= Photoionization cross section
A <photoionization cross section> $\sigma(\nu)$ quantifies the probability that an incident <photon> of frequency $\nu$ ionizes a specified atomic or molecular target. For a weak monochromatic beam with photon number flux $F_\gamma$, the ionization rate per target is $\Gamma=F_\gamma\sigma$; the cross section has dimensions of area.
The cross section depends on photon energy, target state and accessible final channels. In an isotropic radiation field with mean specific intensity $J_\nu$, $\Gamma=\int_{\nu_0}^{\infty}[4\pi J_\nu/(h\nu)]\sigma(\nu)\,d\nu$. Energy-resolved photoelectron measurements probe the resulting channels; interpreting a count rate requires photon-flux normalization and detector calibration.<Photoionization of CO2: Experimental Procedure by the National Institute of Standards and Technology>[Photon-energy selection, electron detection and experimental normalization]{ref}
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