Spontaneous amplified emission of a Na2 diffuse violet band produced through two-photon excitation of sodium vapor

We report observation of enhanced emission of the diffuse violet band at 436.0 nm when sodium vapor is optically pumped with a pulsed dye laser with output wavelength in the 570–595 nm region. A significant enhanced emission is observed in the forward direction along the propagating laser beam when the laser wavelength is tuned to near the two-photon excited 4d state of the Na atom. At a vapor density higher than 10¹⁵ cm^-3 the diffuse violet emission is predominantly produced through processes associated with the excitation of Na₂. The dependence of the enhanced emission on vapor density and laser power density is presented.