Showing papers on "Resonance published in 1981"

Optical heterodyne saturation spectroscopy

Abstract: We discuss a refined, hybrid rf/optical technique for studying sub‐Doppler saturated absorption/dispersion resonances with excellent precision and symmetry. Sensitivity is limited mainly by fundamental noise in the signal. Resonance profiles obtained in I2 are in remarkable agreement with theory. The method promises a new level of accuracy for laser locking to an optical resonance.

[8] Solid state nuclear magnetic resonance of lipid bilayers

Abstract: Publisher Summary This chapter discusses solid state nuclear magnetic resonance of lipid bilayers. The shortcomings of conventional high-resolution spectroscopy when applied to lipid bilayers are presented. The resolution in multiple-pulse experiments has been limited to 1–2 ppm and these techniques have been useful only for studies of small molecules. A method for obtaining high-resolution spectra of solids relies on the spatial factor to attenuate and is referred to as dilute spin spectroscopy. It is found that since the oscillating components are adjusted to the same frequency, the spin systems are in resonance and mutual spin flips can occur and heat can flow between the reservoirs, and they equilibrate to a common spin temperature. It is convenient to add an echo pulse to the experiment at the dilute spin frequency, as is shown in the chapter. The purpose of this pulse is to delay data acquisition until the receiver and filters of the spectrometer recover from the radiofrequency pulse. It is suggested that the sidebands can be used to obtain information on chemical shift and electric field gradient tensors.

Defect states in doped and compensated a -Si: H

Abstract: Luminescence, electron spin resonance (ESR), optical absorption, conductivity, and composition data are measured on doped and compensated hydrogenated amorphous silicon. In material singly doped with boron or phosphorus, a variety of experiments indicates the introduction of a large defect density (up to ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) of the dangling-bond type. Compensation increases the luminescence efficiency, but the luminescence peak shifts strongly to lower energy. Compensation reduces the ESR resonance at $g=2.0055$, but a broad resonance characteristic of a hole trap remains. We deduce that compensation reduces the dangling-bond density, but introduces a new band of localized states above the valence-band edge. We associate these new states with boron-phosphorus complexes whose origin is a chemical interaction occurring during deposition. Changes in the dangling-bond density with doping and compensation lead us to propose an autocompensation mechanism of defect formation. Also reported is the first observation of a metastable light-induced ESR signal in $a$-Si: H.

Calculation of vibrational preionization in H2 by multichannel quantum defect theory: Total and partial cross sections and photoelectron angular distributions

Abstract: Multichannel quantum defect theory has been used to calculate the effect of vibrational preionization on the total and partial oscillator strength distributions and photoelectron angular distribution in H2 for excitation between 790 and 760 A. The total oscillator‐strength distribution obtained agrees well with the high‐resolution photoionization data of Dehmer and Chupka. The partial oscillator strength resonance profiles are predicted to have different shapes in different vibrational ionization channels, while their widths change little with channel. The preionization resonances are also predicted to affect the angular distribution asymmetry parameters b over a broader range than they affect the oscillator strength distribution. The gross features of the preionization resonances are discussed in terms of approximate solutions of the MQD equations.

Measurement of refractive indices of neon, argon, krypton and xenon in the 253.7–140.4 nm wavelength range. Dispersion relations and estimated oscillator strengths of the resonance lines

Abstract: The refractive indices of rare gases have been measured by Fabry-Perot interferometry down to 140 nm. Dispersion relations of the Sellmeier form are given and the results are compared with data found in the literature. For xenon, a value of 0.268±0.008 is derived for the oscillator strength of the first resonance line at 146.9 nm.

Molecular beam electric resonance spectroscopy of the nitric oxide dimer

Abstract: Microwave and radiofrequency spectra have been observed for (NO)2 using the technique of molecular beam electric resonance spectroscopy. Precise values have been determined for rotational, nuclear quadrupole coupling and spin-rotation constants of the ground electronic state treated as a singlet state. The values obtained are A = 25829·4803(20) MHz, B = 5614·3093(4) MHz, C = 4605·4396(12) MHz xaa = -4·0652(2) MHz, xbb - xcc = -8·5498(6) MHz caa = 10·4(5) kHz, cbb = 13·8(4) kHz, ccc = 0·8(4) kHz. The electric dipole moment has also been determined: μ = 0·17120(2) D. From these data (NO)2 is interpreted to have a symmetric cis-planar structure with the structural parameters r NN = 2·33(12) A, r NO = 1·15(1) A, ∠NNO = 95(5), 1/2(r NN + r NO) = 2·444(8) A. The quadrupole coupling constants of the nitrogen nuclei and the dipole moment demonstrate that little electron rearrangement occurs on dimer formation. In addition xaa and xbb - xcc indicate that the unpaired π electrons lie in the plane of the dimer imply...

Resonance Raman scattering of benzene and benzene‐d6 with 212.8 nm excitation

Abstract: The first resonance Raman spectra of benzene and deuterobenzene are reported. Resonance excitation with the second excited singlet transition (1B1u) is obtained with 212.8 nm radiation. These resonance Raman spectra are dominated by the overtones and combinations of the vibronically active e2g modes in this dipole forbidden transition, as well as the fundamental, overtone, and combination of ν1, the a1g ring breathing mode. All of the observed features, including relative intensities and depolarization ratios, are fully rationalized within the context of the vibronic theory once the 3N‐6 multidimensionality of the vibrational sublevels is exposed. The ν8 e2g mode is found to be the most vibronically active mode in the 1B1u transition, in agreement with all calculations. However, in contrast to these calculations, the Raman analysis finds that ν9, not ν6, is the next most vibronically active e2g mode in this transition. These results parallel the observed pattern of vibronic intensities in the 3B1u→1A1g phosphorescence spectra.

Internal electric field distributions of a dielectric cylinder at resonance wavelengths.

Abstract: Calculations of the electric field distribution within and around an infinite circular dielectric cylinder, having a relative index of refraction of 1.53 and illuminated with plane-wave radiation of wavelength λ, have been performed for size parameters ranging from 39 to 51. Results for size parameters that satisfy resonance conditions for various natural modes of oscillation of the cylinder (mode number n = 53; order numbers l = 1–5) clearly illustrate the physical significance of n and l and show that the internally reflected circumferential waves are localized near, but not confined to, the cylinder surface.

Resonance-Enhanced Vibrational Excitation in Electron Scattering from O 2 Multilayer Films

Abstract: Electron-impact intramolecular vibrational and vibronic excitation of disordered multilayer ${\mathrm{O}}_{2}$ films is studied with primary energies varying from 2 to 20 eV The intensity of the $v=1$ to $v=4$ vibrational modes of ground state ${\mathrm{O}}_{2}$ exhibits a broad resonance as a function of electron energy At the resonance maximum the vibrational intensities are enhanced by one to two orders of magnitude The resonance appears to be derived from gas-phase ${\mathrm{O}}_{2}$ quasibound states

Normal mode analysis of lumiflavin and interpretation of resonance Raman spectra of flavoproteins.

Abstract: The normal modes of lumiflavin (10-methyliso-alloxazine) are analyzed with a valence force field constructed with bond length-stretching force constant correlations and bending and interaction force constants transferred from small ring molecules. Observed resonance Raman (RR) bands of flavin are assigned to calculate modes on the basis of frequency and isotope shift matching. The normal mode patterns confirm previous inferences, based on selective effective of chemical substitutions, of localization to certain regions of the molecule. These results are used to interpret the observed variability of the prominent RR bands among different flavoproteins on the basis of protein-isoalloxazine interactions.

Superparamagnetic effects in the ferromagnetic resonance of silica supported nickel particles

Abstract: A silica supported Ni catalyst has been investigated by electron magnetic resonance between 293 and 4.2 K. A relatively narrow absorption signal with linewidth ΔHpp?400 Oe is observed at temperatures ?150 K as a result of motional averaging of the magnetocrystalline anistropy field Ha of the Ni particles resulting from superparamagnetism. Broadening of the resonance occurs at lower temperatures because of a reduction of the influence of thermal fluctuations. A line shape analysis of the observed resonance signals is carried out employing the theory of ferromagnetic resonance in polycrystalline material. The calculated anisotropy field is reduced in the particles as compared to bulk Ni. The Landau–Lifshitz relaxation frequency obtained from the calculated ’’single crystal’’ linewidths are found to be larger than those of bulk Ni single crystals. Concurrently with line broadening, the resonance shifts to lower magnetic fields. The resonance shifts are approximately Ha/2. The appearance of magnetoelastic eff...

Precision acoustic measurements with a spherical resonator: Ar and C2H4

Abstract: The spherical acoustic resonator is a remarkably accurate and convenient tool for the measurement of thermophysical properties of gases at low and moderate densities. The speed of sound (c) in a gas of interest can be measured with an accuracy of 0.02% merely by measuring the frequencies of the radial resonances when the resonator is filled with the gas of interest and then repeating the frequency measurements with a reference gas such as argon. The resonance frequencies of the radial modes are easily measured because these modes have very high Q’s, typically 2000–10 000. In this work the precision and accuracy of speed of sound measurements have been substantially improved by including a detailed acoustic model of the resonator in the analysis. Many of the important parameters of the model can be determined from acoustic measurements: Painstaking mechanical measurements are not required. We have used a spherical resonator to measure the speed and attenuation of sound in C2H4 in the temperature range 0–100 °C and the pressure range 0.15–1.0 MPa. Our measured values of c in C2H4 have a precision of 0.003% and agree with those of Gammon within the scatter of Gammon’s data (±0.02%). This agreement is remarkable when one considers that our spherical resonator is operated in the frequency range 4–13 kHz while Gammon has used a more conventional, cylindrical, variable path, acoustic interferometer operating at 500–2500 kHz. To attain this agreement, we did not have to make any highly accurate measurements other than frequency. Our measured values of the bulk relaxation frequency of C2H4 are within the scatter of the more recent values of the literature. In the course of our ’’calibration’’ measurements with argon we have redetermined the leading acoustic virial coefficient of argon. Our values for the virial are in satisfactory agreement with those in the literature. We include several practical suggestions for increasing the accuracy and/or versatility of spherical resonators.

Theoretical study of low-energy electron and positron scattering on Be, Mg and Ca

Abstract: Static exchange and static exchange plus polarisation calculations are carried out for elastic electron and positron scattering with Be, Mg and Ca. All three atoms are found to have a low-lying 2P resonance in the electron scattering case. Magnesium and calcium are also predicted to have higher lying 2D shape resonances near 2.0 and 0.7 eV, respectively. The 2D resonance in Mg is very weak and broad ( Gamma >3.0 eV) while that in Ca is much more pronounced and is much narrower ( Gamma approximately 0.6 eV). These findings are in agreement with recent experiments. In addition a very broad feature appears in e--Be d-wave scattering near 5 eV. While maxima occur in the p-wave cross sections for positron scattering, these do not appear to be due to resonances. However, evidence is provided for exceptionally broad resonances in d-wave positron scattering and for a virtual (or antibound) state in s-wave scattering.

Quasiclassical theory of laser-induced atomic-beam dispersion

Abstract: We present a theoretical model for the interaction of a beam of two-level atoms with a resonant laser beam. It is shown that the radiation force, while averaging to zero at resonance, produces a transverse spread of the atomic velocities. If radiative damping is ignored, this spread increases linearly with time and finally saturates towards a steady value. Although the spread has a quantum-mechanical origin, we show that it is possible, in the limit $\ensuremath{\hbar}k\ensuremath{\rightarrow}0$, to describe the phenomenon in terms of differential equations of motion for the atomic translational variables, as in the classical case. In this model it is also possible to describe the effects of radiative damping, as well as other additional effects, which may occur in a real experiment. Finally a comparison of our results with experimental data, recently obtained on an Na atomic beam, has been made. Although an order-of-magnitude fit of the experimental data could be obtained, the weights of the different causes, reducing the spread, were found to be ambiguous and have been left partly open.

Intermediate Structure Resonances in Ni 56

Abstract: Detailed excitation functions of angle-integrated cross sections for $^{28}\mathrm{Si}$+$^{28}\mathrm{Si}$ elastic scattering and reactions show narrow, highly correlated structures. These resonances have enhanced partial widths for decay into two $^{28}\mathrm{Si}$ fragments and are interpreted as having quasimolecular or fission-isomeric nature.

Determination of magnetic profiles in implanted garnets using ferromagnetic resonance

Abstract: Magnetic properties of the implanted layer in thin garnet films can be obtained from ferromagnetic resonance experiments. Approximate profiles of implantation induced anisotropy can be obtained using perpendicular resonance alone. For maximum information and accuracy, both perpendicular and parallel resonance spectra are needed, and measurements should be made on a number of samples from which varying amounts of the implanted layer have been removed by ion milling. For narrow linewidth materials it is possible to deduce profiles of H k , 4πM, and A and to determine the value of g in the implanted layer. Methods are presented for the accurate calculation of parallel ferromagnetic resonance (FMR) spectra with depth varying magnetic parameters. This method of analysis has been successfully applied to a yttrium iron garnet (YIG) film substituted with Gd, Tm, Ga, and implanted with He ions at 140 keV with a density of 3 × 1015cm-2.

Determination of the heats of formation of the isomeric xylylenes by ion cyclotron double-resonance spectroscopy

Abstract: Heats of formation of 50 and 53 kcal mol-' have been measured for pand o-xylylene by using pulsed-ion cyclotron double-resonance spectroscopy. In addition a lower bound of 76 kcal mol-' has been obtained for the heat of formation of gaseous m-xylylene. The family of C6H6 hydrocarbons comprises a variety of interesting and much studied compounds, ranging in stability from styrene to the highly strained cubane. One series of C6H6 isomers which have yet to be isolated are the xylylenes (1-3).

Kinetic investigation of the reactions of OH(X2Π) with the hydrogen halides, HCl, DCl, HBr and DBr by time-resolved resonance fluorescence (A2∑+–X2Π)

Abstract: The reaction rates of ground-state OH radicals, X2Π(v″= 0), with the molecules CH4, CO, HCl, DCl, HBr and DBr have been investigated in the gas phase by time-resolved resonance fluorescence at λ= 307 nm [OH(A2∑+–X2Π, (0, 0)]. The OH radicals were generated photochemically by the repetitive vacuum ultraviolet photolysis of water vapour in a flow system, kinetically equivalent to a static system, using a high-intensity, magnetically pinched pulsed light source, and optically excited to the A2∑+ state by means of a resonance source operating in the standard orthogonal arrangement. The resulting time-resolved resonance fluorescence signals were monitored by means of a highly sensitive detection system employing pre-trigger photomultiplier gating, photon counting and signal averaging. Absolute second-order rate constants (k/cm3 molecule–1 s–1, 300 K) were obtained for the reactions of OH with the above molecules as follows: CH4(7.66 ± 0.64)× 10–15 CO (1.46 ± 0.12)× 10–13, HCl(6.66 ± 0.52)× 10–13, DCl (3.48 ± 0.30)× 10–13, HBr (6.01 ± 0.32)× 10–12 and DBr (2.05 ± 0.14)× 10–12, the quoted errors being 2σ. The rate constants for CH4 and CO constituted a kinetic test of the system, being in accord with accepted values for these quantities. The data for the hydrogen halides are compared, where possible, with previously determined rates obtained from monitoring OH directly by various methods. To the best of our knowledge, the absolute rate constant for the reaction with DBr has not been reported hitherto from direct monitoring of OH.

Complex self‐consistent‐field calculations on shape resonances in electron–Mg and electron–Ca scattering

Abstract: The complex self‐consistent‐field (CSCF) method has been used to compute the complex resonance energies and Hartree–Fock wave functions for the low energy 2P shape resonances in electron–magnesium and electron–calcium atom scattering and for the 2D shape resonance in electron–calcium scattering. Comparison with experiment indicates that correlation effects are important in determining the lifetimes of these resonances. It is also demonstrated that it is possible to find a solution of the CSCF equation in some cases by using a real Hamiltonian (no complex coordinates) and only real basis functions.

Pressure-induced degenerate frequency resonance in four-wave light mixing.

Abstract: Phase-matched four-wave mixing in sodium vapor with a helium buffer gas is carried out in a noncoplanar geometry. The directions of the four light beams remain nondegenerate even if all frequencies are equal. A sharp zero-difference frequency resonance in the intensity of the parametrically generated fourth beam by three incident beams is observed that is proportional to the square of the pressure of the buffer gas. A theoretical interpretation of this effect caused by elimination of destructive interference between two coherent pathways is presented.