Subfemtosecond $K$-Shell Excitation with a Few-Cycle Infrared Laser Field

Subfemtosecond bursts of extreme ultraviolet radiation, facilitated by a process known as high-order harmonic generation, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe ultrafast dynamics in the microcosms of atoms, molecules, and solids. These ultrashort pulses are always, and as a by-product of the way they are generated, accompanied by laser-induced recollisions of electrons with their parent ions. By using a few-cycle infrared (${\lambda }_0=2.1\mu \mathrm{m}$) driving laser, we were able to directly excite high-energy ($\sim 870\mathrm{eV}$) inner-shell electrons through laser-induced electron recollision, opening the door to time-resolved studies of core-level and concomitant multielectron dynamics.

Figure 1

High-harmonic spectrum generated with (a) Ne and (d) ${\mathrm{N}}_2$. (b) and (c) show the HHG spectra of Ne through an additional 500 nm of vanadium and iron filters, respectively. Comparing the spectra in (b) and (c) to the spectrum in (a), a clear cut can be seen at the vanadium and iron $L$ edges. The transmission curves of the vanadium and iron filters are presented, as well, as green and pink lines. The insets in (a) and (d) show the averaged absolute photon count rate ($\mathrm{photons}/\sec$) in a $10\%$ bandwidth, taking into account the transmission of the filters and the spectrometer efficiency.

Figure 2

Energy-resolved XUV radiation profiles for two different spectral ranges. The uniform radiation distribution at photon energies of 750–950 eV is an indication that it comes from a fluorescence process and not from harmonic generation. The detector size is $\sim 1\times 1\mathrm{cm}$ and it was placed 60 cm away from the gas target. The rounded upper-right corner is due to the aluminum filter.

Figure 3

The photon yield around the Ne $K$ edge as a function of the backing pressure of the target gas. The red lines in the main figure and in the inset show the predicted incoherent photon yield according to the model of recollision with the parent ion; the blue line represents the prediction of the expanded calculation, taking into account the collisions with neighboring atoms. The yellow circles mark the measured photon yield at a $10\%$ bandwidth around 870 eV (resulting from both coherent and incoherent radiation).