Showing papers on "Resonance published in 1995"

Method for the calibration of atomic force microscope cantilevers

Abstract: The determination of the spring constants of atomic force microscope (AFM) cantilevers is of fundamental importance to users of the AFM. In this paper, a fast and nondestructive method for the evaluation of the spring constant which relies solely on the determination of the unloaded resonant frequency of the cantilever, a knowledge of its density or mass, and its dimensions is proposed. This is in contrast to the method of Cleveland et al. [Rev. Sci. Instrum. 64, 403 (1993)], which requires the attachment of masses to the cantilever in the determination of the spring constant. A number of factors which can influence the resonant frequency are examined, in particular (i) gold coating, which can result in a dramatic variation in the resonant frequency, for which a theoretical account is presented and (ii) air damping which, it is found, leads to a shift of -4% in the resonant frequency down on its value in a vacuum. Furthermore, the point of load on the cantilever is found to be extremely important, since a small variation in the load point can lead to a dramatic variation in the spring constant. Theoretical results that account for this variation, which, it is believed will be of great practical value to the users of the AFM, are given. © 1995 American Institute of Physics.

Adsorption-induced surface stress and its effects on resonance frequency of microcantilevers

Abstract: It is well known that bimetallic microcantilevers can exhibit static deflection as a result of thermal effects, including exothermic adsorption of chemicals on their surfaces. It is shown here that the resonance frequency of a cantilever can change due to a combination of mass loading and change of spring constant resulting from adsorption of chemicals on the surface. Cantilevers also undergo static bending that is induced by differential surface stress. The magnitude of these effects depends upon the chemical properties of the surface and upon the amount of material adsorbed. Hence cantilever deflection as well as resonance frequency change can be used as the basis for development of novel chemical sensors.

Optically detected magnetic resonance of GaN films grown by organometallic chemical-vapor deposition.

Abstract: Photoluminescence (PL) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers grown by organometallic chemical-vapor deposition. Both undoped and Mg-doped GaN films were investigated. Samples were studied from three laboratories to obtain general trends and behavior. The ODMR experiments on the undoped films reveal resonances from both effective-mass and deep-donor states. These two features appear in all of the undoped GaN films and are most likely associated with an intrinsic point defect or defects. The same two donor resonances and a Mg-related quasideep acceptor resonance are found from the ODMR experiments on the Mg-doped samples. The energy level of the deep-donor state is found from ODMR spectral studies to be \ensuremath{\sim}1 eV below the conduction-band minimum. The PL experiments provide evidence for shallow acceptors in the undoped films and in a GaN sample lightly doped by residual Mg in the growth reactor. Models are proposed that describe the capture and recombination among these shallow- and deep-donor and acceptor states. The assignments of the magnetic resonance features are compared with the latest theoretical calculations of defect states in GaN.

Resonances in ultracold collisions of 6Li, 7Li, and 23Na

Abstract: We investigate the resonance properties of ultracold ground state $^{6}\mathrm{Li}$${+}^{6}$Li, $^{7}\mathrm{Li}$${+}^{7}$Li, and $^{23}\mathrm{Na}$${+}^{23}$Na collisions. The locations of various resonances and their corresponding error bounds due to the uncertainty of the interatomic potentials are presented. Also, the resonance widths are computed using rigorous coupled-channel calculations, as well as a modified version of Feshbach theory valid for strong fields.

NMR Study of the Cold, Heat, and Pressure Unfolding of Ribonuclease A

Abstract: The reversible cold, heat, and pressure unfolding of RNase A and RNase A--inhibitor complex were studied by 1D and 2D 1H NMR spectroscopy. The reversible pressure denaturation experiments in the pressure range from 1 bar to 5 kbar were carried out at pH 2.0 and 10 degrees C. The cold denaturation was carried out at 3 kbar, where the protein solution can be cooled down to -25 degrees C without freezing. Including heat denaturation experiments, the experimental data obtained allowed us to construct the pressure--temperature phase diagram of RNase A. The experimental results suggest the possibility that all three denaturation processes (cold, heat, and pressure) lead to non-cooperative unfolding. The appearance of a new histidine resonance in the cold-denatured and pressure-denatured RNase A spectra, compared to the absence of this resonance in the heat-denatured state, indicates that the pressure-denatured and cold-denatured states may contain partially folded structures that are similar to that of the early folding intermediate found in the temperature-jump experiment reported by Blum et al. [Blum, A. D., et al. (1978) J. Mol. Biol. 118, 305]. A hydrogen-exchange experiment was performed to confirm the presence of partially folded structures in the pressure-denatured state. Stable hydrogen-bonded structures protecting the backbone amide hydrogens from solvent exchange were observed in the pressure-denatured state. These experimental results suggest that the pressure-denatured RNase A displays the characteristics of a the inhibitor 3'-UMP show that the RNase A-inhibitor complex is more stable than RNase without the inhibitor.

Observable Properties of X-ray Heated Winds in AGN: Warm Reflectors and Warm Absorbers

Abstract: When an AGN is obscured, the warm reflecting gas nearby can be seen by a combination of bremsstrahlung, intrinsic line emission, and reflection of the nuclear continuum, both by electron scattering and by resonance line scattering. Resonance lines, due both to intrinsic emission and scattering, are particularly prominent in the soft X-ray band. When our line of sight to the nucleus is not obscured, the dominant effect is absorption. In the soft X-ray band, ionization edges of highly ionized species and resonance lines contribute comparably to the opacity; in the ultraviolet, the gas is almost transparent except for a small number of resonance lines. We identify the "warm absorbers" seen in many AGN X-ray spectra with this gas, but argue that most of the UV absorption lines seen must be due to a small amount of more weakly ionized gas which is embedded in the main body of the warm, reflecting gas. Because the ionization equilibration timescales of some ions may be as long as the variability timescales in AGN, the ionic abundances indicated by the transmission spectra may not be well-described by ionization equilibrium.

Compound ES of Cytochrome c Peroxidase Contains a Trp .pi.-Cation Radical: Characterization by Continuous Wave and Pulsed Q-Band External Nuclear Double Resonance Spectroscopy

Abstract: The fully oxidized state of cytochrome c peroxidase (CcP), called ES, contains two oxidizing equivalents, one as an oxyferryl heme and the other as an organic radical on an amino acid residue. The unusual electron paramagnetic resonance spectrum of ES has been shown to be due to a weak distributed exchange coupling between the two paramagnetic redox centers. Various residues have been proposed as the radical site over the years. In this paper continuous wave and pulsed Q-band electron nuclear double resonance (ENDOR) spectroscopy confirms that the radical is located on Trp-191, as previously proposed. The paper completes the characterization of the active site of compound ES as being comprized of an oxyferryl heme coupled to the Trp-191 {pi}-cation radical by a weak spin exchange. 47 refs., 11 figs., 2 tabs.

Thermal‐wave resonator cavity

Abstract: A thermal‐wave resonant cavity was constructed using a thin aluminum foil wall as the intensity‐modulated‐laser‐beam induced oscillator source opposite a pyroelectric polyvilidene fluoride wall acting as a signal transducer and cavity standing‐wave‐equivalent generator. It was shown that scanning the frequency of oscillation produces the fundamental and higher overtone resonant extrema albeit with increasingly attenuated amplitude—a characteristic of thermal‐wave behavior. Experimentally, scanning the cavity length produced a sharp lock‐in in‐phase resonance with simple linewidth dependencies on oscillation (chopping) frequency and intracavity gas thermal diffusivity. The thermal diffusivity of air at 294 K was measured with three significant figure accuracy: 0.211±0.004 cm2/s. The novel resonator can be used as a high‐resolution thermophysical property sensor of gaseous ambients.

A micromachined vibrating gyroscope

Abstract: A vibrating microgyroscope with a thin polysilicon resonator fabricated by silicon surface micromachining is described. The 400 μm × 800 μm resonator is driven in a lateral direction by electrostatic force, and the angular rate is detected as the capacitance change between the resonator and its substrate. Mechanical Q -factors for the driving mode and the detecting mode of the polysilicon resonator are 2800 and 16 000, respectively, at pressures below 0.1 Pa. Methods are presented for modifying the difference between the resonance frequencies of the driving and detecting modes. The test device shows a noise-equivalent rate of 2° s −1 .

Coupling of magnetospheric cavity modes to field line resonances: A study of resonance widths

Abstract: By using a box model for the magnetosphere and by using a matrix eigenvalue method to solve the cold linearized ideal MHD equations, we examine the temporal evolution of the irreversible coupling between fast magnetospheric cavity modes and field line resonances (FLRs). By considering the fast mode frequency to be of the form ωf = ωfr − iωfi, and using a Fourier transform approach, we have determined the full time-dependent evolution of resonance energy widths. We find that at short times the resonances are broad, and narrower widths continue to develop in time. Ultimately, an asymptotic resonance Alfven frequency full width at half maximum (FWHM) of ΔωA = 2ωfi develops on a timescale of τfi = ωfi−1. On timescales longer than τfi, we find that the resonance perturbations can continue to develop even finer scales by phase mixing. Thus, at any time, the finest scales within the resonance are governed by the phase mixing length Lph(t) = 2π (tdωA/dx)−1. The combination of these two effects naturally explains the localisation of pulsations in L shells observed in data, and the finer perturbation scales which may exist within them. During their evolution, FLRs may have their finest perturbation scales limited by either ionospheric dissipation or by kinetic effects (including the breakdown of single fluid MHD). For a continually driven resonance, we define an ionospheric limiting timescale τI in terms of the height-integrated Pedersen conductivity ΣP, and hence derive a limiting ionospheric perturbation scale LI = 2π (τIdωA/dx)−1, in agreement with previous steady state analyses. For sufficiently high ΣP, FLR might be able to evolve so that their radial scales reach a kinetic scale length Lk. For this to occur, we require the pulsations to live for longer than τk = 2π (LkdωA/dx)−1. For t < τk, τI, kinetic effects and ionospheric dissipation are not dominant, and the ideal MHD results presented here may be expected to model realistically the growth phase of ULF pulsations.

Vibrational assignments of trans-n-methylacetamide and some of its deuterated isotopomers from band decomposition of ir, visible, and resonance raman spectra

Abstract: In their reVised Comment,1 Jordan and Spiro (JS) take issue with our recent assessments2,3 of their interpretations4-6 of the nature and the resonance Raman enhancement of two bands of aqueous N-methylacetamide (NMA), viz., the ∼1380 cm-1 CCH3 symmetric bend (sb) and the amide I normal modes. In this reVised Reply, we respond in turn to the two specific points raised in their Comment. 1. Enhancement Mechanism of Amide S. The enhancement of the ∼1380 cm-1 CCH3 sb mode has two aspects that need consideration: the nature of this mode and the origin of its enhancement. We note first that there is no question that this mode in aqueous NMA is predominantly CCH3 sb with a significant contribution of CCH3 antisymmetric bend7 and a small contribution of CC stretch (s).2,8 Therefore, it is not only desirable, as is common practice, that it should be referred to by its major contributing local symmetry coordinate but it is also inappropriate to refer to it as an “amide” mode, which historically9 has been reserved for characteristic normal modes of the CONH group. The amide S designation4 should be dropped. With respect to the enhancement mechanism of this mode, Spiro et al.4-6 have stated that it arises from vibrational mixing with the nearby amide III mode. We have taken exception to this explanation,2 on grounds of both its formulation and its factual basis. We start out with the belief that there is merit in precise formulations of concepts. To imply that there is vibrational mixing between normal coordinates4-6 is indeed “inaccurate”.1,2 In their revised Comment, JS state that they “meant the sharing of internal coordinates between normal modes of similar energy”. While this is closer to the truth, in that a given internal coordinate can contribute to different normal modes, we note that these need not be of “similar energy”: for example, in aqueous NMA8 the CCN deformation coordinate makes small contributions to amide I at 1626 cm-1 and to the CN s mode at 883 cm-1 while its major contribution is to the normal mode at 446 cm-1. With respect to the origin of the enhancement of the CCH3 sb mode, this has been said to be “dependent primarily on the C-N displacement in the excited state” 4 and due to “vibrational mixing between the CN stretch of amide III and (C)CR hydrogen bending displacements”.6 In fact, CN s makes a negligible contribution to the potential energy distribution of the CCH sb mode,7,8 and in terms of the eigenvector components of the enhancement,3 CC s is 60% larger than CN s.10 That the CC s contribution is the crucial one is evidenced by “The lack of resonance enhancement for CCH3sb in [NMA-ND, which] is caused by the reduction and the sign change of the CC s contribution.” 2 It is, thus, clear that CN s, which is a dominant component of amide III, does not play the major role in the resonance Raman enhancement of the CCH3sb mode. In their revised Comment, JS “do not contest” our analysis. 2. Enhancement Mechanism of Amide I. There is a fundamental disagreement between Spiro et al. and our group concerning the origin of the resonance Raman enhancement of the amide vibrations, especially the amide I vibration. These conflicting views result in different conclusions concerning the nature of the amide ππ* excited state and also in different conclusions about the excited state geometry differences between NMA in aqueous solution versus that in non-hydrogen-bonding solvents and in the gas phase. The correct interpretation is needed in order to develop a deep understanding of amide electronic and vibrational spectroscopy and to utilize UV resonance Raman spectroscopy for protein secondary structural investigations. The disagreement is summarized succinctly: Spiro et al. claim that their experimental data for NMA in aqueous solution, in either H2O or D2O, indicate that amide I enhancement does not arise from the lowest energy amide ππ* excited state, which gives rise to the absorption band at ca. 190 nm, even when excitation occurs directly within this absorption band. In contrast, we claim that the previous experimental data simply and directly force the conclusion that the amide I band is resonance enhanced by this ππ* excited state. The ability to reconcile these views was previously clouded by disagreement over the experimental excitation profile results for NMA in H2O and D2O. Although our preresonance excitation profile data above 220 nm3 were similar to those of Wang et al.,5 their Raman cross sections were ca. 2-fold larger than ours for excitation within the amide π f π* transition (below 200 nm). This disagreement has now been reconciled, and JS now concede that their data were not corrected for the π f π* transition self-absorption. Our excitation profile data3 demonstrated that in the region between 192 and 220 nm the amide I Raman cross sections are relatively constant compared to the amide II and III cross sections. In fact, we directly plotted the amide I′/amide II′ ratio in the region between 192 and 235 nm and found the ratio to be essentially constant. JS have now similarly found that the amide I′/amide II′ as well as amide I/amide II intensity ratios are constant in this region. Both groups agree that the amide II and III bands are enhanced dominantly by the 190 nm ππ* excited state. The constant ratio between the amide I and amide II Raman cross sections indicates that these bands have the same frequency dispersion over the region excited; thus, we simply concluded that the amide I and I′ bands are enhanced by the 190 nm ππ* excited state. Any other conclusion requires that the dispersions fortuitously coincide. Spiro et al. continue to argue that the second π f π* transition at ∼165 nm is the only source of enhancement of the amide I band.4 We argued that it is impossible for the preresonance Raman dispersion of amide I from a ca. 165 nm transition to show the same dispersion as the amide II band, which is only enhanced by the 190 nm π f π* transition. In † Permanent address: University of Bremen, FBI-Institute of Experimental Physics, 28359 Bremen, Germany. * To whom correspondence should be addressed. 3992 J. Phys. Chem. A 1997, 101, 3992-3994

Corannulene Reduction: Spectroscopic Detection of All Anionic Oxidation States

Abstract: The reduction of corannulene (1) has been followed by parallel detection of optical absorption, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopies. The major findings are the existence of a diamagnetic dianion that can be observed in NMR and negligible counterion influences in the mono- and dianion. The diamagnetic state of the dianion is also supported by vanishing EPR intensity and semiempirical calculations and exhibits one indistinguishable proton NMR resonance at -5.6 ppm for potassium and lithium as counterions, respectively. The NMR signal is strongly temperature dependent and can only be obtained at low temperature (T < 230 K) where the line width decreases with temperature. Further reduction to the trianion and tetraanion succeeds with Li as the reducing metal, but not with K even at ambient temperatures. The EPR spectra of the trianion show a pronounced temperature dependence with at least two Li cations being strongly coupled. 18 refs., 3 figs., 3 tabs.

Ray chaos and Q spoiling in lasing droplets.

Abstract: The highest Q optical resonators known are dielectric microspheres in which the high Q modes are created by total internal reflection of light circulating just inside the surface of the sphere[1, 2]. These high Q modes are referred to as “whispering gallery” (WG) modes or alternatively as “morphology-dependent resonances” (MDR’s) [2]. If the dielectric is a liquid droplet containing an appropriate dye then the droplet acts as a high Q micro-resonator to support lasing action of the dye when optically pumped [3]. The resonance properties of an ideal spherical dielectric, for which the wave equation separates, are described by Mie theory where the quasi-modes are the product of spherical Bessel functions jl(nkr) (n is the index of refraction) and vector spherical harmonics [4]. The radial equation then contains a repulsive term l(l + 1)/r2 which is the analogue of the angular momentum barrier for light rays and an effectively attractive term associated with the higher index of refraction in the liquid. The combination of the attractive “well” represented by the dielectric and the repulsive angular momentum barrier gives rise to quasi-bound states of the effective potential near the rim of the droplets[4] for certain ratios of l to kR (k is the wavevector in vacuum, R the radius of the spherical droplet). In the ideal sphere these resonances are only broadened by evanescent leakage (“tunneling”), hence their enormous Q values.

Phase effect in taming nonautonomous chaos by weak harmonic perturbations.

Abstract: An additional weak harmonic forcing is added to the Duffing equation, and its ability of suppressing chaos is analyzed in the global parameter space. It is found that the phase difference between the two sinusoidal forces plays a very important role in suppressing chaos. A new type of intermittency characterized by periodic appearance of regular and chaotic motions, called the breathing effect in this Letter, is observed when the two harmonic forces deviate slightly from resonance.

In situ resonance Raman spectra of carotenoids in bird's feathers

Abstract: Micro resonance Raman spectra of a series of synthetic carotenoids and of a large number of bird's feathers which contain various kinds of carotenoids were analysed on the basis of the effective conjugation coordinate (ECC) theory. The data obtained on parrots provide the first structural data on the chemical nature of the pigments embedded in their feathers, and show that these pigments are unmethylated polyene molecules. Data on the molecular orientation of the carotenoids and on the presence of different pigments in the same feather are presented and discussed.

Synchrony between single-unit activity and local field potentials in relation to periodicity coding in primary auditory cortex.

Abstract: 1. We recorded responses from 136 single units and the corresponding local field potentials (LFPs) from the same electrode at 44 positions in the primary auditory cortex of 25 juvenile, ketamine-anesthetized cats in response to periodic click trains with click repetition rates between 1 and 32 Hz; to Poisson-distributed click trains with an average click rate of 4 Hz; and under spontaneous conditions. The aim of the study is to evaluate the synchrony between LFPs and single-unit responses, to compare their coding of periodic stimuli, and to elucidate mechanisms that limit this periodicity coding in primary auditory cortex. 2. We obtained averaged LFPs either as click-triggered averages, the classical evoked potentials, or as spike-triggered averages. We quantified LFPs by initial negative peak-to-positive peak amplitude. In addition, we obtained trigger events from negativegoing level crossings (at approximately 2 SD below the mean) of the 100-Hz low-pass electrode signal. We analyzed these LFP triggers similarly to single-unit spikes. 3. The average ratio of the LFP amplitude in response to the second click in a train and the LFP amplitude to the first click as a function of click rate was low-pass with a slight resonance at approximately 10 Hz, and, above that frequency, decreasing with a slope of approximately 24 dB/octave. We found the 50% point at approximately 16 Hz. In contrast, the LFP amplitude averaged over entire click trains was low-pass with a similar resonance but a high-frequency slope of 12 dB/octave and a 50% point at approximately 12 Hz. 4. The LFP amplitude for click repetition rates between 5 and 11 Hz often showed augmentation, i.e., the amplitude increased in response to the first few clicks in the train and thereafter decreased. This augmentation was paralleled by an increase in the probability of firing in single units simultaneously recorded on the same electrode. 5. We calculated temporal modulation transfer functions (tMTFs) for single-unit spikes and for LFP triggers. They were typically bandpass with a best modulating frequency of 10 Hz and similar shape for both single-unit spikes and LFP triggers. The tMTF per click, obtained by dividing the tMTF by the number of clicks in the train, was low-pass with a 50% cutoff frequency at approximately Hz, similar to that for the average LFP amplitude. 6. the close similarity of the tMTFs for single-unit spikes and LFP triggers suggests that single-unit tMTFs can be predicted from LFP level crossings.(ABSTRACT TRUNCATED AT 400 WORDS)

Measurement of absolute differential cross sections for vibrational excitation of O2 by electron impact

Abstract: New measurements of absolute differential cross sections (DCSS) for vibrational excitation of O2, with improved sensitivity and resolution, are presented. The 90 degrees DCSS are given as a function of energy up to 16 eV for elastic scattering and vibrational excitation of up to v=7. A weak continuous, probably non-resonant, background scattering is found in the v=1 cross section in addition to the well known sharp 2 Pi g resonances dominating vibrational excitation in the energy range up to 2.5 eV. The sharp 2 Pi g resonances interfere coherently with the background scattering both in the v=1 and in the elastic channels. New absolute values for the energy-integrated DCSS for 2 Pi g resonances are given. They exhibit oscillatory Franck-Condon factors in the v=4-7 exit channels. The broad resonance peaking at 9 eV, dominating vibrational DCSS at higher energies, causes excitation of high vibrational levels (up to the dissociation limit) of the X 3 Sigma g- ground state, but not of the a1 Delta g and b1 Sigma g+ electronically excited states, supporting its assignment as the 4 Sigma u- resonance, the selectivity being caused by spin selection rules. The excitation of high vs indicates a relatively narrow autodetachment width for this resonance. The angular dependence at 9 eV is, however, not a simple psigma wave as would be expected for a simple sigma u* shape resonance. Preliminary experiments with a free jet cooled sample, concerned with rotational broadening of the 2 Pi g resonance, are reported.

A frequency scanning method for the identification of harmonic instabilities in HVDC systems

Abstract: A frequency scanning method is introduced in the paper to obtain a more accurate frequency characteristic for identifying harmonic instability in HVDC power systems. An example of the application is used to identify the resonance frequencies in the CIGRE benchmark model. The paper shows that the benchmark model is not tuned to the resonance frequency that it was designed for. Using the scanning method, the resonance frequency of the benchmark model maybe shifted to demonstrate a simulation of core-saturation type instability. >

Temperature Variation of Elastic Properties of α- Quartz up to the α-β Transition

Abstract: The temperature variations of normal mode frequencies of a specimen of α-quartz single-crystal have been measured nearly up to the α-β transition (T0 573°C), and all of the independent elastic constants were determined simultaneously by the rectangular parallelepiped resonance method. The results demonstrate that bulk moduli decrease largely toward T0, while shear moduli show only a slight decrease. The trigonal constant C14 decreases just below T0, but seems to remain a finite value even at T0. It is noted that the isotropic aggregate of α-quartz is a curious and scarce material in the sense that its bulk modulus is smaller than the shear modulus, and Poisson's ratio is very small or even negative at temperatures higher than 450°C. In the measured temperature variations of mode frequencies, a characteristic feature was observed when two branches of Ag modes came close, suggesting mode coupling. Except for this coupling between Ag modes themselves, no definite evidence was observed which was indicative of coupling between modes of different symmetry. So it can be said the present sample is uniform in its property enough to suppress the coupling between modes of different symmetry.

Photon‐assisted tunneling through a quantum dot at high microwave frequencies

Abstract: We investigate the influence of high frequency microwave radiation on single electron tunneling through a quantum dot. Effective coupling of the radiation to the quantum dot is achieved by an on‐chip integrated broadband antenna. Simulations of the current distribution on the antenna are shown. We find that radiation with a frequency of ν=155 GHz, which corresponds to half of the bare charging energy Ec/2 results in an additional conductance peak within the Coulomb blockade regime. This additional resonance is attributed to photon‐assisted tunneling.

Study of Surface States on Cu(110) Using Optical Reflectance Anisotropy.

Abstract: We report spectroscopic measurements of the azimuthal anisotropy in the normal incidence reflectivity of a clean and adsorbate-covered single-crystal metal surface: Reflectance anisotropy spectra were taken between 1.5 and 6 eV from Cu(110). On the clean surface, a sharp resonance is found which is assigned to a transition between two surface states at the $\overline{Y}$ point of the surface Brillouin zone. The resonance can be removed by adsorption, and its energetic position is in good agreement with photoemission and inverse photoemission work.

Pump-Probe Experiments with a Single Molecule: ac-Stark Effect and Nonlinear Optical Response.

Abstract: We have measured the ac-Stark effect on single terrylene molecules in $p$-terphenyl crystals at superfluid helium temperatures. With the pump beam far enough from resonance, the molecular transition shifts as expected, i.e., proportional to the pump intensity and to the inverse pump detuning. Experiments closer to resonance with stronger beams show Autler-Townes-like structures that can be well reproduced by means of Bloch equations with two intense laser fields. These experiments are a demonstration of nonlinear optical effects with the localized electronic states of a single molecule trapped in a solid.

Voltage-induced color-selective absorption with surface plasmons

Abstract: A phenomenon of voltage‐induced color‐selective absorption at metal/liquid crystal interface with surface plasmons is reported. When a white light is incident at a metal/liquid crystal interface, those photons in surface plasmon resonance frequency range are totally absorbed and the reflected light shows the complementary color. If a voltage is used to change the refraction index of the liquid crystal, then the surface plasmon resonance frequency will change, and the reflected light will also show the color change. The reflectance spectrum is calculated from a well‐known solution to the boundary value problem in optics. Good agreement with the observations is obtained. This phenomenon could lead to a new generation of display devices.

A new triple-resonance experiment for the sequential assignment of backbone resonances in proteins.

Abstract: A new protocol is described for obtaining intraresidual and sequential correlations between carbonyl carbons and amide 1H and 15N resonances of amino acids. Frequency labeling of 13CO spins occurs during a period required for the 13Cα-15N polarization transfer, leading to an optimized transfer efficiency. In a four-dimensional version of the experiment, 13Cα chemical shifts are used to improve the dispersion of signals. The resonance frequencies of all backbone nuclei can be detected in a 3D variant in which cross peaks are split along two frequency axes. This pulse scheme is the equivalent of a five-dimensional experiment. The novel pulse sequences are applied to flavodoxin from Desulfovibrio vulgaris.

Dispersive field line resonances on auroral field lines

Abstract: The formation of dispersive Alfven resonance layers is investigated using a three-dimensional, two-fluid, magnetically incompressible model, including electron inertia and finite pressure. The equations are solved in {open_quotes}box{close_quotes} geometry with uniform magnetic field bounded by perfectly conducting ionospheres. Field line resonance (FLR) is stimulated within a density boundary layer with gradient transverse to ambient B; a parallel gradient in the Alfven speed is also included. Numerical results show that the resonance amplitude is largest on the magnetic shell with eigenfrequency matching the frequency of the surface wave propagating on the density boundary layer. Efficient coupling between the resonant Alfven wave and surface wave produces a relatively narrow FLR spectrum, even when the driver is broadbanded. Effective coupling to the external driver occurs only for long-wavelength azimuthal modes. It is shown that the parallel inhomogeneity limits radiation of dispersive Alfven waves by the FLR. The results provide new insights into low-altitude satellite observations of auroral electromagnetic fields and the formation of discrete auroral arcs. 61 refs., 9 figs.

Elimination of optical self-focusing by population trapping.

Abstract: We demonstrate a method for eliminating optical self-focusing and defocusing for a copropagating pair of intense laser beams whose frequencies differ by a Raman resonance. The lasers force the atoms of the medium into a population trapped state, thereby eliminating their contribution to the nonlinear refractive index.

Investigation of microwave π transitions in cesium beam clocks operated with U‐shaped H plane waveguide cavities

Abstract: In this article, we present the characteristics of the ΔF=±1, ΔmF=±1, π microwave transitions which appear in the response of cesium beam resonators operating with U‐shaped waveguide Ramsey cavities bent in the plane of the H field. Such resonances are due to the presence in the cavity of microwave magnetic induction perpendicular to the static field direction. It is shown that the π resonance feature is derived from a pure two‐level atomic system interacting with four spatially separated oscillating fields. Good agreement is found between experimental data and theoretical predictions when we take into account the actual transverse microwave field profile in the cavity obtained with tridimensional electromagnetic field computations.