AbstractThe vibrational structures of photoelectron spectra can reveal the bonding nature of molecular orbitals. For diatomic molecules the bonding nature of molecular orbitals can be rationalized by the concepts of non‐bonding lone pair type and bonding type bond orbitals consisting of Lewis structures.For the simplest polyatomic molecules XY2the concepts of nonbonding lone pair type and bonding type bond orbitals and the method of expanding the canonical molecular orbitals into the bond orbitals are not quite appropriate.Two correlation diagrams, the distance correlation diagram and the angle correlation diagram, provide us with a better theoretical tool. The molecular orbital with the energy curve having positive slope in the distance correlation diagram (or angle correlation diagram) is identified as the R‐bonding (or 9‐bonding) orbital. The removal of an electron from the R‐bonding (or θ‐bonding) gives rise to vibrational excitation with decreasing frequency for the bond stretching (or bending) mode in the photoelectr