Adsorption of NF 3 on Pt(111): interactions with electrons and photons

Abstract

The thermal, electron and photon stimulated chemistries of nitrogen trifluoride adsorbed on Pt(111) are examined using temperature programmed desorption (TPD). There is no evidence for thermal dissociation of NF 3. Reflecting the very weak adsorption on Pt(111), all four resolvable TPD peaks lie below 70 K. For the lowest exposures, there is a peak at 70 K (α state) that continues to grow as a second peak emerges at 65 K (β state); these are attributed to NF 3 attached to Pt(111). When the α and β states saturate, a third peak, attributed to a bilayer, appears at 50 K. Finally, unsaturable multilayer desorption sets in at 47 K. The electron and photon driven chemistries of the α state are significantly different. In both cases, there is evidence for loss of parent without, if any, retention of fragments. However, the NF 3 removal cross-sections differ by orders of magnitude and TPD peaks for the remaining NF 3 shift in opposite directions. The loss of the α state occurs with a cross-section lying between 0.6 and 1.3 × 10 −15 cm 2 for electron activation and a cross-section seven orders of magnitude smaller. 10 −22 cm 2, for photon activation by a Hg-arc source. NF 3 desorbs at higher temperature (80 K) after electron activation and lower temperature (60 K) after photon activation. The cross-sections are discussed in terms of substrate-mediated resonant electron attachment, and also for incident electrons, impact ionization and neutral excitation paths. The temperature shifts are attributed to variations in the structure of the adsorbed NF 3.