Optical Spectrum and Photochemistry of Si2O2.

Abstract

Silicates and silica are the major components of interstellar silicon-based dust grains and mainly composed of silicon and oxygen. Information about their geometric, electronic, optical, and photochemical properties is crucial for developing astrochemical models describing dust grain formation. To this end, we characterize herein the optical spectrum of mass-selected Si₂O₂⁺ cations in the 295-709 nm range using electronic photodissociation (EPD). The EPD spectra are recorded in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser vaporization source and compared to complementary time-dependent density functional theory (TD-DFT) calculations at the UB3LYP-D3/aug-cc-pVQZ level of theory, determining structures, energies, electronic spectra, and fragmentation energies of the low-energy isomers. The EPD spectrum is observed in the lowest-energy fragmentation channel, corresponding to SiO⁺ + SiO. The high calculated dissociation threshold of D₀ = 4.60 eV (37,102 cm^-1) requires two-photon absorption for EPD. The three electronic transitions observed at 19,264, 25,667, and 32,216 cm^-1 are attributed to transitions from the doublet ground state of the most stable rhombic structure of Si₂O₂⁺ (D_2h, ²B_1u) into the first, fourth, and fifth excited doublet states, D₁(²A_g), D₄(²B_2g), and D₅(²B_3u), respectively. The resolved vibronic structure of the D₄ and D₅ state is analyzed by Franck-Condon Herzberg-Teller (FCHT) simulations to suggest vibrational assignments. The calculations indicate the reduction of symmetry from D_2h to C_2v in the D₄ state along the ν₄(b_1u) coordinate (resulting in a flat double minimum potential), while the dipole-forbidden D₅ state gains its vibronic intensity from HT coupling to the same ν₄ mode.