Infrared radiation wavelength/wavenumber

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

By how many kilojoules per mole is the energy of 02 increased when it absorbs ultraviolet radiation with a wavelength of 147 nm How much is the eneigy of C02 increased when it absorbs infrared radiation with a wavenumber of 2 300 cm-1 ... 

Currently, several forms of infrared spectroscopy are in general use, as illustrated in Figure 8.4. The most common form of the technique is transmission infrared spectroscopy, in which the sample consists typically of 10 to 100 mg of catalyst, pressed into a self-supporting disk of approximately 1 cm2 and a few tenths of a millimeter thickness. Transmission infrared spectroscopy can be applied if the bulk of the catalyst absorbs weakly. This is usually the case with typical oxide supports for wavenumbers above about 1000 cm-1, whereas carbon-supported catalysts cannot be measured in transmission mode. Another condition is that the support particles are smaller than the wavelength of the infrared radiation, otherwise scattering losses become important. 

Infrared radiation (X = 2.5-25 pm) is the energy source in infrared spectroscopy. These are somewhat longer wavelengths than visible light, so they are lower in frequency and lower in energy than visible light. Frequencies in IR spectroscopy are reported using a unit called the wavenumber (v) ... 

Calculate the wavenumber of a beam of infrared radiation with a wavelength of 5.00 jam. 

Wavenumber ( ) is another way to describe the frequency of electromagnetic radiation, and the one most often used in infrared spectroscopy. It is the number of waves in one centimeter, so it has units of reciprocal centimeters (cm ). Scientists use wavenumbers in preference to wavelengths because, unlike wavelengths, wavenumbers are directly proportional to energy. The relationship between wavenumber (in cm ) and wavelength (in m) is given by the equation... 

Table A.2.1 gives for radiation of a given energy the color, wavelength, wavenumber and frequency. The visible area corresponds to 400 < A < 700 nm ultraviolet radiation has A < 400 nm, infrared radiation A > 700 nm. In wavenumbers these regions are indicated as follows 14.300

Infrared Absorption is a single-photon process. Here, also, kiR = K 0 applies. Thus, infrared absorption detects only phonons at the F point of the first BZ. In this case, we have oo = L2, where ho) is the quantum energy of the infrared radiation. The frequencies or the wavenumbers of the optical phonons in molecular crystals are of the order of 3 THz or 100 cm" thus the wavelengths of infrared absorption are of the order of 100 /xm. Infrared spectroscopy of phonons in molecular crystals is therefore in fact far-infrared spectroscopy. The symmetry selection rules are complementary to those for Raman scattering for vibrations with u and g states w g transitions are allowed and g g transitions are forbidden. 

The infrared-absorption spectrum of a substance is ideally a plot of the transmittance or the absorbance of a suitable thickness against the wavelength or wavenumber of the infrared radiation. In practice the losses of intensity by reflection at the surfaces of a sample can often be neglected and then, from equations (2.5) and (2.6)... 

By how many joules is the energy of a molecule increased when it absorbs (a) visible light with a wavelength of 500 nm or (b) infrared radiation with a wavenumber of 1 251 cm ... 

Water absorbs infrared radiation that has a frequency of 1595 cm. Convert this wavenumber into a wavelength in microns. 

The above-mentioned wavelength regions of infrared radiation correspond to the wavenumber regions of about 14 000-4000 cm for near-infrared, 4000-400cm for mid-infrared, and400-10cm for far-infrared. 

Infrared radiation is an electromagnetic radiation with wavelengths (t>) between 0.7 and 500pm, corresponding to radiations with wavenumbers (v=l//L) between 1.4x 10 and20cm" , which specifies the number of waves per centimeter. 

The IR absorption spectrum of a polymer sample is usually plotted as a percentage of the transmittance (/// ) or absorbance against a wavelength or wavenumber of infrared radiation. If the sample is very thick, the transmittance will approach zero and the absorbance will tend to infinity for all wavelengths. In order to avoid this issue, it is necessary to work with samples with thickness in the range of 30-300 pm (Bower, 2002). 

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