delta function. Then, $Z_{cg}(R) \equiv Z_{gc}(R) \equiv 0$ since the $g\to c$ transition is forbidden in Fig. \[Fig\_potentials\](a) ; respectively, $Z_{gg}(R) \equiv 0$ in Fig. \[Fig\_potentials\](b). The first nonzero effective potentials in Figs. \[Fig\_potentials\](a) and \[Fig\_potentials\](b) are $Z_{gg}(R)$ and $Z_{cg}(R)$, respectively, and they are plotted in Fig. \[Fig\_effective\_potentials\]. Taking into account that $E_g - \bar{E}_1 \approx E_{cm} = 1$ (a.u.) ($E_g$ being the ground state energy) is the kinetic energy of the center of mass, we may qualitatively conclude from Fig. \[Fig\_effective\_potentials\] that the center of mass needs to tunnel through the barrier \[$Z_{gg}(R)$\] in Fig. \[Fig\_potentials\](a) and flies above the barrier \[$Z_{cg}(R)$\] in Fig. \[Fig\_potentials\](b); thus, the probability of finding the particle in the first quadrant in Fig. \[Fig\_potentials\](b) prevails over the probability of tunneling in Fig. \[Fig\_potentials\](a). Finally, we note that the first nonzero effective potentials in Figs. \[Fig\_potentials\](c) and \[Fig\_potentials\](d) are of the same order; the same statement is valid in cases Figs. \[Fig\_potentials\](e) and \[Fig\_potentials\](f).

![(Color online) Plots of effective potentials $Z_{nn'}(R)$ \[Eq. (\[Effective\_Potential\_Def\])\]. The solid line is $Z_{gg}(R)$ for case (a) of Fig. \[Fig\_potentials\]. The dashed line represents $Z_{cg}(R)$ for case (b) of Fig. \[Fig\_potentials\]. []{data-label="Fig_effective_potentials"}](EffectivePotentials.pdf)

Traces of the forced excitations, which occur in Figs. 