Papers by Nicolais Guevara
Http Dx Doi Org 10 1080 00268970701335771, May 20, 2007
Shannon entropies of one- and two-electron atomic structure factors in the position and momentum ... more Shannon entropies of one- and two-electron atomic structure factors in the position and momentum representations are used to examine the behaviour of the off-diagonal elements of density matrices with respect to the uncertainty principle and to analyse the effects of radial correlation on off-diagonal order. We show that radial correlation induces off-diagonal order in position space which is characterized by larger entropic values. Radial correlation in momentum space is characterized by smaller entropic values as information is forced into regions closer to the diagonal. Related off-diagonal correlation functions are also discussed.
Phys Rev a, 2006
We present a detailed study of the ground state of the Coulomb system (ααee) which corresponds to... more We present a detailed study of the ground state of the Coulomb system (ααee) which corresponds to the He22+ molecular ion in a magnetic field B=0-4.414×1013G in parallel configuration (it is assumed that infinitely massive α particles are situated along a magnetic-field line). One uses a variational method, introducing a trial function which includes electronic correlation in the form exp(γr12) , where γ is a variational parameter. It is shown that the quantum numbers of the lowest total-energy state depend on the magnetic-field strength. This state evolves from the spin-singlet Σg1 metastable state at 0≤B≲0.85a.u. to a repulsive spin-triplet Σu3 state for 0.85≲B≲1100a.u. and, finally, to a stable strongly bound spin-triplet Πu3 state at stronger fields B≳1100a.u. The lowest vibrational energy for the latter case is calculated.
Phys Rev a, 2005
An accurate study of the lowest $1\sigma_g$ and the low-lying excited $1\sigma_u$, $1\pi_{u,g}$, ... more An accurate study of the lowest $1\sigma_g$ and the low-lying excited $1\sigma_u$, $1\pi_{u,g}$, $1\delta_{g,u}$ electronic states of the exotic molecular ion $H_3^{2+}$ in linear configuration parallel to a magnetic field is carried out. The magnetic field ranges from $10^{10}$ G up to $4.414 \times 10^{13}$ G where non-relativistic considerations are justified. The variational method is exploited and the {\it same} trial function is used for different magnetic fields. It is shown that the states of positive $z$-parity $1\sigma_g, 1\pi_u, 1\delta_{g}$ are bound states of the $H_3^{2+}$ exotic ion for all magnetic fields studied. We also demonstrate that for magnetic fields $B\gtrsim 2.35\times 10^{12}$ G the potential energy surface well corresponding to the $1\sigma_g$ state contains at least one longitudinal vibrational state. It is also shown that the negative $z$-parity states $1\sigma_u, 1\pi_g, 1\delta_{u}$, are purely repulsive in the whole range of magnetic fields studied, $B=10^{10}- 4.414 \times 10^{13}$ G.
Conditional entropies and position-momentum correlations in atomic systems
Http Dx Doi Org 10 1080 00268970903153675, Sep 21, 2009
We show via Wigner functions how the sum and difference between Shannon entropies in position and... more We show via Wigner functions how the sum and difference between Shannon entropies in position and in momentum space are related to conditional entropies, and how these quantities are linked to position–momentum correlations. The connection between Fermi, Coulomb and position–momentum correlations on these quantities is made, and the impact of these are discussed in atomic systems.
Relative entropy and atomic structure
Journal of Molecular Structure: THEOCHEM, 2008
Relative entropy, using a hydrogen-like model density as the reference, is studied in atomic syst... more Relative entropy, using a hydrogen-like model density as the reference, is studied in atomic systems. We find that both the position and momentum space measures display the shell effects, being similar in behavior to the expectation value, 〈r〉. We also show that the electronic momentum density is closer to the hydrogen-like reference than is the position space density. A connection
Charge exchange in H^++ He^+ collision
ABSTRACT Charge exchange in H^+ + He^+ collision are investigated theoretically at projectile ene... more ABSTRACT Charge exchange in H^+ + He^+ collision are investigated theoretically at projectile energies below the ionization threshold at about 100 keV/amu. The electron nuclear dynamics (END) method is used to analyze the collision processes. Total charge exchange cross sections were calculated and compared with other theoretical and experimental data.
Charge exchange in C^ 6++ H and C^ 6++ H2 collisions
ABSTRACT In the solar wind, C^6+ ion is one of the most abundant ionic species and its interactio... more ABSTRACT In the solar wind, C^6+ ion is one of the most abundant ionic species and its interaction with comets as well as the atmosphere of planets of the solar system produces several interesting phenomena. The charge exchange reaction is one of the most relevant process as it may provide a possible explanation for the X-ray emission from these objects. Electron capture into a highly excited state of C^5+ ion usually generates radiation in the X-ray region of the spectrum. In the present work, charge exchange in C^6+ + H and C^6+ + H2 collisions are investigated theoretically using electron nuclear dynamics (END) [1] at projectile energies below the ionization threshold. For H2 the one- and two- electron charge exchange cross sections are calculated and compared with other theoretical and experimental data. Orientation effects for the collision with the hydrogen molecules will also be discussed at the conference.
Three-body recombination in He+ H+ H
Beyond the Efimov effect: an infinity of trimer states from subcritical attractive 1/r^ 2 interactions
ABSTRACT We have investigated the spectrum of three identical bosons interacting through a pair-w... more ABSTRACT We have investigated the spectrum of three identical bosons interacting through a pair-wise sum of two-body attractive 1/r^2 potentials. We have found an infinite number of three-body bound states even when the two-body interaction does not support any dimer state. The effect we have found thus resembles the Efimov effect, but it is not the Efimov effect since no scattering length can be defined in the usual sense for an attractive 1/r^2 potential. Moreover, we show that the effective three-body potential is more attractive asymptotically than the Efimov potential. When the two-body interaction is strong enough to support dimer states, a geometrically-spaced Efimov-like spectrum appears yielding an infinite sequence of three-body bound states. We have also found that three identical fermions interacting via a subcritical attractive 1/r^2 potential produces an infinite number of three-body bound energies, geometrically spaced like the Efimov effect. We have thus identified a new class of three-body states distinct from those discussed before, yet possessing some of the peculiar properties of Efimov effect.
A diabatic hyperspherical method for three-body recombination rates: application to H+ H+ He-> H2+ He
ABSTRACT The formation of the hydrogen molecule through three-body recombination reactions was ve... more ABSTRACT The formation of the hydrogen molecule through three-body recombination reactions was very important in the early universe. During the last decade, we have seen important progress in the calculation of three-body recombination rates. Most of them, however, have been focused on ultracold temperatures. Here, we present a method to study three-body recombination reactions that can handle temperatures of astrophysical interest and also many diatomic states. Our method introduces diabatic states in hyperspherical coordinates based on physical arguments to simplify the numerical calculations. In the present work, we have studied the reaction H + H + He -> H2 + He in order to test the utility and efficiency of our approach. Prospects for extending our method to other systems will be discussed.
A new class of three-body states beyond the Efimov effect
ABSTRACT Recently, we have identified a new type of three-body bound state for three identical bo... more ABSTRACT Recently, we have identified a new type of three-body bound state for three identical bosons interacting via attractive two-body 1/r^2 potentials [1]. These three-body states are bound even when the two-body subsystem does not support a dimer state. In fact, there are an infinity of such states. We will present an extension of this work to the system with two identical bosons (B) and one distinguishable particle (X). We have investigated the spectrum of this BBX system assuming only that the B+X interaction is an attractive 1/r^2 potential. We have again found an infinite number of three-body bound states even though the two-body potential does not support a bound state. This effect is shown to exist at large mass ratios (MB/MX) and depends on the strength of the two-body interaction. The most favorable case is the molecular-type system, i.e., MB/MX1. While these new three-body states resemble Efimov states they originate from fundamentally different physics.[4pt] [1] N. L. Guevara, Yujun Wang, and B. D. Esry, arXiv:1110.0476 (2011)
A diabatic hyperspherical method for three-body recombination rates: application to H $+ $ H $+ $ He $\ rightarrow $ H $ _2 $ $+ $ He
ABSTRACT The formation of the hydrogen molecule through three-body recombination reactions was ve... more ABSTRACT The formation of the hydrogen molecule through three-body recombination reactions was very important in the early universe. During the last decade, we have seen important progress in the calculation of three-body recombination rates. Most of them, however, have been focused on ultracold temperatures. Here, we present a method to study three-body recombination reactions that can handle temperatures of astrophysical interest and also many diatomic states. Our method introduces diabatic states in hyperspherical coordinates based on physical arguments to simplify the numerical calculations. In the present work, we have studied the reaction H + H + He -> H2 + He in order to test the utility and efficiency of our approach. Prospects for extending our method to other systems will be discussed.
Electron pair density information measures in atomic systems
Statistical correlation between atomic electron pairs
The statistical correlation between a pair of electrons in Hartree–Fock orbitals is measured by m... more The statistical correlation between a pair of electrons in Hartree–Fock orbitals is measured by mutual information and studied in position and in momentum space. We show that there are same- and opposite-spin orbital pairs where the correlation is larger in momentum space. Among these are opposite-spin valence shell pairs where the correlation arises from the indistinguishability of electron spins.
Molecular Connectivity Indices of Iterated Line Graphs. A New Source of Descriptors for QSPR and QSAR Studies
SAR and QSAR in Environmental Research, 1998
Connectivity indices χ of the line graph L and higher line graphs L , n = 2, 3,…, of the molecula... more Connectivity indices χ of the line graph L and higher line graphs L , n = 2, 3,…, of the molecular graph G are examined as possible structure-descriptors in QSPR and QSAR studies. In the case of alkanes, regression models based on χ(G), χ(L), χ(L), χ(L ) and χ(L4) are found to be significantly better than those based solely on the index χ(G), with χ(G) continuing to play an important role. This has been demonstrated for boiling point, molar volume, molar refraction and surface tension. The same model was then applied for predicting the microsomal p-hydroxylation of aniline by alcohols. The QSAR model obtained by using connectivity indices of higher line graphs represents a significant improvement respect to the model using solely χ(G).
Physical Review Letters, 2012
We calculate the three-body spectrum for identical bosons interacting via attractive 1/r 2 potent... more We calculate the three-body spectrum for identical bosons interacting via attractive 1/r 2 potentials. We have found an infinite number of three-body states even when the pair interactions are too weak to support any two-body states. These new states thus share this surprising scenario with the Efimov effect, but are not themselves Efimov states. Our effect occurs for both identical bosons and identical fermions, and it persists in the presence of two-body bound states.
Physical Review A, 2011
In non-relativistic quantum mechanics we study the Coulomb systems of infinitely massive center o... more In non-relativistic quantum mechanics we study the Coulomb systems of infinitely massive center of charge Z and two-three electrons: (Z, e, e) and (Z, e, e, e). It is shown that in both cases the total energy curve in Z is smooth, without any visible irregularities. Thus, for both systems the physical integer charges Z = 1, 2, . . . do not play a distinguished role as would be associated with charge quantization. By definition, a critical charge Z cr is a charge which separates a domain of the existence of bound states from a domain of unbound ones (continuum). For both systems the critical charges are found, Z cr,2e = 0.91085 and Z cr,3e = 2.009, respectively. Based on numerical analysis, the Puiseux expansion in fractional powers of (Z − Z cr ) is constructed for both systems.
Physical Review A, 2006
A detailed study of the ground state of the Coulomb system (ααee) which corresponds to the He 2+ ... more A detailed study of the ground state of the Coulomb system (ααee) which corresponds to the He 2+ 2 molecular ion in a magnetic field B = 0 − 4.414 × 10 13 G in parallel configuration (infinitely massive α−particles are situated along a magnetic field line) is presented. The variational method is employed using a trial function which includes electronic correlation in the form exp (γr 12 ) where γ is a variational parameter. It is shown that the quantum numbers of the lowest total energy state depend on the magnetic field strength. It evolves from the spin-singlet 1 Σ g metastable state at 0 ≤ B 0.85 a.u. to a repulsive spin-triplet 3 Σ u state for 0.85 a.u.
Shannon-information entropy sum as a correlation measure in atomic systems
Physical Review A, 2003
Shannon-information entropy sum as a correlation measure in atomic systems. Nicolais L. Guevara, ... more Shannon-information entropy sum as a correlation measure in atomic systems. Nicolais L. Guevara, Robin P. Sagar, and Rodolfo O. Esquivel Departamento de Química, Universidad Autónoma Metropolitana, Apartado Postal 55-534, Iztapalapa, 09340 México DF, Mexico. ...
Multiple electron transfer processes in collisions of N^{6+} and O^{7+} with methane
Physical Review A, 2009
ABSTRACT Recent experiments on collision processes of O7+ and N6+ ions colliding with methane at ... more ABSTRACT Recent experiments on collision processes of O7+ and N6+ ions colliding with methane at the same velocity show unexpected differences in the fragmentation cross sections of the methane. Despite the expected similarity of these two processes, as both projectiles are hydrogenic, the mechanisms of electron transfer are different and lead to different fragmentation cross sections. In the present work, the collisions between N6+ and O7+ ions and methane are investigated theoretically at equal velocities corresponding to projectile energies of 30 and 35 keV, respectively. Electron-nuclear dynamics is used to study multiple electron transfer processes occurring in these collisions. Several multiple charge transfer probabilities are calculated and results, averaged over various orientations of the methane molecule, are reported. The collisions proceed in two stages: a fast stage of electron transfer from methane to the ion, and a much slower stage of breakup of the methane. We find and explain the intuitively unexpected result that the total charge transfer cross section for N6+ is slightly larger, but that the O7+ leaves the methane in a higher charged state with higher probability, leading to more fragmentation in the collisions with O7+.
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Papers by Nicolais Guevara