Neon line spectra

A hollow cathode lamp emits an intense line spectrum of the cathode element, of any other element present in the cathode, and of the filler gas (neon or argon). It is therefore necessary to be able to isolate the lines of the determinant element from any other emitted lines. If we do not, the difference between 7t and /0 will be greatly reduced, and the sensitivity unacceptably poor. Moreover, not all lines of the determinant element give equal sensitivity, and it is therefore also desirable to isolate the determinant line at the wavelength which gives the most useful sensitivity from all other lines. This is done with a grating monochromator. Figure 6 illustrates a typical optical layout in the monochromator of an atomic absorption spectrometer. 

Figure 10-3 shows the basic features of a hollow cathode lamp source. Here A is the anode (the plus electrode) and C is the cathode, terminated in the lamp as a hollow cup. The anode can be a wire, such as tungsten, and the cathode cup may be constructed from the element whose spectrum is desired or it may be an inert material into which the desired element or a salt of the desired element is placed. The lamp envelope is made of glass and IT is a window of suitable properties. If an ultraviolet line spectrum is desired, the window may be quartz or a high silica glass. The hollow cathode has an inert gas present, usually neon or argon, at low pressure. 

Several examples of possible spectral line interferences include sodium at 2852.8 A with magnesium at 2852.1 A, iron at 3247.3 A with copper at 3247.3 A, and iron at 3524.3 A with nickel at 3524.5 A. Spectral interferences also are possible from hollow cathode lamps. The fill-gas of a hollow cathode lamp is commonly argon or neon and the lamps emit the line spectrum of the fill-gas as well as that of the hollow cathode material. The fill-gas therefore must be one that does not produce an emission line at the desired wavelength of the hollow cathode element. 

Figure 2.46 illustrates the advantages of this technique. The upper spectrum represents a Lamb peak in the intracavity saturation spectrum of the neon line (l 2p) at A. = 588.2 nm (Sect. 2.3.3). Due to the collisional redistribution of the atomic velocities, a broad and rather intense background appears in addition to the narrow peak. This broad structure is not present in the dichroism and birefrin-gent curves (Fig. 2.46b, c). This improves the signal-to-noise ratio and the spectral resolution. 

For each grating position at least two lines of the Ne lamp could be focused on the detector simultaneously. In this way, after the initial grating drive running with an accuracy of about 2 arcmin, the wavelength calibration routine simply compares the expected to the measured neon line positions and, when required, initiates a final adjustment by successive grating rotation. At the end of the calibration routine the absorption line of interest is fixed with an absolute accuracy of a few percent of a pixel corresponding to about 0.5 arcsec and the spread of the continuum background spectrum is perfectly adapted to the detector dimension. 

Figure 7.41 illustrates the advantages of this technique. The upper spectrum represents a Lamb peak in the intracavity saturation spectrum of the neon line (Is—>2p) at A = 588.2 nm (Sect.7.3.3). Due to the collisional... 

Fig. 2.4. The asymptotic behaviour of the IR spectrum beyond the edge of the absorption branch for CO2 dissolved in different gases (o) xenon (O) argon ( ) nitrogen ( ) neon (V) helium. The points are experimental data, the curves were calculated in [105] according to the quantum J-diffusion model and two vertical broken lines determine the region in which Eq. (2.58) is valid.

Radiation is derived from a sealed quartz tube containing a few milligrams of an element or a volatile compound and neon or argon at low pressure. The discharge is produced by a microwave source via a waveguide cavity or using RF induction. The emission spectrum of the element concerned contains only the most prominent resonance lines and with intensities up to one hundred times those derived from a hollow-cathode lamp. However, the reliability of such sources has been questioned and the only ones which are currently considered successful are those for arsenic, antimony, bismuth, selenium and tellurium using RF excitation. Fortunately, these are the elements for which hollow-cathode lamps are the least successful. 

No spectral lines of neon and argon appear in the photospheric spectrum. The abundances of these elements come from measurements of the Ne/O and Ar/O ratios in coronal material and thus are relatively uncertain. Unfortunately, nucleosynthesis theory does not provide much guidance for either neon or argon. 

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