Direct observation of the symmetric stretching modes of (A)over-tilde(1) A(u) acetylene by pulsed supersonic jet laser induced fluorescence

Rotational analyses are reported for the 2(0)(2) and 1(0)(1) bands of the (A) over tilde (1) A(u) <- (X) over tilde (1) Sigma(+)(g) transition of C2H2 near 45,000 cm(-1) (+2800 cm(-1) relative to T-0) from jet-cooled laser-induced fluorescence spectra. While the 220 band is unperturbed and straightforward to assign, the 1(1) level is strongly perturbed by interactions with the 2(1) B-2 polyad, where upsilon B’ = upsilon(4)’ + upsilon(6)’. 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 1(1)/2(1) B-2 interaction leads to the value v(1)’ = 2880.08 cm(-1) for the fundamental symmetric C-H stretching frequency. Assignments are also reported for the 2 3 and 1 1 2 1 levels, completing all assignments of levels containing excitation in only the totally symmetric vibrational modes up to + 4500 cm(-1). The reassignment of upsilon(1)’ 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.