The photochemical ring-opening of 1,3-cyclohexadiene imaged by ultrafast electron diffraction.

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon-carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale.

High-brightness beam tests of the very high frequency gun at the Advanced Photo-injector EXperiment test facility at the Lawrence Berkeley National Laboratory.

The very-high-frequency gun (VHF-Gun) is a new concept photo-injector developed and built at the Lawrence Berkeley National Laboratory (LBNL) for generating high-brightness electron beams capable of driving X-ray free electron lasers (FELs) at MHz-class repetition rates. The gun that purposely uses established and mature radiofrequency and mechanical technologies has demonstrated over the last many years the capability of reliably operating in continuous wave mode at the design accelerating fields and required vacuum and mechanical performance. The results of VHF-Gun technology demonstration were reported elsewhere [Sannibale et al., Phys. Rev. Spec. Top.-Accel. Beams 15, 103501 (2012)]; here in this paper, we provide and analyze examples of the experimental results of the first high-brightness beam tests performed at the Advanced Photo-injector EXperiment test facility at LBNL that demonstrated the gun capability of delivering the beam quality required for driving high repetition rate X-ray FELs.

Publisher Correction: Beyond a phenomenological description of magnetostriction.

"The technical support from SLAC Accelerator Directorate, Technology Innovation Directorate, LCLS laser division and Test Facility Division is gratefully acknowledged. We thank S.P. Weathersby, R.K. Jobe, D. McCormick, A. Mitra, S. Carron and J. Corbett for their invaluable help and technical assistance. Research at SLAC was supported through the SIMES Institute which like the LCLS and SSRL user facilities is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. DE-AC02-76SF00515. The UED work was performed at SLAC MeV-UED, which is supported in part by the DOE BES SUF Division Accelerator & Detector R&D program, the LCLS Facility, and SLAC under contract Nos. DE-AC02-05-CH11231 and DE-AC02-76SF00515. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515."and"Work at BNL was supported by DOE BES Materials Science and Engineering Division under Contract No: DE-AC02-98CH10886. J.C. would like to acknowledge the support from National Science Foundation Grant No. 1207252. E.E.F. would like to acknowledge support from the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Award No. DE-SC0003678."This has been corrected in both the PDF and HTML versions of the Article.

Beyond a phenomenological description of magnetostriction.

Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction-the underlying magnetoelastic stress-can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the sub-picosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.

Electron-lattice energy relaxation in laser-excited thin-film Au-insulator heterostructures studied by ultrafast MeV electron diffraction.

We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels.

Stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 probed with MeV ultrafast electron diffraction.

Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate l = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0.3 ps time constant. Simultaneously, in the same experiment, the emergence of the incommensurate phase, with a slightly slower 2.0 ps time constant, is determined from the intensity of the CDW satellites aligned around the incommensurate l = 1/3 characteristic stacking order. These results might be of relevance in understanding the metallic character of the laser-induced metastable "hidden" state recently discovered in this compound.

A direct electron detector for time-resolved MeV electron microscopy.

The introduction of direct electron detectors enabled the structural biology revolution of cryogenic electron microscopy. Direct electron detectors are now expected to have a similarly dramatic impact on time-resolved MeV electron microscopy, particularly by enabling both spatial and temporal jitter correction. Here we report on the commissioning of a direct electron detector for time-resolved MeV electron microscopy. The direct electron detector demonstrated MeV single electron sensitivity and is capable of recording megapixel images at 180 Hz. The detector has a 15-bit dynamic range, better than 30-µm spatial resolution and less than 20 analogue-to-digital converter count RMS pixel noise. The unique capabilities of the direct electron detector and the data analysis required to take advantage of these capabilities are presented. The technical challenges associated with generating and processing large amounts of data are also discussed.

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory.

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.

A novel system for measurement of the transverse electron momentum distribution from photocathodes.

The transverse momentum of electrons produced by a photocathode contributes significantly to the performance of several different types of accelerator-based light sources, such as Free Electron Lasers, as well as systems designed for ultrafast electron diffraction and dynamic transmission electron microscopy. Minimization of the transverse emittance from photocathodes is the subject of intensive research, and therefore measurement of this parameter is of great importance. Here, we describe a simple system that offers real time measurements of transverse emittance and can be easily integrated into the photocathode fabrication process.

A stigmatic ultraviolet-visible monochromator for use with a high brightness laser driven plasma light source.

Laser driven plasma light sources offer highly intense output in the UV-visible region combined with a source size as small as 100 µm. In order to effectively use the small source size in high brightness applications, a stigmatic monochromator and focusing system must be used. Here we describe a simple brightness preserving optical system that should be useful across a broad range of applications. The output flux of this system is between 6 × 10(11) ph∕s and 4 × 10(12) ph∕s with a spectra resolution of 1.7 nm and field spot size of 0.1 mm from the UV to the VIS spectra range.

Separation of omphalopagus twins: unique reconstruction using syngeneic cryopreserved tissue.

Omphalopagus twin girls were admitted for evaluation of possible separation and repair at age 7 days. Prenatal sonographic diagnosis occurred late in the third trimester and was followed by cesarean section delivery shortly thereafter. Results of extensive evaluation over the next 7 days including x-rays, computed tomography and ultrasound scan of the head and torso, and cardiac catheterization showed: the gastrointestinal tracts were separate and normal, the livers were joined but had separate biliary and vascular systems, and the hearts were separate with vastly different anatomy and function. One twin (twin A) had a normal heart with a small insignificant VSD. Twin B had a single ventricle, an incompetent A-V valve, stenotic pulmonic valve, ASD, PDA, and congenital heart block. Hemodynamic support of twin B was almost entirely from twin A. The vascular communications between the two consisted of a major connection between the internal mammary arteries and large arterial and venous connections traversing the joined livers. Because of continued deterioration of twin B, separation was undertaken at age 15 days. The separation included dividing the liver and the multiple large vascular connections. Two teams then reconstructed each twin separately. Twin B began showing signs of cardiac decompensation shortly after separation in spite of placement of a pacemaker, pulmonary artery banding, and ligation of the PDA. Cardiac function rapidly deteriorated and she died. Tissue from her chest wall was cryopreserved and placed in the tissue bank. Twin A underwent closure of her abdomen, and received a temporary bovine pericardial patch over the chest defect. She subsequently underwent placement of a graft of twin B's rib cage to bridge the bony chest defect and skin flap closure. She is presently taking a normal diet and thriving at home at age 18 months. The use of cryopreserved tissue from a syngeneic source provides a unique method of reconstruction in this situation.

Glabellar frown lines: direct excision, an evaluation of the scars.

The direct excision of glabellar frown lines is a method that is not new, but neither has it been well evaluated. This paper describes the method, discusses the reason for the problem, and suggests why this method should have long-lasting results. Several examples of the scars are demonstrated. In selected patients, this may be a reasonable alternative to the myriad surgical possibilities in our quest for a solution to this problem.

Connective-tissue disease following breast augmentation: a preliminary test of the human adjuvant disease hypothesis.

It has been proposed that scleroderma in particular or connective-tissue diseases in general may be caused by exposure to silicone-containing materials used for breast augmentation. We performed a historical cohort study to estimate the point prevalence of this potential complication sometimes referred to as "human adjuvant disease." Three-hundred and seventy-eight patients who underwent augmentation mammaplasty with silicone-containing envelope-type prostheses from 1970 through 1981 were surveyed. Regional musculoskeletal syndromes, fibrositis, or osteoarthritis occurred in a third (38 of 125) of our responders. No patient developed an inflammatory systemic rheumatic disease during the period of observation (mean 6.8 years for the 125 subjects). Using calculations based on risk for development of rheumatic disease, it does not appear likely that augmentation mammaplasty is a significant or major inducer of inflammatory connective-tissue diseases in general. However, the number of patients surveyed was small, and our results must be considered preliminary. We could not exclude a specific risk for the development of scleroderma.

Tattoo removal using precise shave excision and dermal overgrafting.

A method of tattoo removal using a precise partial thickness tangential excision of foreign body and dermis followed by dermal overgrafting with thin, split-thickness skin grafts is presented.