John Delos | College of William and Mary (original) (raw)

Papers by John Delos

The Journal of Chemical Physics, 1967

Three quantum-mechanical computational techniques based on energy moments, μk= ∫ dqψ*(q)Hkψ(q), a... more Three quantum-mechanical computational techniques based on energy moments, μk= ∫ dqψ*(q)Hkψ(q), and semimoments, νk(q′)=[Hkψ(q)]q=q′, are formulated. The μ method, which employs the μk, is connected to the method of moments in probability theory, to the variational method, and to eigenvalue spectroscopy. The ν and λ methods, which employ semimoments, are related to local energy methods using one and several configuration points, respectively. An Nth-order calculation, requiring 2N moments or semimoments, yields N approximate eigenvalues and eigenfunctions. In accordance with a conjectured convergence criterion, exact eigenstates are approached in the limit N→∞. From quantities obtained in a moments calculation, a lower bound on the ground-state eigenvalue can also be determined using a refinement of Weinstein's criterion. A computational method for generating moments and semimoments is given and the μ method is applied to the linear harmonic oscillator.

The spin-forbidden reaction revisited. II. Nonadiabatic transition state theory and application

Physical Review A, 1994

ABSTRACT Total electron-detachment cross sections sigmae for collisions of F-, Cl-, Br-, and I- w... more ABSTRACT Total electron-detachment cross sections sigmae for collisions of F-, Cl-, Br-, and I- with atomic hydrogen have been measured for relative collision energies E ranging from 0.1 to 20 eV. For F-, Cl-, and Br-, sigmae is found to decrease with increasing E and is fairly well described by a simple orbiting model in which an effective polarizability is used to describe the interaction of the anion with atomic hydrogen at small internuclear separations. For the system I-+H, sigmae increases with increasing collision energy. The measured sigmae for Cl-+H is also found to agree well with calculations based on an effective-range approximation. Charge-transfer cross sections have been measured for F-, Cl-, and I-+H and are found to be small.

Physical Review A, 1986

The semiclassical local-complex-potential theory has been widely used to describe detachment and ... more The semiclassical local-complex-potential theory has been widely used to describe detachment and ionization in atom-atom and ion-atom collisions. However, it has been shown that the resulting formulas do not conserve probability. In this paper, we show that the problem arises from the inconsistent treatment of the effects of interference, tunneling, and diffraction. A more complete theory is based upon the close-coupling expansion, which leads to an infinite set of coupled equations. A method for solving such sets of equations was developed in earlier work. Here we implement that method using a new iterative numerical scheme, and we show that the iteration converges to results in which probability is conserved.

Physical Review A, 1987

We have calculated the energy spectrum of a highly excited atom in parallel electric and magnetic... more We have calculated the energy spectrum of a highly excited atom in parallel electric and magnetic fields. The eigenvalues were obtained by semiclassical quantization of action variables calculated from first-order classical perturbation theory. For the field strengths studied, the electron moves on a Kepler ellipse whose orbital parameters evolve slowly in time, and first-order perturbation theory reduces the problem to just one degree of freedom. Action variables were calculated from perturbation theory and the eigenvalues were obtained by semiclassical quantization of the action. The semiclassical analysis leads directly to a correlation diagram which connects the eigenstates of the Stark effect to those of the diamagnetic effect. A classification scheme for the eigenstates is proposed. Comparison with first-order degenerate quantum perturbation theory verifies the accuracy of the semiclassical treatment.

Physical Review Letters, 1987

Order Number 8SU390T T h e efFert o f c lo s e d c la s s ic a l o r b it s o tt q u a n t u m s ... more Order Number 8SU390T T h e efFert o f c lo s e d c la s s ic a l o r b it s o tt q u a n t u m s p e c t r a : I o n is a t io n o f a to m s in a m a g n e t ic fie ld 1 >U, M etig Li, E' h .D .

Physical Review Letters, 1995

Numerical calculations have shown that bifurcations of periodic orbits of Hamiltonian systems oft... more Numerical calculations have shown that bifurcations of periodic orbits of Hamiltonian systems often occur in organized groups. Normal-form theory provides an explanation.

Physical Review A, 1993

Measurements have been made of the absorption spectrum of Ba from the 6s6p 'P& level to states ne... more Measurements have been made of the absorption spectrum of Ba from the 6s6p 'P& level to states near the ionization threshold in parallel electric and magnetic fields. The absorption spectrum shows oscillations superposed on a smooth background. Each oscillation is correlated with a closed orbit of the electron. At strong electric fields, trajectories are regular, and closed orbits form orderly patterns. For weak electric fields, trajectories are chaotic, and many more closed orbits are present. Many of these are produced by bifurcations as the electric field is reduced.

Physical Review E, 2014

Integrable Hamiltonian systems are said to display nontrivial monodromy if fundamental action-ang... more Integrable Hamiltonian systems are said to display nontrivial monodromy if fundamental action-angle loops defined on phase-space tori change their topological structure when the system is carried around a circuit. In an earlier paper it was shown that this topological change can occur as a result of time evolution under certain rather abstract flows in phase space. In the present paper, we show that the same topological change can occur as a result of application of ordinary forces. We also show how this dynamical phenomenon could be observed experimentally in classical or in quantum systems.

The Journal of Chemical Physics, 1983

Total cross sections for electron detachment in collisions ofH-and 0with Ne are calculated, using... more Total cross sections for electron detachment in collisions ofH-and 0with Ne are calculated, using a model based on a first•order solution to close•coupled equations. Quantities needed for the calculation are the energy gap and the coupling between bound and free states. The energy gap is taken from previous calculations and the coupling is assumed to be of exponential form, with parameters adjusted to fit experimental data. Special examination is made of isotope effects in the cross sections.

Chaos: An Interdisciplinary Journal of Nonlinear Science, 1993

The semiclassical theory of ballistic electron transport in semiconductor microstructures provide... more The semiclassical theory of ballistic electron transport in semiconductor microstructures provides a description of the quantum conductance fluctuations in terms of the classical distributions for the lengths and directed areas of the scattering trajectories. Because the classical dynamics differs for integrable (circular) and chaotic (stadium) scattering domains, experimental measurements of the conductance of these microstructures provide a unique probe of the quantum properties of classically regular and chaotic systems. To advance these theoretical and experimental studies we compare geometrical formulas for the classical distributions of lengths and areas with numerical simulations for microstructures examined in recent experiments, we assess the effects of lead size and placement, aud we provide a critical analysis of the role of scattering "noise" on the classical and semiclassical predictions. Finally, we present a detailed comparison of the semiclassical theory with recent experimental measurements of the conductance fluctuations in circular-and stadium-shaped microstructures.

Physical Review Letters, 2018

A Hamiltonian system is said to have nontrivial monodromy if its fundamental action-angle loops d... more A Hamiltonian system is said to have nontrivial monodromy if its fundamental action-angle loops do not return to their initial topological state at the end of a closed circuit in angular momentumenergy space. This process has been predicted to have consequences which can be seen in dynamical systems, called dynamical monodromy. Using an apparatus consisting of a spherical pendulum subject to magnetic potentials and torques, we observe nontrivial monodromy by the associated topological change in the evolution of a loop of trajectories.

Physical Review E, Feb 10, 2014

We examine a system consisting of two reservoirs of particles connected by a channel. In the chan... more We examine a system consisting of two reservoirs of particles connected by a channel. In the channel are two oscillating repulsive potential-energy barriers. It is known that such a system can transport particles from one reservoir to the other, even when the chemical potentials in the reservoirs are equal. We use computations and the theory of chaotic transport to study this system. Chaotic transport is described by passage around or through a heteroclinic tangle. Topological properties of the tangle are described using a generalization of homotopic lobe dynamics, which is a theory that gives some properties of intermediate-time behavior from properties of short-time behavior. We compare these predicted properties with direct computation of trajectories.

Physical Review A. Atomic, Molecular, and Optical Physics, Oct 18, 2004

We examine excitation (by a short laser pulse) of a hydrogen atom in parallel electric and magnet... more We examine excitation (by a short laser pulse) of a hydrogen atom in parallel electric and magnetic fields, from an initial tightly bound state to a state above the classical ionization threshold. We predict that the atom ionizes by emitting a train of electron pulses. This prediction is based on the classical dynamics of electron escape. In particular, the pulse train is due to classical chaos, which occurs for nonvanishing magnetic field. We connect the structure of the pulse train to fractal structure in the escape dynamics, and discuss several issues of experimental interest, with a particular emphasis on understanding the resolution of individual pulses. A brief account of this work appeared previously as a Letter [Phys. Rev. Lett. 92, 073001 (2004)].

Physical Review E

Almost everything that happens in classical mechanics also shows up in quantum mechanics when we ... more Almost everything that happens in classical mechanics also shows up in quantum mechanics when we know where to look for it. A phenomenon in classical mechanics involves topological changes in action-angle loops as a result of passage around a "monodromy circuit". This phenomenon is known by the short name "Hamiltonian monodromy" (or, more ponderously, "nontrivial monodromy of action and angle variables in integrable Hamiltonian systems"). In this paper, we show a corresponding change in quantum wave functions: these wave functions change their topological structure in the same way that the corresponding classical action-angle loops change.

Chaos, 2003

We continue our study of the fractal structure of escape-time plots for chaotic maps. In the prec... more We continue our study of the fractal structure of escape-time plots for chaotic maps. In the preceding paper, we showed that the escape-time plot contains regular sequences of successive escape segments, called epistrophes, which converge geometrically upon each end point of every escape segment. In the present paper, we use topological techniques to: ͑1͒ show that there exists a minimal required set of escape segments within the escape-time plot; ͑2͒ develop an algorithm which computes this minimal set; ͑3͒ show that the minimal set eventually displays a recursive structure governed by an ''Epistrophe Start Rule:'' a new epistrophe is spawned ⌬ϭDϩ1 iterates after the segment to which it converges, where D is the minimum delay time of the complex.

Physical Review A, 1977

The formalism developed previously to study electronic transitions by the curve-crossing mechanis... more The formalism developed previously to study electronic transitions by the curve-crossing mechanism is used to obtain a general model for transitions which take place due to the strong coupling of curves which do not cross. This exactly solvable model is a generalization of a model developed by Rosen and Zener. The model is shown to give respectable agreement with exact calculations at low velocities, and to go to the correct resonant-charge-exchange limit at high velocities. Predictions of the model are shown to give good agreement with the measured total cross sections for charge exchange in the sytem Li+ + Na Li+ Na+.

American Journal of Perinatology

Objective This study aimed to evaluate the role of an objective physiologic biomarker, arterial b... more Objective This study aimed to evaluate the role of an objective physiologic biomarker, arterial blood pressure variability, for the early identification of adverse short-term electroencephalogram (EEG) outcomes in infants with hypoxic-ischemic encephalopathy (HIE). Study Design In this multicenter observational study, we analyzed blood pressure of infants meeting these criteria: (1) neonatal encephalopathy determined by modified Sarnat exam, (2) continuous mean arterial blood pressure (MABP) data between 18 and 27 hours after birth, and (3) continuous EEG performed for at least 48 hours. Adverse outcome was defined as moderate–severe grade EEG at 48 hours. Standardized signal preprocessing was used; the power spectral density was computed without interpolation. Multivariate binary logistic regression was used to identify which MABP time and frequency domain metrics provided improved predictive power for adverse outcomes compared with standard clinical predictors (5-minute Apgar scor...

Near-threshold photodetachment from negative atomic ions provides a virtually pointlike source of... more Near-threshold photodetachment from negative atomic ions provides a virtually pointlike source of electrons, and is ideally suited to study electron dynamics in externally applied electric and magnetic fields. These fields govern the motion of the emitted electron wave, and lead to characteristic modulations both in the total photocurrent and in the spatial electron distribution. These changes have been predicted and