Crossed beam studies of the hydrogen exchange reaction: The reaction of H and D atoms with T2 molecules

The dynamics of the hydrogen exchange reaction has been studied in crossed molecular beam experiments using a tritium atom detection scheme. The selectivity and the cumulative feature of T-atom collection provide adequate signal levels and the mass changes introduced by the labeling provide favorable kinematics which allow both a single-valued laboratory↔c.m. transformation and a more efficient laboratory→c.m. conversion of the collision energy. Measured angular distributions of reactively scattered T atoms for the H+T2 and D+T2 systems show that: (1) the product molecules are scattered backwards with respect to the incoming atomic beam implying that these reactions proceed via a direct rebound mechanism, (2) the laboratory angular distributions are quite broad [ranging from ∼70 ° to ∼125 ° FWHM for D(2120 K)+T2 and H(2600 K)+T2, respectively], but agree reasonably well with theoretical predictions if the latter are properly averaged over collision energies; the observed increase in widths of the angular distributions with increasing collision energy appears to be a general property of impulsive collisions involving repulsive barriers, (3) no information is available for the recoil velocity distributions, but the condition E=E′ used in the c.m.→laboratory transformations produce laboratory distributions that provide equally good fits over both branches of the distribution, and (4) under identical conditions, the product T-atom intensity for the H+T2 system is higher than that for the D+T2 system, this difference arises primarily from the different collision energy ranges sampled in the experiments; with a higher temperature H-atom beam, the signal increases substantially, but a quantitative measure of the increase in cross section is precluded by the uncertainty in the H-atom flux. These results are compared with a previous crossed beam study of the D+H2 system and with predictions from various theoretical studies.