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 'doorway' that accelerates the dissociation in this system. In Ar-HCCD, where this 'doorway state' is not Fermi mixed, the dissociation is slower. (C) 1997 Elsevier Science B.V.