SINGLY, DOUBLY AND TRIPLY RESONANT MULTIPHOTON PROCESSES INVOLVING AUTOIONIZING STATES IN MAGNESIUM

Using the R-matrix Floquet theory we have carried out non-perturbative , ab initio one- and two-colour calculations of the multiphoton ioniza tion of magnesium with the laser frequencies chosen such that the init ial state of the atom is resonantly coupled with autoionizing resonanc es of the atom. Good agreement is obtained with previous calculations in the low-intensity regimes. The single-photon ionization from the 3s 3p(1)P(o) excited state of magnesium has been studied in the vicinity of the 3p(2) S-1(e) autoionizing resonance at non-perturbative laser i ntensities. Laser-induced degenerate states (LIDS) are observed for mo dest laser intensities. By adding a second laser which resonantly coup les the 3p2 S-1(e) and 3p3d P-1(o) autoionizing levels, we show that, due to the,small width of the 3p3d P-1(o) state, LIDS occur between th is state and the 3s3p P-1(o) state at intensities of the first laser b elow 10(10) W cm(-2). We next investigate the case in which the first laser induces a resonant two-photon coupling between the ground state and the 3p2 S-1(e) autoionizing state, while the second laser again re sonantly couples the respective 3p2 S-1(e) and 3p3d( 1)P(o) autoionizi ng states. At weak intensities, our calculations compare favourably wi th recent experimental data and calculations. We show that when the in tensity of the first laser is increased, the effect of an additional a utoionizing state, the 4s5s S-1(e) state, becomes significant. This st ate is coupled to the 3p3d P-1(o) autoionizing level by one photon, in ducing a triply resonant processes. We show that LIDS occur among the three autoionizing levels and we discuss their effect on the decay rat e of the ground state. We consider dressed two- and three-level atoms which can be used to model the results of our calculations.