EFFECTS OF MULTI-ELECTRON CORRELATION ON MULTIPHOTON IONIZATION AND HIGH-ORDER HARMONIC GENERATION OF ATOMIC AND MOLECULAR SYSTEMS IN INTENSE ULTRASHORT LASER FIELDS

Abstract

The study of the electron correlation and quantum dynamics of many-electron atoms and molecules in the presence of intense external fields is a subject of much current importance in science and technology. While experimental breakthroughs constantly challenge theorists, the reverse is also true, with theorists suggesting new experimental paths and novel ways to reach exciting regimes where new physics can be explored. For example, we have recently developed ab initio methods and applications to study time dependent quantum dynamics of atoms and molecules which cannot be understood by traditional perturbation theories. Moreover, currently there exist no adequate methods capable of studying the dynamical role of the individual valence electron to the high-order harmonic generation (HHG) and multiphoton ionization (MPI) processes in strong fields. Such a study can provide insights regarding the detailed quantum dynamics and HHG mechanisms, as well as the optimal control of strong-field processes. To advance this strong-field atomic and molecular physics, this dissertation aims at the developing new theoretical formalisms and accurate computational methods for ab initio non-perturbative studies of atomic and molecular processes in intense laser fields. The new methods developed allow in-depth and precision studies of strong-field phenomena for multielectron systems. In this dissertation we investigate the role of electron correlation in dynamics of multielectron systems subject to strong fields. We present a time-dependent density functional theory (TDDFT), with proper asymptotic long-range potential, for nonperturbative treatment of multiphoton processes of homonuclear and heteronuclear diatomic molecules in intense ultrashort laser fields. A time-dependent two-center generalized pseudospectral method is presented for accurate and efficient treatment of the TDDFT equations in space and time. The procedure allows nonuniform and optimal spatial grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split-operator scheme in the energy representation is extended for the time propagation of the individual molecular spin-orbital. The theory is applied to a detailed all-electron study of multiphoton ionization (MPI) and high-order harmonic generation (HHG) processes of N2 and CO molecules in intense laser pulses. The results reveal intriguing and substantially different nonlinear optical response behaviors for N2 and CO, despite the fact that CO has only a very small permanent dipole moment. In particular, we found that the MPI rate for CO is higher than that of N2. Furthermore, while laser excitation of the homonuclear N2 molecule can generate only odd harmonics, both even and odd harmonics can be produced from the heteronuclear CO molecule. Next, we present a complex-scaling (CS)-generalized pseudospectral (GPS) method in hyperspherical coordinates (HSC) for an accurate ab initio and accurate treatment of the electron structure and quantum dynamics of two-electron systems. The six-dimensional coupled hyperspherical adiabatic-channel equations are discretized and solved efficiently and accurately by means of the GPS method. The GPS method allows non-uniform and optimal spatial discretization of the two-electron Hamiltonian in HSC with the use of only a very modest number of grid points. The procedure is applied for the precision calculation of the energies and widths of doubly-excited Rydberg resonance states as well as the ionization rates of He atoms in an external electric field. Lastly, we present a time-dependent generalized pseudospectral (TDGPS) approach in hyperspherical coordinates for fully ab initio nonperturbative treatment of multiphoton dynamics of atomic systems in intense laser fields. The laser-driven two-electron system is described by hyperspherical close coupling scheme. A novel 6D coupled time-dependent generalized pseudospectral method approach in hyperspherical coordinates are developed for single or double ionization without the use of the conventional adiabatic channels. In conclusion, the present dissertation provides new developments in both theoretical and computational techniques, as well as advancements in the essential understanding of strong-field atomic and molecular physics.