Molecular hydrogen formation on interstellar PAHs through Eley-Rideal abstraction reactions

TL;DR

This study experimentally investigates how interstellar PAH cations form molecular hydrogen through Eley-Rideal abstraction reactions, revealing that abstraction plays a more significant role than previously believed.

Contribution

It provides the first detailed experimental analysis of H2 formation on gas-phase PAH cations via abstraction reactions, with quantitative modeling of cross sections.

Findings

Abstraction reactions contribute twice as much as attachment in H2 formation.

Sequential hydrogen attachment observed up to 23 H atoms.

H2 formation on PAHs is more significant than previously estimated.

Abstract

We present experimental data on H2 formation processes on gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by exposing coronene radical cations, confined in a radio-frequency ion trap, to gas phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been observed. Exposure to atomic D instead of H allows one to distinguish attachment from competing abstraction reactions, as the latter now leave a unique fingerprint in the measured mass spectra. Modeling of the experimental results using realistic cross sections and barriers for attachment and abstraction yield a 1:2 ratio of abstraction to attachment cross sections. The strong contribution of abstraction indicates that H2 formation on interstellar PAH cations is an order of magnitude more relevant than previously thought.