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These models include, for example, the rate equations of the ionization and the C ionization cross sections that are directly resulting from quantum mechanical calculations [ 45 , 46 ]. The real-space dynamics of atoms, ions, and the quasi- free electrons resulting from photoionization and Auger decay is described by Newtonian mechanics.

In this methodology, C 60 is modeled as 60 individual carbon atoms. The C atoms were held together by a fullerene-specific classical Brenner force field, and the charges interacted via Coulomb forces. In order to mimic a molecule, the model needed to contain several physical and chemical processes that were revealed through the experiment.

The model originally predicted more abundant C ion charge states, which revealed that there was strong recombination between the released electrons from photoionization and the ions after the FEL pulse ends.

The comparison between the model and the experimental data not only led to the interpretation of our experiment but also led to solid improvement in the MD calculation. This was taken into account by removing the ejected electron from the L-shell of a neighboring C atom, while the 1s vacancy of an ion was filled up with its own L-shell electron; and 4 the recombination of a classically trapped electron if it were no longer delocalized among several C ions but instead localized to only one C ion.

As can be seen, there is no effect with the two different pulse durations, which thus revealed that the dynamics might already be over with the use of a 30 fs X-ray pulse. This means that the physical and chemical processes included might have occurred within 30 fs. The smallest value of the bars corresponds to the best agreement between the model and the experiment. The X-axis displays bars that correspond to the several different physical and chemical processes, which were included in the model.

As described above, each of these processes were included one at a time to display their importance. As can be seen, molecular effects and molecular bonds slightly contributed to a better agreement.

The dramatic effect is obtained in the case of the secondary ionization of C 60 by the photo- and Auger-electrons. This effect is weak in small, isolated molecules and van der Waals clusters; it is completely absent in atoms. As can be seen, the addition of molecular bond-breaking and molecular Auger improved even more the model.

This combined theoretical and experimental work illustrates the successful use of classical mechanics to describe all moving particles in C The work clearly revealed the influence of processes not previously suspected or reported. The impact we aimed to achieve was also realized because this fullerene spectroscopic work quantitatively demonstrated electronic damage due to photoelectron and Auger electrons interacting with the ions as a secondary ionization effect.

Fullerene Collision Reactions | E.E. Campbell | Springer

Our results were corroborated with recent separate calculations [ 39 ]. Finally, one of the goals of the MD model was to build a new approach that would scale with larger systems, such as biomolecules. This work demonstrated that the modeling [ 40 , 44 ] coached by experiment was successful and can be applicable for X-ray interactions with any extended system—even at higher X-ray dose rates that are expected with future FEL sources, such as the soon-to-be available XFEL in Germany and with the future LCLS-II.

The C 60 work described above led us to consider exploring increased complexity by choosing the interaction of endohedral fullerenes also called doped fullerenes with FELs. These systems are even more intriguing than C 60 , because they host a moiety that ranges from an atom to a molecule. Nothing was known about their structure or dynamics when excited with X-ray FEL. As mentioned in the introduction, these nanoscale systems have received attention in part because they can be used for applications ranging from medical usage [ 48 ] to drug delivery, as well as their possible use for quantum computing [ 49 ].

The experiment was carried out at the AMO hutch using a time-of-flight spectrometer [ 50 ] for detecting the ions produced in the interaction of the endohedral fullerenes with the LCLS pulses. The experiment on Ho 3 N C 80 was carried out with eV in order to selectively target the ionization from a specific shell, which was the Ho 3d. The pulse duration of the X-ray pulse was 80 fs with a pulse energy of about 6.

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This experiment had less fluence than the experiment on C 60 by two orders of magnitude due to a different transport of the photon beam through the optics [ 50 ]. This must arise from the fragmentation of the encapsulated Ho 3 N moiety—thus, indicating that the three multiply charged Ho atoms, which were selected to be the most ionized compared to N or C atoms, freed themselves from the C 80 cage.

The loss of C dimers was also observed previously with tabletop experiments [ 2 ]; however, the shrunk cage size to C 50 was never previously observed. This experiment was carried out in the low-fluence regime, since we estimated that about eight photons were absorbed by Ho 3 N C In the C 60 experiment, described in Section 3, we appraised that about photons were absorbed per C 60 molecule.

The photoionization with eV leads to the absorption cross section of Ho to be about 1. Our interpretation of the interaction of Ho 3 N C 80 with eV photon energy was that the Ho atom charges up and gets multi-ionized due to the cyclic photoionization and Auger decay [ 51 ].


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We assumed that as the carbon cage charges up, it would become unstable and will break apart, thus leading to molecular fragment ions. The Ho atoms are about ten times heavier than the C atoms, and, therefore, we assume that they will not move faster than the carbon cage. It is unclear if the moiety first breaks into three Ho atoms and the N atoms and then the carbon cage fragments or if the reverse occurs. It is also unclear how the new bonds have formed. This is to be determined by future time-resolved experiments that might track the ionization and fragmentation dynamics and decipher the mechanisms leading to the final ionic states we observed.

Additionally, we hope that our work will stimulate the development of molecular dynamics simulations suitable for endohedral fullerenes and for even larger molecules exposed to intense XFEL. Pump-probe spectroscopy techniques, championed by tabletop lasers [ 36 ], allow the measurement of dynamics for any system — from atoms to fullerenes and from solids to biological specimens. They are being used extensively, and the ultimate goal is to determine the motions and locations of nuclei and electrons and to determine the energy flow and charge transfer in systems.

Fullerene Collision Reactions

Pump-probe techniques with FELs offer similar opportunities to measure physical and chemical changes in molecules at an atomic spatial resolution on the time scale of atomic motion. With attosecond, the goal is to also measure the electronic motion. These techniques can be used for the study of fullerenes, in order to tackle fundamental questions such as, how do the atoms in the fullerenes move after the photon energy is deposited in the fullerenes?

How do the bonds between the atoms that make the fullerenes break, or what are the pathways for the induced atomic motion in fullerenes? These questions can be asked and answered with FELs using pump-probe techniques. As in standard pump-probe work, to capture the dynamics, the pump pulse initiates the motions, and a probe pulse detects the changes using as many time delays as needed between the pump and probe pulses, ideally in a wide time scale.

In order to achieve this goal, one needs two pulses, which can be generated in a few ways: 1 the accelerator scientists have developed several methods, but the most recent one cuts fresh slices from the electron bunch. They manipulate the electron bunch before it enters a split undulator, so that only its tail is lasing, in order to produce the first X-ray pulse. Next, they delay the electron bunch in order to acquire a time delay and spoil the electron bunch orbit further, by having the head of the bunch lasing and thus using a fresh slice from the electron bunch to produce the second photon pulse [ 52 ].

Fullerene Dynamics with X-Ray Free-Electron Lasers

This scheme is also capable of providing two X-ray colors [ 52 ]; 2 the FEL pulse can be split into two X-ray pulses with an X-ray split and delay tool [ 53 ]. These two schemes were used successfully in experiments in the mode of X-ray pump-X-ray probe; and 3 a third scheme utilizes an X-ray pulse from the FEL as either the pump or the probe, and it is paired with a short tabletop pulse laser [ 54 ] IR or UV. All of these schemes have been used in various FELs. The FEL-based experiments, paired most of the time with pump-probe techniques, are carried out by using various types of spectrometers or imaging detectors for absorption experiments or for diffractive scattering experiments, respectively.

Buy Hardcover. Buy Softcover. FAQ Policy. About this book Fullerene Collision Reactions provides a comprehensive overview of the state-of-the-art of fullerene collision studies. Show all. From the reviews: "The book consists of 10 chapters on pages and deals with topical aspects of atomic, ionic, electronic, cluster and surface collisions involving fullerenes.

Table of contents 10 chapters Table of contents 10 chapters Introduction Pages Experimental Techniques Pages Theoretical Models Pages Collision Induced Dissociation Pages As can be seen, there is no effect with the two different pulse durations, which thus revealed that the dynamics might already be over with the use of a 30 fs X-ray pulse.

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This means that the physical and chemical processes included might have occurred within 30 fs. The smallest value of the bars corresponds to the best agreement between the model and the experiment. The X-axis displays bars that correspond to the several different physical and chemical processes, which were included in the model. As described above, each of these processes were included one at a time to display their importance. As can be seen, molecular effects and molecular bonds slightly contributed to a better agreement. The dramatic effect is obtained in the case of the secondary ionization of C 60 by the photo- and Auger-electrons.

This effect is weak in small, isolated molecules and van der Waals clusters; it is completely absent in atoms. As can be seen, the addition of molecular bond-breaking and molecular Auger improved even more the model. This combined theoretical and experimental work illustrates the successful use of classical mechanics to describe all moving particles in C The work clearly revealed the influence of processes not previously suspected or reported. The impact we aimed to achieve was also realized because this fullerene spectroscopic work quantitatively demonstrated electronic damage due to photoelectron and Auger electrons interacting with the ions as a secondary ionization effect.

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Our results were corroborated with recent separate calculations [ 39 ]. Finally, one of the goals of the MD model was to build a new approach that would scale with larger systems, such as biomolecules. This work demonstrated that the modeling [ 40 , 44 ] coached by experiment was successful and can be applicable for X-ray interactions with any extended system—even at higher X-ray dose rates that are expected with future FEL sources, such as the soon-to-be available XFEL in Germany and with the future LCLS-II. The C 60 work described above led us to consider exploring increased complexity by choosing the interaction of endohedral fullerenes also called doped fullerenes with FELs.

These systems are even more intriguing than C 60 , because they host a moiety that ranges from an atom to a molecule.