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 ${10}^{12}$–${10}^{13}$-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×${10}^{13}$ 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.