Impact Ionization Mass Spectra of Polypyrrole-Coated Anthracene Microparticles: A Useful Mimic for Cosmic Polycyclic Aromatic Hydrocarbon Dust

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are abundantly present in the interstellar medium and in our solar system and lock up a significant fraction of cosmic carbon. They are found to be present in interstellar and interplanetary dust particles. Impact ionization mass spectrometers on future space missions can detect such dust particles and assess their composition; it is essential to understand the impact ionization behavior of PAH-based dust particles impinging on metal targets at relevant velocities. To date, impact ionization studies of fast-moving organic-rich dust particles have been limited to vinyl polymers, such as polystyrene or poly(methyl methacrylate). Recently, PAH anthracene has been prepared in the form of microparticles suitable for use in dust accelerators. Here, we present the first comprehensive study of the impact ionization mass spectra of such anthracene microparticles impinging on a gold target at 2–35 km s^–1. The mass spectra recorded for the resulting ionic plasma are strongly dependent on the incident velocity with impacts at 6–10 km s^–1 being optimal for generating distinctive spectral features that enable the identification of the parent molecule. Under these conditions, the protonated parent ion and doubly protonated radical, C₁₄H₁₁⁺, and C₁₄H₁₂^•+ (as well as other diagnostic cluster species such as (C₁₄H₁₀)(CH)⁺ and (C₁₄H₁₁)(C₂H)⁺) can be reproducibly identified. We find that the impact ionization spectra always differ markedly from the electron impact ionization mass spectra reported for anthracene in the literature regardless of the impact velocity. This study highlights the importance of performing fundamental impact ionization studies of organic particles by using a dust accelerator to enable the interpretation of data collected in future space missions.