Pinch Mode Research Articles

Surface-enhanced Raman scattering and photoemission of C60 on noble-metal surfaces.

Surface-enhanced Raman scattering of monolayer ${\mathrm{C}}_{60}$ deposited in UHV on noble metals yields substantial shifts of the high-frequency, ${\mathit{A}}_{\mathit{g}}$(2) pentagonal pinch mode. These shifts, which increase in the series Au, Cu, and Ag, are attributed, in part, to charge transfer to the fullerene. This behavior is consistent with a decrease in noble-metal work function in this sequence. Confirmation of charge transfer in Cu and Ag is directly obtained by He I, ultraviolet photoemission spectroscopy measurements which indicate formation of a lowest-unoccupied-molecular-orbital (LUMO) -derived band similar to that observed in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. The form and Fermi edge behavior of the LUMO-derived band further imply the formation of a metallic interfacial layer. X-ray photoemission spectroscopy measurements indicate shifts of the C 1s core level which differ from trends noted in Raman scattering, suggesting that interfacial effects beyond charge transfer may be important.

Optical characterization of excited states in films of undoped C 60

We have used various optical characterization techniques to study the electronic excited states in icosahedral clusters of C 60 thin films produced by high-vacuum evaporation onto sapphire substrates. These techniques include c.w. photoluminescence (PL), photomodulation (PM) and spin-dependent PL and PM. We have found a modestly strong PL band at 1.72 eV, with two phonon replicas 185 meV apart. This phonon is the most strongly coupled C 60 pentagonal pinch mode and therefore shows that the PL band is intrinsic to C 60. From its flat modulation frequency response up to 100 kHz, we conclude that the PL lifetime is shorter than 3 μs, implying that it is due to singlet exciton recombination. The PM spectrum contains a single band peaking at about 1 eV, which is correlated with a derivative-like feature around 2.3 eV; no photoinduced IR-active vibrations have been discovered. From the additional study of the frequency and excitation dependencies, we conjecture that the PM is due to charged carriers which recombine with bimolecular kinetics.

Phase separation in KxC60 (0

From in situ Raman measurements during potassium doping of pure ${\mathrm{C}}_{60}$ films to a structure ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ in the range of 0\ensuremath{\le}x\ensuremath{\le}6 we found a continuous change of line intensities for the ${\mathit{A}}_{\mathit{g}}$ modes of the stable phases in this material. No shift of these modes with doping was observed, which proves a strict phase separation. The Raman line at 1467 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for pristine ${\mathrm{C}}_{60}$ is due to oxygen contamination and is assigned to positively charged fullerene states. The intrinsic pinch mode of the pristine material is at 1458 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and has a linewidth of 4 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.

High‐pressure Raman study of fullerite C60

AbstractHigh‐pressure Raman spectra of fullerite, a solid form of C60, were measured at pressures up to 4 GPa and room temperature. A change of slope (dv/dP) of the frequency‐pressure plots for the Ag pentagonal pinch mode and the Ag breathing mode was observed at 1.3 GPa, where a solid‐solid phase transition might occur. The phase transition was observed to be irreversible. The pressure dependence of the Raman mode frequency and the phase transition are discussed in terms of an orientational ordering transition in fullerite.

Resonance Raman scattering and electronic transitions in C 60

We report relative resonance Raman cross sections and self energy as a function of temperature for the pinch mode of C 60 at 1468 cm -1. From a fit to the Albrecht theory optical transitions energies at 2.6 and 3.4 eV were deduced in good agreement with results from LDA calculations. The temperature dependence of the self energy changes characteristically at a temperature which corresponds to the fcc to sc phase transition.

The influence of oxygen on the Raman spectrum of C 60 films

The influence of oxygen exposure on the Raman spectrum of C 60 microscrystalline films under laser irradiation is described. In the frequency region of the symmetric A g pentagonal pinch mode, exposure of vacuum grown C 60 films to 16O 2, 18O 2, or air results in the appearance of a peak at 1467 cm -1, a mode previously attributed to pure C 60. The time dependence of this mode in the laser beam, as well as the Raman spectrum of pristine C 60 are discussed.

The Kelvin-Helmholtz instability of a cylindrical flow with a shear layer

A linear model analysis of the cylindrical shear flow leads to an eigenvalue problem based on a differential equation. A detailed analysis of the solution is presented. From this we extract the following physical principles. (i) Unstable modes only exist within a hemicycle in complex phase space. (ii) Each mode has an associated ‘comoving point’, i.e., there is always some fluid that comoves with the pattern. At the comoving point, neutral modes are oscillating or wave-like and unstable modes are overreflected. (iii) Short wavelengths with large azimuthal numbers or long-wavelength pinch modes can be stabilized by the presence of a shear layer both for ordinary modes and reflecting modes. (iv) Reflecting modes have a band structure in wave-number versus growth rate space. For light jets, the growth rates are very small. (v) A shear layer suppresses pinching modes more effectively.

Stability of magnetohydrodynamic cylindrical flows: Compressible modes

The stability analysis of an ideal magnetohydrodynamic cylindrical configuration subject to compressible perturbations is carried out assuming smooth profiles for both the magnetic field and the mass flow. The adiabatic energy equation is assumed everywhere. Subsonic and transonic regimes up to flow speeds a few times the sound speed are analyzed for models with axial mass flow and for a variety of helical magnetic models. It is shown that compressibility increases the growth rates of pure ideal magnetic models while it lowers those of the Kelvin–Helmholtz instabilities. Beyond the transonic regime, the pinching mode (m=0) is stable for a range of magnetic models wider than that of the incompressible case. The case of uniform mass density is investigated more in detail but some results for models with a nonuniform mass density are also considered.

Spatial stability of the slab jet. I - Linearized stability analysis. II - Numerical simulations

view Abstract Citations (96) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spatial Stability of the Slab Jet. I. Linearized Stability Analysis Hardee, Philip E. ; Norman, Michael L. Abstract A spatial stability analysis of the supersonic slab jet is performed, and expressions for the resonant frequencies, wavelengths, and wave speeds are determined. The results are compared to a spatial stability analysis of the supersonic cylindrical jet. Symmetric (pinch) and antisymmetric (sinusoidal) modes on the slab jet correspond to pinch and helical modes on the cylindrical jet. Higher order fluting modes on the cylindrical jet do not have a counterpart on the slab jet. On the slab jet growing harmonic components can propagate at any angle from 0^deg^ to +/- 90^deg^ with respect to the flow. On the cylindrical jet the propagation angle is uniquely determined by the mode number and the wavelength. The differences between slab and cylindrical geometry are most pronounced for long wavelengths associated with the sinusoidal and helical modes. However, for equal angles of propagation, solutions of the dispersion relation for the slab jet are nearly identical to the results for a cylindrical jet. The spatial stability of a supersonic slab jet provides a reasonable analog for the spatial stability of a supersonic cylindrical jet if the difference in propagation angle is taken into account. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166818 Bibcode: 1988ApJ...334...70H Keywords: Astrophysics; Magnetohydrodynamic Stability; Plasma Jets; Spatial Distribution; Astronomical Models; Linear Equations; Plasma Slabs; Reflected Waves; Supersonic Jet Flow; Astrophysics; GALAXIES: JETS; HYDROMAGNETICS; INSTABILITIES; WAVE MOTIONS full text sources ADS | Related Materials (1) Part 2: 1988ApJ...334...80N

The stability of a magnetically confined radio jet

view Abstract Citations (92) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The stability of a magnetically confined radio jet Cohn, H. Abstract The linear stability against pinching modes of a model for a magnetically confined extragalactic radio jet is investigated. An analytic dispersion relation for these modes is obtained and then solved numerically. For comparison the dispersion relation for the Kelvin-Helmholtz pinching modes of a pressure confined jet is also solved numerically. The behavior of the modes is found to be fairly independent of the confinement mechanism. Subsonic and Mach 1 jets are found to be unstable to a single pinching mode at every wavenumber. In addition to this fundamental mode; supersonic jets have an infinite set of higher order (reflection) pinching modes that enter at points of marginal stability. When the jet velocity is less than hypersonic, these reflection modes have large spatial growth rates and thus may be responsible for the observed knot structure of radio jets. Publication: The Astrophysical Journal Pub Date: June 1983 DOI: 10.1086/161059 Bibcode: 1983ApJ...269..500C Keywords: Galactic Structure; Magnetohydrodynamic Stability; Radio Galaxies; Radio Jets (Astronomy); Jet Flow; Kelvin-Helmholtz Instability; Magnetic Fields; Pinch Effect; Plasma Pinch; Supersonic Jet Flow; Astrophysics full text sources ADS |

Impedance characteristics of diodes operating in the self−pinch mode

The impedance characteristics of large aspect ratio diodes employing hollow field emission cathodes operating in the self−pinch mode have been studied at electron kinetic energies between 400 and 800 keV and currents between 100 and 500 kA. The importance of correcting the anode−cathode gap spacing for closure due to plasma motion is recognized and taken into account in the definition of the diode aspect ratio in impedance calculations. With this correction, the impedance at peak diode voltage and current is found to be nearly independent of voltage and cathode surface area, and inversely proportional to the diode aspect ratio. Its amplitude is approximately 50% higher than that predicted by parapotential flow theory.