X-ray pulses approaching the attosecond frontier

Single soft-x-ray pulses of similar to 90-electron volt (eV) photon energy are produced by high-order harmonic generation with 7-femtosecond (fs), 770 -nanometer (1.6 eV) Laser pulses and are characterized by photoionizing kry pton in the presence of the driver laser pulse. By detecting photoelectrons ejected perpendicularly to the Laser polarization, broadening of the photo electron spectrum due to absorption and emission of Laser photons is suppre ssed, permitting the observation of a Laser-induced downshift of the energy spectrum with sub-laser-cycle resolution in a cross correlation measuremen t. We measure isolated x-ray pulses of 1.8 (+0.7/-1.2) fs in duration, whic h are shorter than the oscillation cycle of the driving Laser Light (2.6 fs ). Our techniques for generation and measurement offer sub-femtosecond reso lution over a wide range of x-ray wavelengths, paving the way to experiment al attosecond science. Tracing atomic processes evolving faster than the ex citing light field is within reach.