Desorption product mass distributions obtained following exposure of a monolayer of (a) PQ and (b) pentacene to increasing H-atom fluences. (a)(i) The mass distribution of the monolayer of PQ (m/z = 308). (a)(ii)−(vi) Mass distributions extending to masses lower and higher than the pristine PQ indicating abstraction and addition reactions, eventually reaching mass 300 which is a fully superhydrogenated pentacene. (b)(i) The mass distribution of the monolayer of pentacene (m/z = 278) with the prominent contaminants dihydropentacene (m/z = 280) and oxygenated pentacene species with m/z = 294 and 308. (b)(ii)−(vi) Mass distributions shifting towards higher masses with increasing H-atom exposure intervals, eventually reaching m/z = 300 corresponding to fully superhydrogenated pentacene.