Doubly excited electron-ion angular momentum transfer in parity-unfavored multiphoton ionization

We observe the parity-unfavored photoelectron emission in multiphoton single ionization of krypton atoms in an intense ultraviolet laser field. With systematic experiments of successively varying the light ellipticity and intensity, we identify that this parity-unfavored emission is associated with the first excited ionic state via a resonant pathway. We reveal an abnormal Coulomb asymmetry of the parity-unfavored photoelectron emission in elliptically polarized fields when the ellipticity is less than 0.3. The simulation using the time-dependent Schrödinger equation based on single-active-electron approximation fails to reproduce the experiment. We attribute the experimental observations to the angular momentum correlation between the resonantly excited electron and the spin-orbit excited ion. Our results demonstrate that the well-known picture of the angular momentum transfer in multiphoton ionization is broken and instead that the electron shares the spin angular momenta of photons with the ion as an effect of electron correlations.

Figure 1

Photoelectron momentum distributions of krypton atoms ionized by a linearly polarized 400-nm light field along the $z$ axis. (a) Two-dimensional momentum slice of the measurement with integrating $p_y$ from −0.15 to 0.15 a.u. (b) TDSE simulation within single-active-electron approximation. In (a) and (b), the ATIs corresponding to the excited (ground) ionic state are marked with the red (green) arrows, and the first- (second-) order ATIs are marked with the solid (dashed) arrows. (c) Measured three-dimensional photoelectron momentum distribution. The isocount surface of the second order ATI is not divided into the two ionic states due to the low counts and their close momentum spacing. (d) The enlarged low-energy part of (c) to strength the anomalous vertically emitted photoelectron structure of the first-order ATI from the excited ionic state. The double-sided arrow indicates the polarization direction of the light field.

Figure 2

(a)–(d) Two-dimensional photoelectron momentum distributions in the polarization plane at the ellipticity of 0.25, 0.5, 0.75, and 1.0, respectively. The momentum $p_y$ is integrated from −0.15 to 0.15 a.u. The white dashed circles are used to mark the abnormal rotation of the down lobe on the first-order ATI from the excited ionic state. The count axis, as well as the color scale, are linear. (e),(f) Ellipticity-resolved photoelectron angular distributions of the first-order ATIs from the excited (e) and the ground (f) ionic states. The integrated energy ranges are labeled on the top of the panels. The white dashed curves depict the rotating trend of the down lobe marked in (a)–(c) with respect to the ellipticity. (g) Angle-integrated photoelectron energy spectra at the ellipticity of 0.0 and 1.0.

Figure 3

(a),(b) TDSE simulations of the photoelectron momentum distributions at the ellipticity of 0.25 for the excited $(J_i=1/2)$ and the ground $(J_i=3/2)$ ionic states, respectively.

Figure 4

(a),(b) Photoelectron energy spectra (angle-integrated) as a function of the light intensity in the linearly polarized field and the circularly polarized field, respectively. The maximum light intensity $I_0$ was calibrated at $\sim 8.0\times {10}^{13}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$. (c),(d) Ionization mechanism in the linearly polarized field and the circularly polarized field, respectively. In (c), the black hollow arrow represents the resonance process and the green hollow arrow represents the autoionization process. In the linearly polarized field, the presence of the resonant $5p$ state gives rise to the autoionization channel of the excited ionic state. In the circularly polarized field, both ionic states are ruled by the direct ionization pathway. (e) Change of the angular momentum vector of the outmost electron during the angular momentum transfer in the autoionization channel. ${\vec{J}}_{\gamma }$ is the spin angular momentum of photon, $\mathrm{\Delta }{\vec{J}}_i$ is the variation of the angular momentum of the ion, and ${\vec{J}}_e$ is the electron angular momentum. (f) The alteration of the angular momentum directions [perpendicular to the orbit plane (dashed ellipse)] of the electron and the equivalent hole, caused by their repulsive interaction during the autoionization.