Analysis of the multielectron dynamics in intense laser-induced ionization of CO by the time-dependent effective potentials for natural orbitals

We simulated the multielectron dynamics of a CO molecule irradiated by near-IR λ = 760 nm two-cycle pulses with different carrier envelope phases by using the multiconfiguration time-dependent (TD) Hartree–Fock (MCTDHF) method. The ionization rate estimated from the simulations is higher when the laser electric field ɛ(t) points from C to O than in the opposite case, in agreement with the results of two-color experiments. The mechanism of the directional anisotropy in tunnel ionization of CO was examined by converting the obtained multielectron dynamics to the representation in terms of TD natural orbitals {ϕ _j(t)}. Within the framework of MCTDHF, we derived the equations of motion for {ϕ _j(t)}. From the derived equations, we defined the TD effective potentials $v_j^{eff}\left(t\right)$ that govern the dynamics of ϕ _j(t). $v_j^{eff}\left(t\right)$ consists of the one-body part v ₁(t) including the interaction with ɛ(t) and the two-body part v _2,j(t) originating from electron–electron interaction. In $v_{5\sigma }^{eff}\left(t\right)$ for the 5σ HOMO, a narrow hump associated with the increase in v _2,5σ(t) is surmounted on the field-distorted barrier of v ₁(t) + v _2,5σ(0) near the nucleus C when ɛ(t) points from C to O, which results in the anisotropic ionization of CO. For 4σ, a high barrier that suppresses ionization is formed in $v_{4\sigma }^{eff}\left(t\right)$ when ɛ(t) points from C to O, which suggests that dynamical correlation exists between 4σ and 5σ electrons on route to ionization. For the opposite phase, $v_{4\sigma }^{eff}\left(t\right)$ becomes barrierless, enhancing high-harmonic generation through 4σ. We also simulated the dynamics for a λ = 380 nm pulse to investigate how $v_{5\sigma }^{eff}\left(t\right)$ reflects the nonadiabatic electronic response to the pulse. We found that the height of the hump in v ₁(0) + v _2,5σ(t) is nearly proportional to the induced dipole moment of ϕ _5σ(t), irrespective of whether the response is adiabatic or not. The $v_{HOMO}^{eff}\left(t\right)$ for LiH is also presented to demonstrate the ubiquity of hump structures. (paper)