Continuum sources

Emission spectrum from a typical continuum source. 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

In addition to these laboratory-based experiments it is interesting to note that the Swan bands of C2 are important in astrophysics. They have been observed in the emission spectra of comets and also in the absorption spectra of stellar atmospheres, including that of the sun, in which the interior of the star acts as the continuum source. 

Promotion of an electron in Hc2 from the (7 15 to a bonding orbital produces some bound states of the molecule of which several have been characterized in emission spectroscopy. For example, the configuration ((J l5 ) ((7 l5 ) ((7 25 ) gives rise to the 2i and bound states. Figure 7.24(a) shows the form of the potential curve for the state. The A-X transition is allowed and gives rise to an intense continuum in emission between 60 nm and 100 nm. This is used as a far-ultraviolet continuum source (see Section 3.4.5) as are the corresponding continua from other noble gas diatomic molecules. 

As early as the 1930s X-ray absorption experiments were being carried out using a continuum source of X-rays (the bremsstrahlung mentioned in Section 8.1.1.1), a dispersive... 

Photomultipliers are appreciably more sensitive sensors than the eye in their response to line or continuum sources. Monochromators are fitted to the light beam in order to be able to operate as substance-speciflcally as possible [5]. Additional filter combinations (monochromatic and cut-off filters) are needed for the measurement of fluorescence. Appropriate instruments are not only suitable for the qualitative detection of separated substances (scanning absorption or fluorescence along the chromatogram) but also for characterization of the substance (recording of spectra in addition to hR and for quantitative determinations. 

It should be noted that when we compare the brightness of a LGS to a NGS, the result depends on the spectral bandwidth, because the LGS is a line source, whereas the NGS is a continuum one. The magnitude scale is a logarithmic measure of flux per spectral interval (see Ch. 15). This means that a (flat) continuum source has a fixed magnitude, no matter how wide the filter is. In contrast, the magnitude of a line source is smaller for narrower bandpasses. It is therefore advisable to use the equivalent magnitude only for qualitative arguments. The photon flux should be used in careful system analyses. 

Excitation-resolved imaging Tunable continuum source... 

Fig. 4.6. (A) Schematic of femtosecond Ti Sapphire laser-pumped tunable continuum source (TCS) (B) fluorescence intensity image and (C) in situ...

McConnell, G. (2004). Confocal laser scanning fluorescence microscopy with a visible continuum source. Opt. Express 12, 2844—50. 

Now, we are not particular experts in X-ray and gamma-ray spectroscopy (nor mass spectroscopy, for that matter), but our understanding of those technologies is that they are used mainly in emission mode. Even when the exciting source is a continuum source, such as is found when an X-ray tube is used to produce the exciting X-rays for an X-ray Fluorescence (XRF) measurement, the measurement itself consists of counting the X-rays emitted from the sample after the sample absorbs an X-ray from the source. These measurements are themselves the equivalent of single-beam measurements and will thus also be Poisson-distributed in accordance with the basic physics of the phenomenon. 

A form of the curve of growth more relevant to stellar (as opposed to interstellar) absorption lines is derived from work by E. A. Milne, A. S. Eddington, M. Min-naert, D. H. Menzel and A. Unsold. In the Milne-Eddington model of a stellar photosphere, the continuum source function (equated to the Planck function in the LTE approximation) increases linearly with continuum optical depth rA and there is a selective absorption i]K, in the line, where rj(Av), the ratio of selective to continuous absorption, is a constant independent of depth given by... 

Fig. 5.3. Energy spectrum of solar neutrinos predicted from a standard solar model (e.g. Bahcall et al. 1982), omitting the undetectably small flux due to the CNO cycle. Fluxes are in units of cm-2 s-1 MeV-1 for continuum sources and cm-2 s-1 for line sources. Detectors appropriate in various energy ranges are shown above the graph. Courtesy J.N. Bahcall.

To make accurate measurements of the integrated absorption associated with such narrow lines requires that the linewidth of the radiation source be appreciably smaller than that of the absorption line. In practice, this could be achieved with a continuum source only if expensive instrumentation of extremely high resolving power were used, and it is doubtful whether conventional photomultiplier detectors would be sufficiently sensitive at the resulting low radiation intensities. An alternative arrangement is to... 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

Spectral line sources are used as light sources in atomic absorption instruments rather than the continuum sources used for UV-VIS molecular absorption instruments, and several atomic emission techniques require no light source at all apart from the thermal energy source. 

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