Light absorption by molecules

The aim of this chapter is to recall the basic principles of light absorption by molecules. The reader is referred to more specialized books for further details. 

In addition, it ought to be kept in mind, considering the classical concept of light absorption by molecules, that, owing to the inertia of molecules, the light field is not capable of turning the molecule and thus excites the vibrations of the dipole oscillator only to the extent to which... 

In this chapter we are concerned primarily with the physical nature of light and the mechanism of light absorption by molecules. We will discuss... 

Figure 5-10. Resonance transfer of excitation from molecule A to molecule B. After light absorption by molecule A, a radiationless transition occurs to the lowest vibrational sublevel of its excited state. Next, resonance transfer of the excitation takes place from A to B, causing the second molecule to go to an excited state, while molecule A returns to its ground state. After a radiationless transition to the lowest vibrational sublevel in the excited state, fluorescence can then be emitted by molecule B as it returns to its ground state. Based on the energy level diagrams (which include the vibrational sublevels for each of these two different pigments), we can conclude that generally the excitation rapidly decreases in energy after each intennolec-ular transfer between dissimilar molecules.

Characterize light absorption by molecules and its role in stratospheric chemistry of ozone and other reactants. 

In discussing light absorption by molecules, we can focus on two MOs. The highest occupied molecular orbital (HOMO) is the MO of highest energy that has electrons in it The lowest unoccupied molecular orbital (LUMO) is the MO of lowest energy that does not have electrons in it. In N2, for example, the HOMO is the a2p MO and the LUMO is the Ttjp MO (Figure 9.43). 

Physical Basis of Light Absorption by Molecules The Franck-Condon Principle... 

The development of photochemistry is actually closely connected to the interpretation of the spectra of light absorption by molecules, which ejqrlairrs whether absorption catrses a chemical change or not. 

Let s consider the influence of gases and particles on the optical properties of the atmosphere. Reduction in visibility is caused by the following interactions in the atmosphere light scattering by gaseous molecules and particles, and light absorption by gases and particles (2). 

Figure 11 Illustration of the interfacial CT processes in a nanocrystalline dye-sensitized solar cell. S / S+/S represent the sensitizer in the ground, oxidized and excited state, respectively. Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of Ti02. The oxidized sensitizer (3) is reduced by the I-/I3 redox couple (4) The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with an oxidized dye molecule (6).

The measurement of light absorption by a solution of molecules is governed by the Beer-Lambert Law, which is written as follows ... 

The basis for the measurement of the strength of light absorption by a molecule at various wavelengths is shown in Fig. 3.11. A parallel monochromatic light beam of wavelength A and power P ] or intensity / , defined as the energy per second striking a unit area,... 

It is emphasized that the terms excimer2 and exciplex3,4 are reserved here for homomolecular and heteromolecular excited double molecules formed after the act of light absorption by one component in a process of photoassociation, in the absence of spectroscopic or cryoscopic evidence for molecular association in the ground state. Recent findings indicate that excimer (or exciplex) formation may also result from triplet-triplet annihilation,5,8 cation-anion combination7 (doublet-doublet-annihilation), and electron capture by the (relatively stable) dimer (or complex) cation8 these processes are discussed in Section VII. 

In the UV curing process, photons from the UV source are absorbed by a chromophoric site of a molecule in a single event. The chromophore is a part of the photoinitiator. The light absorption by the photoinitiator requires that an emission light from the light source overlap with an absorption band of the photoinitiator. 

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