Ultrafast Molecular Imaging by Laser Induced Electron Diffraction

We address the feasibility of imaging geometric and orbital structure of a polyatomic molecule on an attosecond time-scale using the laser induced electron diffraction (LIED) technique. We present numerical results for the highest molecular orbitals of the CO2 molecule excited by a near infrared few-cycle laser pulse. The molecular geometry (bond-lengths) is determined within 3% of accuracy from a diffraction pattern which also reflects the nodal properties of the initial molecular orbital. Robustness of the structure determination is discussed with respect to vibrational and rotational motions with a complete interpretation of the laser-induced mechanisms

Magnitude and variability of process rates in fungal diversity-litter decomposition relationships

There is compelling evidence that losses in plant diversity can alter ecosystem functioning, particularly by reducing primary production. However, impacts of biodiversity loss on decomposition, the complementary process in the carbon cycle, are highly uncertain. By manipulating fungal decomposer diversity in stream microcosm experiments we found that rates of litter decomposition and associated fungal spore production are unaffected by changes in decomposer diversity under benign and harsher environmental conditions. This result calls for caution when generalizing outcomes of biodiversity experiments across systems. In contrast to their magnitude, the variability of process rates among communities increased when species numbers were reduced. This was most likely caused by a portfolio effect (i.e. statistical averaging), with the uneven species distribution typical of natural communities tending to weaken that effect. Curbing species extinctions to maintain ecosystem functioning thus can be important even in situations where process rates are unaffected

Temperature oscillation coupled with fungal community shifts can modulate warming effects on litter decomposition

Diel temperature oscillations are a nearly ubiquitous phenomenon, with amplitudes predicted to change along with mean temperatures under global-warming scenarios. Impact assessments of global warming have largely disregarded diel temperature oscillations, even though key processes in ecosystems, such as decomposition, may be affected. We tested the effect of a 5 degrees C temperature increase with and without diel oscillations on litter decomposition by fungal communities in stream microcosms. Five temperature regimes with identical thermal sums (degree days) were applied: constant 3 degrees and 8 degrees C; diel temperature oscillations of 5 degrees C around each mean; and oscillations of 9 degrees C around 8 degrees C. Temperature oscillations around 8 degrees C (warming scenario), but not 3 degrees C (ambient scenario), accelerated decomposition by 18% (5 degrees C oscillations) and 31% (9 degrees C oscillations), respectively, compared to the constant temperature regime at 8 degrees C. Community structure was not affected by oscillating temperatures, although the rise in mean temperature from 3 degrees to 8 degrees C consistently shifted the relative abundance of species. A simple model using temperature-growth responses of the dominant fungal decomposers accurately described the experimentally observed pattern, indicating that the effect of temperature oscillations on decomposition in our warming scenario was caused by strong curvilinear responses of species to warming at low temperature, particularly of the species becoming most abundant at 8 degrees C (Tetracladium marchalianum). These findings underscore the need to consider species-specific temperature characteristics in concert with changes in communities when assessing consequences of global warming on ecosystem processes

Laser induced electron diffraction: a tool for molecular orbital imaging

We explore the laser-induced ionization dynamics of N2 and CO2 molecules subjected to a few-cycle, linearly polarized, 800\,nm laser pulse using effective two-dimensional single active electron time-dependent quantum simulations. We show that the electron recollision process taking place after an initial tunnel ionization stage results in quantum interference patterns in the energy resolved photo-electron signals. If the molecule is initially aligned perpendicular to the field polarization, the position and relative heights of the associated fringes can be related to the molecular geometrical and orbital structure, using a simple inversion algorithm which takes into account the symmetry of the initial molecular orbital from which the ionized electron is produced. We show that it is possible to extract inter-atomic distances in the molecule from an averaged photon-electron signal with an accuracy of a few percents

Clusters of Exceptional Points for a Laser Control of Selective Vibrational Transfer

When a molecule is exposed to a laser field, all field-free vibrational states become resonances, with complex quasi energies calculated using Floquet theory. There are many ways to produce the coalescences of pairs of such quasi energies, with appropriate wavelength-intensity choices which define Exceptional Points (EP) in the laser parameter plane. We dress for the molecular ion H$_2^+$ an exhaustive map of these exceptional points which appear in clusters. Such clusters can be used to define several vibrational transfer scenarios implying more than a single exceptional point, exchanging single or multiple vibrational quanta. The ultimate goal is molecular vibrational cooling by transferring an initial (thermal, for instance) population on a final (ground, for instance) single vibrational state. When a molecule is exposed to a laser field, all field-free vibrational states become resonances, with complex quasi energies calculated using Floquet theory. There are many ways to produce the coalescences of pairs of such quasi energies, with appropriate wavelength-intensity choices which define Exceptional Points (EP) in the laser parameter plane. We dress for the molecular ion H$_2^+$ an exhaustive map of these exceptional points which appear in clusters. Such clusters can be used to define several vibrational transfer scenarios implying more than a single exceptional point, exchanging single or multiple vibrational quanta. The ultimate goal is molecular vibrational cooling by transferring an initial (thermal, for instance) population on a final (ground, for instance) single vibrational state.Comment: 16 pages, 7 figures, 1 tabl

Systems model for dynamic human error during accident sequences

Statement of responsibility on title page reads: Y. Huang, N. Siu, D. Lanning, J. Carroll, and V. Dang"December 1991."Includes bibliographical references (pages 153-155)Final report: "A systems model for dynamic human error during accident sequences"This report describes a systems-based operating crew model designed to simulate the behavior of an nuclear power plant control room crew during an accident scenario. This model can lead to an improved treatment of potential operator-induced multiple failures, since it deals directly with the causal factors underlying individual and group behavior. It is intended that the model, or more advanced developments of the model, will be used in the human reliability analysis portion of a probabilistic risk assessment study, where careful treatment of multiple, dependent failures is required. The model treats the members of the control room crew as separate, reasoning entities. These entities receive information from the plant and each other, process that information, perform actions that affect the plant, and provide information to the other crew members.The information retrieval, processing, and output activities are affected by the characteristics of the individual operator (e.g., his technical ability) and his relationship (measured in terms of "confidence level") with his fellow operators. Group behavior is modeled as the implicit result of individual operator behavior and the interactions between operators. The model is applied towards the analysis of steam generator tube rupture (SGTR) accidents at a non-U.S. pressurized water reactor, using the SIMSCRIPT 11.5 programming language. Benchmark runs, comparing the model predictions with videotaped observations of the performances of three different crews during SGTR training exercises, are performed to tune a small number of model parameters. The tuned model is then applied in a blind test analysis of a fourth crew. In both the benchmarking and blind test runs, the model performs quite well in predicting the occurrence, ordering, and timing of key events.The model is also employed in a number of sensitivity analyses that demonstrate the robustness of the model (it generates plausible results even when the model parameters are assigned values not representative of observed crews) and the model's usefulness in investigating key issues (e.g., the effect of stress buildup on crew performance). iSupported by the Office of Nuclear Regulatory Research, United States Nuclear Regulatory Commission: NRC-04-89-35