Molecular effects in the ionization of N$_2$, O$_2$ and F$_2$ by intense laser fields

Single ionization of N$_2$, O$_2$ and F$_2$ by intense laser fields is simulated using a time-dependent density functional approach within the exchange-only local density approximation. In particular, the response to laser pulses having a wavelength of $\lambda=390$nm (corresponding to a frequency-doubled Ti:Sapphire laser) is investigated. Single ionization yields of N$_2$ and F$_2$ are seen to agree to within a factor of 3, in keeping with experimental results but in disagreement with predictions based on the symmetry of the highest occupied molecular orbital. The underlying mechanism for this appears to be the importance of multi-electron molecular effects in the single ionization of F$_2$. In addition single ionization of O$_2$ is calculated assuming the ground state configuration to be either a singlet or triplet state. It is discovered that the single ionization process is largely insensitive to the multiplicity of the initial state