Molecular ion emission from single large cluster impacts S.V. Verkhoturov * , R.D. Rickman, C. Guillermier, G.J. Hager, J.E. Locklear, E.A. Schweikert * Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, USA Received 12 September 2005; accepted 15 February 2006 Available online 2 May 2006 Abstract We investigated the emission of the secondary ions stimulated by single impacts of 136 keVAu 400 4+ projectiles. The study was carried out on targets of glycine, phenylalanine, and C 60 . In addition, a target of C 60 was examined with 18 keV C 60 + projectiles. The experiments were performed in the event-by-event bombardment/detection mode. The secondary ions were identified with linear time-of-flight mass spectrometer equipped with an 8-anode detector. The Au 400 4+ projectile induces abundant multi-ion emission, for instance the average number of detected ions (atomic, fragment, molecular and cluster ions) emitted per event from glycine target is 12.5. The glycine intact molecular ion (Gly  ) yield is 1.14. The bombardment of a C 60 target results in the efficient emission of multiple intact C 60  (total yield is 0.15). # 2006 Elsevier B.V. All rights reserved. Keywords: Gold clusters; Secondary ion mass spectrometry; Intact molecules; Multi-ion emission; C 60 1. Introduction The emission of multiple ions from one projectile impact is of great interest for surface analysis. Coincidental ion emission implies that the ejecta originate from molecules co-located within a surface volume perturbed by a single projectile impact [1]. Thus, coincidental ion emission provides an approach for the chemical analysis of nano-domains [2]. The relevance for analysis of nano-structures will depend on the effectiveness of the projectile to cause co-emission of two or more analytically significant ions. There are two ways to achieve the high degree of multi-ion emission. One way is to use larger clusters (collective effect of bombardment), another way is to increase the kinetic energy of the projectiles (advantage of multi- charged clusters) [1]. In this study, we examine multi-ion emission from Au 400 4+ impacts and address additional questions concerning such events. What does the co-emission of multiple molecular ions reveal about the distribution of the corresponding neutral molecules? Are individual emission events, under replicate impact and target conditions, distinctly unique or, on average, equivalent events? 2. Experimental Targets of glycine (M w = 75.07), deuterated phenylalanine (M w = 173.26) and C 60 (M w = 720.64) were bombarded with large and massive gold cluster, ‘‘Au 400 4+ ’’ of 136 keV (340 eV/ atom). ‘‘Au 400 4+ ’’ refers to cluster ions containing 400  30 Au atoms and an overall net charge, q, of +4 [5]. Each target was made by the vapour deposition of analyte (film thickness 1 mm) onto a stainless steel plate. The experiments were run on a setup comprising a liquid metal ion source, a Wien filter for primary ion mass selection, a beam pulser for single projectile bombardment and a linear time-of-flight mass spectrometer [1,3–5]. The liquid metal ion source, LMIS, procured from the Institute of Nuclear Physics Orsay (France) produces gold clusters in wide range of masses [3]. For the mass selection of the primary cluster ions we used a combination of the Wien filter with further selection by time-of-flight. The time-of flight selection of the primary cluster ions was performed with two pulse generators, specifically, a high voltage pulser and a gate generator. The high voltage generator was used to operate in the regime of single projectile bombardment. [4,5]. The gate generator set a time window to edge the continuum of masses around mass 20,000 amu [5]. The ions were detected with a dual micro-channel plate, MCP, assembly with 8-anodes provides the capability of recording, simultaneously, of up to eight ions of the same m/q, provided they strike separate www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 6490–6493 * Corresponding authors. E-mail addresses: verkhoturov@mail.chem.tamu.edu (S.V. Verkhoturov), schweikert@mail.chem.tamu.edu (E.A. Schweikert). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.196