Fluorescence and phosphorescence spectrophotometry

Abstract

Luminescence is the emission of photons from electronically excited states. Fluorescence and phosphorescence are two types of luminescence which differ in the nature of the ground and excited states. A molecule may be excited from a singlet ground state to a singlet excited state by the absorption of radiation in the UV or visible region of the spectrum. Electronic excitation is accompanied by excitation to a higher vibrational energy level. The excited molecules may rapidly lose vibrational energy by collisions with neighbouring molecules. Fluorescence is the emission of light at a longer wavelength than the incident radiation as the molecule returns from a singlet excited state to a singlet ground state after the loss of vibrational energy (Figure 7.1). Phosphorescence is the emission of light at a longer wavelength than the incident radiation as a molecule returns from an excited electronic state to a ground electronic state of different spin multiplicity, generally a triplet excited state returning to a singlet ground state.