Time Resolved Fano Resonances

Recent advances in the generation of sub-fs extreme ultraviolet pulses and attosecond metrology have opened up the possibility to trace the time evolution of electronic wave packets inside atoms in pump-probe experiments. We investigate the feasibility of observing the buildup of a Fano resonance in the time domain by attosecond streaking techniques. A time-resolved resonance is initialized by a sub-fs extreme ultraviolet-pump pulse in the presence of a synchronized phase-controlled probe laser pulse. The time evolution of the coherent superposition of resonant state and continuum is mapped onto a modulation of the electron spectrum as a function of the time delay between pump and probe pulse. (super-)Coster-Kronig transitions with lifetimes of $\sim 400\mathrm{a}\sec $ are identified as prime candidates.

Figure 1

Time-resolved excited Fano resonance, schematically. Excitation by the ultrashort pump pulse (duration ${\tau }_X$) opens two interfering paths from the ground state to the continuum. Arrival in the continuum is monitored by the probe laser pulse with period $T_L=2\pi /{\omega }_L$ and length ${\tau }_L$. The indicated electronic configurations correspond to the Dy atom.

Figure 2

Time-dependent ionization probability $P(E,t)$ as a function of energy and time. Inset 1: spectral distribution at short times $t\le {\tau }_r$. Inset 2: spectral distribution in the scattering limit $t\to \infty$ for $q=1$.

Figure 3

$P_{\Delta t}(E)$ as a function of the time delay between pump and probe pulse and observed in the direction of the polarization of the probe laser pulse. (a) Direct ionization with the resonant excitation suppressed ($q=0$), (b)–(c) autoionizing resonance with both pathways open ($q=2.2$) for different ${\tau }_r$, (d)–(e) complex Fano parameters with $q=2.2e^{i\pi /4}$ and $q=2.2e^{-i\pi /4}$, respectively, and (f) ionization in the absence of a resonance (${\dot{a}}_r=0,V_E=0$).