Franck-Condon factors vibronic selection rules

The synnnetry selection rules discussed above tell us whether a particular vibronic transition is allowed or forbidden, but they give no mfonnation about the intensity of allowed bands. That is detennined by equation (Bl.1.9) for absorption or (Bl.1.13) for emission. That usually means by the Franck-Condon principle if only the zero-order tenn in equation (B 1.1.7) is needed. So we take note of some general principles for Franck-Condon factors (FCFs). 

When a normal mode is distorted but is not displaced, then, according to the selection rules, vibronic transitions with odd n, aO —> bn , are forbidden. For even n, Franck-Condon factors are expressed as follows ... 

The excess photon energy of the order up to a few thousand wavenumbers will end up partly as internal energy of the ion. The resulting vibrational populations depend on the amount of excess energy, the selection rules for transitions from the intermediate vibronic states and the corresponding Franck-Condon factors. 

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