Novel decay channels of carbon cluster ions, C z 40 and C z 41 z  3; 4 R. Parajuli, P. Scheier, V. Grill, S. Matt, O. Echt 1 , T.D. M ark * ,2 Institut f ur Ionenphysik, Leopold Franzens Universit at, Technikerstr. 25, A-6020 Innsbruck, Austria Received 10 August 2000; in ®nal form 19 September 2000 Abstract We have analyzed spontaneous decay reactions of triply and quadruply charged C z 40 and C z 41 cluster ions which are formed from C 60 fullerenes by electron-impact ionization. A new but very weak decay reaction for the even-sized carbon cluster ions is observed, namely loss of C 3 . The odd-sized cluster ions preferentially decay by loss of carbon atoms and, to a lesser degree, trimers. A weak signal due to C 2 loss is observed for C 3 41 . We discuss these decay channels in terms of the geometric structure of these metastable, relatively cold cluster ions. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction A variety of fragmentation channels have been identi®ed for carbon cluster ions. C  60 and other large cluster ions preferentially emit one or more C 2 units with, perhaps, some minor admixture of C 4 [1,2], while small cluster ions preferentially emit neutral C 3 [3,4]. The dierences in the decay pat- terns originate from the dierent structures of these clusters. The patterns are most characteristic if the parent ions are only weakly excited, and fragments are analyzed with considerable delay after the excitation so that the excess energy in the parent ions is randomized over all internal (vi- brational) degrees of freedom [2,5]. In this case, the size of the reaction products provides infor- mation about the geometric structure of the parent ions. In contrast, the decay pattern of highly ex- cited clusters which shatter or ®ssion are complex [4,6±10]; they carry no structural information about the parent ion. Various experiments suggest that the energeti- cally preferred structure of carbon cluster cations critically depends on their size: chains below n  10, planar rings below n  32, fullerenes formed by a closed network of hexagonal plus 12 pentagonal rings of carbon atoms above n  32. Chains and rings may be even- or odd-sized while fullerenes are necessarily even-sized. According to calculations the cross-over from rings to fullerenes already occurs at n  24 or even 20 [11±13], but these studies pertain to neutral clusters. The competition between isomeric structures may change if the clusters are positively charged [14] or if they are excited to as little as 1000 K [15,16]. At any rate, many dierent metastable isomeric structures exist for all cluster sizes; their prevalence in an ensemble of clusters will depend on the ex- perimental conditions. Large even-sized cluster 3 November 2000 Chemical Physics Letters 330 (2000) 53±60 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +43-512-507-2932. E-mail address: tilmann.maerk@uibk.ac.at (T.D. Ma Èrk). 1 Present address: Department of Physics, University of New Hampshire, Durham, NH 03824, USA. 2 Department of Plasma Physics, Comenius University, Mlynska dolina, 84215 Bratislava, Slovak Republic. 0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 8 1 - 2