Structure and photochemistry of a potential precursor of circumstellar dust: The optical spectrum of Si4C2+

Abstract

Mixed silicon carbide clusters are important components of circumstellar shells and their optical properties are fundamental for modelling the chemistry involved in the evolution of dust particles. Herein, we report the ultraviolet photodissociation (UVPD) spectrum of mass-selected Si4C2+ cations obtained in the 208–330 nm range (3.8–6.0 eV) in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser desorption source operating with a SiC rod and a broadly tuneable optical parametric oscillator laser. The UVPD spectrum is observed in the two lowest-energy fragment channels, corresponding to dominant loss of atomic Si at low energy and the rather stable Si2C molecule at high energy. The UVPD spectrum is analyzed by quantum chemical calculations at two levels of density functional theory (CAM-B3LYP, M11) to determine the structures, energies, electronic spectra, and fragmentation energies of the lowest-energy isomers. To locate these isomers on the multidimensional potential energy surface, basin hopping is employed as global optimization approach. The UVPD spectrum is consistent with the optical absorption spectrum computed for the two lowest-energy isomers, one of which is reported for the first time.