Electron-impact dissociative cross sections for H2 and D2 vibrationally excited molecules have been calculated in the frame of the impact-parameter method for dissociative processes involving direct dissociation M2(X'Xg, U;)+e~M2(B 'X"+, C'H")+e~2M+e and radiative cascade M2(X'Xg, v;) +e~Mz (B 'X"+, C'll")+e -+M2(X 'Xg+)+e+hv~2M+e+hv (M2 represents a hydrogen or deuterium molecule). The results show that direct dissociation cross sections strongly depend on the initial vibrational state of the molecule, while an opposite behavior is found in the case of dissociation by radiative cascade. Moreover, for this last process, the fraction of molecules that undergoes dissociation has been evaluated with respect to the total cascade process, yielding in general quite small values in both the H2 and D2 cases.PACS number(s): 34.80.Gs, 52.20.Fs

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