Electrocyclic reactions are characterized by the concerted formation and cleavage of both σ and π bonds through a cyclic structure. This structure is known as a pericyclic transition state for ...thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule α-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated π bonds. The σ bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.
Conformational isomers (conformers) of molecules play a decisive role in biology and organic chemistry. However, experimental methods for investigating chemical reaction dynamics are typically not ...conformer-sensitive. We report on a gas-phase megaelectronvolt ultrafast electron diffraction investigation of α-phellandrene undergoing an electrocyclic ring-opening reaction. We directly imaged the evolution of a specific set of α-phellandrene conformers into the product isomer predicted by the Woodward-Hoffmann rules in real space and time. Our experimental results are in quantitative agreement with nonadiabatic quantum molecular dynamics simulations, which provide considerable detail of how conformation influences the time scale and quantum efficiency of photoinduced ring-opening reactions.
We present the design of an electron velocity map imaging (VMI) spectrometer where the ionizing laser source propagates along the symmetry axis of the spectrometer. The co-axial geometry is useful in ...a variety of experiments, because it provides a unique 2-dimensional projection of the 3-dimensional electron momentum distribution. Initial simulations show that this co-axial VMI can work with both high energy (more than 100 eV) and low energy (tens of eV) electrons. We demonstrate the performance of this co-axial VMI spectrometer at the Linac Coherent Light Source X-ray Free Electron Laser facility.
We report the reflectance, ~1° from normal incidence, of six different mirrors as a function of photon energy, using monochromatic vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation ...with energies between 7.5 eV and 24.5 eV. The mirrors examined included both single and multilayer optical coatings, as well as an uncoated substrate. We discuss the performance of each mirror, paying particular attention to the potential application of suppression and selection of high-order harmonics of a Ti:sapphire laser.
Synopsis We have conducted an experimental study on the photo double ionization (PDI) of carbon dioxide dimers and oxygen dimers, while focusing on the dissociation dynamics upon single photon ...absorption. The results in terms of the kinetic energy and angular distributions of the charged particles show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon dioxide dimers. In the oxygen dimer, the results show that ICD is accompanied by knock-off ionization mechanisms.
European guidelines recommend the routine offer of an HIV test in patients with a number of AIDS-defining and non-AIDS conditions believed to share an association with HIV; so called indicator ...conditions (IC). Adherence with this guidance across Europe is not known. We audited HIV testing behaviour in patients accessing care for a number of ICs. Participating centres reviewed the case notes of either 100 patients or of all consecutive patients in one year, presenting for each of the following ICs: tuberculosis, non-Hodgkins lymphoma, anal and cervical cancer, hepatitis B and C and oesophageal candidiasis. Observed HIV-positive rates were applied by region and IC to estimate the number of HIV diagnoses potentially missed. Outcomes examined were: HIV test rate (% of total patients with IC), HIV test accepted (% of tests performed/% of tests offered) and new HIV diagnosis rate (%). There were 49 audits from 23 centres, representing 7037 patients. The median test rate across audits was 72% (IQR 32-97), lowest in Northern Europe (median 44%, IQR 22-68%) and highest in Eastern Europe (median 99%, IQR 86-100). Uptake of testing was close to 100% in all regions. The median HIV+ rate was 0.9% (IQR 0.0-4.9), with 29 audits (60.4%) having an HIV+ rate >0.1%. After adjustment, there were no differences between regions of Europe in the proportion with >0.1% testing positive (global p = 0.14). A total of 113 patients tested HIV+. Applying the observed rates of testing HIV+ within individual ICs and regions to all persons presenting with an IC suggested that 105 diagnoses were potentially missed. Testing rates in well-established HIV ICs remained low across Europe, despite high prevalence rates, reflecting missed opportunities for earlier HIV diagnosis and care. Significant numbers may have had an opportunity for HIV diagnosis if all persons included in IC audits had been tested.
We present an experimental investigation of symmetry breaking of hydrogen (H2 and D2) molecules after single photoionization due to the electric (i.e., Coulomb) field of the emitted slow electron ...interacting with the parent cation during dissociation. The experiments were carried out by measuring the three-dimensional momentum vectors of the photoelectron and recoiling ion in coincidence using a reaction microscope. For photon energies close to threshold, the low-energy photoelectron influences the dissociation process, which results in an asymmetric molecular frame photoelectron angular distribution. This can be explained by the retroaction of the Coulomb field of the photoelectron on its parent ion and has been recently experimentally demonstrated by M. Waitz et al. Phys. Rev. Lett. 116, 043001 (2016), confirming theoretical predictions by V. V. Serov and A. S. Kheifets Phys. Rev. A 89, 031402(R) (2014). High-momentum resolution and a new series of photon energies just above the dissociation threshold enable the observation of a strong influence of the electron energy and nuclear kinetic energy on the electron localization process for energies below ~100 meV, which so far has neither been observed nor discussed by theory. Exploring the limitations of the retroaction mechanism at our lowest photon energy, we are able to single out a sensitive testbed and present data of non-Born-Oppenheimer dynamics of the simplest molecular system for future benchmark computational treatments.
We present an experimental investigation of symmetry breaking of H_{2} and D_{2} molecules after single photoionization due to the Coulomb field of the emitted slow electron interacting with the ...parent cation during dissociation. The experiments were carried out by measuring the three-dimensional momentum vectors of the photoelectron and recoiling ion in coincidence using a reaction microscope. For photon energies close to threshold, the low-energy photoelectron influences the dissociation process, which results in an asymmetric molecular frame photoelectron angular distribution. This can be explained by the retroaction of the Coulomb field of the photoelectron on its parent ion and has been recently experimentally demonstrated by M. Waitz et al. Phys. Rev. Lett. 116, 043001 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.043001, confirming theoretical predictions by V. V. Serov and A. S. Kheifets Phys. Rev. A 89, 031402(R) (2014)PLRAAN1050-294710.1103/PhysRevA.89.031402. High-momentum resolution and a new series of photon energies just above the dissociation threshold enable the observation of a strong influence of the electron energy and nuclear kinetic energy on the electron localization process for energies below ∼100 meV, which so far has neither been observed nor discussed by theory. Exploring the limitations of the retroaction mechanism at our lowest photon energy, we are able to single out a sensitive testbed and present data of non-Born-Oppenheimer dynamics of the simplest molecular system for future benchmark computational treatments.
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