The infrared spectrum of the He-C2D2 complex

Area of Science:

• Molecular Spectroscopy
• Intermolecular Forces
• Quantum Chemistry

Background:

• Helium-acetylene (He-C2D2) is a weakly bound van der Waals complex.
• Its spectra are challenging to obtain due to its proximity to the free rotor limit.
• Previous studies provided limited assignments in specific spectral regions.

Purpose of the Study:

• To obtain new infrared spectra of the He-C2D2 complex in the nu3 fundamental band.
• To extend spectral analysis to previously unassigned P(1) and R(1) regions.
• To compare experimental energy levels with theoretical calculations from ab initio potential energy surfaces.

Main Methods:

• High-resolution infrared spectroscopy using a tunable optical parametric oscillator laser.
• Pulsed supersonic slit jet expansion from a cooled nozzle.
• Term value approach for experimental energy level determination.
• Ab initio calculations of intermolecular potential energy surfaces.

Main Results:

• New He-C2D2 infrared spectra were obtained and analyzed for the nu3 fundamental band.
• Analysis was extended to the weaker P(1) and R(1) spectral regions.
• Experimental energy levels were compared with theoretical predictions from two different potential energy surfaces, revealing small but significant discrepancies.
• Rovibrational energies for the related He-C2H2 complex were also computed.

Conclusions:

• The study provides a more comprehensive understanding of the He-C2D2 complex's energy levels and intermolecular interactions.
• Discrepancies between experimental data and theoretical models highlight areas for refinement in ab initio potential energy surfaces.
• A Coriolis model aids in interpreting spectral intensities and energy level patterns, with comparisons to similar complexes like He-CO.