Electronic transitions of molecules

The easiest method for creating many vibrational excitations is to use convenient pulsed visible or near-UV lasers to pump electronic transitions of molecules which undergo fast nonradiative processes such as internal conversion (e.g. porjDhyrin [64, 65] or near-IR dyes [66, 62, 68 and 62]), photoisomerization (e.g. stilbene [12] or photodissociation (e.g. Hgl2 [8]). Creating a specific vibrational excitation D in a controlled way requires more finesse. The easiest method is to use visible or near-UV pulses to resonantly pump a vibronic transition (e.g. 

The most characteristic type of primary activations are the electronic transitions of molecules which are much faster than other response of the irradiated medium. This enables one to consider separately the physical stage of radiolysis, at the end of which a certain ensemble of excited and ionized molecules is formed in the medium. Each of the activated molecules possesses a particular amount of energy available for subsequent processes. The initial distribution and yields of individual primary activations are dealt with by the theory of primary radiation chemical yield (PRCY). We have studied the application of this theory to the radiolysis of gases in detail during the last years (16, 17, 18, 19, 20). Thus, in the formal expression—see (5), for the yield G(X) = %ngncompetitive reaction ways and remain much more obscure at the present. 

Recently quite a number of theories have been advanced to explain optical dephasing in electronic transitions of molecules in solids. 

Fig. 11. Effect of polar and non-polar solvents on the energies of electronic transitions of molecules in which the (n, ti ) and (tt, tz ) states are close together.

Many band spectra, which arise from electron transitions of molecules and radicals as well as internal molecular vibrations and rotations, appear as a continuum since the monochromators commonly used for flame... 

The origin of FA is the polarization of electronic transition of molecules to each transition is associated a vector called transition moment (see Sect. 1.5) which as a given orientation with respect to the molecular structure, In Fig. 6.15, the absorption transitions Sq —> 5i and 5o —> S2 of perylene are depicted. Transition 5i —> Sq responsible for the fluorescence is almost parallel to 5q —> 5i. In general, when the deactivation of an excited state takes place radiatively, the emitted photon is polarized parallel to the transition moment. Hence, if a single molecule is observed, the polarization of the emitted light is parallel to the direction defined by the transition responsible for the fluorescence. 

Herzberg G, Lagerquist A and Malmberg C 1969 New electronic transitions of the C2 molecule absorption in the vacuum ultraviolet region Can. J. Phys. 47 2735-43... 

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

The development of molecular orbital theory (MO theory) in the late 1920s overcame these difficulties. It explains why the electron pair is so important for bond formation and predicts that oxygen is paramagnetic. It accommodates electron-deficient compounds such as the boranes just as naturally as it deals with methane and water. Furthermore, molecular orbital theory can be extended to account for the structures and properties of metals and semiconductors. It can also be used to account for the electronic spectra of molecules, which arise when an electron makes a transition from an occupied molecular orbital to a vacant molecular orbital. 

Since all the photopolymerizable monomers (A) contain two conjugated double bonds, the tt-tt electronic transition of a dimer and a molecule larger than a dimer (B) is shifted to a higher energy level than that of A. The reaction scheme is as shown in (6)-(ll) (Tamaki et al., 1972), where A and B represent the species A and B, respectively, in the tt-tt excited state. 

Electron configurations of transition metal complexes are governed by the principles described in Chapters. The Pauli exclusion principle states that no two electrons can have identical descriptions, and Hund s rule requires that all unpaired electrons have the same spin orientation. These concepts are used in Chapter 8 for atomic configurations and in Chapters 9 and 10 to describe the electron configurations of molecules. They also determine the electron configurations of transition metal complexes. 

Resonance Raman scattering occurs when the incident photon of light (El) has enough energy to approach or become resonant with an electronic transition of the molecule (Eq + - e,) so that the first term in equation (3.2) becomes predomi-... 

When both electron donor (D) and acceptor (A) groups are attached to a w-electron system it is not possible to consider the transition in terms of the excitation of one electron since it is a composite of several different one-electron excitation types. An example of a molecule which has electronic transitions of this type is Michler s ketone ... 

In ultraviolet and visible region, electronic transition of atoms and molecules are observed. This is why it is called electronic spectroscopy. In infrared region the absorption of radiation by an organic compound causes molecular vibrations and so it is called vibrational spectroscopy. 

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