Isolated attosecond pulses from ionization-gated high harmonics for molecular spectroscopy

Ionization gating of high harmonic emission on the leading edge of the driver pulse affords a convenient route to isolated attosecond pulses. The gating technique is based on a sub-femtosecond loss of phase matching for the high-harmonic generation process due to the rising plasma density during the driver pulse. Several techniques of attosecond spectroscopy are used to characterize the harmonic emission, including half-cycle cutoff analysis (Haworth et al., \textit{Nat. Phys.} \textbf{3}, 52), CEP-scanning (Pfeifer et al., \textit{Opt. Lett.}, \textbf{34},1819), and time-resolved photoelectron streaking (Kienberger et al., \textit{Nature}, \textbf{427}, 817). Ionization gating can generate a cleanly isolated attosecond pulse with $430\pm15$ as duration, limited here by the bandwidth of the reflective x-ray optic employed. We discuss the advantages of ionization gating, including driver pulse duration scalability, and wavelength tunability, and increased x-ray bandwidth over the traditional technique. Further, the ionization gated harmonic radiation can be used to initiate ultrafast molecular electronic dynamics.