Ellen B. Stechel

R‐matrix solution of coupled equations for inelastic scattering

The R‐matrix propagation method for solving the close coupled equations for inelastic scattering is presented. The method is shown to be fast, accurate, and stable without the need for stabilizing transformations. We demonstrate the use of this method for two model close coupling problems, and investigate the rate of convergence of the solutions.

A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production

We describe and analyze the efficiency of a new solar-thermochemical reactor concept, which employs a moving packed bed of reactive particles produce of H2 or CO from solar energy and H2O or CO2. The packed bed reactor incorporates several features essential to achieving high efficiency: spatial separation of pressures, temperature, and reaction products in the reactor; solid–solid sensible heat recovery between reaction steps; continuous on-sun operation; and direct solar illumination of the working material. Our efficiency analysis includes material thermodynamics and a detailed accounting of energy losses, and demonstrates that vacuum pumping, made possible by the innovative pressure separation approach in our reactor, has a decisive efficiency advantage over inert gas sweeping. We show that in a fully developed system, using CeO2 as a reactive material, the conversion efficiency of solar energy into H2 and CO at the design point can exceed 30%. The reactor operational flexibility makes it suitable for a wide range of operating conditions, allowing for high efficiency on an annual average basis. The mixture of H2 and CO, known as synthesis gas, is not only usable as a fuel but is also a universal starting point for the production of synthetic fuels compatible with the existing energy infrastructure. This would make it possible to replace petroleum derivatives used in transportation in the U.S., by using less than 0.7% of the U.S. land area, a roughly two orders of magnitude improvement over mature biofuel approaches. In addition, the packed bed reactor design is flexible and can be adapted to new, better performing reactive materials.

Quantum ergodicity and a quantum measure algebra

A quantum ergodic theory for finite systems (such as isolated molecules) is developed by introducing the concept of a quantum measure algebra. The basic concept in classical ergodic theory is that of a measure space. A measure space is a set M, together with a specified sigma algebra of subsets in M and a measure defined on that algebra. A sigma algebra is closed under the formation of intersections and symmetric differences. A measure is a nonnegative and countably additive set function. For this to be further classified as a dynamical system, a measurable transformation is introduced. A measurable transformation is a mapping from a measure space into a measure space, such that the inverse image of every measurable set is measurable. In conservative dynamical systems, a measurable transformation is measure preserving, which is to say that the inverse image of every measurable set has the same measure as the original set. Once the measure space and the measurable transformation are defined, ergodic theory...

Electron-stimulated production of NO2(g) from O2 coadsorbed with NO on Pt(111)

Using laser resonance‐enhanced ionization spectroscopy, we have detected O(3PJ=2,1,0) and NO X 2Π3/2,1/2 (ν=5) above a 6–350 eV electron beam‐irradiated Pt(111) surface containing coadsorbed O2 and NO at 90 K. Both product yields have the same chemisorbed NO coverage dependence at saturation O2 precoverage as well as the same ≊10 eV excitation threshold. We conclude that both O(3PJ) and NO(ν=5) are laser‐induced photodissociation fragments of NO2(g). This is established by the observation of identical O(3P2) and NO(ν=5) time‐of‐flight distributions that correspond to NO2 desorption from the surface. The NO2(g) is probably the reaction product of a collision between an O atom (created by electron‐stimulated dissociation of adsorbed O2) and NO(a). We correlate the 10 eV NO2 production threshold with the dissociative ionization of the 3σg molecular bonding orbital of O2(a).

Rotational dynamics and electronic energy partitioning in the electron‐stimulated desorption of NO from Pt(111)

Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurements which show an equal partitioning between the two lowest NO electronic states (2Π1/2 and 2Π3/2), and a preference for the π level which is perpendicular (antisymmetric) to the plane of rotation.Using laser resonance ionization, we have examined the rotational dynamics of neutral NO molecules desorbed by electron impact from a cold (80 K) Pt(111) surface. Previously [A. R. Burns, E. B. Stechel, and D. R. Jennison, Phys. Rev. Lett. 58, 250 (1987)], we observed ‘‘hot’’ rotational temperatures in the range 481–642 K for the ν=0, 1, 2, and 3 vibrational levels. We now present more detailed data which do not reveal shifts or broadening in the rotationally selected NO velocity distributions as a function of rotational level J in the range J=2.5–29.5. The rotationally independent velocity distributions, peaked at 0.05 eV, as well as the rotational temperature can be understood within the framework of ‘‘electronically stimulated adsorbate rotation’’ (Burns et al., above). The model also predicts complete rotational alignment in a plane normal to the surface. In the laboratory, no alignment is observed, but this is most likely due to the presence of stray magnetic fields. We will also discuss our measurem...

A novel wave function factorization simplifying the matrix representation of the Schrödinger equation

In quantum scattering theory, coordinate systems with nontrivial Jacobians may arise and cause difficulty in the reduction of close coupled equations to the desired, simple, obviously Hermitian form {−I (d2/dx2)+W (x)}f=0 with W =W° . We consider x to be the translational coordinate, orthogonal to the surface coordinates, and the Schrodinger equation is represented in a basis of surface functions. We introduce a novel wave function factorization which permits reduction to the above form if the basis is (locally) independent of x for arbitrary Jacobian and general weight function for the surface functions. This factorization is compared with the more common factorization where all coordinates are treated equally. Applications to wave function matching and the calculation of surface integrals are mentioned. Several three‐dimensional, orthogonal coordinate systems provide examples simplified by the novel factorization.

Coverage dependence in the electron‐stimulated desorption of neutral NO from Pt(111)

We report a strong coverage dependence in the yield of a neutral molecule desorbed by low‐energy excitations produced by electron impact. For NO/Pt(111), there are two distinct desorption channels revealed by state‐specific translational energy distributions, measured by time‐of‐flight laser resonance ionization. The dominant low‐energy channel (Etrans =∼0.05 eV) has been previously assigned to a simple 5σ excitation. The high‐energy channel (Etrans =∼0.35 eV) involves complex (multielectron) excitations. Here we present the coverage dependence of the specific yield for the two channels. Strikingly different behavior is seen in that a strong inverse dependence occurs for low‐energy NO but relative independence of coverage is seen for high‐energy NO. A model is developed which describes the coverage dependence in terms of varying localization (or self‐trapping) probabilities as a function of nearest‐neighbor site occupancy. Comparison to experiment using a random vacancy model is rather good, with deviatio...

Resonance ionization detection of neutral O(3PJ) produced from stimulated surface processes

We report the first quantum‐state specific detection of ground‐state atomic oxygen O(3PJ ) above an electron irradiated adsorbate‐covered surface using laser resonance‐enhanced multiphoton ionization spectroscopy. Electron‐stimulated desorption of O(3PJ ) is not observed from an O‐covered Pt(111) surface, but O(3PJ ) fragments are produced from electron‐stimulated dissociation of NO2(a) coadsorbed with O (θ0=0.75 ML) on Pt(111). The ∼9–10 eV threshold for O(3PJ ) formation correlates with that observed for NO2(a) dissociation. The measured O(3PJ ) spin‐orbit state distribution is (5.0):(2.5):(1.0) for J=2, 1, and 0, respectively. This is within experimental error of the 2J+1 statistical (T→∞) limit. No gas phase O(3PJ ) is detected from the stimulated dissociation of adsorbed O2 on Pt(111); however, we have utilized the O(3PJ) detection capability as an indirect probe of NO2 production from an electron‐stimulated surface reaction between coadsorbed O2 and NO on Pt(111). The NO2 is detected by laser photol...

Charge--transfer screening and the dynamics of electronically stimulated adsorbate dissociation and desorption

We present examples of substrate screening effects on two electronically stimulated surface processes: (1) The dissociation of NO2 on clean Pt(111); and (2) the desorption of NO from clean and O‐covered Pt(111). For each set of experiments, we discuss quantum‐resolved data obtained by time‐of‐flight laser resonance‐ionization on the directly desorbed NO products. Two dominant NO2 dissociation channels have been assigned to 3b−12 and 1a−22 excitations on the basis of threshold measurements and lifetime arguments. Screening occurs through substrate charge transfer into the hybridized antibonding 6a1 molecular level. The 1a−22 channel has more NO vibrational and rotational internal energy than the 3b−12 channel due to the greater 6a1 charge density. However, screening is only partial, thus the internal excitations are a small fraction of those observed in gas phase photodissociation. The specific desorption yield from the 5σ−1 excitation of NO with coadsorbed O (≤0.75 ML) is enormously enhanced (>100×), whil...

Laser-induced damage in step-index multimode fibers

In the present study, fiber damage experiments were conducted using a new approach for injecting the laser beam into fiber samples. The purpose of this approach was to achieve a test configuration that was independent of the mode structure of the particular laser being used. The method developed was successful in eliminating internal damage within the first few hundred fiber diameters, but resulted in the fiber entrance face being the most likely damage site. This feature, however, proved quite useful for re-examining the effects of CO2-laser polishing on damage thresholds at the entrance face. Using a test sequence similar to that used previously, a particular laser-polishing schedule was found that significantly improved damage resistance. In addition, preliminary damage experiments were performed during which fiber samples were subjected to much higher bending stresses under carefully controlled conditions.