Radiation modes dipole excitation

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. 

An ordered monolayer of molecules having a large dynamical dipole moment must not be regarded as an ensemble of individual oscillators but a strongly coupled system, the vibrational excitations being collective modes (phonons) for which the wavevector q is a good quantum number. The dispersion of the mode for CO/Cu(100) in the c(2 x 2) structure has been measured by off-specular EELS, while the infrared radiation of course only excites the q = 0 mode. 

A molecule can only absorb infrared radiation if the vibration changes the dipole moment. Homonuclear diatomic molecules (such as N2) have no dipole moment no matter how much the atoms are separated, so they have no infrared spectra, just as they had no microwave spectra. They still have rotational and vibrational energy levels it is just that absorption of one infrared or microwave photon will not excite transitions between those levels. Heteronuclear diatomics (such as CO or HC1) absorb infrared radiation. All polyatomic molecules (three or more atoms) also absorb infrared radiation, because there are always some vibrations which create a dipole moment. For example, the bending modes of carbon dioxide make the molecule nonlinear and create a dipole moment, hence CO2 can absorb infrared radiation. 

Fig. 9.3 Same as Fig. 9.2, now cast in the dressed states (eigenstates oF/Zm +HR)form. 0> = g k) corresponds to the molecule in the ground state with a single photon of mode k.. v) describes the molecule in an excited state and the radiation field in its vacuum state. The coupling between 0) and x> is proportional to the dipole matrix element /j.obetween the corresponding molecular states. 

In Eq. (12.25), 6 is the angle between the polarization of the incident radiation (A ) and the direction of propagation of the scattered wave k ), R is the position of the detector, is the dynamic polarizability of the segment, and p iv) is the density of modes of the incident radiation at frequency v (Eqs. 12.9,12.12, and B12.1.14). The factor sin(5)/IAI is the same factor that determines the amplitude of the field from an oscillating electric dipole (Figs. 3.1 and 3.2), and the fluorescence from an excited molecule whose transition dipole is oriented along a fixed axis (Sect. 5.9). The polarizability Uaa can be obtained from the difference between the dielectric constant of the solution and that of the pure solvent. 

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