We describe a phase plate for transmission electron microscopy taking advantage of a hitherto-unknown phenomenon, namely a beam-induced Volta potential on the surface of a continuous thin film. The Volta potential is negative, indicating that it is not caused by beam-induced electrostatic charging. The film must be heated to ∼200°C to prevent contamination and enable the Volta potential effect. The phase shift is created "on the fly" by the central diffraction beam eliminating the need for precise phase plate alignment. Images acquired with the Volta phase plate (VPP) show higher contrast and unlike Zernike phase plate images no fringing artifacts. Following installation into the microscope, the VPP has an initial settling time of about a week after which the phase shift behavior becomes stable. The VPP has a long service life and has been used for more than 6 mo without noticeable degradation in performance. The mechanism underlying the VPP is the same as the one responsible for the degradation over time of the performance of thin-film Zernike phase plates, but in the VPP it is used in a constructive way. The exact physics and/or chemistry behind the process causing the Volta potential are not fully understood, but experimental evidence suggests that radiation-induced surface modification combined with a chemical equilibrium between the surface and residual gases in the vacuum play an important role.TEM | phase plate | Volta potential | phase contrast | cryo-EM

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O( 3 P j ), jϭ0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

Photodissociation in the Herzberg continuum of molecular oxygen has been studied at 236, 226 and 204 nm. Using ion-imaging and monitoring of O(3 P j), jϭ0, 1, and 2 product-atom angular distributions, the amount of parallel character of the transition was measured. In order to interpret these data, analyses of the photoabsorption oscillator strengths and the parallel-perpendicular nature of the Herzberg I, II and III bands, and extrapolation of these properties into the Herzberg-continuum region have been performed. Our measured fine-structure-averaged angular distributions are found to be consistent with this photoabsorption model. In addition, the dynamics of the dissociation process is discussed, based on the O-atom fine-structure distributions.

MicroED has recently emerged as a powerful method for the analysis of biological structures at atomic resolution. This technique has been largely limited to protein nanocrystals which grow either as needles or plates measuring only a few hundred nanometers in thickness. Furthermore, traditional microED data processing uses established X-ray crystallography software that is not optimized for handling compound effects that are unique to electron diffraction data. Here, we present an integrated workflow for microED, from sample preparation by cryo-focused ion beam milling, through data collection with a standard Ceta-D detector, to data processing using the DIALS software suite, thus enabling routine atomic structure determination of protein crystals of any size and shape using microED. We demonstrate the effectiveness of the workflow by determining the structure of proteinase K to 2.0 Å resolution and show the advantage of using protein crystal lamellae over nanocrystals.

A novel design is described for an aperture that blocks a half-plane of the electron diffraction pattern out to a desired scattering angle, and then – except for a narrow support beam – transmits all of the scattered electrons beyond that angle. Our proposed tulip-shaped design is thus a hybrid between the single-sideband (ssb) aperture, which blocks a full half-plane of the diffraction pattern, and the conventional (i.e. fully open) double-sideband (dsb) aperture. The benefits of this hybrid design include the fact that such an aperture allows one to obtain high-contrast images of weak-phase objects with the objective lens set to Scherzer defocus. We further demonstrate that such apertures can be fabricated from thin-foil materials by milling with a focused ion beam (FIB), and that such apertures are fully compatible with the requirements of imaging out to a resolution of at least 0.34 nm. As is known from earlier work with single-sideband apertures, however, the edge of such an aperture can introduce unwanted, electrostatic phase shifts due to charging. The principal requirement for using such an aperture in a routine data-collection mode is thus to discover appropriate materials, protocols for fabrication and processing, and conditions of use such that the hybrid aperture remains free of charging over long periods of time.