Ionization rate vs. laser intensity determined from ion count vs. peak intensity due to neutral gas exposure to an 800 nm ultrashort pulsed laser

The optical cycle-averaged ionization rate of Ar, O${}_2$, and N${}_2$ vs. local instantaneous laser intensity $I$ for linear polarized $800$ nm light is determined up to approx. $300$ TW/cm${}^2$ by numerically inverting published time-of-flight ion spectrometer data. The published Ar${}^{+}$ collection efficiency of the microchannel plate (MCP) at the end of the spectrometer and its $I_0$ scale are recalibrated by fitting it to its high $I_0$ solution. The relative collection efficiencies of the other species are determined by published MCP cathode data. Results for O${}_2$ are consistent with a reevaluation of published data used to determine its cross section ${\sigma }_8$ in the multiphoton (low $I$) regime.