The multiphoton ionization spectrum of complexes of benzene and acetylene

Weakly bound complexes of benzene and acetylene were formed in a supersonic molecular beam and were studied using resonantly enhanced multiphoton ionization spectroscopy. Spectral features produced by different complexes were overlapped but were resolved using a time-of-flight mass spectrometer detector. The 1B2u ← 1A1g transition of benzene is forbidden, but the 0–0 band of this transition was observed in the benzene–acetylene complex, the transition being weakly allowed under the lower symmetry of the complex. Features near the 610 band of benzene were observed and assigned to two different geometric isomers of the benzene–acetylene complex. Features assigned to the mixed trimer benzene–(C2H2)2 were also observed. Analysis of the rotational band contours of the benzene–acetylene origin transition indicated that in that isomer, the acetylene is located above the plane of the benzene off of the sixfold axis. Vibrational predissociation of the neutral complex was observed when one of the isomers of φ-C2H2 was excited to the 6111 vibrational level (1440 cm−1 of vibrational energy) but was not observed when this isomer was excited to the 61 level (520 cm−1 of vibrational energy). The second isomer did not vibrationally predissociate from 6111 but did predissociate from 6112 (2365 cm−1 vibrational energy).