Question

Estimate which absorption transition will be the most intense. Explain, using the Franck-Condon principle, your statement. Answer: (b) Estimate which fluorescence transition will be the most intense. Explain, using the Franck-Condon principle, your statement. Answer: (c) In this example, is the geometry of the excited state close to that of the ground state or not? Explain your answer.

Answer 1

According to Frank Codon principle, not every possible transition is as preferred as the others and this ultimately decides the intensities of the spectral lines of every transition. It essentially states that a transition happening from the lowest ground state [both electronic and vibronic] to the excited electronic state is most probable when their wavefunctions overlap.

Here, transition from v = 0 of ground state and v = 2 of the first excited electronic state will be the most intense.

For fluorescence, transition from vibronic excited state of the electronic ground state to the ground vibronic state of excited electronic state will be most intense. This happens for v = 0 of first electronic excited state to v = 3 of electronic state.

The excited state is displaced from the ground state because of the nuclear motion during an electronic transition.