Surprise in Highly Correlated Two-Electron System: Extended Secondary Plateau in X-ray High Harmonic Generation from Helium Due to Double Electron Recombination

We observe experimentally, for the first time to our knowledge, a secondary plateau in UV-driven high harmonic generation in the X-ray regime, extending the conventional cutoff from 130 eV to 300 eV, due to emission of a single X-ray photon at double recombination of highly-correlated electrons in helium atoms. Intense 400 nm UV laser pulses, with a duration of 28 fs and energies of up to 2.8 mJ at 1 kHz, derived through a second harmonic upconversion of a Ti:Sapphire laser amplifier, are used to drive high harmonic generation inside a gas-filled hollow waveguide (Fig. 1A). Bright emission extending up to 120–150 eV from He, Ne, and Ar is observed within the effective phase-matching cutoffs for UV driving lasers, where low quantum diffusion of the rescattering electron and high linear refractive indices of atoms and ions contribute favorably to increase the single atom and ion yield, and the phase-matching efficiency [1]. Surprisingly, at laser intensities greater than $2.5\times 10^{15}$ W/cm ² , the X-ray emission from He exhibits two signature plateau regions – a conventional combined plateau extending up to 130 eV from He atoms and He ⁺ ions, determined using TDSE calculations including two active electrons [2], as well as a secondary plateau extending - well beyond the conventional $3.2U_{p}$ cutoff - up to 300 eV (Fig. 1B, C, D), where $U_{p}$ is the ponderomotive energy. Numerical simulations predict that double-electron-recombination (DER) is the physical process of highest probability to yield a secondary plateau. In this highly-correlated electron system, fractions of the two electron wavefunctions are ionized at different laser half-cycles and travel along different trajectories, mitigating strong repulsion during local space-time correlation in the presence of the strong UV field, but recombine with the parent ion at the same instant to produce a single X-ray photon. The observed DER plateau in He is >4 orders of magnitude weaker, and has a characteristic stronger dependence on the ellipticity of the UV driver, due to the longer rescattering time (Fig. 1B). A laser intensity scan indicates that the DER cutoffs scale as $5.5U_{p}$ (Fig. 1C), in agreement with earlier theoretical scaling as $hv_{DER}={I_{p}}^{(1)}+{I_{p}}^{(2)}+5.5U_{p}$ or ${I_{p}}^{(1)}+{I_{p}}^{(2)}+4.7U_{p}$ for ionization of the two electrons separated by a full or a half-laser cycle [3], in contrast to the single-electron recombination (SER) - $hv_{SER}={I_{p}}^{(i)}+3.2U_{p}$ where ${I_{p}}^{(1)}=24.4\ \text{eV}$ and ${I_{p}}^{(2)}=54.6\ \text{eV}$ are the ionization potentials of He.