Molecular electronic transitions

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

CLASSIFICATION OF MOLECULAR ELECTRONIC TRANSITIONS AND EXCITED STATES... 

Taylor, H., Simons, J. A different view of molecular electronic transitions. J. Phys. Chem. 1986, 90, 580-3. 

Figure 2.1 Schematic diagram showing possible molecular electronic transitions, and vibrational and rotational energy levels...

The role of curve crossing in atomic and molecular collisions is a subject of considerable interest at the present time. Fairly recently there was an international conference entirely devoted to this subject [170]. A fine review article on radiationless molecular electronic transitions has also appeared rather recently [171]. 

In both techniques, the spectrum of the emitted radiation is characterised by lower energy than the excitation spectrum (Fig. 4.4), as the excitation process usually requires not only energy for a molecular electronic transition but also an increase in vibrational energy. 

By the Franck-Condon principle, a molecular electronic transition is much faster than a molecular vibration. 

Vibrational Relaxation. Stochastic processes, including vibrational relaxation in condensed media, have been considered from a theoretical standpoint in an extensive review,502 and a further review has considered measurement of such processes also.503 Models have been presented for vibrational relaxation in diatomic liquids 504 and in condensed media,505 using a master-equation approach. An extensive development of quantum ergodic theory for relaxation processes has been published,506 and quantum resonance effects in electronic to vibrational energy transfer have been considered.507 A paper has also considered the coupling between vibrational relaxation and molecular electronic transitions.508 A theory has also been outlined for the time-resolved electronic absorption spectrum of a molecule undergoing collisional vibrational relaxation.509... 

Figure 5.9 An electronic transition occurs over a band of energy due to the multiple vibrational and rotational sublevels associated with each electronic state. This schematic depicts four of the many possible transitions that occur. The length of the arrow is proportional to the energy required for the transition, so a molecular electronic transition consists of many closely spaced transitions, resulting in a band of energy absorbed rather than a discrete line absorption.

The fluorescence, ultraviolet and visible spectra of the series were reported in Russian [78] spectral shifts are related directly to LCAO MO calculations. Following the correlations between carcinogenicity and theoretical molecular electronic transition energies made by Mason [79—81], Birks [82], and Steele, Cusachs and McGlynn [83, 84] and in part experimentally confirmed by Sung and Lazar [85], the Russian results emphasise the importance of electronic transitions in the carcinogenic involvement of the series.]... 

This relationship may be used for calculation of the absorption coefficient otabs and extinction coefficient K from measured values of D. A typical absorption spectrum of a liquid crystalline substance in the isotropic phase is shown in Fig. 11.9a. In the UV part of the spectrum, the absorption originates from molecular electronic transitions (with vibronic structure). Except for dyes, the long-wave edge of organic compounds is situated at about 250-350 nm depending on particular molecular structure. As a rule, liquid crystalline materials are transparent in the... 

UV-Vis spectroscopy is an analytical method that measures molecular electronic transitions to characterize mainly organic molecules. For applying this method, the investigated species (analyte) has to absorb in the UV or visible wavelength range (200-780 nm). Consequently, this method is restricted to certain compounds such as... 

The extraction of the molecidar parameters requires an assumption on the orientation distribution for the p angle and on the relative magnitude of the different elements of the molecular hyperpolarizabihty. This is achieved with the knowledge of the symmetry of the molecular electronic transitions lying in the vicinity of the fundamental and the harmonic wavelengths. Finally, working with relative intensities on the macroscopic susceptibihty tensor, one can extract the orientation angle 9, usually determined for a narrow distribution, and the ratio of the two dominant hyperpolarizability tensor elements. 

Even in the visible or ultraviolet range, atomic or molecular electronic transitions with very small transition probabilities exist. In a dipole approximation these are forbidden transitions. One example is the 2s -o-Is transition for the hydrogen atom. The upper level 2s cannot decay by electric dipole transition, but a two-photon transition to the Is ground state is possible. The natural lifetime is t = 0.12 s and the natural linewidth of such a two-photon line is therefore 8v = 1.3 Hz. 

It was emphasized in the introduction to this chapter that molecular electronic transitions are generally accompanied by simultaneous changes in vibrational and rotational states. A calculation of the transition energy of a particular spectroscopic line thus requires knowledge of the rovibrational energy for a diatomic with vibrational and rotational quantum numbers v" and J" in the lower diatomic state... 

In contrast to the nonretarded treatment, which is solely based on the electrostatic interaction potential V(r), we equate the electric potential to zero in the retarded case. The electric interaction is completely covered by the vector potential A(r). The transverse modes under investigation enable the Lorentz gauge to be used for vanishing electric potential. This concept turns out especially useful with respect to the Schrodinger formalism presented in the next Chapter. We may ascribe the retarded interaction between electrons located at different particles solely to the vector potential A(r). In addition to providing the proper multipole susceptibilities, quantum theory still has to answer the question regarding statistics. Are the localized modes which are strictly coupled to molecular electron transitions, still Bosons ... 

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