Electronic spectroscopies dipole-forbidden transitions

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

For dipole forbidden transitions, e.g., from a p core state to a 4f final state, higher-order terms in the exponential expansion lead to a finite contribution to the cross section. This is in general small unless q 1/r, where is the core orbital radius, but such conditions may be satisfied both in transmission and reflection geometry (Schnatterly 1979, Grunes and Leapman 1980, Ludeke and Koma 1975), and will be of importance in rare earth electron loss spectroscopy. Thus even within the Born-Bethe regime monopole, quadrupole and octopole transitions may be prominent. 

The ultraviolet spectroscopy of formaldehyde has been studied almost exhaustively, and there is an excellent review on this subject (171). A majority of the bands in the electric-dipole-forbidden vibronically allowed A 2 +X Aj transition have been assigned mostly due to the work of Brand (37), Robinson and DiGiorgio (196), Callomon and Innes (44), and Job, et al. (124). As briefly mentioned earlier, the ground electronic state (X) is planar and the first excited singlet state (A) is pyramidal. It is valid to use the C2V point group symmetry for both electronic states, rather than the C2 point group symmetry (see ref. 171), although the emission could certainly be treated as a -A" - 1A transition. 

Electron energy-loss spectroscopy at low excitation energies is a surface sensitive technique to study the electronic structure by exciting collective oscillations or electrons from occupied into unoccupied states. In metals with a high density of states arising from d electrons, the excitation of plasmon losses has a relatively low probability. Therefore, the spectra are dominated by interband or intraband transitions. In rare earth metals, excitations of the partially filled/shell are observed that are assigned to be dipole-forbidden 4/4/transitions. These transitions are enhanced near the 4ii-4/threshold [56]. 

One example is the spectroscopy of highly forbidden transitions, which becomes possible because of the long interaction time. Another aspect is a closer look at the chemistry of cold trapped molecules, where the reaction rates and the molecular dynamics are dominated by tunneling and a manipulation of molecular trajectories seems possible. Experiments on testing time-reversal symmetry via a search for a possible electric dipole moment of the proton or the electron [1212] are more sensitive when cold molecules are used [1213, 1214]. 

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

The continuum emission in this spectrum is due to thermal emission from hot dust that has been heated by absorbing starlight. The sharp emission lines are due to radiation from various atoms and atomic ions. Atomic designations with square brackets such as [Nell] indicate the emission is due to a forbidden transition. A forbidden transition is a transition via the atom s electronic quadrapole moment or magnetic dipole moment rather than the electronic dipole moment. The broader emission features designated by PAH (polycylic aromatic hydrocarbons) are emission by dust components. One of the major discoveries with ISO spectroscopy has been that these PAH emission features... 

Yttrium oxide crystallizes in the Bixbyite (C) structure, with two inequivalent types of yttrium sites in the unit cell these sites have C2 and Csi symmetry. As electric-dipole transitions are forbidden in Csi sites (with an inversion symmetry), data on the energy levels and other optical properties of these sites are sparse and do not cover a wide range in energy. For this reason, our tabulation of data concentrates on the C2 sites. In particular, all the crystal-field parameters and energy levels given here are those appropriate for C2 sites. The C3i sites (and their interaction with C2 sites) have been studied primarily by means of far-infrared spectroscopy (Bloor and Dean, 1972), electron paramagnetic resonance (Schafer, 1969), energy-transfer studies (Heber et al., 1970), and similar methods. 

NH radicals in higher electronic states can be detected by emission as mentioned above for the A state at 336 nm [54, 58, 62, 97] this A Tl-X emission was also observed with Fourier transform spectroscopy [47, 53]. Those electronically excited NH states, which have allowed transitions to lower electronic states, can easily detected by emission, but also a spin-forbidden, dipole-allowed radiative transition b S - X was observed... 

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