Interaction d'un jet moléculaire avec un gaz tampon : relaxation collisionnelle d'un système à deux niveaux d'énergie

Using a molecular beam, we study the transitions induced by collisions with molecules of a scattering gas. The experimental method uses a H₂CO beam apparatus which allows the observation of rotational transitions . The target gas is NH₃. The application of a static electric field E considerably modifies the collision induced transition probabilities; the transitions become forbidden when Eis intense. An experimental method is deduced to select transitions on a molecular beam. We initially prepare the system to be in the level (i) before entering the scattering chamber. After scattering, the population ratio n _f/n _i is measured as a function of the target gas pressure P _T. We determine the state to state cross-section of H₂CO. When the pressure P _T becomes intense, the effect is non linear. It is shown that this effect results of many collisions. We compare theory with experimental results. This experimental method gives a model for relaxation process studies in two energy level systems. Re?u le 30 juin 1999 et re?u sous forme finale le 14 décembre 1999