Optical Spectroscopy and Photochemistry of Silicon Oxide Cations: The Case of Triatomic Si₂O⁺ and SiO₂⁺

Small silicon oxide compounds are considered crucial in the formation and evolution of stardust, particularly particles with silica and silicate cores. Thus, detailed information on the geometry, energy, electronic structure, reactivity, and photochemistry of small silicon oxides is essential for unraveling the fundamental mechanisms involved in the production and processing of stardust. Herein, the optical spectra of size-selected triatomic Si2O+ and SiO2+ cations are obtained in the range 289.9–709.4 nm (1.75–4.28 eV, 14,100–34,500 cm–1) by means of electronic photodissociation (EPD) in a tandem mass spectrometer coupled to a laser vaporization source. The EPD spectra are assigned by comparison to density functional theory calculations. The EPD spectrum of Si2O+ observed in the lowest-energy Si+ fragment ion channel is characterized by two band systems A and B with maxima observed at 25,202(5) and 30,609(5) cm–1. Bands A and B are assigned to transitions into the excited D3(2B2) and D6(2B2) doublet electronic states of the bent isomer II with C2v symmetry. Resolved vibronic structure of band B is attributed to anharmonic progressions of the symmetric stretching and bending modes, ω1 = 707(2) and ω2 = 804(3) cm–1. The predicted more stable linear isomer I with D∞h symmetry (ΔE0 = 0.23 eV) does not have any allowed transition expected in the spectral range investigated and is not observed. In line with the computational prediction, the EPD spectra measured for linear SiO2+ do not reveal any electronic transition, because of its vanishing absorption cross section in the considered spectral range.