Bimodal Energy Research Articles

Ne2+[II (1/2)u]: radiative decay and electronic predissociation

Metastable fragmentation of neon dimer ions has been investigated by measuring and analyzing high resolution kinetic energy release distributions. Data were obtained by a modified MIKE (mass-analyzed ion kinetic energy) scan technique. Due to the high energy resolution it was possible to distinguish two different reaction mechanisms in the micros time regime which produce Ne+ ions with different kinetic energy distributions. Theoretical studies based on ab initio calculations of potential energy curves allowed the assignment of the reactions to specific electronic transitions in the excited Ne2+ ion. The unusual bimodal kinetic energy release distribution arises from competition between radiative and non radiative decay of the long-lived Ne2+ II(1/2)u state.

C–Cl bond fission dynamics and angular momentum recoupling in the 235 nm photodissociation of allyl chloride

The photodissociation dynamics of allyl chloride at 235 nm producing atomic Cl((2)P(J);J=1/2,3/2) fragments is investigated using a two-dimensional photofragment velocity ion imaging technique. Detection of the Cl((2)P(1/2)) and Cl((2)P(3/2)) products by [2+1] resonance enhanced multiphoton ionization shows that primary C-Cl bond fission of allyl chloride generates 66.8% Cl((2)P(3/2)) and 33.2% Cl((2)P(1/2)). The Cl((2)P(3/2)) fragments evidenced a bimodal translational energy distribution with a relative weight of low kinetic energy Cl((2)P(3/2))/high kinetic energy Cl((2)P(3/2)) of 0.097/0.903. The minor dissociation channel for C-Cl bond fission, producing low kinetic energy chlorine atoms, formed only chlorine atoms in the Cl((2)P(3/2)) spin-orbit state. The dominant C-Cl bond fission channel, attributed to an electronic predissociation that results in high kinetic energy Cl atoms, produced both Cl((2)P(1/2)) and Cl((2)P(3/2)) atomic fragments. The relative branching for this dissociation channel is Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))]=35.5%. The average fraction of available energy imparted into product recoil for the high kinetic energy products was found to be 59%, in qualitative agreement with that predicted by a rigid radical impulsive model. Both the spin-orbit ground and excited chlorine atom angular distributions were close to isotropic. We compare the observed Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))] ratio produced in the electronic predissociation channel of allyl chloride with a prior study of the chlorine atom spin-orbit states produced from HCl photodissociation, concluding that angular momentum recoupling in the exit channel at long interatomic distance determines the chlorine atom spin-orbit branching.

Adsorption isotherms of the fullerenes C 60 and C 70 on a tetraphenylporphyrin-bonded silica

The single-component adsorption isotherms of the C 60 (from 0 to 15 g/L) and C 70 (from 0 to 8 g/L) buckminsterfullerenes on a tetraphenylporphyrin-bonded silica were acquired by frontal analysis, using a solution of toluene–1-methylnaphthalene (40:60, v/v) as the mobile phase. The best isotherm model derived from the fitting of these adsorption data was the bi-Langmuir model, a choice supported by the bimodal affinity energy distribution (AED) obtained for C 60. The isotherm parameters derived from the inverse method (IM) of isotherm determination (by fitting calculated profiles to experimental overloaded band profiles of C 60 and C 70) are in very good agreement with those derived from the FA data. According to the isotherm parameters found by these three methods (FA, AED, IM), the tetraphenylporphyrin-bonded silica can adsorb 54 and 42 mmol/L of C 60 and C 70 fullerenes, respectively, a result that is consistent with the relative molecular size of these two compounds. The 20% lower surface accessibility for C 70 is compensated by a three times higher equilibrium constant on the low-energy sites, giving a selectivity α C 70 / C 60 = 3.6. Large volumes (0.2, 0.8 and 1.7 mL) of mixtures of C 60 (3.2 g/L) and C 70 (1.3 g/L) were injected and their elution profiles compared to those calculated from the competitive bi-Langmuir model derived from the single-component isotherm data. A good agreement is obtained between calculated and experimental profiles, which supports the two-site adsorption mechanism derived from the single-component adsorption data. The measurements of the influence of the pressure on the retention of C 60 and C 70 demonstrate that the partial molar volumes of the two buckminsterfullerenes are 12 mL/mol larger in the stationary than in the mobile phase.

Retention of ionizable compounds in reversed-phase liquid chromatography. Effect of the ionic strength of the mobile phase and the nature of the salts used on the overloading behavior.

The retention mechanism of the protonated cation in propranolol chloride on C18-Xterra was investigated using mobile phases of various compositions. Accurate adsorption data were measured by frontal analysis, with a mixture of methanol and water (25% methanol), with no salt, as the mobile phase. The experimental isotherm has at least two inflection points, at concentrations of about 0.2 and 6.0 g/L, respectively. This precludes the modeling of these data with a simple convex-upward isotherm (e.g., Langmuir). The adsorption energy distribution or relationship between the number of sites on the adsorbent surface and the energy of adsorption on these sites was calculated by assuming Moreau isotherm behavior (S-shaped isotherm). This model has never been applied to describe the surface heterogeneity of any RPLC adsorbent. The calculation converged toward a bimodal energy distribution. Accordingly, the bi-Moreau model is the simplest theoretical model accounting for the adsorption data of propranolol from a mobile phase without salt. The complex-overloaded band profiles of propranolol measured in the presence of increasing concentrations of a supporting salt (KCl) in the mobile phase demonstrate that the same isotherm model applies also under these conditions, as was merely assumed in a previous work. The elution band profiles of propranolol calculated with the bi-Moreau isotherm model for solutions of salts of different natures (CaCl2, CsCl, Na2SO4) in the same mobile phase agree very well with the experimental band profiles.

Spatial and temporal variations in wave characteristics across a reef platform, Warraber Island, Torres Strait, Australia

A field experiment was conducted at Warraber Island, Torres Strait, Australia to investigate spatial and temporal variations in wave characteristics and energy across a mesotidal coral reef platform. Measurements of water depth were obtained using five pressure sensors deployed across a 2.7-km section of reef flat from July 3–5, 2001. The reef surface was uneven and consisted of an outer reef flat, a central reef flat depression, an inner reef ramp, a palaeo-reef surface and the shoreline. Water levels decreased landward across the platform with tide ranges at the shoreline being almost 50% lower than at the outer reef flat. Rising and falling tides were characterised by a bimodal energy distribution with both short-period (0–3 s) and wind (3–8 s) waves present. Higher water levels were dominated by wind waves. The highest waves occurred at high tides associated with nocturnal tidal cycles with Hs decreasing from 0.5 to 0.2 m from the outer reef flat to the shoreline. Wave energy at swell (8–20 s) and infragravity (>20 s) frequencies was negligible across the reef platform although there was evidence of wave groups at higher water levels.Reef geometry and changes in water level determine the magnitude of wave energy on the reef platform. Up to 85–95% of incident wave energy is attenuated by the central reef flat depression at high and low tide, respectively, and strong linear relationships exist between Hs and h at all locations. Both wave height and wave type are strongly depth dependent. Critical reef rim depths required to produce Hs of a given size vary spatially across the reef rim due to variations in reef topography. A distinct depth related threshold exists at which short-period and wind wave dominance reverses. Over a 14-day spring-neap tidal cycle, the time of occurrence of wave action diminishes across the reef platform to the shoreline. Larger waves (Hs=0.2 m) occur for only 9% of time at the outer reef flat and for less than 0.5% over the remaining reef platform. This implies that on mesotidal reef platforms, sediment entrainment and transport are severely constrained under normal wave energy conditions and significant change is likely restricted to extreme events.

A washboard with moment of inertia model of gas-surface scattering.

A washboard with moment of inertia (WBMI) model for gas atom scattering from a flexible surface is proposed and applied. This model is a direct extension of the washboard model [J. Chem. Phys. 92, 680 (1990)] proposed for gas atom scattering from relatively rigid, corrugated surfaces. In addition, a moment of inertia is incorporated in the original washboard model to describe the flexibility of softer, more highly corrugated surfaces such as polymer or liquid surfaces. The moment of inertia of the effective surface object introduces a dependence of the efficiency of energy transfer on the position and direction of impact, a feature that has been shown to be critical by molecular dynamics simulations. The WBMI model is solved numerically by Monte Carlo integration, which makes the implementation of multiple impacts between a colliding atom and the surface very efficient. The model is applied to Ne and Ar atoms scattering from an alkylthiolate self-assembled monolayer surface and reproduces the major results obtained by classical trajectory simulation of the same system, i.e., a bimodal translation energy distribution P(E(f)) with the low-energy component well-fit with a Boltzmann distribution, but with a temperature that may (Ar) or may not (Ne) be the same as the surface temperature. This indicates that the WBMI model, with well-motivated physical assumptions and simplified interaction, reveals many of the major aspects of the gas-surface collision dynamics, though it does not take into account the real-time dynamics explicitly.

Determination of the single component and competitive adsorption isotherms of the 1-indanol enantiomers by the inverse method

The inverse method of isotherm determination consists in calculating the numerical values of the coefficients of an isotherm model that give a set of chromatographic profiles in best possible agreement with the set of experimental profiles available. This method was applied to determine the adsorption isotherms of the 1-indanol enantiomers on a cellulose tribenzoate chiral stationary phase. Both single-component and competitive isotherms were determined by using no more than one or two overloaded band profiles. The isotherms determined from the overloaded band profiles agreed extremely well with the isotherms determined by frontal analysis. Several isotherm models were used and tested. The best-fit isotherm was selected by means of statistical evaluation of the results. The results show that the adsorption is best characterized with a model describing heterogeneous adsorption with bimodal adsorption energy distribution.

A Wavepacket Study on Translational Energy Distributions of the Photo-stimulated Desorbed Xe from an Oxidized Si(001) Surface

We report a quantum wavepacket study on the characteristic bimodal translational energy distribution of photostimulated desorbed Xe from an oxidized silicon (001) surface observed by Watanabe and Matsumoto, Faraday Discuss. 117 (2000) 203. We have simulated the theoretical translational energy distributions based on wavepacket calculations with a sudden transition and averaging model to reproduce the experiment. We discuss the desorption mechanism and suggest a very strong position dependence of the deexcitation processes for Xe/oxidized Si(001).

Decay dynamics of the vibrationally activated OH–CO reactant complex

The decay dynamics of the OH–CO reactant complex have been examined following infrared excitation in the OH overtone region using various IR pump–UV probe methods. The time scale for overall decay of the OH–CO (2vOH) complex has been bracketed between 0.19 and 5 ns through linewidth and direct time-domain measurements. The inelastically scattered OH (v=1) fragments exhibit a bimodal internal energy distribution, which reveals that vibrational predissociation proceeds through two pathways. The dominant inelastic decay channel involves vibrational energy transfer from OH to CO with little excess energy remaining for rotational excitation of the OH fragment, while a slower secondary channel releases most of the excess energy as OH rotational excitation. Intermolecular bending excitation of the OH–CO complex through combination bands results in increased rotational excitation of the OH fragments. The most probable OH product states display a strong lambda-doublet preference indicating that the singly occupied pπ orbital of OH is aligned perpendicular to the OH rotation plane following vibrational predissociation of the complex. These product states also minimize the translational recoil of the fragments and maximize the rotational angular momentum of the OH fragment. Abrupt cutoffs in the OH (v=1) fragment internal energy distributions are utilized to determine an upper limit for the ground state binding energy of OH–CO, D0⩽410 cm−1, which is in good accord with ab initio predictions. Finally, a comparison of infrared band intensities obtained using action and depletion detection methods suggests that geared bend and H-atom bend excitation may promote reactive decay of the OH–CO reactant complex.

An energy-based beam hardening model in tomography

As a consequence of the polychromatic x-ray source, used in micro-computer tomography (µCT) and in medical CT, the attenuation is no longer a linear function of absorber thickness. If this nonlinear beam hardening effect is not compensated, the reconstructed images will be corrupted by cupping artefacts.In this paper, a bimodal energy model for the detected energy spectrum is presented, which can be used for reduction of artefacts caused by beam hardening in well-specified conditions. Based on the combination of the spectrum of the source and the detector efficiency, the assumption is made that there are two dominant energies which can describe the system. The validity of the proposed model is examined by fitting the model to the experimental datapoints obtained on a microtomograph for different materials and source voltages.

Bimodal energy sources for laparoscopic hysterectomy

Bimodal energy sources for laparoscopic hysterectomy

Heterogeneity of Surface Energies in Reversed-Phase Perfusive Packings

The surface energetic heterogeneity of the packing media used in perfusion chromatography was investigated based on the liquid–solid adsorption information of phenol. The adsorption isotherms on two perfusive packings, POROS R1 and POROS R2, were measured by stepwise frontal experiments at varying mobile-phase concentrations and temperatures. The isosteric heat of adsorption was calculated from the isotherm data and the adsorption energy distributions (AEDs) were obtained numerically by the expectation maximization (EM) method. The adsorption isotherms corroborated the Langmuir–Freundlich (LF) isotherm well, suggesting the heterogeneity of surface energies of the perfusive medium. The analysis of the isosteric heat of adsorption indicated that this surface heterogeneity might have arisen from the differences in the pore size distributions of the perfusive particles. This observation was further confirmed by the numerically obtained adsorption energy distribution functions. Bimodal energy distribution functions were obtained for the perfusive packings, which is a reflection of the bimodal pore size distributions of the materials. In addition, it has also been found that the temperature dependence of the surface energetic heterogeneity is insignificant for such polymer-based support used in perfusion chromatography, whereas its dependence on the acetonitrile concentration in the mobile phase is rather pronounced.

UV photodissociation dynamics of ethyl radical via the à 2A′(3s) state

H-atom channels in the photodissociation of jet-cooled ethyl radical (C2H5) via the à 2A′(3s) state are studied near 245 nm by using the high-n Rydberg-atom time-of-flight technique. Bimodal product translational energy release and energy-dependent angular distribution suggest two dissociation pathways. A slow (〈fT〉∼0.35) and isotropic channel corresponds to unimolecular dissociation of the radical, presumably after internal conversion. A previously unobserved fast (〈fT〉∼0.78) and anisotropic (β=0.5±0.1) channel is consistent with direct H-atom scission via a nonclassical H-bridged transition state from the 3s state to yield H+C2H4(X̃ 1Ag). The fast/slow branching ratio is ∼0.2. Site-selective loss of the β hydrogen atom is confirmed by using the partially-deuterated CH3CD2 radical.

The photodissociation dynamics of nitric acid studied at 193 nm by LIF and REMPI–TOF methods

The 193 nm photodissociation of HNO 3 in a supersonic jet has been investigated by LIF and REMPI–TOF techniques probing the OH( X 2Π) and the O( 3 P) products, respectively. The measured rotational state distribution of OH is shown to be dominated by a bimodal distribution consistent with the previously reported bimodal translational energy distribution of OH. An additional decay channel, yielding O( 3 P) fragments with high kinetic energy, has been observed and tentatively assigned to HNO 3+hν → O( 3 P)+ HONO( X ̃ 1 A ′) . Energetic considerations indicate that about one third of the HONO products are unstable, decaying further to OH + NO.

Ab Initio Investigation of the Temporary Anion States of Perfluoroethane

The stabilization method is used to characterize the temporary anion states of C2F6. Based on the theoretical results, we assign the lowest-energy feature in the electron transmission spectrum of C2F6 to the 2Eu ground-state anion and the second feature to overlapping 12Au, 12A1g, 22Au, and 12Eg anion states. It is also proposed that the bimodal kinetic energy distribution of the F- ions produced in the dissociative attachment of near 5 eV electrons on C2F6 is due to the Jahn−Teller effect.

Maximum-entropy calculation of energy distributions

We use the maximum-entropy method to calculate molecular energy distributions from the moments of the distribution which in turn can be obtained from the temperature dependence of the heat capacity. If one knows the temperature expansion of the heat capacity through the nth power of the temperature, this then gives the exact first (n+2) energy moments. We illustrate the method for the ideal gas (the Maxwell–Boltzmann distribution of kinetic energy) and then use a model function to show that if one knows four or more moments of the energy distribution this allows one to resolve two or more distinct peaks in this function. We examine argon above the critical pressure, a one-dimensional model, and the protein barnase, all of which exhibit bimodal energy distributions.

Ab initio based study of the ArO− photoelectron spectra: Selectivity of spin–orbit transitions

A combined ab initio atoms-in-molecule approach was implemented to model the photoelectron spectra of the ArO− anion. The lowest adiabatic states of Σ and Π symmetry of ArO and ArO− were investigated using the fourth-order Møller–Plessett perturbation theory including bond functions. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The complex of Ar with atomic oxygen is only weakly bound, primarily by dispersion interaction. The Π state possesses a deeper minimum (Re=3.4 Å,De=380 μEh) than the Σ state (Re=3.8 Å,De=220 μEh). In contrast, the anion complex is fairly strongly bound, primarily by ion-induced dipole induction forces, and the Σ state possesses a deeper minimum at shorter interatomic distances (Re=3.02 Å,De=3600 μEh) than the Π state (Re=3.35 Å,De=2400 μEh). The Σ–Π splittings in both systems are mainly due to differences in the exchange repulsion terms. Atoms-in-molecule models were used to account for the spin–orbit interaction, and to generate adiabatic relativistic potentials and wave functions. Collisional properties, diffusion, and mobility coefficients of O and O− in Ar, and absolute total Ar+O scattering cross sections, were calculated and found to agree well with the available experimental data. The photoelectron spectra were simulated within vibronic model, and were found in excellent agreement with the experimental measurements. The bimodal electron kinetic energy distribution was shown to stem from the strong selectivity of spin–orbit transitions, which split into two dense groups, depending on the initial electronic state of the anion. The latter feature cannot be described without explicit consideration of electronic intensity factor.

MODELS AND REALITY: TIME–ENERGY TRADE-OFFS IN PECTORAL SANDPIPER (CALIDRIS MELANOTOS) MIGRATION

We used a combination of modeling and field studies to determine the spring migration strategy of Pectoral Sandpipers (Calidris melanotos). We developed a dynamic programming model to predict patterns that should be detected along the migratory route if Pectoral Sandpipers use a strategy of early arrival at the breeding grounds (time minimization) or arrival at the breeding grounds with excess energy reserves (energy maximization). The predictions were then compared to data collected at stopover sites in the mid-continent of North America and at the breeding grounds in Alaska over a 5-yr period (1992–1996). During spring migration to their Arctic breeding grounds, Pectoral Sandpipers stop periodically to feed. The length-of-stay at such stopovers, for both time minimizers and energy maximizers, was predicted to vary inversely with date and body fat, and to vary directly with invertebrate abundance. We observed that: (1) length-of-stay was negatively correlated with capture date in Missouri and Nebraska, but not in Texas; (2) length-of-stay was not correlated with body fat at any site; and (3) length-of-stay was positively related to invertebrate abundance at the Nebraska and Missouri sites. As the population moves northward in the spring, three regional patterns are diagnostic of migration strategy. Length-of-stay was predicted to be bimodal (energy maximizer) or constant (time minimizer) with respect to latitude, but neither pattern was observed. The migration window, or period of time during which spring migrants occur, was predicted to decrease with increasing latitude for time minimizers, a pattern that was seen for both males and females. Body fat was predicted to increase with latitude for energy maximizers, a pattern that was seen for females but not males. The evidence suggests that males and females differ in their spring migration strategies. Both sexes attempt to arrive in the Arctic as early as possible after ice breakup in the spring. Additionally, females gain significantly higher fat loads than males (up to 60% body fat for females) during migration, and these energy reserves may later enhance female reproductive success. However, females gained large fat loads only during 1993 and 1995, which had above normal spring precipitation along the migration route. We believe that the correlation between female body fat and precipitation reflects an abundance of high-quality stopover habitat during wet springs. This view is supported by model sensitivity analyses showing that the spacing and quality of stopover habitat can strongly influence observed migration patterns. Our results suggest the need to focus additional research on the landscape-level features of the flyway through which shorebirds migrate.

Energy distribution of ions bombarding biased electrodes in high density plasma reactors

A compact retarding field ion energy analyzer has been designed and built to measure the energy distribution of ions bombarding the wafer surfaces placed on radio frequency (rf) biased electrodes in high-density plasma reactors. The analyzer was used to measure the energy distribution of ions impinging on the rf-biased electrostatic chuck in a high-density transformer coupled plasma (TCP) reactor. The effects of TCP power, rf bias, gas composition, and ion mass on the ion energy distributions (IEDs) were demonstrated through Ar, Ne, Ar/Ne, O2 and CF4/O2 discharges. In the operating range studied, the average ion energy increased linearly with increasing rf bias while the ion flux remained constant indicating that independent control of ion flux and energy was achieved in the TCP reactor. Bimodal ion energy distributions resulting from ion energy modulation in the sheath were observed and multiple peaks in the IEDs measured in gas mixtures were identified as ions with different masses falling through the sheath.

Guided ion beam studies of the reactions of Crn+ (n=1–18) with CO2: Chromium cluster oxide bond energies

The kinetic energy dependence of the reactions of Crn+ (n=1–18) with CO2 are studied in a guided ion beam mass spectrometer. The primary product ions are CrnO+, which then decompose by sequential loss of chromium atoms as the kinetic energy is increased. Simple collision-induced dissociation to form the Crn−1+ product ions is also observed. Large cluster ions, n⩾9, form the CrnCO2+ adduct at low kinetic energies. For many cluster sizes, the cross section for the primary reaction, Crn++CO2→CrnO++CO, exhibits an interesting bimodal energy behavior that is discussed in some detail. Crn+–O bond energies are measured and found to compare well with measurements obtained from guided ion beam studies of the Crn++O2 systems. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values.