If the temperature coefficient of the rate of adsorption is very small, the adsorption process is said to be unactivated (i.e. to have a negligible activation energy). In this case the sticking coefficient at low coverages may be near unity particularly for smaller molecules.
Source:
PAC, 1976, 46, 71. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis' on page 79 (https://doi.org/10.1351/pac197646010071)