Theoretical and experimental studies of the isotope effects for He-CO₂ and Ne-CO₂ complexes

In this work, we have studied the isotope effects for the He-CO₂ and Ne-CO₂ complexes by means of theoretical calculations and experimental measurements, which were carried out using a distributed quantum cascade laser to probe a pulsed supersonic jet expansion. Firstly, infrared spectra have been recorded for the He/Ne-¹²C¹⁸O₂ complexes. Spectroscopic parameters including band origin ν₀, rotational constants A, B, C, and centrifugal distortion constants Δ_JK were obtained by fitting a Watson A-reduced Hamiltonian with 13 assigned rovibrational transitions for He-¹²C¹⁸O₂. For Ne-¹²C¹⁸O₂, the observed spectrum produces a set of spectroscopic parameters including the band origin, rotational constants and all the quartic centrifugal distortion constants with more than 100 rovibrational transitions (40 new transitions). Secondly, we have calculated the rovibrational energy levels, vibrational shifts, and rotational constants for the He/Ne-CO₂ complexes based on potential energy surfaces (PESs) and bound state calculations for ground and vibrationally excited states. The obtained results show that the spectroscopic characteristics (vibrational shifts and rotational constants) for Ne-CO₂ are analogous to those of Ar-CO₂, while those for He-CO₂ show some differences especially for the rotational constants. Finally, according to the available experimental data and our theoretical calculations, infrared spectra were predicted for six isotopologues with C_2v symmetry of Ne-CO₂ complex.