Bimodal Energy Distribution Research Articles

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.

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.

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.

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.

Pulse-contrast effects on energy distributions of C 1+ to C 4+ ions for high-intensity 100-fs laser-ablation plasmas

A time-resolved spherical-sector electrostatic analyzer is used to measure the energy spectra of carbon ions from ablation plumes of graphite that are generated by high-intensity ultrafast laser pulses. The time-resolved spectra from the E/ q analyzer are used to construct energy distributions of the individual charge states of various carbon ions. The measurement range of the detector is from 0.05 to 20 keV per unit charge, allowing for observation of up to 80 keV at charge state four. Data are presented for laser intensities of 5×10 14 to 2×10 15 W/cm 2 at a wavelength of 780 nm. Charge states and energy distributions are observed to be strongly dependent on the laser pulse contrast and/or presence of a pre-pulse. A bimodal energy distribution is obtained for high contrast pulses and is interpreted in terms of a plasma hydrodynamic model for the lower ion energies (few hundred eV) and an accelerated ion contribution for the higher energies (several keV).

Equilibrium thermodynamics of homopolymers and clusters: Molecular dynamics and Monte Carlo simulations of systems with square-well interactions

The thermodynamics of homopolymers and clusters with square-well interactions of up to 64 particles are studied with constant-temperature discontinuous molecular dynamics (DMD) simulations; for comparison Monte Carlo (MC) simulations are also reported. Homopolymers composed of more than five beads are found to exhibit two or more equilibrium transitions. In the long chain limit, these multiple transitions correspond to gas-to-liquid, liquid-to-solid, and solid-to-solid transitions. In particular, the liquid-to-solid-like disorder-to-order transition for isolated 32mers and 64mers is strongly first order (bimodal energy distribution) at the reduced square-well diameter λ=1.5. As λ decreases from 1.5 to 1.3, the bimodal distribution becomes unimodal. The use of Lindemann’s rule for solids indicates that the structure formed right below the liquid-to-solid transition temperature has a solid core but a liquid surface. Comparing the homopolymer results with those for square-well clusters indicates that the bonding constraint in homopolymers increases the temperatures of transitions but decreases their strength. The solid structure of an isolated 64mer is nearly identical to that of a cluster of 64 beads. Possible approaches to the experimental observation of the solid-state for an isolated chain are discussed.

Local interactions and protein folding: A three-dimensional off-lattice approach

The thermodynamic behavior of a three-dimensional off-lattice model for protein folding is probed. The model has only two types of residues, hydrophobic and hydrophilic. In absence of local interactions, native structure formation does not occur for the temperatures considered. By including sequence independent local interactions, which qualitatively reproduce local properties of functional proteins, the dominance of a native state for many sequences is observed. As in lattice model approaches, folding takes place by gradual compactification, followed by a sequence dependent folding transition. Our results differ from lattice approaches in that bimodal energy distributions are not observed and that high folding temperatures are accompanied by relatively low temperatures for the peak of the specific heat. Also, in contrast to earlier studies using lattice models, our results convincingly demonstrate that one does not need more than two types of residues to generate sequences with good thermodynamic folding properties in three dimensions.

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Laser-induced desorption of metal atoms at low rate has been studied for pulsed excitation with wavelengths of λ = 266, 355, 532 and 1064 nm. For this purpose sodium adsorbed on quartz served as a model system. The detached Na atoms were photo-ionized with the light of a second laser operating at λ = 193 nm and their kinetic energy distribution was determined by time-of-flight measurements. For λ = 1064 nm a distribution typical of thermal bond breaking is observed. If desorption, however, is stimulated with light of λ = 266 or 532 nm, the kinetic energy distribution is non-thermal with a single maximum atEkin = 0.16 ± 0.02 eV. For λ = 355 nm the non-thermal distribution is even bimodal with maxima appearing atEkin = 0.16 ± 0.02 and 0.33 ± 0.02 eV. These values of the kinetic energies actually remain constant under variation of all experimental parameters. They appear to reflect the electronic and geometric properties of different binding sites from which the atoms are detached and thus constitute fingerprints of the metal surface. The non-thermal desorption mechanism is discussed in the framework of the Menzel-Gomer-Redhead scenario. The transition from non-thermal to thermal desorption at large fluentes of the laser light could also be identified.

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

Non-thermal laser-induced desorption of metal atoms with bimodal kinetic energy distribution

State resolved translational energy distributions of Cl and HCl in the ultraviolet photodissociation of chloroethylenes

Velocity-aligned Doppler spectroscopy was used to measure the speed distribution functions of HCl(v″=0–2,J″) produced in the 193 nm photodissociation of vinyl chloride. A mechanism which is consistent with our present data as well with our earlier work is three center elimination of HCl followed by concerted but nonsynchronous isomerization of the vinylidene fragment, with some of the isomerization energy converted to kinetic energy of the recoiling fragments. In order to explain the vibrational dependence of the translational and rotational energy distributions, the mechanism is assumed to be vibrationally adiabatic. In addition, magic angle Doppler spectroscopy was used to measure the speed distribution functions of Cl atoms produced in the photodissociation of vinyl chloride and the three dichloroethylene isomers. Bimodal energy distributions were observed for both spin–orbit states, with the Cl(2P1/2) fragment having more kinetic energy than Cl(2P3/2). The data are consistent with competitive reaction on two potential energy surfaces. Adiabatic correlation, with some scrambling in the asymptotic region, provide a qualitative explanation of the spin–orbit branching ratios.

Dependence of the collisional relaxation of highly vibrationally excited polyatomic molecules on the population distribution function

The influence of the population distribution function on the collisional relaxation of highly vibrationally excited polyatomic molecules is analyzed in a non-reactive system. Unimodal and bimodal energy distributions are considered. Calculations made with unimodal energy distributions showed that the energy decay is almost independent of its initial shape and also of the collisional transition probability models, provided they have the same dependence on energy. When the initial distribution is bimodal, the rate of energy decay, for constant average energy transferred, depends on the fraction of molecules excited, but if the decay is exponential, the energy loss profile is almost independent of the fraction q of molecules excited. These results are discussed in relation to the use of IR multiphoton absorption as an experimental technique for the study of energy transfer processes.

Comparison of advanced plasma sources for etching applications. III. Ion energy distribution functions for a helicon and a multipole electron cyclotron resonance source

As part of our research effort in evaluating the etching performance of high density plasma sources, we measured ion energy distribution functions near the wafer surface for a helicon and a multipole electron cyclotron resonance source (ECR). The following two salient results stand out: first is the remarkable similarity in behavior of the two sources which was also observed in previous studies comparing etching rates, profile control, and Langmuir probe diagnostics; and second is the surprising level of coupling between the applied rf bias and the bulk plasma. For both sources, the ion flux increases strongly with source power, decreases by 20%–40% as the reactor pressure increases from 2.0 to 5.0 mTorr, and is weakly modified by the applied rf bias. The mean ion energy is strongly influenced by the applied rf-bias and is relatively insensitive to source power and pressure. The ion flux exhibits high uniformity for both sources, with the helicon exhibiting slightly better uniformity. However, we note that instabilities in the ECR discharge from mode jumps caused by different operating conditions and changing reactor wall conditions, such as temperature, result in poorer uniformity. The behavior of ions with respect to applied source and rf-bias powers follows roughly the trends expected of quiescent, high density plasmas in contact with a rf-biased electrode (i.e., independent control of ion flux and mean ion energy). However, there exist subtle effects upon the ion flux, such as bimodal energy distributions, brought about by the coupling of the rf-bias power into the bulk plasma. This coupling may be an essential parameter in wafer platen design that must be addressed in order to obtain high etching rate uniformity.

Double-resonance measurements of vibrational levels populated by infrared multiphoton excitation of CF 3I in a molecular beam

The infrared multiphoton excitation of CF 3I by CO 2 laser radiation between 1068 and 1082 cm −1 (9R4 to 9R26 lines) is monitored under collision-free conditions by (2 + 1) resonance-enhanced multiphoton ionization through the C state. The vibrational levels populated are measured as a function of infrared laser frequency. The vibrational level resonant at the first absorption step is seen only partially to determine which higher combinations are populated. Population of vibrational levels up to four times the infrared photon energy is seen, with no discernible transitions from levels with a higher vibrational energy. The results are consistent with previous reports of bimodal energy distributions in CF 3I.

Infrared multiphoton excitation dynamics of CF3I. II. Collisional effects on vibrational and rotational state populations

Time-resolved resonantly enhanced multiphoton ionization spectroscopy of I(2P3/2) and I(2P1/2) dissociation products of CF3I can be used to characterize populations of highly-excited CF3I*. This technique has been applied to monitor infrared multiphoton excitation dynamics of CF3I in the presence of collisions. Compared to collision-free situations, collisions tend to destroy bimodal vibrational energy distributions and to enhance the overall pumping rate. Collisional energy transfer could be monitored for molecules above the dissociation threshold. Collisional relaxation of highly-excited molecules has been followed and analyzed with respect to average energies transferred per collision.

Bimodal internal energy distribution as a consequence of Cul‐de‐sac isomerization reactions

AbstractA simple model is developed to examine the internal energy distribution for metastable ions and the probability of reaction in the field‐free regions of sector mass spectrometers in systems where the reactant ions in the ion source react by dissociation in competition with reversible isomerization to non‐dissociating species. The internal energy distribution for the reacting ions is strongly dependent on whether dissociation is preceded by isomerization and back‐isomerization, and it is shown that a range of reasonable combinations of rate constants for the forward and reverse isomerization result in a bimodal distribution of the internal energy for the ions that dissociate in the field‐free regions. The isomerization effectively provides a kinetic reservoir of higher energy reactant ions. The reaction probability of the metastable ions varies strongly with internal energy, and the back‐isomerized ions may in some instances be responsible for most of the reactions observed. A bimodal energy distribution will give rise to composite metastable peaks even though neither non‐statistical behavior nor multiple reaction channels are involved.

Electronically stimulated desorption of neutral atoms from Ar films on Ru(001): Desorption mechanisms and energy-transfer processes derived from distributions of kinetic energy.

Desorption yields as well as kinetic-energy distributions of desorbing argon atoms were investigated for pure argon films of variable thickness (annealed and not annealed), and for Ar samples doped with ${\mathrm{N}}_{2}$, ${\mathrm{O}}_{2}$, and NO additives of (a) 1% and 5% concentration in the bulk, and (b) 1/4 to 1/2 monolayer on the surface. The preparation-dependent prevalence of different desorption processes is investigated. Bimodal energy distributions are obtained from clean, annealed samples which are drastically changed by dopants as well as lattice defects. From thick, annealed samples the yield ratio of fast to slow particles is found to be 1.6, which is much larger than reported for previous experiments. This indicates that perfect vacuum conditions are decisive for the study of such systems if mechanistic conclusions are to be drawn.

Mobility and the Glass Transition in Mixed Polymer-Diluent Glasses by NMR

AbstractPhosphorous and proton line shape studies of a trioctyl phosphate diluent in polycarbonate and in a fifty-fifty blend of polystyrene with polyphenylene oxide have been performed to monitor diluent dynamics. Both the phosphorous and proton line shapes are superficially bimodal with a narrow, mobile component of the line shape appearing at temperatures well below the thermal glass transition temperatures. Plots of the h.ction of mobile material versus temperature show two step increases separated by a plateau region. This behavior is analyzed in terms of a distribution of activation energies which results in a bimodal description. The bimodal activation energy distribution corresponds to a bimodal correlation time distribution. The more mobile material is identified with diluent molecules in contact with other diluent molecules in a lattice description which assumes a single phase system is assumed with a purely random distribution of diluent and polymer on the lattice.

A macroscopic profile of the typical quasi‐perpendicular bow shock: Isee 1 and 2

Field and particle properties of the bow shock recorded on November 5, 1977, identified earlier as stable features by comparison of Isee 1 and 2 data, are examined macroscopically. The extent to which the shock was in a ‘typical’ state is described, and multidiagnostic observations are combined to define the shock profile, which, in this known case, is reminiscent of composite portraits assembled from earlier studies. A foot, a principal wave front gradient, and a postfront overshoot characterized the magnetic field. Various particle measurements revealed corresponding structural elements, including a bimodal ion energy distribution whose two peaks were at roughly the solar wind streaming energy and 2 to 3 times the streaming energy. The electrons were thermalized before the principal sharp magnetic gradient; the ions were partially randomized at the gradient, but the ion thermalization process was not completed until long after the gradient was crossed. The second peak of the ion distribution represented a separate group of reflected particles upstream of the main magnetic gradient that retained its identity for an appreciable distance behind the front before being merged by thermalization into the general distribution. This separate group included a northward proton flux in front of, but not immediately behind, the front. The group ultimately became thermalized through a series of very regular oscillations. The decrease in solar wind bulk velocity at the front exceeded the sum of that calculated from an idealized first‐order model of the rise in potential opposing the flow, plus the proton heating, leaving proton reflection and wave radiation and dispersion as candidates to account for the discrepancy. Plasma wave activity, at ion acoustic frequencies, was most intense in the foot and at the front and appeared to be enhanced at the outer boundary of the reflected particles.

Uni- and bimodal product energy distributions for the reactions H + Cl 2 (υ = 1) and D + Cl 2 (υ = 1)

Vibrotational reaction probabilities for the reactions H + Cl 2 (υ = 1) and D + Cl 2 (υ = 1) at translational energies close to 0.1 eV are predicted to be bimodal, with the D reaction showing the more pronounced bimodality. The corresponding rotational and vibrational product distributions may be either uni- or bimodal. The results are from accurate quantum calculations for the collinear reactions, together with an information theoretic 1D → 3D transformation. An optimized, extended LEPS surface is used.