Emission spectrum limitations

Section 6.13.2 and illustrated in Figure 6.5. The possible inaccuracies of the method were made clear and it was stressed that these are reduced by obtaining term values near to the dissociation limit. Whether this can be done depends very much on the relative dispositions of the various potential curves in a particular molecule and whether electronic transitions between them are allowed. How many ground state vibrational term values can be obtained from an emission spectrum is determined by the Franck-Condon principle. If r c r" then progressions in emission are very short and few term values result but if r is very different from r", as in the A U — system of carbon monoxide discussed in Section 7.2.5.4, long progressions are observed in emission and a more accurate value of Dq can be obtained. 

Separate the light from the emission spectrum of the Sun and you will see the familiar rainbow colour spectrum but how small a wavelength difference can be detected Is it possible to tell between 500 nm and 501 nm The spectral resolution limits the ability of a telescope to tell the difference between two spectral lines and hence two different molecules. The smallest separation that allows two wavelengths to be distinguished is limited by the physics of dispersion and for sources of the same intensity, Lord Rayleigh determined that the dip between the two peaks should be 8/7r 2 or about 19 per cent. 

Alternatively, a standard fluorescent compound can be used whose corrected emission spectrum has been reported3 . Comparison of this spectrum with the technical spectrum recorded with the detection system provides the correction factors. The wavelength range must obviously cover the fluorescence spectrum to be corrected. Unfortunately, there is a limited number of reliable standards. 

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. 

Figure 1.2. An image produced by exciting hydrogen gas and separating the outgoing light with a prism, reprinted from [Her. Fig. 1. p. 5]. Specifically, this is the emission spectrum of the hydrogen atom in the visible and near ultraviolet region. The label marks the position of the limit of the series of wavelengths.

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

The first term in Eq. (4.3) is reminiscent of Eq. (3.2) for the spontaneous emission spectrum. It represents a doorway wavepacket created by the pump in the excited state, which is then detected by its overlap with a window. The only difference is that the role of the gate in determining the window in SLE is now played by the probe Wigner function W2. In addition, the pump-probe signal contains a second term that does not show up in fluorescence. This term represents a wavepacket created in the ground state (a hole ) that evolves in time as well and is finally determined by a different window Wg [24]. In the snapshot limit, defined in the preceding section, we have... 

A breaking off of the branches has been observed in the emission spectrum of HNO by Clement and Ramsay25 and, correspondingly, higher rotational lines have been found to be diffuse in absorption.28 These observations lead to an upper limit of 2.11 eV for D(H—NO). 

Figure 7.10 shows a simplified emission spectrum of anthracene as it would be observed on a grating fluorimeter over a wide wavelength range. The real emission at around 400-500 nm is reproduced with lower intensity in the 800-1000 nm region which corresponds to the first harmonic. Higher order harmonics are not observed because of the limitations of the sensitivity... 

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