Microwave resonance technique

TM9-1910 (1955), 41-2 (Rate of deton) 55) E.F. Pound M.A. Cook, "The Measurement of Detonation Velocity by Microwave Resonator Techniques , Univ of Utah, TechRept 44(1955), Contract N7-onr-45107 56) H.T, Knight ... 

M.A.Cook, "The Measurement of Detonation Velocity by Microwave Resonator Techniques ,... 

Why microwave resonance techniques are so attractive for the spectroscopy of Rydberg states becomes clear when we estimate how much microwave power is required to drive the transitions. To drive the electric dipole ml— ni transition A... 

Measuring the M intervals by microwave resonance techniques generally yields the fine structure intervals as well. However, A = 0 transitions between the fine structure levels can also be examined by several other techniques. The first of these is rf resonance. Since the transition involves no change in Z it is not an electric dipole transition but rather a magnetic dipole transition, and a straightforward approach is magnetic resonance, which has been used by Farley and Gupta36 to measure the 6f and 7f fine structure intervals in Rb. Their approach is... 

In this paper we outline the theory necessary to understand the important magnetic interactions in a molecule that has zero electronic spin and orbital angular momenta in the ground electronic state. We concentrate on a description of the magnetic interactions that are normally measured by microwave resonance techniques.1,2... 

Experimental methods of measuring the Lamb shift can be broadly classified into two basic types. The first is the microwave resonance technique originally used by Lamb and Retherford, in which the 2s - 2p transition is observed directly in a microwave cavity. The prime factor limiting the accuracy is the precision with which the resonance line center can be located. Since the width r of the 2pi state is about one tenth of the Lamb shift, the line center must-be located to a precision of lOx ppm relative to F for a precision of x ppm n the Lamb shift. Line narrowing can be achieved by use of Ramsey s separated oscillatory fields technique, but at the expense of a lower signaj-to-noise ratio. Since the Lamb shift increases in proportion to Z, the f quency lies outside the micro-wave region for ions heavier than Li. A tunable laser can then be used in conjunction ith a fast ion beam as further discussed below. 

Johnson SG, Fearey BL (1993) Spectroscopic study of thorium using continuous-wave resonance ionization mass-spectrometry with rrltraviolet ionization. Spectrochim Acta Part B 48 1065-1077 Knoll GF (1989) Radiation Detection and Measurement. J. Wiley and Sons, New York Kuss HM (1992) Applications of microwave digestion technique for elemental analyses. Fresenins J Anal Chem 343 788-793... 

From a study of the microwave spectrum of 2-methylselenophene, the second-order Stark effect in the ground state was determined.11 The technique used was double radiofrequency-microwave resonance. For the identification by the double resonance method transitions of chiefly the A-state were chosen. From these observations the components of the dipole moment of 2-methylselenophene and the total dipole moment were determined. 

To resolve hf and nuclear quadrupole interactions which are not accessible in the EPR spectra, George Feher introduced in 1956 a double resonance technique, in which the spin system is simultaneously irradiated by a microwave (MW) and a radio frequency (rf) field3. This electron nuclear double resonance (ENDOR) spectroscopy has widely been applied in physics, chemistry and biology during the last 25 years. Several monographs2,4 and review articles7 11 dealing with experimental and theoretical aspects of ENDOR have been published. 

Infrared, Raman, microwave, and double resonance techniques turn out to offer nicely complementary tools, which usually can and have to be complemented by quantum chemical calculations. In both experiment and theory, progress over the last 10 years has been enormous. The relationship between theory and experiment is symbiotic, as the elementary systems represent benchmarks for rigorous quantum treatments of clear-cut observables. Even the simplest cases such as methanol dimer still present challenges, which can only be met by high-level electron correlation and nuclear motion approaches in many dimensions. On the experimental side, infrared spectroscopy is most powerful for the O—H stretching dynamics, whereas double resonance techniques offer selectivity and Raman scattering profits from other selection rules. A few challenges for accurate theoretical treatments in this field are listed in Table I. 

Buschmuller et have demonstrated that microwave resonance can be used effectively as means to monitor the moisture levels in a fluidized-bed dryer during the granulation process. The penetration depth of microwave resonance may be limited to a few microns, and hence this technique may not have any real advantages over NIR which has also been used for monitoring moisture in dryers, and has the advantage of providing chemical information such as solvent levels in addition to water, and other important properties such as polymorphic form, and particle size. 

Microwaves. Among the lowest frequencies of interest in collisional absorption are radio- and microwaves. As will be seen below, the absorption coefficient a is extremely small at low frequencies because absorption falls off to zero frequency as of2 see Chapter 5 for details. As a consequence, it has generally been necessary to use sensitive resonator techniques for the measurement of the loss tangent, tan <5 = s"/s, where s and s" are the real and imaginary part of the dielectric constant. The loss tangent is obtained by determination of the quality factors Qa, Qo, of the cavity with and without the gas filling, as (Dagg 1985)... 

Intercollisional interference. We note that at the lowest frequencies the simple proportionality between absorption coefficient and product of gas densities breaks down. Under such conditions, certain many-body interactions affect the observations and modify the shape or intensities of the binary spectra, often quite strikingly. An example is shown in Fig. 3.3, a measurement of the absorption in a neon-xenon mixture in the microwave region, at the fixed frequency of 4.4 cm-1. Because of the frequency-dependent factor of g(v) that falls off to zero frequency as v2, absorption is extremely small at such frequencies, Eq. 3.2. As a consequence, it has generally been necessary to use sensitive resonator techniques for a measurement of the absorption at microwave frequencies... 

Non resonant techniques are only of limited use to determine microwave losses with high precision, in particular when the losses are very small. Flowever, for the investigation of nonlinear absorption phenomena (i.e. rf power dependent on surface impedance or loss tangent) by intermodulation distortion measurements broad-band test devices are more common. Typically, a planar transmission line with an impedance of 50 Ohms can be employed for intermodulation... 

In this section we have described in considerable detail just one aspect of the spectroscopy of OH, namely, the measurement of zl-doubling frequencies and their nuclear hyperfine structure. This has led us to develop the theory of the fine and hyperfine levels in zero field as well as a brief discussion of the Stark effect. We should note at this point, however, that OH was the first transient gas phase free radical to be studied by pure microwave spectroscopy [121], We will describe these experiments in chapter 10. We note also that magnetic resonance investigations using microwave or far-infrared laser frequencies have also provided much of the most important and accurate information these studies are described in chapter 9, where we are also able to compare OH with the equally important radical, CH, a species which, until very recently, had not been detected and studied by either electric resonance techniques or pure microwave spectroscopy. 

There are several other important aspects of the experiment which should be mentioned. The waveguide cell is surrounded by a solenoid coil which can produce a magnetic field parallel to the ion beam direction the magnitude of this field (up to 50 G) is often sufficient to produce observable Zeeman splittings which greatly assist spectroscopic assignment, as we will see. ft is also possible to expose the molecular ion beam to two different microwave frequencies this so-called double resonance technique enables two different microwave transitions to be connected, if they share a... 

ENDOR (Electron Nuclear Double Resonance) involves the simultaneous application of a microwave and a radio frequency signal to the sample. This is a technique invented by Feher in 1956. The original studies were on phosphorous-doped silicon. A description of the experimental results and apparatus used is presented in two Physical Review articles [24, 25], An excellent treatment of EPR double resonance techniques and theory is given in the book by Kevan and Kispert [26], What follows here is the theory and application of ENDOR used the in analysis of single crystal data with the goal of identifying free radical products in DNA constituents. 

The microwave spectra and ab initio calculations (MP2/6-31G ) of 1-nitropyrazole indicate the planar structure of the molecule [1022], The initial assignment of the spectrum was carried out using a radio frequency-microwave double resonance technique. 

B. Historical Development of Microwave-Optical Multiple-Resonance Techniques... 

Both CO2-HF and CO2-HCI complexes were first examined in the gas phase by Klemperer and co-workers, using the molecular beam electric resonance technique, with the radio-frequency and microwave spectra of CO2-HF and CO2-HCI indicating nearly linear, hydrogen-bonded structures [35, 36], Accurate determinations of rotational constants allowed the separations between the centers-of-mass of each sub-unit to be obtained, and assuming that neither CO2 nor HX underwent intramolecular change, the O—H bond lengths could be estimated for each complex. A comparison of CO2-HF and CO2-HCI showed the O—H separation in the former to be 10% less than in the latter. Equilibrium structures for CO2-HF and CO2HCI are shown in Fig. 10, and Table 1 lists structural parameters for all known CO2-HX complexes. 

A double resonance technique in which transitions between spin sublevels are detected by optical means. Usually these are sublevels of a triplet, and the transitions are induced by microwaves. 

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