Adsorption of D₂ on Si (100) was studied by using mol. beam techniques. The dependence of the D₂ sticking coeff. (S) on surface temp. (Ts) and nozzle temp. (Tn) was characterized. The sticking coeff. increases gradually in the range 300 < Tn < 1040 K. With an incident translational energy of .apprx. 65 meV, S rises with increasing Ts from a value insignificantly different from the null level to a value of (1.5 +- 0.1)*10-5 at Ts = 630 K. Having established that S increases with both increasing mol. energy and increasing Ts, the authors demonstrate directly that the adsorption of mol. H on Si is activated and that lattice excitations play an important role in the adsorption process. The implications of these results for the calcn. of the Si-H bond strength are discussed.