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Register a new userExperimental evidence for the influence of charge on the adsorption capacity of carbon dioxide on charged fullerenes
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We show, with both experiment and theory, that adsorption of $CO_2$ is sensitive to charge on a capturing model carbonaceous surface. In the experiment we dope superfluid helium droplets with $C_{60}$ and $CO_2$ and expose them to ionising free electrons. Both positively and negatively charged $C_{60}(CO_2)_n^{+/-}$ cluster ion distributions are observed with a high-resolution mass spectrometer and these show remarkable and reproducible anomalies in intensities that are strongly dependent on the charge. The highest adsorption capacity is seen with $C_{60}^+$. Complementary density functional theory calculations and molecular dynamics simulations provided insight into the nature of the interaction of charged $C_{60}$ with $CO_2$ as well as trends in the packing of $C_{60}^+$ and $C_{60}^-$. The quadrupole moment of $CO_2$ itself was seen to be decisive in determining the charge dependence of the observed adsorption features. Our findings are expected to apply to adsorption of $CO_2$ by charged surfaces in general.
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Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some experiments have shown promising results. We investigate the upper bounds of hydrogen storage capacities of $C_{60}Cs$ clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. On bare $C_{60}^{+}$, a commensurate phase with 32 $H_2$ molecules was identified in previous experiments. Doping $C_{60}$ with a single cesium atom leads to an increase in relative ion abundance for the first 10 $H_2$ molecules, and the closure of the commensurate phase is shifted from 32 to 42 $H_2$ molecules. Density functional theory calculations indicate that thirteen energetically enhanced adsorption sites exist, where six of them fill the groove between Cs and $C_{60}$ and 7 are located at the cesium atom. We emphasize the large effect of the quantum nature of the hydrogen molecule on the adsorption energies, i.e. the adsorption energies are decreased by around 50% for $(H_2)C_{60}Cs$ and up to 80% for $(H_2)C_{60}$ by harmonic zero-point corrections, which represent an upper bound to corrections for dissociation energies ($D_e$ to $D_0$) by the vibrational ground states. Five normal modes of libration and vibration of $H_2$ physisorbed on the substrate contribute primarily to this large decrease in adsorption energies. A similar effect can be found for H2 physisorbed on benzene and is expected to be found for any other weakly $H_2$-binding substrate.
Adsorption of para-Hydrogen on the outer surface of a single fullerene is studied theoretically, by means of ground state Quantum Monte Carlo simulations. We compute energetics and radial density profiles of para-Hydrogen for various coverages on a variety of small fullerenes. The equilibrium adsorbed monolayer is commensurate with the surface of the fullerene; as the chemical potential is increased, a discontinuous change is generally observed, to an incommensurate, compressible layer. Quantum exchanges of Hydrogen molecules are absent in these systems.
We have calculated photoionization cross-section of endohedral atoms A@CN. We took into account the polarizability of the fullerene electron shell CN that modifies the incoming photon beam and the one-electron wave functions of the caged atom A. We employ simplifi
In this Letter, we investigate the variation of endohedral A@CN potential due to addition at the center of it a positive charge, for example, in the process of atom A photoionization. Using a reasonable model to describe the fullerenes shell, we managed to calculate the variation that is a consequence of the monopole polarization of CN shell. We analyze model potentials with flat and non-flat bottoms and demonstrate that the phenomenological potentials that properly simulates the C60 shell potential should belong to a family of potentials with a non-flat bottom. As concrete example, we use the Lorentz-bubble model potential. By varying the thickness of this potential, we describe the various degrees of the monopole polarization of the C60 shell by positive electric charge in the center of the shell. We calculated the photoionization cross-sections of He, Ar and Xe atoms located at the center of C60 shell with and without taking into account accompanying this process monopole polarization of the fullerenes shell. Unexpectedly, we found that the monopole polarization do not affect the photoionization cross sections of these endohedral atoms.
A novel experimental scheme has been developed in order to measure the heat capacity of mass selected clusters. It is based on controlled sticking of atoms on clusters. This allows one to construct the caloric curve, thus determining the melting temperature and the latent heat of fusion in the case of first-order phase transitions. This method is model-free. It is transferable to many systems since the energy is brought to clusters through sticking collisions. As an example, it has been applied to Na_90^+ and Na_140^+. Our results are in good agreement with previous measurements.
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