Spectral evidence of vibronic Rabi oscillations in the resonance-enhanced photodissociation of ${\mathrm{MgH}}^{+}$

We study by real-time wave-packet simulations the sequential two-photon dissociation of the ${\mathrm{MgH}}^{+}$ molecule, under intense ultraviolet laser pulses of narrow spectral width. By a resonant coupling, one-photon Rabi oscillations are generated between two vibrational levels of the molecule, belonging to two different bound electronic states. These vibronic Rabi oscillations are probed upon absorption of further photons from the same laser pulse, that gradually promote the molecule to a dissociative electronic state. Calculating the energy spectrum of the photofragments explicitly analytically, we elucidate the physical origin of the main spectral features—such as the splitting, shifting, asymmetry, and multipeak pattern—identified by accurate numerical simulations. We find that the pronounced spectral intensity modulations result from the interference of fragment amplitudes that are generated during the distinct Rabi cycles in the upper and lower dynamically dressed states, formed upon the resonant coupling with the laser pulse. These intensity modulations are sensitively influenced by the resonant and nonresonant Stark effects, and can be manipulated by the parameters of the applied laser pulse. Our results contribute to the understanding and control of ultrafast coherent phenomena via the energy spectrum of particles emitted during the dynamical process under investigation.