Polarization-dependent pump-probe studies in atomic fine-structure levels:towards the production of spin-polarized electrons

The precession of orbital and spin angular momentum vectors has been observ ed in a pump-probe study of the 4P fine-structure states of atomic potassiu m. A femtosecond pump pulse prepared a coherent superposition of the two fi ne-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the i on signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives phys ical insight into the cause of the half-period phase shift observed when th e relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation o f the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale.