With reference to absorption spectroscopy,we deal herewith photon absorption by electrons
distributed within specific orbitals in a population of molecules. Upon absorption, one
electron reaches an upper vacant orbital of higher energy. Thus, light absorption would
induce the molecule excitation. Transition from ground to excited state is accompanied
by a redistribution of an electronic cloud within the molecular orbitals. This condition
is implicit for transitions to occur. According to the Franck–Condon principle, electronic
transitions are so fast that they occur without any change in nuclei position, that is, nuclei
have no time to move during electronic transition. For this reason, electronic transitions
are always drawn as vertical lines.
The energy of a pair of atoms as a function of the distance separating them is given by
the Morse curve. Re is the equilibrium bond distance. At this distance, the
molecule is in its most stable position, and so its energy is called the molecular equilibrium
energy, which is expressed as E0 or Ee. Stretching or compressing the bond induces an
energy increase. On the left-hand side of Re, the two atoms become increasingly closer,
inducing repulsion forces. Thus, an energy increase will be observed as a consequence of
these repulsion forces. On the right-hand side of Re, the distance between the two atoms
increases, and there will be attraction forces so that an equilibrium distance can be reached.
Thus, an energy increase will be observed as result of the attraction forces. In principle, a
harmonic oscillation should be obtained, but this is not the case. In fact, beyond a certain
distance between the two atoms, the attraction forces will exert no more influence, and
attraction energy will reach a plateau. Therefore, the Morse curve is anharmonic.