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"Koch, H."
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Probing ultrafast ππ/nπ internal conversion in organic chromophores via K-edge resonant absorption
Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed
ππ
* to a dark
nπ
* state. The
ππ
*/
nπ
* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the
n
orbital during
ππ
*/
nπ
* internal conversion, the absorption spectrum at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that
ππ
*/
nπ
* internal conversion takes place within (60 ± 30) fs. High-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.
Many photo-induced processes such as photosynthesis occur in organic molecules, but their femtosecond excited-state dynamics are difficult to track. Here, the authors exploit the element and site selectivity of soft X-ray absorption to sensitively follow the ultrafast
ππ
*/
nπ
* electronic relaxation of hetero-organic molecules.
Journal Article
Synthesis of structurally well-defined and liquid-phase-processable graphene nanoribbons
The properties of graphene nanoribbons (GNRs) make them good candidates for next-generation electronic materials. Whereas ‘top-down’ methods, such as the lithographical patterning of graphene and the unzipping of carbon nanotubes, give mixtures of different GNRs, structurally well-defined GNRs can be made using a ‘bottom-up’ organic synthesis approach through solution-mediated or surface-assisted cyclodehydrogenation reactions. Specifically, non-planar polyphenylene precursors were first ‘built up’ from small molecules, and then ‘graphitized’ and ‘planarized’ to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a bottom-up solution synthesis of long (>200 nm) liquid-phase-processable GNRs with a well-defined structure and a large optical bandgap of 1.88 eV. Self-assembled monolayers of GNRs can be observed by scanning probe microscopy, and non-contact time-resolved terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs may prove useful for fundamental studies of graphene nanostructures, as well as the development of GNR-based nanoelectronics.
Liquid-phase-processable graphene nanoribbons (GNRs) over 200 nm long and with well-defined structures have now been synthesized by a bottom-up method, and are found to have a large optical bandgap of 1.88 eV. Scanning probe microscopy revealed highly ordered self-assembled monolayers of the GNRs, and the high intrinsic charge-carrier mobility of individual ribbons was characterized by terahertz spectroscopy.
Journal Article
Search for Axionlike Dark Matter through Nuclear Spin Precession in Electric and Magnetic Fields
We report on a search for ultralow-mass axionlike dark matter by analyzing the ratio of the spin-precession frequencies of stored ultracold neutrons and Hg199 atoms for an axion-induced oscillating electric dipole moment of the neutron and an axion-wind spin-precession effect. No signal consistent with dark matter is observed for the axion mass range 10−24≤ma≤10−17eV . Our null result sets the first laboratory constraints on the coupling of axion dark matter to gluons, which improve on astrophysical limits by up to 3 orders of magnitude, and also improves on previous laboratory constraints on the axion coupling to nucleons by up to a factor of 40.
Journal Article
Quantum chemical “Aufbau” principles: how to estimate the shape of highly flexible (bio-)polymers? A recursively extendable “chemion picture” of Euler-Hückel-type
An outline is given of how to split the n-dimensional space of torsion angles occurring in flexible (bio-)polymers (like alkanes, nucleic acids, or proteins, for instance) into n one-dimensional potential curves. Forthcoming applications will focus on the “protein folding problem,” beginning with polyglycine.
Context
In accordance with Euler’s topology rules, molecules are considered to be composed of “vertices” (atoms, ligands, bonding sites, functional groups, and bigger fragments). Following Hückel, each vertex is represented by only one basis function. Starting from the “monofocal” hydrids
CH
4
,
NH
3
,
OH
2
, FH, and
SiH
4
,
PH
3
,
SH
2
, ClH as anchor units, “chemionic” Hamiltonians (of individual “chemion ensembles” and proportional nuclear charges) are constructed recursively, together with an appropriate basis set for the first five (normal) alkanes and some related oligomers like primary alcohols, alkyl amines, and alkyl chlorides.
Methods
Standard methods (“Restricted Hartree-Fock RHF” and “Full Configuration Interaction FCI”) are used to solve the various stationary Schrödinger equations. Two software packages are indispensable: “SMILES” for integral evaluations over Slater-type orbitals (STO), and “Numerical Recipes” for matrix diagonalizations and inversions. While managing with only two-center repulsion integrals, “implicit multi-center integrations” lead us to the non-empirical fundament of Hoffmann’s “Extended-Hückel Theory.”
Journal Article
Multidisciplinary management of people with spina bifida across the lifespan
The average worldwide prevalence of neural tube defects (NTDs) is 1.0 per 1000 births. Its development is multifactorial due to genetic and non-genetic factors. Spina bifida (SB) is one of main representatives of NTD. The spinal cord lesion level is the main determinant of the level of paralysis, numbness, and difficulties with bladder/bowel functions. Myelomeningocele prenatal repair reduces hydrocephalus and hindbrain herniation and improves motor function. The severity of hydrocephalus is associated with poorer neurodevelopmental outcomes whether operated on prenatally or after birth. People with SB tend to have a lower IQ and cognitive difficulties. Early diagnosis, proactivity, and lifelong multidisciplinary follow-up are key protective issues. Invasive urological interventions should be considered in selected patients after failure of conservative treatment. Transition to adult care should be well planned as it is challenging. Health literacy is directly associated with success at transition. Sexuality and fertility should be addressed before/during puberty. Overall, the rates of fecal and urinary continence and skin breakdown increase with age, whereas the ability to ambulate declines with age. Bowel and urinary incontinence are independent predictors of lower health-related quality of life (HRQoL) in adults with SB. Bowel incontinence has negative impact on HRQoL regardless of frequency or amount. Long-term caregiver support should be offered at diagnosis. Survival at a mean of 50 years is poor, at 32%, due to central nervous system deaths, cancer, urological disease, and sepsis. Challenges to implementation of recommended practices exist, especially in low and middle-income countries.
Journal Article
Investigation of the lightest hybrid meson candidate with a coupled-channel analysis of p¯p-, π-p- and ππ-Data
A sophisticated coupled-channel analysis is presented that combines different processes: the channels π0π0η, π0ηη and K+K-π0 from p¯p annihilations, the P- and D-wave amplitudes of the πη and πη′ systems produced in π-p scattering, and data from ππ-scattering reactions. Hence our analysis combines the data sets used in two independent previous analyses published by the Crystal Barrel experiment and by the JPAC group. Based on the new insights from these studies, this paper aims at a better understanding of the spin-exotic π1 resonances in the light-meson sector. By utilizing the K-matrix approach and realizing the analyticity via Chew-Mandelstam functions the amplitude of the spin-exotic wave can be well described by a single π1 pole for both systems, πη and πη′. The mass and the width of the π1-pole are measured to be (1623±47-75+24)MeV/c2 and (455±88-175+144)MeV.
Journal Article
Denser forests across the USA experience more damage from insects and pathogens
Forests across much of the United States are becoming denser. Trees growing in denser stands experience more competition for essential resources, which can make them more vulnerable to disturbances. Forest density can be expressed in terms of basal area, a metric that has been used to assess vulnerability of some forests to damage by certain insects or pathogens. A raster map of total tree basal area (TBA) for the conterminous United States was compared with annual (2000–2019) survey maps of forest damage due to insects and pathogens. Across each of four regions, median TBA was significantly higher within forest areas defoliated or killed by insects or pathogens than in areas without recorded damage. Therefore, TBA may serve as a regional-scale indicator of forest health and a first filter for identifying areas that merit finer-scale analysis of forest conditions.
Journal Article
Evaluating fuelbreak strategies for compartmentalizing a fire-prone forest landscape in Alberta, Canada
Large wildfires, the dominant natural disturbance type in North American forests, can cause significant damage to human infrastructure. One well-known approach to reduce the threat of wildfires is the strategic removal of forest fuels in linear firebreaks that segment forest landscapes into distinct compartments. However, limited human and financial resources can make it difficult to plan compartmentalization effectively. In this study, we developed a simulation-optimization approach to assist with the planning of wildfire risk mitigation efforts in the Red Rock-Prairie Creek area of Alberta, Canada, a rugged, fire-prone landscape. First, we used a spatial fire growth model to calculate a matrix of fire spread likelihoods between all pairs of locations in the landscape and used this matrix to guide the allocation of firebreaks. Then, we formulated a firebreak compartmentalization problem to reduce the fire spread potential in the landscape. We depicted the landscape as a network of patches containing hazardous fuels and solved a critical edge removal linear programming problem (CERP) to partially fragment the landscape and minimize the potential of wildfires to spread to adjacent areas. We compared the CERP with other fuel treatment strategies intended to minimize fire-threat measures such as burn likelihood and fuel exposure. Compared to these strategies, the CERP solutions demonstrated better capacity to segment the landscape into evenly spaced compartments and effectively minimized fire spread along the prevailing wind paths. Our solutions provide several strategies for reducing the risk of wildfires to forest habitat and could assist strategic planning of wildfire mitigation activities in other regions.
Journal Article
Obesity Facts and Their Influence on Renal Function Across the Life Span
Obesity is a chronic disease, with a rapidly increasing prevalence worldwide. Body mass index (BMI) provides the most useful population-level measure of overweight and obesity. For adults, overweight is defined as a BMI (Kg/m 2 ) ≥ 25, and obesity as a BMI ≥ 30, for non-Asians and ≥ 27.5 for Asians. Abdominal obesity can be defined as a waist circumference equal to or higher than 102 cm for men and ≥88 cm for women. The definition of children and adolescents BMI changes with age and sex. Obesity may be exogenous or endogenous obesity, the latter is multifactorial and predominantly manifested during childhood. Presently, overweight and obesity are linked to more deaths worldwide than underweight. The total kidney glomerular filtration rate (GFR) is determined by the sum of nephrons and the GFR within each nephron or single nephron GFR. In clinical practice, GFR is more frequently calculated by GFR estimating equations based upon the plasma levels of creatinine, cystatin C, or both. The measured value of plasma creatinine is strongly influenced by non-GFR factors, by its tubular and gastrointestinal secretion, and by the problems associated with the lack of standardization of creatinine's laboratory assay discrediting it as an ideal GFR biomarker. Unlike creatinine, cystatin C plasma levels are mainly determined by GFR. Obesity may affect the kidney, via development of systemic arterial hypertension and/or diabetes mellitus, or directly, by ectopic accumulation of adipose tissue in the kidney. As obesity is a clinical condition associated with altered body composition, creatinine may not be the ideal biomarker for GFR measurement in obese individuals.
Journal Article