An investigation of the ionization of dense sodium vapour in the presence of an intense resonant laser beam

Ionization in an effusive jet of sodium, excited by continuous or pulsed resonant laser radiation (D2 resonance), is studied by electron spectrometry. Associative ionization (IA) in the collision of two excited Na (3p) atoms is directly detected. -the populating of highly excited nl states in the 'energy association' collisions of two Na (3p), followed by the collisional ionization (IC) (Penning) in the Na (nl) + Na (3p) system. -heating' of the electrons by 1, 2 or 3 superelastic collisions with Na (3p). For both cases of excitation (continuous or pulsed source), the analysis of the experimental results leads to a description of the whole ionised medium, characterised as a low-density plasma, seat of ambipolar diffusion of charged particles. A very simplified model accounts for the kinetic and electrokinetic equilibrium (continuous case), in which the different populations of electrons of distinct energies (low energy 'primary' electrons, 'hot' electrons), non-thermalized, play distinct specific roles. Within the framework of the model, the cross-sections associated with (IA) and (IC) are estimated from the experimental results