Photodissociation dynamics of C2H2 at 193 nm: Vibrational distributions of the CCH radical and the rotational state distribution of the Ã(010) state by time-resolved Fourier transform infrared emission

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Abstract
Time-resolved Fourier transform infrared (FTIR) emission is used to study the formation of CCH in the photodissociation of C2H2 at 193 nm. Excitation of C2H2 at 193 nm is known to populate the 10ν3 level of the trans–bent electronically excited state of acetylene, which undergoes decomposition. State-resolved infrared emission is obtained from the CCH radicals that are produced. Only vibronic levels which originate or borrow oscillator strength from the low-lying electronically excited state of CCH, Ã 2Π, are observed in this study. The relative intensities of these bands are measured and the rotational state distribution for the Ã(010) state is obtained. The observed average rotational energy of CCH is ∼156 cm−1, which is less than the average rotational energy of the acetylene precursor. A kinematic model which can account for a rotational cooling effect in the Ã(010) state is described. The model incorporates the angular momentum generated by the impulsive kick due to the release of the H atom and the cooling due to the orbital angular momentum carried away by the tangential velocity of the H atom, which is imparted by the initial C2H2 rotation.