Hengtai Yu

Intense field ionization of molecules with ultra-short laser pulses - enhanced ionization and barrier-suppression effects

Exact numerical solutions of the time-dependent Schr?dinger equation have been performed in order to calculate ionization rates in intense short laser pulses for linear three-dimensional and linear one-dimensional . All molecular ions exhibit unusually large ionization rates at large critical interfragment distances . Expressions are derived for based on quasistatic models of field-induced suppression of electron Coulomb barriers in the molecules. The predicted agree satisfactorily with the exact numerical results. In the symmetric case, laser-induced electron localization can also enhance ionization. The present numerical results show that large or enhanced ionization rates exceeding those of nonionized fragments will dominate for laser pulses which are sufficiently long to allow dissociative ionization fragments to reach such critical distances.

Three‐dimensional Cartesian finite element method for the time dependent Schrödinger equation of molecules in laser fields

A finite element (FE) method in three‐dimensional Cartesian coordinates is described to solve the time dependent Schrodinger equation for H+2, H2, and H+3 in the presence of time dependent electromagnetic fields. The ionization rates and harmonic generation spectra have been calculated for these molecules for field directions parallel or perpendicular to the molecular axis. Nonlinear optical susceptibilities of H+2 have been also obtained for different laser field directions. The time dependent Hartree–Fock results are compared to frozen core calculations for H2. Comparisons of present FE numerical results with previously published calculations show the FE method reproduces perturbative results and can also treat nonperturbatively the effect of intense short laser pulses as the method includes both bound and continuum electronic states.

High-order harmonic generation at long range in intense laser pulses

The physics of high-order harmonic generation in single-electron molecular ions at long range (large internuclear distances) is examined by exact numerical solutions of the three-dimensional time-dependent Schrodinger equation. These numerical solutions show that photon energies can be produced with energies exceeding the atomic cut-off law , where is the ionization energy and is the ponderomotive energy. It is shown that preferential high-order harmonic generation occurs at critical distances , and , where is the ponderomotive radius, by collision of electrons with neighbouring ions as opposed to recollision with the parent ion. The most efficient harmonic generation occurs at due to less electron wavefunction expansion.

Three-dimensional finite element method for electronic properties of small polyatomic molecules: H+2, H2, H2+3 and H+3

Abstract A fully numerical solution of the SCF equations is presented for the small polyatomic molecules: H + 2 , H 2 , H 2+ 3 and H + 3 by a finite element method in Cartesian coordinates in three dimensions. A new method to remove the singularities of nuclear potentials is given. A block Lanczos method is used to avoid large matrix storage problems for large finite element basis sets.

Application of the finite element method to time-dependent quantum mechanics: I. H and He in a laser field

A finite element approach is described to solve the time- dependent Hartree-Fock equation of atoms in the presence of time-dependent electromagnetic fields. Time-dependent energy changes, ionization rates and high order nonlinear optical polarizabilities, χ2n+1 (n >, 0) for the atoms H and He have been calculated. The finite element method is shown to be easily adaptable to treat intense short pulses and includes automatically both bound and continuum electronic states.

Laser Control of Electrons in Molecules

Coherent superposition of electronic states can be achieved by simultaneous laser excitation at different frequencies and phases. A three level excitation scheme is analyzed and applied to phase control of electrons or equivalently charge transfer in complex molecules. Ab initio molecular orbitals are used to demonstrate the principle in the charge transfer molecule DMABN, 4-(N,N-dimethylamino)benzonitrile.

Charge Resonance Enhanced Ionization (CREI) of Molecules in Intense Laser Fields

Short (τ≤100 fs) intense (I≥ 1014 W/cm2) laser pulses are shown from nonperturbative numerical simulations of the time‐dependent Schroedinger equation to induce unusually large ionization rates in molecules often exceeding those of the dissociated fragments. Linear one electron, H2+, H3++ and two electron systems H2, H3+ have been studied numerically in order to understand this phenomenon. It will be shown that enhanced ionization is due to the existence of large divergent transition moments in these symmetric molecules due to degeneracies of electronic orbitals upon dissociation. Such degenerate states called charge resonance states in 1939 by Mulliken, give rise to charge resonance oscillations. We show that these charge resonance states are responsible for charge resonance enhanced ionization, CREI. A static field picture of CREI will be shown to explain adequately the enhanced ionization and the critical distances at which it occurs through field induced barrier suppression of the electron‐nuclear cou...

Application of the Finite Element Method to the 3-D Hydrogen Atom in an Intense Laser Field

Present laser technology allows one to subject atoms and molecules to ever increasing radiation intensities so that eventually perturbation theory breaks down, i.e., renormalization of the electronic spectrum by the external field must be taken into account. Highly nonlinear effects such as above threshold ionization (ATI) readily occur in the case of atoms [1] and recently in molecules also [2–3]. Laser induced avoided crossings of molecular electronic potential curves is another result of such nonperturbative effects [2–6]. Clearly nonperturbative radiative effects predominate whenever laser field intensities approach atomic electric fields in an atom. The atomic unit (a.u.) of the electric field e2/a 0 2 ; corresponds to a field intensity of 6 x 1016 W/cm2 [7]. Such intensities are currently being attained, so that perturbative approaches to atom-laser interactions are no longer adequate.