Influence of autoionizing states on the pulse-length dependence of strong-field Ne+ photoionization at 38.4 eV

Linda Hamonou (1) , Michael A Lysaght (1) , Hugo W van Der Hart (1)

1 Centre for Theoretical Atomic, Molecular and Optical Physics

Using the time-dependent R-matrix approach, we investigate ionization of ground-state Ne +, irradiated by laser light with a photon energy of 38.4 eV at intensities 10 13 W cm −2, 2 × 10 13 W cm −2 and 10 14 W cm −2 as a function of pulse length. Although the photon energy is below the threshold for single-photon ionization, we obtain a significant contribution from single-photon ionization to the ionization probability due to the finite duration of the pulse. The two-photon ionization rates deduced from the calculations are consistent with those obtained in R-matrix-Floquet rate calculations. The ionization probability oscillates with pulse length, which is ascribed to population and depopulation of autoionizing states just above the Ne 2+ ground state, reached after absorption of a single photon. At an intensity of 10 14 W cm −2, pulse lengths longer than 50 cycles are required for two-photon ionization to dominate the ionization probability. Letter to the Editor 2 The development of free-electron lasers operating in the VUV and the X-ray domain has given experimentalists new ways of investigating multi-electron dynamics in strong laser fields. These new laser facilities have, for example, enabled experimentalists to investigate two-photon double ionization of Ne in the photon-energy regime where direct two-photon double ionization is energetically allowed, but sequential (Ne → Ne + → Ne 2+) double ionization is energetically not allowed since the photon energy is not sufficient to ionize Ne + with a single photon (eg. Sorokin et al 2007). At larger photon energies, sequential double ionization is allowed, but also in this process one can find signatures of the fact that the two different emission processes are not independent of each other (Fritzsche et al 2008). One of the grand challenges in theoretical atomic physics is the description of multielectron dynamics in complex atoms irradiated by intense laser pulses. Over the last 15 years, great progress has been made in the description of pure two-electron systems in intense laser fields, for example for two-photon double ionization processes (see, for example, Colgan et al 2002, Feng and van der Hart 2003, Laulan et al 2005, Feist et al 2008) as well as for multiphoton double ionization at 390 nm (Parker et al 2006). These calculations require substantial computational resources, such that the direct extension of these techniques to systems with more than two electrons, like Ne, is unfeasible at present. Other approaches are required to describe the behaviour of complex atoms in intense light fields. The most successful approach for the description of complex atoms in intense laser light at present is the R-matrix-Floquet approach. This approach was designed from the outset to treat complex atoms in intense light fields by combining the R-matrix approach with the Floquet Ansatz (Burke et al 1991). It has been applied to a wide range of problems, ranging from strong-field ionization of Ne and Ar at 390 nm, requiring absorption of at least eight and six photons respectively, (van der Hart 2006) to twophoton emission of the inner 1s electron from ground-state Li − (van der Hart 2005). More recently, the R-matrix-Floquet approach has been instrumental in indicating the importance of detailed atomic structure in two-photon ionization of Ne + (Hamonou et al 2008, Hamonou and van der Hart 2008). The theoretical investigation of ionization processes in Ne + is of particular relevance at the moment, due to the large number of strong-field multiple ionization studies on Ne at photon energies in the range between 38 and 50 eV. Ionization yields of various Ne ions were obtained by Sorokin et al (2007) at photon energies below the Ne + ionization threshold and above this threshold. Moshammer et al (2007) obtained detailed recoil momentum spectra for two-photon double ionization of Ne at 44 eV. Rudenko et al (2008) found that these recoil-ion momentum distributions differed strongly from the recoil-ion momentum distributions for two-photon double ionization of He at 44 eV. At 44 eV, sequential double ionization is allowed for Ne, whereas it is not allowed for He. The dominance of sequential double ionization of Ne was demonstrated experimentally by Kurka et al (2009). Whereas in previous work (Hamonou et al 2008, Hamonou and van der Hart 2009),

Mots clés: Physical Sciences

Linda Hamonou, Michael A Lysaght, Hugo W van Der Hart. Influence of autoionizing states on the pulse-length dependence of strong-field Ne+ photoionization at 38.4 eV. Journal of Physics B: Atomic, Molecular and Optical Physics, 2010, 43 (4), pp.45601. ⟨10.1088/0953-4075/43/4/045601⟩. ⟨hal-00569867⟩