  We investigate the resonance-enhanced few-photon ionization of atomic lithium
by linearly polarized light whose frequency is tuned near the 2s-2p transition.
Considering the direction of light polarization orthogonal to the quantization
axis, the process can be viewed as an atomic "double-slit experiment" where the
2p states with magnetic quantum numbers m_l=+-1 act as the slits. In our
experiment, we can virtually close one of the two slits by preparing lithium in
one of the two circularly polarized 2p states before subjecting it to the
ionizing radiation. This allows us to extract the interference term between the
two pathways and obtain complex phase information on the final state. The
experimental results show very good agreement with numerical solutions of the
time-dependent Schroedinger equation. The validity of the two-slit model is
also analyzed theoretically using a time-dependent perturbative approach.
