Sub-femtosecond X-ray pulse generation and measurement

We report the generation and measurement of isolated soft-X-ray pulses (λ_X = 14 nm) with a duration of τ_X = 650 ± 150 attoseconds (as) by using few-cycle intense visible/near-infrared (λ₀ = 750 nm) laser pulses. For the temporal characterization of the X-ray pulses, a cross-correlation technique relying on laser field assisted X-ray photoemission from krypton atoms was employed. The experimental results bear direct evidence of the X-ray pulse being synchronized to the field oscillations of the visible-light pulse with attosecond precision and of bound-free electronic transitions from the 4p state of krypton responding to 90-eV excitation on an attosecond time scale. As a first demonstration of attosecond metrology, the synchronized single sub-fs X-ray pulses were used for tracing the electric field oscillations in a visible-light wave with a resolution of better than 150 as.