When molecules are driven by intense laser fields linearly polarized along the internuclear axis, their ionization rate vs the internuclear separation $R$ is known to have a maximum at some critical intermediate separation $R_c$. We show that such a maximum should occur only in molecules whose highest occupied molecular orbitals (HOMOs) are such that the electron cloud is concentrated along the internuclear axis (i.e., $\sigma$-type HOMOs). For molecular systems whose HOMO symmetry prevents electron probability distribution along this axis, we find that the ionization rate increases monotonically with increasing $R$ until saturation occurs. In other words, for such molecular systems, there is no critical $R_c$ at which molecular ionization is maximum.

• Received 28 October 2006

DOI:https://doi.org/10.1103/PhysRevA.75.041401