Multiphoton ionization spectroscopy of the sodium dimer

We report an investigation of the role of molecular multiphoton ionization in the production of ${\mathrm{Na}}_2^{+}$ when sodium vapor is subjected to intense optical radiation. Previous authors attribute the source of much of the ion dimer signal to laser-induced associative ionization of atom sodium. In this experiment, we distinguish the molecular process from atomic collisional mechanisms by producing an intense molecular beam created through free-jet expansion of the metal vapor. The beam of nearly 50% dimers cooled to their low rotational and vibrational states allow us to obtain a simplified three-photon ionization spectrum. We find that the spectrum displays two-photon resonances corresponding to known Rydberg level transitions and that the $A$ state, acting as virtual intermediate, plays a crucial role in the large peak-to-peak intensity variations. We employ a simple model of multiphoton ionization which uses a rate-equation approach to generate a calculated spectrum. Based on the experimental results and the success of the model in reflecting them, we conclude that much of the highly structured component of the dimer ion signal reported previously under different experimental conditions is probably due to molecular multiphoton ionization but that this structure rides on a slowly varying broad signal envelope due to laser-induced associative ionization.