Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels. Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.

Figure 1: The three types of energy levels in a diatomic molecule.

A molecule can absorb energy in the form of a photon from electromagnetic radiation and use that energy to excite the molecule into a higher energy state. During this process, changes can occur in the rotation around a bond, the frequency of bond vibration, or the transition of an electron from its ground state (the lowest energy state) to an excited state (a higher energy level). When the excited-state molecule returns to its ground state, it emits radiation. The energy of the absorbed or emitted photon is equivalent to the energy gap between the two energy levels involved in the transition. Therefore, each transition is dependent on the wavelength or frequency of the radiation.

Due to the varying magnitudes of energy gaps, the wavelengths of radiation absorbed during these transitions differ. For example, the energy consumed or released during the transition of a specific rotation in a molecule is in the range of microwave radiation. In contrast, the energy of infrared radiation corresponds to changes in bond vibrations. Furthermore, photons in the UV–visible range can excite an electron to a different orbital, particularly in the case of molecules with conjugated double bonds.