 3 excited metastable level. As the experiments have a self-evident metastable component in their ground state measurement, averaging over the initial levels allows for a more consistent and realistic comparison to be made. In the wider context, the absence of many detailed electron-impact excitation (EIE) experiments for tungsten and its multi-charged ion stages offers current photoionization measurements and theory to provide a road-map for future electron-impact excitation, ionization and di-electronic cross section calculations by identifying the dominant

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2019

Photoionization of Rb 2+ ions in the valence energy region 37–44 eV

Absolute photoionization cross sections for the Rb2+ ion were recently measured at high resolution over the energy range 37.31–44.08 eV, with autoionizing Rydberg resonance series identified, using the photon-ion merged-beam setup at the Advanced Light Source (als) in Berkeley (Macaluso D A et al 2016 J. Phys. B: At. Mol. Opt. Phys. 49, 235002; 2017 50, 119501). Detailed photon-energy scans taken at 13.5 ± 2.5 meV bandwidth illustrated multiple Rydberg resonance series associated with the ground and metastable states. Here we present theoretical cross section results obtained using the Dirac–Coulomb R-matrix approximation with a detailed analysis of the resonances. The calculations were performed for the $3{d}^{10}4{s}^{2}4{p}^{5}{}^{2}{{{\rm{P}}}^{{\rm{o}}}}_{J}$, $J=\tfrac{3}{2}$ ground state and the corresponding $3{d}^{10}4{s}^{2}4{p}^{5}{}^{2}{{{\rm{P}}}^{{\rm{o}}}}_{J},J=\tfrac{1}{2}$ metastable level. Finally, results from the large-scale calculations are benchmarked against the als high-resolution measurements and reproduce the dominant resonance features in the spectra, providing confidence in the theoretical work for astrophysical applications.

74 ATOMIC AND MOLECULAR PHYSICS↗