Changes in fluorescence induced by infrared multiphoton excitation of optically excited SO2

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Abstract
Photophysical properties of excited SO2 were studied by the UV+MPE technique: single‐photon UV excitation together with IR multiphoton excitation (MPE). Compared to the fluorescence in the absence of IR irradiation, the fluorescence following MPE showed a decrease of the main fluorescence peaks, accompanied by the appearance of broad fluorescence features to the high‐energy side of the excitation frequency. Different CO2 laser lines were about equally effective. The dependence on laser fluence can be accounted for quantitatively using a simple stepladder model for MPE. The cross section for absorption of CO2 laser radiation by SO2, excited to around 32 000 cm−1, was found to be σ = 1.5×10−21 cm2. The effective decay time of the fluorescence with IR‐on is increased twofold, compared to that with IR‐off. This increase seems to parallel the reported dependence of the lifetimes on excitation frequency. It may be interpreted as due to more efficient coupling of excited vibronic states with highly‐excited vibrational states in the ground electronic state.

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