Phosphorescence

The temporally delayed emission of light after excitation with light energy is referred to as phosphorescence. The time delay observed in solutions is between 0.001 and 10 seconds. It is often possible to use a fluorescence detector to measure the phosphorescence (as well as the Chemiluminescence). In this case, the emitting light is measured several milliseconds after the excitation flash.

 Note:

The wavelength of the emitted light is always higher than the excitation wavelength. In HPLC applications, the phosphorescence spectrum is independent of the excitation wavelength.

The Molecular Orbital (MO) theory provides a more exact explanation:

Similar to Fluorescence, molecules are excited by the absorption of light. Absorption arises from the ground state, the lowest vibrational level of the lowest singlet state (S0) and terminates in different excited vibrational levels of the next higher singlet state (S1).

where:

h = Plank's constant

where:

c = speed of light

l = wavelength

The excited vibrational states of S1 transit via a radiationless intersystem crossing into the lowest triplet state (T1). The transition from the triplet state into the ground state is spin prohibited. Therefore, it is temporally delayed and phosphorescent light is emitted:

 Notes:

Compared to fluorescence spectra, phosphorescence spectra are shifted to the red because the electronic T1 states are energetically below the S1 states.

Due to the fast relaxation in solution the emission spectra of electronically excited molecules in solution is independent of the excitation wavelength (also for excitation of, e.g., S0®S2), according to the Kasha rule.