Frequency of radiation

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sfflnple through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compar ed with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted as a spectrum, which consists of a series of peaks at characteristic frequencies. Its interpretation can furnish structural information. Each type of spectroscopy developed independently of the others, and so the data format is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 

Such efforts have met with limited success, and the reason usually advanced is our lack of understanding of the frequency dependence of molecular NLO properties. In classical electromagnetism, we refer to properties that depend on the frequency of radiation as dispersive and we say that (for example) dispersion is responsible for a rainbow. The blue colour of the sky is a dispersion effect, as is the red sky at night and morning. There is more to it than that, and you might like to read a more advanced text (Hinchliffe and Munn, 1985). 

Example -CH2CH2CH2-. black body An object that absorbs and emirs all frequencies of radiation without favor, black-body radiation The electromagnetic radiation emitted by a black body. 

Bohr frequency condition The relation between the change in energy of an atom or molecule and the frequency of radiation emitted or absorbed ... 

Microwaves have wavelengths between 1 mm and 1 m and hence have similar frequencies to radar and telecommunication devices. So as not to cause interference with these systems the frequency of radiation that can be emitted by household and industrial appliances is strictly regulated, with most appliances operating at a fixed frequency of 2.45 GHz. To some extent this reduces the flexibility of such equipment. 

In Eq. (4.9), V is the frequency of radiation and a>. and (Ox are the statistical weights of the initial and final states. It should be remembered that Eq. (4.9) refers to the photoionization cross section, not the total photoabsorption cross section (see Sect. 4.2). 

When a compound is irradiated with monochromatic radiation, most of the radiation is transmitted unchanged, but a small portion is scattered. If the scattered radiation is passed into a spectrometer, we detect a strong Rayleigh line at the unmodified frequency of radiation used to excite the sample. In addition, the scattered radiation also contains frequencies arrayed above and below the frequency of the Rayleigh line. The differences between the Rayleigh line and these weaker Raman line frequencies correspond to the vibrational frequencies present in the molecules of the sample. For example, we may obtain a Raman line at 1640 cm-1 on either side of the Rayleigh line, and the sample thus possesses a vibrational mode of this frequency. The frequencies of molecular vibrations are typically 1012—1014 Hz. A more convenient unit, which is proportional to frequency, is wavenumber (cm-1), since fundamental vibrational modes lie between 4000 and 50 cm-1. 

Letter from G. N. Lewis to Paul Ehrenfest, undated but probably 1925, G. N. Lewis Correspondence, BL.UCB. G. N. Lewis and D. F. Smith promised in their paper, "The Theory of Reaction Rate," JACS 47 (1925) 15081520, to publish a demonstration that a range of frequencies of radiation affecting degrees of freedom in a molecule is responsible for chemical reaction. This paper was the subject of the letter, with anonymous referee s report, from Arthur B. Lamb to G. N. Lewis, 28 February 1925, G. N. Lewis Papers, BL.UCB. The referee said "No real unimolecular reaction has actually been observed they have been shown to be merely catalytic the idea that a unimolecular reaction is due to collision between a quantum and a molecule is not original with Lewis."... 

Waveform can be defined in at least two different ways which are relevant to spectroscopic measurements. Wavelength (X.) is defined as the distance between successive peaks (Figure 2.1) and is measured in subunits of a metre, of which the most frequently used is the nanometre (10 9m). An angstrom unit (A) is not acceptable in SI terminology but is still occasionally encountered and is 10-10m (i.e. 10 A = 1 nm). The frequency of radiation (nu, v) is defined as the number of successive peaks passing a given point in 1 second. Hence the relationship between these two units of measurement is ... 

A study of the wavelength or frequency of radiation absorbed or emitted by an atom or a molecule will give information about its identity and this technique is known as qualitative spectroscopy. This information is usually reported as the wavelength of radiation involved and is most easily represented as an absorption or emission spectrum (Figure 2.2). Measurement of the total amount of radiation will give information about the number of absorbing or emitting atoms or molecules and is called quantitative spectroscopy. 

We now ask die question, for what values of m is Ic 1 large Given a particular frequency of radiation zu, die magnitude of c , will be large if ci) p is close to ca, thereby making the denominator in the second term in brackets very small (note diat even when ct) ,p is equal to ct), the expansion coefficient is well behaved because of the way the numerator approaches zero, cf. Section 10.5.2). This result is consistent with the notion that a photon of energy hv... 

The technique which determines the relationship between the wave lengrh(frequency) of radiation and its attentuation by absorption upon passage through a particular medium, is called absorption spectrophotometry... 

Atoms and molecules absorb only specific frequencies of radiation dictated by their electronic configurations. Under suitable conditions they also emit some of these frequencies. A perfect absorber is defined as one which absorbs all the radiation falling on it and, under steady state conditions, emits all frequencies with unit efficiency. Such an absorber is called a black body. When a system is in thermal equilibrium with its environment rates of absorption and emission are equal (Kirchhoff s law). This equilibrium is disturbed if energy from another source flows in. Molecules electronically excited by light are not in thermal equilibrium with their neighbours. 

Infrared thermographic techniques can be used to identify hot spots on process equipment The camera works on the theory that the hotter the object, the higher the frequency of radiation. For off-line corrosion monitoring, horoscopes for inspecting tubes, pumps, compressors, and other equipment may be used. Spot chemical testing can indicate the... 

Bohr frequency condition The relation between the change in energy of an atom or molecule and the frequency of radiation emitted or absorbed AE = hv. Bohr radius a0 In an early model of the hydrogen atom, the radius of the lowest energy orbit now a specific combination of fundamental constants (aG =... 

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