Eur. Phys. J. D 34, 205–209 (2005) DOI: 10.1140/epjd/e2005-00111-0 T HE EUROPEAN P HYSICAL JOURNAL D A femtosecond laser study of the endohedral fullerenes Li@C 60 and La@C 82 A. Lassesson 1, a , K. Hansen 1 , M. J¨ onsson 1 , A. Gromov 1 , E.E.B. Campbell 1 , b , M. Boyle 2 , D. Pop 2 , C.P. Schulz 2 , I.V. Hertel 2 , A. Taninaka 3 , and H. Shinohara 3 1 Department of Physics, G¨oteborg University, 41296 G¨oteborg, Sweden 2 Max-Born-Institut f¨ ur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Str. 2a, 12489 Berlin, Germany 3 Department of Chemistry, Nagoya University, 464-8602 Nagoya, Japan Received 6 September 2004 Published online 13 July 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. The endohedral fullerenes La@C82 and Li@C60 have been studied in the gas phase with fem- tosecond laser excitation. The two molecules show qualitatively the same behaviour with respect to ion yield vs. pulse energy, but markedly different fragmentation patterns, with La@C82 fragmenting via the shrink-wrap mechanism and Li@C60 predominantly losing the metal atom in the first fragmentation step. The ion yields and electron energy distributions of La@C82 agree well with a recently developed model for fs laser ionisation of C60. PACS. 61.48.+c Fullerenes and fullerene-related materials – 36.40.Qv Stability and fragmentation of clusters – 31.15.Bs Statistical model calculations (including Thomas-Fermi and Thomas-Fermi-Dirac models) 1 Introduction Endohedral fullerenes are among the most fascinating fullerene-based derivates with a wide range of atom and molecule containing fullerenes having been produced and studied [1,2]. Even although many different endohedral fullerenes have been available in macroscopic amounts for at least a decade, little is known about their behavior when excited to high internal energies in the gas phase. The processes that occur (ionisation, fragmentation) are normally studied in mass spectrometry experiments. A large number of experiments of this type have been per- formed on C 60 , and, as more information about the pro- cesses of this molecule at high internal energies becomes available, it is increasingly interesting to test hypotheses and theories by studying other similar systems. Many en- dohedral fullerenes, such as the well-studied metal con- taining C 82 based species, are suited for this purpose as they are as stable as C 60 under the conditions and time scales of the mass spectrometry experiments. We have for example recently shown that it is possible to model the delayed ionisation of laser desorbed excited La@C 82 , ap- plying unimolecular decay theories previously applied to excited C 60 [3]. Similarly the fragmentation of highly ex- cited La@C 82 on microsecond timescales resembles that of C 60 which experiences loss of the fullerene structure above approximately 80 eV [4]. a Present address: Institut f¨ ur Physik, Ernst-Moritz-Arndt- Universit¨at Greifswald, 17487 Greifswald, Germany. b e-mail: Eleanor.Campbell@fy.chalmers.se In comparison with the nanosecond lasers primarily used for these earlier studies, femtosecond lasers are capa- ble of creating multiply charged cluster ions on time scales comparable to and lower than the coupling time between the excited electronic subsystem and the vibrational de- grees of freedom. They are thus useful for studies of very fast interactions, such as energy redistributions, in excited fullerenes and have previously been used for investigations of C 60 [5–8]. In this letter we present results from a femtosecond laser investigation of the molecules La@C 82 and Li@C 60 and analyse the results using theories recently developed for C 60 [9]. 2 Experimental The endohedral fullerenes were prepared with two dif- ferent methods. The Li@C 60 material was produced us- ing low energy Li + ion bombardment of thin fullerene films under high vacuum conditions [10]. The result- ing thin films contained 10% endohedral Li@C 60 . The arc-discharge production method was used for the preparation of La@C 82 [1]. Both fullerene species were purified to above 95% purity using High-Pressure-Liquid- Chromatography (HPLC). The vacuum apparatus used for the ionisation and fragmentation experiments has been described previ- ously [7,8]. Briefly, the main excitation source is a