Knowledge Bank

The infrared spectrum of the argon---acetylene van der Waals complex has been re-investigated in the three micron wavelength region. Two bands, corresponding to the $v3$ and $v2+v4+v5$ Fermi diad of the monomer, have been recorded. The infrared data were combined with recently published ground state microwave data in a least squares analysis using the Watson S reduction of the asymmetric top Hamiltonian. Some general features of the rotational constants obtained include: very large, positive inertial defects; a large decrease in A on going from the ground to the excited vibrational states; and inverted K=2 asymmetry doublels in the excited states. This last feature, where the $J2,J-1$ energy levels lie above the $J2,J-2$ levels, is caused by $d2=-0.18$ MHz. All of these features are related to large amplitude motions, which are being analyzed in a dynamic model which couples internal rotation of the HCCH monomer with the overall rotation of the complex.