Polymath Research (United States)

Utilization of graphene-based materials for selective carbon dioxide capture has been demonstrated recently as a promising technological approach. In this study we report results from density functional theory calculations and molecular dynamics simulations on the adsorption of CO2, N2, and CH4 gases on a graphene sheet. We calculate adsorption isotherms of ternary and binary mixtures of these gases and reproduce the larger selectivity of CO2 to graphene relative to the other two gases. Furthermore it is shown that the confinement to two-dimensions, associated with adsorbing the CO2 gas molecules on the plane of graphene, increases their propensity to form clusters on the surface. Above a critical surface coverage (or partial pressure) of the gas, these CO2-CO2 interactions augment the effective adsorption energy to graphene, and, in part, contribute to the high selectivity of carbon dioxide with respect to nitrogen and methane. The origin of the attractive interaction between the CO2 molecules adsorbed on the surface is of electric quadrupole-quadrupole nature, in which the positively-charged carbon of one molecule interacts with the negatively-charged oxygen of another molecule. The energy of attraction of forming a CO2 dimer is predicted to be around 5-6 kJ mol-1, much higher than the corresponding values calculated for N2 and CH4. We also evaluated the adsorption energies of these gases to a graphene sheet and found that the attractions obtained using the classical force-fields might be over-exaggerated. Nevertheless, even when the magnitudes of these (classical force-field) graphene-gas interactions are scaled-down sufficiently, the tendency of CO2 molecules to cluster on the surface is still observed.

Summary form only given. The physics of kinetic electrostatic electron nonlinear (KEEN) waves is elucidated by detailed diagnoses of Vlasov-Poisson simulations. Here we explain the scaling of the multimode non-stationary density response and the phase space partitioned dynamics as a function of drive amplitude and using long time simulations with increasing velocity resolution. In addition, by using 2x-2v simulations, two-dimensional KEEN waves are driven and shown to retain their character found in 1x-1v simulations. Different transverse widths of ponderomotive drive will be compared. Also, Lenard-Bernstein collisions are included to show that the general features of phase space partition and multimode phase locked response both persist but now with a weakly decaying amplitude in time. Thus, this work proves the robustness of KEEN wave results found previously and extends them in dimension and with the addition of velocity space collisions.KEEN-KEEN and KEEN-EPW (electron plasma wave) interactions are considered with novel resonances involving multiple harmonics of KEEN wave interacting simultaneously with an EPW at resonance. This disallows EPW trapped particle states. But at different frequency ratios, their interactions are non-resonant and they are shown to be able to coexist. These are nonlocal interactions in phase space mediated by a common, self-consistent, nonstationary, multimode electrostatic field.

An adaptive method of controlling parametric instabilities in laser produced plasmas is proposed. It involves fast temporal modulation of a laser pulse on the fastest instability's amplification time scale, adapting to changing and unknown plasma conditions. These pulses are comprised of on and off sequences having at least one or two orders of magnitude contrast between them. Such laser illumination profiles are called STUD pulses for Spike Trains of Uneven Duration and Delay. The STUD pulse program includes scrambling the speckle patterns spatially in between the laser spikes. The off times allow damping of driven waves. The scrambling of the hot spots allows tens of damping times to elapse before hot spot locations experience recurring high intensity spikes. Damping in the meantime will have healed the scars of past growth. Another unique feature of STUD pulses on crossing beams is that their temporal profiles can be interlaced or staggered, and their interactions thus controlled with an on-off switch and a dimmer.

We demonstrate operation of an optical parametric amplifier using a chirped quasi-phase-matching grating. Such devices enable engineering of gain spectra over wide bandwidths, applicable to femtosecond pulse amplification and other wide-band devices.

We present analytic results on optical parametric amplifiers and generators where the pump pulse is short enough that its pulse shape dictates the dynamics of the energy transfer to signal and idler waves. We include the group velocity mismatches between the three waves as well as a phase mismatch. A Laplace transform-in-space and WKB-in-time asymptotic formalism is applied to co-propagating waves with a short pump pulse and any group velocity mismatch. Possible generalizations of this formalism to include the effects of nonuniform phase mismatch are indicated. An explicit evaluation of our results with Gaussian pulses is given.

We report the observation of nonlinear saturation of Langmuir waves produced by a probe laser beam interacting with a high intensity pumping laser beam. Amplification of the probe beam is observed and interpreted as scattering of pump energy by a Langmuir wave that is produced by the beating of the two beams. It is found that, as the probe beam amplitude is increased, the scattering and Langmuir wave amplitude do not increase proportionally, demonstrating that the wave is nonlinearly saturated consistent with saturation by secondary-ion-wave instabilities. {copyright} {ital 1999} {ital The American Physical Society}

Three previously reported algorithms for the evaluation of the fourth-order triple excitation energy component in many-body perturbation theory have been compared. Their implementation on current Intel distributed memory parallel computers has been investigated. None of the algorithms, which were developed for shared memory computer architectures, performed well since they lead to prohibitive IO demands. A new algorithm suitable for distributed memory machines is suggested and its implementation on two Intel i860 supercomputers is described. A high level of parallelism is obtained.

A set of large-scale ab initio molecular orbital calculations on the title molecules and their protonated forms has been performed. The aim of the present study has been to help establish very accurate absolute proton affinities for each of these molecules. For each molecule a series of calculations was performed using increasingly larger atomic natural orbital (ANO) one-particle spaces. The energetics of protonation were then evaluated using four methods. These include self-consistent-field (SCF), second-order perturbation theory (MP2), the singles and doubles coupled-cluster (CCSD) ansatz, and the CCSD(T) method, which includes a perturbational estimate of connected triple excitations. At each of these levels of theory the incompleteness of the one and N-particle spaces was ascertained by an evaluation of the basis set superposition error (BSSE) for the protonation reaction. We believe that the final proton affinities all attain chemical accuracy in that they contain <1 kcal/mol error. Our results are in extremely good agreement and consistent with the recently revised absolute proton affinity scale.