Impact Ionization Properties of Polypyrrole Nanoparticles

Upcoming space missions flying dust impact ionization mass spectrometers will detect and analyze dust grains that are partially organic in composition. These organic components are expected to include mixtures of polycyclic aromatic hydrocarbons, heterocyclic compounds (containing oxygen, sulfur, and nitrogen), and additional functionalized condensed species. Dust impact ionization is a strongly velocity-dependent process that produces atomic and molecular ions reflective of the composition of the impacting particle. In this work, we characterize the impact ionization response of the nitrogen-bearing heterocyclic polymer polypyrrole (PPy). Because of its electrical conductivity, PPy is commonly used as a coating material for both mineral and organic dust particles in electrostatic dust accelerator studies. PPy nanoparticles were accelerated to velocities of 2-30 km s^-1, and the resulting time-of-flight mass spectra were analyzed as a function of impact velocity with additional care paid to spectral variations with particle mass. The resultant mass spectra produced by impacts under roughly 8 km s^-1 are dominated by smaller PPy-derived molecular fragments at masses 27, 28, 56, and 63u, in addition to common contaminants such as Na⁺ (23u) and K⁺ (39u). Some of these molecular fragments can be understood as originating from pyrrole, i.e., the species from which PPy is derived, while others appear to be unique to PPy. At higher velocities, the impact ionization of PPy produces two homologous series of fragment ions with the general form C _n H _m ⁺ and C _n NH _m ⁺, alongside the molecular fragments. This study refines our understanding of impact ionization processes for organic heterocyclic compounds and provides essential reference data for interpreting dust spectra from upcoming interstellar and interplanetary missions.