FUNDAMENTALS OF ATOMIC AND MOLECULAR SPECTROSCOPY IN INSTRUMENTAL ANALYSIS S. PAM • 1. INTRODUCTION The atomic and molecular spectroscopy is a powerful tool to analyse chemical composition or structure of a substance, which may be a pure compound or a simply mixture or solution of two or more different phases of a crystalline or amorphous material. The chemis- try and industry people sometimes also talk of minerals, ores, and pollutants, but these comprise the same crystalline or amorphous structures. They are characterized using the same instrumental tech- niques. The various techniques explored to characterise these materials using the atomic and molecular spectroscopy constitute a wide sub- ject of basic and applied sciences. Those deal with the interaction of electric or magnetic field or electromagnetic radiation (including the electron beam) with matter through : (i) spin or orbital motion of valence electron(s), (ii) excitation of an electron from a core level of an atom or molecule, (iii) vibration of nuclie about their equilibrium positions in the molecule, or (iv) rotation of molecule about the symmetry axes. The statistical distribution of the interaction probability over the energy scale is called the spectrum and the discipline under which these interactions are studied is called spectroscopy. Thus, according • Materials Processing Division, National Metallurgical Laboratory, Jamshedpur-831 007. V-1