Extreme-Ultraviolet-Initated High-Order Harmonic Generation: Driving Inner-Valence Electrons Using Below-Threshold-Energy Extreme-Ultraviolet Light

We propose a novel scheme for resolving the contribution of inner- and outer-valence electrons in extreme-ultraviolet (XUV)-initiated high-harmonic generation in neon. By probing the atom with a low-energy (below the $2s$ ionization threshold) ultrashort XUV pulse, the $2p$ electron is steered away from the core, while the $2s$ electron is enabled to describe recollision trajectories. By selectively suppressing the $2p$ recollision trajectories, we can resolve the contribution of the $2s$ electron to the high-harmonic spectrum. We apply the classical trajectory model to account for the contribution of the $2s$ electron, which allows for an intuitive understanding of the process.

Figure 1

The smoothed harmonic yield from Ne driven by an 18 fs, $1.8\mu \mathrm{m}$, $1.2\times 10^{14}{\mathrm{W}\mathrm{cm}}^{-2}$ pulse, in combination with a (a) 1 fs and (b) 250 as XUV pulse of energy 19 (black dotted line), 31 (red dot-dashed line), 45 (green dashed line), and 59 eV (blue solid line). The cutoff of the $2p$ harmonics as predicted by the model (see the text) are marked by yellow squares. The predicted $2s$ cutoff is marked for the 45 (circle) and 59 eV (plus) XUV pulses.

Figure 2

The smoothed cutoff harmonics from Ne driven by an 18 fs, $1.8\mu \mathrm{m}$, $1.2\times 10^{14}{\mathrm{W}\mathrm{cm}}^{-2}$ pulse, in combination with a 250 as (lower two lines) and 1 fs (upper two lines), 45 eV XUV pulse timed to coincide with the penultimate maximum of the IR field. Calculations include the action of both the $2s$ and $2p$ electrons (solid lines) or neglecting the action of the $2s$ (dotted lines).

Figure 3

The maximum emitted harmonic photon energies that would result from electrons being released at the times shown along the horizontal axis for both $2p$ (blue) and $2s$ (green) electrons from Ne in an 18 fs, $1.8\mu \mathrm{m}$, $1.2\times 10^{14}{\mathrm{W}\mathrm{cm}}^{-2}$ pulse (red dashed line). Trajectories are initiated by a (a) 250 as and (b) 1 fs, 45 eV XUV pulse. Trajectories of $2s$ and $2p$ which lead to recollision are localized in time and therefore can be resolved in the short-pulse case (a). For a long XUV pulse (b), both electrons contribute.

Figure 4

The XIHHG spectrum for Ne as a function of the time delay between the XUV and IR pulse peaks for RMT calculations both (a) neglecting and (b) including the action of a $2s$ electron.