  Chirality causes symmetry breaks in a large variety of natural phenomena
ranging from particle physics to biochemistry. We investigate one of the
simplest conceivable chiral systems, a laser-excited, oriented, effective
one-electron Li target. Prepared in a polarized p state with |m|=1 in an
optical trap, the atoms are exposed to co- and counter-rotating circularly
polarized femtosecond laser pulses. For a field frequency near the excitation
energy of the oriented initial state, a strong circular dichroism is observed
and the photoelectron energies are significantly affected by the
helicity-dependent Autler-Townes splitting. Besides its fundamental relevance,
this system is suited to create spin-polarized electron pulses with a
reversible switch on a femtosecond timescale at an energy resolution of a few
meV.
