Isolated high-harmonic XUV photon absorption and NIR strong-field tunnel ionization

Extreme ultraviolet (XUV) pulses with a duration of tens of femtoseconds initiate 4s(-1) or 4p(-1) photoionization of krypton, which populates highly excited satellite states through the electron correlation. The excited ions are then tunnel ionized to Kr2+ 4s(-1)4p(-1) or 4p(-2) by a strong-field near-infrared (NIR) pulse of a similar duration. The XUV pulses are produced by high harmonic generation in a gas jet and we employ a state-of-the-art time-preserving monochromator to isolate individual XUV harmonic orders. An enhancement of the Kr2+ yield as a function of harmonic photon energy and XUV-pump NIR-probe delay is observed and compared with a two-step model, which allows the population of the satellite states to be inferred. Furthermore, relative 4s and 4p satellite excitation cross-sections are predicted at the photon energies studied. This proof-of-principle experiment demonstrates that isolated harmonics can be employed to pump specific electronic states, which will be highly complementary to synchrotron, attosecond and x-ray free-electron laser studies of complex systems.