Page 320 - Instant notes

Physical Chemistry 306

vibrational energy levels which are more closely spaced than the electronic energy levels.
Therefore, rotational,

Fig. 1. The electromagnetic spectrum
and the type of spectroscopy
associated with different spectral
regions.

vibrational and electronic spectroscopy are usually associated with electromagnetic
radiation in the microwave, infrared and visible/ultraviolet regions of the spectrum,
respectively (Fig. 1).

Selection rules
Whether a transition between a particular pair of energy levels is allowed or forbidden,
is determined by the selection rule(s) for the type of atomic or molecular energy under
consideration.
A physical selection rule specifies the general features that the molecule must have in
order to exhibit any non-zero transition probabilities for a particular type of energy
transition. The magnitude of the transition probability determines the intensity of a
transition (see Topic I2).
A specific selection rule specifies exactly which pairs of quantum states are linked by
allowed transitions, provided that transitions are allowed by the physical selection rule.