Signature of molecular symmetry in the plateau region of the photoelectron spectra: Above-threshold ionization of the C2 molecule

By analyzing angular and energy distributions of the photoelectrons emitted in strong-laser-field-induced ionization of molecules, one can obtain information about the molecular structure and the ground-state symmetry. High-energy part of the photoelectron spectra in the above-threshold ionization (ATI) is characterized by a plateau region in which the ionization probability is practically energy independent. The photoelectron yield drops off exponentially for electron energies higher than some critical energy, i.e. the mentioned plateau is followed by an abrupt cutoff. We investigate the influence of the molecular ground state symmetry on this plateau region and show that, analyzing the corresponding high-order ATI spectra, one can obtain information about the highest occupied molecular orbitals (HOMOs) of the considered molecules. We present results for different homonuclear diatomic molecules: N2, O2, Ar2 and C2 having, respectively, the σ g , π g , σ u and π u symmetries of the HOMO. Particular attention is devoted to the C2 molecule since high-order ATI spectra for this molecule have not been analyzed yet. We consider ATI by a linearly polarized laser field for which the mentioned plateau can be well-developed, depending on the orientation of the molecular axis with respect to the laser-field polarization axis. The HOMO-symmetry-dependent (dis)appearance of the plateau is particularly pronounced for the parallel and perpendicular orientations. Our findings are valid for a wide range of the laser-field intensities and wavelengths, which is important for realization of the suggested experiments. Using the improved molecular strong-field approximation, the theory which is particularly suitable for the analysis of high-energy ATI spectra, for the case of the C2 molecule and different molecular and laser parameters, we investigate various features of the plateau, such as its length and the interference minima and their positions.