Characterizing attosecond pulses in the soft x-ray regime

Attosecond x-ray pulses offer unprecedented opportunities for probing and triggering new types of ultrafast motion. At the same time, pulse characterization of x-rays presents new challenges that do not exist in the UV regime. Inner-shell ionization is the dominant ionization mechanism for x-rays and it is followed by secondary processes like fluorescence, Auger decay, and shake-up. In general, we find that inner-shell ionization and secondary processes can create additional delay-dependent modulations that will affect pulse reconstruction schemes. Our recently proposed pulse characterization method (Pabst and Dahlström 2016 Phys. Rev. A 94 013411), where a bound electron wavepacket is sequentially photoionized by the attosecond pulse, can be adapted to mitigate the impact of these effects, thus opening up an avenue for reliable pulse reconstruction in the x-ray regime.