Spectral lines forbidden

In Mn, an electron can only go from one 3 c/level to another if it changes its spin. Transitions in which an electron changes its spin are forbidden and so give rise to weak spectral lines. 

In various pure coupling schemes the intensities of spectral lines may differ significantly. Some lines, permitted in one coupling scheme, are forbidden in others. Comparison of such theoretical results with the relevant experimental data may serve as an additional criterion of the validity of the coupling scheme used. 

In the limit of large chemical shift differences, the three-spin system displays a first-order spectrum, consisting of 12 lines, the other three being forbidden. The spectral line positions can be derived from the ABX spectrum or simply by application of repetitive first-order splittings. 

The above elementary treatment of molecular spectra is sufficient for our present purpose and for a more extended discussion of the coupling of the vibrational and rotational modes of oscillation, the polarization and intensity of spectral lines, permitted and forbidden transitions, the Frank-Condon principle and the application to thermodynamics, the reader is referred to the many monographs which have been published... 

If, for example we used in g R) only the long-range asymptote of this function, that is, (R) = —CjR (if we consider dipole-forbidden excitation), we immediately obtain the expression derived by Margenau [11] and others [12], which describes the behavior of the spectral line of a molecule dissolved in an ideal gas ... 

From this, one can also obtain the parity violating energy difference ApvS. Because the highly resolved spectra of both parity isomers x (+) and x (-) are different because of the electric dipole selection rule, one can obtain the concentration c,i of X(+) through the determination of the increase of the initially forbidden spectral lines Cs (t = 0) = 0). For short times with sin x x, the following approximation holds... 

If one calculates the total number of spectral lines that should occur involving all values of n and k and compares this number with the number of spectral lines actually observed, it is obvious that certain transitions do not occur. The analysis of spectra led to the conclusion that some transitions were permitted and some were forbidden. The following rules were formulated as a result. 

Experimentally, free-ion spectra (both neutral and ionic species) are usually observed in emission, and the energy level structure is deduced from coincidences of energy differences of pairs of spectral lines, subject to verification by isotope shift, hyperfine structure and magnetic gf-factor tests. In condensed phases, spectra are more commonly measured in absorption. Relative intensities associated with parity-allowed and forbidden transitions are reflected in the nature of two processes transitions in which the initial and final states belong to electronic configurations of different parity (parity-allowed transitions, e.g. 5f - 5f 6d) and those in which both states belong to the same configuration (parity-forbidden transitions, e.g. 5f 5f ). The latter are weak and sharp. The... 

Vibrational transitions do not occur without rotational transitions, because A / = 0 is forbidden for diatomic molecules. Figure 23.8 shows the allowed transitions that occur between the ground vibrational state (u = 0) and the first excited vibrational state (u = 1) of a diatomic molecule. The resulting set of spectral lines is called the... 

Figure 12. Electronic spectra and the results of open-shell PPP-like semiempirical calculations for radical ions. The vertical lines represent the allowed transitions, the wavy lines with arrows the forbidden ones. The right side scales denote the calculated spectral intensities, where f stands for the oscillator strength. Top left the absorption curve (146) redrawn to the log e vs. 0 (cm ) form calculations are taken from (59). Top right taken from (11). Bottom left taken from (143). Bottom right taken from (136), the absorption curve redrawn to the log e vs, 0 (cm" ) form.

---