Double Resonance Spectrometers

Earlier work by Lee and White was repeated by Fehse et al using a double resonance crossed Fabry-Perot spectrometer with frequency modulation of the probe source. Both worked in the 26-40 GHz region for the probe frequency, with 15 GHz and 23 GHz pump radiation respectively. The work was not reported in any great depth but served to illustrate that the technique was viable according to both sets of workers, if less sensitive than conventional Stark spectrometry. 

The approach is interesting, because with double resonance techniques it is possible in principle to uniquely identify a component from a single measurement in even a complex gas mixture. This arises because the probability of two spectral lines each from two molecules in the same mixture overlapping is so vanishingly small.Double resonance methods have therefore a certain charm for the analytical scientist who is always seeking methods showing absolute identification possibility. The methods as configured, e.g. Andrews et aV and references therein, are, however, complex and expensive. They would be probably too 

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Figure 11.38. Block diagram of the molecular beam pump/probe microwave/optical double resonance spectrometer, developed by Fletcher, Jung, Scurlock and Steimle [78]. 

N. Boden, J. Capart, W. Derbyshire, H. S. Gutowsky, and J. R. Hansen, "Frequency sweep, field-frequency stabilized double resonance spectrometer," Rev. Sci. Instrum. 39, 805-816 (1968). 

W. Fehse, D. Christen and W. Zeil, Microcomputer Controlled Microwave-Microwave Double Resonance Spectrometer Incorporating Two Crossed Fabry-Perot Resonators, J. Mol. Struct, 1983, 97, 263-270. 

W.E. Ernst, S. Kindt, A molecular betim laser-microwave double resonance spectrometer for precise measurements of high temperature molecules. Appl. Phys. B 31,79 (1983)... 

The " N nucleus, because of its widespread occurrence in all types of systems (especially biologically active systems), is of particular interest in studying electron density distribution, molecular reorientations and intermolecular time-dependent interactions. It seems that such studies will acquire more and more importance in the future and will occur more frequently, especially with the availability of double resonance spectrometers and new data processing techniques such as the maximum entropy method. The examples discussed do not, of course, exhaust the potential of NQR as a tool for structure and chemical bonding. These are only simple illustrations of the applied aspects of NQR spectroscopy. 

This chapter concludes with a brief description of one advanced technique, Electron Nuclear Double Resonance (ENDOR), the capabilities for which, unlike pulsed methods, may be added as a relatively minor modification to commercial CW ESR spectrometers. 

Potentially, extremely large SNR enhancements are obtainable with double resonance NMR spectrometers with H frequencies 1000 times greater than the 14N NQR frequencies in TNT are available. Of course, such equipment is completely unsuitable for... 

Triple resonance experiments are used when double resonance experiments (either ID or 2D) fail to provide the needed answer, whether it be assignment of 29Si lines, their resolution, or separation of NMR parameters. A number of determinations, however, can be carried out using pseudo-triple resonance that is less demanding on the spectrometer hardware as described in Sections V.B, VI.A, and VI.C.2. 

The newly developed 600 MHz NMR Spectrometer is used to characterize coal-derived liquids and their chromatographically separated fractions. The distinct and well resolved proton resonance lines in both aromatic and aliphatic regions and IR analysis have been used to identify the major compounds and compound types. Double resonance technique has been applied for the chemical shift identification of donor protons (or-CHg, p-CH,) of partially hydrogenated polynuclear aromatic compounds. An NMR difference technique is applied to determine specific compositional changes in upgraded liquids derived under identical process conditions, but from different coal sources. 

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