Absolute measurement of photoionization cross sections of excited atoms: Application to determination of atomic beam densities

A method of simultaneously determining the magnitude of the cross section for photoionization of an excited atom and the atomic density of the ground state is presented. Measurements of $\sigma$ for $\mathrm{Ba}\left(6s6p{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}\right)$, $\mathrm{K}\mathrm{}\left(4p\right)\mathrm{}$, and $\mathrm{Na}\mathrm{}\left(3p\right)\mathrm{}$ at photon ionizing energy 3.51 eV (3533 Å) are reported and compared with previous values. Application of this technique to the determination of the density in an atomic beam is discussed.