Excited States of Atoms or Molecules, Fluorescence and Phosphorescence

 Light rays consist of tiny particles called photons. The energy carried by a photon is called

quantum. Light rays of shorter wavelengths contain more energy per photon of light than the light rays of

longer wavelengths. for example, one photon of blue light contains about 70K.cal. of energy while one photon

of red light contains about 40k.cal. of energy only.

Diagram showing excitation of molecules of fluorescence or phosphorescence

The normal state of the molecules or atoms is called as ground state or singlet state. When an electron

of a molecule or atom absorbs a quantum of light, it is raised to a higher energy level, which is called as

exciting second singlet state. This is unstable and has a half-life of 10-12 seconds.

* The electron comes to the next higher energy level by the loss of some of its extra energy in the form

of heat. This is called an exciting first singlet state and is also unstable with a half-life of 10-9

second.

* From the first singlet state, the excited electron may return to the ground state in two ways.

a. either losing its remaining extra energy in the form of heat or

b. by losing extra energy in the form of radiant energy. The latter process is called as

Fluorescence. The substances, which show this property of fluorescence, emit fluorescent

light only during the period they are exposed to incident light. Secondly, the fluorescent light is of

a higher wavelength than the incident light.

* The excited molecule or the atom may also lose its electronic excitation energy by internal conversion

and comes to another excited state called as triplet state which is metastable with the half-life of

10-3 seconds.

* From the triplet state, the excited molecule or the atom may return to the ground state in three ways:

a. by losing its remaining extra energy in the form of heat.

b. by losing the extra energy in the form of radiant energy. This latter process is called as

Phosphorescence. The substances, which show this property of phosphorescence, emit

phosphorescent light even after the incident radiant light is cut-off. Secondly, phosphorescent

light is of a higher wavelength than the incident light and fluorescent light.

c. the electron carrying the extra energy may be expelled from the molecule and is consumed in some

further chemical reaction and a fresh normal electron returns to the molecule, or the extra energy is

utilized in the reaction, and the same electron which has now become excited and returns to the

molecule. This is what exactly happens with the excited Triplet State of chlorophyll a molecule, which

takes part in the primary photochemical reaction in photosynthesis.