Fast and exceptionally slow vibrational energy transfer in acetylene and phenylacetylene in solution

The excitation of the CH-stretching mode (ν₃) of C ₂H₂ in CCl₄ leads to fast population (<2 ps) and very slow depopulation (240 ps) of the symmetric CC-stretching mode (ν₂). The long lifetime of ν₂ is due to the absence of anharmonic interaction to neighboring states (symmetry forbidden). The corresponding CC-stretching mode of C₆H₅-C₂H has a lifetime of 15 ps. The phenyl group relaxes the symmetry restrictions and introduces new decay channels.