Definition of fluorescence Generally, at room temperature molecules are in their ground electronic state and vibrational level (Sections Q1 and Q3). Absorption of energy at an appropriate wavelength results in electronic excitation to the singlet state and the upper vibrational level associated with this electronic transition. To regain the equilibrium or ground state, several

processes may take place, enabling energy loss. The first event is that molecules in vibrational levels above the lowest in the excited state lose their excess energy (usually in the form of heat), moving to a lower vibrational level, but still in the excited electronic state. From this state spontaneous emission can occur, and the light that is emitted is fluorescence (Figure 1).