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 1S 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 1S state. The amount of above threshold ionization observation 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.