Photochemical properties of a potential interstellar dust precursor: the electronic spectrum of Si3O2+

Abstract

Silicon oxide compounds are considered as precursors for silicon-based interstellar dust grains which consist mainly of silica and silicates. Knowledge of their geometric, electronic, optical, and photochemical properties provides crucial input for astrochemical models describing the evolution of dust grains. Herein, we report the optical spectrum of mass-selected Si₃O₂⁺ cations recorded in the 234–709 nm range by means of electronic photodissociation (EPD) in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser vaporization source. The EPD spectrum is observed predominantly in the lowest-energy fragmentation channel corresponding to Si₂O⁺ (loss of SiO), while the higher-energy Si⁺ channel (loss of Si₂O₂) provides only a minor contribution. The EPD spectrum exhibits two weaker unresolved bands A and B near 26 490 and 34 250 cm⁻¹ (377.5 and 292 nm) and a strong transition C with a band origin at 36 914 cm⁻¹ (270.9 nm) which shows vibrational fine structure. Analysis of the EPD spectrum is guided by complementary time-dependent density functional theory (TD-DFT) calculations at the UCAM-B3LYP/cc-pVTZ and UB3LYP/cc-pVTZ levels to determine structures, energies, electronic spectra, and fragmentation energies of the lowest-energy isomers. The cyclic global minimum structure with C_2v symmetry determined previously by infrared spectroscopy can explain the EPD spectrum well, with assignments of bands A–C to transitions from the ²A₁ ground electronic state (D₀) into the 4th, 9th, and 11th excited doublet states (D_4,9,11), respectively. The vibronic fine structure of band C is analyzed by Franck–Condon simulations, which confirm the isomer assignment. Significantly, the presented EPD spectrum of Si₃O₂⁺ corresponds to the first optical spectrum of any polyatomic Si_nO_m⁺ cation.