Microwave basic principles

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Polyatomic molecules have more complex microwave spectra, but the basic principle is the same any molecule with a dipole moment can absorb microwave radiation. This means, for example, that the only important absorber of microwaves in the air is water (as scientists discovered while developing radar systems during World War II). In fact, microwave spectroscopy became a major field of research after that war, because military requirements had dramatically improved the available technology for microwave generation and detection. A more prosaic use of microwave absorption of water is the microwave oven it works by exciting water rotations, and the tumbling then heats all other components of food. 

This section reviews the basic principles of physics pertaining to microwave heating. 

Phase-transfer catalysis (PTC) is the most widely used method for solving the problem of the mutual insolubility of nonpolar and ionic compounds. Basic principles, synthetic uses, industrial applications of PTC, and its advantages over conventional methods are well documented [1-3]. PTC has become a powerful and widely accepted tool for organic chemists due to its efficiency, simplicity, and cost effectiveness. The main merit of the method is its universality. It may be applied to many types of reactions involving diverse classes of compounds. An important feature of PTC is its computability with other methods for the intensification of biphasic reactions (sonolysis, photolysis, microwaving, etc.) as well as with other types of catalysis, in particular, with transition-metal-complex catalysis. Homogeneous metal-complex catalysis under PTC conditions involves the simul-... 

A major disadvantage is the greater sophistication required in instrumentation. Our use of a liquid solution model to demonstrate the basic principles of ENDOR is perhaps misleading, as most ENDOR studies are done on solid samples at very low temperatures where long relaxation times allow ENDOR to be done at microwave and radio frequency powers that are easily obtained. ENDOR studies have been done on free radicals in solution,1"5 but the high radio frequency power levels needed require special instrumentation. 

In this introductory book chapter, several modem extraction techniques will be described, including supercritical fluid extraction, pressurized liquid extraction, pressurized hot Avater extraction, microwave assisted extraction, membrane-assisted solvent extraction, solid phase micro extraction and stir-bar sorptive extraction. These are techniques that meet many of today s requirements in terms of environmental sustainability, speed and automation. Basic principles of operation as well as method optimization will be discussed and compared for the different techniques. Both analytical and industrial applications will be discussed, together with commercial instruments available on the market. Key references will be given, and conclusions regarding applicability of the different techniques with respect to sample e, target-molecules and analytical vs. large-scale applications. 

Although the basic principles of heating and drying at dielectric and microwave frequencies are the same, the methods of generation and eqnipment are different. These will be described separately later in the chapter. In other cases, the two terms may be used interchangeably in the text. 

The basic principle involved in electron spin echo spectroscopy is the formation of an echo. The simplest spin echo experiment uses two microwave pulses to form an echo, which is also... 

This emitted radiation falls within the radio wave or microwave frequency region. By using a radio wave receiver which can detect this radiation we can detect this quantum transition. However, the transition is so fast that at the moment when the sample is placed into the magnetic field all the protons will reorient themselves so quickly that is becomes very difficult to register signals. To avoid this difficulty the instrument, the nmr spectrometer has an oscillator which irradiates the sample with radio or microwaves by which the spins are constantly reoriented into disfavored orientations. The basic principle of the construction of nmr spectrometer is represented in the next figure. 

The Rabi technique of radio frequency or microwave spectroscopy in atomic or molecular beams [10.14-10.17] has made outstanding contributions to the accurate determination of ground state parameters, such as the hfs splittings in atoms and molecules, small Coriolis splitting in rotating and vibrating molecules, or the narrow rotational structures of weakly bound van der Waals complexes [10.18]. Its basic principle is illustrated in Fig. 10.9. A collimated beam of molecules with a permanent dipole moment is deflected in a static... 

Doppler Radar. Doppler radar was first used in England in 1953 to pick up the movement of small storms. The basic principle behind Doppler radar is that the back-scattered radiation frequency detected at a certain location changes over time as the target, such as a storm, moves. The mode of operation requires a transmitter that is used to send short but powerful microwave pulses. When a foreign object... 

Although the basic principles of ESR and NMR are similar, practical difficulties mean that pulse methods are less useful in ESR. This is because the pulse power required to produce the frequency span of a typical ESR spectrum would be several kilowatts and the pulse very short (nanoseconds). The pulse or FT (Fourier transform) ESR spectrometer is usually based on a standard CW instrument because it is often useful to record a standard ESR spectrum before carrying out pulse experiments. The pulse microwave source is usually a travelling wave tube other essentials... 

In this chapter we have reviewed all the advancements of the wet-chemical synthesis approaches to colloidal metal NCs highlighting the basic principles which govern this part of chemistry and specifically, how their elegant manipulation influences the synthesis outcomes. Beyond this, we have tried to delineate the synthetic effectiveness of thermal energy when delivered via traditional heat transfer modes or via microwaves. 

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