The high-temperature rotation-vibration spectrum and rotational clustering of silylene (SiH2)

Abstract

A rotation-vibration line list for the electronic ground state (X 1 A 1 ) of SiH 2 is presented. The line list, named CATS, is suitable for temperatures up to 2000 K and covers the wavenumber range 0-10 000 cm −1 (wavelengths > 1.0 µm) for states with rotational excitation up to J = 52. Over 310 million transitions between 593 804 energy levels have been computed variationally with a new empirically refined potential energy surface, determined by refining to 75 empirical term values with J ≤ 5 and a newly computed high-level ab initio dipole moment surface. This is the first, comprehensive high-temperature line list to be reported for SiH 2 and it is expected to aid the study of silylene in plasma physics, industrial processes and possible astronomical detection. Furthermore, we investigate the phenomenon of rotational energy level clustering in the spectrum of SiH 2 . The CATS line list is available from the ExoMol database (www.exomol.com) and the CDS database.

The 2020 release of the ExoMol database: Molecular line lists for exoplanet and other hot atmospheres

“…A new line list for SiH 2 [175] has been recently constructed in its ground electronic state using the standard ExoMol technology [93] . The set of experimental data or their quality were, however, very limited which may affect the quality of the hot bands, which basically has to rely on the quality of the ab initio PES.…”

Modelling the non-local thermodynamic equilibrium spectra of silylene (SiH2)

“…The calculated temperature dependent populations N J,k,n (T) in eqn (1) can be then combined with a molecular line list for SiH 2 to simulate absorption or emission spectra of this molecule under the non-LTE conditions as defined by N J,k,n (T) in eqn (1). Here we use the ExoMol line list CATS by Clark et al 72 as provided by ExoMol (www.exomol.com). Technically this is done by incorporating the non-LTE vibrational densities N into the ExoMol States file as described in Section 4 (the ExoMol file formats are discussed extensively elsewhere 73 ).…”

“…The independent treatment of the two stretching populations is partly justified by the local mode character of the vibrational degrees of freedom of SiH 2 due to the near 901 bond angle (see, e.g. Jensen 110 and Clark et al 72 ). The asymmetric vibrational modes of SiH 2 (B 2 in C 2v ) are non populated in this 1D approximation.…”

“…Recently we have computed an accurate ro-vibrational line list for SiH 2 , named CATS. 72 It covers a large range of rotational and vibrational excitation, capable of modelling very hot spectra of this molecule (up to T = 2000 K) as part of the ExoMol database. 73 The CATS line list was produced using the program TROVE, 74,75 which solves the nuclear motion Schro ¨dinger equation variationally.…”

“…The study by Clark et al 72 forms the basis for the present work, where we utilize the CATS line list, wavefunctions, purposebuilt numerical basis set, and the CATS computational TROVE setup to model non-LTE spectroscopic properties of SiH 2 produced from dissociation of Si 2 H 6 through different reaction channels. Using a simplified 1D Harmonic oscillator wavefunctions (see Pastorek et al 6 ) and more sophisticated 3D vibrational CATS wavefunctions from accurate variational calculations, the non-LTE ro-vibrational populations of SiH 2 are generated and used to produce non-LTE spectroscopic spectra of different dissociation channels of disilane.…”