  High-resolution photoelectron momentum distributions of Xe atoms ionized by
800-nm linearly polarized laser fields have been traced at intensities from
1.1*1013 to 3.5*1013W/cm2 using velocity-map imaging techniques. At certain
laser intensities, the momentum spectrum exhibits a distinct double-ring
structure for low-order above-threshold ionization, which appears to be absent
at lower or higher laser intensities. By investigating the intensity-resolved
photoelectron energy spectrum, we find that this double-ring structure
originates from resonant multiphoton ionization involving multiple Rydberg
states of atoms. Varying the laser intensity, we can selectively enhance the
resonant multiphoton ionization through certain atomic Rydberg states. The
photoelectron angular distributions of multiphoton resonance are also
investigated for the low-order above-threshold ionization.
