Absorption of infrared radiation

The model chosen to describe the absorption process pictures a molecule in which the individual atoms, held together by chemical bonds, are in vibratory motion along these bonds, while the molecule as a whole is rotating. Thus we are concerned with both the vibrational and the rotational energy of the molecule. We describe a 

Relatianshfp Between Band Contours and Vibrational-Rotational Energy 

to represent a band or indicate how energy transitions occur when radiation is absorbed and an infrared band formed, one must diagram rotational as well as vibrational energy levels. 

Rotational energy levels are also quantized and are indicated by the quantum number J. 

The magnitude of rotational energy changes in molecules is less than that of changes in vibrational energy, and therefore rotational 

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Energy level diagram showing difference between the absorption of Infrared radiation (left) and ultravlolet-visible radiation (right). 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

As indicated above, the penetration depth is on the order of a micrometer. That means that in ATR, absorption of infrared radiation mostly occurs within a distance 8 of the surface and ATR is not as surface sensitive as some other surface analysis techniques. However, ATR, like all forms of infrared spectroscopy, is very sensitive to functional groups and is a powerful technique for characterizing the surface regions of polymers. 

As the isoquinoline molecule reorients in the order listed above, the absorption of infrared radiation by the in-plane vibrational modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease (in accordance with the surface selection rule as described above). In the flat orientation there is no component of the dipole moment perpendicular to the surface for the in-plane modes, and under the surface selection rule these modes will not be able to absorb any of the incident radiation. However, as mentioned above, infrared active modes (and in some cases infrared forbidden transitions) can still be observed due to field-induced vibronic coupled infrared absorption (16-20). We have determined that this type of interaction is present in this particular system. 

Each dip in the spectrum is called a band or peak. The transmittance is 0 % if all the radiation is absorbed and with no absorption the transmittance is 100%. The absorption of infrared radiations can be expressed either in terms of wave length (A.) or in wave number (y). Mostly infra red spectra of organic compounds are plotted as percentage transmittance against wave number. The relationship between wave length and wave number is as follows ... 

These iodometric calibration methods are based on the assumption that there is a stoichiometric reaction between ozone and the iodine in the various potassium iodide procedures. Three essentially independent methods have been used to test the accuracy of this assumption measuring the absorption of ultraviolet radiation at 254 nm by ozone in air, measuring the absorption of infrared radiation at 9,480 nm by ozone in air, and determining the ozone concentration in air by titration with nitric oxide. 

The absorption of infrared radiation causes bonds within a molecule to vibrate and infrared spectroscopy can be used to identify functional groups in an organic molecule. 

Gray SI unit to measure the amount of radiation dose equal to deposition of one joule of energy into one kilogram of matter Greenhouse Effect warming of the Earth due to absorption of infrared radiation by particular atmospheric gases such as H O, CO, and CH ... 

Absorption of infrared radiation by characteristic vibrations of a surface can be used to obtain information about that surface, by comparison with known absorption frequencies in molecules of known structure. Surface sensitivity is obtained by using small particles ) and thin films or, better, a multiple-reflection arrangement with optimized angles of incidence and reflection in particular making work on single-... 

Greenhouse effect A warming effect exerted by the earth s atmosphere (particularly CO2 and H2O) due to the thermal energy retained by the absorption of infrared radiation. 

V. N. Filimonov. Induced absorption of infrared radiation by molecules. Sov. Phys. USPEKHI, 2 894, 1960. 

A photoconductive detector is a semiconductor whose conductivity increases when infrared radiation excites electrons from the valence band to the conduction band. Photovoltaic detectors contain pn junctions, across which an electric field exists. Absorption of infrared radiation creates electrons and holes, which are attracted to opposite sides of the junction and which change the voltage across the junction. Mercury cadmium telluride (Hg,. Cd/Te, 0 < x < 1) is a detector material whose sensitivity to different wavelengths is affected by the stoichiome-try coefficient, x. Photoconductive and photovoltaic devices can be cooled to 77 K (liquid nitrogen temperature) to reduce thermal electric noise by more than an order of magnitude. 

Let us first consider the case of Y/D 1. This means that at certain values of the compound nucleus excitation energy, individual levels of the compound nucleus can be excited (i.e., when the excitation energy exactly equals the energy of a given CN level). When this happens, there will be a sharp rise, or resonance, in the reaction cross section akin to the absorption of infrared radiation by sodium chloride when the radiation frequency equals the natural crystal oscillation frequency. In this case, the formula for the cross section (the Breit-Wigner single-lev el formula) for the reaction a + A —> C b + B is... 

Absorption of infrared radiation causes transitions between vibrational energy states of a molecule. A simple diatomic molecule, such as H—Cl, has only one vibrational mode available to it, a stretching vibration somewhat like balls on the ends of a spring ... 

Absorption of infrared radiation over the range from 600 cm-1 to 3600 cm-1 corresponds to energy-level differences, as in Figure 9-10, of 1.7 kcal mole-1 to 10.3 kcal mole-1. 

The techniques considered in this chapter are infrared spectroscopy (or vibrational spectroscopy), nuclear magnetic resonance spectroscopy, ultraviolet-visible spectroscopy (or electronic spectroscopy) and mass spectrometry. Absorption of infrared radiation is associated with the energy differences between vibrational states of molecules nuclear magnetic resonance absorption is associated with changes in the orientation of atomic nuclei in an applied magnetic field absorption of ultraviolet and visible radiation is associated with changes in the energy states of the valence electrons of molecules and mass spectrometry is concerned... 

Very recently, Bailey and Richards (23) have shown that a high degree of sensitivity for adsorbed species can be achieved by measuring the absorption of infrared radiation on a thin sample cooled to liquid helium temperature. The optical arrangement used in these studies is shown in Figure 10. The modulated beam produced by the interferometer is introduced into the UHV sample chamber and reflected off a thin slice of monocrystalline alumina covered on one side by a 1000 k film of nickel or copper. Radiation absorbed by the sample is detected by a doped germanium resistance thermometer. The minimum absorbed power detected by this device when operated at liquid helium temperature is 5 x 10 14 W for a 1 Hz band width. With this sensitivity absorbtivities of 10"4 could be measured. 

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