Multiphoton ionization of magnesium via an autoionizing state

Multiphoton single and double ionization of magnesium was studied by measuring electron energy spectra and ion mass spectra using 1-ps laser pulses in the 580-595-nm wavelength and 10¹²-10¹³-W/cm² intensity range. In single ionization the (3p)^{2 1}S doubly excited autoionizing state, resonant at the four-photon level, is found to play an important role. Single ionization leaving the Mg⁺ ion in the 3p excited state is strongly enhanced when resonant with the (3p)^{2 1}S state. The amount of above-threshold ionization observed also varies strongly in the wavelength range under study. At an intensity of 2 x 10¹³ W/cm², sequential double ionization is found to occur dominantly via the excited ionic state Mg⁺(3p). The first ionization step populates this state less than the ion ground state, Mg⁺(3s), but this is compensated by the lower order of the second ionization step.