LVe present a study of cold collisions in a sample of magneto-optically trapped sodium atoms througli the technique of two-color photoassociative ionization spectroscopy. We demonstrate the inhibition of the process by adding an auxiliary %uppressor" laser beam and analyze tlie dependence of this optical shielding effect on the laser frequency and intensity. The possibility of using photons to control the atomic collision and to modify the thermodynamic properties of tlie cold gas is discussed. Tlie study of collisions involving laser cooled and trapped atorns has been the subject of intense investigations during the past few years. TIiis interest has been motivated by tlie desire of achieving the regime of a degenerated quantum gas and also by intrinsic features it presents, inainly tliat in tlie presence of light, collisions involving excited state atoms have duration comparable to the spontaneous emission time. In this regime, the exchange of energy between the atom ,and the modes of the radiation field (including vacuum) provides a prototype for studying the properties of nonequilibrium open systems coupled to reservoirs. As the kinetic energy of a two-body collision approaches zero, the number of partia1 waves coiitributing to the elastic collision reduces to one, the s-wave, and the information about the atomic interaction is comyletely contained in the scattering length a. The properties of a cold gas are therefore dependent on the scattering lengtli and its sign can determine the behavior of the system. Tlie recente achievement of the quantum gas regime iii a sample of laser cooled and magnetic trapped rubidium atoms[lI has opened new possibilities of studies involving the thermodymanics of a gas in such regime. Controlling the atomic interaction with photons can provide the condition to stabilize tlie gas in a single quantum state of the confining potential and also the manipulation of its thermodynamical properties. We liave recently demonstrated the possibility of controlling the atomic interaction by using photo-associative ionization (PAI) in a sample of cold sodium atoms held in a trapt2]. In this paper we present the studies of cold collisions using PAI as a prototype and the route we liave taken towards the demonstration of optical control of cold collisions. We discuss the possibility of using photons as a to01 to cliange the course of the atomic encounter, including the modification of attraction into repulsion aiid its applications. Photo-associative ionization as a prototype of cold collisions Conventional associative ionization occurring at ordinary temperatures proceeds in two distinct steps: excitation of isolated atoms followed by the collisional interaction between excited atomic states. Tlie collision event is fast compared to the radiative relaxation and the two steps are decoupled. In contrast, PAI starts with ground state partners moving sufficiently slow that they have time to absorb and spontaneously emit photons prior to the final ionizing interaction. The partners inust be close enough when the initial absorption takes place such tliat a significant fraction of the excited population survives to relaxation back to the ground state. Thus, PAI starts by promoting the ground state of the colliding species, designated by [Na,,Na], to a
