Absorption of IR Radiation by Molecules

Molecules absorb radiation when a bond in the molecule vibrates at the same frequency as the incident radiant energy. After absorbing radiation, the molecules have more energy and vibrate at increased amplitude. The frequency absorbed depends on the masses of the atoms in the bond, the geometry of the molecule, the strength of the bond, and several other factors. Not all molecules can absorb IR radiation. The molecule must have a change in dipole moment during vibration in order to absorb IR radiation. 

Collected on a Thermo Scientific 6700 FTIR spectrometer with a deuterated triglycine sulfate (DTGS) detector. ( Thermo Fisher Scientific, www.thermofisher.com. Used with permission.) 

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The basis of this technique is absorption of ir radiation by molecules over a wide spectrum of wavelengths to give a characteristic fingerprint spectrum providing both qualitative and quantitative data on the substance. This versatile technique owes its success in occupational hygiene to the development of a portable spectrometer. Table 9.8 lists some compounds detectable by one type of portable ir analyser. 

The requirements for the absorption of IR radiation by molecules can be summarized as follows ... 

Fig. 2.13 Radiation and heat balance of the system earth-atmosphere. Percentages are given in relation to the incoming solar radiation (100% = 343 W m ). I interception of solar radiation by molecules/particles (p) clouds (c), and earth surface (e), D diffuse radiation by molecules/particles (p) clouds (c), R reflexion (albedo) by molecules/particles (p) clouds (c), and earth surface (e), IR infrared dissipation to space by molecules/particles (p) clouds (c), T terrestrial radiation back to space (without absorption), A absorption of terrestrial radiation by molecules, AB atmospheric back-radiation, SH and LH sensible and latent heat, resp.

IR spectroscopy is an analytic method based on the absorption of IR radiation by vibrational excitation of lattices, surface groups, molecules, etc. in each physical condition. The absorptions are always associated with a change in the dipole moment of the molecule/material. Consequently, vibrational and/or rotational modes of molecules. 

The greenhouse effect is a well-known phenomenon that results in warming of Earth s atmosphere by absorption of infrared (IR) radiation by molecules in the atmosphere such as carbon dioxide, water vapor, and methane. Figure 6.2 shows that infrared radiation accounts for about 53% of the radiation coming from our Sun. About 8% is higher-energy ultraviolet (UV) radiation and 39% is visible (Vis) radiation. What is the impact of this radiation as it travels through our atmosphere and then strikes Earth ... 

Activation of the vibrational energy of ions can also be induced by the absorption of IR radiations. A popular type of IR radiation source is far-IR laser. In fact, many molecules have a broad IR absorption band. The most widely used IR source is a continuous wave (c.w.) CO2 laser, with the wavelength of 10.6 pm. This wavelength corresponds to an energy of 0.3 eV per laser photon. Because decomposition of a chemical bond requires >1 eV, laser excitation has to extended over hundreds of milliseconds to allow ions to absorb multiple IR photons. This method is known as infrared multiphoton dissociation (IRMPD). Another type of similar technique is black-body infrared radiative dissociation... 

One of the most fruitful application of laboratory microwave spectroscopy over the last twenty years is the analysis of the molecular content of interstellar clouds. These clouds contain gas (99% in mass) which has been mostly studied by radioastronomy, and dust, whose content has been analysed mostly by IR astronomy. The clouds rich in molecular content are dense or dark clouds (they present a large visual extinction), with a gas density of 10 -10 molecules cm", and temperatures of T < 50K. At these low temperatures only the low-lying quantum states of molecules can be thermally (or collisionally) excited, i.e. rotational levels. Spontaneous emission from these excited states occurs at microwave wavelengths. In some warm regions of dense clouds (star formation cores) the absorption of IR radiation produces rotational emission in excited vibrational states. Other rich chemical sources are the molecular clouds surrounding evolved old stars, such as IRC-i-10216, and called circumstellar clouds. 

The method has been applied, for example, in electrochemical investigations (110) and also for surface catalytic reactions in the presence of a gas phase 111). When PM-IRRAS is used with a thin-layer cell, as depicted in Fig. 37, the contribution from dissolved molecules in the liquid phase can be minimized. Still, the layer thickness has to be small to prevent complete absorption of the IR radiation by the solvent. The combination of polarization modulation and ATR for metal films was demonstrated recently and applied in an investigation of self-assembled octadecylmercaptan monolayers on thin gold films 112). This combination could emerge as a valuable technique for the investigation of model catalysts. 

Fluorescence is a three-stage process excitation via the absorption of a radiation, excited state for a very short time -(10 9 sec), and emission. The excitation of a molecule is achieved by the absorption of a light quantum of an appropriate wavelength, promoting, in a simplified view, a ir or nonbonding (n) electron to a 7r antibonding orbital. The quantum yield is a fundamental molecular property that describes the ratio of the number of emitted photons to the number of photons absorbed. 

Absorption of microwave radiation to excite molecular rotation is allowed only if the molecule has a permanent dipole moment. This restriction is less severe than it may sound, however, because centrifugal distortion can disturb the molecular symmetry enough to allow weak absorption, especially in transitions between the higher rotational states which may appear in the far IR (c. 100cm-1). Microwave spectroscopy can provide a wealth of other molecular data, mostly of interest to physical chemists rather than inorganic chemists. Because of the ways in which molecular rotation is affected by vibration, it is possible to obtain vibrational frequencies from pure rotational spectra, often more accurately than is possible by direct vibrational spectroscopy. 

The vibrational modes of adsorbed molecules on a surface are studied by monitoring the absorption or emission of IR radiation from thermally excited modes as a function of energy. 

Infrared (IR) spectroscopy A spectroscopic instrumental technique measuring the absorption of infrared radiation over a range of frequencies by molecules of a substance. The infrared spectrogram (or spectrum ) is produced as an analytical record of this absorption it is unique to each compound (when performed under standard conditions) and can be useful as a fingerprint for comparative identifications. 

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