High-resolution anion photoelectron spectroscopy of cryogenically cooled 4-atom silicon carbides

Abstract

High-resolution anion photoelectron spectra of cryogenically cooled ${\mathrm{Si}}_3{\mathrm{C}}^{-}$, $\text{S}{\text{i}}_2{\text{C}}_2^{-}$, and ${\text{SiC}}_3^{-}$ obtained using slow photoelectron velocity-map imaging (cryo-SEVI) are presented, providing insight into the geometries, energetics, and vibronic structure of the anionic and the neutral clusters. These spectra yield accurate vibrational frequencies for the neutral clusters. They also yield refined adiabatic detachment energies (ADEs) for the ground states of ${\mathrm{Si}}_3{\mathrm{C}}^{-}$ and $\text{S}{\text{i}}_2{\text{C}}_2^{-}$ of 1.5374(6) eV and 1.9019(4) eV, respectively, while the ADE of a low-lying isomer of ${\text{SiC}}_3^{-}$ is found to be 1.9050(7) eV. The cryo-SEVI spectra show that the ground state of $\text{S}{\text{i}}_2{\text{C}}_2^{-}$ is a distorted trapezoid, and represent the first confirmation of the distorted trapezoid structure of ${\mathrm{Si}}_2{\mathrm{C}}_2$, the only low-lying isomer of this cluster with a permanent dipole moment. Additional transitions are observed from two low-lying anion isomers: a linear structure and a rhombus. The spectrum of ${\text{SiC}}_3^{-}$, in combination with electronic structure calculations, suggests that the true ground state of $\mathrm{Si}{\mathrm{C}}_3$ is a ring structure with a transannular C–C bond, addressing a longstanding controversy surrounding this cluster. All three spectra exhibit Franck-Condon forbidden transitions; these are attributed to Herzberg-Teller coupling in ${\mathrm{Si}}_3\mathrm{C}$ and $\mathrm{Si}{\mathrm{C}}_3$ and autodetachment from an excited electronic state of $\text{S}{\text{i}}_2{\text{C}}_2^{-}$.

GRAPHICAL ABSTRACT

KEYWORDS:

• Photoelectron spectroscopy
• ion spectroscopy
• clusters

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Acknowledgments

M.C.B. thanks the Army Research Office for a National Defense Science and Engineering Graduate fellowship. M.C.B. acknowledges NIH Grant No S10OD023532 for funding the computational facilities used in this work. We would like to thank Luke Bertels for productive conversations regarding ${\mathrm{Si}}_2{\mathrm{C}}_2$.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research is funded by the Air Force Office of Scientific Research under grant number FA9550-19-1-0051.