In the "standard" adiabatic approximation, this work is the first time to report the isotope effect in the electron impact dissociation of H-2+(1ssigmag) and D2+(1ssigmag). The various dissociative cross sections of the Born asymptote calculated according to the Bethe theory due to Inokuti et al are given for studying the isotope effect. There is almost no isotope effect in the electron impact ionization and a 2% of isotope effects in the electron impact dissociation at incident energies greater-than-or-equal-to 200 eV. The good agreement between theory and experiment for the isotope effect in the total ion production is found. This work also points out that accurate experimental data of the rotational-vibrational(R-V) state distributions of H-2+(1ssigmag) and D2+(1ssigmag) with an error less than 5% are required to study the small isotope effects reported here. The isotope effect of experimental measurements in the electron impact dissociation of H-2 and D2 is identified to be much larger than the theoretical values deduced from the study of total inelastic scattering at high incident energies (> 500 eV). The large isotope effects in the electron impact dissociation is argued mainly due to the contribution from predissociation via the "superexcited" state of molecules. The nonadiabatic approach to understanding the coupling between electronic and nuclear motions based on the multichannel quantum defect theory (MQDT) is outlined to discuss the large isotope effects.
