Ultrafast Laser Diagnostics to Interrogate High Pressure, Highly Collisional Plasma Environments*

The implementation and demonstration of laser-collision induced fluorescence (LCIF) generated in atmospheric pressure helium environments is presented in this communication. As collision times are observed to be fast (\textasciitilde 10 ns), ultrashort pulse laser excitation (\textless 100 fs) of the 2$^{\mathrm{3}}$S to 3$^{\mathrm{3}}$P (388.9 nm) is utilized to initiate the LCIF process. Both neutral induced and electron induced components of the LCIF are observed in helium afterglow plasma as the reduced electric field (E/N) is tuned from \textless 0.1 Td to over 5 Td. Under the discharge conditions presented in this study (640 Torr He), the lower limit of electron density detection is \textasciitilde 10$^{\mathrm{12\thinspace }}$ e/cm$^{\mathrm{3}}$. Spatial profiles of the 2$^{\mathrm{3}}$S helium metastable and electrons are presented as functions of E/N to demonstrate the spatial resolving capabilities of the LCIF method.