Intense-laser-field ionization of the hydrogen molecular ions H2+ and D2+ at critical internuclear distances.

Fragmentation of H2+ and D2+ in ion beams has been studied with short intense laser pulses (100 fs, I=5x10(13)-1x10(15) W/cm2) and by a high-resolution two-dimensional velocity imaging technique. In the Coulomb explosion channel, at intensities just above the threshold for this process, we observe a clear structure in the kinetic energy spectra not previously found or predicted. The peaks can be attributed to single vibrational levels. We interpret this observation as a dissociative allocation of the electron to a proton followed by enhanced ionization at a well-defined "critical" overstretched internuclear distance. When using longer pulses we observe three separate Coulomb explosion velocity groups corresponding to critical distances of about 8, 11, and 15 a.u.