Sub-femtosecond X-ray pulse generation and measurement
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We report the generation and measurement of isolated soft-X-ray pulses (λX= 14 nm) with a duration of τX=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.