New rovibrational subbands of Ar-D₂O complex in the D₂O bending mode region

Abstract

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 could provide a wealth of information on these intermolecular interactions. The precise experimental data can be used to test the accuracy and efficiency of various theoretical methods for constructing the intermolecular potential energy surfaces and calculating the bound states. In this work, the high-resolution infrared absorption spectrum of the Ar-D<sub>2</sub>O complex in the <em>v</em><sub>2</sub> bending region of D<sub>2</sub>O was measured using an external cavity quantum cascade laser. A segmented rapid-scan data acquisition method was employed. The Ar-D<sub>2</sub>O compex was generated in a slit supersonic jet expansion by passing Ar gas through a vessel containing liquid D<sub>2</sub>O. Four new rovibrational subbands are assigned in the spectral range of 1150-1190 cm<sup>-1</sup>, namely <img src="20221728_O_editing.files/image059.png" alt="" />，<img src="20221728_O_editing.files/image094.png" alt="" />，<img src="20221728_O_editing.files/image090.png" alt="" /> and <img src="20221728_O_editing.files/image091.png" alt="" />. The first two subbands belong to the <em>otho</em>- species of Ar-D<sub>2</sub>O, while the latter two belong to the <em>para</em>- 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<sup>-1</sup> for the infrared transitions was set in the global fitting when using Pickett’s program SPFIT, respectively. Molecular constants including vibrational substate energies, rotational and centrifugal distortion constants, and Coriolis coupling constant, are determined accurately. The previous results for the <img alt="" /> substate are found to be likely wrong. The energies of the <img src="20221728_O_editing.files/image081.png" alt="" />和<img alt="" />substates are determined experimentally for the first time. The band origin of Ar-D<sub>2</sub>O in the D<sub>2</sub>O <em>v</em><sub>2</sub> bending mode region is determined to be 1177.92144(13) cm<sup>-1</sup>, which is red-shifted from that of D<sub>2</sub>O monomer by about 0.458 cm<sup>-1</sup>. The experimental vibrational substate energies are compared with their theoretical values based on a four-dimensional intermolecular potential energy surface which includes the normal coordinate of the D<sub>2</sub>O <em>v</em><sub>2</sub> 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 the theoretical calculations still have the space to be improved.