Radiofrequency electronics

High-power Radiofrequency Electronics. - The system must produce and transmit r.f. power levels of several hundreds of watts, and the probe circuit must not arc during the high-power pulses. Modern instrumentation is designed to conform to the necessary standards. 

Electromagnetic and Radiofrequency Shielding. Because bismuth is highly diamagnetic, its ahoys are quite useful in appHcations where electronic equipment must be protected from outside interference or where equipment can cause outside interference. 

In two other implementations of electron impact SNMS, a plasma is generated in the ionizer volume to provide an electron gas sufFiciendy dense and energetic for efficient postionization (Figure 2c). In one instrument, the electrons are a component of a low-pressure radiofrequency (RF) plasma in Ar, and in the second, the plasma is an electron beam excited plasma, also in Ar. The latter type of electron-gas SNMS is still in the developmental stages, while the former has been incorporated into commercial instmmentation. 

When LiMn204 electrodes are deposited as thin films on a platinum substrate, either by electron-beam evaporation or radiofrequency (rf) sputtering, structures are sometimes formed that exhibit unusual electrochemical behavior [146, 147]. Such electrodes have been evaluated in solid-... 

Electron energy distribution function The distribution function of electrons in a plasma. That of a low-pressure radiofrequency plasma generally consists of two Maxwellian distributions, that is, fast and slow electrons. 

In Chapter 2, ENDOR (electron-nuclear double resonance) was briefly described. To perform an ENDOR experiment it is necessary to apply both a radiofrequency and a microwave frequency, effectively performing simultaneous NMR and ESR, respectively, on the sample. The experiment is performed at a fixed magnetic field, with the ESR saturating frequency centered on a... 

Electron gain > 5 million. Lower gains are workable, but external amplification is then required and the greater is the susceptibility to radiofrequency pickup from the source. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

A chemical shift is defined as a displacement in the magnetic resonance frequency of a nucleus as a consequence of the electronic environment in which the nucleus resides. Because moving electrons generate their own magnetic fields, a nucleus surrounded by these electrons experiences an effective field, which is defined by (1 - a)H , where a is the so-called screening constant and Ho is the applied magnetic field. A chemical shift is typically reported as a dimensionless displacement (units = parts per million, or simply ppm) from a reference standard. If the magnetic field is varied while the radiofrequency V is held constant, then the chemical shift... 

The physical chemist of today has a wide variety of methods at his disposal for the experimental investigation of electronic structure and all of them have been used in attempts at obtaining evidence of the participation of outer d-orbitals in bonding. One such group of methods is constituted by the various techniques of radiofrequency spectroscopy, which have the advantage that they yield information about the molecule in its ground state. In this they have a distinct superiority over, say, electronic absorption spectra where it is necessary to consider both ground and excited states. Moreover much of the data derived from radiofrequency spectroscopic methods concerns essentially just one part of the molecule so that attention can be concentrated on those atoms of interest in whatever study happens to be under way. 

The immediate object of this review is the results of the application of radiofrequency spectroscopic methods to the valence-shell expansion problem but it is hoped that this will serve as an illustration of the more general one of their application to any problem in electronic structure. 

Almost all the parameters yielded by the various types of radiofrequency spectroscopy arise from the interaction of nuclear magnetic or electrostatic moments with the magnetic or electrostatic fields produced by the surrounding electrons. A consideration of the way these interactions arise shows that they fall into two groups one of the groups contains terms proportional to the electron density at the nucleus, N, itself, Vn(0), and consequently reflects only the s-character of the wave-function centered on N, v>n while the other is proportional to the value for all or some of the electrons surroimding the nucleus N (Table 1). This latter term vanishes for s-type orbitals and for p, d, f orbitals of the same principal quantum number has values in the order p > > d > > f. In practice this means that in a first approximation, only p-electrons contribute to and that the direct effect of the d-orbitals is only... 

The ENDOR technique refers to electron-nuclear double resonance. This consists of the effect on a partially saturated ESR line of simultaneously irradiating the sample with a radiofrequency to induce nuclear resonance transitions of hyperfine coupled nuclei. It may enable one to obtain information about signs of coupling constants. ELDOR is the technique corresponding to electron-electron double resonance. Such techniques, coupled with TRIPLE resonance, have been utilized and well described in a discussion of pyridine and 4,4-bipyridyl radical anion ESR spectra measured in sodium/liquid ammonia (80JMR<41)17). 

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