Spectral Model for Femtosecond Laser-Induced Breakdown Spectroscopy of Near-Vacuum Gases

A model has been created for the emissions spectrum from a laser-induced plasma ionized by a femtosecond laser pulse in near-vacuum gases. This forward model will be used to inform future inversion of femtosecond laser-induced breakdown spectroscopy (fs-LIBS) signals from near-vacuum gases that are thermally desorbed from pulsed power electrodes. Recent work shows that fs-LIBS produces a sufficiently high signal-to-noise ratio in these conditions to enable single-shot detection on ultrashort timescales. Based on the femtosecond timescales, the model assumes thermal nonequilibrium, that the emission spectrum is dominated by electronic transitions, and that plasma evolution and recombination of dissociated species are negligible. Spectra were modeled using monatomic and diatomic species produced during the ionization of gaseous H₂, H₂O, and CH₄ at single-digit mTorr pressures, with corrections for instrumentation effects. The model has been validated experimentally against fs-LIBS measurements of these same gases at these conditions.