Appropriate frequency

Detailed All <0.25 X OEL <1.25 X OEL Arithmetic mean <0.5 xOEL Mean >0.5 x OEL (or with individual results scattered above the limit) None if exposure is as low as reasonably practical. Investigate, take remedial action and repeat survey. Consider routine monitoring and the appropriate frequency. Investigate, assess control measures, improve where possible, repeat survey and consider routine monitoring. 

Valid data are an absolute prerequisite of vibration monitoring and analysis. Without accurate and complete data taken in the appropriate frequency range, it is impossible to interpret the vibration profiles obtained from a machine-train. 

Arithmetic mean <0.5 xOEL Consider routine monitoring and the appropriate frequency. 

The electron spin resonance spectrum of a free radical or coordination complex with one unpaired electron is the simplest of all forms of spectroscopy. The degeneracy of the electron spin states characterized by the quantum number, ms = 1/2, is lifted by the application of a magnetic field, and transitions between the spin levels are induced by radiation of the appropriate frequency (Figure 1.1). If unpaired electrons in radicals were indistinguishable from free electrons, the only information content of an ESR spectrum would be the integrated intensity, proportional to the radical concentration. Fortunately, an unpaired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The arrow in Figure 1.1 shows the transitions induced by 0.315 cm-1 radiation. 

This expression provides the basis of a spectroscopic method The transition of electron or nuclear spins between energy levels ("change of spin") may be associated with the emission or absorption of energy in the form of radiation with the appropriate frequency. Since the frequency is proportional to the applied field, spin spectra can in principle be studied in any region of the electromagnetic spectrum, merely by choosing an appropriate field strength. For practical reasons the fields are normally of the order of 1.5 tesla for nuclei and 0.3 tesla for electrons. 

Some reactions occur much faster if the reacting system is exposed to incident radiation of an appropriate frequency. Thus, a mixture of hydrogen and chlorine can be kept in the dark, and the reaction to form hydrogen chloride is very slow however, if the mixture is exposed to ordinary light, reaction occurs with explosive rapidity. Such reactions are generally called photochemical reactions. 

Sometimes the atoms (or molecules) in molecular beams are put into selected electronic, vibrational and rotational states. The initial state selection can be made with lasers. A laser beam of appropriate frequency is shined onto a molecular beam and the molecule goes onto an appropriate excited state. The efficiency of selection depends upon the absorption coefficient. We can attain sufficient absorption to get highly vibrationally excited molecule with the laser. A spin forbidden transition can also be achieved by using a laser. 

Luminescence is, in some ways, the inverse process to absorption. We have seen in the previous section how a simple two-level atomic system shifts to the excited state after photons of appropriate frequency are absorbed. This atomic system can return to the ground state by spontaneous emission of photons. This de-excitation process is called luminescence. However, the absorption of light is only one of the multiple mechanisms by which a system can be excited. In a general sense, luminescence is the emission of light from a system that is excited by some form of energy. Table 1.2 lists the most important types of luminescence according to the excitation mechanism. 

If an atomic transition is optically pumped by a beam of laser radiation having the appropriate frequency, the population in the upper state can be considerably enhanced along the path of the beam. This causes an intensification of the spontaneous emission from this state, which contains information about the conditions within the pumped region, since the exponential decay time for the intensified emission depends upon both the electron number density and the electron temperature. The latter can be obtained from the intensity ratio of the fluorescence excited from two different lower levels, if local thermal equilibrium is assumed. This method has been dis-... 

We now intend to derive the Bloch equations in order to express Ti and T2 according to spectral densities at appropriate frequencies. The starting point is the evolution equation of an elementary magnetic moment p subjected to a random field b... 

Because the coefficients of absorbance and emission are identical, a microwave photon of the appropriate frequency would be equally likely to cause emission of an identical photon from an excited molecule as to be absorbed by a molecule in the ground state if equal numbers of molecules were present in each state. Therefore, net absorbance of energy depends on the difference in populations between the ground state and the excited state. 

Stimulated absorption of photons. In this case, the electronic transition takes place from state 1 to state 2 in response to the action of an external radiation of the appropriate frequency. Atomic absorption spectrometry (AAS) is based on this process. On the other hand, atomic fluorescence spectrometry (AES) corresponds to the sequential combination of a stimulated absorption followed by spontaneous emission. 

Stimulated emission of photons. This process consists of electronic transitions from the excited energy level to the lower one stimulated by an external radiation of the appropriate frequency ( 2 - E fh and constitutes the basis of the laser (light amplification by stimulated emission of radiation) phenomenon. 

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