1. Department of Chemistry, University of Durham, Durham DH1 3LE, UK
2. Istituto di Struttura della Materia – Consiglio Nazionale delle Ricerche, Area della Ricerca di Roma – Tor Vergata, via del Fosso del Cavaliere 100, 00133 Rome, Italy
3. Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA
4. Present address: School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK.

Correspondence to: Stuart C. Althorpe^1,2 Correspondence and requests for materials should be addressed to S.C.A. (e-mail: Email: s.c.althorpe@ex.ac.uk).

Extensive theoretical^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13} and experimental^{2, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22} studies have shown the hydrogen exchange reaction H + H₂  H₂ + H to occur predominantly through a 'direct recoil' mechanism: the H–H bonds break and form concertedly while the system passes straight over a collinear transition state, with recoil from the collision causing the H₂ product molecules to scatter backward. Theoretical predictions agree well with experimental observations of this scattering process^{15, 16, 17, 18, 19, 20, 22}. Indirect exchange mechanisms involving H₃ intermediates have been suggested to occur as well^{8, 9, 10, 11, 12, 13}, but these are difficult to test because bimolecular reactions cannot be studied by the femtosecond spectroscopies²³ used to monitor unimolecular reactions. Moreover, full quantum simulations of the time evolution of bimolecular reactions have not been performed. For the isotopic variant of the hydrogen exchange reaction, H + D₂  HD + D, forward scattering features²¹ observed in the product angular distribution have been attributed^{21, 12} to possible scattering resonances associated with a quasibound collision complex. Here we extend these measurements to a wide range of collision energies and interpret the results using a full time-dependent quantum simulation of the reaction, thus showing that two different reaction mechanisms modulate the measured product angular distribution features. One of the mechanisms is direct and leads to backward scattering, the other is indirect and leads to forward scattering after a delay of about 25 femtoseconds.