The mechanisms of anisotropic near-IR tunnel ionization and high-order harmonic generation (HHG) in a CO molecule are theoretically investigated by using the multiconfiguration timedependent Hartree-Fock (MCTDHF) method developed for the simulation of multielectron dynamics of molecules. The multielectron dynamics obtained by numerically solving the equations of motion (EOMs) in the MCTDHF method is converted to a single orbital picture in the natural orbital representation where the first-order reduced density matrix is diagonalized. The ionization through each natural orbital is examined and the process of HHG is classified into different optical paths designated by a combinations of initial, intermediate and final natural orbitals. The EOMs for natural spin-orbitals are also derived within the framework of the MCTDHF, which maintains the first-order reduced density matrix to be a diagonal one throughout the time propagation of a many-electron wave function. The orbital dependent, timedependent effective potentials that govern the dynamics of respective time-dependent natural orbitals are deduced from the derived EOMs, of which the temporal variation can be used to interpret the motion of the electron density associated with each natural spin-orbital. The roles of the orbital shape, multiorbital ionization, linear Stark effect and multielectron interaction in the ionization and HHG of a CO molecule are revealed by the effective potentials obtained. When the laser electric field points to the nucleus O from C, tunnel ionization from the C atom side is enhanced; a hump structure originating from multielectron interaction is then formed on the top of the field-induced distorted barrier of the HOMO effective potential. This hump formation, responsible for the directional anisotropy of tunnel ionization, restrains the influence of the linear Stark effect on the energy shifts of bound states.

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 {ϕj(t)}. Within the framework of MCTDHF, we derived the equations of motion for {ϕj(t)}. From the derived equations, we defined the TD effective potentials that govern the dynamics of ϕj(t). consists of the one-body part v1(t) including the interaction with ɛ(t) and the two-body part v2,j(t) originating from electron–electron interaction. In for the 5σ HOMO, a narrow hump associated with the increase in v2,5σ(t) is surmounted on the field-distorted barrier of v1(t) + v2,5σ(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 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, becomes barrierless, enhancing high-harmonic generation through 4σ. We also simulated the dynamics for a λ = 380 nm pulse to investigate how reflects the nonadiabatic electronic response to the pulse. We found that the height of the hump in v1(0) + v2,5σ(t) is nearly proportional to the induced dipole moment of ϕ5σ(t), irrespective of whether the response is adiabatic or not. The for LiH is also presented to demonstrate the ubiquity of hump structures.

Using the framework of multiconfiguration theory, where the wavefunction Φ(t) of a many-electron system at time t is expanded as Φ(t)=Σ(I)C(I)(t)Φ(I)(t) in terms of electron configurations {Φ(I)(t)}, we divided the total electronic energy E(t) as E(t)=Σ(I)|C(I)(t)|(2)E(I)(t) . Here E(I)(t) is the instantaneous phase changes of C(I)(t) regarded as a configurational energy associated with Φ(I)(t). We then newly defined two types of time-dependent states: (i) a state at which the rates of population transfer among configurations are all zero; (ii) a state at which {E(I)(t)} associated with the quantum phases of C(I)(t) are all the same. We call the former time-dependent state a classical stationary state by analogy with the stationary (steady) states of classical reaction rate equations and the latter one a quantum stationary state. The conditions (i) and (ii) are satisfied simultaneously for the conventional stationary state in quantum mechanics. We numerically found for a LiH molecule interacting with a near-infrared (IR) field ε(t) that the condition (i) is satisfied whenever the average velocity of electrons is zero and the condition (ii) is satisfied whenever the average acceleration is zero. We also derived the chemical potentials μ(j)(t) for time-dependent natural orbitals ϕ(j)(t) of a many-electron system. The analysis of the electron dynamics of LiH indicated that the temporal change in Δμ(j)(t) ≡ μ(j)(t) + ε(t) · d(j)(t) - μ(j)(0) correlates with the motion of the dipole moment of ϕ(j)(t), d(j)(t). The values Δμ(j)(t) are much larger than the energy ζ(j)(t) directly supplied to ϕ(j)(t) by the field, suggesting that valence electrons exchange energy with inner shell electrons. For H2 in an intense near-IR field, the ionization efficiency of ϕ(j)(t) is correlated with Δμ(j)(t). Comparing Δμ(j)(t) to ζ(j)(t), we found that energy accepting orbitals of Δμ(j)(t) > ζ(j)(t) indicate high ionization efficiency. The difference between Δμ(j)(t) and ζ(j)(t) is significantly affected by electron-electron interactions in real time.

We propose an effective model called the "charge model", for the half-filled one-dimensional Hubbard and extended Hubbard models. In this model, spin-charge separation, which has been justified from an infinite on-site repulsion (U ) in the strict sense, is compatible with charge fluctuations. Our analyses based on the many-body Wannier functions succeeded in determining the optical conductivity spectra in large systems. The obtained spectra reproduce the spectra for the original models well even in the intermediate U region of U = 5-10T , with T being the nearestneighbor electron hopping energy. These results indicate that the spin-charge separation works fairly well in this intermediate U region against the usual expectation and that the charge model is an effective model that applies to actual quasi-one-dimensional materials classified as strongly correlated electron systems. N n=1 σ,σ c † n,σ c n,σ c † n+1,σ c n+1,σ .The termK(t) describes the transfer of electrons, where c † n,σ (c n,σ ) creates (annihilates) an electron of spin σ at site n, and A(t) is the dimensionless vector potential at time t. The electron-field coupling has been introduced into the transfer integral as a Peierls phase.The termV describes the Coulomb interaction, where V is the Coulomb interaction energy between neighboring sites. A periodic boundary condition is imposed in that c N +1,σ = c 1,σ

We have investigated the terahertz (THz)-pulse induced dynamics of tetrathiafulvalene-pchloranil near the boundary between the ionic and neutral phases with the use of exact numerical calculations of an extended Hubbard model coupled with lattice motion. For the ionic phase, when the applied electric field of the THz-pulse opposes the electronic contribution to the electric polarization (electronic polarization)P el of the ground state and the maximum amplitude of electric field is greater than a threshold value, the THz-pulse excited state changes as I A → N → I B → N → I A → N → • • • (I A ground state case) or I B → N → I A → N → I B → N → • • • (I B ground state case), where N shows the neutral state, I A (I B) shows the ionic state withP el < 0 (P el > 0), and the phase of the bond length alternation of I A is opposite to that of I B. For the neutral phase, when the maximum amplitude of the electric field is greater than a threshold value, the THz-pulse excited state changes as N → I B → N → I A → N → I B → • • • (positive electric field case) or N → I A → N → I B → N → I A → • • • (negative electric field case). The phase transitions and electronic polarization reversal are driven by time variation of the lattice order parameter, which indicates the magnitude and phase of the bond length alternation, and the lattice motion is induced by THz-pulse excitation through the electron-lattice coupling. Transitions between the ionic and neutral phases occur and electronic polarization reverses on a picosecond time scale together with the realizable magnitude of the THz pulse both in the ionic and neutral phases near the phase boundary.