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.^[ref]