Eur. Phys. J. D 16, 69–72 (2001) T HE EUROPEAN P HYSICAL JOURNAL D c  EDP Sciences Societ` a Italiana di Fisica Springer-Verlag 2001 Decay reactions of rare gas cluster ions: Kinetic energy release distributions and binding energies R. Parajuli 1 , S. Matt 1 , O. Echt 1,2, a , A. Stamatovic 1,3 , P. Scheier 1 , and T.D. M¨ ark 1,4 1 Institut f¨ ur Ionenphysik, Leopold Franzens Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria 2 Department of Physics, University of New Hampshire, Durham NH 03824, USA 3 Faculty of Physics, P.O. Box 638, Yu-11001 Beograd, Yugoslavia 4 Kaytedra fyziky plazmy, Univerzita, Mlynska dolina F2, SK-84248 Bratislava, Slovak Republic Received 20 November 2000 Abstract. We have carried out measurements on metastable fragmentation of mass selected argon cluster ions which are produced by electron impact ionization of a neutral argon cluster beam. From the shape of the fragment ion peaks (MIKE scan technique) one can deduce information about the distribution of kinetic energy that is released in the decay reaction. In this study, for Ar + 5 to Ar + 15 , it is Gaussian and thus we can calculate from the peak width the mean kinetic energy release 〈KER〉 of the corresponding decay reactions. Using finite heat bath theory we calculate from these data the binding energies of the decaying cluster ions. PACS. 36.40.Qv Stability and fragmentation of clusters – 33.15.Fm Bond strengths, dissociation energies – 34.30.+h Intramolecular energy transfer; intramolecular dynamics; dynamics of van der Waals molecules 1 Introduction Mass spectrometric studies of spontaneous (metastable) decay reactions and of dissociative reactions of mass- selected cluster ions induced by photons, electrons or sur- face collisions have provided a wealth of information about structure, stability and energetics of these species (see the review on this subject [1]). Surprisingly few studies, how- ever, have been reported concerning measurements of the kinetic energy release (KER) distribution for the decay of metastable, weakly bound atomic (rare gas) or molecu- lar cluster ions. Exceptions to this are the measurements by Stace and co-workers of the average kinetic energy re- lease 〈KER〉 of carbon dioxide [2] and argon [3] cluster ions. The 〈KER〉 was derived from metastable peaks aris- ing from decays in the field free region between the ion source and an analyzing magnetic sector field. In a later paper Stace and co-workers repeated the argon measure- ments with a double focusing sector field instrument em- ploying the mass analyzed ion kinetic energy (MIKE) scan technique [4]. This method had already been used before by Bowers and co-workers [5,6] to obtain 〈KER〉 values for small cluster ions (mainly dimers) of water, ammo- nia and carbon dioxide, and by Lifshitz and co-workers for protonated ammonia and methanol cluster ions up to size n = 8 [7,8]. Moreover, Castleman and co-workers em- ployed a reflectron-type time-of-flight mass spectrometer to determine the energy release of decaying protonated a e-mail: olof.echt@unh.edu ammonia clusters from the arrival time peak shapes [9]. In addition to these studies the 〈KER〉 has been measured for metastable and electron induced carbon cluster ion de- cay reactions [10–12]. Vibrational predissociation is a very likely mechanism for the dissociation of metastable cluster ions [1]. Lifshitz et al. [7] have argued that results obtained on the depen- dence of 〈KER〉 on cluster size in the case of ammonia demonstrates indeed the statistical nature of the dissoci- ations. Whether vibrational predissociation is the domi- nating channel at all times is not known, nevertheless the kinetic energy release as a function of cluster size has been modeled with statistical theories. In a pioneering study Engelking [13] showed that the binding energy of a clus- ter constituent within a cluster may be determined within a QET/RRK type statistical model from the measure- ment of cluster evaporative lifetime and average kinetic energy release. Whereas stringent demands are placed on the accurate determination of the 〈KER〉, only moderate demands are placed on the lifetime. Engelking has applied this method to calculate binding energies for argon and carbon dioxide cluster ions using the 〈KER〉 data from [2, 3]. Castleman and co-workers [9] applied the same model to their own data to obtain binding energies for ammonia cluster ions. Moreover, evaporation from small particles has been treated theoretically by Klots [14] in the so-called finite heat bath theory. He showed that the relative binding en- ergies of a series of cluster ions can be calculated by fitting