Photoexcitation Dynamics of Vibrationally Excited Molecules

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Excited state dynamics of rovibronically selected acetylene has been investigated by means of the laser-induced fluorescence(LIF) and the dispersed fluorescence(DF) spectroscopies.When acetylene in the electronic ground state(X^^-) is phtoexcited to levels in the electronic excited state(A^^-), most intense spectral transitions form a progression of trans-GH bending mode V^n(n=4-6). In the vicinity of the V^6 level in the A state, two near-lying levels were found to exist 4.5cm^<-1> apart. The DF spectra from these two levels were found to show signiicantly different spectral patterns : One DF spectrum is characterized by the trans-CH bending and CC stretching progressions, while the other one the cis-CH bending. This fact was interpreted as that the zero-order V^6 level interacts with a dark level including the cis-CH bending mode through anharmonic resonance, and thus, it is concluded that these two levels are located above the trans-cis isomerization barrier.The IR-UV double resonance spectroscopy was applied to observe the ungerade vibronic levels of acetylene in the A state to which one photon transition is forbidden from the vibronic ground level. The observed rotational line structure indicates a strong coupling between the cis in-plane CH bending(v^<S'>_6) and the out-of-plane CH bending(v^'_4) modes, though some unassignable lines remain to be discussed in terms of another type of interactions.