In Fig. 12.1, the thick lines represent electronic energy levels which are separated from one another by energies in the range of 0.5 to a few electron volts. “Within” (or between two) each electronic energy levels there are several vibrational energy levels and within each vibrational level, there are several rotational energy levels. These energy levels are not equally placed and the spacings between higher levels decrease. The range of energy levels and the region of the electromagnetic radiation that causes transitions between these levels is shown in Table 12.1

Table 12.1 Molecular Energy levels and the Regions of Electromagnetic Radiation