Ion formation in laser-irradiated sodium vapor

The formation of ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_2^{+}$ in laser-irradiated sodium vapor has been studied using a nitrogen-laser-pumped dye laser. The effects of irradiation by this pulsed laser are substantially different from those reported previously using a cw device, but otherwise identical experimental conditions. In contrast to the earlier experiments, in which ions were observed only when the cw laser was tuned to a $D$ line, relatively large "background" ion signals are observed at all excitation wavelengths, a result of multiphoton processes involving the ${\mathrm{Na}}_2$ component of the vapor. It is shown that the laser bandwidth and pulse duration are extremely important parameters; a wide bandwidth effectively magnifying the importance of the dimer component of the vapor and a short pulse duration minimizing the effects of interactions between excited atoms and molecules.