New rovibrational subbands of Ar-D<sub>2</sub>O complex in the D<sub>2</sub>O bending mode region

The intermolecular interactions involving the water molecule play important roles in many fields of physics, chemistry, and biology. High-resolution spectroscopy of Van der Waals complexes formed by a rare gas atom and a water molecule can provide a wealth of information about these intermolecular interactions. The precise experimental data can be used to test the accuracies and efficiencies of various theoretical methods of constructing the intermolecular potential energy surfaces and calculating the bound states. In this work, the high-resolution infrared absorption spectrum of the Ar-D₂O complex in the v₂ bending region of D₂O is measured by using an external cavity quantum cascade laser. A segmented rapid-scan data acquisition method is employed. The Ar-D₂O complex is generated in a slit supersonic jet expansion by passing Ar gas through a vessel containing liquid D₂O. Four new rovibrational subbands are assigned in the spectral range of 1150–1190 cm^–1, namely \begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right) \leftarrow \Sigma \left( {{1_{11}}} \right)$\end{document}, \begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right) \leftarrow \Pi \left( {{1_{11}}} \right)$\end{document}, \begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right) \leftarrow \Pi \left( {{1_{10}}} \right)$\end{document} and \begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right) $\end{document}\begin{document}$\leftarrow \Pi \left( {{1_{01}}} \right) $\end{document}. The first two subbands belong to the otho- species of Ar-D₂O, while the latter two belong to the para- species. The observed rovibrational transitions together with the previously reported pure rotational spectra having the common lower vibrational sub-states are analyzed by a weighted least-squares fitting using a pseudo-diatomic effective Hamiltonian. An experimental error of 10 kHz for the far-infrared transitions and 0.001 cm^–1 for the infrared transitions are set in the global fitting when using Pickett’s program SPFIT, respectively. The molecular constants including vibrational substate energy, rotational and centrifugal distortion constants, and Coriolis coupling constant, are determined accurately. The previous results for the \begin{document}$\Pi \left( {{1_{11}}, {v_2} = 0} \right)$\end{document} substate are found to be likely incorrect. The energy of the \begin{document}$\Sigma \left( {{0_{00}}, {v_2} = 1} \right)$\end{document}and \begin{document}$\Sigma \left( {{1_{01}}, {v_2} = 1} \right)$\end{document}substates are determined experimentally for the first time. The band origin of Ar-D₂O in the D₂O v₂ bending mode region is determined to be 1177.92144(13) cm^–1, which is a red shift about 0.458 cm^–1 compared with the head of D₂O monomer. The experimental vibrational substate energy is compared with its theoretical value based on a four-dimensional intermolecular potential energy surface which includes the normal coordinate of the D₂O v₂ bending mode. The experimental and theoretical results are in good agreement with each other. But the calculated energy levels are generally higher than the experimental values, so, there is still much room for improving the theoretical calculations.