Direct observation of the symmetric stretching modes of à 1 A u acetylene by pulsed supersonic jet laser induced fluorescence

Abstract

Rotational analyses are reported for the and bands of the transition of C2H2 near 45,000 cm−1 (+2800 cm−1 relative to T 0) from jet-cooled laser-induced fluorescence spectra. While the band is unperturbed and straightforward to assign, the 11 level is strongly perturbed by interactions with the 21 B 2 polyad, where υ B ′ = υ4′ + υ6′. In order to assign the lines of this band, a population-labelling technique was used, employing an infrared laser to deplete the population in selected ground state rotational levels before probing with the ultraviolet laser. Deperturbation of the 11/21 B 2 interaction leads to the value cm−1 for the fundamental symmetric C–H stretching frequency. Assignments are also reported for the 23 and 1121 levels, completing all assignments of levels containing excitation in only the totally symmetric vibrational modes up to +4500 cm−1. The reassignment of implies that some of currently accepted assignments above 47,000 cm−1 are in error and suggests that the interpretation of some aspects of the near-threshold photodissociation measurements of Mordaunt et al. [J. Chem. Phys. 108, 519 (1998)] may need to be revisited.