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Description
Synopsis High-order Harmonic Generation (HHG) driven by few-cycle near-infrared (NIR) pulses produces a comb of spectrally broad odd harmonics in the eXtreme Ultra-Violet (XUV) range. Owing to the spectral width of the harmonics, electron wave packets (EWPs) singly photoionized to the continuum interfere with EWPs tak-ing a two-photon path (XUV+NIR). We analytically discuss the relationship between these interferences and Fano’s propensity rules in helium.
High-order Harmonic Generation (HHG) driven by near-infrared (NIR) pulses generates phase-locked odd-order harmonics of the driving laser frequency in the eXtreme Ultra-Violet (XUV) range. These can be used to produce electron wave packets (EWPs) via photoionization that are separated by twice the NIR energy. In the RABBIT (Reconstruction of Attosecond Beatings By Interference of Two-photon transitions) technique [1], a weak delayed replica of the NIR laser couples the EWPs generated by the consecutive harmonics to the same final energy through the absorption/emission of an additional NIR photon. The resulting interferences exhibit oscillations at twice the NIR frequency and can be used to extract information about the photoionization process.
In this work, we consider RABBIT with few-cycle NIR pulses in helium. Ascribed to the short pulse duration, the harmonics are spectrally broad, allowing EWPs singly photoionized to the continuum by the XUV (1-photon path) to interfere with EWPs that undergo an additional transition by absorbing or emitting a NIR photon (two-photon path). When angular resolution is available, this translates into oscillations at the laser frequency. We propose a link between the modulations and Fano’s propensity rules[2,3].
For this, we analytically describe the angular-dependence of the EWP interferences originating from the one-photon/two-photons parity mixing in helium using a partial wave expansion. We show that the odd expansion coefficients $h_{2n+1}$ (n=0,1) allow extracting the radial two-photon transition amplitudes from a same intermediate state.
References
[1] P.M. Paul et al., Science 292, 1689-1692, (2001)
[2] D. Busto et al, Phys. Rev. Lett. 123, 133201, (2019)
[3] M. Bertolino et al, J. Phys. B: At. Mol. Opt. Phys. 53, 144002, (2020)
