Combination bands spectroscopy

Infrared Spectroscopy. The infrared spectroscopy of adsorbates has been studied for many years, especially for chemisorbed species (see Section XVIII-2C). In the case of physisorption, where the molecule remains intact, one is interested in how the molecular symmetry is altered on adsorption. Perhaps the conceptually simplest case is that of H2 on NaCl(lOO). Being homo-polar, Ha by itself has no allowed vibrational absorption (except for some weak collision-induced transitions) but when adsorbed, the reduced symmetry allows a vibrational spectrum to be observed. Fig. XVII-16 shows the infrared spectrum at 30 K for various degrees of monolayer coverage [96] (the adsorption is Langmuirian with half-coverage at about 10 atm). The bands labeled sf are for transitions of H2 on a smooth face and are from the 7 = 0 and J = 1 rotational states Q /fR) is assigned as a combination band. The bands labeled... 

Most infrared spectroscopy of complexes is carried out in tire mid-infrared, which is tire region in which tire monomers usually absorb infrared radiation. Van der Waals complexes can absorb mid-infrared radiation eitlier witli or without simultaneous excitation of intennolecular bending and stretching vibrations. The mid-infrared bands tliat contain tire most infonnation about intennolecular forces are combination bands, in which tire intennolecular vibrations are excited. Such spectra map out tire vibrational and rotational energy levels associated witli monomers in excited vibrational states and, tluis, provide infonnation on interaction potentials involving excited monomers, which may be slightly different from Arose for ground-state molecules. 

Valence Band Spectroscopy. Optical and electronic properties of UPD metal flms on metal electrodes have been studied in situ by means of differential- and electroreflectance spectroscopy [98], Optical absorption bands, however, reflect a combined density of electronic states at a photon energy which is the energetic difference of... 

The latter applies to NIR spectroscopy used for the non-invasive determination of blood glucose by means of a fibre-optical measuring-head (Jagemann et al. [1995] Muller et al. [1997] Danzer et al. [1998]). In addition to the weak overtone and combination bands resulting from glucose, strongly disturbing absorption of water, that is the main component... 

Near-infrared spectroscopy is quickly becoming a preferred technique for the quantitative identification of an active component within a formulated tablet. In addition, the same spectroscopic measurement can be used to determine water content since the combination band of water displays a fairly large absorption band in the near-IR. In one such study [41] the concentration of ceftazidime pentahydrate and water content in physical mixtures has been determined. Due to the ease of sample preparation, near-IR spectra were collected on 20 samples, and subsequent calibration curves were constructed for active ingredient and water content. An interesting aspect of this study was the determination that the calibration samples must be representative of the production process. When calibration curves were constructed from laboratory samples only, significant prediction errors were noted. When, however, calibration curves were constructed from laboratory and production samples, realistic prediction values were determined ( 5%). 

Another potential source of peaks in the NIR is called Fermi resonance. This is where an overtone or combination band interacts strongly with a fundamental band. The math is covered in any good theoretical spectroscopy text, but, in short, the two different-sized, closely located peaks tend to normalize in size and move away from one another. This leads to difficulties in first principle identification of peaks within complex spectra. 

The challenges in this work concerned the fundamental limits of NIR spectroscopy. Eirst, would NIR, with its typically broad and highly overlapped bands, have enough spectral resolution to distinguish the ortho, meta, and para isomers from each other and from the extractant Second, would NIR, with its typically weak overtone and combination bands, have enough sensitivity to quantify the minor components of the stream, especially the ortho isomer, typically present at only 1% ... 

When pairs of lines of different intensity occur in a region where only a single one is expected on the basis of normal mode calculations (cf. below), these need not indicate a site splitting, but are often indicative of Fermi-resonances, that is, a (near) coincidence of an IR active fundamental with a combination band that borrows intensity from the former. Such phenomena can often only be detected by virtue of the high resolution that is available in matrix isolation IR spectroscopy. 

Although the Fourier compression method can be effective for reducing data into frequency components, it cannot effectively handle situations where the dominant frequency components vary as a function of position in the spectrum. For example, in Fourier transform near-infrared (FTNIR) spectroscopy, where wavenumber (cm-1) is used as the x-axis, the bandwidths of the combination bands at the lower wavenumbers can be much smaller than the bandwidths of the overtone bands at the higher wavenumbers.31,32 In any such case where relevant spectral information can exist at different frequencies for different positions, it can be advantageous to use a compression technique that compresses based on frequency but still preserves some position information. The Wavelet transform is one such technique.33... 

A very important type of combination band is obtained when a transition, say 0- l of a vibration of high frequency (like vj combines with 1 — 1, 2- 2, 3- 3, etc. transitions of a vibration of low frequency (like v3 or the bridge deformation modes.) Then a series of bands is obtained which is analogous to sequences known from electronic spectroscopy. These are denoted as (vt + n v3 — n"v3) or, in this case,... 

Comparable to CO2 is the molecule CSc2. It was investigated by high-resolution FTIR spectroscopy because precise structural determination of free molecules by microwave spectroscopy are limited to molecules which have a permanent dipole moment. Hence, other molecules have to be investigated by electron diffraction or high-resolution infrared techniques. Two recently studied examples are the Dooh species CSc2 and HF. The following molecular constants with the standard error in parentheses of C °Se2 were measured ui = 369.1331(12) cm (obtained from a combination band), 1 2 = 313.0539(10) cm , = 1301.8774(5) cm From the rotational fine structure the equilibrium bond... 

Ethylene has no dipole moment and a center of symmetry and therefore the Raman spectrum is an important source of structural information. After the early work on the rotational (Dowling and Stoicheff, 1959) and rovibrational Raman spectrum (Feldman et ah, 1956) these spectra were thoroughly studied in a series of publications (Hills and Jones, 1975 Hills et ah, 1977 Foster et ah, 1977). Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et ah (1985). The Q-branch of the U2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment (Byer et ah, 1981). 

Compared to the measurement of VCD the measurement of optical activity in the Raman spectrum offers all the well known advantages that Raman spectroscopy has over infrared spectroscopy the use of the inexpensive glass as the sample cell, and the occu-rance of fewer bands, overtones and combination bands are reduced in intensity, thereby diminishing the possibility of overlap. Very important also is the fact that water is usable as solvent. 

I.r. laser spectroscopy and quadrupole mass spectrometry were used by Fischer et al. to study vibrational predissociation of clusters of C2H4, and CsHg, but-l-ene, cis- and trans-but-2-ene, and isobutene. They obtained spectra in the range 2900—3200 cm and for C2H4 clusters predissociation was observed to result from excitation near the v-i, and vg fundamentals and the i 2 + V12 combination band. The vibrational bands were observed to have Lorentzian lineshapes with IWHM of ca. 5 cm. A homogeneous broadening mechanism was assumed and the widths were used to calculate excited-state lifetimes. Valentini and co-workers studied the predissociation of C2H4 clusters at 950 cm in a crossed laser/molecular beam apparatus. 

The final two examples of the determination of excited state distortions are large bimetallic compounds whose electronic absorption spectra are broad and featureless. We must turn entirely to resonance Raman spectroscopy to measure the distortions because all of the information in the electronic spectrum is buried under the envelope. Fortunately, the resonance Raman profiles contain a great deal of information. These molecules were chosen as illustrative examples precisely because the resonance Raman spectra are so rich. The spectrum contains long overtone progressions and combination bands. Excitation profiles of not only the fundamentals but also of overtones and combination bands will be used to determine the distortions. The power of time-dependent theory from Section III.F and experimental examples of the effects of A on fundamentals, overtones, and combination bands are shown. The calculated distortions provide new insight about the orbitals involved in the electronic transition. 

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