1,720,986 research outputs found
Confined H(1s) and H(2p) under different geometries
No full textIn this paper the diffusion Monte Carlo method is applied to the confined hydrogen atom with different confinement geometries. This approach is validated using the much studied spherical and cylindrical confinements and then applied to cubical and squared ones, for which data are not available, as new applications of the method relevant to solid state physics. The energy eigenvalues of the ground state and one low-lying excited state are reported as a function of the characteristic confinement length
Spherically confined H2+: 2σ +g and 2σ +u states
No full textThe molecular ion H2+ is studied under strong confinement conditions produced by a spherical barrier centered in the gravity center of the molecule. Results for the potential curves are obtained by diffusion Monte Carlo methods for the ground state (X) and the first excited state (A), and reported as functions of the internuclear distance d for different values of the confinement radius. Results show that the compressed states corresponding to both and present deep minima in their potential curves, due to the increased space for electron wave-functions when the protons are displaced from the barrier surface
Thermal decomposition rate of MgCO3 as an inorganic astrobiological matrix in meteorites
No full textCarbonate minerals, likely of hydrothermal origins and included into orthopyroxenite, have been extensively studied in the ALH84001 meteorite. In this meteorite, nanocrystals comparable with those produced by magnetotactic bacteria have been found into a carbonate matrix. This leads naturally to a discussion of the role of such carbonates in panspermia theories. In this context, the present work sets the basis of a criterion to evaluate whether a carbonate matrix in a meteor entering a planetary atmosphere would be able to reach the surface. As a preliminary step, the composition of carbonate minerals in the ALH84001 meteorite is reviewed; in view of the predominance of Mg in these carbonates, pure magnesite (MgCO3) is proposed as a mineral model. This mineral is much more sensitive to high temperatures reached during an entry process, compared with silicates, due to facile decomposition into MgO and gaseous carbon dioxide (CO2). A most important quantity for further studies is therefore the decomposition rate expressed as CO2 evaporation rate J (molecules/m2 s). An analytical expression for J(T) is given using the Langmuir law, based on CO2 pressure in equilibrium with MgCO3 and MgO at the surface temperature T. Results suggest that carbonate minerals rich in magnesium may offer much better thermal protection to embedded biological matter than silicates and significantly better than limestone, which was considered in previous studies, in view of the heat absorbed by their decomposition even at moderate temperatures. This first study can be extended in the future to account for more complex compositions, including Fe and Ca.I minerali carbonatici, probabilmente di origine idrotermale e inclusi nell'ortopropossenite, sono stati ampiamente studiati nel meteorite ALH84001. In questo meteorite, i nanocristalli paragonabili a quelli prodotti dai batteri magnetotattici sono stati trovati in una matrice carbonatica. Ciò porta naturalmente a una discussione sul ruolo di tali carbonati nelle teorie della panspermia. In questo contesto, il presente lavoro pone le basi di un criterio per valutare se una matrice carbonatica in una meteora che entra in un'atmosfera planetaria sarebbe in grado di raggiungere la superficie. Come fase preliminare, viene rivista la composizione dei minerali di carbonato nel meteorite ALH84001; in considerazione della predominanza di Mg in questi carbonati, la magnesite pura (MgCO3) viene proposta come modello minerale. Questo minerale è molto più sensibile alle alte temperature raggiunte durante un processo di entrata, rispetto ai silicati, a causa della facile decomposizione in MgO e anidride carbonica gassosa (CO2). Una quantità molto importante per ulteriori studi è quindi il tasso di decomposizione espresso come tasso di evaporazione della CO2 J (molecole / m2 s). Un'espressione analitica per J (T) è data usando la legge di Langmuir, basata sulla pressione di CO2 in equilibrio con MgCO3 e MgO alla temperatura superficiale T. I risultati suggeriscono che i minerali di carbonato ricchi di magnesio possono offrire una protezione termica molto migliore alla materia biologica incorporata rispetto a silicati e significativamente migliori del calcare, che era stato considerato in studi precedenti, in considerazione del calore assorbito dalla loro decomposizione anche a temperature moderate. Questo primo studio può essere esteso in futuro per tenere conto di composizioni più complesse, tra cui Fe e Ca
Lexical access to large vocabularies for speech recognition
No full textA large vocabulary isolated word recognition system based on the hypothesize-and-test paradigm is described. The system has been, however, devised as a word hypothesizer for a continuous speech
understanding system able to answer to queries put to a geographical database. Word preselection is achieved by segmenting and classifying the input signal in terms of broad phonetic classes. Due to low redundancy of this phonetic code for lexical access, to achieve high performance, a lattice of phonetic segments is generated, rather than a single sequence of hypotheses. It can be organized as a graph, and word hypothesization is obtained by matching this graph against the models of all vocabulary words. A word model is itself a phonetic representation made in terms of a graph accounting for deletion, substitution, and insertion errors. A modified Dynamic Programming (DP) matching procedure gives an efficient solution to this graph-to-graph matching problem. Hidden Markov Models (HMM's) of subword units are used as a more detailed knowledge in the verification step. The word candidates generated by the previous step are represented as sequences of diphone-like subword units, and the Viterbi algorithm is used for evaluating their likelihood. To reduce storage and computational costs, lexical knowledge is organized in a tree structure where the initial common
subsequences of word descriptions are shared, and a beam-search
strategy carries on the most promising paths only. The results show
that a complexity reduction of about 73 percent can be achieved by
using the two pass approach with respect to the direct approach, while the recognition accuracy remains comparable
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