The metallofullerene Tm@C82: isomer-selective electronic structure

82 using photoemission and electron energy-loss spectroscopy in transmission. In particular, the impact of the isomer symmetry of the C₈₂ cage (C_s or C_3v) on the electronic structure and on the optical properties is analysed in detail. For both isomers, combined XPS and XAS measurements of the Tm 4f and 4d states offer conclusive proof of a divalent encaged Tm ion. No significant change of the valence of the rare earth ion upon changes in the cage symmetry is observed. From the C 1s excitation spectra we find a more pronounced LUMO for the C_3v(8) isomer, as is expected for the higher symmetry of the cage. Additional information about the optical properties was obtained from the low-energy loss function. With use of a Kramers–Kronig analysis, the dielectric function, ε, and the optical conductivity, σ, have been derived. ε(0) and the onset of the spectral weight are found to be 7.7 [12.8] and 0.6 eV [0.8 eV] for the C_s(6) {C_3v(8)} isomer, respectively. Received: 14 October 1997/Accepted: 28 October 1997     

Pulsed ESR investigations of anisotropic interactions in M@C82 (M=Sc,Y,La)

82 , Y@C₈₂, and La@C₈₂ in frozen solutions. We were able to determine the g tensors of these molecules by analysing magnetic field spectra at X-band (9.5 GHz) and W-band (94 GHz) frequencies. Moreover, in Y@C₈₂ we have investigated the hyperfine interaction of the ⁸⁹Y nuclear spin (I=1/2) with the electron spin on the C₈₂ cage. The principal values of the hyperfine tensor A and the relative orientation of g and A tensors were determined by applying three- and four-pulse electron spin echo envelope modulation techniques (ESEEM). Received: 3 September 1997/Accepted: 10 November 1997     

Production, isolation, and characterization of group-2 metal-containing endohedral metallofullerenes

80 , Ca/Sr/Ba@C₈₂, Ca/Sr/Ba@C₈₄) are investigated for the first time by UV-Vis-NIR absorption spectroscopy and high-resolution ¹³C NMR. Direct evidence of the endohedral nature and cage structures are revealed by these measurements. Furthermore, we have found that each metallofullerene has 2–4 structural isomers, which have been isolated by multistage high-performance liquid chromatography. These isomers have different cage structures and give characteristic UV-Vis-NIR absorption spectra. Received: 22 September 1997/Accepted: 16 October 1997     

Structures and energetics of carbon bridged C 60 clusters

The structures and energetics of carbon bridged C₆₀ clusters (C ₆₀ ) _n C_m have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp² addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp² addition” dimers, a pentagon-linked C₁₂₁ isomer and a hexagon-linked C₁₂₂ isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C₆₀ joints on a single cage are not too close to each other. Large C₆₀ clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C₆₀-carbon polymers. Received 17 January 2001 and Received in final form 26 February 2001     

A spectroscopic study of M@C82 metallofullerenes: Raman, far-infrared, and neutron scattering results

82 metallofullerenes have been studied at room temperature by Raman (for M=La, Y, Ce, Gd), far-infrared (FIR) (for M=La, Y, Ce), and inelastic neutron scattering (INS) (for M=La, Y) spectroscopy. Raman and FIR spectra suggest that these metallofullerenes have a common dominant, if not a single, structure of the C₈₂ cage and a similar bonding of the encapsulated metal ion, i.e. the bonding is primarily electrostatic and the metal atoms are in the same oxidation state (+3). The metal ion vibrations are located around 160 and 50 cm^-1. INS reveals no gap between internal vibrational and external vibrational and rotational modes in the range ∼50–200 cm^-1 as is typically observed for other fullerides and also predicted by our model calculations. Presumably this is due to strong intermolecular interactions between M@C₈₂ units in the bulk sample. The studied metallofullerenes are air sensitive, and degradation in air could be followed by changes in the Raman spectra. Received: 24 August 1997/Accepted: 26 September 1997     

161Dy Mössbauer study of the endohedral metallofullerenes Dy@C n (n = 80, 82, 84)

Mössbauer studies of Dy@C _n (n = 80, 82, 84) metallofullerenes were performed at 4.2, 9.6 and 78 K with the ¹⁶¹Dy (25.6 keV) resonance. The observed spectra consist of two subspectra splitted by magnetic hyperfine fields near to the full moment value of trivalent Dy. Paramagnetic relaxation is observed even at 4.2 K. The observed isomer shift is consistent with the Dy³⁺ state and indicates a full charge transfer to the fullerene cage.     

The incorporation of Nickel and Phosphorus dopants into Boron-Carbon alloy thin films

5 C) alloy thin films grown by plasma-enhanced chemical vapor deposition have been examined. The Ni-doped boron–carbon alloys were grown using closo-1,2-dicarbadodecaborane (C₂B₁₀H₁₂) as the boron–carbon source compound and nickelocene(Ni(C₅H₅)₂) as the nickel source. The phosphorus-doped alloys were grown using the single-source compound: dimeric chloro-phospha(III)-carborane ([C₂B₁₀H₁₀PCl]₂). Nickel doping increased the conductivity, relative to undoped B₅C, by six orders of magnitude from 10^-9 to 10^-3 (Ω cm)^-1 and transformed the material from a p-type semiconductor to an n-type. Phosphorus doping decreased the conductivity, relative to undoped B₅C, by two orders of magnitude and increased the band gap from 0.9 eV for the undoped material to 2.6 eV. Infrared absorption spectra of the nickel- and phosphorus-doped B₅C alloys were relatively unchanged from those of undoped B₅C. X-ray diffraction suggests that the phosphorus-doped material may be a different polytype from the Ni-doped and undoped B₅C alloys. Received: 23 April 1997/Accepted: 3 November 1997     

Ab initio approach to the structure and dynamics of metallofullerenes

60 , and La@C₈₂. Structural, electronic, and vibrational properties are determined and compared with experiment whenever possible. In the C₆₀ case, clear characteristic and measurable features are found that distinguish between exo- and endo-complexes. Also in C₈₂, fingerprints of the presence of the guest atom, its position, and charge state are identified in measurable spectra, as well as of the structure of the cage. Received: 26 November 1997/Accepted: 28 November 1997     

Nuclear spins, magnetic moments and α-decay spectroscopy of long-lived isomeric states in 185Pb

Alpha-decay properties of the neutron-deficient isotope ¹⁸⁵Pb were studied at the PSB-ISOLDE (CERN) on-line mass separator using the resonance ionisation laser ion source (RILIS). The nuclei of interest were produced in a 1.4 GeV proton-induced spallation reaction of a uranium graphite target. In contrast to previous studies, two α-decaying isomeric states were identified in ¹⁸⁵Pb. The relative production of the isomers, monitored by their α-counting rates, could be significantly changed when a narrow-bandwidth laser at the RILIS setup was used to scan through the atomic hyperfine structure. Based on the atomic hyperfine structure measurements, along with the systematics for heavier odd-mass lead isotopes, the spin and the parity of these states were interpreted as 3/2^- and 13/2⁺ and their nuclear magnetic moments were deduced. The α-decay energy and half-life value for the I ^π = 13/2⁺ isomer are E _α = 6408(5) keV, T _1/2 = 4.3(2) s, respectively; while for the I ^π = 3/2^- isomer ( T _1/2 = 6.3(4) s) two α-decays with E _α1 = 6288(5) keV, I _α1 = 56(2)% and E _α2 = 6486(5) keV, I _α2 = 44(2)% were observed. By observing prompt α-γ coincidences new information on the low-lying states in the daughter isotope ¹⁸¹Hg was obtained. Received: 7 February 2002 / Accepted: 19 February 2002     

Optical spectra and spin-Hamiltonian parameters of trivalent ytterbium in lead tungstate

By using crystal-field theory, the optical spectra and spin-Hamiltonian parameters (abbr. SH parameters, i.e. the anisotropic g factors g _∥, g _⊥, and hyperfine structure constants A _∥, A _⊥) of ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes in the tetragonal PbWO₄ are calculated. The theoretical results agree well with the experimental values. The crystal-field parameters and the signs of the hyperfine structure constants for both ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes are determined. The validities of the theoretical results are discussed.     

The electronic state of scandium trimer encapsulated in a C82 cage

The electron spin resonance spectrum of scandium trimer encapsulated in a C₈₂ cage (Sc₃@C₈₂) was measured at low temperatures. The spectrum exhibited a specific pattern due to the strongly anisotropic hyperfine tensor of the scandium trimer. The electronic state of the Sc₃@C₈₂ was determined from an analysis of the hyperfine tensor.     

Vibrational dynamics of fullerene molecules adsorbed on metal surfaces studied with synchrotron infrared radiation

Infrared (IR) spectroscopy of chemisorbed C₆₀ on Ag (111), Au (110) and Cu (100) reveals that a non-IR-active mode becomes active upon adsorption, and that its frequency shifts proportionally with the charge transferred from the metal to the molecule by about 5 cm^-1 per electron. The temperature dependence of the frequency and the width of this IR feature have also been followed for C₆₀/Cu (100) and were found to agree well with a weak anharmonic coupling (dephasing) to a low-frequency mode, which we suggest to be the frustrated translational mode of the adsorbed molecules. Additionally, the adsorption is accompanied by a broadband reflectance change, which is interpreted as due to the scattering of conduction electrons of the metal surface by the adsorbate. The reflectance change allows determination of the friction coefficient of the C₆₀ molecules, which results in rather small values (∼2×10⁹ s^-1 for Ag and Au, and ∼1.6×10⁹ s^-1for Cu), consistent with a marked metallic character of the adsorbed molecules. Pre-dosing of alkali atoms onto the metal substrates drastically changes the IR spectra recorded during subsequent C₆₀ deposition: anti-absorption bands, as well as an increase of the broadband reflectance, occur and are interpreted as due to strong electron–phonon coupling with induced surface states. Received: 6 June 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

A neutron diffraction and Mössbauer study of NdFe9.25Si1.75Cx compounds

The crystallographic structures, magnetic properties and hyperfine interactions of NdFe_9.25Si_1.75C_x have been studied using neutron diffraction, magnetic and ⁵⁷Fe Mössbauer spectra measurements. The refinement of the powder neutron diffraction demonstrates that NdFe_9.25Si_1.75C_1.5 crystallizes in the BaCd₁₁-type structure with space group I4₁/amd and four formula units per unit cell. Nd atoms occupy 4a sites, and Fe atoms distribute on 4b, 8d and 32i sites. Si and C atoms occupy 8d and 8c sites, respectively. The saturation magnetization, the isomer shift (IS) and the hyperfine field reach their highest values at the carbon content x=1.0. With a set of phenomenological formulas, the contribution of the magneto-volume effect and the chemical bonding effect to the hyperfine parameters are successfully separated. The pure magneto-volume effect and the pure chemical bonding effect influence the hyperfine field and the IS differently. The former one increases both of them and the latter one decreases them. This is consistent with the rather strong bond between Fe(32i) and C(8c).     

Thermal tuning and modulation of a DFB fibre laser with a thin-film heater

Results are given for thermal tuning and modulation of a 1556-nm distributed feedback fibre laser by resistive heating of a thin silver film chemically deposited on the fibre. Without reaching the limits of performance, linear tuning is demonstrated at a rate of 1.72 pm/mW up to about 200 pm, and a peak-to-peak modulation of 100 MHz up to modulation frequencies of 60 Hz. The heat flow is analyzed, and the coated fibre is characterized in terms of the static and dynamic wavelength response to the applied electric power. The performance of the scheme is tested by recording part of the ν₁+ν₃ combination band spectrum of ¹³C₂H₂ with thermal modulation and scanning of the fibre laser. Received: 12 March 2002 / Revised version: 24 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +45/4593-1137, E-mail: sus@dfm.dtu.dk     

Atomic nitrogen in C60:N@C60

60 (atomic nitrogen inside C₆₀) is produced by ion implantation. Two different production methods are employed: Kaufman ion source and glow discharge. After the bombarded material is dissolved in toluene or CS₂ and is filtered, several milligrams of C₆₀ containing N@C₆₀ in a concentration of 10^-4 to 10^-5 are obtained. N@C₆₀ gives a very clear hyperfine-split electron paramagnetic resonance signal. The most prominent features of N@C₆₀ are: (i) Nitrogen in C₆₀ keeps its atomic electronic configuration and occupies the on-center position. (ii) N@C₆₀ is stable at ambient conditions, the thermal instability starts at 260 °C. (iii) The complex survives exohedral addition reactions and is a sensitive detector of cage distortions caused by addends. (iv) C₆₀ and N@C₆₀ exhibit slightly different retention times in column chromatography, thus permitting an enrichment of N@C₆₀ by this method. Received: 2 October 1997/Accepted: 16 October 1997     

X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C60 polymer films

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon [ _]K near-edge X-ray-absorptionfine structure spectroscopy (NEXAFS) results of plasma-polymerized C₆₀. In comparison with evaporated C₆₀ the C 1s peak is broader and asymmetric for the C₆₀ polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π^* antibonding orbitals of the C₆₀ polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C₆₀, the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. Received: 28 May 1999 / Accepted: 31 August 1999 / Published online: 8 March 2000     

Nuclear charge radii and electromagnetic moments of scandium isotopes and isomers in the f7/2 shell

Collinear laser spectroscopy experiments on the ScII transition 3d4s ³D₂→3d4p ³F₃ at λ ≈ 363.1 nm were performed on the ^42–46Sc isotopic chain using an ion guide isotope separator with a cooler–buncher. Isotope and isomer shifts and hyperfine structures of five ground states and two isomers were measured. Preliminary results on the nuclear moments and charge radii changes deduced from these measurements are reported.     

EPR investigation of endofullerenes in solution

82 and Lu@C₈₂ were prepared by arc burning and subsequent HPLC purification. EPR spectra of Lu@C₈₂ could be interpreted as arising from unresolved hyperfine interaction with the I=7/2 nuclear spin of ¹⁷⁵Lu. At temperatures above 250 K, a thermally activated process, which is tentatively attributed to a hopping process of the encapsulated ion or to time-dependent population of close-lying electronic states, leads to pronounced line broadening. For the Ho@C₈₂ sample, no EPR signals could be detected, indicating a high spin state of this molecule. Spin relaxation data of N@C₆₀, which was prepared by ion bombardment, could be interpreted by assuming that collision-induced deformation of the carbon shell leads to a fluctuating zero-field splitting, sensed by the quartet spin state of the central encapsulated nitrogen atom. Received: 25 August 1997/Accepted: 6 October 1997     

Laser-induced fluorescence study of a xenon Hall thruster

2 ⁰→6p[3/2]₂(³P₂-¹D₂) transition at 823.2 nm and the xenon-ion 5d[3]_7/2→6p[2]_5/2 ⁰(⁴D_7/2-⁴P_5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s^-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar⁺-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s^-1 to 400 m s^-1 and ion velocities as high as 12 km s^-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm^-1. Received: 20 January 1997/Revised version: 12 May 1997     

ESR study on the reactivity of two isomers of LaC82 with disilirane

We observed photochemical and thermal addition of disilirane to the LaC₈₂-A isomer, which give new ESR active species. Both ESR spectra of the adducts in photo- and thermal reactions are well simulated by at least six octet components, and the isotropic La coupling constants and g-factors of the six octet components are determined. On the other hand, the reaction of the LaC₈₂-B isomer with disilirane gives no ESR active species. This fact is also verified by mass spectroscopic measurements. The different reactivities with disilirane of the two isomers are not interpreted by a popular view that the two isomers have a different C₈₂ cage structure, but we rather suggest that the origin of the isomers of metallofullerenes should be re-examined.