# Optical Spectrum and Photochemistry of Si2O2+

## TL;DR

This study investigates the optical spectrum and photochemistry of Si2O2+ ions to better understand silicon-based dust grains in space.

## Contribution

The paper reports the first experimental and theoretical characterization of the optical spectrum of Si2O2+ cations.

## Key findings

- The EPD spectrum corresponds to fragmentation into SiO+ + SiO.
- Three electronic transitions are identified and attributed to specific excited states.
- Vibronic structures of D4 and D5 states are analyzed using FCHT simulations.

## 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+ (D2h, 2B1u) into the first, fourth,
and fifth excited doublet states, D1(2Ag), D4(2B2g), and D5(2B3u), 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 D2h to C2v in the
D4 state along the ν4(b1u)
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.