Circa conservation of vibrational energy among three strongly coupled modes of a cyanine dye molecule studied by quantum-beat spectroscopy with a 7 fs laser

Real-time energy exchange among the three vibrational modes forming a triplet with frequencies ω1, ω2, and ω3 satisfying a ‘circa sum-frequency relation’, namely ω1 + ω2 ≅ ω3 in a cyanine dye molecule has been studied with a 7-fs pulse laser. There are two triplets with average frequencies of the components of (146, 279, 432 cm−1) and (279, 432, 738 cm−1). Even though the circa sum-frequency relation is satisfied, circa-conservation of the vibrational energy holds among the three components only in the latter triplet. The difference between the two is ascribed to the symmetry of the molecular vibrational modes.