In the 1980s Demkov, Kondratovich, and Ostrovsky and Kondratovich and
Ostrovsky proposed an experiment based on the projection of slow electrons
emitted by a photoionized atom onto a position-sensitive detector. In the case
of resonant excitation, they predicted that the spatial electron distribution
on the detector should represent nothing else but a magnified image of the
projection of a quasibound electronic state. By exciting lithium atoms in the
presence of a static electric field, we present in this Letter the first
experimental photoionization wave function microscopy images where signatures
of quasibound states are evident. Characteristic resonant features, such as
(i) the abrupt change of the number of wave function nodes across a resonance
and (ii) the broadening of the outer ring of the image (associated with
tunneling ionization), are observed and interpreted via wave packet
propagation simulations and recently proposed resonance tunneling mechanisms.
The electron spatial distribution measured by our microscope is a direct
macroscopic image of the projection of the microscopic squared modulus of the
electron wave that is quasibound to the atom and constitutes the first
experimental realization of the experiment proposed 30 years ago
