2,908 research outputs found
Testing protoplanetary disc dispersal with radio emission
No full textWe consider continuum free–free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of extreme-ultraviolet (EUV) and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free–free emission from protoplanetary discs. In general, the free–free luminosity scales roughly linearly with ionizing luminosity in both EUV- and X-ray-driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14–18?GHz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free–free emission from the ionized disc in either the EUV- or the X-ray-driven scenario. Finally, we suggest a possible route to testing the EUV- and X-ray-driven dispersal model of protoplanetary discs, by combining observed free–free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find an M?2? scaling with free–free flux in the case of EUV-driven disc dispersal or an ?* scaling in the case of X-ray-driven disc dispersa
Signatures of hydrogen atom quantum diffusion: h + n2o reaction in solid parahydrogen
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Previous issue date: 26In 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles.\footnote{A. F. Andreev and I. M. Lifshitz, \textit{Sov. Phys. JETP. }\textbf{29}, 1107-1113 (1969).} The term “quantum crystal” was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal.\footnote{J. de Boer, \textit{Physica }\textbf{14}, 139-148 (1948).} The main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communication\footnote{F. M. Mutunga, S. E. Follett, D. T. Anderson, \textit{J. Chem. Phys. }\textbf{139}, 151104-4 (2013).} on the kinetics of the H + NO reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the \textit{in situ} photodissociation of NO and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze NO doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + NO reaction shows an inverse dependence on the NO concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogen\footnote{T. Kumada et. al., \textit{J. Chem. Phys. }\textbf{116}, 1109-1119 (2002).} and more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions
Signatures Of Hydrogen Atom Quantum Diffusion: H + N<sub>2</sub>o Reaction In Solid Parahydrogen
No full textIn 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles.\footnote{A. F. Andreev and I. M. Lifshitz, \textit{Sov. Phys. JETP. }\textbf{29}, 1107-1113 (1969).} The term “quantum crystal” was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal.\footnote{J. de Boer, \textit{Physica }\textbf{14}, 139-148 (1948).} The main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communication\footnote{F. M. Mutunga, S. E. Follett, D. T. Anderson, \textit{J. Chem. Phys. }\textbf{139}, 151104-4 (2013).} on the kinetics of the H + NO reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the \textit{in situ} photodissociation of NO and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze NO doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + NO reaction shows an inverse dependence on the NO concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogen\footnote{T. Kumada et. al., \textit{J. Chem. Phys. }\textbf{116}, 1109-1119 (2002).} and more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions.Made available in DSpace on 2021-09-24T21:08:59Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-23Made available in DSpace on 2022-01-21T16:11:07Z (GMT). No. of bitstreams: 4
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Previous issue date: 2021-06-2
Creighton School of Law Class of 1953
No full textGraduates|Adcock, Orie C.; Benton, Donald E.; Borer, John J., Jr.; Egan, Daniel J.; Fennell, John F.; McNair, Dan B.; Merriman, James E.; Naviaux, John L.; O'Brien, David F.; O'Brien, Richard J., Jr.; Pedersen, Henry F., Jr.; Peters, Peter J.; Rasmussen, Wendell A.; Stehno, Richard G.; Strom, Lyle E.; Toland, Paul; Verzani, Phyllis M.; Wolbers, Donald G.; Bloch, Norman (not pictured); Elworth, Herbert J. (not pictured); Welch, Edward J. (not pictured)|17 x 27 in. (portrait
The temperature dependence of the H + N2O reaction in solid hydrogen
No full textMade available in DSpace on 2019-07-15T22:16:33Z (GMT). No. of bitstreams: 2
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Previous issue date: 2019-06-18Made available in DSpace on 2020-01-25T19:30:37Z (GMT). No. of bitstreams: 4
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Previous issue date: 2019-06-18In the late 1960s Andreev and Lifshitz predicted that at sufficiently low temperatures defects in quantum crystals such as solid parahydrogen should move freely through the crystal possessing the property of superfluidity.\footnote{A. F. Andreev and I. M. Lifshitz, \textit{Sov. Phys. JETP. }\textbf{29}, 1107-1113 (1969).} The hydrogen atom (H-atom) is an ideal candidate for such a defect owing to its small mass and neutral charge. In 2013 our group published a communication\footnote{F.M. Mutunga, S.E. Follett, D.T. Anderson, \textit{J. Chem. Phys. }\textbf{139}, 151104-4 (2013).} on the kinetics of the H + NO reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the \textit{in situ} photodissociation of NO and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze NO doped parahydrogen solids with a short duration of 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample after photolysis we observe an abrupt onset to the reaction at temperatures below 2.4 K. This change in the reaction kinetics is fully reversible with temperature. We have subsequently improved our experimental apparatus such that we can record the sample temperature with millisecond time resolution while we measure the reaction kinetics using FTIR spectroscopy. We have now performed a number of additional kinetic experiments at constant temperatures of 1.5 K, 4.0 K, and intermediate temperatures within the range from 1.5 to 4.0 K. These measurements have shown that the reaction yield changes dramatically over this temperature range, but the kinetic rate coefficients do not change significantly. The remarkable change in the reaction kinetics with temperature is not as abrupt as originally thought, but now has been reproduced under a variety of conditions. This strange behavior is intimately linked to the motion and reactivity of H-atoms in solid parahydrogen and the most recent experiments and analysis will be presented
Nuclear spin conversion of propyne in solid parahydrogen
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Previous issue date: 2019-06-18We report observations of propyne (\chem{H_3CCCH}) nuclear spin conversion (NSC) in solid parahydrogen (para-\chem{H_2}) at 1.7 K via high-resolution matrix isolation infrared spectroscopy. A rapid vapor deposition technique is used to codeposit room temperature \chem{H_3CCCH} and precooled para-\chem{H_2} gas streams onto a cold substrate maintained below 2.4 K with flow rates that ensure the expeditious growth of monomer-doped solids. This study will focus on the and modes of propyne in the methyl \chem{C–H} stretching region near 3.4 m, which correspond to parallel and perpendicular rovibrational bands, respectively. For both bands, temporal changes in peak intensities are detected that are characteristic of NSC. In this way, NSC can be used to assign peaks originating from \textit{K}=0 (\textit{A}, \textit{I}=3/2) and \textit{K}=1 (\textit{E}, \textit{I}=1/2) levels, even when absorptions are strongly overlapping. Based on these observations, the fine structure observed in these two bands can be assigned to \textit{K}-rotational structure. At these temperatures, the \textit{K}=1 rotational state should not be populated without nuclear spin restrictions on the total wavefunction. Thus, the slow NSC process allows the \textit{K}=1 level population to be partially trapped in the low-temperature solid. The observation of this NSC process means that the \textit{K} rotational quantum number is at least partially conserved, indicating \chem{H_3CCCH} rotates about its symmetry axis in the para-\chem{H_2} matrix. The extracted time constant for NSC (=270(10) min) is within an order of magnitude of measurements for other methyl-rotors (\chem{H_3CX}; \chem{X} = \chem{H};\footnote{Y. Miyamoto, M. Fushitani, D. Ando, T. Momose, \textit{J. Chem. Phys. }\textbf{128}, 114502 (2008).} \chem{F};\footnote{Y.-P. Lee, Y.-J. Wu, J.T. Hougen, \textit{J. Chem. Phys. }\textbf{129}, 104502 (2008).} \chem{OH};\footnote{Y.-P. Lee, Y.-J. Wu, R.M. Lees, L.-H. Xu, J.T. Hougen, \textit{Science }\textbf{311}, 365 (2006).} \chem{C(O)CH=COHCH_3}\footnote{R.R. Lozada-Garcia, J. Ceponkus, M. Chevalier, W. Chin, J.-M. Mestdagh, C. Cr\'{e}pin, \textit{Angew. Chem. Int. Ed. }\textbf{51}, 6947 (2012).}) trapped in para-\chem{H_2} matrices, however, this is the fastest rate of relaxation measured to date. These findings are discussed in light of accepted models for NSC and the various rovibrational selection-rules for the above-mentioned molecules
High Resolution Infrared Spectroscopy Of Oc-hoh Trapped In Solid Parahydrogen: Coherent Tunneling In A Quantum Solid
No full textOne of the main objectives in the study of weakly bound complexes is to provide a quantitative description of the (ro)vibrational dynamics which can be dominated by nuclear quantum effects. For example, water and carbon monoxide form a weakly bound complex where the HO moiety can undergo a large-amplitude tunneling motion within the complex that exchanges the free and bound hydrogen atoms in the intermolecular bond. The exchange symmetry of identical particles entangles the spin and spatial quantum states of HO such that in the ground vibrational state, the symmetric tunneling state A correlates with para-HO (I=0), while the antisymmetric tunneling state B correlates with ortho-HO (I=1). The gas phase water-CO complex has been extensively studied by microwave\footnote{D. Yaron, K. I. Peterson, D. Zolandz, W. Klemperer, F. J. Lovas, R. D. Suenram, \textit{J. Chem. Phys. }\textbf{92}, 7095 (1990).}\footnote{R. E. Bumgarner, S. Suzuki, P. A. Stockman, P. G. Green, G. A. Blake, \textit{Chem. Phys. Lett. }\textbf{176}, 123 (1991).} and IR spectroscopy\footnote{M. D. Brookes, A. R. W. McKellar, \textit{J. Chem. Phys. }\textbf{109}, 5823 (1998).}\footnote{L. Oudejans, R. E. Miller, \textit{Chem. Phys. Lett. }\textbf{306}, 214 (1999).}\footnote{A. J. Barclay, A. van der Avoid, A. R. W. McKellar, N. Moazzen-Ahmadi, \textit{Phys. Chem. Chem. Phys. }\textbf{21}, 14911 (2019).} and when compared with full-dimensional quantum bound state calculations on a nine-dimensional potential energy surface, the agreement is very good.\footnote{P. M. Felker, Z. Bacic, \textit{J. Chem. Phys. }\textbf{153}, 074107 (2020).} We have completed analogous IR studies of the tunneling splittings of the water-CO complex when the complex is isolated in a parahydrogen quantum solid. We can estimate the tunneling splittings in the ground and excited (CO stretch, water stretch, and water bend) vibrational states to see how the tunneling motion is perturbed by the presence of the quantum solid. Furthermore, we can examine the nuclear spin conversion kinetics between the two tunneling levels in the ground vibrational state by rapidly changing the temperature of the sample. Nuclear spin conversion has not been reported in the previous gas phase studies and thus these are the first results for this water-CO complex.Made available in DSpace on 2021-09-24T21:09:58Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-2
The Role of Self-regulation in Doctoral Students' Status of All But Dissertation
No full textA large amount of research has been conducted on self-regulated learning as it relates to academic achievement. Further, there is a large body of literature regarding doctoral candidates who do not complete their dissertations and are classified as All But Dissertation. However, there is a paucity of research regarding the synthesis of these two disparate bodies of research. The purposes of this study were to: (a) assess self-regulated learning as it applied to the time to completion of the dissertation, (b) determine the relationship between self-regulated learning and the intrinsic task value of the dissertation, (c) assess the impact of intrinsic task value upon time to completion of the dissertation, (d) determine whether there was a linear relationship between self-regulated learning and time to complete the dissertation, (e) assess whether differences existed between the levels of self-regulated learning strategies exhibited by those who were classified as All But Dissertation and those who had recently completed their dissertations and (f) assess whether there were differences in the time since comprehensive exams were taken for those who were All But Dissertation and those participants who had recently completed their dissertations.
The researcher developed an online survey (titled the Dissertation Enablers Scale) for this purpose. This survey included four subscales to operationalize the variables and covariates (the Self-regulated Learning Scale, the Intrinsic Task Value Scale, the Research Self-efficacy Scale and the Social Support Scale). This survey was validated through comparison with like instruments, expert review, exploratory factor analyses and reliability estimates.
The results of hierarchical regressions indicated that self-regulated learning did predict time to completion of the dissertation, yet intrinsic task value did not predict time to completion of the dissertation. Self-regulated learning was significantly correlated with task value. The findings suggest that there was a linear relationship between self-regulated learning and time to completion of the dissertation. A discriminant function analysis revealed that there were no differences in the use of self-regulated learning strategies between those classified as All But Dissertation and those participants who had recently completed their dissertations.
Additional factors that emerged as important to dissertation completion were the importance of financial support and the potential mediating role of intrinsic task value upon self-regulated learning
Stromové indexační metody pro podobnostní vyhledávání
No full textTitle: Tree-based Indexing Methods for Similarity Search in Metric and Nonmetric Spaces Author: Mgr. Jakub Lokoč Department: Department of Software Engineering Faculty of Mathematics and Physics Charles University in Prague Supervisor: Doc. RNDr. Tomáš Skopal, Ph.D. Author's e-mail address: [email protected] Supervisor's e-mail address: [email protected] Abstract: The M-tree is a well-known indexing method enabling efficient similarity search in metric spaces. Although the M-tree is an aging method nowadays, we believe it still offers an undiscovered potential. We present sev- eral approaches and directions that show how the original M-tree algorithms and structure can be improved. To allow more efficient query processing by the M-tree, we propose several new methods of (parallel) M-tree construction that achieve more compact M-tree hierarchies and preserve acceptable con- struction cost. We also demonstrate that the M-tree can be simply extended to a new indexing method - the NM-tree, which allows efficient nonmetric similarity search by use of the TriGen algorithm. All these experimentally verified improvements show that the M-tree can still be regarded as an im- portant dynamic metric access method suitable for management of large collections of unstructured data. Moreover, all the improvements can be...Název: Stromové indexační metody pro podobnostní vyhledávání v metrických a nemetrických prostorech Autor: Mgr. Jakub Lokoč Katedra: Katedra softwarového inženýrství Matematicko-fyzikální fakulta Univerzita Karlova v Praze Školitel: Doc. RNDr. Tomáš Skopal, Ph.D. Email autora: [email protected] Email školitele: [email protected] Abstrakt: M-strom je dnes již klasická indexační metoda používaná pro efektivní podobnostní vyhledávání v metrických prostorech. Ačkoliv M- strom již nepatří mezi nejnovější metody, věříme, že stále nabízí zatím neob- jevený potenciál. V této práci se proto zaměřujeme na způsoby, jak vylepšit jeho původní algoritmy a strukturu. Abychom umožnili rychlejší zpracování dotazů pomocí M-stromu, navrhli jsme několik nových metod jeho konstrukce (i paralelních), které vedou k vytváření kompaktnějších metrických hierar- chií a přitom nejsou extrémně drahé. Dále jsme ukázali snadný způsob, jak rozšířit M-strom na novou indexační metodu NM-strom, která slouží k efektivnímu nemetrickému podobnostnímu vyhledávání za pomocí algo- ritmu TriGen. Všechna tato experimentálně ověřená vylepšení prokazují, že můžeme M-strom stále ještě považovat za důležitou dynamickou met-...Department of Software EngineeringKatedra softwarového inženýrstvíFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult
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