High-order harmonic generation of heteronuclear diatomic molecules in intense ultrashort laser fields: An all-electron TDDFT study

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 used 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. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007