Electron stimulated desorption and conversion of N 2 adsorbed on a tungsten (110) plane

Abstract

The response of γ-N 2 on W(110) to electron impact has been investigated. The desorption products are principally neutral N 2 and small amounts of neutral N, which may come from decomposition of N 2 in the mass analyzer. N + 2 was not detected. N + is seen for electron energies above 60 eV. Approximately 32% of the initial saturated γ-N 2 layer (in terms of N atoms) is converted by electron impact to chemisorbed atomic N at E _ = 150 eV, corresponding to N/ W ≈ 0.50. It was possible, by thermal desorption after electron impact for different times to follow both the appearance of N and the desorption of N 2. It turns out that the cross section for conversion to adsorbed atomic N is 2.5 × 10 −18 cm 2, virtually independent of N 2 coverage remaining. The ESD signal for N 2 is not proportional to the amount of N 2 remaining on the surface. The data indicate that along with desorption there is conversion to a electron impact desorption inactive state (not atomic N) which however is either N 2 or desorbs thermally like N 2. UPS measurements show that such a new molecular state, with 5σ1π shifted to higher binding energy by ∼ 1 eV is formed by electron impact. The desorption cross section is 1 × 10 −17 cm 2, the conversion to inactive N 2, 8.5 × 10 −18 cm 2. The threshold for N 2 desorpt E - = 7–8 eV, suggesting creation of a 5σ −1 state as the desorption mechanism near threshold. Conversion to adsorbed N occurred at E - ⩾ 12.5 eV within our detection limit (XPS of the N 1s level of atomic N) suggesting a low energy intramolecular N 2 excitation. N + is not formed from chemisorbed N but by Coulomb explosion of N 2, probably after creation of a 3σ −2 hole state. The weak binding of N 2 and the resultant large N 2-W separation are postulated to account for the absence of N + 2 : at the large N 2-W distance in the ground state vertical transitions lead to the attractive part of any ionic curves reached, so that N + 2 is propelled toward the surface and neutralized before it can desorb as an ion. Similarities and differences with the ESD behavior of CO are rationalized in terms of the differences in 5σ and 2π orbital distributions in the two cases.