Direct XUV Probing of Attosecond Electron Recollision

We demonstrate that the recolliding electron wave packet, fundamental to many strong field phenomena, can be directly imaged with sub-Å spatial and attosecond temporal resolution using attosecond extreme ultraviolet (XUV) pulses. When the recolliding electron revisits the parent ion, it can absorb an XUV photon yielding high energy electron and thereby providing a measurement of the electron energy at the moment of recollision. The full temporal evolution of the recollision wave packet can be reconstructed by measuring the photoelectron spectra for different time delays between the driving laser and the attosecond XUV probe. The strength of the photoelectron signal can be used to characterize the spatial distribution of the electron density in the longitudinal direction. Elliptical polarization can be used to characterize the electron probability in transversal direction.

Figure 1

The attosecond XUV pulse and atomic core act as a temporal and spatial gates of the electron dynamics.

Figure 2

XUV ionization spectrum for a variety of arrival times $t_X$ of the XUV pulse showing both streaking and depletion of the XUV electron peaks for both cw- and 5-fs pulses. ${\tau }_X=10\mathrm{a}.\mathrm{u}.$ (242 asec).

Figure 3

Reconstructed energy of recollision for different durations of the XUV pulse from the top to the bottom: 300 asec (red), 250 asec (cyan), 200 asec (green), 150 asec (blue). SFA result for recollision energy Eq. (4) (dashed line), simple man’s predictions for the recollision energy (dash-dotted line).

Figure 4

Photoelectron spectra of 3D hydrogen for 800 nm laser field with intensity $2.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and 200 asec XUV pulse with carrier ${\omega }_X=2\mathrm{a}.\mathrm{u}.$ and intensity $3.5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ for $t_X=3\pi /2{\omega }_L$. Black (inner) circle marks one-photon XUV absorption from the ground state, which is at the level of ${10}^{-6}$ arb. units, red (intermediate) circle marks two-photon absorption from the ground state (at the level of ${10}^{-12}$ arb. units) and white (outer) circle marks the recollision signal (at the level of ${10}^{-12}$ arb. units). Arrow indicates the direction of laser polarization.

Figure 5

Heterodyne-type enhancement: interference between the XUV and ATI electrons helps to detect the recollision signal (dashed circle at the level of ${10}^{-5}$ arb. units). 800 nm laser field with intensity $2.8\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and 200 asec XUV pulse with carrier ${\omega }_X=1\mathrm{a}.\mathrm{u}.$ and intensity $3.5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ arriving at $t_X=3\pi /2{\omega }_L$ were used.