The analysis of the absorption spectrum of naphthalene vapour.—Structure and activation of the molecule of naphthalene

The study of the adsorption spectrum of a substance in the state of vapour enables us to widen our knowledge of the movements and structure of molecules. The ultra-violet adsorption spectra of a large number of different substances in the state of vapour show a great variety of structures. We must find a general classification which holds good for all types observed till now, and this classification must be based on the mechanism causing absorption spectra. Bohr's theory shows us the general mechanism of all absorption and emission phenomena. An atom or molecule in the normal state possesses a value of internal energy E0 . Under the influence of different agents, such as radiation, thermal motion, electrical discharges or chemical reactions, etc., this energy can be changed. This change in internal energy is called the "activation" or excitation of the atom or molecule. When the activation is produced by absorption of radiation (frequency =v ), the second postulate of Bohr then says thathv = E' - E0 .