The product translational energy distribution P(ET) for acetylene photodissociation at 193 nm was obtained from the time-of-flight spectrum of the H atom fragments. The P(ET) shows resolved structure from the vibrational and electronic excitation of the C2H fragment; comparison of the translational energy release for given excited states of C2H with the known energy levels of these states gives D0(HCC–H)=131.4±0.5 kcal/mol. This value is in agreement with that determined previously in this group from analogous studies of the C2H fragment and with the latest experimental and theoretical work. The high resolution of the experiment also reveals the nature of C2H internal excitation. A significant fraction of the H atoms detected at moderate laser power were from the secondary dissociation of C2H. The P(ET) derived for this channel indicates that most of the C2 is produced in excited electronic states.
When expansion-cooled acetylene is excited to the ν″1+3ν″3 vibrational level (4 quanta of CH-stretch) and then photodissociated at 248.3 nm, the dominant product channel is C2H(Ã 2Π). This differs markedly from one-photon 193.3 nm photodissociation, which provides 1200 cm−1 less energy and yields C2H(X̃ 2Σ+) as the primary product. Photodissociation at 121.6 nm yields C2H(Ã 2Π) exclusively.
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