Overtone bands

In addition to bands in the infrared and Raman spectra due to Au = 1 transitions, combination and overtone bands may occur with appreciable intensity, particularly in the infrared. Care must be taken not to confuse such bands with weakly active fundamentals. Occasionally combinations and, more often, overtones may be used to aid identification of group vibrations. 

The above, of course, is a very simplified picture, as many bands of much weaker intensities occur at shorter wavelengths (these are known as overtone bands and combination bands), but these are unlikely to be confused with the... 

Another interesting facet of the vibrational IETS is the weakness of overtone and combination bands. There are sound theoretical reasons to expect that overtone bands should be very weak in IETS [46, 47]. To our knowledge, there has been no theoretical investigation of the intensities of combination bands in tunneling spectra. To be sure, there are experimental papers that contain tunneling band assignments that include assignments as combination and overtone bands. Most... 

Vibrational overtone bands within the ground electronic state of OH, known as Meinel bands, result from the reaction of H atoms with ozone,... 

Chemiluminescence is observed from several different emitting species, depending on the analyte and reaction conditions. Vibrational overtone bands of HF in the wavelength region of =500-900 nm are observed under nearly all conditions and are often the dominant spectral feature, the (3,0), (4,0) (5,1), and (6,2) bands being the most intense, while for some reaction conditions emissions from levels up to v = 8 are observed [63], It is likely that hydrogen atoms are produced in the reaction and form vibrationally excited HF in the reaction reported by Mann et al. [62] ... 

Fig. 9. (a) Infrared spectra of outgassed thin pellets of Ti-free silicalite (curve 1) and TS-1 with increasing Ti content x (curves 2-5). Spectra were normalized by means of the overtone bands between 1500 and 2000 cm-1 (not shown) and vertically shifted for clarity. The thick horizontal line represents the fwhm of the 960 cm-1 band for sample 2. By assuming that this band has a constant fwhm for any x, the absorbance W obtained is plotted as the ordinate in panel b, where the band has the same fwhm as in curve 2 (horizontal thin lines), (b) Intensity W of the 960 cm-1 infrared band (normalized absorbance units) as a function of x (full squares) and corresponding Raman counts (open squares) [Reprinted from Ricchiardi et al. (41) with permission. Copyright (2001) American Chemical Society]. 

There are two effects of the anharmonicity of the quantized energy levels described above, which have signiflcance for NIRS. First, the gap between adjacent energy levels is no longer constant, as it was in the simple harmonic case. The energy levels converge as n increases. Second, the rigorous selection rule that An = +1 is relaxed, so that weak absorptions can occur with n = 2 (flrst overtone band), or +3 (second overtone band), etc. 

Absorbance signals seen in NIR consist of combination and overtone bands of hydrogen bonds such as C-H, N-H, 0-H, and S-H, which are aroused by large force constants and small mass. NIR spectra thus cover precious information on chemical as well as physical properties of analyzed samples due to characteristic reflectance and absorbance patterns [121-123], which makes this analysis method applicable to the characterization of monolithic stationary phases. 

Overtone bands of mono-substituted benzene ring... 

Furthermore, polyatomic molecules consisting of n atoms have 3n - 6 vibrational degrees of freedom (or 3n — 5 in the special case of a linear polyatomic molecule), instead of just one as in the case of a diatomic molecule. Some or all of these may absorb infrared radiation, leading to more than one infrared absorption band. In addition, overtone bands (Av > 1)... 

Weak combination and overtone bands appear in the 2000-1650 cm-1 region. The pattern of the overtone bands is not a reliable guide to the substitution pattern of the ring. Because they are weak, the overtone and combination bands are most readily observed in spectra obtained from thick samples. The spectrum of Figure 3.13 is that of a typical aromatic (benzenoid) compound. 

The most intense bands, as well as overtone bands at 1744 and 1281 cm"1, are used as purity probes for sulfur tetrafluoride.9 The mass spectrum of sulfur tetrafluoride shows a weak molecular peak and strong fragmentation peaks corresponding to SF/, SF2+, SF+ and F+ ions.10... 

Fig. 1. First overtone band of the surface OH-groups of alumosilicagel, immersed into carbon tetrachloride (1), chlorobenzene (2), benzene (3), toluene (4) and acetone (5). (From top to bottom)...

If molecular gases are considered, infrared spectra richer than those seen in the rare gases occur. Besides the translational spectra shown above, various rotational and rotovibrational spectral components may be expected even if the molecules are non-polar. Besides overlap, other induction mechanisms become important, most notably multipole-induced dipoles. Dipole components may be thought of as being modulated by the vibration and rotation of the interacting molecules so that induced supermolecular bands appear at the rotovibrational frequencies. In other words, besides the translational induced spectra studied above, we may expect rotational induced bands in the infrared (and rotovibrational and electronic bands at higher frequencies as this was suggested above, Eq. 1.7 and Fig. 1.3). Lines at sums and differences of such frequencies also occur and are common in the fundamental and overtone bands. We will discuss the rotational pair and triplet spectra first. 

Overtone bands. The induced first overtone band of H2 is shown in Fig. 3.37 at a variety of temperatures, observed in pure hydrogen gas using long absorption paths. Instead of the three components Q, S(0) and S(l) seen in the fundamental band, we now observe much richer structures, especially at the lower temperatures. This fact suggests that a number of double transitions take place. If one constructs a rotovibrational term scheme of the H2 molecule, like Fig. 3.32, which includes the lowest rotational levels of the v = 2 vibrational state, this is obvious. Various... 

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