Single and double ionization of magnesium via four-photon excitation of the 3p(2) S-1(0) autoionizing state: Experimental and theoretical analysis

摘要

Single and double ionization of ground state Mg atoms is observed in an atomic beam experiment in the 584-596 nm spectral range with a maximum intensity of 9x10(11) W/cm(2) and pulses of ns duration. The 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 3p(2) S-1(0) state. For intensities higher than 2x10(11) W/cm(2) a considerable double ionization yield is obtained. The Mg2+ 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 3p(1/2,3/2) Mg+ states (with negligible contribution from the 3s(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 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 3p(2) S-1(0) and 3p3d P-1(1) autoionizing states and employing calculated parameters for both Mg and 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.