Q1. How many possible orientations can two parallel spins adopt with respect to the external magnetic field? Show by schematic diagram.

Q2. What are the spin multiplicity and the spin of triplet state?

Q3. Why two electrons are always present in two different orbitals of the molecule in triplet state?

Q4. Internal conversion (takes place at a time scale of 10-12 s) is faster than the fluorescence emission. Then how does radiative emission (fluorescence) compete with the non-radiative transition? (Hint: Significantly larger energy gap between S1 and S0 and longer lifetime of S1.)

Q5. Transitions between states of different multiplicity are formally forbidden. What are the mechanisms that make the intersystem crossing probable?

Q6. How can you justify that the relaxation to ground state occurs via 'vertical' transition?

Q7. In most of the cases, the S1 potential energy curve shifts to the right with respect to S0 potential energy curve. Justify it. (Hint: In the excited state, the electron is promoted to an anti-bonding orbital.)

Q8. Why are absorption and fluorescence much faster than phosphorescence?