Spin-multiplicity selection rule

Regardless of the nature of the space parts, Q vanishes if V spin V spm- If Q vanishes, so does /. Thus we have the so-called spin-selection rule which denies the possibility of an electronic transition between states of different spin-multiplicity and we write AS = 0 for spin-allowed transitions. Expressed in different words, transitions between states of different spin are not allowed because light has no spin properties and cannot, therefore, change the spin. 

Excited states formed by light absorption are governed by (dipole) selection rules. Two selection rules derive from parity and spin considerations. Atoms and molecules with a center of symmetry must have wavefunctions that are either symmetric (g) or antisymmetric (u). Since the dipole moment operator is of odd parity, allowed transitions must relate states of different parity thus, u—g is allowed, but not u—u or g—g. Similarly, allowed transitions must connect states of the same multiplicity—that is, singlet—singlet, triplet-triplet, and so on. The parity selection rule is strictly obeyed for atoms and molecules of high symmetry. In molecules of low symmetry, it tends to break down gradually however,... 

For more complicated spin systems than the one presented in Fig. 1, the number of EPR lines becomes much greater than the number of ENDOR lines. This is due to the different selection rules for EPR and ENDOR transitions. In an EPR spectrum (Ams = 1, Ami = 0) the number of lines increases multiplicatively with the number of nonequivalent nuclei, whereas in ENDOR (Ams = 0, Amt = 1) the corresponding increase is only additive7. Since on the other hand the total spectral widths in EPR and ENDOR are of the same order of magnitude (expressed in units of energy), the average spectral... 

Spin selection rule The spin selection rule, AS = 0, specifies that there should be no change in the spin multiplicity. Weak spin-forbidden bands may occur when spin-orbit coupling is possible. Spin-forbidden transitions are more intense in complexes of heavy atoms as these lead to a larger spin-orbit coupling. 

Selection rules for the electronic energy transfer by dipole-dipole interactions are the same as those for corresponding electric dipole transitions in the isolated molecules. The spin selection rule requires that the total multiplicity of the donor arid the acceptor, prior to and after the act of transfer, must be preserved. This implies that M0. - Mq and MA — MA where M s denote the multiplicity of the states (Section 2.5.1). 

The spin selection rule states that no transition can occur between states of different multiplicity i,e. AS = 0. Transitions which violate this rule are generally so weak that they can usually be ignored. 

A second selection rule states that anv transition for which S 0 is forbidden, i.e.. in order to be allowed, a transition must involve no change in spin stale. Looking at the correlation diagram for a d configuration in an octahedral field (Fig. 11.35). we note that the ground stale has a multiplicity of 3 (5 = I) and that there are three excited stales with this same multiplicity 372) . and Ttu (from the 3P). Thus we can envision three transitions that are spin allowed ... 

As atomic weights increase, selection rules are less rigorously obeyed so that many transitions occur with violation of one or more of them. This is particularly true for the transitions with change in spin, so that AS = 1 (or a change, for example, of multiplicity from singlet to triplet or vice versa) is often found for heavy atoms. Transitions for which the rules are obeyed always occur with higher probability than those for which one or more of the rules is disobeyed. 

Intensities of absorption bands are governed by probabilities of electronic transitions between the split 3d orbital energy levels. The probabilities are expressed by selection rules, two of which are the spin-multiplicity selection rule and the Laporte selection rule. 

Laporte and spin-multiplicity selection rules ( 3.7) and have intensities 103 to 104 times higher than those of crystal field transitions (table 3.6), their absorption edges may extend well into the visible region and overlap crystal field spin-allowed and spin-forbidden peaks. 

Spin Selection Rules. The spin selection rule for energy transfer by coulombic interaction is very restrictive, since no change of spin in either the donor or the acceptor is allowed, i.e., the multiplicities Mv> must equal Md and Ma equal Ma. ... 

Formal rules, known as selection rules, may be used to decide whether or not an electric dipole transition between two states may take place. Perhaps the most important is the rule governing spin multiplicity spin must not change during an electronic transition. The usual way to write rules of this kind is... 

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