Spectra Generated by Dipole Transitions

A spectrum is generated by transitions between different energy states according to certain selection rules. The selection rules for allowed transitions essentially reflect the requirement of conservation of angular momentum for the atom/molecule-photon system. Considerations of symmetry for the 

The selection rules will not be derived here, but stated with some comments. We will then see how spectra are generated by allowed transitions between energy levels described in Chaps. 2 and 3. It is best to treat atoms and molecules separately, but first some general features will be discussed in connection with Fig.4.5. 

A special type of emission, phosphorescence, can be obtained from certain molecules that are excited from the ground state to a higher-lying state 

For one-electron systems we have the following selection rules A = l,  

In Fig.4.7 allowed transitions between a p and an sp configuration are indicated as an example. 

A spectrum is generated by transitions between different energy states according to certain selection rules. The selection rules for allowed transitions essentially reflect the requirement of conservation of angular momentum for the atom/molecule-photon system. Considerations of symmetry for the wave ftmetions describing the states involved are also important. For electric dipole transitions the establishing of selection rules is equivalent to a determination of the conditions mrder wliich the matrix element (n r ) is non-zero. Since r is an odd operator, we can immediately state that the two considered states must be of opposite parity. 

Spectra of atoms and molecules resulting from absorption or emission can be studied. In absorption., a wavelength continuum is used, of which certain wavelengths are absorbed. Emission spectra may be generated in a discharge in a light source where the excited levels ai e populated by, for example, electron collisions. K atoms or molecules are irradiated by light of 

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Figure 2.7 shows the optical spectra generated from the SAC-CI calculations for each of the systems studied. The spectra were obtained non-perturbatively, by explicitly computing transition dipole moment and oscillator strength for... 

The effect of quantum interference on spontaneous emission in atomic and molecular systems is the generation of superposition states that can be manipulated, to reduce the interaction with the environment, by adjusting the polarizations of the transition dipole moments, or the amplitudes and phases of the external driving fields. With a suitable choice of parameters, the superposition states can decay with controlled and significantly reduced rates. This modification can lead to subnatural linewidths in the fluorescence and absorption spectra [5,10]. Furthermore, as will be shown in this review, the superposition states can even be decoupled from the environment and the population can be trapped in these states without decaying to the lower levels. These states, known as dark or trapped states, were predicted in many configurations of multilevel systems [11], as well as in multiatom systems [12],... 

A second important application of this technique is the determination of molecular orientation within an adsorbed overlayer. Since the X-rays generated in the electron synchrotron by deflecting electrons with energies in the 1 GeV range are polarized with the E-vector orientated parallel to the plane of the synchrotron, the X-ray absorption dichroism can be conveniently measured by recording absorption spectra for normal and grazing incidence, respectively. From measurements at different angles of incidence one can thus obtain the orientation of the transition dipole moment with respect to the surface normal, from which in turn the orientation of the molecule can be derived. 

The MORBID-like expansions (4th order) given by Eqs. (4,5) were used by Jensen et al. [24] to represent an ab initio (CASSCF) dipole moment of C3. Vibrational transition moments were computed for a large number of vibrational states and low T intensities were generated to simulate the spectra of a supersonic expansion, where the rotational and vibrational temperature were taken as 10 K and 50 K, respectively. 

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