Ionization Mechanisms in a Laser-Produced Plasma for Single Particle Aerosol Mass Spectrometers

Single particle aerosol mass spectrometers (SPAMS) are an emerging technology which can provide high sensitivity mass spectra for aerosols. For example, SPAMS could enable real-time measurements of pollution rather than collection on filters and processing in a laboratory. Obtaining mass spectra for individual particles rather than an average also provides more information than the average alone. In this presentation, the ionization mechanisms and plasma chemistry which occur in a SPAMS system are investigated using computational modeling. A 1 $\mu $m aluminum sphere is vaporized, and the resulting gas is ionized by a 248 nm laser with an 8 mJ, 8 ns pulse. The initial vaporization is investigated using a hydrodynamics model, and upon transition to gas phase, the plasma chemistry is modeled with a 0-dimensional model. It was found that pressure broadening can lead to direct absorption of laser photons, despite the laser wavelength being 9 nm from resonance with a transition. Photoionization of electronic excited states also plays a significant role. The effects of particle diameter and laser intensity on the ionization fraction and dominant ionization mechanisms are discussed. \textit{SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.}