Experimental Studies for structure and Isomerization Dynamics of Triplet Acetylene

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The triplet states of normal and deuteride acetylene were generated by mercury photo-sensitized techniques and were detected by using near infrared diode laser spectroscopy. A thousand of absorption lines were observed in Doppler limited resolution in the 7300-7500 cm^-^1 and were assigned to the electronic transition from a ^3B_2 to d ^3A_2 state. The intensity alternation due to two equivalent protons or deuterons appeared in the asymmetry (K-)doubling clearly showed that the structure of the triplet acetylene is cis-bent. The band origin and rotational constants agrees well to a previous low resolution study and recent theoretical results. Spin rotation and spin-spin interaction constants determined are consistent among C_2H_2. C_21-D and C_2D_2.A kinetic future of the triplet state was also studied in order to reveal the interaction between the triplet and singlet states. The transient absorption profiles along the time axis were measured by using diode laser kinetic spectroscopy combined with a pulsed mercury photo-sensitized method. The lifetime at the collision free condition was extrapolated by Stern-Volmer analysis. The natural lifetime of a ^3B_2 state of C_2H_2 is about 4xl0^-^5 second. which is independent to the rotational states and spin states(F_1.F_2, F_3). However, the lifetime of C_2D_2 is three times longer than that of C_2H_2. These results would be a key point for understanding the internal conversion mechanism from the bent triplet excited state to the linear singlet ground state.