Fano line shape metamorphosis in resonant two-photon ionization

Two-photon atomic ionization driven by time-locked XUV and IR pulses allows to study dynamics of Fano resonances in time and energy domains. Different time evolution of the two interfering pathways leading to a Fano resonance can be exploited to turn the Fano profile of the two-photon XUV/IR ionization into a symmetric Gaussian once the directly ejected photoelectron leaves the parent ion and cannot any longer absorb an IR photon. This line shape transformation allows for the direct determination of the resonant lifetime from the spectroscopic measurements without need for an extremely fine energy resolution. Ubiquitous nature of Fano resonances makes this determination a universal tool in diverse quantum systems ranging from nuclei to nano-fabricated solids.

💡 Research Summary

The paper investigates the ultrafast dynamics of auto‑ionizing (Fano) resonances using a pair of time‑locked extreme‑ultraviolet (XUV) and infrared (IR) laser pulses. Traditional RABBITT (Reconstruction of Attosecond Beating By Interference of Two‑photon Transitions) experiments employ an attosecond pulse train (APT) as the XUV pump, which limits the usable XUV–IR delay to half an IR optical cycle (≈2.6 fs for 800 nm). This restriction makes it difficult to resolve long‑lived resonances whose lifetimes exceed the available delay window.

To overcome this limitation, the authors replace the APT with a single, ultrashort XUV pulse whose duration is much shorter than the resonance lifetime τ, and they use a weak, Gaussian‑enveloped IR probe pulse of controllable duration T (≈7 fs). The XUV pulse instantaneously populates the auto‑ionizing state; the subsequent IR pulse, arriving after a variable delay t₀, can promote the bound component of the wave packet into the continuum, creating sidebands (SBs) displaced from the main photo‑electron line by ±ℏω (the IR photon energy). Two quantum pathways contribute to each SB: (i) a direct, non‑resonant continuum–continuum transition (continuum‑continuum, CC) and (ii) a resonant pathway that first excites the auto‑ionizing state and then absorbs an IR photon (resonant‑resonant, R). The interference of these pathways yields the characteristic asymmetric Fano line shape described by σ(ε)∝(ε+q)²/(ε²+1).

The authors derive an analytical expression for the SB amplitude, \