  We have theoretically studied the effect of nuclear mass on electron
localization in dissociating H_2^+ and its isotopes subjected to a few-cycle
3-{\mu}m laser pulse. Compared to the isotopic trend in the near-infrared
regime, our results reveal an inverse isotopic effect in which the degree of
electron-directed reactivity is even higher for heavier isotopes. With the
semi-classical analysis, we find, for the first time, the pronounced electron
localization is established by the interferences through different channels of
one- and, more importantly, higher-order photon coupling. Interestingly, due to
the enhanced high-order above-threshold dissociation of heavier isotopes, the
interference maxima gradually become in phase with growing mass and ultimately
lead to the anomalous isotopic behavior of the electron localization. This
indicates that the multi-photon coupling channels will play an important role
in controlling the dissociation of larger molecules with midinfrared pulses.
