Optical—optical double-resonant multiphoton ionization spectra of Rydberg states of nitrogen dioxide

The optical—optical double-resonant multiphoton ionization (OODR-MPI) technique has been applied to the study of the Rydberg states of nitrogen dioxide. The results show that, although the OODR-MPI spectra of NO₂ are composed of regular progression bands at different pump laser intensities, their ionization pathways are different. The NO₂ molecule is ionized through the (3+1+1) double-resonant process as the pump laser intensity is in a high value, or else it is through the (1+2+1) process. The final resonant states in the two ionizing processes have been attributed to different Rydberg states.