Photoionization and Fragmentation Dynamics ofC60

Abstract

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...

Ionization and Fragmentation ofC60via Multiphoton-Multiplasmon Excitation

“…Although the resolution of our TOF spectrometer is not sufficient to distinguish delayed emission of a single C 2 , as observed in Ref. [7], FIG. 1.…”

“…Some rather puzzling facts have been discovered in studies addressing these aspects of fullerenes by, e.g., photoexcitation, electron-impact, and ion-impact excitation, including the existence of both direct and delayed ionization [1][2][3][4][5] and fragmentation [6,7] processes. Several experiments indicate that fragmentation of C 60 , which generally occurs via the loss of one or more C 2 segments, happens only after excitation above a certain threshold energy.…”

The repopulation of electronic states upon vibrational excitation of niobium carbide clusters

“…"Bulk-like" effects such as non-adiabatic energy transfer between the electronic and vibrational systems in clusters have been studied for decades [7][8][9][10][11][12][13] . Usually a direct excitation of the cluster electronic system is performed with subsequent observation of the relaxation time through the energy exchange with the nuclear system.…”

Infrared multiphoton ionization of superhot C60: Experiment and model calculations

“…6 Those are all impulsive events that occur on the subpicosecond time scale. The optical excitation is usually induced via UV/visible ͑VIS͒ multiphoton absorption where the time scale for the energy deposition is dictated by the laser pulse duration and extends from the femtosecond 7 to the nanoseccond [8][9][10] range. Under some experimental conditions, the delayed emission of electrons after electronic excitation using UV/VIS lasers can be observed.…”