Excitation of H₂⁺ with one-cycle laser pulses: Shaped post-laser-field electronic oscillations, generation of higher- and lower-order harmonics

Non Born-Oppenheimer quantum dynamics of H$_{2}^{+}$ excited by shaped one-cycle laser pulses linearly polarized along the molecular axis have been studied by the numerical solution of the time-dependent Schr\"odinger equation within a %three-body three-dimensional model, including the internuclear separation, $R$, and the electron coordinates $z$ and $\rho$. Laser carrier frequencies corresponding to the wavelengths $\lambda_{l}=25$~nm through $\lambda_{l}=400$~nm were used and the amplitudes of the pulses were chosen such that the energy of H$_{2}^{+}$ was close to its dissociation threshold at the end of any laser pulse applied. It is shown that there exists a characteristic oscillation frequency $\omega_{\rm osc} \simeq 0.2265$~au (corresponding to the period of $\tau_{\rm osc} \simeq 0.671$~fs and the wavelength of $\lambda_{\rm osc} \simeq 200$~nm) that manifests itself as a "carrier" frequency of temporally shaped oscillations of the time-dependent expectation values $\langle z \rangle$ and $\langle \partial V/\partial z \rangle$ that emerge at the ends of the laser pulses and exist on a timescale of at least 50~fs. Time-dependent expectation values $\langle \rho \rangle$ and $\langle \partial V/\partial \rho \rangle$ of the optically-passive degree of freedom, $\rho$, demonstrate post-laser-field oscillations at two basic frequencies $\omega^{\rho}_{1} \approx \omega_{\rm osc}$ and $\omega^{\rho}_{2} \approx 2\omega_{\rm osc}$. Power spectra associated with the electronic motion show higher- and lower-order harmonics with respect to the driving field.