Gas-phase C60 photoionization and photofragmentation experiments were performed using a sub-50 fs Ti Sapphire laser system and reflectron time-of-flight (RETOF) mass spectrometer. The dependence of the C60+ and C602+ signals on the laser intensity for the fundamental (795 nm) and second harmonic (ca. 400 nm) has been determined. For low laser intensities, before the onset of fragmentation, single ionization is a direct multiphoton process. Double ionization is a sequential process in which C602+ originates from already singly ionized fullerenes. At laser intensities beyond the onset of C602+ there is considerable metastable fragmentation indicating a strong coupling of electronic excitation energy into vibrational degrees of freedom that appears to be in competition with multiple ionization.

Photoionization of C60 by 15 ns laser pulses at 193 and 308 nm and by ca. 100 fs pulses at 310 and 620 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is seen to be the ejection of C", n )2, as opposed to successive C2 evaporation. The fragment ions produced in this initial fragmentation step have sufficient internal energy to cool, by the emission of a C2 molecule in the field-free region of the mass spectrometer. Pump-probe experiments with 90 fs laser pulses at 620 nm give additional jnsights into the ionization mechanism.PACS numbers: 36.90.+f, 33.80.Eh, 33.90.+h One of the great puzzles in fullerene research is the fragmentation mechanism that leads to the well known bimodal fragment-ion mass spectra. This distribution was first observed in the photofragmentation experiments of the Smalley group in 1987 [1]. In this pioneering paper photofragmentation of mass-selected carbon-cluster ions produced by laser desorption of graphite was studied at various laser wavelengths and fluences. The bimodal fragmentation pattern of C60+ showed the large fragment ions separated by C2 ending abruptly at C32+. The small fragment distribution appeared predominantly between C7+ and C25+ with mass peaks separated by C+. The authors discussed the sequential loss of C2 but concluded that the level of excitation that would be needed to obtain the very high order fragmentations observed were prohibitively large and instead suggested an "unzipping" mechanism, in which neutral carbon chains could be produced. These experiments were carried out with laser pulse widths on the order of nanoseconds so that one could not be sure whether the fragments observed were produced directly from the parent fullerene or were arising from fragments after a second absorption process in the later part of the laser pulse. In the meantime, similar fragmentation patterns have also been observed in experiments in which pure buckminsterfullerene samples have been excited by collisions, either with atoms or molecules [2 -4], with electrons [5], or at surfaces [6]. In the collision experiments the appearance of the large fragments (from Css+ down to C50+ or further) can often be plausibly explained by a successive statistical "evaporation" of C2 molecules from the highly vibrationally excited C60+ [3,4,7,8]; however, it is not possible to explain the presence of the smaller fragments using such a model. In this Letter we report femtosecond photoionization and fragmentation experiments of C60 in the gas phase and compare them with novel nanosecond laser studies, combining both excitation schemes with a reflectron timeof-tlight (RETOF) mass spectrometer.This allows us also to obtain information on the relative importance of fast vs metastable fragmentation processes and thus gives information on the content of internal energy of the fragments.These results help to shed new light on the ionization and fragmentation dynamics and on the time scale for electronic to vibrational coupling within free fullere...

Abstract.Photo-ionisation and -fragmentation of C6o by 15 ns excimer laser pulses at 308 nm and 193 nm as well as 0.8 ps laser pulses at 193 nm has been studied with reflectron time-of-flight mass spectrometry. The initial fragmentation process is ejection of C,,, n > 2, as opposed to successive Cz evaporation. Studies of the relative intensities of metastable fragmentation processes compared with direct fragmentation provide new insight into the fragmentation mechanism and provide a thermometer for the internal energy of C~-o prior to fragmentation. The proposed mechanism is in agreement with measurements of the fragment ion kinetic energies. The results are compared with molecular dynamics simulations.

The relative stability of clusters of fullerenes has been investigated. By heating the clusters before ionization we have obtained mass spectra where only the monomer and (C60)13 are present in significant amounts. An approximately 20% increase of the activation energy for evaporation of a monomer from (C60)13 compared to that from (C60)14 explains the experimental results.

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