Molecular bands, atomic spectroscopy

However, the comparison of the whole series of experimental facts involving IR-spectroscopy of adsorption of molecular and atomic hydrogen as well as the change in electric conductivity of adsorbent is indicative of a more complex phenomenon. For instance, in paper [97] both the spectra of adsorption of adsorbed molecular hydrogen were studied together with those of hydrogen atoms adsorbed from gaseous phase. In case when H2 are adsorbed in a dissociative manner one would have expected a manifestation of the same bands 3498 and 1708 cm or at least one of them inherent to adsorption of H-atoms in the spectrum of ZnO. 

More than sixty elements can be determined by atomic-absorption or flame-emission spectroscopy, many at or below about 1 ppm [4]. Only metals and metalloids can be determined by usual flame methods, because the resonance lines for nonmetals occur in the vacuum-ultraviolet region however, a number of indirect methods for determining nonmetals have been described. For example, chloride can be determined by precipitating it with silver ion and then measuring either the excess or the reacted silver. Phosphorus (525.9 nm) and sulfur (383.7 nm) species (e.g., Sj) exhibit sharp molecular-band emission in the argon-hydrogen flame. 

Although atomic spectroscopy requires the presence of free atoms, highly stable radicals or molecules are also present in a radiation source and contribute to the background emission. The atomic and ionic Hnes are then superimposed on the molecular bands. Common species encountered in plasmas are CN, NH, NO OH, and N2 or Nj, but refractory reaction products may also be observed (e. g. AlO, TiO+, or YO ). The dissociation of the molecular species within the plasma is an equihbrium reaction. It can be described by a formula similar to the Saha equation ... 

Some molecular spectra have been used by atomic spectroscopists for analytical purposes. These include the electronic band spectra of CaO, MgO, S2, and C2. More often the band spectra encountered in analytical atomic spectroscopy adversely affect atomic spectroscopy since they tend to mask or obscure useful atomic spectral lines. Bands such as those produced by CN, N2, NH, and OH are particularly troublesome in the region from 3000 to 4000 A, a region containing many useful atomic lines. For example, the most sensitive lines of copper fall in the OH band region of the spectrum. 

The spectroscopic analysis and study of molecules is related to atomic spectroscopy in that spectral line positions provide information about the molecular structure. However, the non destructive method of transmission spectroscopy is much more prevalent for molecular species allowing the determination of characteristic spectral information not only for gas phase but also liquid and solid phase substances. The majority of rotation vibration absorption bands of molecules occur in the infrared region of the spectrum. 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms iavolved ia the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of iaertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting ia very high specificity. The vibrational spectmm of any molecule is unique, except for those of optical isomers. Every molecule, except homonuclear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption ia the iafrared. Several texts treat iafrared iastmmentation and techniques (22,36—38) and thek appHcations (39—42). 

An important difference between atomic and molecular spectroscopy is the width of absorption or emission bands. Spectra of liquids and solids typically have bandwidths of — 100 nm, as in Figures 18-7 and 18-14. In contrast, spectra of gaseous atoms consist of sharp lines with widths of —0.001 nm (Figure 21-3). Lines are so sharp that there is usu-... 

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