Energy- and Momentum-Resolved Photoelectron Spin Polarization in Multiphoton Ionization of Xe by Circularly Polarized Fields

We perform a joint experimental and theoretical study on momentum- and energy-resolved photoelectron spin polarization in multiphoton ionization of Xe atoms by circularly polarized fields. We experimentally measure the photoelectron momentum distributions of Xe atoms in circularly polarized near-infrared (800 nm) and ultraviolet (400 nm) light, respectively. We analyze the momentum- and energy-resolved photoelectron spin polarization by comparing the experimental photoelectron momentum distributions with the simulations, although we cannot derive the spin polarization solely from the experiment. We show that the use of circularly polarized ultraviolet light at 400 nm can create better than 90% spin polarization with focal volume effect considered, which enables the separation of the spin states by momentum gating. This paves the way to produce high-degree spin-polarized electron sources from strong-field multiphoton ionization.

Figure 1

The measured (blue solid line) and simulated (black solid line) photoelectron energy spectra in near-infrared circularly polarized light at 800 nm. The maximum data have been normalized to unity. The green dashed line and red dash-dotted line represent the simulated energy distributions of spin-up and spin-down electrons, respectively. Focal volume effect has been taken into account.

Figure 2

Measured (a) and simulated (b) two-dimensional photoelectron momentum distributions in laser polarization plane corresponding to the energy spectra in Fig. 1. The maximum data have been normalized to be unity. For the measured distribution we restrict the momentum along the laser propagation direction so that $|p_z|<0.1(\mathrm{a}.\mathrm{u}.)$. The white circle in (a) indicates the position of the transition point of spin polarization. (c),(d) Simulated momentum distributions of spin-up and spin-down electrons, respectively.

Figure 3

(a),(b) Calculated 2D momentum- and energy- resolved spin polarization. (c),(d) The corresponding momentum- and energy-resolved normalized yield asymmetry, respectively. Both the spin polarization and the normalized yield asymmetry are obtained from the simulation.

Figure 4

The measured (a) and simulated (b) 2D photoelectron momentum distributions in laser polarization plane in ultraviolet circularly polarized light at 400 nm. For the measured distribution we restrict the momentum along the laser propagation direction so that $|p_z|<0.1(\mathrm{a}.\mathrm{u}.)$. (c),(d) Simulated momentum distribution of spin-up and spin-down electrons, respectively. Red arrows indicate the ATI peaks corresponding to the ${{}^{2}P}_{3/2}$ channel, and white arrows indicate those corresponding to the ${{}^{2}P}_{1/2}$ channel. Both the measured and simulated spectra are in logarithmic scale.

Figure 5

(a),(b) 2D momentum- and energy-resolved spin polarization in ultraviolet circularly polarized light at 400 nm, respectively. (c),(d) The corresponding momentum- and energy-resolved normalized yield asymmetry. Both the spin polarization and the normalized yield asymmetry are obtained from the simulation.