130 research outputs found
Full-dimensional treatment of short-time vibronic dynamics in molecular high-harmonics generation process in methane
We present derivation and implementation of the Multi-Configurational
Strong-Field Approximation with Gaussian nuclear Wave Packets (MC-SFA-GWP) -- a
version of the molecular strong-field approximation which treats all electronic
and nuclear degrees of freedom, including their correlations,
quantum-mechanically. The technique allows, for the first time, realistic
simulation of high-harmonic emission in polyatomic molecules without invoking
reduced-dimensionality models for the nuclear motion or the electronic
structure. We use MC-SFA-GWP to model isotope effects in high-harmonics
generation (HHG) spectroscopy of methane. The HHG emission in this molecule
transiently involves strongly vibronically-coupled electronic state of
the cation. We show that the isotopic HHG ratio in methane
contains signatures of: a) field-free vibronic dynamics at the conical
intersection (CI); b) resonant features in the recombination cross-sections; c)
laser-driven bound-state dynamics; as well as d) the well-known short-time
Gaussian decay of the emission. We assign the intrinsic vibronic feature (a) to
a relatively long-lived ( fs) vibronic wave packet of the singly-excited
() and () vibrational modes, strongly coupled to the
components of the electronic state. We demonstrate that these physical
effects differ in their dependence on the wavelength, intensity, and duration
of the driving pulse, allowing them to be disentangled. We thus show that HHG
spectroscopy provides a versatile tool for exploring both conical intersections
and resonant features in photorecombination matrix elements in the regime not
easily accessible with other techniques
A perturbative approximation to DFT/MRCI: DFT/MRCI(2)
We introduce a perturbative approximation to the combined density functional
theory and multireference configuration interaction (DFT/MRCI) method. The
method, termed DFT/MRCI(2), results from the application of quasi-degenerate
perturbation theory and the Epstein-Nesbet partitioning of the DFT/MRCI
Hamiltonian matrix. This results in the replacement of the diagonalization of
the large DFT/MRCI Hamiltonian with that of a small effective Hamiltonian, and
affords orders of magnitude savings in terms of computational cost. Moreover,
the DFT/MRCI(2) approximation is found to be of excellent accuracy, furnishing
excitation energies with a root mean squared deviation from the DFT/MRCI values
of less than 0.03 eV for an extensive test set of organic molecules
Ultrafast X-ray spectroscopy of conical intersections
Ongoing developments in ultrafast X-ray sources offer powerful new means of
probing the com- plex non-adiabatically coupled structural and electronic
dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are
governed by general electronic degeneracies termed conical in- tersections
which play a key role, analogous to that of a transition state, in the
electronic-nuclear dynamics of excited molecules. Using high level ab initio
quantum dynamics simulations, we studied time-resolved X-ray absorption and
photoelectron spectroscopy (TRXAS and TRXPS, respectively) of the prototypical
unsaturated organic chromophore, ethylene, following excitation to its S2
state. The TRXAS in particular is highly sensitive to all aspects of the
ensuing dynamics. These X-ray spectroscopies provide a clear signature of the
wavepacket dynamics near conical intersections, related to charge localization
effects driven by the nuclear dynamics. Given the ubiquity of charge
localization in excited state dynamics, we believe that ultrafast X-ray
spectroscopies offer a unique and powerful route to the direct observation of
dynamics around conical intersections.Comment: 5 pages, 4 figure
Expression of Epstein–Barr Virus–Encoded Small RNA (by the EBER-1 Gene) in Liver Specimens from Transplant Recipients with Post-Transplantation Lymphoproliferative Disease
Epstein-Barr virus (EBV)—associated post-transplantation lymphoproliferative disease (PTLD) develops in 1 to 10 percent of transplant recipients, in whom it can be treated by a reduction in the level of immunosuppression. We postulated that the tissue expression of the small RNA transcribed by the EBER-1 gene during latent EBV infection would identify patients at risk for PTLD. We studied EBER-1 gene expression in liver specimens obtained from 24 patients 2 days to 22 months before the development of PTLD, using in situ hybridization with an oligonucleotide probe. Control specimens were obtained from 20 recipients of allografts with signs of injury due to organ retrieval, acute graft rejection, or viral hepatitis in whom PTLD had not developed 9 to 71 months after the biopsy. Of the 24 patients with PTLD, 17 (71 percent) had specimens in which 1 to 40 percent of mononuclear cells were positive for the EBER-1 gene. In addition, 10 of these 17 patients (59 percent) had specimens with histopathological changes suggestive of EBV hepatitis. In every case, EBER-1—positive cells were found within the lymphoproliferative lesions identified at autopsy. Only 2 of the 20 controls (10 percent) had specimens with EBER-1—positive cells (P<0.001), and such cells were rare. EBER-1 gene expression in liver tissue precedes the occurrence of clinical and histologic PTLD. The possibility of identifying patients at risk by the method we describe here and preventing the occurrence of PTLD by a timely reduction of immunosuppression needs to be addressed by future prospective studies. (N Engl J Med 1992;327:1710–4.), POST-TRANSPLANTATION lymphoproliferative disease (PTLD), either polyclonal or monoclonal, complicates the clinical course of 1 to 10 percent of organ-transplant recipients.123 Immunohistochemical studies have demonstrated that the lymphoid cells within the lesions of PTLD almost invariably contain Epstein–Barr virus (EBV), primarily in a state of latent infection.4,5 The EBER-1 gene is expressed early during latent EBV infection and codes for a small messenger RNA (mRNA) expressed at up to 107 copies per cell.6 We and others have previously demonstrated the value of the detection of EBER-1 RNA for identifying EBV-infected cells in formalin-fixed paraffin-embedded tissues.7,8 In the current investigation, we used… © 1992, Massachusetts Medical Society. All rights reserved
Модель Портера как стратегический инструмент анализа (на примере рынка мороженого Республики Беларусь)
Разработка стратегии предприятия основывается на анализе микросреды с помощью модели Портера, который позволяет сформулировать основные угрозы иразработать способы защиты от них. На примере рынка мороженого было проведено выявление и анализ угроз и сформулированы направления работы по защите
Tabletop imaging of structural evolutions in chemical reactions
The introduction of femto-chemistry has made it a primary goal to follow the
nuclear and electronic evolution of a molecule in time and space as it
undergoes a chemical reaction. Using Coulomb Explosion Imaging we have shot the
first high-resolution molecular movie of a to and fro isomerization process in
the acetylene cation. So far, this kind of phenomenon could only be observed
using VUV light from a Free Electron Laser [Phys. Rev. Lett. 105, 263002
(2010)]. Here we show that 266 nm ultrashort laser pulses are capable of
initiating rich dynamics through multiphoton ionization. With our generally
applicable tabletop approach that can be used for other small organic
molecules, we have investigated two basic chemical reactions simultaneously:
proton migration and C=C bond-breaking, triggered by multiphoton ionization.
The experimental results are in excellent agreement with the timescales and
relaxation pathways predicted by new and definitively quantitative ab initio
trajectory simulations
Assessing the influence of the built environment on physical activity for utility and recreation in suburban metro Vancouver
<p>Abstract</p> <p>Background</p> <p>Physical inactivity and associated co-morbidities such as obesity and cardiovascular disease are estimated to have large societal costs. There is increasing interest in examining the role of the built environment in shaping patterns of physical activity. However, few studies have: (1) simultaneously examined physical activity for leisure and utility; (2) selected study areas with a range of built environment characteristics; and (3) assessed the built environment using high-resolution land use data.</p> <p>Methods</p> <p>Data on individuals used for this study are from a survey of 1602 adults in selected sites across suburban Metro Vancouver. Four types of physical activity were assessed: walking to work/school, walking for errands, walking for leisure and moderate physical activity for exercise. The built environment was assessed by constructing one-kilometre road network buffers around each respondent's postal code. Measures of the built environment include terciles of recreational and park land, residential land, institutional land, commercial land and land use mix.</p> <p>Results</p> <p>Logistic regression analyses showed that walking to work/school and moderate physical activity were not associated with any built environment measure. Living in areas with lower land use mix, lower commercial and lower recreational land increased the odds of low levels of walking for errands. Individuals living in the lower third of land use mix and institutional land were more likely to report low levels of walking for leisure.</p> <p>Conclusions</p> <p>These results suggest that walking for errands and leisure have a greater association with the built environment than other dimensions of physical activity.</p
A Field Trial of Alternative Targeted Screening Strategies for Chagas Disease in Arequipa, Peru
In the wake of emerging T. cruzi infection in children of periurban Arequipa, Peru, we conducted a prospective field trial to evaluate alternative targeted screening strategies for Chagas disease across the city. Using insect vector data that is routinely collected during Ministry of Health insecticide application campaigns in 3 periurban districts of Arequipa, we separated into 4 categories those households with 1) infected vectors; 2) high vector densities; 3) low vector densities; and 4) no vectors. Residents of all infected-vector households and a random sample of those in the other 3 categories were invited for serological screening for T. cruzi infection. Subsequently, all residents of households within a 15-meter radius of detected seropositive individuals were invited to be screened in a ring case-detection scheme. Of 923 participants, 21 (2.28%) were seropositive. There were no significant differences in prevalence across the 4 screening strategies, indicating that household entomologic factors alone could not predict the risk of infection. Indeed, the most predictive variable of infection was the number of years a person lived in a location with triatomine insects. Therefore, a simple residence history questionnaire may be a useful screening tool in large, diverse urban environments with emerging Chagas disease
Prevalence of concurrent deep vein thrombosis in patients with lower limb cellulitis: a prospective cohort study
- …