Proposal for a quantum-degenerate electron source

We propose a pulsed electron source capable of 6D brightness orders-of-magnitude greater than that of existing sources. It could deliver average current up to 0.5 pA and achieve an emittance approaching the quantum limit in each spatial dimension. It could be employed to advantage in electron microscopy, inverse photo-emission, precision low-energy scattering experiments, and electron holography. This source could make possible pump-probe experiments with Angstrom spatial and subpicosecond time resolution. Here, we present the basic concepts of the source, including a generalized expression for the brightness that can be used in the quantum limited case and the analysis of the main issues that must be addressed for successful construction and operation. We have begun an experiment to demonstrate its essential features.     

Observation of the 6 2P12−7 2P12 M1 transition in atomic thallium vapor

The $\text{6}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?7}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ (292.7 nm) M1 transition in Tl is observed and its matrix element $\text{m = 〈7}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{|μ}{}_{\mathrm{Z}}\text{| 6}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{〉}$ is measured. Tl vapor is illuminated by a pulsed linearly polarized laser beam (λ = 292.7 nm). Interference between m and the Stark-induced $\text{6}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?7}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ E1 amplitude in finite electric field E results in circular polarization of 535 nm fluorescence proportional to m/E, which is detected. The result is m = ?(2.11 ± 0.30) × 10^?5 | eh/2m_ec| in agreement with theory. The ultimate goal of this research is to detect parity violation in the neutral weak interaction by observation of a helicity dependence in the $\text{6}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{?7}{}^2\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ absorption when the laser light is circularly polarized.     

New limit on the electron electric dipole moment

We present the result of our most recent search for T violation in 205Tl, which is interpreted in terms of an electric dipole moment of the electron d(e). We find d(e) = (6.9 +/- 7.4)x10(-28)e cm, which yields an upper limit /d(e)/ < or = 1.6x10(-27)e cm with 90% confidence. The present apparatus is a major upgrade of the atomic beam magnetic-resonance device used to set the previous limit on d(e).     

Is a Bent Crystal Still a Single Crystal?

The mention of the word “crystal” invokes images of minerals, gems, and rocks, all of which are inevitably solid, hard, and durable entities with well‐defined smooth faces and straight edges. With the discovery in the first half of the 20th century that many molecular crystals are soft and can be deformed in a similar way as rubber or plastic, this perception is changing, and both the concept and formal definition of what a crystal is may require reinterpretation. The seemingly naïve question posed in the title of this Minireview does not have a simple answer. Here, we discuss how the effects of the elastic and plastic deformation of molecular crystals on the diffraction signature give primary evidence of their degree of crystallinity. In most cases, the definition of a crystal holds for both elastically and plastically deformed crystals and, unless there is significant or complete physical separation of the crystal during the deformation, they can safely be considered (deformed) single crystals with a high concentration of defects.     

Self‐Healing Molecular Crystals

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self‐healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid‐state organic chemists. The first evidence of self‐healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self‐healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid‐state compounds.     

Synthesis of bridged molecular gyroscopes with closed topologies: triple one-pot macrocyclization

We describe the synthesis and characterization of six bridged molecular gyroscopes with m-alkoxy-substituted trityl stators and dialkynylphenylene rotators. All of the bridged molecular gyroscopes were synthesized convergently to form the phenolic stator-rotator framework, while the alkyl and benzophenone bridges were installed in one step by relatively efficient one-pot reactions to form macrocyclic diether or diester linkages. The isolated yield per bond-forming reaction varied from ca. 42% to 80%, with one exception where macrocyclization failed to produce the desired product. The molecular structure and crystal packing of each of the bridged molecular gyroscopes were determined via single crystal X-ray diffraction. Like most molecular gyroscopes with open topologies previously studied, the singly bridged structures pack by interdigitating one trityl stator in one molecule next to the rotator of an adjacent molecule in the lattice. In contrast, the triply bridged molecular gyroscopes were found to pack in lamellar sheets that prevent the rotator-stator interdigitation of adjacent molecules. However, solvent molecules and conformationally flexible bridges tend to fill in the packing volume by collapsing next to the rotator or by extending one of their bridges into the cavity of a neighboring molecule.