Experimental observation of the elusive double-peak structure in R-dependent strong-field ionization rate of H2+

Abstract

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 ionization"(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.