Field modulation of Rydberg-state populations of NO studied by femtosecond time-resolved photoelectron imaging

Femtosecond time-resolved velocity map imaging combined with multiphoton ionization was applied to study the optical field modulation of NO Rydberg-state populations. The A(2)Sigma +(upsilon = 2) state is populated by absorption of one 271-nm photon. Two peaks in the photoelectron kinetic energy spectra, centered at 0.82 and 2.35 eV, are caused by ionization from the A(2)Sigma(+) (upsilon = 2) state by time-delayed one-color and two-color multiphoton ionization, respectively. In the overlap region of the pump and probe light, the C-2 Pi (upsilon = 4) state is populated by a 1 + 1' excitation. When the pump laser intensity is increased, other Rydberg states (E-2 Sigma(+), F-2 Delta, and D-2 Sigma(+)) are moved into resonance by a laser-induced Stark shift. These states can be populated only within the temporal overlap region of the pump and probe light. When the intensity of the pump laser is higher than 2.9 x 10(12) W/cm(2), Rydberg-valence coupling between the A(2)Sigma(+)(upsilon = 2) and B-2 Pi (upsilon = 4) states may play a key role, resulting in photoelectrons with kinetic energy of 0.37 eV. The coupling strength increases with increasing pump laser intensity.