The modern theory of the structure of the atom began with the discovery
of the electron by J. J. Thomson in 1897. In the early 1900s, Bohr,
Rutherford and Sommerfeld, amongst many others, constructed the first
electron-nucleus **models** of the atom. The nucleus is a tiny
positive core surrounded by negatively charged electrons moving in
elliptical orbits. Our current model combines these early ideas with **Quantum theory ** and ** Relativity**. The motion of each electron is now described by wave functions that satisfy Schrödinger's equation,
plus a correction for relativity. The square of these functions
gives the likelihood of an electron being found at a particular
location in the atom.

Atomic spectroscopy studies the interaction, absorption or emission, of
electromagnetic radiation by atoms. In particular, the interaction of
photons with electrons of the atoms of interest is observed and
measured. Three main photon-electron interactions of the molecule are
considered here: absorption, emission and stimulated emission . Light
scattering is not considered in this paragraph.

A ground state atom
(E0) can be provided with a certain amount of energy so that an
electron located in one energy level reaches a new energy state (E1).

The energy needed to pass from E0 to E1 is known as the **excitation energy**
and can be provided as heat, electric field, radiations… Once excited,
the electron is not stable and seeks to return to the lower energy (not
excited) level so that the energy difference ΔE = E_{1}-E_{0} can be emitted as electromagnetic radiation of frequency ν = ΔE / h, where h is the Planck’s constant.

First published on the web:15 November 1999

**Authors**
of the latest version (March 2008): Lara Lobo Revilla and Karl Preiss .
The text is based on a lecture given by Prof E.B. Steers at the first Gladnet training course in Antwerp Sept. 2007