Single and double ionization of strontium in the vicinity of four-photon excitation of the 5p(2) (1)S(0) doubly excited state (Journal article)

Liontos, I./ Cohen, S./ Bolovinos, A.

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of similar to 5 ns duration, maximum intensity similar to 4 x 10(11) 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 5p(2) (1)S(0) state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr(2+) ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr(2+) yield at the location of the 5p2 1S0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p(1/2,3/2), 6s(1/2) and 5d(3/2,5/2) 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 5p(1/2,3/2) ionic states unambiguously proves the absorption of two photons above the first 5s(1/2) atomic threshold while that of the 5d(3/2),(5/2) and 6s(1/2) 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.
Institution and School/Department of submitter: Πανεπιστήμιο Ιωαννίνων. Σχολή Επιστημών και Τεχνολογιών. Τμήμα Βιολογικών Εφαρμογών και Τεχνολογιών
Keywords: multiphoton double ionization,laser optogalvanic spectroscopy,above-threshold ionization,multi-photon ionization,autoionizing states,even-parity,theoretical-analysis,picosecond pulses,charged ions,ba atoms
ISSN: 0953-4075
Link: <Go to ISI>://000253547600025
Appears in Collections:Άρθρα σε επιστημονικά περιοδικά ( Ανοικτά)

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