Intense source of spin-polarized electrons using laser-induced optical pumping

Abstract

A source of spin-polarized electrons based on a laser-pumped flowing helium afterglow is described. He(23S) atoms contained in the afterglow are optically pumped using circularly polarized 1.08-μm (23S→23P) radiation provided by a NaF (F2+)* color-center laser. Spin angular momentum conservation in subsequent chemi-ionization reactions with CO2 produces polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of ∼70%–80% are achieved. At higher currents the polarization decreases, falling to ∼40% at 50 μA. The spin polarization can be simply reversed (P→−P) and the source is suitable for use in the majority of low-energy spin-dependent scattering experiments proposed to date.

Spin-polarized electrons produced by a sequence of two femtosecond pulses. Calculation of differential and global polarization rates

“…The maximum value is reached at a time t p = (2p + 1)π/ω 21 where a maximum of population is transferred from the bright state to the dark one. Since the moduli of the dipole moments µ 1 and µ 2 are different, the wavepacket cannot entirely be transferred to the dark state, which limits the value of the polarization rate to 8 9 . Expression (5) has been obtained in the ideal case where the pulse duration is negligible compared with the oscillation period 2π/ω 21 .…”

“…For example, resonant two-photon ionization of trivalent atoms with circularly polarized light [3], threephoton ionization of alkali atoms [4][5][6] or laser selective induced interference [7]. Alternative methods using the optical pumping of helium atoms have led successfully to the design of polarized electron sources [8,9]. These methods provide high polarization rates (up to 87%) at low currents (0.1 µA), but the rate drops (≈ 50%) at higher currents (70 µA).…”

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

“…For example, resonant two-photon ionization of trivalent atoms with circularly polarized light [21], three-photon ionization of alkali atoms [22][23][24] or laser selective induced interference [25]. Alternative methods using the optical pumping of helium have permitted the design of polarized electron sources [26,27]. These methods provide high polarization rates (up to 87%) at low currents (0.1 µA), but much lower rates (∼ 50%) at higher currents (70 µA).…”

Ultrafast Magnetization of a Dense Molecular Gas with an Optical Centrifuge

“…Most notably, such gases are used for converting their electronic polarization to the polarization of nuclear spins [1], one of the key elements in nuclear magnetic resonance imaging [2]. Equally important, they provide a source of spin-polarized electrons [3,4] for particle-physics experiments [5], as well as for probing molecular dynamics during chemical reactions and analyzing the electronic properties of materials [6].…”