Phase differences in the photoemission from krypton in the fine-structure-split ionization channels ${}^{2}P_{3/2}$ and ${}^{2}P_{1/2}$

Using the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) technique, a nonzero phase difference is observed in the two-photon ionization transition of krypton atoms into the ionization channels ${}^{2}P_{1/2}$ and ${}^{2}P_{3/2}$ when an electron is removed from the $4p$ shell. The phase difference is studied for excitation energies ranging from $\approx 18\mathrm{eV}$ up to $\approx 28\mathrm{eV}$. The phase differences are found to depend on the relative directions of linear polarization of the photoionization laser pulses and the pulses inducing the subsequent continuum-continuum transition. The experimental results are compared to phase differences calculated on the basis of dipole matrix elements for photoionization and scattering phases known for krypton from earlier experiments. The results indicate that the RABBITT phase differences do not give an obvious direct access to time-delay differences in the primary photoionization step of the atom.

Figure 1

The RABBITT excitation scheme of the krypton atom.

Figure 2

Representative RABBITT traces measured for the sideband ${\mathrm{Sb}}_{14}$ in the ionization channels with the Kr ion core in the states ${}^{2}P_{1/2}$ (blue circles) and ${}^{2}P_{3/2}$ (black squares). The axes of linear polarization of the harmonics and IR beams are parallel. The solid lines represent sine-curve fits to the data points.

Figure 3

Measured and calculated RABBITT TDDs ${\tau }_a$ for photoemission of a Kr $4p$ electron into the ionization channels with the ion core in the states ${}^{2}P_{1/2}$ and ${}^{2}P_{3/2}$. Black squares represent the experimental results for parallel linear polarization of the harmonics and IR beams and red circles those for crossed polarizations. The black solid line displays the calculated TDDs for parallel polarizations and the dashed red line that for crossed polarizations. On the horizontal axis, the kinetic energy of that photoelectron is displayed that leaves the ion core in the ${}^{2}P_{3/2}$ state.