Vibrational Dynamics of Electronic Excited Molecules

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Vibrational and reaction dynamics of electronic excited acetylene have been investigated.1. The vibrational level structure of acetylene in the electronic excited A^^〜 state was believed to be the progressions of the 2^n3^m vibrational states. However, in this work it was proved that the V_3' mode couples anharmonically with the ungerade v_4'/v_6' mode to form a polyad structure composed of the 2^m3^n, 2^m3^<n-1>(4/5)^2, 2^m3^<n-2>(4/6)^4, ………. The same type of the anharmonic coupling was found for the ungerade vibrational states which were investigated by an IR-UV double resonance spectroscopy. In this case, the (4/6) states having odd quantum numbers couple with the 3^n state.2. The predissociation rate constants of acetylene in respective rovibronic levels of the A^^〜 state were determined through observation of the spectral line-widths. The rate constants were found to be independent of the vibrational parity gerade or ungerade. This can be attributed to the anharmonic coupliig between the V_3' and V_4'/V_6' modes which causes the gerade and ungerade states to have a similar vibrational chracteristics.3. It was found that the triplet state-mediated predissociation of excited acetylene toward C_2H(X^^〜)+H ceases around the excitation energy of 51100 cm^<-1> which is far beyond the adiabatic dissociation threshold of the A state. At this energy, acetylene still predissociates at a rate of around 10^<10> s^<-1> to form C_2H(A^^〜) + H.4. A new experimental technique, Differential Temperature Laser Induced Fluorescence Spectroscopy, was developed which is capable of separating overlapping cold and hot bands of a LIF spectrum.