Multiphoton CO₂ laser vibrational heating of the triplet state of biacetyl

Multiple infrared photon excitation of the metastable triplet state of biacetyl is carried out by irradiating the sample with an intense CO₂ laser pulse following cutoff of the optical excitation at 4579 Å. Changes in the luminescence characteristics of the electronically excited molecules induced by the infrared pulse are studied in various visible spectral regions. In the region characteristic of the biacetyl phosphorescence, a fast decay of the emission is observed. A burst of delayed fluorescence, characterized by a very fast rise time, is observed in the short-wavelength emission region. Both the fluorescence and phosphorescence signals exhibit the same decay time, which is inversely proportional to the infrared fluence in the CO₂ laser beam. The experimental results are explained in terms of mixed singlet-triplet vibronic wave functions characterizing the triplet vibrational manifold, with vibrational energy exceeding the ¹A_u-³A _u separation. A calculation of the decay times as a function of the absorbed CO₂ laser energy, which is based on a recent model for delayed fluorescence due to Jortner and Kommandeur, gives good agreement with the experimental data. The possibility of multiphoton induced emission from vibrationally excited ground-state molecules is also considered.