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Laser induced strong-field phenomena in atoms and molecules on the femtosecond (fs) time scale have been almost exclusively investigated with traveling wave fields. In almost all cases, approximation of the strong electromagnetic field by an electric field purely oscillating in time suffices to describe experimental observations.
Spatially dependent electromagnetic fields, as they occur in a standing light wave, allow for strong energy and momentum transfer and are expected to extend strong-field dynamics profoundly.
Here we report a strong-field version of the Kapitza-Dirac effect for neutral atoms where we scatter neutral He atoms in an intense short pulse standing light wave with fs duration and intensities well in the strong-field tunneling regime. We observe substantial longitudinal momentum transfer concomitant with an unprecedented atomic photon scattering rate greater than 10$^{16}$~s$^{-1}$.
In conclusion we report extreme longitudinal acceleration of He atoms located in an intense short-pulse standing wave. Besides the absorption of energy corresponding to roughly 15 photons from the standing light wave we also measure the strong momentum transfer equivalent to more than 800 photon momenta during the short laser pulse. The Kapitza-Dirac scattering of neutral He atoms takes place with an unprecedented scattering rate exceeding 10$^{16}$~s$^{-1}$. The investigation opens up new perspectives in strong-field physics by focusing on the importance of the magnetic field and on field gradients.
Particularly, the existence of spatially separated pure electric and pure magnetic fields might allow for probing matter with intense magnetic fields at optical oscillation frequencies unperturbed by electric fields. At lower wavelength, the gradient of the electric field alters the excitation and ionization dynamics on the atomic length scale. Furthermore, the possibility to vary the polarization properties on an atomic scale might allow to investigate chirality of molecules in the strong-field domain (superchiral light)\cite{TaC:11}. Finally, we expect our experimental work to stimulate strong-field quantum mechanical calculations beyond the dipole approximation.
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