Observing the Transition from Stark-Shifted, Strong-Field Resonance to Nonadiabatic Excitation<xref ref-type="fn" rid="fn800"></xref>

Time-dependent Hartree-Fock simulations for a linear triatomic molecular monocation (CO2) interacting with a 5 fs, 800 nm, strong field laser pulse were performed to explore the excitation mechanisms in a molecular cation. Fourier analysis of the time-dependent residual dipole moment reveal a nonmonotonic behavior in the amplitude of the 5.18 eV feature in the excitation spectra of the molecular monocation with increasing intensity, suggesting a change in the excitation mechanism. Calculations performed for different carrier frequencies of the laser field reveal that the mechanism changes from resonant multiphoton excitation to nonadiabatic excitation. In the resonant multiphoton excitation regime, a slight variation of the laser pulse duration (fwhm) reveals a nonlinear increase in the peak height of the multiphoton resonant state, and Stark-shifting of the multiphoton resonant state is observed for an increase in intensity. Nonadiabatic excitation from TDHF compares well with the analytical theory for nonadiabatic excitation.