Volume 206, number $6 CHEMICAL PHYSICS LETTERS 18 June 1993 Negative particle emission from a Cs/Ru (000 1) surface during exposure to NO and NO2 A. Biittcher, R. Grobecker, T. Greber ’ and G. Ertl Fritz-Haber-lnstituf der Max-Planck-Gesellxhajt,Faradayweg 4- 6, W - 1000 Berlin 33, Germany Received I March 1993; in final form I3 April 1993 Exposure of a monolayer of Cs adsorbed on a Ru (0001) surface to NO leads at first to dissociative chemisorption and in a later stage to molecular adsorption. During this latter stage emission ofexoelectrons signals the possibility for non-adiabatic processes, while with a clean surface efficient quenching of electronically excited states via resonance ionisation dominates. The behavior of NO closely resembles that of 02; however, in the latter case O- ions are also ejected. Interaction with NOz leads to qualitatively different results: Emission of exoelectrons is observed during the initial stage of dissociative chemisorption while at higher expo- sures NO? ions are ejected as well. The experimental findings are rationalized in terms of a model which takes the different electron affinities of the incident particles into account. The observation of a pronounced dependence of the yield of exoelectrons on the substrate temperature suggests the participation of a metastable intermediate phase. The interaction of O2 with Cs films was found to be associated with electronic excitations which man- ifest themselves in the emission of electrons (“exo- electrons”) [l-4] or even of O- ions [ 51. In order to clarify the underlying mechanisms, these studies were extended to other oxidizing molecules, namely NO and NOZ. The starting situation was always a monolayer (1 ML) of Cs chemisorbed on a Ru ( 000 1) surface. This phase corresponds to a cov- erage &=0,33 (corresponding to 5.3~ lOI Cs at- oms/cm’) and is associated with an ordered fix fi R30” structure whose properties have been in- vestigated previously [ 61. A detailed account of findings on the interaction of O2 with this system will be given elsewhere [ 41. The experiments were performed with a UHV sys- tem containing standard facilities for surface prep- aration and characterization as described previously [ 21. In particular, information about the electronic properties of the outermost atomic layer could be obtained by means of metastable deexcitation spec- trosocopy (MDS) _ The currents and energies of the negative particles were measured by means of an electrostatic energy analyzer which allowed us to ’ A. von Humboldt Fellow. measure the work function changes. The work func- tion was determined from the energetic width of He I ultraviolet photoelectron spectra, and negative ions ejected from the surface were monitored by a Balzers QMG 421 mass spectrometer, while a QMG 112 served for recording thermal desorption spectra (TDS). Fig. 1 shows the total current of negative particles and the variation of the work function A$ emitted from the Cs/Ru( 0001) sample as a function of ex- posure to gaseous NO at a sample temperature of 190 K. This current was exclusively due to exoelectrons: the ejection of negative ions was below the detection limit. By contrast, with O2 immediately after gas ex- posure the emission of O- (albeit with low proba- bility of about 1 O-* per incident O2 molecule) started and the yield rapidly decayed with progressing oxi- dation. However, the characteristic features for ex- oelectron emission were similar for both adsorbates: Exoelectron emission commenced when the work function had passed through a minimum and was in- creasing again. Also the total electron yield was of comparable magnitude for both NO and 02, al- though pronounced temperature effects came addi- tionally into play, as will be outlined below. Ob- viously the clean surface is inactive, which had been 404 0009-2614/93/$06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.