Gradient-enhanced heteronuclear correlation spectroscopy: theory and experimental aspects. 1992

A detailed theoretical and experimental treatment is given for gradient-enhanced heteronuclear correlation spectroscopy. Both multiple-quantum and single-quantum sequences are described. In addition to a comparison with conventional experiments using phase cycling, the effects of different gradient combinations are examined with respect to artifacts occurring in the heteronuclear dimension. The influence of gradient performance and diffusion on sensitivity is discussed. Approaches to attain phase-sensitive spectra are also analyzed.     

Gradient-enhanced FAWSETS perfusion measurements

This work describes the use of custom-built gradients to enhance skeletal muscle perfusion measurements acquired with a previously described arterial spin labeling technique known as FAWSETS (flow-driven arterial water stimulation with elimination of tissue signal). Custom-built gradients provide active control of the static magnetic field gradient on which FAWSETS relies for labeling. This allows selective, 180 degrees modulations of the phase of the perfusion component of the signal. Phase cycling can then be implemented to eliminate all extraneous components leaving a signal that exclusively reflects capillary-level perfusion. Gradient-enhancement substantially reduces acquisition time and eliminates the need to acquire an ischemic signal to quantify perfusion. This removes critical obstacles to application of FAWSETS in organs other than skeletal muscle and makes the measurements more desirable for clinical environments. The basic physical principles of gradient-enhancement are demonstrated in flow phantom experiments and in vivo utility is demonstrated in rat hind limb during stimulated exercise.     

Laser spectroscopy : 5. Raman spectroscopy

This article in the series reviews the application of lasers in Raman spectroscopy.     

Laser spectroscopy 4. Nonlinear high resolution spectroscopy

This paper in the series gives a brief outline of the main ideas, methods and types of nonlinear laser spectrometer used for atomic and molecular spectroscopy without Doppler broadening. This is the most developed area of laser spectroscopy and has recently been the subject of several papers^1,2 to which readers are reffered for more detailed information.     

Laser spectroscopy : 3. Linear laser spectroscopy

The three basic methods of linear laser spectroscopy are reviewed: absorption, opto-acoustic, and fluorescence spectroscopy. The characteristics of each method and their areas of application are presented and compared.     

Dynamic-angle spinning of quadrupolar nuclei. 1990

In dynamic-angle spinning (DAS), a sample spins around an axis inclined at an angle θ(t) with respect to the magnetic field such that the averages of P_n(cosθ) are zero. The simplest case is where θ(t) assumes two discrete values θ₁ and θ₂ (complementary DAS angles) such that the averages of P₂(cosθ) and P₄(cosθ) are zero, thereby removing second-order quadrupolar (and dipolar) broadening. Examples of DAS complementary angles are θ₁=37.38° and θ₂=79.18°. Experimental details for DAS experiments are provided and applications to sodium-23 and oxygen-17 NMR illustrate the enhanced resolution achieved by removing the second-order broadening inherent in magic-angle spinning.     

Contents volume 17 no. 9 September 1990

Contents volume 17 no. 9 September 1990     

Variable temperature photoacoustic spectroscopy. I. Instrumentation

To use the advantages of photoacoustic spectroscopy for low temperature measurements, e.g. in the study of frozen liquids or suspensions, a new high performance photoacoustic cell has been constructed which allows continuous variation of the temperature between 90 K and 320 K. The frequency response of the cell at 90 K and 295 K is examined by means of a carbon black sample.     

Penning ionization electron spectroscopy and photo-electron spectroscopy of molecular solids. II. Ammonia and water

The Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) of ammonia and water molecules condensed on a cold metal substrate have been measured using thermal He*(2³S, 2¹S) metastable atoms and He(I) (58.4 nm) photons. The shifts of the observed positions of the PIES peaks relative to those of the UPS peaks in the condensed phase are roughly equal to the corresponding shifts in the gas phase. The relative intensities of the 3a₁ and 1e orbital peaks are reversed in the PIES and UPS for both gaseous and condensed ammonia; the origin of this reversal is interpreted as the difference between the interactions with metastable atoms and photons. On the other hand, the relative intensities of the 3a₁ orbital peak in the PIES and UPS for condensed water decrease as compared to the gas-phase spectra. This implies participation of the 3a₁ orbital of water in the hydrogen bonding.     

Laser spectroscopy 8. Picosecond processes

Picosecond laser spectroscopy is a rapidly developing field, the applications of which extend to the adjacent subjects of chemistry and biology. Here, the various methods and areas of application are classified and compared. Some experiments and results are mentioned for illustration.     

E.S.R. spectroscopy of non-cyclic enones

The π-bond model is applied to the calculation of the proton chemical shifts of a range of benzenoid hydrocarbons. The simple calculations give results which compare well with experiment and with calculations based on the ring current model. Least squares fitting of the predicted chemical shifts to the observed values can give an indication of the occurrence of local aromatic groups in larger molecules. This procedure also yields an empirically adjusted estimate of the anisotropy in the molar magnetic susceptibility from the experimental proton chemical shifts. A simple criterion for aromaticity is also given. The overall results suggest that there is no need to associate a special ring current attribute to π conjugation in aromatic molecules.     

Three-dimensional triple-resonance NMR Spectroscopy of isotopically enriched proteins. 1990

Four new and complementary three-dimensional triple-resonance experiments are described for obtaining complete backbone ¹H, ¹³C, and ¹⁵N resonance assignments of proteins uniformly enriched with ¹³C and ¹⁵N. The new methods all rely on ¹H detection and use multiple magnetization transfers through well-resolved one-bond J couplings. Therefore, the 3D experiments are sensitive and permit relatively rapid recording of 3D spectra (l–2 days) for protein concentrations on the order of 1 mM. One experiment (HNCO) correlates the amide ¹H and ¹⁵N shifts with the ¹³C shift of the carbonyl resonance of the preceding amino acid. A second experiment (HNCA) correlates the intraresidue amide ¹H and ¹⁵N shifts with the Cα chemical shift. This experiment often also provides a weak correlation between the amide NH and ¹⁵N resonances of one amino acid and the Ca resonance of the preceding amino acid. A third experiment (HCACO) correlates the Hα and Cα shifts with the intraresidue carbonyl shift. Finally, a 3D relay experiment, HCA(CO)N, correlates Ha and Cal resonances of one residue with the ¹⁵N frequency of the succeeding residue. The principles of these experiments are described in terms of the operator formalism. To optimize spectral resolution, special attention is paid to removal of undesired J splittings in the 3D spectra. Technical details regarding the implementation of these triple-resonance experiments on a commercial spectrometer are also provided. The experiments are demonstrated for the protein calmodulin (16.7 kDa).     

Laser spectroscopy 9. Prospects and applications

This concluding article in the series discusses the prospects of realizing the ultimate parameters — spectral resolving power and sensitivity — and some fields of application — nuclear physics, chemical kinetics and molecular physics, quantum metrology — of laser spectroscopy.     

Laser spectroscopy : 1. Principal problems

Laser spectroscopy has made it possible to solve, or at least to begin solving, a number of important problems to which classical spectroscopy could not be applied. These problems are defined in this, the first in a series of articles reviewing the subject of laser spectroscopy. This is followed by an outline of the present status of tunable lasers — the basic instruments used in developing the techniques of laser spectroscopy.     

Applied Physics A 51, no. 2 (1990)

Applied Physics A 51, no. 2 (1990)     

Strength distributions and statistical spectroscopy. I. General theory

The strength distribution for an arbitrary excitation is given in terms of a double expansion, and its sum rules by single expansions, in polynomials defined by the initial and final energy spectra. In model spaces which are not too large, a rapid convergence, to within fluctuations, is assured by the action of a central limit theorem, as is shown in particular by considering the response of the system under infinitesimal deformations of the Hamiltonian. When larger spaces are decomposed into subspaces defined by a partitioning of the single-particle space a similar convergence results. At the same time, close contact is made with, and important corrections are found to, intuitive procedures which are often used for approximating strength distributions. The general features of the distribution are often easily understood in termsof a simple geometry made effective in the model space by the central limit theorem, and further features by exploiting the connection of this geometry to the unitary group of transformations in the single-particle space. Extensions are given for multipole strengths and sum rules, appropriate when the angular momenta (and isospins) are specified for the states involved in the transitions. Measures for the RMS fluctuations in the sum-rule quantities, and correlations between them, are given by combining the low-order-polynomial (statistically smoothed) strengths with an assumed Porter-Thomas distribution for the (high-order) strength fluctuations.