Understanding Spin-Allowed and Spin-Forbidden Transitions in Spectroscopy

Spin-allowed and spin-forbidden transitions refer to the selection rules governing transitions in atomic or molecular systems, particularly in the context of spectroscopy.
Spin-Allowed Transition:
Spin-allowed transitions are those in which the total spin angular momentum (spin) of the electrons remains conserved.
These transitions are more likely to occur and are typically associated with a higher probability.
For example, in electronic transitions (e.g., electronic absorption or emission spectra), a transition from one electronic energy level to another is often spin-allowed if the change in the total spin quantum number (∆S) is 0. This means that the total spin of the initial and final states remains the same.
Spin-Forbidden Transition:
Spin-forbidden transitions are those in which the total spin angular momentum of the electrons does not remain conserved.
These transitions are less likely to occur and are associated with lower probabilities.
Spin-forbidden transitions often involve a change in the total spin quantum number (∆S) that is not equal to 0.
The selection rules for these transitions are less permissive, making them less intense in spectroscopic observations.
These terms are commonly used in the context of electronic, vibrational, and rotational spectroscopy to describe the probability and intensity of transitions between different energy levels or states in atoms or molecules.
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