Effect of bound electron wave packet displacement on the multiphoton ionization of a lithium atom

We study numerically the strong-field single ionization of a Li atom over a wide range of wavelengths (400 nm-1600 nm) in the multiphoton regime. By observing the behavior of the ionization probabilities and the occupation of excited states, we found that the occupied 2p state in different wavelength regimes exhibit different bound electron wave packet motion. The motion is in phase in the long-wavelength regime, out of phase in the short-wavelength regime and lagged when the wavelength is close to resonance with the 2p state. We then demonstrate how these different motions of the bound electron wave packet affect the ionization probability.