  The dissociative ionization of $H_2^+$ in linearly polarized, 400 nm laser
pulses is simulated by solving a three-particle time-dependent Schr\"odinger
equation in full dimensionality. The joint energy spectra (JES) are computed
for $\cos^8$ and flat-top envelopes using the time-dependent surface flux
(tSurff) methods. In JES, the energy sharing $n$ photon energies $\omega$ of
nuclear kinetic energy release (KER) $E_N$ and electronic KER $E_e$ are well
represented by $E_N+E_e=n\omega-U_p+E_0$ for $\cos^8$ pulses, but satisfy
$E_N+E_e=n\omega+E_0$ for flat-top envelope, exposing a deviation of the
ponderomotive energy $U_p$, which has been observed in experiments, where $E_0$
is the ground energy of $H_2^+$. The analysis of the wavefunction for electrons
and protons after the pulse are presented, where we find $U_p$ is absorbed by
the Freeman resonances between two excited ungerade states.
