Strong-Field Many-Body Physics and the Giant Enhancement in the High-Harmonic Spectrum of Xenon

We resolve an open question about the origin of the giant enhancement in the high-harmonic generation spectrum of atomic xenon around 100 eV. By solving the many-body time-dependent Schrödinger equation with all $4d$, $5s$, and $5p$ orbitals active, we truly demonstrate the enhancement results from the collective many-body excitation induced by the returning photoelectron via two-body interchannel interactions. Without the many-body interactions, which promote a $4d$ electron into the $5p$ vacancy created by strong-field ionization, no collective excitation and no enhancement in the high-harmonic generation spectrum exist.

Figure 1

Schematic illustration of the HHG process in xenon (spin states are excluded). (1) The electron is tunnel-ionized mainly from the $5p_0$ orbital by the strong-field driving pulse. (2) The electron is driven back to the ion by the oscillating electric field. In the third step, the electron recombines with the ion in two different ways: (3.1) the electron recombines with the very same hole that was generated in step 1, or (3.2) the electron exchanges energy with the ion by promoting an inner shell electron from the $4d$ shell in the $5p_0$ hole via Coulomb interaction before recombining in a more tightly bound $4d$ orbital. All five $4d_m$ orbitals ($m=-2,\dots ,2$) contribute to the Coulomb interaction.

Figure 2

The HHG spectrum of xenon for different theoretical models: (solid blue) the full TDCIS model ($\mathrm{\text{intrachannel}}+\mathrm{\text{interchannel}}$ interactions) with all orbitals in the $4d$, $5s$, and $5p$ shells active, (dashed green) the full TDCIS model with only $m=0$ electrons in the $4d$, $5s$, and $5p$ shells active, and (dotted red) the intrachannel TDCIS model, excluding interchannel coupling, with all orbitals in the $4d$, $5s$, and $5p$ shells active. The difference between the two models including interchannel effects and the model including only the intrachannel interactions is highlighted with the according colors.

Figure 3

The partial photoionization cross sections of the $5p_0$ orbitals and the total photoionization cross section of all orbitals for the three different theoretical models (see Fig. 2).

Figure 4

The returning electron spectrum $W(\omega )$ calculated by using the factorization $W(\omega )=S(\omega )/\sigma (\omega )$, where $S(\omega )$ is the HHG spectrum and $\sigma (\omega )$ is the photoionization cross section. The electron spectra shown for the three different theoretical models (see Fig. 2) illustrate the system dependence of the returning electron wave packet—especially when interchannel interactions are not negligible.