Prismane C_8: A New Form of Carbon?

Our numerical calculations on small carbon clusters point to the existence of a metastable three-dimensional eight-atom cluster C$_8$ which has a shape of a six-atom triangular prism with two excess atoms above and below its bases. We gave this cluster the name "prismane". The binding energy of the prismane equals to 5.1 eV/atom, i.e., is 0.45 eV/atom lower than the binding energy of the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the binding energy of the bulk graphite or diamond. Molecular dynamics simulations give evidence for a rather high stability of the prismane, the activation energy for a prismane decay being about 0.8 eV. The prismane lifetime increases rapidly as the temperature decreases indicating a possibility of experimental observation of this cluster.Comment: 5 pages (revtex), 3 figures (eps

Theoretical study of the stable states of small carbon clusters Cn (n = 2-10)

Both even- and odd-numbered neutral carbon clusters Cn (n = 2-10) are systematically studied using the energy minimization method and the modified Brenner potential for the carbon-carbon interactions. Many stable configurations were found and several new isomers are predicted. For the lowest energy stable configurations we obtained their binding energies and bond lengths. We found that for n < 6 the linear isomer is the most stable one while for n > 5 the monocyclic isomer becomes the most stable. The latter was found to be regular for all studied clusters. The dependence of the binding energy for linear and cyclic clusters versus the cluster size n (n = 2-10) is found to be in good agreement with several previous calculations, in particular with ab initio calculations as well as with experimental data for n = 2-5.Comment: Submitted to Phys. Rev.

Superconductivity in Fullerides

Experimental studies of superconductivity properties of fullerides are briefly reviewed. Theoretical calculations of the electron-phonon coupling, in particular for the intramolecular phonons, are discussed extensively. The calculations are compared with coupling constants deduced from a number of different experimental techniques. It is discussed why the A_3 C_60 are not Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates of the Coulomb pseudopotential $\mu^*$, describing the effect of the Coulomb repulsion on the superconductivity, as well as possible electronic mechanisms for the superconductivity are reviewed. The calculation of various properties within the Migdal-Eliashberg theory and attempts to go beyond this theory are described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to Review of Modern Physics, more information at http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm

SPECTROSCOPY OF C3C_{3} AND SiC2SiC_{2} MOLECULES IN NEON AND ARGON MATRICES

$^{2}$B. Kleman, Astrophys, J. 125, 162 (1956). $^{1}$L. Gausset, G. Herzberg, A. Lagerquist and B. Rosen, Faraday Society (in press)Author Institution: Union Carbide Research Institute“The spectrum of the $C_{3}$ molecule trapped in neon and argon matrices at $4^{\circ} K$ and $20^{\circ} K$ has been studied in absorption in the infrared and near-ultravoilet regions and in fluorescence in the visible region. Many features of the well-known cometary emission spectrum beginning at 4050\AA and strikingly reproduced in the neon absorption spectrum. $C$ substitution has also been used to prove that $C_{3}$ is isolated is isolated under the extreme conditions prevailing during the preparation of the matrices. Analysis of the near-ultraviolet bands yields the excited-state frequencies: $\nu_{1} = 1100\;cm^{-1},\;\nu’_{2} = 420\;cm^{-1},\;\nu’_{3} = ?$. Fluorescence and infrared measurements give the ground-state frequencies: $\nu_{1}’’ = 1235\;cm^{-1}\;\nu_{3}’’ = 2042\;cm^{-1}$. Our ultraviolet spectra are in accord with the low bending frequency, $\nu_{2}’’ \cong 70\;cm^{-1}$, recently proposed by Gausset, Herzberg, Lagerquist, and $Rosen.^{1}$ Other complexities in the spectrum indicate that the observed transition may be $^{3}\Pi_{u} \longleftrightarrow X ^{3} \Sigma_{g}^{-}$- rather than the expected $^{1} \Pi_{u} \longleftrightarrow {^{1}}\Sigma_{g}^{+}$. A similar study has been made of the $SiC_{2}$ molecule which has been observed in stellar spectra and produced in the laboratory by $Kleman^{2}$. The matrices have been prepared by trapping the vapour effused from hot silicon carbide. The absorption spectrum in a neon matrix begins at 4963{\AA} (as compared to 4977{\AA} in the gas) and contains some weak bands not observed by Kleman. The matrix may also exhibit the spectra of the $Si_{2},\;Si_{2}C$, and $Si_{2}C_{3}$ molecules, depending upon the conditions of vaporization.

SPECTROSCOPY OF TRANSITION-METAL OXIDE MOLECULES IN RARE-GAS MATRICES AT 4∘4^{\circ} AND 20∘K20^{\circ}K.

Author Institution: Union Carbide Research InstituteThe infrared, visible, and near-ultraviolet spectra of TiO, ZrO, HfO, WO, $WO_{2}$ (and $WO_{3}, W_{2}O_{6}, W_{3}O_{8}, W_{3}O_{9}$) have been observed in neon and argon matrices at $4^{\circ}$ and $20^{\circ}K$ with $^{16}O$ and $^{18}O$ substitution. A discussion will be given of the spectra and the molecular orbitals and ground states of the diatomic and triatomic molecules. The larger tungsten oxide molecules have been observed in the infrared only; the particular trapped species varying with the conditions of vaporization. It is found that (1) the spectra confirm a $^{3}\Delta_{***}$ ground state for $TiO,^{1}$ (2) ZrO, and probably HfO, have a $^{1}\Sigma^{+}$ ground state, (3) the $W^{16}O$ electronic spectra exhibit many vibrational perturbations not present in the $W^{18}O$ spectra, (4) the $WO_{2}$ spectra are like those of $TaO_{2}$, so that $WO_{2}$ is bent in the ground and excited $states,^{2}$ (5) each of the numerous infrared bands of the $W_{a}O_{m}$ molecules can usually be assigned to particular species with the help of mass spectrometric $data.^{3} {^{1}}$ J. G. Phillips, Astrophys, J. 115, 567 (1962). $^{2}$ W. Weltner, Jr. and D. McLeod, Jr., J. Chem. Phys. 42, 882 (1965). $^{3}$ J. Berkowitz, W. A. Chupka, M. G. Inghram, J. Chem. Phys. 27, 85 (1957); R. J. Ackermann and E. G. Rauh, J. Phys. Chem. 67, 2596 (1963)

ESR OF THE BO AND AlO MOLECULES: EVIDENCE FOR A AlO Kr MOLECULE AT 4∘K4^\circ K

$^{1}$L. B. Knight, JR., W. C. Easley, and W. Weltner, JR., J. Chem. Phys. 52 1607 (1970).""Author Institution: Department of Chemistry, University of FloridaBO and AlO molecules have been trapped in rare-gas matrices at $4^\circ K$ in their $^{2}\Sigma$ ground states. Hyperfine (hf) interactions, g tensor, and matrix effects are strikingly different for these two species. BO exhibits a large unpaired spin density at the boron nucleus, is highly oriented in a ncon matrix, and has g values near $g_{e} = 2.0023$ in all matrices. In the ionic AlO molecule, only 20% of the odd electron is on the aluminum atom, and $k_{1}$ and $A_{1}$ (hf splitting of $^{27}Al$) are extremely sensitive to the rare-gas $employed.^{1}$ $^{83}Kr$ hf structure has been resolved in that matrix associated with an extra set of lines that vanish above $35^\circ K$ and reappear upon cooling to $4^\circ K$. The molecule formed between AlO and one Kr atom is presumably linear and exhibits a high degree of preferential orientation in the matrix. Its Al hfs is greater than that of AlO in a non-complexed lattice site