Molecular bands

The purpose of the monochromator is to select a given emission line and to isolate it from other lines, and occasionally, from molecular band emissions. 

Apart from the interferences which may arise from other elements present in the substance to be analysed, some interference may arise from the emission band spectra produced by molecules or molecular fragments present in the flame gases in particular, band spectra due to hydroxyl and cyanogen radicals arise in many flames. Although in AAS these flame signals are modulated (Section 21.9), in practice care should be taken to select an absorption line which does not correspond with the wavelengths due to any molecular bands because of the excessive noise produced by the latter this leads to decreased sensitivity and to poor precision of analysis. 

My approach to this issue has been to use the molecular bands of CH, CN and now NH to study the star-to-star abundance variations of C and of N. Since these bands are strong enough to be observed at moderate resolution, I can use the multiplexing capability of the Low Resolution Imaging Spectrograph at Keck (Oke et al 1995) to build up large samples. This effort is being undertaken jointly with Michael Briley of the University of Wisconsin at Oshkosh and with Peter Stetson of the National Research Council, Victoria, Canada. 

Various spectral features can be used to derive the nitrogen abundance in dwarfs. Unfortunately weak high excitation (x=10.34 eV) near-infrared NI lines at 7468.31, 8216.34, 8683.4, 8703.25 and 8718.83 A disappear at metallicities [Fe/H] < -1 and for the analysis of N in metal-poor stars we are left with the CN and NH molecular bands at 3883 and 3360 A, respectively. It must be mentioned... 

The abundance analyses for metal lines and molecular bands of CH and C2 demonstrates that (1) our sample covers the metallicity range from [Fe/H]= —3.5 to —1.7, and (2) 39 objects have [C/Fe]> +0.5. Here we regard these 39 stars as carbon-enhanced, metal-poor (CEMP) stars. 

Fig. 3.18. Synthetic spectrum of a red giant, Tes = 4500 K, log g = 2.25 in the region of the strong Mg i b lines (see Fig. 3.9). The upper spectrum is the same with atomic lines switched off and shows molecular bands of MgH. The horizontal lines show the central and side bands of the Lick Mg2 index. Adapted from Mould (1978).

Incomplete dissociation of alkaline-earth oxides and hydroxides often causes unwanted molecular bands to be stronger than atomic lines. This can often be cured by altering flame temperatures or fuel/oxidant ratio, to shift the equilibrium in the desired direction. 

We now investigate the long chain limit. In this case (n ), within the molecular band ( — e < 2t), the denominator in equation (39) is dominated by the term proportional to n, and we obtain ... 

Transmittance within the band is a simple quadratic function of energy. Its maximum is at the middle of the band, and it goes to zero at the edges of the band. In the long chain limit, transmittance (like the band width) becomes independent of system size. Within the molecular band, sudden changes in the conductance... 

Spectral interferences from the overlap of molecular bands and lines (e g. the calcium hydroxide absorption band on barium at 553.55 nm) cannot be so easily dismissed. Lead seems to be particularly prone to such non-specific absorption problems at the 217.0 nm line (e g. sodium chloride appears to give strong molecular absorption at this wavelength). This type of problem is encountered in practical situations, but can sometimes be removed by the technique of background correction (see Section 2.2.5.2). 

Wavelength Molecular Band Wavelength Atomic Line... 

W. R. Jarmain, Transition Probabilities of Molecular Band Systems, University of Western Ontario report No. GRD-TN-60-498, 1960. 

Several types of interference effects may contribute to inaccuracies in the determination of major and minor elements. The interferences can be classified as spectral, physical, and chemical. Spectral interferences involve an overlap of a spectral line from another element, unresolved overlap of molecular band spectra, background contribution from continuous or recombination phenomena, and background contribution from stray light from the line emission of high-concentration elements. The second effect may require selection of an alternative wavelength. The third and fourth effects can usually be compensated by a background correction adjacent to the analyte line. 

Spectral interferences from ion-atom recombination, spectral line overlaps, molecular band emission, or stray light can occur that may alter the net signal intensity. These can be avoided by selecting alternate analytical wavelengths and making background corrections. 

Figure 2.8 Schematic of a typical interfacial supramolecular assembly, A-L-B, when A is a metal electrode, illustrating the relative molecular band structures of A and B, respectively. Ef is the fermi level...

Astronomical observations in red-giant stars of molecular bands in the molecules TiO and TiC have provided a sensitive test of the transition from an O-rich star to a C-rich star ( carbon star ). Ifthe star s atmosphere contains more oxygen than carbon, free oxygen remains after chemically forming as much of the tightly bound CO molecule as is possible. Such stars reveal bands ofTiO in their spectra. Conversely, in carbon stars, excess C remains after the formation of the maximum amount of CO molecules. These stars reveal bands of TiC in their spectra. And it is from the atmospheres of such stars that the mainstream SiC presolar grains are recovered in meteorites. 

Other reports deal with individual elements, such as Ni [1, 86, 87] or Fe [11,84]. The efficiency [71—73] of flame methods (AAS) has been compared with flameless techniques (NFAAS) (Table 6). Because of their significance there have been attempts to determine the elements P [38] and S [78] directly with AAS. This, however, requires a device which can measure ultraviolet lines (ca. 180 nm) with sufficient sensitivity. Good results can also be achieved by gas chromatographic separation and successive AAS determination [92] and simultaneous multielement analysis with a Vidicon-detector has been tried [68] because the speed with which the information is gained can be very important in practice. Some work [39, 53] reports on the problem of molecular bands which can appear when working with... 

Fig. 4.10. Intensity ratio of molecular bands in hydrogen and deuterium plasmas as a function of graphite temperature (left) and ion energy (right)...

The reasons for these discrepancies are still unclear - there might be stronger varying rates with density and temperature than published, problems in the molecular physics, and last but not least, influences from the emitting surfaces themselves on the measured molecular bands. Therefore, a clarification of the quantities used is urgently necessary as follows ... 

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