  We investigate the non-parametric, pure ac driven dynamics of nonlinear
Klein-Gordon solitary waves having an internal mode of frequency $\Omega_i$. We
show that the strongest resonance arises when the driving frequency
$\delta=\Omega_i/2$, whereas when $\delta=\Omega_i$ the resonance is weaker,
disappearing for nonzero damping. At resonance, the dynamics of the kink center
of mass becomes chaotic. As we identify the resonance mechanism as an {\em
indirect} coupling to the internal mode due to its symmetry, we expect similar
results for other systems.
