Each yield is calculated over 2000 trajectories, each trajectory is initiated with a randomly placed H and D in each adsorption site using the same (4 × 4) unit cell. The fluence was 50 J/m 2 in all cases. Desorption of H 2 is more likely and faster compared to HD and D 2 as also observed in experiments. The isotope ratio, defined as Y(H 2 )/Y(D 2 ) is experimentally found to be ∼20 for a fluence of 50 J/m 2 while we obtain ∼1.6. Experimentally, 4 a so-called dynamical promotion / suppression effect was found, where desorption of D 2 was larger and desorption of H 2 was smaller than expected from second order rate equations, in which a desorbing molecule is assumed to be unaffected by neighboring adsorbates (see ref 17 for details). The results of the second order rate equations are shown in Figure as solid lines.