Using a new approach the authors have determined the internal (vibrational) energy distribution of the molecular ions formed in the collisional associative ionization (AI) between thermal Na*(3p) atoms. The method is based on the wavelength dependent measurement of the Na⁺ ions formed in the photodissociation of the Na₂⁺ ions. The authors find that there is a substantial vibrational excitation and even a population inversion, with the upsilon =2 to 4 levels predominantly populated. These findings are in good agreement with previous experimental results obtained through electron detection, and with the detailed ab initio calculations by Dulieu et al (1991). The present method should prove itself suitable under conditions where other methods are difficult to apply, e.g. in collisions between cold atoms.