Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Vibrational Spectroscopy”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Ligand-Structure-Dependent Coherent Vibrational Wavepacket Dynamics in Pyrazolate-Bridged Pt(II) Dimers

Bimetallic transition metal complexes have gained increasing attention because of their versatile functions in solar energy conversion and photonics applications arising from inter-metal electronic coupling. In bimetallic platinum (Pt) complexes, electronic communication between the Pt-centered and ligand-centered moieties have been shown to be critical for defining their excited-state dynamic trajectories undergoing either localized ligand centered (LC)/metal-to-ligand-charge-transfer (MLCT) transitions or delocalized metal-metal-to-ligand-charge-transfer (MMLCT) transitions. The branching of the excited-state intersystem crossing (ISC) trajectories are modulated through structural factors that alter the relative energies of the different states. In this study, we investigated the correlation of the structural factors influencing the excited state trajectories. Using femtosecond broadband transient absorption (fs-BBTA) spectroscopy, ultrafast dynamics in the excited state of two select Pt(II) dimers have been mapped out using their coherent vibrational wavepacket signatures in corresponding transient absorption spectra. To examine how the ligand moieties of the Pt(II) dimers influence excited-state dynamics and the coherent vibrational wavepacket behavior, here we carried out comparative studies on two pyrazolate-bridged Pt(II) dimers of the general formula [Pt( t Bu 2 Pz)(N^C)] 2 ( t Bu 2 Pz = 3,5-di-tert-butylpyrazole); N^C = 7,8-benzoquinoline (bzq, 1) or 1-phenylisoquinoline (piq, 2)). We found that photoexcitation into the low energy absorption bands of 1 and 2 respectively induce the formation of 1 MMLCT states from which ultrafast ISC proceeds, resulting in stimulated emission quenching and decoherence of the vibrational wavepacket motions. The results obtained in this study suggest that both energetics and the structural rigidity of the aromatic cyclometalating ligands in 1 and 2 can significantly influence dynamics along the excited state trajectory characterized by dephasing of the coherent oscillations. The collective results provide direct evidence of how ligand structure alters electronic dynamics along excited state trajectories associated with ISC processes, providing insight into using ligand design to steer photochemical processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1980

A laboratory study on the dissociative recombination of vibrationally excited O2/+/ions

The dissociative recombination of vibrationally excited O2(+) ions is studied in light of the possible importance of this reaction in upper atmospheric chemistry. A plasma spectroscopy experiment was performed in a microwave cavity filled by an argon-oxygen mixture, with O(1S) production monitored by measurements of the 5577-A afterglow, the O2(+) density and the electron concentration. Plasma and optical data reveal the predominant afterglow ions to be Ar2(+) and O2(+), with an effective O(1S) dissociative recombination coefficient of 2.1 x 10 to the -8th cu cm/sec, corresponding to a quantum yield of 10%. Experiments with an argon-krypton-oxygen mixture reveal that vibrationally excited O2(+) ions are the chief source of the O(1S) atoms, with a specific recombination coefficient for the dissociation of O2(+)(2 pi g) into O(1S) and O(1D) of 4.2 x 10 to the -9th cu cm/sec. A comparison of the laboratory results with Atmospheric Explorer data on the 5577-A airglow implies that O2(+) ions in the sunlit ionosphere are vibrationally excited to the same degree as in the laboratory, with the vibrational relaxation of these ions much slower than dissociative recombination. Results also predict a dawn-twilight asymmetry in the effective O(1S) yield due to the normal variation of electron content.

Zipf, E. C.↗
DOE OSTI2022

A two-directional vibrational probe reveals different electric field orientations in solution and an enzyme active site

We report the catalytic power of an electric field depends on its magnitude and orientation with respect to the reactive chemical species. Understanding and designing new catalysts for electrostatic catalysis thus requires methods to measure the electric field orientation and magnitude at the molecular scale. We demonstrate that electric field orientations can be extracted using a two-directional vibrational probe by exploiting the vibrational Stark effect of both the C=O and C–D stretches of a deuterated aldehyde. Combining spectroscopy with molecular dynamics and electronic structure partitioning methods, we demonstrate that, despite distinct polarities, solvents act similarly in their preference for electrostatically stabilizing large bond dipoles at the expense of destabilizing small ones. In contrast, we find that for an active-site aldehyde inhibitor of liver alcohol dehydrogenase, the electric field orientation deviates markedly from that found in solvents, which provides direct evidence for the fundamental difference between the electrostatic environment of solvents and that of a preorganized enzyme active site.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1994

Near-IR diode laser absorption for measurement of tropospheric HO2

The possibility of using tunable lead salt diode lasers in the infrared for measurement of tropospheric HO2 has been frequently considered. Although the sensitivity of diode laser absorption has been improved through the use of high frequency detection techniques, nature has been unkind in that the HO2 absorption cross sections are weak. Even using the most optimistic assumptions about attainable path length and detectable absorbance, measurement of tropospheric HO2 by diode laser absorption in the mid-IR appears marginal. A possible alternative method for measuring HO2 is by absorption at near-infrared wavelengths. Several absorption bands of HO2 occur in the wavelength region between 1.2 and 1.6 micron due to electronic transitions and overtones of the fundamental vibrational modes. InGaAsP diode lasers operate in this wavelength region and can be used for high resolution spectroscopy in a manner analogous to the lead salt lasers. A diode laser system in the near-IR offers some advantages.

Stanton, Alan C.↗
NASA NTRS1996

Atmospheric Oxygen Photoabsorption

The work conducted on this grant was devoted to various aspects of the photophysics and photochemistry of the oxygen molecule. Predissociation linewidths were measured for several vibrational levels in the O2(B3 Sigma(sub u)(sup -)) state, providing good agreement with other groups working on this important problem. Extensive measurements were made on the loss kinetics of vibrationally excited oxygen, where levels between v = 5 and v = 22 were investigated. Cavity ring-down spectroscopy was used to measure oscillator strengths in the oxygen Herzberg bands. The great sensitivity of this technique made it possible to extend the known absorption bands to the dissociation limit as well as providing many new absorption lines that seem to be associated with new O2 transitions. The literature concerning the Herzberg band strengths was evaluated in light of our new measurements, and we made recommendations for the appropriate Herzberg continuum cross sections to be used in stratospheric chemistry. The transition probabilities for all three Herzberg band systems were re-evaluated, and we are recommending a new set of values.

Slanger, Tom G.↗
NASA NTRS1994

Rotational Relaxation Analysis of Nitrogen in Low-Density Freejet Expansions

The phenomena of rotational relaxation of nitrogen has been examined by numerous investigators over many years. One of the experiments which has been performed examines nonequilibrium flow in low-density free jet expansions. Data have been taken in such flows using a variety of techniques, including time-of-flight methods and electron beam fluorescence spectroscopy. The direct flow properties measured in these different investigations, such as density and translational, rotational and vibrational temperatures generally show reasonable agreement. However, this kind of correlation from experiment to experiment tends to be lost when these data are analyzed to obtain rotational relaxation time or collision number. The goal of such data analyses is to generate a succinct model for rotational relaxation in nitrogen which is essential for the computation of nonequilibrium rarefied flows. The objective of the present work is to process a large body of experimental data in a consistent manner to yield relaxation model parameters of the greatest utility for flow computations.

Gochberg, Lawrence A.↗
DOE OSTI2020

One-colour (~220 nm) resonance-enhanced (S 1 –S 0 ) multi-photon dissociation of acetylene: probe of the C 2 A 1 Π u – X 1 Σ + g band by frequency-modulation spectroscopy

In a recent paper, we demonstrated that one-colour (~220 nm), resonance-enhanced (S 1 –S 0 ), photodissociation of acetylene generates strong $C_2$ Swan band ($d^3Π_g–a^3Π_u$) and $C_2$ Deslandres-d'Azambuja band ($C^1Π_g–A^1Π_u$) fluorescence, and long-lived (>3 µs) fluorescence from an electronically-excited $C_2$H * species. It was not known whether the $C_2A^1Π_u$ and $X^1Σ^+_g$ states are also directly populated in this process. In this paper, multiple vibration-rotation transitions between the $C_2A$-state v = 2 and the X-state v = 0 level are examined by time-resolved frequency-modulation (FM) spectroscopy. The photolysis laser wavelength is tuned into resonance at the one-photon level with S 1 –S 0 transitions that populate individual rotational levels of the S 1 $_{trans}$-conformer $3^2,3^3,$ and $3^4$ vibrational states. By comparing the phase of the FM signals from the $C_2A–X$ transitions with that from the Rb D 1 -line absorption transition, we determine that, for all of the probed A–X transitions, the X-state level is more populated than the A-state level. We propose that the acetylene S 1 level is excited by the second photon to an acetylene dissociation precursor state, which undergoes sequential C-H bond-breaking to produce the $C_2X$ state. The dissociation precursor is assigned as the $1^1B_g(trans-bent)/1^1B_1(cis-bent)$ valence state, which correlates to a doubly-excited configuration, $(1π_u)^2(1π_g)^2$, at linear geometry. Based on the rotational distributions of the $C_2X$-state fragments, we believe that at least one of the transition states involved in the photolysis via S 1 $3^4$ has a larger CC-H bend-angle for the departing H-atom than that involved in the S 1 $3^2$ and $3^3$ photolysis.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Direct Visualization of Localized Vibrations at Complex Grain Boundaries

Abstract Grain boundaries (GBs) are a prolific microstructural feature that dominates the functionality of a wide class of materials. The functionality at a GB results from the unique atomic arrangements, different from those in the grain, that have driven extensive experimental and theoretical studies correlating atomic‐scale GB structures to macroscopic electronic, infrared optical, and thermal properties. In this work, a SrTiO 3 GB is examined using atomic‐resolution aberration‐corrected scanning transmission electron microscopy and ultrahigh‐energy‐resolution monochromated electron energy‐loss spectroscopy, in conjunction with density functional theory. This combination enables the correlation of the GB structure, nonstoichiometry, and chemical bonding with a redistribution of vibrational states within the GB dislocation cores. The new experimental access to localized GB vibrations provides a direct route to quantifying the impact of individual boundaries on macroscopic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Conical intersections shed light on hot carrier cooling in quantum dots

Experimental observations of vibronic coherences in electronically excited colloidal semiconductor nanocrystals offer a window into the ultrafast dynamics of hot carrier cooling. In previous work, we showed that, in amine-passivated quantum dots (QDs), these coherences arise during relaxation through a cascade of conical intersections between electronically excited states. Here, in this work, we demonstrate the generality of this framework by application to QDs with surface-bound carboxylate ligands. A model involving a similar cascade of conical intersections accurately reproduces the frequencies of vibronic coherences observed with broadband multidimensional spectroscopy. The impact of ligands on the relaxation dynamics is attributed to two distinct mechanisms involving either electronic or vibrational coupling between the core and ligands. Compared to the amine-passivated QDs studied previously, the electronic coupling mechanism is less prominent in carboxylate-passivated QDs. Furthermore, comparison of acetate and formate ligands reveals that truncating the ligand alkyl chains alters the relaxation behavior predicted by the model.

14 SOLAR ENERGY↗
DOE OSTI2021

Broadband 2DES detection of vibrational coherence in the S x state of canthaxanthin

The nonadiabatic mechanism that mediates nonradiative decay of the bright S 2 state to the dark S 1 state of carotenoids involves population of a bridging intermediate state, S x , in several examples. The nature of S x remains to be determined definitively, but it has been recently suggested that S x corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S 2 potential surface. Here, the electronic and vibrational dynamics accompanying the formation of S x in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of S x in <20 fs following excitation of the main absorption band. S x is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN’s isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that S x undergoes displacements from the Franck–Condon S 2 state along out-of-plane coordinates as it passes to the S 1 state. The results are consistent with the conclusion that CAN’s carbonyl-substituted β-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from S x to S 1 compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S 1 state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the S x state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1986

An assessment of OH airglow interference on the remote sensing of stratospheric HCl via limb sounding in the near-I.R.

Chemiluminescent emission from the formation of vibrationally excited OH at an altitude of approximately 85 km presents a barrier to earth-limb measurements of NIR emissions originating in the stratosphere. High-resolution measured spectra of OH airglow have not been reported, and thus it is difficult to assess its impact on moderately-high-resolution earth-limb spectroscopy capable of looking between strong OH lines. Low-resolution rocketborne spectrometer earth-limb measurements of OH airglow are mathematically inverted to recover vibrational-state populations that are used to simulate a higher resolution spectrum in the neighbourhood of 2843/cm. Implications for remote sensing HCl are presented.

Mergenthaler, J. L.↗
DOE OSTI2021

Electronic structure of reduced CeO 2 (111) surfaces interacting with hydrogen as revealed through electron energy loss spectroscopy in comparison with theoretical investigations

Based on both, ab-initio cluster calculations as well as periodic density functional based loss function calculations, we have assigned the origins of the valence electron excitation regime in electron energy loss spectra of well-ordered ceria films in (111) orientation at various states of reduction as well as after exposing the reduced films to hydrogen from the gas phase (Li et al, Angew. Chem. Int. Ed., 58 (2019) 14686–14693). The explicit calculation of intensity distributions using the dipole approximation allow us to draw conclusions about the nature of oxygen vacancies, which occur upon reduction and how those interact with hydrogen to form hydride species. Herein, we also reports a brief discussion of vibrational excitations of hydrogen loaded ceria and corroborates previous interpretations, based on inelastic neutron scattering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Ultrafast solvent-to-solute proton transfer mediated by intermolecular coherent vibrations

Ultrafast photoinduced excited-state proton transfer (ESPT) plays a crucial role in protecting biomolecules and functional materials from photodamage. However, the influence of solute-solvent interactions on ESPT dynamics remains under active investigation. Here, we present an ultrafast spectroscopic study of ESPT in the photobase 2-(2´-pyridyl)benzimidazole (PBI) in methanol. Ultrafast absorption spectroscopy, supported by quantum chemical calculations, reveals three distinct kinetic steps: (1) a 2.2 ps solvent-to-solute proton transfer, (2) subsequent nonradiative relaxation to the ground state within 31 ps, producing a vibrationally hot ensemble with substantial excess kinetic energy, and (3) equilibration as this energy dissipates into the surrounding solvent bath over 186 ps. Femtosecond-resolved dynamics exhibit oscillatory signals indicative of coherent wavepacket motion on the S 1 potential energy surface. A phase flip in the excited-state absorption maximum confirms this assignment. Fourier analysis resolves two dominant periods (∼117 fs and ∼340 fs), corresponding to in-plane and out-of-plane vibrational modes coupled between PBI and the hydrogen-bonded methanol molecule. The rapid dephasing ( < 300 fs) suggests that the nuclear wavefunction evolves on an anharmonic potential energy surface while traversing the ESPT reaction coordinate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Disentangling the confounding spectroscopy of C 1 molecules: Without symmetry as a guide, everything is allowed

The spectra of C 1 molecules are confounding in that each of the fundamental vibrational modes transform as the same irreducible representation (A) and hence each band consists of a seemingly random distribution of a-, b-, and c-type transitions. This is in contrast to higher symmetry molecules for which band types are readily deduced by simple symmetry rules. Herein, we present a method to simulate the convoluted rotational contours in the gas-phase spectra of C 1 molecules by combining existing ab initio calculations with Colin Western’s PGOPHER rotational contour program. Specifically, ab initio calculations in the NWC HEM suite of programs were employed to predict the components of the dipole moment derivatives along the principal axes of the moments of inertia. This information was then input into PGOPHER to model the fundamental band contours as a sum of a-, b-, and c-type transitions. This method was applied to simulate the rotational contour spectra of a series of representative C 1 molecules which were then compared against both ab initio stick spectra and experimentally measured broadband IR spectra from the Pacific Northwest National Laboratory infrared gas-phase database. In addition to providing further insight beyond what is revealed in a typical stick spectrum, the simulated contours showed good agreement with the measured spectra.

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

Structural and vibrational behavior of 2 H -NbSe 2 at high pressures

In this work, the high-pressure structural and vibrational properties of a layered transition metal dichalcogenide 2 H -NbSe 2 were investigated using single-crystal x-ray diffraction and Raman spectroscopy, demonstrating its structural stability up to 35 GPa. The lattice compressibility changes character from being highly anisotropic at low pressures to largely isotropic at high pressures. Concomitantly, the interatomic bonds demonstrate highly anisotropic compression behavior with the Se-Se interlayer bonds compressing by >20%, while the intramolecular Se-Se distance shows a nonmonotonic pressure dependence with a maximum at-12 GPa. The nearest-neighbor central force lattice vibrational model yields pressure dependencies of the interatomic forces in qualitative agreement with bond length compression, providing insight into the vibrational properties of 2 H -NbSe 2 at high pressures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

In-operando surface-sensitive probing of electrochemical reactions on nanoparticle electrocatalysts: Spectroscopic characterization of reaction intermediates and elementary steps of oxygen reduction reaction on Pt

In this paper we present an in-operando spectroscopic study of the reaction intermediates involved in ORR on electrified Pt nanoparticle electrocatalysts. To accomplish this, we have synthesized Ag-Pt core–shell nanoparticles, that contain a thin shell of Pt (~1 nm) uniformly deposited on a larger Ag core (~60 nm). We show that these nanostructures are ideal platforms for in-operando Plasmon Enhanced Raman Spectroscopy (PERS) of ORR on Pt. We combined these PERS studies with DFT calculations to assign the measured vibrational spectra to relevant surface intermediates as a function of electrocatalyst potential. Overall, these vibrational assignments were further verified and validated by performing PERS measurements using de-oxygenated and heavy oxygen (O 2 -18) saturated electrolyte. These studies allowed us to identity the presence of critical reaction intermediates (OH, O 2 , OOH, and H 2 O) as the function of operating voltage on the Pt surface and derive voltage-dependent elementary step mechanisms of the reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗