The single active electron approximation for carbon monoxide in strong laser fields; a comparison with the multi-electron response*

Ultrafast, high intensity laser pulses make it possible to create electric fields comparable to those internal to molecular structure. For example, internal electric fields of the HOMO for neutral diatomic molecules are of order 10^-1 atomic units, corresponding to a laser intensity of  3.5×10¹⁴ W/cm². We calculate the molecular electron wave function for carbon monoxide interacting with a 40 femtosecond, 800nm pulse. We employ single active electron (SAE) and many active electron (MAE) Hartree-Fock wave functions for the calculations. The SAE response does not accurately account for the polarizability or Stark shift of the electron orbitals in the external field. Using MAE, the anisotropy of the many-electron wave function creates orientation dependent enhancement and suppression of the ionization yields observed experimentally. Given the large spatial overlap of the HOMO electron wave functions with the more tightly bound (HOMO-n) electrons in molecules, it is arguable that MAE calculations are required for many molecular systems in laser fields.