Radiation field, interaction with molecules

The situation becomes even worse when the Boltzmann formula is used to interpret the absorption of radiant energy by molecules. Electromagnetic radiation considered as a fluctuating electric field interacts with electrons in... 

Interaction of infrared radiation with a vibrating molecule is only possible if the electric vector of the radiation field oscillates with the same frequency as does the molecular dipole moment. A vibration is infrared active only if the molecular dipole moment is modulated by the normal vibration,... 

Radicals are molecules which are normally diamagnetic, but which for one reason or another (because of chemical reactions or photolysis) have lost or gained one electron. The pairing balance is therefore lost one electron is unpaired and possesses a magnetic moment which in a magnetic field interacts with electromagnetic radiation as described above. 

The interaction terms are defined in equation 9. The simplest interpretation of this term is that the radiation field interacts directly with the last two moments (it sees the whole molecule effectively as a point) with the quantity in curly brackets providing a strong perturbative glue of dipolar interactions between electric transition moments (each of which is symmetry allowed)on A and B, so that the radiation field sees the two chromophores as a single system. Further discussion of this term is given elsewhere (5). ... 

Fig. 1.17. The radiation s electric field interacting with the rotating dipole moment of a rotating diatomic molecule.

The electrical field associated with the electromagnetic radiation will interact with the molecule to change its electrical properties. Some molecules (for example, HCl) have a dipole moment due to charge separation and will interact with the field. Others may acquire a dipxrle when they vibrate. For example, methane, CH4, has no dipxrle, but when one of the CH bonds stretches, the molecule will develop a temporary dipxrle. 

Photochemical reactions are initiated by the interaction of a molecule with the local radiation field, perhaps from an embedded star. In cold dark clouds the radiation field may be rather small but as stars begin to form and emit radiation, initially at low energy but of every-increasing energy, photochemistry becomes important in generating a wide variety of reactive species to add to the diversity of species available for the chemical networks. 

Any periodic distortion that causes polarization of a molecule can also cause interaction with the electric field component of radiation. An example is the asymmetric stretching vibration of the CO2 molecule, that creates a fluctuating dipole moment as shown below. 

A molecule must have a permanent dipole moment to be micro-wave active. As it rotates, the changing dipole moment interacts with the oscillating electric field of the electromagnetic radiation, resulting in absorption or emission of energy. This requirement means that homonuclear molecules such as H2 are microwave inactive, but heteronuclear molecules such as SO3, S02, NO and, of course, H20 are active. 

The study of behavior of many-electron systems such as atoms, molecules, and solids under the action of time-dependent (TD) external fields, which includes interaction with radiation, has been an important area of research. In the linear response regime, where one considers the external held to cause a small perturbation to the initial ground state of the system, one can obtain many important physical quantities such as polarizabilities, dielectric functions, excitation energies, photoabsorption spectra, van der Waals coefficients, etc. In many situations, for example, in the case of interaction of many-electron systems with strong laser held, however, it is necessary to go beyond linear response for investigation of the properties. Since a full theoretical description based on accurate solution of TD Schrodinger equation is not yet within the reach of computational capabilities, new methods which can efficiently handle the TD many-electron correlations need to be explored, and time-dependent density functional theory (TDDFT) is one such valuable approach. 

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