The accuracy of the resonance of a photo-associative collision is essentially given by the relative kinetic energy of colliding species, which varies from a few thousand Gigahertz at ordinary temperature to a few Megahertz for cesium cooled atoms. By using Cs cold atoms obtained in a Magneto-Optical Trap (MOT), we will develop a high resolution spectroscopy of long range molecular states of Cs2. In a cesium MOT, the photo-associative collisional process of two free cesium atoms in the dimer molecule Cs2*, in a rotovibrational excited state, will be realized by a first laser. Signature of photo-associative process will be obtained by photo-ionization of Cs2 in Cs2+ using a second leser. The ions will be selectively detected through a time-of-flight mass spectrometer. The theoretical interpretation of the spectroscopic lines will give access to the long range molecular potential curves of Cs2.