We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 1011 W cm−2 and within the 710–740 nm wavelength range. The Sr+ spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p2 1S0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr2+ ion. A weakly evident but reproducible inflection point (‘knee’ structure) appears in the intensity dependence of the Sr2+ yield at the location of the 5p2 1S0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p1/2,3/2, 6s1/2 and 5d3/2,5/2 excited Sr+ states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr+ spectrum. The population accumulation in the 5p1/2,3/2 ionic states unambiguously proves the absorption of two photons above the first 5s1/2 atomic threshold while that of the 5d3/2,5/2 and 6s1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.
