We show that continuously illuminated single europium ions incorporated in yttrium oxide (Eu 3+ :Y 2 O 3 ) nanocrystals (5-15 nm diameter) undergo on-off blinking on a variable time scale ranging from hundreds of milliseconds to several seconds. We observe both a pump-intensity dependent "duty factor" (on-time as a percentage of total measurement time), and quantum jumps between at least three well-defined luminescence intensity levels (bright states) from individual nanoparticles. Interesting switching or oscillation between different bright levels was also observed with a modulation rate that is dependent on pump-laser intensity. These features of single-ion luminescence are not observed for larger particles with multiple chromophores. We propose that these effects derive from pump laser-induced fluctuations between different quasi-stable Eu 3+ symmetry sites that effectively modulate the electric dipole transition moment.

SUMMARY: Molecular dynamics simulations for nanometer scale polyethylene (PE) particles generated with up to 12000 atoms are presented to gain insight into some thermodynamic properties of ultra fine polymer powders. By computing molecular volume and total energy as a function of temperature, we obtained melting point, glass transition temperature, and heat capacity. The results of our simulations predict an interesting reduction of the melting point in comparison with the PE bulk system.

2000 luminescence, fluorescence luminescence, fluorescence (solids and liquids) D 6540 -009On-Off Blinking and Multiple Bright States of Single Europium Ions in Eu 3+ : Y 2 O 3 Nanocrystals.-Eu 3+ :Y 2 O 3 nanocrystals are prepared by an inverse micelle technique using 1:10 Eu 3+ :Y 2 O 3 stoichiometry. The nanoparticles ranging in size from 2 to 15 nm are characterized by AFM. It is expected that a significant fraction of the nanoparticles contain only a single Eu 3+ ion. The prepared nanoparticles are continuously illuminated with the 514.5 nm line from an argon ion laser. The illuminated single Eu 3+ ions undergo on-off blinking on a variable time scale. Switching between different bright levels is also observed with a modulation rate that is dependent on pump-laser intensity. These features of single ion luminescence are not observed for larger particles with multiple chromophores.-(BARNES, M. D.; MEHTA, A.; THUNDAT, T.; BHARGAVA, R. N.; CHHABRA, V.; KULKARNI, B.; J.

Alfvén eigenmodes driven by energetic particles are routinely observed in tokamak plasmas. These modes consist of poloidal harmonics of shear Alfvén waves coupled by inhomogeneity in the magnetic field. Further coupling is introduced by 3D inhomogeneities in the ion density during the assimilation of injected pellets. This additional coupling modifies the Alfvén continuum and discrete eigenmode spectrum. The frequencies of Alfvén eigenmodes drop dramatically when a pellet is injected in JET. From these observations, information about the changes in the ion density caused by a pellet can be inferred. To use Alfvén eigenmodes for MHD spectroscopy of pellet injected plasmas, the 3D MHD codes Stellgap and AE3D were generalised to incorporate 3D density profiles. A model for the expansion of the ionised pellet plasmoid along a magnetic field line was derived from the fluid equations. Thereby, the time evolution of the Alfvén eigenfrequency is reproduced. By comparing the numerical frequency drop of a toroidal Alfvén eigenmode (TAE) to experimental observations, the initial ion density of a cigar-shaped ablation region of length 4cm is estimated to be n * = 6.8×10 22 m −3 at the TAE location (r/a ≈ 0.75). The frequency sweeping of an Alfvén eigenmode ends when the ion density homogenises poloidally. Modelling suggests that the time for poloidal homogenisation of the ion density at the TAE position is τ h = 18 ± 4 ms for inboard pellet injection, and τ h = 26 ± 2 ms for outboard pellet injection. By reproducing the frequency evolution of the elliptical Alfvén eigenmode (EAE), the initial ion density at the EAE location (r/a ≈ 0.9) can be estimated to be n * = 4.8 × 10 22 m −3. Poloidal homogenisation of the ion density takes 2.7 times longer at the EAE location than at the TAE location for both inboard and outboard pellet injection. MHD spectroscopy, Alfvén eigenmodes, pellet injection ‡ See the author list of "Overview of the JET preparation for Deuterium-Tritium Operation" by E.

Optimization of molecular detection efficiencies is of central importance in analytical applications involving single molecule detection.1 In addition to limitations imposed on the fraction of molecules which can be detected by the average signal-to-noise ratio, experimental factors such as excitation inhomogeneity and molecular diffusion conspire to further limit "molecular detectability." Recent single molecule detection experiments in microdroplets suggest that such experimental limitations can be significantly reduced primarily because the molecule cannot diffuse away from the excitation volume. However, unlike fluorescence detection from bulk streams where the fluorescence intensity is isotropic in space, the large refractive index change at the surface of microdroplets implies that the fluorescence intensity collected by a lens will be strongly dependent on the position of the molecule within the droplet. In addition, the same refractive index discontinuity at the droplet surface produces a complicated excitation intensity distribution within the droplet as a result of interference between refracted and totally-internally-reflected rays. Thus, issues such as whether molecules near the surface of the sphere can "hide" from the detector as a result of total internal reflection of emission near the droplet surface, or poor excitation efficiency due to the molecule being located in a "shadow" region of the droplet will have a potential effect on molecular detection efficiencies. These questions are nontrivial to address in a quantitative way. Here we discuss development of numerical tools for modeling the fluorescence collected from a single molecule within a microdroplet as a function of position, orientation, and detection geometry based on the semiclassical electrodynamics formalism developed by Chew2 for light scattering in dielectric microspheres. In addition we also examine effects of excitation inhomogeneity within the sphere, molecular diffusion, and transition rate modification in order to obtain a realistic model of molecular detection efficiencies in microdroplets.

We describe experiments probing single europium and terbium ions in isolated nanocrystals (2–15 nm diam.) using fluorescence microscopy techniques. Emission pattern imaging also shows patterns that are distinctly characteristic of single dipoles.