Excitation of H$_{2}^{+}$ with one-cycle laser pulses: Shaped post-laser-field electronic oscillations, generation of higher- and lower-order harmonics

摘要

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