Surface Science 425 (1999) 81–89 Ion K+ N time-of-flight angular distributions for K beam scattering and cluster formation at graphite surfaces J. Wang, L. Holmlid * Reaction Dynamics Group, Department of Chemistry, University of Go ¨ teborg, SE-412 96 Go ¨teborg, Sweden Received 22 August 1998; accepted for publication 14 January 1999 Abstract Angular resolved ion time-of-flight distributions are reported for the K–graphite (0001) system over the range −90° to +90° relative to the surface normal. A low intensity K thermal atom beam strikes the graphite surface at 900 K at −45°. Ions and field ionisable Rydberg states which desorb or scatter from the surface are detected. The size distribution of clusters K* N has its maximum at N=4, while N>6 is unlikely. A complex formation collision process K+K* N K* N+1 at the surface is now identified from the skewed angular distributions and the dip at ca 6°. Such a process has not previously been identified at a graphite surface, where the rapid diffusion into the bulk keeps the surface density low. A scattering process due to collisions K++K* within the desorbing flux is observed, as symmetrically placed tails with lower velocity at increasing angle towards the normal. Both collision processes are estimated to have cross-sections in the order of 106 nm2, which is so large that resonant transfer involving Rydberg species is likely to be involved in the scattering. These collision processes take place close to the surface where the density is higher. © 1999 Elsevier Science B.V. All rights reserved. Keywords: K beam scattering; Ion time-of-flight angular distributions 1. Introduction the collisions between a K atom beam and the K N cluster flux from a zirconia surface [5]. Other studies of alkali Rydberg species at surfaces have Recently, the first time-of-flight ( TOF ) mass appeared recently, like the angular distribution spectrometry based study of the K N clusters formed study at graphite (0001) in Ref. [6 ], the bulk at a graphite surface was reported from our group diffusion study on the same surface in Ref. [7] and [1]. This was a further step in a series of studies the nanosecond desorption study on graphite films from our group on the formation of Rydberg in Ref. [8]. Now, more detailed results with TOF species and their reaction processes at surfaces. mass spectrometry are presented in the form of Earlier steps include the observation of excited angular resolved TOF distributions over the full clusters by field ionization detectors [2] and angu- angular range. lar distributions [3]. The scattering due to such The research concerned with Rydberg clusters clusters was studied in Ref. [4]. Recently, a com- and Rydberg states [9,10] in clusters is diversifying plex formation scattering process was detected in and increasing in importance at present. There are several reasons for this, of which the most impor- * Corresponding author. Fax: +46-31 7723107; e-mail: holmlid@phc.chalmers.se. tant may be the higher reactivity expected for the 0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0039-6028(99)00187-9