A 4-mm probe for (13)C CP/MAS NMR of solids at 21.15 T

The design of a broadband 4-mm magic-angle spinning (MAS) X-(1)H/(19)F double resonance probe for cross-polarization (CP)/MAS NMR studies at 21.15 T ((1)H at 900 MHz) is described. The high-frequency (1)H/(19)F channel employs a new and efficient transmission line tuning design. The first (13)C CP/MAS NMR spectra recorded at 21.15 T have been obtained with this probe and exhibit the best S/N per milligram sample of hexamethylbenzene achieved so far for a 4-mm rotor.     

Biomolecular solid state NMR with magic-angle spinning at 25K

A magic-angle spinning (MAS) probe has been constructed which allows the sample to be cooled with helium, while the MAS bearing and drive gases are nitrogen. The sample can be cooled to 25 K using roughly 3 L/h of liquid helium, while the 4-mm diameter rotor spins at 6.7 kHz with good stability (±5 Hz) for many hours. Proton decoupling fields up to at least 130 kHz can be applied. This helium-cooled MAS probe enables a variety of one-dimensional and two-dimensional NMR experiments on biomolecular solids and other materials at low temperatures, with signal-to-noise proportional to 1/T. We show examples of low-temperature ¹³C NMR data for two biomolecular samples, namely the peptide Aβ_14–23 in the form of amyloid fibrils and the protein HP35 in frozen glycerol/water solution. Issues related to temperature calibration, spin–lattice relaxation at low temperatures, paramagnetic doping of frozen solutions, and ¹³C MAS NMR linewidths are discussed.     

Straightforward, effective calibration of SPINAL-64 decoupling results in the enhancement of sensitivity and resolution of biomolecular solid-state NMR

We describe a simple yet highly effective optimization strategy for SPINAL-64 1H decoupling conditions for magic-angle spinning solid-state NMR. With adjustment of the phase angles in a coupled manner, the optimal conditions resulting from three parameter optimizations can be determined with adjustment of a single phase. Notably, echo T? relaxation times for 13C and 1?N show significant enhancement (up to 64%), relative to the previous described SPINAL-64 conditions, under the moderate 1H decoupling levels (60-100 kHz) and MAS rate (13.3 kHz) commonly employed for high-resolution SSNMR spectroscopy of proteins. Additionally, we also investigated the effect at higher spinning rate (33.3 kHz) and compared the results with other 1H decoupling schemes (TPPM, XiX), as well as SPINAL-64 with the originally reported optimal values.     

Sensitivity enhancement, assignment, and distance measurement in 13C solid-state NMR spectroscopy for paramagnetic systems under fast magic angle spinning

Despite success of previous studies, high-resolution solid-state NMR (SSNMR) of paramagnetic systems has been still largely unexplored because of limited sensitivity/resolution and difficulty in assignment due to large paramagnetic shifts. Recently, we demonstrated that an approach using very-fast magic angle spinning (VFMAS; spinning speed 20kHz) enhances resolution/sensitivity in (13)C SSNMR for paramagnetic complexes [Y. Ishii, S. Chimon, N.P. Wickramasinghe, A new approach in 1D and 2D (13)C high resolution solid-state NMR spectroscopy of paramagnetic organometallic complexes by very fast magic-angle spinning, J. Am. Chem. Soc. 125 (2003) 3438-3439]. In this study, we present a new strategy for sensitivity enhancement, signal assignment, and distance measurement in (13)C SSNMR under VFMAS for unlabeled paramagnetic complexes using recoupling-based polarization transfer. As a robust alternative of cross-polarization (CP), rapid application of recoupling-based polarization transfer under VFMAS is proposed. In the present approach, a dipolar-based analog of INEPT (dipolar INEPT) methods is used for polarization transfer and a (13)C signal is observed under VFMAS without (1)H decoupling. The resulting low duty factor permits rapid signal accumulation without probe arcing at recycle times ( approximately 3 ms/scan) matched to short (1)H T(1) values of small paramagnetic systems ( approximately 1 ms). Experiments on Cu(dl-Ala)(2) showed that the fast repetition approach under VFMAS provided sensitivity enhancement by a factor of 8-66 for a given sample, compared with the (13)C MAS spectrum under moderate MAS at 5kHz. The applicability of this approach was also demonstrated for a more challenging system, Mn(acac)(3), for which (13)C and (1)H paramagnetic shift dispersions reach 1500 and 700 ppm, respectively. It was shown that effective-evolution-time dependence of transferred signals in dipolar INEPT permitted one to distinguish (13)CH, (13)CH(2), (13)CH(3), (13)CO2- groups in 1D experiments for Cu(DL-Ala)(2) and Cu(Gly)(2). Applications of this technique to 2D (13)C/(1)H correlation NMR under VFMAS yielded reliable assignments of (1)H resonances as well as (13)C resonances for Cu(DL-Ala)(2) and Mn(acac)(3). Quantitative analysis of cross-peak intensities in 2D (13)C/(1)H correlation NMR spectra of Cu(DL-Ala)(2) provided distance information between non-bonded (13)C-(1)H pairs in the paramagnetic system.     

Tuning curves and pitch matches in a listener with a unilateral, low-frequency hearing loss

Psychoacoustical tuning curves and interaural pitch matches were measured in a listener with a unilateral, moderately severe hearing loss of primarily cochlear origin below 2 kHz. The psychoacoustical tuning curves, measured in a simultaneous-masking paradigm, were obtained at 1 kHz for probe levels of 4.5-, 7-, and 13-dB SL in the impaired ear, and 7-dB SL in the impaired ear, and 7-dB SL in the normal ear. Results show that as the level of the probe increased from 4.5- to 13-dB SL in the impaired ear, (1) the frequency location of the tip of the tuning curve decreased from approximately 2.85 to 2.20 kHz and (2) the lowest level of the masker required to just mask the probe increased from 49- to 83-dB SPL. The tuning curve in the normal ear was comparable to data from other normal listeners. The interaural pitch matches were measured from 0.5 to 6 kHz at 10-dB SL in the impaired ear and approximately 15- to 20-dB SL in the normal ear. Results show reasonable identity matches (e.g., a 500-Hz tone in the impaired ear was matched close to a 500-Hz tone in the normal ear), although variability was significantly greater for pitch matches below 2 kHz. The results are discussed in terms of their implications for models of pitch perception.     

An effective stochastic excitation strategy for finding elusive NMR signals from solids

The versatility of using a stochastic pulse sequence to elucidate peaks with a wide range of shifts, peak widths, and T(1) relaxation times is demonstrated. A stochastic sequence is combined with high speed magic angle spinning (MAS) to obtain the broad and largely shifted peak associated with (31)P in LiNiPO(4). A stochastic sequence is also used to obtain a spectrum of 85% H(3)PO(4), which has a much longer T(1) value. The signal-to-noise was comparable for spectra of 85% H(3)PO(4) obtained with either a stochastic sequence or an optimized Ernst angle experiment. Experimental parameters for the stochastic experiment are set depending only on the ringdown of the probe and not on any inherent qualities of the sample. A stochastic sequence, therefore, combined with MAS provides a useful strategy for finding peaks with unknown T(1) relaxation constants, peak widths, and shifts.     

中国散裂中子源铁氧体加载腔的性能研究

利用同轴线理论分析了扫频工作的中国散裂中子源（CSNS）铁氧体加载同轴谐振腔的主要性能参数。给出了铁氧体加载腔在三维电磁场仿真计算中的建模过程,在场分析的基础上提出了陶瓷介质等效谐振电容的方法。将模拟计算得到的腔体的谐振特性与同轴线理论计算和实际腔体测量结果相比较,三者吻合得非常好。通过对腔体样机母排引入的寄生模的实验和模拟研究,提出了解决寄生模问题的方法;通过对腔体内铁氧体环的填充系数的研究,给出了短尺寸腔的扫频工作方案。     

Electron paramagnetic resonance W-band spectrometer with a low-noise amplifier

The Mark II W-band (94 GHz) EPR spectrometer with a low-noise millimeter-wave amplifier is described. The microwave bridge is of a high-sensitivity homodyne design. Signal-to-noise ratios were measured for a number of detectors with and without the low-noise amplifier. The signal-to-noise ratio was determined not only by the type of detector but also how well it was matched. Without a microwave preamplifier, a hot-electron bolometer provides the best signal-to-noise ratio. Addition of a low-noise microwave preamplifier to the CW homodyne bridge gives a 10 dB improvement in the noise figure of the receiver at a modulation frequency of 100 kHz. A greater improvement in the signal-to-noise ratio is seen at low modulation frequencies (1–10 kHz), making the low-noise amplifier useful for systems with large linewidths. This allows larger modulation amplitudes to be used without causing significant cavity heating or microphonics. The W-band spectrometer is capable of rapid sweeps from 0 to 7 T, as well as narrower (0.1 T) high-resolution sweeps. It is suitable for a wide variety of samples including liquids and samples cooled to sub-liquid-helium temperatures.     

RF characters study of a pick-up parallel-setting HOM coupler

A pick-up parallel-setting high order mode coupler(pps-HOM coupler)has been fabricated at Peking University.It has been tested on a coaxial transmission line and a 2-cell TESLA-shape copper cavity.The test result of the 2-cell TESLA cavity shows that the coupler can cut off the fundamental mode TM010 and absorb HOMs electively.As its RF properties are insensitive to the probe dip length,there is no need to adjust the probe repeatedly during practical operation.     

基于固体核磁共振技术的固体酸结构、酸性及活性分析

固体酸是工业烃转化和生物质精炼中应用最广泛的非均相催化剂之一，了解它们的局部结构和酸性等性质有利于合理设计高效绿色固体酸催化剂，从而提高目标反应的活性和稳定性.近年来，固体核磁共振波谱在定性和定量表征固体酸的局部结构和酸性方面已显示出巨大的应用潜力，甚至可作为一种标准方法.二维固体核磁共振波谱的应用可以进一步揭示固体酸表面位点的结构对称性和不同位点的空间构效关系，从而加深对“催化剂结构-酸性-活性关系”的理解.在这篇综述中，我们总结了用于固体酸表征的固体核磁共振波谱方法和常规实验操作流程，并着重阐述了在使用和不使用探针分子的情况下，固体核磁共振波谱应用于固体酸局部结构和酸性性质研究的进展.     

An alternative tuning approach to enhance NMR signals

By using spin-noise type measurement we show that the resonance frequency of the reception circuit of classical NMR spectrometers does not match the Larmor frequency even if, in emission, the electronic circuit is perfectly tuned at the Larmor frequency and matches the amplifier impedance. We also show that this spin-noise method can be used to ensure a match between the Larmor frequency and the reception circuit resonance frequency. In these conditions, (i) the radiation damping field is in perfect quadrature to the magnetization and (ii) the NMR signal level and potentially the signal-to-noise ratio, are enhanced. This choice induces a change of the probe resonance frequency by several hundreds of kHz for 500 or 700 MHz spectrometer. We show that the resulting mismatch condition for emission can be removed by adding other tuning and matching degrees of freedom located on the excitation line (or by symmetry on the reception line) decoupled to the probe resonance circuit by the crossed diodes.     

A general protocol for temperature calibration of MAS NMR probes at arbitrary spinning speeds

A protocol using (207)Pb NMR of solid lead nitrate was developed to determine the temperature of magic-angle spinning (MAS) NMR probes over a range of nominal set temperatures and spinning speeds. Using BioMAS and FastMAS probes with typical sample spinning rates of 8 and 35 kHz, respectively, empirical equations were devised to predict the respective sample temperatures. These procedures provide a straightforward recipe for temperature calibration of any MAS probe.     

Why does PMLG proton decoupling work at 65 kHz MAS?

Schemes such as phase-modulated Lee–Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution ¹H spectra at high magic-angle spinning (MAS) frequencies of 50–70 kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here, a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be successful at high MAS frequencies.     

Investigations of Advanced Coaxial Gyrotrons at IAP RAS

Main results of experimental testing of the 140 GHz/1.5 MW coaxial gyrotron are summarized. High selective properties of the coaxial cavities and a possibility to increase considerably the efficiency of a coaxial gyrotron only by applying independently an appropriate voltage on its electrodes (without any design modification) have been confirmed. Successful uses of the two-potential connection scheme for the frequency step tuning and output power modulation in a coaxial gyrotron has been also demonstrated.     

Remote tuning of NMR probe circuits

There are many circumstances in which the probe tuning adjustments cannot be located near the rf NMR coil. These may occur in high-temperature NMR, low-temperature NMR, and in the use of magnets with small diameter access bores. We address here circuitry for connecting a fixed-tuned probe circuit by a transmission line to a remotely located tuning network. In particular, the bandwidth over which the probe may be remotely tuned while keeping the losses in the transmission line acceptably low is considered. The results show that for all resonant circuit geometries (series, parallel, series-parallel), overcoupling of the line to the tuned circuit is key to obtaining a large tuning bandwidth. At equivalent extents of overcoupling, all resonant circuit geometries have nearly equal remote tuning bandwidths. Particularly for the case of low-loss transmission line, the tuning bandwidth can be many times the tuned circuit's bandwidth, f(o)/Q.     

15N spin diffusion rate in solid-state NMR of totally enriched proteins: the magic angle spinning frequency effect

As demonstrated by means of the one-dimensional solid-state MAS exchange experiment (CODEX), the rate of the proton driven spin diffusion between backbone (15)N nuclei in totally enriched protein depends strongly on the magic angle spinning (MAS) frequency: spin diffusion at MAS frequency 16 kHz is about 4-5 times slower as compared to that at MAS frequency 1 kHz which is due to the averaging of the homo- and hetero-nuclear dipolar interactions by MAS. It is important that even at the highest MAS frequencies used in our experiments the spin diffusion rate is comparable or larger than typical values of the spin-lattice relaxation rates of backbone nitrogens in solid proteins. Thus, the precise quantitative analysis of (15)N T(1)'s in totally enriched solid proteins may lead to wrong quantitative results. On the other hand, the effectiveness of the (15)N-(15)N correlation and structure determination experiments making use of (15)N-(15)N distances can be increased by decreasing the MAS frequency as far as possible, which is counter intuitive to the commonly applied fast MAS conditions in order to reduce the dipolar-broadened line widths for increased spectral resolution.     

Sliding backshorts for planar circuits

The Superconductor-Insulator-Superconductor (SIS) tunnel junction is an extremely sensitive heterodyne detector at millimeter and submillimeter wavelengths. The large inherent capacitance associated with this device results in a substantial impedance mismatch with typical antennas and, therefore, requires a tuning circuit for optimum results. At frequencies where waveguide dimensions are realizable, impedance matching can be accomplished by embedding the detector in a waveguide circuit with adjustable waveguide backshorts. At higher frequencies, where waveguide dimensions become prohibitively small, a planar transmission line embedding circuit provides a reasonable alternative. Typically, such planar circuits offer no post-fabrication adjustability, resulting in demanding materials and design requirements. An adjustable planar embedding circuit based on coplanar transmission lines with movable noncontacting shorting elements has been developed. The shorting elements each consist of a thin metallic plate with an optimized arrangement of rectangular holes, placed along the insulated metallic transmission line to provide a periodic variation of the line impedance. A scadel (1–5 GHz) has shown that a large reflection coefficient, |s ₁₁|–0.5 dB, can be achieved with these sliding elements. A low frequency tuning circuit incorporating these shorting elements has been tested to demonstrate practical tuning ranges.     

Stop band characteristics of a TESLA cavity coaxial-type HOM coupler

A TTF-type coaxial higher order modes (HOM) coupler has been used in a TESLA 9 cell cavity. It is impossible to measure the stop band characteristics of the HOM coupler with the cavity. A measurement device for the coaxial transmission line type for the HOM coupler has been designed at Peking University. Experimentally it was shown that the average voltage standing wave ratio of the coaxial transmission line is smaller than 1.08. The experimental results of the stop band characteristics of the TTF-type HOM coupler have been fitted for the simulation. This paper describes the design of the measurement device and discusses the experimental and simulation results of stop band characteristics of the HOM coupler.     

High-speed femtosecond pump-probe spectroscopy with a smart pixel detector array

A new femtosecond pump-probe spectroscopy technique is demonstrated that permits the high-speed, parallel acquisition of pump-probe measurements at multiple wavelengths. This is made possible by use of a novel, two-dimensional smart pixel detector array that performs amplitude demodulation in real time on each pixel. This detector array can not only achieve sensitivities comparable with lock-in amplification but also simultaneously performs demodulation of probe transmission signals at multiple wavelengths, thus permitting rapid time- and wavelength-resolved femtosecond pump-probe spectroscopy. Measurements on a thin sample of bulk GaAs are performed across 58 simultaneous wavelengths. Differential probe transmission changes as small as approximately 2 x 10(-4) can be measured over a 5-ps delay scan in only approximately 3 min. This technology can be applied to a wide range of pump-probe measurements in condensed matter, chemistry, and biology.