Attosecond photoionization dynamics in the vicinity of the Cooper minima in argon

Using a spectrally resolved electron interferometry technique, we measure photoionization time delays between the $3s$ and $3p$ subshells of argon over a large 34-eV energy range covering the Cooper minima in both subshells. The observed strong variations of the $3s-3p$ delay difference, including a sign change, are well reproduced by theoretical calculations using the two-photon two-color random-phase approximation with exchange. Strong shake-up channels lead to photoelectrons spectrally overlapping with those emitted from the $3s$ subshell. These channels need to be included in our analysis to reproduce the experimental data. Our measurements provide a benchmark for multielectronic theoretical models aiming at an accurate description of interchannel correlation.

Figure 1

One-photon ionization cross sections of the $3p$ (blue/black) and $3s$ (red/medium gray) channels [34, 36], together with that of the satellite shake-up states $3s^{2}3p^4{(}^{1}D)4p{(}^{2}P)$ (cyan/light gray) [47] and $3s^{2}3p^4{(}^{1}D)3d{(}^{2}S)$ (green/dark gray dots for experiments [48] and solid line for theory [47]) labeled $4p$ and $3d$, respectively. The ionization thresholds for $3p$, $3s$, $4p$, and $3d$ are 15.76, 29.24, 37.15, and 38.60 eV, respectively.

Figure 2

(a) $\mathrm{XUV}+\mathrm{IR}$ photoelectron spectrum (black), obtained in Saclay by integrating the RABBIT spectrogram over the delay, as compared to the XUV-only spectrum (gray), and corresponding multiphoton transitions for the $3s$ and $3p$ channels. Spectral amplitude (b) and phase (c) of the $2{\omega }_0$ oscillations obtained from a Fourier transform at each energy of the RABBIT spectrogram. In panel (b), the amplitudes related to the $3p$ ($3s$) channel are highlighted in hatched blue (filled red), respectively. The red dashed lines indicate the spectral regions with dominant $3s$ contributions. In panel (c), the measured phase evolution (black line) is compared to the simulated one (orange/gray line) [46].

Figure 3

Atomic delay difference between $3s$ and $3p$ ionization channels in the vicinity of the $3s$ CM and $3p$ CM (highlighted in red/medium gray and blue/dark gray, respectively). (a) Experimental data from Lund (magenta/dark gray dots) and Saclay (orange/light gray markers, circles for sidebands, squares for harmonics) compared with simulations using the 2P2C-RPAE model (green/dark gray line). (b) Decomposition of the simulated delay-difference curve (green/dark gray line) into the $3s$ (red/gray dashed line) and $3p$ (blue/black dashed line) delay curves.

Figure 4

(a) Estimated modulation strength $B_{n,i}$ of the RABBIT signal for the $3s$ (red/medium gray line), $4p$ (cyan/light gray line), and $3d$ (green/dark gray line) channels, and effective modulation strength $B_t$ of the incoherently combined $3s$ and $4p$ contributions, denoted $3s+4p$ (black line). (b) Theoretical atomic delays for the $3s$ (red/medium gray line) and $4p$ (cyan/light gray line) channels, and effective atomic delay for the incoherent $3s+4p$ case (black line). The experimental points as well as the previously published data [11, 12] are plotted for comparison (in this region, ${\tau }_{3p}^{\mathrm{A}}\sim 0$).