Infrared spectra of Ne-C2D2 are observed in the region of the ?3 fundamental band (asymmetric C-D stretch, ∼2440 cm-1) using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion from a cooled nozzle. Like helium-acetylene, this system lies close to the free rotor limit, making analysis tricky because stronger transitions tend to pile up close to monomer (C2D2) rotation-vibration transitions. Assignments are aided by predicted rotational energies calculated from a published ab initio intermolecular potential energy surface. The analysis extends up to the j = 3 ← 2 band, where j labels C2D2 rotation within the dimer, and is much more complete than the limited infrared assignments previously reported for Ne-C2H2 and Ne-C2HD. Two previous microwave transitions within the j = 1 state of Ne-C2D2 are reassigned. Coriolis model fits to the theoretical levels and to the spectrum are compared. Since the variations observed as a function of C2D2 vibrational excitation are comp able to those noted between theory and experiment, it is evident that more detailed testing of theory will require vibrational averaging over the acetylene intramolecular modes.
The Journal of Chemical Physics143, no. 20, 204307 (25 November 2015).