  When a diatomic molecule is ionized by an intense laser field, the ionization
rate depends very strongly on the inter-nuclear separation. That dependence
exhibits a pronounced maximum at the inter-nuclear separation known as the
critical distance. This phenomenon was first demonstrated theoretically in H2+
and became known as charge-resonance enhanced ionization (CREI, in reference to
a proposed physical mechanism) or simply enhanced ionisation (EI). All
theoretical models of this phenomenon predict a double-peak structure in the
R-dependent ionization rate of H2+. However, such double-peak structure has
never been observed experimentally. It was even suggested that it is impossible
to observe due to fast motion of the nuclear wavepackets. Here we report a
few-cycle pump-probe experiment which clearly resolves that elusive double-peak
structure. In the experiment, an expanding H2+ ion produced by an intense pump
pulse is probed by a much weaker probe pulse. The predicted double-peak
structure is clearly seen in delay-dependent kinetic energy spectra of protons
when pump and probe pulses are polarized parallel to each other. No structure
is seen when the probe is polarized perpendicular to the pump.
