Calculation of electric quadrupole linestrengths for diatomic molecules: Application to the H2, CO, HF and O2 molecules

Journal of Chemical Physics | November 2021

We present a unified variational treatment of the electric quadrupole (E2) matrix elements, Einstein coefficients, and line strengths for general open-shell diatomic molecules in the general purpose diatomic code Duo. Transformation relations between the Cartesian representation (typically used in electronic structure calculations) to the tensorial representation (required for spectroscopic applications) of the electric quadrupole moment components are derived. The implementation has been validated against accurate theoretical calculations and experimental measurements of quadrupole intensities of 1H2 available in the literature. We also present accurate electronic structure calculations of the electric quadrupole moment functions for the X 1Σ+ electronic states of CO and HF at the CCSD(T) and MRCI levels of theory, respectively, as well for the a 1Δg - b 1Σg+ quadrupole transition moment of of O2 with MRCI level of theory. Accurate infrared E2 line lists for 12C16O and 1H19F are provided. A demonstration of spectroscopic applications is presented by simulating E2 spectra for 12C16O, 1H19F and 16O2 (Noxon a 1Δg - b 1Σg+ band).

Clouds spoil UK's view of Saturn and Jupiter's 'great conjunction'

The Guardian | January 2021

This was a short interview I gave to the Guardian newspaper on the closest night of Saturn and Jupiter's historic conjunction.

The 2020 release of the ExoMol database

J. Quant. Spectrosc. Rad. Transf. | November 2020

The ExoMol database provides molecular data for studies of planetary and exoplanetary atmospheres, as well as a wide range of other research and industrial applications.

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. 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 High-Resolution Linelist for Molecular Oxygen

Europlanet Science Congress | September 2020

Molecular oxygen (O2) is of particular interest in exoplanetary observations not least of all as an important biosignature on habitable planets. In this work we develop and apply ab initio computational techniques to produce an accurate electric quadrupole spectrum of molecular oxygen presented for use in atmospheric retrievals across a range of temperatures, and made available through the ExoMol database.

Higher Order Transition Moments in the Spectra of Diatomic Molecules

ESA ARIEL SMC2020 | January 2020

The ARIEL space telescope is the European Space Agency's M4 mission in the ESA Cosmic Vision longterm plan. The launch is planned in 2029 by the Ariane 62 launch vehicle for a nominal four year mission to observe and characterise exoplanetary atmospheres.

We consider contributions from higher order transition moments. Principally the electric quadrupole and magnetic dipole moments - responsible for the infrared spectrum of many diatomic molecules - and also the electric polarisability, which generates Raman spectra.