The Astrochemistry Low-energy Electron Cross-Section (ALeCS) database

I. Semi-empirical electron-impact ionization cross-section calculations and ionization rates

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg 412 96, Sweden
e-mail: brandt.gaches@chalmers.se
² Center of Planetary Systems Habitability, The University of Texas at Austin, USA
³ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁴ Departamento de Físico-Química, Facultad de Ciencias Químicas, Universidad de Concepción, Concepción, Chile
⁵ Atomistic Simulations, Italian Institute of Technology, 16152 Genova, Italy
⁶ Department of Chemistry, University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, UK
⁷ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción Av. Esteban Iturra s/n Barrio Universitario, Casilla 160, Concepción, Chile
⁹ INAF–Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹⁰ Chemistry Department, Sapienza University of Rome, P.le A. Moro, 00185 Rome, Italy
¹¹ Department of Chemistry and Molecular Biology, University of Gothenburg, 41296 Gothenburg, Sweden

Received: 16 October 2023
Accepted: 8 February 2024

Abstract

Context. Electron–molecule interaction is a fundamental process in radiation-driven chemistry in space, from the interstellar medium to comets. Therefore, knowledge of interaction cross sections is key. There have been a plethora of both theoretical and experimental studies of total ionization cross sections spanning from diatomics to complex organics. However, the data are often spread over many sources or are not public or readily available.

Aims. We introduce the Astrochemistry Low-energy Electron cross-section (ALeCS) database. This is a public database for electron interaction cross sections and ionization rates for molecules of astrochemical interest. In particular, we present here the first data release, comprising total ionization cross sections and ionization rates for over 200 neutral molecules.

Methods. We include optimized geometries and molecular orbital energies at various levels of quantum chemistry theory. Furthermore, for a subset of the molecules, we have calculated ionization potentials. We computed the total ionization cross sections using the binary-encounter Bethe model and screening-corrected additivity rule, and we computed ionization rates and reaction network coefficients for molecular cloud environments.

Results. We present the cross sections and reaction rates for >200 neutral molecules ranging from diatomics to complex organics, with the largest being C₁₄H₁₀. We find that the screening-corrected additivity rule cross sections generally significantly overestimate experimental total ionization cross sections. We demonstrate that our binary-encounter Bethe cross sections agree well with experimental data. We show that the ionization rates scale roughly linearly with the number of constituent atoms in the molecule.

Conclusions. We introduce and describe the public ALeCS database. For the initial release, we include total ionization cross sections for >200 neutral molecules and several cations and anions calculated with different levels of quantum chemistry theory, the chemical reaction rates for the ionization, and network files in the formats of the two most popular astrochemical networks: the Kinetic Database for Astrochemistry, and UMIST. The database will be continuously updated for more molecules and interactions.