Dynamics of triplet states in beam-isolated benzaldehyde

The radiative and nonradiative dynamics of benzaldehyde in the 0-3750-cm^-1 energy interval above the T₁ origin were explored. The relative radiative rates were extracted by combining two different simultaneous measurements: laser-induced phosphorescence (LIP) and surface ejection of electrons by laser excited metastables (SEELEM). The ratio of the LIP signal and the SEELEM signal is proportional to the pure radiative rate. The details of the dynamics were rationalized as a development in the coupling of three electronic states. The T₁(³nπ*)-T₂( ³ππ*) vibronic coupling increases regularly in the region between 400 and ≃2000 cm^-1 of excess vibrational energy above the T₁ origin. Starting from the excess energy of the S₁ origin (1730 cm^-1), the S₁ excitations are coupled to the sparse T₁-T₂ background. We have shown that the line profile and dynamics of the S₁ manifold in the 1730-2500-cm^-1 region can be described in terms of the interaction of an optically active state with a background of almost dark states, in accordance with the theory of radiationless transitions. From our spectroscopic data we have estimated the density of coupled states to be about 200 states/cm^-1 and the coupling strength to be 0.05 cm^-1. The S₁ content in the coupled states is gradually diluted in the 1730-2500-cm^-1 region due to the congestion of the triplet states. Finally, the T₂ character is eroded probably due to the larger density of T₁ states. The T₁-T₂ and the S₁-T₁ coupling mechanisms were studied.