Isotope effects and bond softening in intense-laser-field multiphoton dissociation ofH2+

Isotope effects in the ${\mathrm{H}}_2^{+\mathrm{-}}$ ${\mathrm{D}}_2^{+}$ fragmentation by intense laser fields offer the possibility of a multiphoton interpretation of the bond-softening mechanism. Surprisingly, the calculations indicate that the one-photon dissociation of ${\mathrm{D}}_2^{+}$ is favored with respect to that of ${\mathrm{H}}_2^{+}$. This cannot be understood, as has previously been done, by a single-photon mechanism following tunneling through a lowered potential barrier, obviously more transparent for the lighter ${\mathrm{H}}_2^{+}$. It is rather a competition between this single-photon mechanism and a five-photon mechanism which is suggested for a more realistic interpretation.