Question

The following question involves many electron atoms and their emission spectra.

d. What are the selection rules for many electron atom spectra? Give an example of a forbidden transition between term symbols and the selection rule that is broken by the forbidden transition.

Answer 1

SELECTION RULE FOR ELECTRONIC TRANSITIONS IN MANY ELECTRON ATOMS:

LAPORTE SELECTION RULE:

In a centrosymmetric molecular environment, the transition between the same atomic orbitals is forbidden.

SS, PP, D​​​​​​​D, & FF   ΔL = ±1

SPIN SELECTION RULE :

The allowed transition occurs without changing the spin of the electron. ΔS = 0

FORBIDDEN TRANSITION:

Rausell Saunder's SELECTION RULES FOR ALLOWED TRANSITIONS: ΔS = 0,  ΔL=±1 [TERM SYMBOLS]

¹S ³P here ΔS is not equal to zero hence forbidden transition.

spectra shown by the d-d transition is spin forbidden but is partially allowed because of spin-orbit coupling.

In crystal field theory d orbital splits to t₂g & eg. The t₂g orbitals have a partial p orbital character thus a mixture of g and u character thus t₂g to eg transitions are partially allowed