Absolute photoionization cross-section measurement of selectively excited magnesium

We report the first absolute measurement of the cross section for photoionization from a selectively excited short-lived atomic singlet state. Employing two simultaneous tunable laser pulses, the $3s3\mathrm{}p{}_{}{}^1{}_{}{}^{}P_1^o{}_{}{}^{}-3p^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ autoionization resonance in MgI was found to have a peak cross section of (8±4)×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at $\lambda 300.9$ nm and a half-width of 2.5 nm. The measured ratio of the cross sections to the $3p^2{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ states was 14:1. The effect of polarization of the lasers has been investigated and shown to provide a means of identifying the angular momentum states of autoionizing levels.