SEARCH · Engineering Papers
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.
Explore the source record for details and available documents.
We investigated the structure-dependent vibrational properties of different Mg(BH 4 ) 2 polymorphs (α, β, γ, and δ phases) with a combination of neutron vibrational spectroscopy (NVS) measurements and density functional theory (DFT) calculations, with emphasis placed on the effects of the local structure and orientation of the BH 4 - anions. DFT simulations closely match the neutron vibrational spectra. The main bands in the low-energy region (20–80 meV) are associated with the BH4 - librational modes. The features in the intermediate energy region (80–120 meV) are attributed to overtones and combination bands arising from the lower-energy modes. The features in the high-energy region (120–200 meV) correspond to the BH 4 - symmetric and asymmetric bending vibrations, of which four peaks located at 140, 142, 160, and 172 meV are especially intense. There are noticeable intensity distribution variations in the vibrational bands for different polymorphs. We can explain these differences using the spatial distribution of BH 4 - anions within various structures. An example of the possible identification of products after the hydrogenation of MgB 2 , using NVS measurements, is presented. Our results provide fundamental insights of benefit to researchers currently studying these promising hydrogen-storage materials.
Changes in the electronic structure of pigments in protein environments and of polar molecules in solution inevitably induce a re-adaption of molecular nuclear structure. Both changes of electronic and vibrational energies can be probed with visible or infrared lasers, such as two-dimensional electronic spectroscopy or vibrational spectroscopy. The extent to which the two changes are correlated remains elusive. The recent demonstration of two-dimensional electronic-vibrational (2DEV) spectroscopy potentially enables a direct measurement of this correlation experimentally. However, it has hitherto been unclear how to characterize the correlation from the spectra. In this paper, we present a theoretical formalism to demonstrate the slope of the nodal line between the excited state absorption and ground state bleach peaks in the spectra as a characterization of the correlation between electronic and vibrational transition energies. We also show the dynamics of the nodal line slope is correlated to the vibrational spectral dynamics. Additionally, we demonstrate the fundamental 2DEV spectral line-shape of a monomer with newly developed response functions.
We investigate the vibrational predissociation dynamics of the hydrogen-bonded 3-aminophenol-ammonia cluster (3-AP-NH{sub 3}) in the OH and NH stretching regions. Vibrational excitation provides enough energy to dissociate the cluster into its constituent 3-AP and NH{sub 3} monomers, and we detect the 3-AP fragments via (1 + 1) resonance-enhanced multiphoton ionization (REMPI). The distribution of vibrational states of the 3-AP fragment suggests the presence of two distinct dissociation pathways. The first dissociation channel produces a broad, unstructured feature in the REMPI-action spectrum after excitation of any of the OH or NH stretching vibrations, pointing to a nearly statistical dissociation pathway with extensive coupling among the vibrations in the cluster during the vibrational predissociation. The second dissociation channel produces distinct, resolved features on top of the broad feature but only following excitation of the OH or symmetric NH{sub 3} stretch in the cluster. This striking mode-specificity is consistent with strong coupling of these two modes to the dissociation coordinate (the O–H⋯N bond). The presence of clearly resolved transitions to the electronic origin and to the 10a{sup 2} + 10b{sup 2} state of the cis-3-AP isomer shows that vibrational excitation is driving the isomerization of the trans-3-AP-NH{sub 3} isomer to the cis-3-AP-NH{sub 3} isomer in the course of the dissociation.
Franck-Condon vibrational overlap integrals for the A{sup ~1}A{sub u}—X{sup ~1}Σ{sup +}{sub g} transition in acetylene have been calculated in full dimension in the harmonic normal mode basis. The calculation uses the method of generating functions first developed for polyatomic Franck-Condon factors by Sharp and Rosenstock [J. Chem. Phys. 41(11), 3453–3463 (1964)], and previously applied to acetylene by Watson [J. Mol. Spectrosc. 207(2), 276–284 (2001)] in a reduced-dimension calculation. Because the transition involves a large change in the equilibrium geometry of the electronic states, two different types of corrections to the coordinate transformation are considered to first order: corrections for axis-switching between the Cartesian molecular frames and corrections for the curvilinear nature of the normal modes at large amplitude. The angular factor in the wave function for the out-of-plane component of the trans bending mode, ν{sub 4}{sup ″}, is treated as a rotation, which results in an Eckart constraint on the polar coordinates of the bending modes. To simplify the calculation, the other degenerate bending mode, ν{sub 5}{sup ″}, is integrated in the Cartesian basis and later transformed to the constrained polar coordinate basis, restoring the conventional v and l quantum numbers. An updated A{sup ~}-state harmonic force field obtained recently in the R. W. Field research group is evaluated. The results for transitions involving the gerade vibrational modes are in qualitative agreement with experiment. Calculated results for transitions involving ungerade modes are presented in Paper II of this series [G. B. Park, J. H. Baraban, and R. W. Field, “Full dimensional Franck–Condon factors for the acetylene A{sup ~1}A{sub u}—X{sup ~1}Σ{sup +}{sub g} transition. II. Vibrational overlap factors for levels involving excitation in ungerade modes,” J. Chem. Phys. 141, 134305 (2014)].
Two-dimensional electronic-vibrational (2DEV) spectroscopy is an experimental technique that shows great promise in its ability to provide detailed information concerning the interactions between the electronic and vibrational degrees of freedom in molecular systems. The physical quantities 2DEV is particularly suited for measuring have not yet been fully determined, nor how these effects manifest in the spectra. In this work, we investigate the use of the center line slope of a peak in a 2DEV spectrum as a measure of both the dynamic and static correlations between the electronic and vibrational states of a dye molecule in solution. We show how this center line slope is directly related to the solvation correlation function for the vibrational degrees of freedom. We also demonstrate how the strength with which the vibration on the electronic excited state couples to its bath can be extracted from a set of 2DEV spectra. These analytical techniques are then applied to experimental data from the laser dye 3,3′-diethylthiatricarbocyanine iodide in deuterated chloroform, where we determine the lifetime of the correlation between the electronic transition frequency and the transition frequency for the backbone C = C stretch mode to be ∼1.7 ps. Furthermore, we find that on the electronic excited state, this mode couples to the bath ∼1.5 times more strongly than on the electronic ground state.
The heat capacity of natural (Cu{sub 12−x} (Fe, Zn, Ag){sub x}(Sb, As){sub 4}S{sub 13}) and synthetic (Cu{sub 12−x}Zn{sub x}Sb{sub 4}S{sub 13} with x = 0, 1, 2) tetrahedrite compounds was measured between 2 K and 380 K. It was found that the temperature dependence of the heat capacity can be described using a Debye term and three Einstein oscillators with characteristic temperatures that correspond to energies of ∼1.0 meV, ∼2.8 meV, and ∼8.4 meV. The existence of localized vibrational modes, which are assigned to the displacements of the trigonally coordinated Cu atoms in the structure, is discussed in the context of anharmonicity and its effect on the low lattice thermal conductivity exhibited by these compounds.
Multi-shaker vibration testing is gaining interest from structural dynamics test engineers as it can provide a much more accurate match to complicated field vibration responses than traditional single-axis shaker tests. However, the force capabilities of the small modal shakers typically used in multi-shaker vibration tests has limited the achievable response levels. To date, most multi-shaker vibration tests have been performed using a variety of standard, commercially-available control systems. While these control systems are adequate for a wide range of multiple-input/multiple-output tests, their control algorithms have not been tailored for the specific problem of multi-shaker vibration tests: efficiently coordinating the various shakers to work together to achieve a desired response. Here, a new input estimation algorithm is developed and demonstrated using simulations and actual test data. This algorithm, dubbed shape-constrained input estimation, is shown to effectively coordinate multiple shakers using a set of constraint vectors based on the deflection shapes of the test structure. This is accomplished by using the singular vector shapes of the system frequency response matrix, which allows the constraint vectors to automatically change as a function of frequency. Simulation and test results indicate a significant reduction in the input forces required to achieve a desired response. Finally, the results indicate that shape-constrained input estimation is an effective method to achieve higher response levels from limited shaker forces which will enable higher level multi-shaker vibration tests to be performed.
Here, the C 1 B 2 state of SO 2 has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. However, low-lying levels with odd quanta of antisymmetric stretch (b 2 vibrational symmetry) have not previously been observed because transitions into these levels from the zero-point level of the X ~ state are vibronically forbidden. We use IR-UV double resonance to observe the b 2 vibrational levels of the C state below 1600 cm –1 of vibrational excitation. This enables a direct characterization of the vibrational level staggering that results from the double-minimum potential. In addition, it allows us to deperturb the strong c-axis Coriolis interactions between levels of a 1 and b 2 vibrational symmetry, and to determine accurately the vibrational dependence of the rotational constants in the distorted C electronic state.
Even though in principle the frequency-domain and time-domain spectroscopic measurement should generate identical information for a given molecular system, inhomogeneous character of surface vibrations in the sum-frequency generation vibrational spectroscopy (SFG-VS) studies has only been studied with the time-domain SFGVS by mapping the decay of the vibrational polarization using ultrafast lasers, due to the lack of SFG vibrational spectra with high enough spectral resolution and accurate enough line shape. Here with recently developed high-resolution broadband SFG-VS (HR-BB-SFG-VS) we show that the inhomogeneous line shape can be obtained in the frequency-domain, for the anchoring CN stretch of the 4-n-octyl-4'-cyanobiphenyl (8CB) Langmuir monolayer at the air-water interface, and that an excellent agreement with the time-domain SFG free-induction-decay (FID) results can be established. We found that the 8CB CN stretch spectrum consists of a single peak centered at 2234.00 + * 0.01 cm-1 with a total line width of 10.9 + - 0.3 cm-1 at half maximum. The Lorentzian contribution accounts only for 4:7 + -0:4 cm-1 to this width and the Gaussian (inhomogeneous) broadening for as much as 8:1+*0:2 cm-1. Polarization analysis of the -CN spectra showed that the -CN group is tilted 57 + - 2 degrees from the surface normal. The large heterogeneity in the -CN spectrum is tentatively attributed to the -CN group interactions with the interfacial water molecules penetrated/accomodated into the 8CB monolayer, a unique phenomenon for the nCB Langmuir monolayers reported previously.
Not provided.
Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (ν{sub CN}) and either a ligand-to-metal charge transfer transition ([Fe{sup III}(CN){sub 6}]{sup 3−} dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN){sub 5}Fe{sup II}CNRu{sup III}(NH{sub 3}){sub 5}]{sup −} dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific ν{sub CN} modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems.
We employed ultrafast transient absorption and broadband 2DIR spectroscopy to study the vibrational dynamics of aqueous hydroxide solutions by exciting the O–H stretch vibrations of the strongly hydrogen-bonded hydroxide solvation shell water and probing the continuum absorption of the solvated ion between 1500 and 3800 cm{sup −1}. We observe rapid vibrational relaxation processes on 150–250 fs time scales across the entire probed spectral region as well as slower vibrational dynamics on 1–2 ps time scales. Furthermore, the O–H stretch excitation loses its frequency memory in 180 fs, and vibrational energy exchange between bulk-like water vibrations and hydroxide-associated water vibrations occurs in ∼200 fs. The fast dynamics in this system originate in strong nonlinear coupling between intra- and intermolecular vibrations and are explained in terms of non-adiabatic vibrational relaxation. These measurements indicate that the vibrational dynamics of the aqueous hydroxide complex are faster than the time scales reported for long-range transport of protons in aqueous hydroxide solutions.
Coherence oscillations measured in two-dimensional (2D) electronic spectra of pigment-protein complexes may have electronic, vibrational, or mixed-character vibronic origins, which depend on the degree of electronic-vibrational mixing. Oscillations from intrapigment vibrations can obscure the inter-site coherence lifetime of interest in elucidating the mechanisms of energy transfer in photosynthetic light-harvesting. Huang-Rhys factors (S) for low-frequency vibrations in Chlorophyll and Bacteriochlorophyll are quite small (S ≤ 0.05), so it is often assumed that these vibrations influence neither 2D spectra nor inter-site coherence dynamics. In this work, we explore the influence of S within this range on the oscillatory signatures in simulated 2D spectra of a pigment heterodimer. To visualize the inter-site coherence dynamics underlying the 2D spectra, we introduce a formalism which we call the “site-probe response.” By comparing the calculated 2D spectra with the site-probe response, we show that an on-resonance vibration with Huang-Rhys factor as small as S = 0.005 and the most strongly coupled off-resonance vibrations (S = 0.05) give rise to long-lived, purely vibrational coherences at 77 K. We moreover calculate the correlation between optical pump interactions and subsequent entanglement between sites, as measured by the concurrence. At 77 K, greater long-lived inter-site coherence and entanglement appear with increasing S. This dependence all but vanishes at physiological temperature, as environmentally induced fluctuations destroy the vibronic mixing.
Explore the source record for details and available documents.
Recently, nuclear vibrational contribution signatures in two-dimensional (2D) electronic spectroscopy have attracted considerable interest, in particular as regards interpretation of the oscillatory transients observed in light-harvesting complexes. These transients have dephasing times that persist for much longer than theoretically predicted electronic coherence lifetime. As a plausible explanation for this long-lived spectral beating in 2D electronic spectra, quantum-mechanically mixed electronic and vibrational states (vibronic excitons) were proposed by Christensson et al. [J. Phys. Chem. B 116, 7449 (2012)] and have since been explored. In this work, we address a dimer which produces little beating of electronic origin in the absence of vibronic contributions, and examine the impact of protein-induced fluctuations upon electronic-vibrational quantum mixtures by calculating the electronic energy transfer dynamics and 2D electronic spectra in a numerically accurate manner. It is found that, at cryogenic temperatures, the electronic-vibrational quantum mixtures are rather robust, even under the influence of the fluctuations and despite the small Huang-Rhys factors of the Franck-Condon active vibrational modes. This results in long-lasting beating behavior of vibrational origin in the 2D electronic spectra. At physiological temperatures, however, the fluctuations eradicate the mixing, and hence, the beating in the 2D spectra disappears. Further, it is demonstrated that such electronic-vibrational quantum mixtures do not necessarily play a significant role in electronic energy transfer dynamics, despite contributing to the enhancement of long-lived quantum beating in 2D electronic spectra, contrary to speculations in recent publications.
Quasi-classical molecular dynamics simulations were used to study the energy relaxation of an initially non-rotating, vibrationally excited ( v = 4) hydroxyl radical (OH) in an Ar bath at 300 K and at high pressures from 50 atm to 400 atm. A Morse oscillator potential represented the OH, and two sets of interaction potentials were used based on whether the Ar-H potential was a Buckingham (Exp6) or a Lennard-Jones (LJ) potential. The vibrational and rotational energies were monitored for 25 000-90 000 ps for Exp6 trajectories and 5000 ps for LJ trajectories. Comparisons to measured vibrational relaxation rates show that Exp6 rates are superior. Simulated initial vibrational relaxation rates are linearly proportional to pressure, implying no effect of high-pressure breakdown in the isolated binary collision approximation. The vibrational decay curves upward from single-exponential decay. A model based on transition rates that exponentially depend on the anharmonic energy gap between vibrational levels fits the vibrational decay well at all pressures, suggesting that anharmonicity is a major cause of the curvature. Due to the competition of vibration-to-rotation energy transfer and bath gas relaxation, the rotational energy overshoots and then relaxes to its thermal value. Approximate models with adjustable rates for this competition successfully reproduced the rotational results. These models show that a large fraction of the vibrational energy loss is initially converted to rotational energy but that fraction decreases rapidly as the vibrational energy content of OH decreases. Finally, while simulated rates change dramatically between Exp6 and LJ potentials, the mechanisms remain the same.
A full-dimensional Franck-Condon calculation has been applied to the A{sup ~} {sup 1}A{sub u}—X{sup ~1}Σ{sub g}{sup +} transition in acetylene in the harmonic normal mode basis. Details of the calculation are discussed in Part I of this series. To our knowledge, this is the first full-dimensional Franck-Condon calculation on a tetra-atomic molecule undergoing a linear-to-bent geometry change. In the current work, the vibrational intensity factors for levels involving excitation in ungerade vibrational modes are evaluated. Because the Franck-Condon integral accumulates away from the linear geometry, we have been able to treat the out-of-plane component of trans bend (ν{sub 4}{sup ′′}) in the linear X{sup ~} state in the rotational part of the problem, restoring the χ Euler angle and the a-axis Eckart conditions. A consequence of the Eckart conditions is that the out-of-plane component of ν{sub 4}{sup ′′} does not participate in the vibrational overlap integral. This affects the structure of the coordinate transformation and the symmetry of the vibrational wavefunctions used in the overlap integral, and results in propensity rules involving the bending modes of the X{sup ~} state that were not previously understood. We explain the origin of some of the unexpected propensities observed in IR-UV laser-induced fluorescence spectra, and we calculate emission intensities from bending levels of the A{sup ~} state into bending levels of the X{sup ~} state, using normal bending mode and local bending mode basis sets. Our calculations also reveal Franck-Condon propensities for the Cartesian components of the cis bend (ν{sub 5}{sup ′′}), and we predict that the best A{sup ~}-state vibrational levels for populating X{sup ~}-state levels with large amplitude bending motion localized in a single C–H bond (the acetylene↔vinylidene isomerization coordinate) involve a high degree of excitation in ν{sub 6}{sup ′} (cis-bend). Mode ν{sub 4}{sup ′} (torsion) populates levels with large amplitude counter-rotational motion of the two hydrogen atoms.