Surface Science 128 (1983) 197-204 LI97 North-Holland Publishing Company SURFACE SCIENCE LETTERS MECHANISMS OF He*(2ts) QUENCHING ON OXYGEN-COVERED Ni(l I 1) SURFACES Ferenc BOZSO *, Ciaran P. HANRAHAN, Jose ARIAS, Horia METIU **, John T. YATES, Jr. * and Richard M. MARTIN Department of Chemistry, University of California, Santa Barbara, California 93106, USA Received 15 February 1983 Ni(lll) surfaces exposed to 0.8 L 02 and 3.2 L 02 were studied using He*(21S) metastable quenching spectroscopy (MQS). The electron spectrum from the fully covered surface (3.2 L O2) showed that quenching takes place by ionization of the He*(2IS) followed by Auger neutralization of the ion lather than by Penning ionization as has been observed for other adsorbates. The chemisorbed oxygen spectrum on a partially covered surface changes with temperature between 90 and 200 K, and this is attributed to a phase transition in the chemisorbed oxygen layer. Surface metastable quenching spectroscopy (MQS) has been recently used [1-3] to probe the electronic structure of adsorbed molecular layers. The method analyzes the kinetic energy distribution of the electrons emitted when metastable noble gas atoms (e.g. He*(21S)) are deexcited at a surface. So far, two mechanisms have been documented experimentally [1-3]. In one of them [4] (resonance ionization + Auger neutralization, RI + AN), an electron is transferred from the excited atom into an empty resonant orbital of the metal surface, forming a noble gas ion. This is then Auger neutralized by the surface, giving a ground state noble gas atom and an electron emitted from the metal. The kinetic energy distribution of the electrons emitted by the surface is essentially the same as that obtained in ion neutralization spectroscopy (INS) [5]. This quenching mechanism has been found [1-3] to be dominant when the metastable collides wi~h a clean metal surface whose work function is larger than the ionization potential of the metastable. If the surface is covered with adsorbed molecules, the quenching mechanism is usually different. The metastable induces Penning ionization of the ad- sorbate [6]. The kinetic energy distribution of the Penning electrons gives * Present address: Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA. ** Alfred P. Sloan Fellow and Camille and Henry Dreyfus Teacher-Scholar. 0039-6028/83/0000-0000/$03.00 © 1983 North-Holland