Unravelling the electronic structure of the silicon dimer using threshold photoelectron spectroscopy

Abstract

The low-lying electronic states of silicon dimer (Si${}_2$) and its cation (Si${}_2^{+}$) have been studied by single-photon photoelectron spectroscopy combining a flow-tube reactor, vacuum-ultraviolet synchrotron radiation, and a double imaging photoelectron/photoion spectrometer. The energy range covered in this study (7.0−9.5 eV) allowed to observe several photoionising transitions involving the three lowest electronic states of Si${}_2$ (${\mathrm{X}}^3{\mathrm{\Sigma }}_{\mathrm{g}}^{-}$, ${\mathrm{D}}^3{\mathrm{\Pi }}_{\mathrm{u}}$, ${\mathrm{a}}^1{\mathrm{\Delta }}_{\mathrm{g}}$) and five of the six lowest states of Si${}_2^{+}$ (${\mathrm{X}}^{+4}{\mathrm{\Sigma }}_{\mathrm{g}}^{-}$, ${\mathrm{a}}^{+2}{\mathrm{\Pi }}_{\mathrm{u}}$, ${\mathrm{b}}^{+2}{\mathrm{\Delta }}_{\mathrm{g}}$, ${\mathrm{c}}^{+2}{\mathrm{\Sigma }}_{\mathrm{g}}^{-}$, and ${\mathrm{e}}^{+2}{\mathrm{\Pi }}_{\mathrm{u}}$). Using ab initio calculations and Franck-Condon simulations, several electronic transitions are identified which bring new elements in the description of the dense electronic landscapes of the silicon dimer and its cation. Interestingly, one of the most intense transitions is spin-forbidden (${\mathrm{X}}^{+4}{\mathrm{\Sigma }}_{\mathrm{g}}^{-}\leftarrow {\mathrm{a}}^1{\mathrm{\Delta }}_{\mathrm{g}}$) and is most probably observed through autoionisation processes by spin interactions.

GRAPHICAL ABSTRACT

Keywords:

• Threshold photoelectron
• silicon dimer
• flow-tube reactor
• radical production
• VUV spectroscopy

Acknowledgments

We are grateful to the whole staff of SOLEIL for running the facility. The authors acknowledge S. Pratt for usefull discussions. Ning L. Chen acknowledges the support from the Paris Ile-de-France Region (DIM ACAV+) for her Ph.D. grant.

Disclosure statement

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

Additional information

Funding

This work was performed on the DESIRS Beamline at SOLEIL synchrotron under Proposal No. 20200996. This work has also received financial support from the French Agence Nationale de la Recherche (ANR) under Grant No. ANR- 12-BS08-0020-02 (Project SYNCHROKIN). This work was supported by the Programme National Physique et Chimie du Milieu Interstellaire (PCMI) of CNRS/INSU with INC/INP cofunded by CEA and CNES.