Ionization of N₂, O₂, and linear carbon clusters in a strong laser pulse

Multiphoton ionization of linear molecules is studied using the strong-field S-matrix approach. Numerical calculations for the angular distribution and the energy spectrum of the photoelectron as well as the total ionization rates and yields in an intense linearly polarized laser pulse are performed. Results are obtained for molecules aligned along the polarization axis and for ensembles of molecules having a random orientation of the molecular axis with respect to the polarization direction. Signatures of the molecular geometry and the orbital symmetry are identified and discussed with reference to molecular imaging and alignment. It is found that these signatures are clearly marked for the diatomic molecules C₂, N₂, and O₂, but are much weaker for the group of linear carbon clusters due to the polyatomic character as well as to the competing contributions from different valence shells of these molecules. Finally, predictions of the present theory for the dependence of the total ionization rates and yields on the orientation of the molecule are compared with other theoretical models and recent experimental data.