A. General introduction

Within the limits of Planck’s quantum theory, any kind of energy input to an atom or molecule can cause one or more of its electrons to rise to a higher-energy state. In returning to a lower state the electron will emit a photon — the quantum of electromagnetic radiation — the energy of which is

\[\label{eq1} E_q=h\nu\;\;\;\;\mbox{J} \tag{1}\]

where h is Planck’s quantum constant = 6.62 × 10^–34 J s and ν the frequency of the electromagnetic wave in s^–1.

As long as only single atoms are considered there are a limited number of discrete states of energy, the differences of which correspond with the frequencies of the photons that can be emitted or absorbed. These frequencies or spectral lines are characteristic for the kind of atom in question.