Suchergebnisse

2010 Spring. ; Includes bibliographic references. ; Covers not scanned. ; Print version deaccessioned 2022. ; Energetic materials have a wide variety of industrial, civil, and military applications. They include a number of organic compounds such as RDX (1,3,5- trinitroheahydro-s-triazine), HMX (octahydro-1,3,5,7-tetranitro-l ,3,5,7-tetrazocine), DAAF (3,3'-diamino-4,4'-azoxyfurazan), DAATO35 (3,3'-azobis(6-amino-l,2,4,5- tetrazine)-mixed N-oxides), etc. These materials release huge chemical energy during their decomposition. The decomposition of energetic materials is initiated with a shock or compression wave or a spark. Such events in solids generate molecules in the excited electronic states. Hence, in order to maximize release of the stored chemical energy in the most efficient and useful manner and to design new energetic materials, the unimolecular decomposition mechanisms and dynamics from excited electronic states should be understood for these systems. This thesis describes understanding about unimolecular decomposition of energetic materials from their excited electronic states. A few fundamental questions at molecular level dealing with electronic excitation of energetic materials are addressed here: (a) what happens immediately after electronic excitation of energetic molecules?; (b) how is excess energy partitioned among product molecules following electronic excitation?; (b) what are the mechanism and dynamics of molecular decomposition?; (d) does nonadiabatic chemistry (a process that span multiple electronic potential energy surfaces) through conical intersection (crossing of different potential energy surfaces) dominate system behavior? Both energy and time resolved spectroscopic techniques are used in this effort. The product internal state (rotational and vibrational) distributions are probed using mass and energy resolved spectroscopic techniques using time-of-flight mass spectrometry (TOFMS) and laser induced fluorescence (LIF) spectroscopy. Analyzing the product internal state distributions, the mechanisms of unimolecular decomposition of energetic molecules from excited electronic states are determined. The femtosecond pump-probe spectroscopic technique is utilized to determine ultrafast decomposition dynamics of these molecules. From a theoretical point of view, multiconfigurational methodologies such as, CASSCF and CASMP2 are used to model the processes involving excited electronic states of energetic molecules. Influence of nonadiabatic chemistry in the overall decomposition of energetic molecules is also theoretically judged. The primary energetic systems whose nonadiabatic chemistry discussed here are the nitramine (e.g., RDX, HMX), furazan (e.g., DAAF), and tetrazine-N-oxide (e.g., DAATO3.5) based energetic species. A number of model systems, which are simple analogue molecules of the large and more complex energetic materials, are studied in detail to understand nonadiabatic energetic behavior of a single energetic moiety of particular class. Subsequently, the decomposition mechanism for more complex energetic systems are studied and compared with that of their model systems. Nitramine energetic materials and model systems undergo nitro-nitrite isomerization followed by IV NO elimination. Nitramine energetic materials dissociates in the ground state generating rotationally cold (20 K) distribution of the NO product. Nitramine model systems dissociates in the excited state surface producing rotationally hot (-120 K) distribution of the NO product. The nitro-nitrite isomerization happens through conical intersection. Furazan based model molecules (e.g., furazan) possess two different pathways of decomposition: ring contraction and ring opening. These two pathways are electronically nonadiabatic. The ring contraction mechanism generates rotationally cold (20 K) product NO and the ring opening mechanism generates rotationally hot (100 K) product NO. Furazan based energetic material (DAAF), however, dissociates only through a ring contraction mechanism. Thus nonadiabatic pathways control the decomposition of furazan based molecules. Decomposition of tetrazine-2,4-dioxide based molecules involves a ring contraction mechanism through (Si/So)ci, producing only rotationally cold (20 K) but vibrationally hot (1200 K) distributions of the NO product. Tetrazine-l,4-dioxde undergoes similar decomposition pathway through (Si/So)ci; however, it produces rotationally hotter (50 K) but vibrationally colder distribution of the NO product. Thus the relative position of the oxygen atoms attached to the tetrazine ring is important parameter along with their nonadiabatic chemistry controlling their final energetic reactivity. Decomposition dynamics of all energetic materials is faster than 180 fs. Considering the influence of conical intersections in the excited electronic state decomposition of energetic materials, rotationally cold N2 product is predicted to be the major decomposition product of high nitrogen content energetic species. The present work infers that generation of internally cold product is an important characteristics of a true energetic molecule. Presence of low lying chemically relevant conical intersections provides a direct pathway of ultrafast decomposition chemistry of energetic molecules. The energy barrier to the low lying chemically relevant conical intersection, in principle, would be a point of interest to make a system more or less energetic.

Abstract. Glacial lake outburst floods (GLOFs) are a major concern throughout High Mountain Asia, where societal impacts can extend far downstream. This is
particularly true for transboundary Himalayan basins, where risks are expected to further increase as new lakes develop. Given the need for
anticipatory approaches to disaster risk reduction, this study aims to demonstrate how the threat from a future lake can be feasibly assessed
alongside that of worst-case scenarios from current lakes, as well as how this information is relevant for disaster risk management. We have focused on two previously identified dangerous lakes (Galongco and Jialongco), comparing the timing and magnitude of simulated worst-case outburst events from these lakes both in the Tibetan town of Nyalam and downstream at the border with Nepal. In addition, a future scenario has been assessed, whereby an avalanche-triggered GLOF was simulated for a potential large new lake forming upstream of Nyalam. Results show that large (>20×106 m3) rock and/or ice avalanches could generate GLOF discharges at the border with Nepal that are more than 15 times larger than what has been observed previously or anticipated based on more gradual breach simulations. For all assessed lakes, warning times in Nyalam would be only 5–11 min and 30 min at the border. Recent remedial measures undertaken to lower the water level at Jialongco would have little influence on downstream impacts resulting from a very large-magnitude GLOF, particularly in Nyalam where there has been significant development of infrastructure directly within the high-intensity flood zone. Based on these findings, a comprehensive approach to disaster risk management is called for, combining early warning systems with effective land use zoning and programmes to build local response capacities. Such approaches would address the current drivers of GLOF risk in the basin while remaining robust in the face of worst-case, catastrophic outburst events that become more likely under a warming climate.

Funding: This study was supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDA20100300) and the Swiss National Science Foundation (200021E_177652/1). NN received funding from the European Union's Horizon 2020 programme (No. 689443). ; Knowledge about the long-term response of High Mountain Asian glaciers to climatic variations is paramount because of their important role in sustaining Asian river flow. Here, a satellite-based time series of glacier mass balance for seven climatically different regions across High Mountain Asia since the 1960s shows that glacier mass loss rates have persistently increased at most sites. Regional glacier mass budgets ranged from −0.40 ± 0.07 m w.e.a−1 in Central and Northern Tien Shan to −0.06 ± 0.07 m w.e.a−1 in Eastern Pamir, with considerable temporal and spatial variability. Highest rates of mass loss occurred in Central Himalaya and Northern Tien Shan after 2015 and even in regions where glaciers were previously in balance with climate, such as Eastern Pamir, mass losses prevailed in recent years. An increase in summer temperature explains the long-term trend in mass loss and now appears to drive mass loss even in regions formerly sensitive to both temperature and precipitation. ; Publisher PDF ; Peer reviewed