ECAMP 15

Attosecond spectroscopy have been mostly performed to date using table-top experimental set ups employing high-harmonic generation (HHG) techniques. The success of this technology was recognized with the Nobel Prize in Physics in 2023. Additionally, in the last decade, free electron laser (FEL) facilities have been commissioned all over the world, generating high brilliance and high intense pulses with incomparable frequency tunability, and, in some cases, even producing coherent attosecond pulses after the introduction of self-amplified spontaneous emission (SASE) schemes. The combination of these intense XUV sources with advanced detection devices that enable coincident measurements of all charged fragments enables a complete dynamical characterization of non-linear phenomena in the XUV and X-Ray regimes that remained experimentally inaccessible until now [1]. This technological progress thus calls for accurate and reliable theoretical methods to unravel the role of nuclear motion and electron correlation in the excitation and ionization process. We here present our first results obtained from a full dimensional solution for the two-photon double ionization of H2 molecule. Very few theoretical works have addressed this problem due to difficulty and computational cost of achieving an accurate evaluation of the strong correlation between all fragments in the four-body Coulomb breakup, and only frozen-nuclei approaches have been employed until now [2]. In this work, we have implemented a new computational tool to describe, for the first time, the multiphoton double ionization of H2 including electronic and nuclear degrees of freedom at equal footing, i.e., working beyond the Born-Oppenheimer approximation [4,5]. We employ a numerical representation of the molecular wave function directly written in a basis set of FE-DVR (finite elements combined with a discrete variable representation) and apply an exterior complex scaling procedure to impose the appropriate many-body Coulomb boundary conditions [3]. Accurate angle and energy differential two-photon double ionization yields show a significant energy displacements in the photoelectrons spectra with respect to frozen nuclei approaches. More interestingly, counterintuitive angularly resolved double ionization yields with respect to its atomic analog are found, due to novel interferences that arise from sequential two-photon absorption paths through different cationic states [5].

[1] “Coulomb explosion imaging of small polyatomic molecules with ultrashort x-ray pulses”, X. Li et al., Phys. Rev. Research 4, 013029 (2022)
[2] “Alignment and pulse-duration effects in two-photon double ionization of H2 by femtosecond XUV laser pulses”, X. Guan et al., Phys. Rev. A 90, 043416 (2014)
[3] “Practical calculations of quantum breakup cross sections”, C. W. McCurdy and T. N. Rescigno, Phys. Rev. 74, 052702 (2000) & “Double photoionization of aligned H2”, Phys. Rev. A 74, 052702 (2006)
[4] “A pump probe scheme with a single chirped pulse to image electron and nuclear dynamics in molecules”, D Jelovina, J Feist, F Martín and A Palacios, New J . Phys. 20, 123004 (2018)
[5] “Strong Electron-Electron-Nuclei Correlations in Two-Photon Double Ionization of H2”, K Arteaga, J Feist, D Jelovina, F Martín, A Palacios, Physical Review Letters 133 (12), 123201 (2024)