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At least 1,063 records · Page 59Linked to original sources
PubMed2005

Quantum vibrational state-dependent potentials for classical many-body simulations.

In this paper, we present a method for constructing simple state-dependent many-body potentials for quantum vibrations in a classical bath. The approach is based on an adiabatic separation between high-frequency quantum vibrational modes of the solute and the lower frequency classical motion of the solvent, and on a first-order perturbation theory description of the dependence of the quantum energies on bath configuration. In the simplest realization of the method, the delocalized quantum probability density of the vibrational mode is approximated by a sum of two delta functions, with positions and weights chosen to represent the lowest three moments of the exact distribution. Thus, in the many-body description of the system, each atom describing the quantum vibration is represented by a pair of particles. These quantum particles are held in rigid relative position and interact with the bath via potentials the magnitudes of which are modified by the delta-function weights. The resulting approach allows the classical molecular dynamics of molecules in arbitrary quantum vibrational states to be simulated with a little more effort than a purely classical description. The applicability of the method is illustrated in many-body simulations of the dephasing of vibrational superposition states of I(2) in a cryogenic krypton matrix, yielding results in good agreement with experiment.

Journal Article↗
PubMed2005

Time-resolved infrared absorption studies of the solvent-dependent vibrational relaxation dynamics of chlorine dioxide.

We report a series of time-resolved infrared absorption studies on chlorine dioxide (OClO) dissolved in H2O, D2O, and acetonitrile. Following the photoexcitation at 401 nm, the evolution in optical density for frequencies corresponding to asymmetric stretch of OClO is measured with a time resolution of 120+/-50 fs. The experimentally determined optical-density evolution is compared with theoretical models of OClO vibrational relaxation derived from collisional models as well as classical molecular-dynamics (MD) studies. The vibrational relaxation rates in D2O are reduced by a factor of 3 relative to H2O consistent with the predictions of MD. This difference reflects modification of the frequency-dependent solvent-solute coupling accompanying isotopic substitution of the solvent. Also, the geminate-recombination quantum yield for the primary photofragments resulting in the reformation of ground-state OClO is reduced in D2O relative to H2O. It is proposed that this reduction reflects enhancement of the dissociation rate accompanying vibrational excitation along the asymmetric-stretch coordinate. In contrast to H2O and D2O, the vibrational-relaxation dynamics in acetonitrile are not well described by the theoretical models. Reproduction of the optical-density evolution in acetonitrile requires significant modification of the frequency-dependent solvent-solute coupling derived from MD. It is proposed that this modification reflects vibrational-energy transfer from the asymmetric stretch of OClO to the methyl rock of acetonitrile. In total, the results presented here provide a detailed description of the solvent-dependent geminate-recombination and vibrational-relaxation dynamics of OClO in solution.

Journal Article↗
PubMed2005

Vibrational coupling in carboxylic acid dimers.

The vibrational level splitting in the ground electronic state of carboxylic acid dimers mediated by the doubly hydrogen-bonded networks are investigated using pure and mixed dimers of benzoic acid with formic acid as molecular prototypes. Within the 0-2000-cm(-1) range, the frequencies for the fundamental and combination vibrations of the two dimers are experimentally measured by using dispersed fluorescence spectroscopy in a supersonic jet expansion. Density-functional-theory calculations predict that most of the dimer vibrations are essentially in-phase and out-of-phase combinations of the monomer modes, and many of such combinations show significantly large splitting in vibrational frequencies. The infrared spectrum of the jet-cooled benzoic acid dimer, reported recently by Bakker et al. [J. Chem. Phys. 119, 11180 (2003)], has been used along with the dispersed fluorescence spectra to analyze the coupled g-u vibrational levels. Assignments of the dispersed fluorescence spectra of the mixed dimer are suggested by comparing the vibronic features with those in the homodimer spectrum and the predictions of density-functional-theory calculation. The fluorescence spectra measured by excitations of the low-lying single vibronic levels of the mixed dimer reveal that the hydrogen-bond vibrations are extensively mixed with the ring modes in the S1 surface.

Journal Article↗
PubMed2005

Photoelectron spectroscopy of S1 toluene: I. Photoionization propensities of selected vibrational levels in S1 toluene.

Laser photoelectron spectra have been obtained following the preparation of eight vibrational states in S(1) toluene. For four of the vibrational states (up to approximately 550 cm(-1) excess energy) excitation and ionization with nanosecond laser pulses give rise to photoelectron spectra with well-resolved vibrational peaks. For the other states (>750 cm(-1) excess energy) the photoelectron spectra show a loss of structure when nanosecond pulses are used, as a result of intramolecular dynamics [see Whiteside et al., J. Chem. Phys. 123, 204317 (2005), following paper]. A number of vibrational peaks in the photoelectron spectra are assigned, and we find that the common series of ion vibrational peaks observed following the ionization of p-fluorotoluene in various S(1) vibrational states is not reproduced in toluene.

Journal Article↗
PubMed2006

Optimal control in a dissipative system: vibrational excitation of CO/Cu(100) by IR pulses.

The question as to whether state-selective population of molecular vibrational levels by shaped infrared laser pulses is possible in a condensed phase environment is of central importance for such diverse fields as time-resolved spectroscopy, quantum computing, or "vibrationally mediated chemistry." This question is addressed here for a model system, representing carbon monoxide adsorbed on a Cu(100) surface. Three of the six vibrational modes are considered explicitly, namely, the CO stretch vibration, the CO-surface vibration, and a frustrated translation. Optimized infrared pulses for state-selective excitation of "bright" and "dark" vibrational levels are designed by optimal control theory in the framework of a Markovian open-system density matrix approach, with energy flow to substrate electrons and phonons, phase relaxation, and finite temperature accounted for. The pulses are analyzed by their Husimi "quasiprobability" distribution in time-energy space.

Journal Article↗
PubMed2006

Infrared overtone spectroscopy and vibrational analysis of a Fermi resonance in nitric acid: Experiment and theory.

High resolution infrared spectra of nitric acid have been recorded in the first OH overtone region under jet-cooled conditions using a sequential IR-UV excitation method. Vibrational bands observed at 6933.39(3), 6938.75(4), and 6951.985(3) cm(-1) (origins) with relative intensities of 0.42(1), 0.38(1), and 0.20(1) are attributed to strongly mixed states involved in a Fermi resonance. A vibrational deperturbation analysis suggests that the optically bright OH overtone stretch (2nu1) at 6939.2(1) cm(-1) is coupled directly to the nu1 + 2nu2 state at 6946.4(1) cm(-1) and indirectly to the 3nu2 + nu3 + nu7 state at 6938.5(1) cm(-1). Both the identity of the zero-order states and the indirect coupling scheme are deduced from complementary CCSD(T) calculations in conjunction with second-order vibrational perturbation theory. The deperturbation analysis also yields the experimental coupling between 2nu1 and nu1 + 2nu2 of -6.9(1) cm(-1), and that between the two dark states of +5.0(1) cm(-1). The calculated vibrational energies and couplings are in near quantitative agreement with experimentally derived values except for a predicted twofold stronger coupling of 2nu1 to nu1 + 2nu2. Weaker coupling of the strongly mixed states to a dense background of vibrational states via intramolecular vibrational energy redistribution is evident from the experimental linewidths of 0.08 and 0.25 cm(-1) for the higher energy and two overlapping lower energy bands, respectively. A comprehensive rotational analysis of the higher energy band yields spectroscopic parameters and the direction of the OH overtone transition dipole moment.

Journal Article↗
PubMed2006

The vibrational structures of furan, pyrrole, and thiophene cations studied by zero kinetic energy photoelectron spectroscopy.

The vibrational structures of the electronic ground states ((approximately)X (2)A(2)) of furan, pyrrole, and thiophene cations have been studied by zero kinetic energy (ZEKE) photoelectron spectroscopic method. In addition to the strong excitations of the symmetric a(1) vibrational modes, other three symmetric vibrational modes (a(2), b(1), and b(2)) have been observed unambiguously. These results which cannot be explained by the Franck-Condon principle illustrate that the vibronic coupling and the Coriolis coupling may play important roles in understanding the vibrational structures of the five-membered heterocycle cations. The vibrationally resolved ZEKE spectra are assigned with the assistance of the density function theory calculations, and the fundamental frequencies for many vibrational modes have been determined for the first time. The first adiabatic ionization energies for furan, pyrrole, and thiophene were determined as 8.8863, 8.2099, and 8.8742 eV, respectively, with uncertainties of 0.0002 eV.

Journal Article↗
PubMed2005

Detection of chiral sum frequency generation vibrational spectra of proteins and peptides at interfaces in situ.

In this work, we demonstrate the feasibility to collect off-electronic resonance chiral sum frequency generation (SFG) vibrational spectra from interfacial proteins and peptides at the solid/liquid interface in situ. It is difficult to directly detect a chiral SFG vibrational spectrum from interfacial fibrinogen molecules. By adopting an interference enhancement method, such a chiral SFG vibrational spectrum can be deduced from interference spectra between the normal achiral spectrum and the chiral spectrum. We found that the chiral SFG vibrational spectrum of interfacial fibrinogen was mainly contributed by the beta-sheet structure. For a beta-sheet peptide tachyplesin I, which may be quite ordered at the solid/liquid interface, chiral SFG vibrational spectra can be collected directly. We believe that these chiral signals are mainly contributed by electric dipole contributions, which can dominate the chiroptical responses of uniaxial systems. For the first time, to our knowledge, this work indicates that the off-electronic resonance SFG technique is sensitive enough to collect chiral SFG vibrational spectra of interfacial proteins and peptides, providing more structural information to elucidate interfacial protein and peptide structures.

Animals↗
PubMed1991

Crystal structure and thermal vibrations of cholesteryl acetate from neutron diffraction at 123 and 20 K.

Cholesteryl acetate (C29H48O2) at 20 K is monoclinic, space group P2(1) with a = 16,521 (4), b = 9,220 (2), c = 17,620 (5) A, beta = 107.18 (2) degrees and Z = 4 (two molecules in the asymmetric unit). The crystal structure, earlier determined by X-ray diffraction at 123 K, has been redetermined at 123 and 20 K by neutron diffraction (7447 reflections at 123 K, 7281 at 20 K; sin theta/lambda less than 0.69 A-1) and refined by full-matrix least squares with 1423 variables to give R(F2) = 0.10 at 123 K, 0.06 at 20 K. Results at 20 K are more accurate not only because nuclear thermal vibrations have reduced amplitudes but also because a larger crystal was used for data collection. At 20 K, the average of 44 methylene C-H bond lengths is 1,102 (8) A (sigma from the observed distribution) and 1.103 (6) A for 16 methine groups. The 22 methylene H-C-H angles [106.2 (9) degrees] all fall within a narrow range. Analysis of nuclear anisotropic thermal parameters shows that intramolecular vibrations of the H nuclei are highly significant with similar mean-square amplitudes at 123 and 20 K. Torsional vibrations around the C-CH3 bonds of the C18 and C19 methyl groups have r.m.s. amplitudes of about 7 degrees. For methylene groups, mean-square amplitudes are a minimum for C-H bond stretching (0.006 A2) and a maximum for CH2 in-plane vibrations (0.024 A2). Values of Beq for the H-atom intramolecular vibrations are 0.8, 1.2 and 1.6 A2 for methine, methylene and methyl H atoms respectively. These results can be used to estimate Beq for H atoms in crystal structure determinations carried out by X-ray diffraction. The anisotropic intramolecular vibrations of the H nuclei contribute to the effective shape of the H atoms which, from the observed anisotropy in the distribution of intermolecular H...H distances, appears to be flattened along the direction of the C-H bond.

Cholesterol Esters↗
PubMed2003

Thickness-shear vibrations of a quartz plate under time-dependent biasing deformations.

Incremental thickness-shear vibrations of a Y-cut quartz crystal plate under time-harmonic biasing extensional deformations are studied using the two-dimensional equations for small fields superposed on finite biasing fields in an electroelastic plate. It is shown that the incremental thickness-shear vibrations are governed by the well-known Mathieu's equation with a time-dependent coefficient. Both free and electrically forced vibrations are studied. Approximate analytical solutions are obtained when the frequency of the biasing deformation is much lower than that of the incremental thickness-shear vibration. The incremental thickness-shear free vibration mode is shown to be both frequency and amplitude modulated, with the frequency modulation as a first-order effect and the amplitude modulation a second-order effect. The forced vibration solutions show that both the static and motional capacitances become time-dependent due to the time-harmonic biasing deformations.

Journal Article↗
PubMed2005

Theoretical, numerical, and experimental investigation on resonant vibrations of piezoceramic annular disks.

In this study, vibration characteristics of thin piezoceramic annular disks with stress-free boundary conditions are investigated by theoretical analysis, numerical simulation, and experimental measurement. The nonaxisymmetric, out-of-plane (transverse), and axisymmetric in-plane (tangential and radial extensional) vibration modes are discussed in detail in terms of resonant frequencies, mode shapes, and electrical currents. Two optical techniques, amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) and laser Doppler vibrometer (LDV), as well as the electrical impedance measurement are used to validate the analytical results. Both theoretical and experimental results indicate that the transverse and tangential vibration modes cannot be determined by the impedance analysis; hence, only resonant frequencies of extensional vibration modes are presented from the impedance analyzer. The LDV system is used to measure the resonant frequencies of transverse vibrations. However, both the transverse and extensional vibration modes and resonant frequencies of piezoceramic annular disks are obtained by the AF-ESPI method, and the interferometric fringes are produced instantly by a video recording system. Numerical results obtained by finite-element calculations are compared with those from theoretical analysis and experimental measurements. It is shown that the theoretical predictions of resonant frequencies and the corresponding mode shapes agree well with the experimental results. Good agreement between the predicted and measured electrical impedance also is found. The dependence of resonant frequencies and dynamic electromechanical coupling coefficients on the inner-to-outer radius ratio also is analyzed and discussed in this study.

Journal Article↗
PubMed1986

The cobalt-nitrosyl stretching vibration as a sensitive resonance Raman probe for distal histidine-nitrosyl interaction in monomeric hemoglobins.

The Co-NO stretching vibration has been assigned in the resonance Raman spectra of various cobalt-substituted monomeric hemoglobins by employing isotope-labeling of nitrosyl (14N16O, 15N16O, 14N18O). Monomeric hemoglobins with a distal histidine (sperm whale myoglobin and leghemoglobin) exhibit this vibration at 573-575 cm-1, whereas hemoglobins without distal histidine (elephant myoglobin and insect hemoglobin from Chironomus thummi thummi, CTT III) show this vibration in the range of 553-558 cm-1. The Fe-NO stretching vibration which occurs in the range of 554-556 cm-1 does not reflect the distal histidine-ligand interaction. Therefore, the Co-NO moiety which is isoelectronic with the Fe-O2 moiety is a good monitor for distal effects on the exogenous ligand of hemoglobins, especially due to the fact that in hemoglobins with distal histidine the Fe-O2 stretching vibration (567-572 cm-1) is similar to the Co-NO stretching vibration.

Animals↗
PubMed2001

Experimental and numerical investigations of resonant vibration characteristics for piezoceramic plates.

Electronic speckle pattern interferometry (ESPI) is a full field, non-contact technique for measuring the surface displacement of a structure subjected to static loading or, especially, to dynamic vibration. In this article we employ an optical system called the amplitude-fluctuation ESPI with out-of-plane and in-plane measurements to investigate the vibration characteristics of piezoceramic plates. Two different configurations of piezoceramic plates, namely the rectangular and the circular plates, are discussed in detail. As compared with the film recording and optical reconstruction procedures used for holographic interferometry, the interferometric fringes of AF-ESPI are produced instantly by a video recording system. Because the clear fringe patterns will be shown only at resonant frequencies, both the resonant frequencies and the corresponding mode shapes are obtained experimentally at the same time by the proposed AF-ESPI method. Excellent quality of the interferometric fringe patterns for both the in-plane and out-of-plane vibration mode shapes is demonstrated. The resonant frequencies of the piezoceramic plates are also measured by the conventional impedance analysis. From experimental results, we find that the out-of-plane vibration modes (type A) with lower resonant frequencies cannot be measured by the impedance analysis and only the in-plane vibration modes (type B) will be shown. However, both the out-of-plane (bending) and in-plane (extensional) vibration modes of piezoceramic plates are obtained by the AF-ESPI method. Finally, the numerical finite element calculations are also performed, and the results are compared with the experimental measurements. It is shown that the numerical calculations and the experimental results agree fairly well for both the resonant frequencies and the mode shapes.

Journal Article↗
PubMed2002

Experimental study of vibrational behavior of laminated annular disks.

The purpose of this paper is to present an experimental study of the vibrational behavior of laminated annular disks, and effects of laminations on the vibrations of the disks. The vibrations of a series of solid annular disks were calculated using the finite-element method in order to provide a basis for comparison with experimental data. An extensive range of experiments was performed on both a series of solid disks and a series of laminated disks under a range of normal clamping pressures. Based on the calculated and experimental results, it was found that the vibrational behavior of the laminated disks was dominated by that of the individual disk, of which the laminated disks were composed. Laminations had great effects on the vibrational behavior of the laminated disks and the effects depended upon the mode type, the clamping pressure, and the number of disk assembly. Laminations increased damping and reduced the amplitude of frequency response function for both the transverse modes and in-plane modes of disks. The resonant frequency of transverse modes shifted higher because of the effects of laminations. For the in-plane vibrational modes, the effects on the resonant frequency could be neglected and the resonant frequency could be considered to be a constant.

Journal Article↗
PubMed2003

Reed vibration in lingual organ pipes without the resonators.

Vibrations of plucked and blown reeds of lingual organ pipes without the resonators have been investigated. Three rather surprising phenomena are observed: the frequency of the reed plucked by hand is shifted upwards for large-amplitude plucking, the blown frequency is significantly higher than the plucked one, and peaks halfway between the harmonics of the fundamental frequency appear in the spectrum of the reed velocity. The dependence of the plucked frequency on the length of the reed reveals that the vibrating length at small vibrations is 3 mm shorter than the apparent free length. The frequency shift for large-amplitude plucking is explained by the periodic change of the vibrating length during the oscillation. Reed vibrations of the blown pipe can be described by a physical model based on the assumption of air flow between the reed and the shallot. Aerodynamic effects may generate and sustain the oscillation of the reed without acoustic feedback. The appearance of subharmonics is explained by taking into account the periodic modulation of the stress in the reed material by the sound field. Therefore, a parametric instability appears in the differential equation of vibration, leading to the appearance of subharmonics.

Journal Article↗
PubMed2003

Modal characteristics of in-plane vibration of circular plates clamped at the outer edge.

The equations of in-plane vibration in thin flat plates are solved for free vibration in circular plates clamped at the outer edge. The mode shapes are represented by trigonometric functions in the circumferential direction and by series summation of Bessel functions in the radial direction. Accuracy of the predictions of natural frequencies and mode shapes is assessed by comparisons with finite-element predictions and with previously reported results. The present solution gives very accurate predictions. The work also highlights the nature of coupling between the different circumferential and radial modes and the response of different vibrational modes at the center of the plate. It is shown that the center point of the plate vibrates only for modes with unity circumferential wave number (number of nodal diameters). Nondimensional frequency parameters are listed and the radial mode shapes of natural vibration are depicted to illustrate the free-vibration behavior in the frequency range of practical interest.

Journal Article↗
PubMed1994

DETECTION OF AIRBORNE SOUND BY A COCKROACH 'VIBRATION DETECTOR': A POSSIBLE MISSING LINK IN INSECT AUDITORY EVOLUTION

Extracellular recordings from nerve 5 in metathoracic legs of Periplaneta americana disclose a sense organ that is extremely responsive to vibration but also detects sound (best response near 1.8 kHz) with a sensitivity similar to some insect auditory organs. The energy required from an auditory signal for a criterion response is similar or even smaller than for an optimal vibratory input. Responses originate in the subgenual organs (SGO) in the proximal tibiae, and cross-modal adaptation indicates that the same cells respond to both vibration and sound. Sound is picked up directly on some internal structure, not via sound-induced substratum vibration. Adaptation at different frequencies discloses no frequency-selectivity in the SGO. The nerve response is a burst of synchronized impulses at a frequency, fR, of approximately 300 Hz, that is practically invariant both with sound intensity and within the burst, suggesting that fR might represent some underlying resonance phenomenon, either of the SGO or of air in the tracheal system. The latter possibility is ruled out by observations made while the tracheae are perfused with He­O2. Similar responses can be recorded from the pro- and mesothoracic legs. Although Periplaneta is thought to be deaf and appears to ignore loud tones presented to the home colony, a more sensitive assay detects small leg movements in response to sound, confirming the presence of a functional auditory sense. The SGO is suspended from an expansion of the leg trachea, which may function to enhance sensitivity to vibration. This linkage preadapts the SGO to detect airborne sound transmitted in the tracheal system, and contact vibration may also stimulate the system in part by deforming the tracheae. It is proposed that auditory organs of crickets evolved from an ancestral SGO that already possessed dual responsiveness by subsequently developing effective vibration-isolating filters.

Journal Article↗
PubMed1993

Spontaneous cellular vibrations in the guinea-pig temporal-bone preparation.

Mechanical vibrations of Hensen cells were measured with a laser-heterodyne interferometer in the guinea-pig temporal-bone preparation without the application of an external acoustic stimulus. Smoothed periodograms (spectral-density estimates v. frequency) were constructed from the velocity v. time waveforms recorded from individual cells. Several peaks were seen in the periodograms at levels as high as 10 dB above the noise floor, indicating the presence of spontaneous vibrations. The frequencies at which the peaks were located differed in different preparations, indicating that the observed peaks were not caused by the presence of ambient noise or ambient vibrations. Furthermore, vibrations were seen only in fresh preparations. The tuning curves of cells from which spontaneous vibrations were measured (determined by applying an external stimulus to the ear) had single principal peaks. Several peaks in the periodogram were found to be located within the principal-peak region of the tuning curve. The spontaneous response does not arise from noise filtered through the tuning curve which would have a single peak. We propose that these spontaneous vibrations originate at the outer hair cells and are the source of spontaneous otoacoustic emissions in the ear.

Animals↗