sipated as heat in 10–^11 – 10 –^15 s by collisions between solute and solvent molecules, may be followed by
the slower (10–^9 to over 1 s) mechanisms of internal conversion and intersystem crossing. Of analytical
significance is the possibility of photoluminescence, a general term for relaxation in which radiation of
a longer wavelength (lower energy) than that originally absorbed is re-emitted after about 10–^8 s or
longer and following vibrational relaxation. Fluorescence and phosphorescence are two distinct forms
of photoluminescence, both giving rise to analytical techniques of very high sensitivity. A summary of
relaxation mechanisms is shown diagrammatically in Figure 9.11(a).
Figure 9.11(a)
Relaxation mechanisms from excited electronic states.