Single and double ionization of magnesium via four-photon excitation of the3p2S01autoionizing state: Experimental and theoretical analysis

Single and double ionization of ground state $\mathrm{Mg}$ atoms is observed in an atomic beam experiment in the $584\char21{}596\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ spectral range with a maximum intensity of $9\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ and pulses of ns duration. The ${\mathrm{Mg}}^{+}$ spectrum consists of a single broad resonance whose position is very sensitive to laser intensity and it is attributed to the four-photon excitation of the doubly excited $3{p}^{2}\phantom{\rule{0.3em}{0ex}}^{1}S_{0}$ state. For intensities higher than $2\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ a considerable double ionization yield is obtained. The ${\mathrm{Mg}}^{2+}$ spectra are composed of two lobes, each one dominating at different intensities. These observations can be interpreted by a sequential mechanism in which doubly charged ions are created by multiphoton resonant ionization of the excited $3{p}_{1∕2,3∕2}$ ${\mathrm{Mg}}^{+}$ states (with negligible contribution from the $3{s}_{1∕2}$ ionic ground state) after the single ion production has saturated. The population of these excited ionic states suggests the absorption of two photons above the first ionization threshold of $\mathrm{Mg}$. This interpretation is unambiguously confirmed by a complementary fluorescence experiment. Moreover, both ionization and fluorescence spectra as well as ionization yields are very well reproduced by a theoretical model based on a combination of density matrix and rate equations, taking into account the near-resonant coupling between the $3{p}^{2}\phantom{\rule{0.3em}{0ex}}^{1}S_{0}$ and $3p3d\phantom{\rule{0.3em}{0ex}}^{1}P_{1}$ autoionizing states and employing calculated parameters for both $\mathrm{Mg}$ and ${\mathrm{Mg}}^{+}$. It is thus pointed out that for visible ns pulses of moderate intensity the autoionizing resonances play a crucial role in the multiphoton sequential double ionization of alkaline-earth-metal atoms.