Deshaping of the laser beam observed on the collisionally induced fluorescence of Na atoms in the nozzle expansion

The excitation spectrum ofD ₁ fluorescence and collisionally inducedD ₂ fluorescence of sodium in the early stage of the beam formation near the nozzle is studied. We observe strong reabsorption and imprisonment ofD ₁ photons influencing collisionally inducedD ₂ fluorescence. For high pressure of sodium vapour the excitation profile ofD ₂ fluorescence is dominated by the collisional broadening effect and shows unusual maxima and minima. These we connect with the laser beam focusing and defocusing effect, changing the number of photons penetrating the high density region.