Near-infrared laser spectroscopy of the Ar–C2HD complex:: Fermi resonance assisted vibrational predissociation

The rotationally resolved infrared spectrum is reported for the C–H stretching vibration of Ar–HCCD. A bending combination band is also observed from which we obtain an accurate value for the low-frequency intermolecular bending frequency. The vibrational predissociation lifetime of Ar–HCCD is found to be considerably longer than that of Ar–C2H2. This difference is attributed to the strong Fermi resonance in Ar–C2H2 which is absence in Ar–HCCD. We propose that the perturbing level in the Ar–C2H2 complex provides a `doorwayʹ that accelerates the dissociation in this system. In Ar–HCCD, where this `doorway stateʹ is not Fermi mixed, the dissociation is slower.