Search

In this paper we provide a highly accurate value for the binding energy of benzene to proton-ordered crystalline water ice (XIh), as a model for interstellar ices. We compare our computed value to the latest experimental data available from temperature programmed desorption (TPD) experiments and find that our binding energy value agrees well with data obtained from binding to either crystalline or amorphous ice. Importantly, our new value is lower than that used in most astrochemical networks by about nearly half its value. We explore the impact of this revised binding energy value for both an AGB outflow and a protoplanetary disk. We find that the lower value of the binding energy predicted here compared with values used in the literature (4050 K versus 7587 K) leads to less depletion of gas-phase benzene in an AGB outflow, and leads to a shift outwards in the benzene snowline in the midplane of a protoplanetary disk. Using this new value, the AGB model predicts lower abundances of benzene in the solid phase throughout the outflow. The disk model also predicts a larger reservoir of gas-phase benzene in the inner disk, which is consistent with the recent detections of benzene for the first time in protoplanetary disks with JWST.

A new silicon monoxide ($^{28}$Si$^{16}$O) line list covering infrared, visible and ultraviolet regions called SiOUVenIR is presented. This line list extends the infrared EBJT ExoMol line list by including vibronic transitions to the $A\,{}^{1}\Pi$ and $E\,{}^{1}\Sigma^{+}$ electronic states. Strong perturbations to the $A\,{}^{1}\Pi$ band system are accurately modelled through the treatment of 6 dark electronic states: $C\,{}^{1}\Sigma^{-}$, $D\,{}^{1}\Delta$, $a\,{}^{3}\Sigma^{+}$, $b\,{}^{3}\Pi$, $e\,{}^{3}\Sigma^{-}$ and $d\,{}^{3}\Delta$. Along with the $X\,{}^{1}\Sigma^{+}$ ground state, these 9 electronic states were used to build a comprehensive spectroscopic model of SiO using a combination of empirical and ab initio curves, including the potential energy (PE), spin-orbit (SO), electronic angular momentum (EAM) and (transition) dipole moment curves. The ab initio PE and coupling curves, computed at the multireference configuration interaction (MRCI) level of theory, were refined by fitting their analytical representations to 2617 experimentally derived SiO energy levels determined from 97 vibronic bands belonging to the $X$-$X$, $E$-$X$ and $A$-$X$ electronic systems through the MARVEL procedure. 112 observed forbidden transitions from the $C$-$X$, $D$-$X$, $e$-$X$, and $d$-$X$ bands were assigned using our predictions, and these could be fed back into the MARVEL procedure. The SiOUVenIR line list was computed using published ab initio transition dipole moments for the $E$-$X$ and $A$-$X$ bands; the line list is suitable for temperatures up to 10,000 K and for wavelengths longer than 140 nm. SiOUVenIR is available from www.exomol.com and the CDS database.

The rovibronic (rotation-vibration-electronic) spectrum of the calcium monohydroxide radical (CaOH) is of interest to studies of exoplanet atmospheres and ultracold molecules. Here, we theoretically investigate the $\tilde{A}\,^2\Pi$--$\tilde{X}\,^2\Sigma^+$ band system of CaOH using high-level \textit{ab initio} theory and variational nuclear motion calculations. New potential energy surfaces (PESs) are constructed for the $\tilde{X}\,^2\Sigma^+$ and $\tilde{A}\,^2\Pi$ electronic states along with $\tilde{A}$--$\tilde{X}$ transition dipole moment surfaces (DMSs). For the ground $\tilde{X}\,^2\Sigma^+$ state, a published high-level \textit{ab initio} PES is empirically refined to all available experimental rovibrational energy levels up to $J=15.5$, reproducing the observed term values with a root-mean-square (rms) error of 0.06~cm$^{-1}$. Large-scale multireference configuration interaction (MRCI) calculations using quintuple-zeta quality basis sets are employed to generate the $\tilde{A}\,^2\Pi$ state PESs and $\tilde{A}$--$\tilde{X}$ DMSs. Variational calculations consider both Renner-Teller and spin-orbit coupling effects, which are essential for a correct description of the spectrum of CaOH. Computed rovibronic energy levels of the $\tilde{A}\,^2\Pi$ state, line list calculations up to $J=125.5$, and an analysis of Renner-Teller splittings in the $\nu_2$ bending mode of CaOH are discussed.

We present a hybrid CCSD(T)+PBE-D3 approach to calculating the vibrational signatures for gas phase benzene and benzene adsorbed on an ordered water-ice surface. We compare the results of our method against experimentally recorded spectra and calculations performed using PBE-D3-only approaches (harmonic and anharmonic). Calculations use a proton ordered XIh water-ice surface consisting of 288 water molecules, and results are compared against experimental spectra recorded for an ASW ice surface. We show the importance of including a water ice surface into spectroscopic calculations, owing to the resulting differences in vibrational modes, frequencies and intensities of transitions seen in the IR spectrum. The overall intensity pattern shifts from a dominating $\nu_{11}$ band in the gas-phase to several high-intensity carriers for an IR spectrum of adsorbed benzene. When used for adsorbed benzene, the hybrid approach presented here achieves an RMSD for IR active modes of 21~cm$^{-1}$, compared to 72~cm$^{-1}$ and 49~cm$^{-1}$ for the anharmonic and harmonic PBE-D3 approaches, respectively. Our hybrid model for gaseous benzene also achieves the best results when compared to experiment, with an RMSD for IR active modes of 24~cm$^{-1}$, compared to 55~cm$^{-1}$ and 31~cm$^{-1}$ for the anharmonic and harmonic PBE-D3 approaches, respectively. To facilitate assignment, we generate and provide a correspondence graph between the normal modes of the gaseous and adsorbed benzene molecules. Finally, we calculate the frequency shifts, $\Delta\nu$, of adsorbed benzene relative to its gas phase to highlight the effects of surface interactions on vibrational bands and evaluate the suitability of our chosen dispersion-corrected density functional theory.

This paper sets out a robust methodology for modelling spectra of polyatomic molecules produced in reactive or dissociative environments, with vibrational populations outside local thermal equilibrium (LTE). The methodology is based on accurate, extensive ro-vibrational line lists containing transitions with high vibrational excitations and relies on the detailed ro-vibrational assignments. The developed methodology is applied to model non-LTE IR and visible spectra of silylene (SiH$_2$) produced in a decomposition of disilane (Si$_2$H$_6$), a reaction of technological importance. Two approaches for non-LTE vibrational populations of the product SiH$_2$ are introduced: a simplistic 1D approach based on the Harmonic approximation and a full 3D model incorporating accurate vibrational wavefunctions of SiH$_2$ computed variationally with the TROVE (Theoretical ROVibrational Energy) program. We show how their non-LTE spectral signatures can be used to trace different reaction channels of molecular dissociations.

The ExoMol database (www.exomol.com) provides molecular data for spectroscopic studies of hot atmospheres. While the data is intended for studies of exoplanets and other astronomical bodies, the dataset is widely applicable. The basic form of the database is extensive line lists; these are supplemented with partition functions, state lifetimes, cooling functions, Land\'e g-factors, temperature-dependent cross sections, opacities, pressure broadening parameters, $k$-coefficients and dipoles. This paper presents the latest release of the database which has been expanded to consider 80 molecules and 190 isotopologues totaling over 700 billion transitions. While the spectroscopic data is concentrated at infrared and visible wavelengths, ultraviolet transitions are being increasingly considered in response to requests from observers. The core of the database comes from the ExoMol project which primarily uses theoretical methods, albeit usually fine-tuned to reproduce laboratory spectra, to generate very extensive line lists for studies of hot bodies. The data has recently been supplemented by line lists deriving from direct laboratory observations, albeit usually with the use of ab initio transition intensities. A major push in the new release is towards accurate characterisation of transition frequencies for use in high resolution studies of exoplanets and other bodies.

A rotation-vibration line list for the electronic ground state ($\tilde{X}^{1}A_{1}$) 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 $>1.0$ $\mu$m) 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 $J\leq 5$ 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.