The thresholds for laser-induced intrinsic damage in several alkali halide crystals are calculated by means of separate avalanche and multiphoton ionization models, as well as by a combination of the two. The appearance of 10 1 8 electrons/cm 3 in the conduction band is taken as the criterion for damage. Numerical calculations are performed for nanosecond and picosecond pulse durations, with wavelengths ranging from 1.064 to 0.355 μm. Upon comparing these theoretical predictions with available experimental results, it is found that, in general, the avalanche mechanism yields damaging optoelectric field strengths in good agreement with the experimentally measured values. The field strengths required for damage solely due to multiphoton ionization are substantially higher. However, when the photon multiplicity is less than or equal to four, the damage fields predicted by the multiphoton model are often smaller than those calculated for catastrophic avalanche multiplication, and are in better agreement with the experimentally measured damage thresholds. Furthermore, It is noted that neither the avalanche nor the multiphoton ionization model, when considered alone, can satisfactorily explain the experimentally observed frequency and pulse length dependence of the damage fields over the entire reported range of these parameters. It is thus suggested that it is more appropriate to combine these two mechanisms.
Abstract NOTE: The first page of text has been automatically extracted and included below in lieu of an abstract Session 1653 Freshman Engineering Student Success Indicators Patrick E. Devens, Thomas D. Walker Virginia Polytechnic Institute & State University Abstract: Every year, up to 1300 freshman engineering students from around the world arrive at Virginia Tech's1 College of Engineering with varying backgrounds, experiences, and degrees of academic expertise. Many fail to meet first year engineering expectations even though college entrance requirements have increased. The question is why and how can the college improve retention? This paper provides results at the two-year-point of an ongoing study of first year engineering students at Virginia Tech. The purpose of the study is to attempt to discern student success predictors so that appropriate interventions / corrective actions can be taken to increase retention in the program. The study includes the analysis of student Scholastic Aptitude Test (SAT) scores and a baseline math test. This data is then compared with their first semester engineering course grade. This paper only addresses correlations between final course grades and student SAT scores. Overall student performance in the initial freshman-engineering course is analyzed in addition to female, Afro-American, and Hispanic subgroups. The goal of the study is to better accommodate student needs by identifying how to allocate existing resources more. I. Introduction Significant numbers of freshman engineering students at Virginia Tech do not perform at a satisfactory level in their first semester engineering course. The questions are, "Why?" and "How can the college increase retention of the students without decreasing performance standards?" To help answer these questions, the Engineering Fundamentals Division (EF) continues to perform an analysis of freshman-engineering student performance during their first engineering course and on two tests. The tests are the national SAT test and in-house developed mathematics pre & post tests. The objective is to identify trends and/or indicators of poor student performance that can facilitate the development of programs to increase student performance and subsequent retention. Other authors have attempted to analyze correlations between student SAT scores and academic performance. Most of these studies are, however, comparatively old and few are targeted at success in engineering education. For example, Gilbert reported in 1960 that " . . . scores on the SAT-V, SAT-M, and the Advanced Mathematics Test and scores on a science test (Physics and Chemistry) do not seem to provide a very sound basis for Proceedings of the 2001 American Society for Engineering Education Annual Conference & Exposition Copyright O 2001, American Society for Engineering Education
Focusing on Nicaragua after the 1990 Sandinista electoral defeat, this book is a comprehensive, multidisciplinary study of one of the most unusual cases of regime transition in the late 20th century. It shows the similarities and differences between Nicaragua's regime and those of other countries.
Research conducted in the United States (U.S.) suggests that many police professionals are unaware of, or resistant to, empirical research, and see little value in adopting evidence-based approaches for tackling policing issues. To determine whether similar views are held by Canadian police professionals, 598 police professionals (civilians and officers) from seven police services across Canada were surveyed. The survey was designed by Lum and Telep (n.d. Officer receptivity survey on evidence-based policing. Fairfax, VA: Center for Evidence-Based Crime Policy, George Mason University) to determine respondents' knowledge of, and support for, evidence-based policing (EBP). Using their survey allowed us to compare our results to the data they collected in the U.S. Although Canadian respondents had similar concerns regarding EBP as those in the U.S., in several ways, Canadian police professionals were more open to the idea of EBP. The results are encouraging, but still suggest a lack of buy-in from some police professionals in certain regards. Potential reasons for the cross-national discrepancies, and the consequences of the findings, are discussed.
In this paper the theories of avalanche ionization, multiphoton absorption, and impurity-initiated laser-induced damage are treated. At first inspection, none of these theories adequately describe the experimental observations of the variation of pulsed laser damage threshold in optical thin films with changes in material property, laser wavelength, pulse length, or film thickness. However, it is shown that the inclusion of a Mie absorption cross section for a range of dielectric impurity sizes provides a good description of the database with the impurity model. It is also shown that the thermal properties of the host film material and impurity are of considerable importance in explaining observed experimental data.
Anthropogenic emissions of the greenhouse gases carbon dioxide and methane have stimulated a rise in global surface temperature of 0.76 �C since the turn of the 20th Century. Such climate warming has already had significant impacts on the terrestrial biosphere, raising concerns that ecosystems will feed back to future climate by altering the balance of carbon flow between the land and atmosphere. It is well established that warming can directly affect rates of photosynthesis and ecosystem respiration, which together dictate the carbon balance of most ecosystems. However, warming is also causing shifts in the productivity and composition of vegetation, and there is growing recognition that this can have indirect effects on carbon cycling via its influence over soil properties and the activity of the soil microbial community. Despite this, much uncertainty currently surrounds the effects of warming on vegetation, both between individuals of the same species and at the plant community scale. Furthermore, the consequences of vegetation change at either scale for carbon dynamics are not well understood when considered in tandem with warming.Northern peatlands are of particular relevance to ecosystem climate feedbacks, holding one third of global soil carbon in regions vulnerable to rapid temperature change. The aim of this thesis was to explore how different scales of vegetation change regulate peatland carbon cycle responses to warming. This was achieved using field experiments across Europe, including manipulations, reciprocal transplants and gradient studies, which integrated a range of approaches for detecting responses from the genetic to ecosystem scale. By measuring the growth responses of dominant plant species to warming and the presence or absence of different plant functional types (Chapter 2), I reveal interdependencies between peatland plants that will cause community change over decades to centuries. I suggest that such responses occur due to both plant-plant interactions and the influence of vegetation over microclimate, the latter of which I also show to be regulated by vegetation composition (Chapter 3). Additionally, I demonstrate that warming at different temporal scales has contrasting effects on the metabolism, photosynthesis and growth of Eriophorum vaginatum (Chapter 5), causing decreases in growth over seasons to years through phenotypic plasticity and increases in growth over centuries to millennia through natural selection. I show that such adaptation of individual plants to rising temperature has potential consequences for peatland carbon dynamics. Moreover, I reveal that changes in vegetation composition at the community level could destabilise the peatland carbon stock under warming by accelerating decomposition of ancient carbon (Chapter 4). Through these findings, I provide novel insight into the scales and mechanisms by which vegetation responses to warming impact carbon dynamics. Given the key role of northern peatlands in the global carbon cycle, I suggest that warming effects on peatland vegetation may have considerable consequences for future climate by controlling liberation of peatland carbon into the atmosphere.
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