Recombination continuum

Apart from the atomic and ion lines of the species present in a plasma source an emission spectrum has a continuum on which the emission lines are superimposed. This extends over the whole spectrum. It is due to the interactions between free electrons ( Bremsstrahlung ) and to the interaction of free and bound electrons ( recombination continuum ). The former is particularly important in the UV spectral region, whereas the latter is important at longer wavelengths. The spectral intensity distribution for the continuum radiation is given by ... 

At complete ionization of the hydrogen (e.g. when added to a plasma with another gas as the main constituent) ne = p/(2 x k x Te) has a maximum at a wavelength of X — (7.2 x 107)/Te or at a fixed wavelength, the maximum intensity is found at a temperature Te = (5.76 x 107)/2. Thus, the electron temperature can be determined from the wavelength dependence of the continuum intensity. As Te is the electron temperature, absolute measurements of the background continuum emission in a plasma, e.g. for the case of hydrogen, allow determination of the electron temperature in a plasma, irrespective of whether it is in local thermal equilibrium or not. Similar methods also make use of the recombination continuum and of the ratio of the Bremsstrahlung and the recombination continuum. 

Whereas the rotational and the gas temperature are particularly relevant to the evaporation processes in the plasma, the electron temperature, being a measure of the kinetic energy of the electrons, is relevant to the study of excitation and ionization by collisions with electrons. This is an important process for generation of the analyte signal both in optical atomic emission and in mass spectrometry. The electron temperature can be determined from the intensity of the recombination continuum or of the Bremsstrahlung , as described by Eq. (57). 

It is important for all excitation and ionisation processes that are initiated by collision with electrons. Electron temperatures are often determined from the intensity of the recombination continuum or from the energy distribution of the brems-strahlung. 

Certain continua have been observed in flames which are due to the direct recombination of radicals and atoms. The best known of these are the alkali metal-hydroxyl recombination continuum, which extends throughout the visible range. This continuum is obscured by the very intense resonance lines of Na and Li. It accounts for the majority of the visible radiation from a K laden flame, where the first and second resonance doublets are at the limits of visibility. James and Sugden showed that the integrated intensity of this continuum was proportional to both the [OH], derived from their measurements of [H] atoms, and to the concentration of free alkali metal, and that the intensity could therefore be used as a relative measure of [OH] concentration. The natural continumn arising from the reaction H + OH -> HjO -1- hv has also been used, as has the H -b Cl continuiun. The direct measurement of the intensity of the OH (306 nm) band can be used to determine the OH profile in a flame however Hollander has pointed out that there is considerable overexcitation and the method is unreliable near the reaction zone. 

Most flame photometric studies reveal the presence of a faint background continuum due to radical recombination, and the continuum may be enhanced by additives— the pale lilac colour of the potassium flame test in elementary analysis is due in large measure to the K—OH recombination continuum. V le not strictly a problem in chemical kinetics, it is interesting to survey some of the results obtained from the study of continua. 

Typical elemental detection limits are listed in Table 1. The detection limit is the concentration that produces the smallest signal that can be distinguished from background emission fluctuations. The continuum background is produced via radiative recombination of electrons and ions e — M+ hv or M + e + e — ... 

The cage effect was also analyzed for the model of diffusion of two particles (radical pair) in viscous continuum using the diffusion equation [106], Due to initiator decomposition, two radicals R formed are separated by the distance r( at / = 0. The acceptor of free radicals Q is introduced into the solvent it reacts with radicals with the rate constant k i. Two radicals recombine with the rate constant kc when they come into contact at a distance 2rR, where rR is the radius of the radical R Solvent is treated as continuum with viscosity 17. The distribution of radical pairs (n) as a function of the distance x between them obeys the equation of diffusion ... 

In gas clouds containing one or more hot stars (7 cn > 30 000 K), hydrogen atoms are ionized by the stellar UV radiation in the Lyman continuum and recombine to excited levels their decay gives rise to observable emission lines such as the Balmer series (see, for example, Fig. 3.22). Examples are planetary nebulae (PN), which are envelopes of evolved intermediate-mass stars in process of ejection and... 

Fig. 3.30 is believed to be in the Sedov phase, with the shock propagating at a high velocity (730 km s-1) in the rarefied intercloud ISM ( h 0.9 cm-3) and at a much slower velocity (< 140 km s-1) in nearby dense molecular clouds (nu 150 cm-3) where it gives rise to optical and UV emission. The X-ray continuum is a sum of recombination radiation from various ions plus thermal bremsstrahlung, aka free-free emission, the latter being given for an ion with charge Ze by... 

The heart of a Fourier transform infrared spectrophotometer is the interferometer in Figure 20-26. Radiation from the source at the left strikes a beamsplitter, which transmits some light and reflects some light. For the sake of this discussion, consider a beam of monochromatic radiation. (In fact, the Fourier transform spectrophotometer uses a continuum source of infrared radiation, not a monochromatic source.) For simplicity, suppose that the beamsplitter reflects half of the light and transmits half. When light strikes the beamsplitter at point O, some is reflected to a stationary mirror at a distance OS and some is transmitted to a movable mirror at a distance OM. The rays reflected by the mirrors travel back to the beamsplitter, where half of each ray is transmitted and half is reflected. One recombined ray travels in the direction of the detector, and another heads back to the source. 

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