Emission spectra quality

An emission spectrum for pure mercury obtained from a mercury lamp. It is easy to see that mixed sources, and higher energy excitation will produce very complex patterns of lines, demanding high quality optical... 

Yellow flame color is achieved by atomic emission from sodium. The emission intensity at 589 nanometers increases as the reaction temperature is raised there is no molecular emitting species here to decompose. Ionization of sodium atoms to sodium ions will occur at very high temperatures, however, so even here there is an upper limit of temperature that must be avoided for maximum color quality. The emission spectrum of a yellow flare is shown in Figure 7.2. 

The value of the ratio Ne/N0 does not imply that all excited atoms return to their initial state as they emit a photon. As the temperature increases, the emission spectrum becomes more complex, particularly due to the emission of lines emanating from ionised atoms (see Fig. 14.3). It thus becomes necessary to have good quality optics in order to use this technique. The corresponding instruments are atomic emission spectrophotometers, which will be discussed in Chapter 15. 

The value of the ratio N /Nq does not imply that all of the excited atoms return to their initial state as they emit photons. When they fall back down, they can lose their excess energy by other means. Otherwise, the more the temperature increases, the more complex the emission spectrum becomes, mainly due to the emergence of lines originating from ionized atoms (Figure 13.3). To study these complex spectra instruments that possess optics of very high quality are required (cf. Chapter 14). 

The total enhancement integrated over wavelength, rather than the enhancement at the resonance wavelength, is relevant for many practical devices. On resonance, the emission is enhanced along the axis of the cavity. However, sufficiently far off resonance, the emission is suppressed. Because the natural emission spectrum of the active medium (without a cavity) can be much broader than the cavity resonance, it is, a priori, not clear whether the integrated emission is enhanced. To calculate the wavelength-integrated enhancement, the spectral width of the cavity resonance and the spectral width of the natural emission spectrum must be determined. The resonance spectral width can be calculated from the finesse of the cavity or the cavity quality factor. 

To measure a high-quality emission spectrum, the sample should be prepared carefully. The following points should be handled in an appropriate manner. 

Emissivity is strongly dependent on the surface quality. The emissivity of a rough surface is greater than that of a smooth surface, increasing the rate of absorption. Emissivity values are found in textbooks. Care must be taken when using these values, as they usually denote total emissivities. The emissivity is considered constant in the spectrum, and this may be a poor approximation. 

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