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

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 C2v 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 (D4,9,11), 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 SinOm+ cation.

The Electronic Spectrum of Si2.

The optical spectrum of Si2+ is presented. The two electronic band systems observed near 430 and 270 nm correspond to the two lowest optically allowed transitions of Si2+ assigned to 4Σu-(I) ← X4Σg- and 4Σu-(II) ← X4Σg-. The spectra were measured via photodissociation spectroscopy of mass-selected ions at the level of vibrational resolution, and the determined spectroscopic constants provide detailed information about the geometric and electronic structure, establishing molecular constants of this fundamental diatomic cation that enable astrophysical detection on, for example, hot rocky super-Earth-like exoplanets.

Near-Infrared Spectrum of the First Excited State of Au2.

Au2 + is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au2 + ground state (X2 Σg + ) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A2 Σu + ). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar-tagged Au2 + cations. This exceptionally low-lying and well isolated A2 Σ(u) + ←X2 Σ(g) + transition occurs in the near-infrared range. The observed band origin (5738 cm-1 , 1742.9 nm, 0.711 eV) and harmonic Au-Au and Au-Ar stretch frequencies (201 and 133 cm-1 ) agree surprisingly well with those predicted by standard time-dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au2 + , because the Au2 + ⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au-Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σu * orbital (HOMO-1) into the bonding σg orbital (SOMO), the Au-Au bond contracts substantially (by 0.1 Å).