Characterization of S₂O and S₂O^- by Vibrational and Photoelectron Spectra Using a Quantum Mechanical Approach

In this work, accurate potential energy surfaces (PESs) of the neutral molecule S₂O(X̃¹A') and the anion S₂O^-(X̃²A″) were constructed using high-level explicitly correlated CCSD(T)-F12 and MRCI-F12+Q methods with the cc-pCVQZ-F12 basis set, in which the PESs were represented by the neural network approach. Based on the PESs, we investigated the vibrational spectra of S₂O(X̃¹A') and S₂O^-(X̃²A″), the photoelectron spectrum of the S₂O^-(X̃²A″) anion with hot bands, and associated isotope effects involving ³²S, ³³S, ³⁴S, ³⁶S, ¹⁶O, and ¹⁸O via a rigorous quantum mechanical approach. It was found that the calculated isotope ratios of S₂O(X̃¹A')/S₂O^-(X̃²A″) and the photoelectron spectrum for ³²S₂¹⁶O^- exhibit excellent agreement with experimental results. Associated isotope effects in the photoelectron spectrum are observed to be notably small, manifested by minor shifts in both peak positions and peak intensities. Vibrational wave function analysis reveals that the photoelectron spectrum of S₂O^-(X̃²A″) is dominated by a strong vibrational progression, 3₀ⁿ, corresponding to the S-S stretching vibrational mode. Relatively weaker progressions are assigned to coupled vibrational states involving v₃ (S-S stretching) and v₁ (S-O stretching) modes, namely, the 3₀ⁿ1₀¹ and 3₀ⁿ1₀² progressions. The mode specificity in the photoelectron spectrum of S₂O^-(X̃²A″) is qualitatively consistent with the relative amplitudes of the normal coordinate displacement (ΔQ) for vibrational modes during the ionization process.