1,720,967 research outputs found
Axially nonadiabatic channel treatment of low-energy capture in ion-rotating diatom collisions
No full textThe quantum version of an axially nonadiabatic channel (ANC) approximation, introduced in an earlier article for the calculation of complex-formation cross sections and rate constants in ion-diatom collisions [Maergoiz, A. I.; Nikitin, E. E.; Troe, J.; Ushakov, V. G. J. Chem. Phys. 2002, 117, 4201-4213] is tested against accurate quantum results. Cross sections and rate constants are determined for several representative systems with the participation of a diatom in the state j = 1, assuming various long-range potentials between the collision partners, such as anisotropic ion-induced dipole, second-order charge-permanent dipole, and first-order charge-quadrupole interaction. The ANC approximation well reproduces accurate quantum results in the perturbed rotor limit, while the standard quantum adiabatic channel (AC) approximation fails at low energy due to neglect of Coriolis coupling. However, the low-energy extrapolation of classical adiabatic channel results (ACCI) provides a reasonable approximation both to accurate and quantal ANC results down to collision energies when only few partial cross sections determine the total capture cross section. This unexpected feature of the ACCI approximation is due to two effects: (a) an artificial simulation of tunneling transmission and overbarrier reflection at centrifugal barriers by introducing a continuous distribution over total angular momenta and (b) a slight effective lowering of the centrifugal barriers compared to centrifugal barriers within the AC model. Low-temperature quantum rate constants are also presented
Energy dependence of the intramultiplet mixing cross section in Hg-N(2) collisions: Conical intersection mechanism, indication at tunneling and interference
No full textExperimental results on the energy dependence of the cross section for the intramultiplet transitions in Hg atom induced by collisions with the rotationally cold nitrogen molecules, (3)P(1),nu = 0 --> (3)P(0), nu = 1, nu being the vibrational quantum number of N(2) (M. Okunishi, J. Hashimoto, H. Chiba, K. Ohmori, K. Ueda and Y. Sato, J. Phys. Chem. A, 1999, 103, 1742.) are interpreted within rotationally sudden versions of the Landau-Zener and generalized Landau-Zener-Teller models. The broad threshold region of the cross section is ascribed to noticeable dependence of the avoided crossing seam energy on the collision angle between two conical intersection points (linear and T-shaped configurations) and to the tunneling/interference effects in the transition probability. The experimental energy dependence of the cross section is consistent with the anticipated values of the crossing-point energy at the optimal configuration
Classical diffusion model of vibrational predissociation of van der Waals complexes: truncated mean first passage time approximation
No full textA classical diffusion theory of the vibrational predissociation for a triatomic van der Waals (vdW) complex A-BC is presented under the condition that the mean square energy transferred to the low-frequency oscillator per one "encounter' with the repulsive part of the A-BC potential is small compared with the square of its dissociation energy. Unlike the case of a thermal dissociation of diatomic molecules in a heat bath, or from the case of fission of a strong bond in an isolated polyatomic molecule when the dissociation rate is given by the inverse of the mean first passage time, in the considered case of vibrational predissociation the dissociation energy is lower than the "thermal' energy of the heat bath. For the present situation, a truncated first mean time approximation is developed and tested against numerical solutions of the respective diffusion equation. It is shown that the truncated first mean time approximation provides a reasonable approximation to the predissociation rate, provided that the relevant diffusion coefficient depends comparatively strongly on the energy of the vdW oscillator. This approach allows one to relate directly the mean square energy transferred to the vdW bond per one encounter in the initial energy state of the vdW oscillator to the rate of predissociation from this state
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Classical diffusion model of vibrational predissociation of van der Waals complexes
No full textThe quantum and classical expressions for the vibrational predissociation rates of two-degree-of-freedom triatomic van der Waals complexes are compared. The classical rate is approximated by its diffusional counterparts (Parts I and II of this series), while the expression for the quantum rate is based on the generalization of the Landau method (E. E. Nikitin and L. Pitaevski, Phys. Rev. A, 1994, 49, 695). The ratio of the classical to quantum rates is expressed by an analytical formula which shows the reason for the difference between the classical and quantum rates and which explains a similar trend of both rates with the change of the key parameters of the vibrational predissociation event. We conclude that the occasional numerical closeness of classical and quantum rates results from the delicate interplay between the classical diffusion through a narrow energy layer and quantum tunneling through the broad energy range
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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