Nonlinear time-dependent density-functional-theory study of ionization and harmonic generation in CO{sub 2} by ultrashort intense laser pulses: Orientational effects

Time-dependent density-functional-theory (TDDFT) methods are used to calculate the orientational dependence of ionization and molecular high-order harmonic generation (MHOHG) in the CO{sub 2} molecule as a function of laser intensity I{sub 0{>=}}10{sup 14} W/cm{sup 2} for few-cycle 800 nm laser pulses. A time-series analysis is used to confirm the recollision model in MHOHG for different density potentials. It is found that at intensities I{sub 0}>3.5x10{sup 14} W/cm{sup 2}, lower highest occupied molecular orbitals (HOMO's) contribute significantly to ionization and to the MHOHG process. This is due to the symmetry of these orbitals. Even though such lower orbitals have higher ionization potentials (IP), ionization and MHOHG processes occur when orbital densities are maximum with laser polarization direction.