Improved pulse schemes for separated local field spectroscopy for static and spinning samples

An improved pulse sequence for SLF experiments based on the magic sandwich (MS) scheme for homo-nuclear dipolar decoupling is proposed. The sequence incorporates a double MS, both on I and S spins and has been named as EXE-MS2. The proposed scheme which has a scaling factor of 1 is observed to be free from low intensity artifacts and provides better line-widths particularly for S spins labeled at multiple sites. The pulse sequence which has been applied on static oriented samples incorporates the EXE scheme where direct polarization of the S spin in the B(0) field is utilized in the place of polarization inversion and is observed to perform well without any loss of sensitivity while ensuring considerable reduction in rf power input into the sample. The EXE scheme has also been tested for solid samples under MAS.     

Proton-detected separated local field spectroscopy

PISEMO, a separated local field experiment that can be performed with either direct (15)N (or (13)C) detection or indirect (1)H detection, is demonstrated on a single crystal of a model peptide. The (1)H signals modulated by (1)H-(15)N heteronuclear dipole-dipole couplings are observed stroboscopically in the windows of the multiple-pulse sequence used to attenuate (1)H-(1)H homonuclear dipole-dipole couplings. (1)H-detection yields spectra with about 2.5 times the signal to noise ratio observed with (15)N-detection under equivalent conditions. Resolution in both the (15)N chemical shift and (1)H-(15)N heteronuclear dipole-dipole coupling dimensions is similar to that observed with PISEMA, however, since only on-resonance pulses are utilized, the bandwidth is better.     

A superior pulse sequence for two-dimensional chemical shift correlation spectroscopy

A sequence comprising two ¹³C pulses and two proton pulses is justified theoretically and experimentally as providing a new form of ¹³C-¹H chemical shift correlation spectroscopy. The sequence is superior to polarization transfer sequences as it contains fewer pulses and distinguishes between carbon groups with different numbers of bonded protons.     

High-resolution local field spectroscopy with internuclear correlations

Establishing correlations among distant (>3 Å) spins remains an outstanding problem for both spectral assignment and elucidation of interhelical contacts in solid-state NMR of oriented membrane proteins. Here we present a pulse sequence which incorporates the previously established mismatched Hartmann–Hahn mixing of dilute spins via the proton bath together with high-resolution local field spectroscopy. In addition to providing structural information, the use of dipolar couplings in the indirect dimension helps eliminate the spectral crowdedness compared to the standard homonuclear correlation techniques. The proposed pulse sequence may find its use in assigning protein spectra in uniaxially oriented membrane environments.     

2-D homonuclear correlation and separated local field experiments for solids with strong homonuclear dipolar couplings

An experiment for acquiring two-dimensional homonuclear correlation spectra of nuclei in solids in the presence of strong homonuclear dipolar couplings is described. The experiment utilizes a multiple-pulse homonuclear decoupling sequence with an effective precession axis parallel to the rotating frame z-axis during the evolution and detection periods. A multiple-pulse sequence that suppresses chemical shift and heteronuclear dipolar coupling evolution and scales the static homonuclear dipolar coupling is proposed for the mixing period. The evolution during the mixing period is analogous to the dynamics of the mixing period in solution-state TOCSY experiments, and can be interpreted as the oscillatory exchange of longitudinal magnetization between coupled spins. For nuclides with large gyromagnetic ratios, the static homonuclear dipolar interaction will be substantially larger than the mechanisms used to develop internuclear correlations in solution state 2-D experiments, which should make it possible to establish correlations over much longer distances and with significantly shorter mixing times. Extensions to separated local field experiments are discussed.     

A narrow-linewidth optical parametric oscillator for mid-infrared high-resolution spectroscopy

We present a narrow-linewidth, singly-resonant cw optical parametric oscillator, emitting more than 1 W in the 2.7–4.2?µm range. The OPO is pumped by a narrow linewidth (40?kHz) fibre-laser system and the signal frequency is locked to a high-finesse Fabry–Pérot cavity in order to increase the spectral resolution, thus obtaining a residual linewidth of 70?kHz for the signal. We tested the spectral performance of our OPO on several transitions in the ν₁ rovibrational band of CH₃I, measuring line intensities and showing sub-Doppler dip detection.     

A programmable digital pulse generator for pulsednmr spectroscopy

The pulse programmer generates highly stable pulses and is capable of producing various scheme of pulses (i.e. a given specific number of pulses with independently variable widths and separations) employed in pulsednmr spectroscopy. The construction and working of the programmer is described with reference to the two most commonly used schemes, namely, a two-pulse sequence and the Carr-Purcell sequence.     

Homonuclear Hartmann-Hahn transfer with reduced relaxation losses by use of the MOCCA-XY16 multiple pulse sequence

Homonuclear Hartmann-Hahn transfer is one of the most important building blocks in modern high-resolution NMR. It constitutes a very efficient transfer element for the assignment of proteins, nucleic acids, and oligosaccharides. Nevertheless, in macromolecules exceeding approximately 10 kDa TOCSY-experiments can show decreasing sensitivity due to fast transverse relaxation processes that are active during the mixing periods. In this article we propose the MOCCA-XY16 multiple pulse sequence, originally developed for efficient TOCSY transfer through residual dipolar couplings, as a homonuclear Hartmann-Hahn sequence with improved relaxation properties. A theoretical analysis of the coherence transfer via scalar couplings and its relaxation behavior as well as experimental transfer curves for MOCCA-XY16 relative to the well-characterized DIPSI-2 multiple pulse sequence are given.     

An analysis of the intrinsic resonance offset dependence of magnetization generated by stimulated echo pulse sequences for noncoupled spins.

It is demonstrated that the basic radiofrequency pulse train used to generate stimulated echoes (90x-tau TE-90x-tau TM-90x-tau TE-Acq.) is in general characterized by strong amplitude and phase modulations of the transverse magnetization as a function of the resonance offset. Two dephasing techniques which eliminate the modulations are investigated both theoretically and experimentally, and a simple formula is derived for calculating the relative modulation across a spectrum as a function of gradient strength and duration, echo delay, and spectral linewidth.     

A hollow cathode for high-resolution optogalvanic spectroscopy: Application to uranium

Summary We report the design criteria and performance characteristics of a hollow-cathode tube developed especially for high-resolution Doppler-free laser spectroscopy. It is simple to construct and is easily demountable. Narrow homogeneous line widths of refractory and highly reactive-element transitions have been observed by applying intermodulated detection scheme. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

A line‐scan hyperspectral Raman system for spatially offset Raman spectroscopy

Spatially offset Raman spectroscopy (SORS) is a technique that can obtain subsurface layered information by collecting Raman spectra from a series of surface positions laterally offset from the excitation laser. Currently optical fiber probes are used as major tools in SORS measurement, which are either slow (single fiber probe with mechanical movement) or restricted in selecting offset range and interval (fiber probe array). This study proposes a new method to conduct SORS measurement based on a newly developed line‐scan hyperspectral Raman imaging system. A 785‐nm point laser was used as an excitation source. A detection module consisting of an imaging spectrograph and a charge‐coupled device camera was used to acquire line‐shape SORS data in a spectral region of −592 to 3015 cm⁻¹. Using a single scan, the system allowed simultaneous collection of a series of Raman spectra in a broad offset range (e.g. 0–36 mm in two sides of the incident laser) with a narrow interval (e.g. 0.07 mm). Four layered samples were created by placing butter slices with thicknesses of 1, 4, 7, and 10 mm on top of melamine powder, providing different individual Raman characteristics to test the line‐scan SORS technique. Self‐modeling mixture analysis (SMA) was used to analyze the SORS data. Raman spectra from butter and melamine were successfully retrieved for all four butter‐on‐melamine samples using the SMA method. The line‐scan SORS measurement technique provides a flexible and efficient method for subsurface evaluation, which has potential to be used for food safety and quality inspection. Copyright © 2015 John Wiley & Sons, Ltd.     

Probeheads and instrumentation for pulse EPR and ENDOR spectroscopy with chirped radio frequency pulses and magnetic field steps

Probeheads and instrumentation for modern X-band pulse EPR and ENDOR experiments with chirped radio-frequency pulses and rapidB ₀-field pulses are described. The resonant frequency, the quality factor and, for the first time, the response of a pulse ENDOR resonator structure to a microwave pulse in the subnanosecond time scale have been calculated. The performance of the probeheads for time-domain chirp ENDOR and electron Zeeman-resolved EPR is demonstrated.     

A sequential HNCA NMR pulse sequence for protein backbone assignment

The conventional HNCA pulse sequence suffers from the ambiguity that it cannot distinguish inter- and intraresidue correlations because the one-bond and two-bond J(NC(alpha)) coupling constants are of similar magnitude. This paper presents a novel pulse sequence, sequential HNCA, that leads to a spectrum exhibiting exclusively interresidue correlations. This important sequential information has so far usually been obtained by an HN(CO)CA experiment that for medium field strengths typically also is more sensitive than HNCA. However, for increasing static magnetic fields the chemical shift anisotropy relaxation mechanism of carbonyl carbons becomes more and more efficient, leading to a degradation of the HN(CO)CA sensitivity. Hence there is a point where the sequential HNCA experiment becomes the most sensitive option for sequential N-C(alpha) correlation.     

A two frequency,separated oscillatory field technique for multiple quantum transitions

We have developed a new two frequency technique to apply Ramsey's method of separated oscillatory fields to multiple quantum transitions, and have applied it to observing the 3S-3D double quantum transition in atomic hydrogen.     

Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

An injection-seeded optical parametric oscillator (OPO), based on periodically poled KTiOPO4 and pumped by a frequency-doubled, nanosecond-pulsed Nd:YAG laser, generates continuously tunable, single-longitudinal-mode, pulsed output at approximately 842 nm for high-resolution spectroscopy. Optical-heterodyne measurements show that the OPO frequency chirp increases linearly with detuning from the free-running (unseeded) OPO frequency and can be maintained as low as 10 MHz. Other factors affecting chirp are identified.     

A high-resolution Fourier transform spectrometer for far infrared magneto-optic spectroscopy of magnetic materials

We describe a high-resolution (0.015 cm⁻¹) Fourier transform spectrometer which has been developed to investigate bulk and surface magnetic polaritons in magnetic media by far infrared magneto-optic spectroscopy. The spectrometer uses a novel combination of laser-controlled sampling of the interferogram and phase modulation of the infrared beam to combine very accurate sampling and low signal-to-noise ratio. The spectrometer is coupled to a liquid helium cryostat with a 7 T superconducting magnet, and a liquid helium-cooled silicon bolometer is used as the detector. Samples can be mounted in the cryostat for polarised oblique incidence reflection measurements in the Voigt geometry with the applied magnetic field vertical. Measurements on surface polaritons are made by using attenuated total reflection (ATR) spectroscopy with silicon prisms to obtain the necessary wave vector enhancement. The resolution of the instrument is demonstrated with measurements on the rotational lines of water vapour, and a selection of measurements on a bulk single crystal of FeF₂, a uniaxial antiferromagnet, is presented to illustrate the performance of the instrument as a probe of magnetic excitations.     

A continuously tunable long-wavelength cw IR source for high-resolution spectroscopy and trace-gas detection

A new widely tunable source in the infrared for use in high-resolution spectroscopy and trace-gas detection is described. This spectroscopic source is based on Difference Frequency Generation (DFG) in gallium selenide (GaSe) and is continuously tunable in the 8.8–15.0 m wavelength region. Such a DFG source operates at room temperature which makes it a useful alternative to a lead-salt diode-laser- based detection system that requires cryogenic temperatures and numerous individual diode lasers.Prof. F. P. Schäfer on the occasion of his 65th birthday.