Introduction

Abstract

This chapter discusses clearly how the spectra of molecules differ from those of atoms and how this difference is explained through the introduction of Born–Oppenheimer approximation. This approximation is shown to split the complex molecular wave equation into two parts, one belonging to the electronic motion and the other to the nuclear motion. The nuclear motion is composed of rotations and vibrations of molecules. It is shown that the rotational energy quantum is less than the vibrational energy quantum which again is less than the energy arising from electronic motion. These three motions give rise to three types of spectrum, respectively, lying in the microwave, infrared and visible/ultraviolet regions of electromagnetic radiation. The validity of this approximation and also the condition under which it breaks down are also discussed.