Ultrasonic absorption in aqueous solutions of amino acids at neutral pH

Ultrasonic absorption coefficients in aqueous solutions of glycine, L-alanine, imidazole, L-phenylalanine, L-histidine and L-tryptophan at neutral pH were measured in the range from 0.8 to 220 MHz at 25 degrees C. A characteristic ultrasonic relaxation phenomenon was observed only in the solution of L-histidine with a relaxation frequency at around 2 MHz at neutral pH. It was proposed from the concentration independent relaxation frequency and the linear concentration dependence of the maximum absorption per wavelength that the relaxation mechanism was associated with a perturbation of the rotational isomeric equilibrium of the L-histidine molecule. The existence of two rotational isomeric forms of L-histidine in water was examined by semiempirical quantum chemical methods, in order to determine the free energy difference between the two states. The forward and backward rate constants were determined from the relaxation frequency and the energy change. Also, the standard volume change of the reaction was estimated from the concentration dependence of the maximum absorption per wavelength. It was speculated that L-histidine fulfills a specific function among amino acids because of the rotational motion in the molecule, in addition to its well-established acid-base properties.     

Ultrasonic absorption of bovine serum albumin solutions in the frequency range 60 to 160 kHz

The ultrasonic absorption of the globular protein bovine serum albumin in aqueous solution has been measured in the frequency range 60 to 160 kHz using a 2-1 spherical resonator. The effect of pH change and of the denaturants urea, guanidine hydrochloride, and sodium dodecyl sulfate on the absorption loss has been studied. It is concluded that a significant ultrasonic absorption process exists which is related to structural helix-coil transition equilibria. For native protein the maximum loss appears to occur at a frequency at least as low as 70 kHz for a pH of about 4.2. This loss process is distinct from those arising from proton-transfer equilibria perturbations which are manifest at pH 3.2 and 11.6 and at peak frequencies of 400 kHz and above.     

New designed helical resonator to improve measurement accuracy of magic radio frequency

Based upon the new designed helical resonator, the resonant radio frequency (RF) for trapping ions can be consecutively adjusted in a large range (about 12 MHz to 29 MHz) with high Q-factors (above 300). We analyze the helical resonator with a lumped element circuit model and find that the theoretical results fit well with the experimental data. With our resonator system, the resonant frequency near magic RF frequency (where the scalar Stark shift and the second-order Doppler shift due to excess micromotion cancel each other) can be continuously changed at kHz level. For ⁸⁸Sr⁺ ion, compared to earlier results, the measurement accuracy of magic RF frequency can be improved by an order of magnitude upon rough calculation, and therefore the net micromotion frequency shifts can be further reduced. Also, the differential static scalar polarizability Δα₀ of clock transition can be experimentally measured more accurately.     

Tunable high-<Emphasis Type="Italic">Q</Emphasis> surface-acoustic-wave resonator

The feasibility of a tunable two-port surface-acoustic-wave resonator is analyzed theoretically and verified experimentally. The center frequency of the resonator is tuned by varying the equivalent length of its cavity. The variation of the center frequency achieved in the experiment is 25 kHz, with the width of the resonance peak being ≈4 kHz at a level of ?3 dB (the loaded Q factor of the resonator is ≈24600) and a frequency of ≈98 MHz. As a result of this study, a tunable resonator controlled by a static electric field that realizes the parameters predicted theoretically is designed. This resonator may serve as a basis for a stable tunable oscillator with a low phase noise.     

Wideband acoustic absorption characteristics of rubberized coir for underwater applications

Reflection and transmission coefficients of rubberized coir pads over the frequency band 200 kHz to 4 MHz are presented in this Paper. These results are compared with those reported for neoprene, paraffin wax, rubber car mat and plastic door mat¹. The rubberized coir pads were found to possess wideband absorption characteristics. It has been experimentally found that 0.05 m thick coir pads have almost 100% absorption in the frequency range 800 kHz-3 MHz with a maximum at 2.35 MHz. We have used this material for lining the water tank for underwater acoustic studies.     

Microwave frequency modulation in CW EPR at W-band using a loop-gap resonator

Loop-gap resonator (LGR) technology has been extended to W-band (94GHz). One output of a multiarm Q-band (35GHz) EPR bridge was translated to W-band for sample irradiation by mixing with 59 GHz; similarly, the EPR signal was translated back to Q-band for detection. A cavity resonant in the cylindrical TE011 mode suitable for use with 100 kHz field modulation has also been developed. Results using microwave frequency modulation (FM) at 50 kHz as an alternative to magnetic field modulation are described. FM was accomplished by modulating a varactor coupled to the 59 GHz oscillator. A spin-label study of sensitivity was performed under conditions of overmodulation and gamma2H1(2)T1T2<1. EPR spectra were obtained, both absorption and dispersion, by lock-in detection at the fundamental modulation frequency (50 kHz), and also at the second and third harmonics (100 and 150 kHz). Source noise was deleterious in first harmonic spectra, but was very low in second and third harmonic spectra. First harmonic microwave FM was transferred to microwave modulation at second and third harmonics by the spins, thus satisfying the "transfer of modulation" principle. The loaded Q-value of the LGR with sample was 90 (i.e., a bandwidth between 3 dB points of about 1 GHz), the resonator efficiency parameter was calculated to be 9.3 G at one W incident power, and the frequency deviation was 11.3 MHz p-p, which is equivalent to a field modulation amplitude of 4 G. W-band EPR using an LGR is a favorable configuration for microwave FM experiments.     

Ultrasound absorption studies in the critical region of lithium-ammonia solutions

Measurement of the absorption of ultrasound in the critical region of lithium-ammonia solutions as a function of concentration, frequency, and temperature are presented. The absorption reaches a maximum at 4.2 mole percent lithium and is a function of the frequency. The frequency dependence can be explained either by a single relaxation at around 20 MHz or by using Fixman's theory with a correlation length of 51.3 Å.     

Zur Deutung der Protonenspinrelaxation in Glycerin

In contrast with the usual assumption that proton spin relaxation in glycerol originates from rotational molecular diffusion, quantitative evaluation of the temperature and Larmor frequency dependence in the ranges ?30 °C to+70 °C and 10 kHz to 120 MHz, respectively, gives extensive agreement with Fick's translational random walk model. This observation is supported by direct measurements of the self-diffusion constant by means of the pulse gradient method, which reveals the same activation barrier as relaxation spectroscopy.     

Frequency stabilization of blue extended cavity diode lasers by external cavity optical feedback

Optical feedback from a high-finesse V-resonator, developed for this study, results in efficient coupling with an extended cavity diode laser, stabilizing its emission frequency and strongly decreasing the laser linewidth. This in turn enhances resonator output power, thus increasing the signal-to-noise ratio when used for the detection of gas phase species by absorption spectroscopy. This effect was directly measured by heterodyning two extended cavity diode lasers at a wavelength of 409 nm with and without the influence of optical feedback from a high-finesse V-resonator. The heterodyne signal of freely running lasers is composed of a set of sharp peaks whose envelope shows a width on the order of 4.5 MHz at a sweep rate of 80 MHz/0.8 s, leading to a laser linewidth of 3 MHz. Optical feedback from the high-finesse V-resonator reduces the heterodyne signal to a single peak with a mean width of 10 kHz, leading to a laser linewidth of 7 kHz. This is the lowest value of linewidth, reported thus far, for diode lasers operating in this wavelength region.     

Ultrasonic absorption in polymer gel dosimeters

Ultrasonic absorption in polymer gel dosimeters was investigated. An ultrasonic interferometer was used to study the frequency (f) dependence of the absorption coefficient (alpha) in a polyacrylamide gel dosimeter (PAG) in the frequency range 5-20 MHz. The frequency dependence of ultrasonic absorption deviated from that of an ideal viscous fluid. The presence of relaxation mechanisms was evidenced by the frequency dependence of alpha/f(2) and the dispersion in ultrasonic velocity. It was concluded that absorption in polymer gel dosimeters is due to a number of relaxation processes which may include polymer-solvent interactions as well as relaxation due to motion of polymer side groups.The dependence of ultrasonic absorption on absorbed dose and formulation was also investigated in polymer gel dosimeters as a function of pH and chemical composition. Changes in dosimeter pH and chemical composition resulted in a variation in ultrasonic dose response curves. The observed dependence on pH was considered to be due to pH induced modifications in the radiation yield while changes in chemical composition resulted in differences in polymerisation kinetics.     

Saturation spectroscopy of CO2 and frequency stabilization of an optical parametric oscillator at 2.77 μm

We report the frequency stabilization of a CW single-frequency, singly resonant optical parametric oscillator (OPO) to the saturation absorption center of the (12)C(16)O2[10°1,02°1](II)>←00°0 P(14) line at 2.77 μm. The CO2 molecules were excited by the OPO idler wave, and the absorption signal was monitored through the fluorescence at 4.3 μm using a gold-coated longitudinal cell. The idler frequency was stabilized onto the line center by wavelength modulation method. The linewidth of the saturation dip was estimated to be 4.7 MHz, and the achieved frequency stability was 3.9 kHz (3.6×10(-11)).     

The effect of temperature on sound wave absorption in a sediment layer

The effect of temperature on sound velocity, absorption, and reflection coefficient in the seabed sediment layer is investigated. Experimental measurements of sound speed, absorption, and the reflection coefficient in a sandy sediment layer have been carried out at several temperatures. An absorption reduction of 75 dB/m and a velocity increase of 65 m/s have been measured at a frequency of 1 MHz when the temperature increases from 5 to 25 degrees C. Because of the absorption temperature dependence the amplitude of the reflected wave from the back surface of the sub-bottom layer after going back and forth across the layer increases with the temperature.     

Dielectric and electrooptical properties of hockey-stick-shaped liquid crystal with a negative dielectric anisotropy

We investigated dielectric properties of a hockey-stick-shaped liquid crystal (HLC). Two dielectric relaxation modes were observed at 0.91 kHz and 4.51 MHz. The low frequency relaxation modes in isotropic, smectic, and nematic phases are related to the motion of ions, collective tilt fluctuation, and rotation around the long molecular axis, respectively. Meanwhile, the high frequency relaxation modes in nematic and smectic phases was due to the rotation around the short axis of the molecules and hindered by the resistance of the ITO layers. We also examined the electrooptical response of the 5.0 wt% HLC-doped commercial nematic liquid crystal (LC) mixtures. The birefringence of the LC mixture was slightly increased, while the falling time and the rotational viscosity was decreased in the nematic phase.     

Magnetic field dependence of spin-lattice relaxation enhancement using piperidinyl nitroxyl spin-labels

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.     

高动态范围声光接收机

介绍了一种高动态范围声光接收机，这种接收机具备实现宽带射频信号幅度，频率和相位的信道比探测能力，该接收机在０．６３２８μｍ激光器的工作条件下（１．０ｍＷ），声光布拉格盒在５０ｍＷ射频信号区动下，在１４０ＭＨｚ的中心频率上２０ＭＨdisplay status     

低β162.5 MHz taper型HWR腔的机械性能分析（英）

北京大学正在设计=0.09, 频率为162.5 MHz taper型的二分之一波长射频超导谐振腔（HWR腔）, 这种腔针对高流强质子束（约100 mA）和氘束（约50 mA）的加速而设计。对于这种超导腔而言机械性能分析是十分重要的, 可以通过机械性能分析来估计由于腔体的形变带来的频率偏移。用ANSYS分析了由于液氦压力不稳定造成的麦克风效应以及洛伦兹力造成的腔体失谐, 并且对沿腔体轴线方向的调谐进行了分析。模拟结果显示这只腔压力敏感系数为31.1 Hz/kPa, 洛伦兹力系数为－0.41 Hz/(MVm-1)2。 腔体的调谐范围达到177 kHz, 足够补偿腔体可能的频率偏移。腔体的机械性能满足腔体正常运行的要求。     

千周和兆周范围的马氏体相变内耗以及预马氏体相变内耗

在千周和兆周两个频段分别测量了含Cd为46.1at％,47.5at％,50.0at％和52.5at％的AuCd合金中与马氏体转变有关的内耗和杨氏模量。实验结果表明,在千周范围,由于马氏体转变引起的Q^-1(内耗)是静滞后型的。Q^-1峰的形成可以用弹性软化及界面位错的静滞后阻尼机制来解释。而兆周范围的超声衰减是与频率有关的,表明尚有共振型及弛豫型阻尼的成份。此外,实验还观察到AuCd合金的等温马氏体转变Q^-1峰以及预马氏体相变Q^-1峰。 关键词：     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Dielectric relaxation and related studies of 4-ethylphenol-methanol mixtures using time domain reflectometry

The dielectric relaxation studies of 4-ethylphenol-methanol mixtures have been carried out at various temperatures ranging from 10°C to 40°C using time domain reflectometry in the frequency range 10 MHz to 10 GHz. The relaxation mechanism in these systems is explained by Cole-Davidson model. The excess dielectric parameters, Kirkwood correlation factor and activation energy have been calculated and discussed with respect to molecular arrangements, and microdynamics of the binary mixture composed of both the associative type of liquids.