PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences Ionisation of fullerenes and fullerene clusters using ultrashort laser pulses Eleanor E. B. Campbell,* a Klavs Hansen, a Martin Hed´ en, a Mikael Kjellberg a and Alexander V. Bulgakov b Received 4th September 2006, Accepted 18th October 2006 First published as an Advance Article on the web 3rd November 2006 DOI: 10.1039/b612749e We give a brief review of the literature concerning the ultra-short pulse ionisation of fullerenes in the gas phase. Emphasis is placed on the excitation time dependence of different ionisation regimes as manifested by photoelectron spectroscopy. The ionisation rates are modelled for the intermediate situation where the excitation energy is equilibrated between electronic degrees of freedom but not yet coupled to vibrational degrees of freedom. The model is shown to describe many aspects of the experiments. New results are presented on the intra-cluster molecular fusion of fullerene molecules when van der Waals bound clusters of fullerenes are exposed to ultra-short laser pulses. Pump–probe measurements give a decay time constant for the intra-cluster fusion reaction of 520 ± 55 fs. A comparison with monomer ionisation results suggests that the time window for the fusion reaction is influenced by the coupling of the electronic excitation energy to vibrational degrees of freedom of the molecules in the cluster. Introduction Fullerenes have been used since their first discovery as model systems for studying the behaviour of highly excited molecules or clusters with a large number of degrees of freedom. Gas phase studies allow the properties of the individual molecules to be probed in detail without the complications arising from interactions with liquid or solid surroundings. For the past 15 years, the ionisation and fragmentation behaviour of highly excited fullerenes in the gas phase have been the subject of much attention. 1–5 In this context “highly excited” typically refers to the situation where the molecules have more internal excitation energy than the energy required to remove an electron. A number of inter- esting and, initially, surprising phenomena were observed in such experiments such as the observation of delayed ionisation occuring up to many microseconds after excitation with ns laser pulses, 6 the observation of a very large kinetic shift in the metastable fragmentation of fullerene ions 7 and the occurrence of black- body-like radiation from laser desorbed fullerenes. 8 It is now fairly well established that all of these phenomena can be understood by considering statistical decay mechanisms of the molecules, assuming that the excitation energy is equilibrated among all degrees of freedom, 5 although a few uncertainties concerning the details remain. 9 Fullerenes are especially interesting and almost unique in this context since it is possible to observe the three main decay channels (electron emission, fragmentation and radiation) on the microsecond timescales that are easily accessed by mass spectrometers. 2 The reason for this is that the ionisation potential (7.59 eV for C 60 ) 10 is much less than the energy required for fragmentation (10.8 ± 0.3 eV for C 2 emission from neutral C 60 ). 11 Statistical behaviour is observed for excitation of fullerenes with ns laser pulses as well as in many collision experiments. 12 a Department of Physics, G¨ oteborg University, SE-41296, G¨ oteborg, Sweden. E-mail: eleanor.campbell@physics.gu.se; Fax: +46 31 772 3496; Tel: +46 31 772 3272 b Institute of Thermophysics, SB-RAS, 1Acad Lavrentyev Ave., 630090, Novosibirsk, Russia Our interest in recent years has been to explore how the statistical behaviour develops with time. On what timescale can we consider the excitation energy to be statistically distributed among the degrees of freedom of the molecule? In the following we will briefly review the results using ultra-short laser pulse excitation and statistical modeling that provide information on such processes. In addition, we will describe results concerning the excitation of clusters of fullerenes. We show that if sufficient energy is absorbed by fullerene clusters on a sub-picosecond timescale then it is possible to observe intra-cluster molecular fusion where the individual fullerene molecules in the weakly-bound cluster fuse together to produce large fullerenes. Intense ultra-short pulse laser excitation of clusters could prove to be a useful tool to study chemical reactions under extreme excitation conditions. Experimental All of the results presented here have been obtained with the kind of equipment schematically illustrated in Fig. 1. Fullerene beams are produced by evaporating “gold grade” C 60 powder in an oven at a temperature of typically 500 ◦ C. Beams containing clusters of fullerenes are produced in a cluster aggregation source, cooled by liquid nitrogen and using helium as a carrier gas. 13 The monomer and cluster fullerenes that emerge from this source have a vibrational temperature of 100 K or slightly less. Both fullerene sources are oriented at right angles to the flight axes of a reflectron ion time-of-flight mass spectrometer and a l-metal Fig. 1 Schematic diagram of a typical experimental setup to study the ionisation and fragmentation of fullerenes with ultra-short laser pulses. This journal is © The Royal Society of Chemistry and Owner Societies 2006 Photochem. Photobiol. Sci., 2006, 5, 1183–1189 | 1183