ADSORPTION OF OXYGEN AND HYDROGEN ON THE TUNGSTEN SINGLE-CRYSTAL SURFACE AT LOW-TEMPERATURES (DOWN TO 5-K)

Adsorption of oxygen and hydrogen is studied on tungsten at low temper atures down to 5 K, using molecular beam and thermodesorption spectros copy techniques. A <<black>> ultra-high-vacuum chamber is used. Low-te mperature TD spectra of molecular oxygen adsorption states have three peaks at 25, 27 and 45 K corresponding to the states of multilayer con densation, physisorption and weak molecular chemisorption, respectivel y. The TD spectra are independent of the substrate crystallographic or ientation. On the contrary, the low-temperature TD spectrum of the hyd rogen molecular states are sententive to the orientation of the surfac e: 8 K peaks are common for W(100) and W(111). but 60 K molecular peak is present for W(110) only. Dissociative chemisorption of oxygen and hydrogen takes place on all surfaces studied on the formation of the f irst monolayer down to 5 K. The corresponding height of the activation barrier is less than or similar to 10 meV. This rule is broken only i n the case or H-2 on W(10), where the molecular adsorption state at 60 K and the atomic adsorption state are filled simultaneously from the very beginning. The study of the dependences of sticking probability o n coverage, the substrate temperature and the kinetic energy of imping ing molecules revealed different mechanisms of adsorption with and wit hout precursor states.