Photoionization of small neutral polycyclic aromatic hydrocarbons

Testing photoelectric models of interstellar dust

¹ Synchrotron SOLEIL, L’Orme des Merisiers, 91192 Gif sur Yvette Cedex, France
² Present address: LISA UMR 7583 Université Paris-Est Créteil and Université de Paris, Institut Pierre et Simon Laplace, 61 Avenue du Général de Gaulle, 94010 Créteil, France
³ Institut d’Astrophysique Spatiale (IAS), CNRS, Univ. Paris Sud / Université Paris-Saclay, 91405 Orsay, France
⁴ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, 91405 Orsay, France

^★ Corresponding author: hhrodmarsson@lisa.ipsl.fr

Received: 24 February 2025
Accepted: 30 April 2025

Abstract

Context. In the interstellar medium (ISM) and circumstellar environments, photoionization, or the photoelectric effect, emerges as a prevalent phenomenon. In regions exposed to UV photons, either stellar or secondary photons induced by cosmic rays, polycyclic aromatic hydrocarbons (PAHs) liberate electrons through the photoelectric effect, efficiently contributing to the heating budget of the gas. In contrast to shielded areas, neutral and cationic PAHs can scavenge electrons and retain a significant portion of the cloud’s electronic charge. This dual behavior of PAHs not only influences the gas thermal and dynamical behavior but also plays a pivotal role in shaping the chemistry of the environments.

Aims. Our aim was to measure the photoelectron kinetic energy distribution (KED) of PAHs of varying sizes, symmetries, and C:H ratios to describe the photoelectron KED with a law that can be implemented in astrophysical photoelectric models that describe gas heating.

Methods. We used a double-imaging photoelectron photoion coincidence spectrometer coupled with the DESIRS VUV beamline at the SOLEIL synchrotron to record the gas-phase spectra of a series of sublimated PAHs with different sizes and structures in the 13–20 C atom range. We then compared our data to current astrophysical dust photoelectric models used to describe the PAH charge and gas photoelectric heating in the ISM. In particular, we extended the Kimura 2016, (MNRAS, 459, 2751) model (eK16) to take into account the KED of the photoelectron and its interaction with the grain.

Results. We show that although subtle differences between the molecules in our dataset arise from individual electronic structures, the photoelectron KED of PAHs of different sizes and symmetry display remarkable similarities. A general law can thus be implemented in sophisticated ISM astrochemical models to describe their photoelectron KED behavior. We find that the eK16 photoelectric model closely reproduces the present photoionization cross sections of neutral, small PAHs as well as literature data obtained on cationic PAHs. It is noteworthy that the eK16 model, unlike former models, matches the absolute scale of the measured photoionization cross sections. We show that the eK16 model predicts a maximum photoelectric efficiency significantly lower than the previous models, implying a lower interstellar gas temperature and less emission.