Practice question

Electronic excitation occurs orders of magnitude faster than nuclear motion, about femtoseconds versus hundred femtoseconds for vibrations. Franck-Condon principle states nuclear coordinates remain essentially frozen during transition, so absorption appears vertical on potential energy diagram, reaching excited vibrational levels with greatest wavefunction overlap. After excitation, nuclei relax to new equilibrium. This concept explains vibrational progressions in absorption and why emission arises from lowest S1. Stokes and Kasha describe energy loss and emission rule, Förster concerns energy transfer, not vertical nature.