A background-free intensity autocorrelation measurement for 10-keV femtosecond x-ray pulses is demonstrated using x-ray second harmonic generation. The narrow phase-matching condition of x-ray second harmonic generation allows for the selective generation of either the autocorrelation or background signals in a quasicollinear geometry with a crossing angle of $\sim 300\mathrm{\mu }\mathrm{rad}$, achieving a geometric time resolution of $\sim 1$ attosecond. The measured autocorrelation traces exhibit zero background and clearly reveal temporal modifications through an Si(311) monochromator, verifying x-ray temporal shaping with perfect crystals. The applicability of this method to attosecond x-ray pulses is also discussed.

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• X-ray beams & optics
• X-ray lasers
• X-ray techniques