Multiphoton ionization of magnesium via an autoionizing state.

Abstract

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 ${10}^{12}$--${10}^{13}$-W/${\mathrm{cm}}^{2}$ 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 ${\mathrm{Mg}}^{+}$ ion in the 3p excited state is strongly enhanced when resonant with the (3p${)}^{2}$ $^{1}$S state. The amount of above threshold ionization observation also varies strongly in the wavelength range under study. At an intensity of 2\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$, sequential double ionization is found to occur dominantly via the excited ionic state ${\mathrm{Mg}}^{+}$(3p). The first ionization step populates this state less than the ion ground state ${\mathrm{Mg}}^{+}$(3s), but this is compensated by the lower order of the second ionization step.