Observation of blue-shifted ultralong-range Cs2_{2} Rydberg molecules

We observe ultralong-range blue-shifted Cs$_{2}$ molecular states near $ns_{1/2}$ Rydberg states in an optical dipole trap, where $31\leq n\leq34$. The accidental near degeneracy of $(n-4)l$ and $ns$ Rydberg states for $l>2$ in Cs, due to the small fractional $ns$ quantum defect, leads to non-adiabatic coupling among these states, producing potential wells above the $ns$ thresholds. Two important consequences of admixing high angular momentum states with $ns$ states are the formation of large permanent dipole moments, $\sim 15-100\,$Debye, and accessibility of these states via two-photon association. The observed states are in excellent agreement with theory. Both projections of the total angular momentum on the internuclear axis are visible in the experiment

Surface charging of phosphors and its effects on cathodoluminescence at low electron energies

Measurements of the threshold for secondary electron emission and shifts of the carbon Auger line position have been used to deduce the surface potential of several common phosphors during irradiation by electrons in the 0.5--5.0 keV range. All of the insulating phosphors display similar behavior: the surface potential is within {+-}1 V of zero at low electron energies. However, above 2--3 kV it becomes increasingly negative, reaching hundreds of volts within 1 keV of the turn-on energy. The electron energy at which this charging begins decreases dramatically after Coulomb aging at 17 {micro}A/cm{sup 2} for 30--60 min. Measurements using coincident electron beams at low and high electron energies to control the surface potential were made to investigate the dependence of the cathodoluminescence (CL) process on charging. Initially, the CL from the two beams is identical to the sum of the separate beam responses, but after Coulomb aging large deviations from this additivity are observed. These results indicate that charging has important, detrimental effects on CL efficiency after prolonged e-beam irradiation. Measurements of the electron energy dependence of the CL efficiency before and after Coulomb aging will also be presented, and the implications of these data on the physics of the low-voltage CL process will be discussed

Immodest and proud

In his ‘Ambitious, Yet Modest, Metaphysics’, Hofweber (Metametaphysics, Oxford University Press, Oxford, pp 260–289, 2009a) puts forward arguments against positions in metaphysics that he describes as ‘immodest’; a position he identifies as defended by Jonathan Lowe. In this paper I reply to Hofweber’s arguments, offering a defence of immodest metaphysics of the type practiced by Lowe (The possibility of metaphysics, Oxford University Press, Oxford, 1998) inter alia

Atomic structure and vibrational properties of icosahedral B4_4C boron carbide

The atomic structure of icosahedral B$_4$C boron carbide is determined by comparing existing infra-red absorption and Raman diffusion measurements with the predictions of accurate {\it ab initio} lattice-dynamical calculations performed for different structural models. This allows us to unambiguously determine the location of the carbon atom within the boron icosahedron, a task presently beyond X-ray and neutron diffraction ability. By examining the inter- and intra-icosahedral contributions to the stiffness we show that, contrary to recent conjectures, intra-icosahedral bonds are harder.Comment: 9 pages including 3 figures, accepted in Physical Review Letter

Presentism remains

Here I examine some recent attempts to provide a new way of thinking about the philosophy of time that question the central role of ‘presentness’ within the definition of presentism. The central concern raised by these critics turns on the intelligibility and theoretical usefulness of the term ‘is present’ (cf. Correia and Rosenkrantz in Thought 4:19–27, 2015; Deasy in Nous, 2017. https://doi.org/10.1111/nous.12109; Williamson in Modal logic as metaphysics, OUP, Oxford, 2013). My overarching aim is to at least challenge such concerns. I begin with arguments due to Deasy (Nous, 2017. https://doi.org/10.1111/nous.12109). Deasy develops a view that he calls ‘transientism’ and that he takes to be a well-motivated version of presentism. I show that both this way of thinking about presentism and the argument supposedly motivating it all fail. I then move to an argument due to Correia and Rosenkrantz (Thought 4:19–27, 2015). Correia and Rosenkrantz purport to show that presentism can be salvaged without making recourse to the term ‘is present’. I demonstrate that their arguments fail. I then move on to a view, proposed and defended by Merricks (Truth and ontology, OUP, Oxford, 2007), Tallant (Erkenntnis 79:479–501, 2014), and Zimmerman (Philos Pap 25:115–126, 1996), and show that it has the wherewithal to meet the challenges raised by Williamson (Modal logic as metaphysics, OUP, Oxford, 2013) who, as noted above, raises genuine concerns about our capacity to define presentism

Nature of the green luminescent center in zinc oxide

EPR, optical absorption, and photoluminescence spectroscopies are used to characterize a wide range of different ZnO phosphor powders. A good correlation is generally observed between the 510-nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. Both quantities are found to peak at a free-carrier density of about 1.4x10{sup 18}cm{sup -3}. The green emission intensity can be strongly influenced by free-carrier depletion at the particle surface, especially for small particles and/or low doping. The data suggest that the green PL in ZnO phosphors is due to recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band