Single and double ionization of strontium in the vicinity of four-photon excitation of the 5p2 1S0 doubly excited state

Abstract

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.