Deriving the Spectrum, Ch Electronic spectra of diatomic molecules
Problem
A sample of naphthalene frozen in a rigid glass at low temperature is illuminated with ultraviolet light and then the light source is switched off. Two emissions are observed. A fast component decays with a lifetime of about 100ns and has its 0-0 band at 315nm. After this fast component has died away, a much weaker glow persists, decaying with a lifetime of about 2.5s, with its 0-0 band at 475nm.
((a)) Identify which emission is fluorescence and which is phosphorescence, and explain in terms of intersystem crossing and spin why the second one survives so much longer.
((b)) Predict, qualitatively, what happens to the intensity of the long-lived glow if dissolved oxygen (itself a ground-state triplet, and an efficient quencher of triplet states) is admitted to the sample, and why.
((c)) Using the two 0-0 band positions, estimate the energy gap between the singlet state responsible for the fast emission and the triplet state responsible for the slow one.