Abstract
We investigate the interaction of ground-state Mg atoms with tuneable laser radiation of similar to 5 ns duration and moderate intensity (<= 10(12) W cm(-2)), capable of exciting the 3p(2) S-1(0) autoionizing state by four photons. For the corresponding photon energy range, this level is strongly one-photon coupled both with bound (third photon) and, primarily, a multitude of other autoionizing states (fifth photon). The coupling is strong enough to induce the absorption of up to six photons before the atom is ionized, creating thus population into the first excited 3p(j) Mg+ levels which subsequently decay radiatively to the 3s(1/2) ionic ground state. The earlier studies devoted to this excitation scheme were thorough but partial. Particularly, the issues concerning the five-photon excited levels were not addressed. In this work, the examined wavelength range is greatly extended to include these states. Moreover, the interaction is characterized in more detail by employing an array of experimental techniques, namely Mg+ ion and photoelectron spectroscopy (including photoelectron angular distributions (PADs) from four-and five-photon ionization) as well as fluorescence spectroscopy where the 3p(j)-> 3s(1/2) decay is monitored. The experimental data show that most of these five-photon excited levels undergo blue ac Stark shifts which are occasionally nonlinear with respect to the laser intensity and comparable to or even larger in magnitude than the large redshift of the 3p(2) S-1(0) state itself. Finally, for the latter state, the wavelength dependence of four-photon PAD measurements is suggestive of a situation reminiscent of an ac Stark splitting picture.