Infrared spectroscopy and vibrational predissociation of C2H2–HF

The optothermal detection method has been used to obtain a rotationally resolved infrared spectrum of the ν1 band of C2H2–HF. This spectrum is characteristic of a parallel band of a slightly asymmetric rotor and has been completely assigned. A fit to the observed spectrum gives excited vibrational state rotational constants A1=1.130 20±0.0009 cm−1, B1=0.161 13±0.0002 cm−1, and C1=0.141 05±0.0002 cm−1. The fundamental vibrational frequency has also been accurately determined to be 3794.3646±0.004 cm−1. Intensity alternations resulting from the nuclear spin statistics clearly show that the molecule has a C2 axis, thus confirming the T-shaped geometry. The individual rotational lines are observed to be substantially broader (200 MHz FWHM) than the instrumental resolution (15 MHz FWHM). From these linewidths, the lifetime of the vibrationally excited state has been determined to be 0.8 ns. Stark spectra have also been obtained for the R(0) and P(1) transitions of the complex from which it has been possible to determine the dipole moment in the vibrationally excited state ( μ1=2.559 D±0.003 D).