Momentum transfer in the ponderomotive potential of near-infrared laser pulses leads to sizable energy shifts and electron-wavepacket squeezing in time-resolved ARPES

We observe momentum transfer in the ponderomotive potential of near-infrared (NIR) laser pulses in time- and angle-resolved photoemission spectroscopy (tr-ARPES) experiments with ultrashort extreme ultraviolet probe pulses. Acceleration of photoelectrons in the transient grating provided by an intense laser pulse reflected at a surface leads to delay-dependent oscillations of electric kinetic energies. Photon and electron momenta determine the oscillation frequency. We experimentally observe and theoretically simulate electron yield modulations driven by a novel electron-energy bunching effect. Measurement results are simulated and fitted with high accuracy. Complete reversion of the ponderomotive momentum transfer allows for retrieval of the undisturbed initial state and the transient band structure for overlapping pump and probe pulses.