Analytical Chem istry - DePauw University

614 Analytical Chemistry 2.0S 2vrvrvrS 1vricvrvrvrT 1vriscic ecic ecvrvrvrisc ecvrvrvrvrvrvrvrvrvrvrvrvrvrvrvrvrvrvrS vr0vrvrvrvrvrvrvrvrabsorptionfluorescenceabsorptionFigure 10.48 Energy level diagram for a molecule showing pathways for the deactivation of an excited state: vr is vibrationalrelaxation; ic is internal conversion; ec is external conversion; and isc is an intersystem crossing. The lowestvibrational energy for each electronic state is indicated by the thicker line. The electronic ground state is shown in blackand the three electronic excited states are shown in green. The absorption, fluorescence, and phosphorescence of photonsalso are shown.fluorescenceFigure 10.48. Absorption of a photon excites the molecule to one of severalvibrational energy levels in the first excited electronic state, S 1 , or the secondelectronic excited state, S 2 , both of which are singlet states. Relaxationto the ground state occurs by a number of mechanisms, some involving theemission of photons and others occurring without emitting photons. Theserelaxation mechanisms are shown in Figure 10.48. The most likely relaxationpathway is the one with the shortest lifetime for the excited state.phosphorescenceRadiationless De a c t i v a t i o nWhen a molecule relaxes without emitting a photon we call the processradiationless deactivation. One example of radiationless deactivationis vibrational relaxation, in which a molecule in an excited vibrationalenergy level loses energy by moving to a lower vibrational energy level inthe same electronic state. Vibrational relaxation is very rapid, with an averagelifetime of