bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of sputtered Cu atoms and ions in a direct current glow discharge: Combined fluid and Monte Carlo model
Authors: Bogaerts A. , Gijbels R.
Published: July 2002

The Cu atoms sputtered from the cathode and the corresponding Cu ions in an argon direct current glow discharge are described by a combination of two models: a fluid model for their overall behavior (diffusion and migration) in the entire discharge and a Monte Carlo model for the explicit transport of the Cu ions in the cathode dark space. The models are combined with other models described previously for the electrons, Ar ions, fast Ar atoms, and Ar metastables, in order to obtain an overall picture of the glow discharge. Results of the fluid model are the densities and fluxes of the Cu atoms and ions. At 100 Pa and 1000 V the Cu atom and ion densities are of the order of 1012–1013 and 1010–1011 cm−3, respectively. The ionization degree is hence about 1%, which is much higher than for Ar. The Cu ion to Ar ion density is about 6% and the Cu ion to Ar ion flux is about 5%. The energy distribution of the Cu ions bombarding the cathode is calculated with the Monte Carlo model and shows good agreement with experiment. It is characterized by a peak at maximum energy, in contrast to the energy distribution of Ar ions and fast atoms. Since sputtering increases with the bombarding energy, the amount of self-sputtering is significant, although still clearly lower than the contribution of Ar ions and fast atoms. The influence of pressure, voltage, and current on all these quantities is investigated.
The effects of flux, fluence and temperature on amorphization in ion implanted semiconductors
Authors: Carter G.
Published: July 2002

The Avrami–Johnson–Mehl [P. G. Shewmon, Transformation in Metals (McGraw Hill, New York (1969)] approach to phase transformations is extended to include finite area amorphous zone nucleation or generation by ion impact onto semiconductors together with bombardment induced zone expansion and thermal annealing zone contraction (recrystallization) in order to predict the variation of amorphized material fraction with ion fluence. The ion flux and substrate temperature are found to play decisive roles in determining this variation. The model predictions are shown to agree qualitatively with currently available experimental data with respect to the system variables of ion flux, fluence, and substrate temperature.
Can spin valves be reliably deposited for magnetic recording applications? (invited)
Authors: Gurney Bruce A. , Speriosu Virgil S. , Wilhoit Dennis R. , Lefakis Harry , Fontana Robert E. , Heim David E. , Dovek Moris
Published: July 2002

The tolerance of the expected read-back signal of spin valve giant magnetoresistance based structures to varying deposition and process conditions are described. We determine if spin valves can be produced reliably, and evaluate which thicknesses and properties are most critical. First, the dependence of spin valve properties on layer thickness are experimentally determined. Next, the variation of read-back signal and transfer curve characteristics with spin valve properties is calculated from micromagnetic modeling. Finally, these are convolved with the expected reproducibility of layer thickness to obtain an effective “yield” of structures within 10% of the mean amplitude. We find that spin valves can be reliably deposited, with “yields” well in excess of 90% likely.
Burst magnetostriction in Tb0.3Dy0.7Fe1.9
Authors: Armstrong William D.
Published: July 2002

The magnetostriction and magnetic induction calculated by a continuous, anisotropic, anhysteric, magnetization model are compared with magnetostriction and magnetic induction measurements on burst and nonburst magnetostrictive Tb0.3Dy0.7Fe1.9 twinned single crystal rods. The model shows that the magnetostriction and permeability suppression occurring at low applied field is the result of the rotation, and subsequent capture, of initial field antiparallel magnetization into field transverse [111̄] or [1̄1̄1] local magnetoelastic energy minima. The model further shows that the interval of high magnetostriction applied field derivative, dλ/dH, characteristic of burst magnetostrictive material, is the result of the rotation of field transverse [111̄] or [1̄1̄1] oriented magnetization into the [111] near field magnetocrystalline minima. The occurance of burst magnetostriction is therefore contingent on obtaining sufficient magnetocrystalline anisotropy and sufficiently tight magnetization energy distribution in experimental Tb0.3Dy0.7Fe1.9 twinned single crystal rods so as to minimize the applied field interval over which this magnetization rotation process occurs. A final analysis shows that the present model is able to correctly approximate the applied field dependence of the burst magnetostriction response and the applied field dependence of the simultaneous magnetostriction and permeability suppression with a single set of parameters for a range of constant [112] applied compressive stresses. The model additionally exhibits approximately correct saturation magnetostrictions for a range of experimentally applied compressive stresses. However, the model fails to match the experimental behavior above a simultaneous dλ/dH, permeability and field hysteresis transition, located approximately 1000 microstrain from the saturation magnetostriction. The experimental transition clearly indicates a change in magnetization mechanism not accommodated by the present model.
Analysis of a high-Tc hot-electron superconducting mixer for terahertz applications
Authors: Karasik B. S. , McGrath W. R. , Gaidis M. C.
Published: July 2002

The prospects of a YBa2Cu3O7−δ hot-electron bolometer mixer for a THz heterodyne receiver are discussed. The modeled device is a submicron bridge made from a 10-nm-thick film on a high thermal conductance substrate. The mixer performance expected for this device is analyzed in the framework of a two-temperature model which includes heating both of the electrons and the lattice. Also, the contribution of phonon diffusion from the film through the substrate and from the film to the normal metal contacts is evaluated. The intrinsic conversion efficiency and the noise temperature have been calculated as functions of the device size, local oscillator (LO) power, and ambient temperature. Assuming thermal fluctuations and Johnson noise to be the main sources of noise, a minimum single sideband mixer noise temperature of ≅2000 K is predicted. For our modeled device the intrinsic conversion loss at an intermediate frequency of 2.5 GHz is less than 10 dB and the required LO power is ∼1–10 μW.
Nondestructive determination of the C13 content in isotopic diamond by nuclear resonance fluorescence
Authors: Beck O. , Ruf T. , Finkelstein Y. , Cardona M. , Anthony T. R. , Belic D. , Eckert T. , Jäger D. , Kneissl U. , Maser H. , Moreh R. , Nord A. , Pitz H. H. , Wolpert A.
Published: July 2002

Nuclear resonance fluorescence excited with continuous electron bremsstrahlung from the 4.3 MV Stuttgart Dynamitron accelerator is used as a nondestructive method to determine the C13 content x of bulk isotopic diamonds (12C1−x13Cx). The smallest detectable amount of C13 in carbon or low Z matrices is on the order of 0.5 mg. The relative accuracy of absolute mass determinations is about ±7%. Errors are mainly due to uncertainties in the natural widths Γ of the C13 nuclear levels at 3089 and 3684 keV used in the measurements. The results confirm a previous calibration which is based on Raman scattering and the destructive determination of x by mass spectroscopy.
Dependence of the density of defects in the oxide on Czochralski silicon on its thickness
Authors: Itsumi Manabu , Maeda Masahiko , Ueki Takemi
Published: July 2002

The dependence of the density of defects in the oxide on Czochralski silicon substrates on its thickness is discussed quantitatively in terms of the shape and the size of the octahedral void defects. Oxide thinning at corners of the void defects may be a dominant factor in the oxide thickness dependence when oxide thickness is smaller than 50 nm. The size distribution (80–160 nm) of the void defects may be a dominant factor in the oxide thickness dependence when oxide thickness is larger than 50 nm. Model is presented to explain the mechanism of the two factors.
Elastic and piezoelectric fields around a buried quantum dot: A simple picture
Authors: Davies John H.
Published: July 2002

The elastic field around a buried, strained quantum dot is solved with a scalar potential that obeys Poisson’s equation. Standard methods from electrostatics can therefore be used. The lattice mismatch and displacement are analogous to the charge density and electric field. The dilation is proportional to the local lattice mismatch and therefore vanishes outside a dot. Expressions are also given for the piezoelectric potential. The results agree remarkably well with previous numerical calculations for a pyramidal dot. Thermoelasticity provides another analogy with many useful solutions available. These results are for an isotropic medium but cubic symmetry is considered briefly.
Determination of negative-ion and electron parameters in an Ar/SF6 plasma
Authors: Stamate E. , Ohe K.
Published: July 2002

A new method for evaluating negative ion and electron parameters from the current–voltage characteristics of electric probes is presented. A theoretical model and its related numerical procedures are established and errors included are estimated. Temperatures and densities of negative ions and electrons in a magnetic multipolar-confined plasma of Ar and Ar/SF6 mixtures are determined with allowable errors for various density ratios of the negative ion to the electron.
Quantitative vector magnetometry using generalized magneto-optical ellipsometry
Authors: Berger A. , Pufall M. R.
Published: July 2002

We recently developed the method of generalized magneto-optical ellipsometry (GME), which enabled us to achieve a complete magneto-optical characterization of a magnetic material, including the magnetization orientation. For the present study, we have modified our original setup to allow for measurements at arbitrary external fields including entire field scans. Consequently, GME can be used as a tool to perform vector magnetometry measurements. In this article, we present our first experimental results on polycrystalline Co films which demonstrate the feasibility of GME as a magneto-optical vector magnetometer.
Strain distributions in quantum dots of arbitrary shape
Authors: Andreev A. D. , Downes J. R. , Faux D. A. , O’Reilly E. P.
Published: July 2002

A method based on the Green’s function technique for calculating strain in quantum dot (QD) structures has been developed. An analytical formula in the form of a Fourier series has been obtained for the strain tensor for arrays of QDs of arbitrary shape taking into account the anisotropy of elastic properties. Strain distributions using the anisotropic model for semiconductor QDs are compared to results of a simplified model in which the elastic properties are assumed to be isotropic. It is demonstrated that, in contrast to quantum wells, both anisotropic and isotropic models give similar results if the symmetry of the QD shape is less than or equal to the cubic symmetry of the crystal. The strain distribution for QDs in the shape of a sphere, cube, pyramid, hemisphere, truncated pyramid, and flat cylinder are calculated and analyzed. It is shown that the strain distributions in the major part of the QD structure are very similar for different shapes and that the characteristic value of the hydrostatic strain component depends only weakly on the QD shape. Application of the method can considerably simplify electronic structure calculations based on the envelope function method and plane wave expansion techniques.
Properties of the second-order nonlinear optical susceptibility χ(2) in asymmetric undoped AlGaAs/InGaAs double quantum wells
Authors: Park Tae-ik , Gumbs Godfrey , Chen Y. C.
Published: July 2002

The second-order nonlinear optical susceptibility χ(2) for second-harmonic generation is calculated for the 11H transition of a graded double quantum well (DQW) structure of undoped GaAs/AlxGa1−xAs. These results are compared with the single quantum well (QW). Our results show that the values of χ(2) have optimal magnitudes dependent on the width, depth and separation between the QWs in a DQW structure. When the electric field increases, the dipole moment increases due to the increasing separation between the electron and hole wave functions. On the other hand, the oscillator strength of the 11H transition is reduced as a result of the decrease in the overlap of the electron and hole envelope functions. These two competing factors give rise to optimal conditions for the enhancement of the second-order nonlinear susceptibility χ(2). It is demonstrated that χ(2) for the DQW structure is more enhanced than for the biased single QW.
Surface loss probabilities of hydrocarbon radicals on amorphous hydrogenated carbon film surfaces
Authors: Hopf C. , Schwarz-Selinger T. , Jacob W. , von Keudell A.
Published: July 2002

The surface loss probabilities of hydrocarbon radicals on the surface of amorphous hydrogenated carbon (C:H) films are investigated by depositing films inside a cavity with walls made from silicon substrates. This cavity is exposed to a discharge using different hydrocarbon source gases. Particles from the plasma can enter the cavity through a slit. The surface loss probability β is determined by analysis of the deposition profile inside the cavity. This surface loss probability corresponds to the sum of the probabilities of effective sticking on the surface and formation of a nonreactive volatile product via surface reactions. By comparing the deposition profiles measured in CH4, C2H2, C2H4, C2H6 discharges one obtains for C2H radicals β=0.80±0.05, for C2H3 radicals β=0.35±0.1, and for C2H5 radicals β<10−3. The growth rate of C:H films is, therefore, very sensitive to any contribution of undersaturated C2Hx species in the impinging flux from a hydrocarbon discharge.
Power effects in polarization spectroscopy on a water solution of malachite green
Authors: Marcano O. A. , Golding F. García
Published: July 2002

An experimental evidence of shape deformation of PS spectrum and a nonquadratic pump power dependence of the PS signal for a water solution of malachite green is presented. A calculation of nonlinear polarization at all orders in the pump and linear in the probe was needed to interpret the data.
Vibronic effects in magnetic vibrational circular dichroism of the triply degenerate vibration. Theoretical analysis for the ν6 vibration of tungsten hexacarbonyl
Authors: Pawlikowski M. , Devine T. R.
Published: July 2002

The MVCD and infrared absorption spectra of a triply degenerate vibration are discussed in the frame of a simple molecular model applicable for molecules of cubic symmetry. The presented results show that vibronic contribution to infrared intensity can dominate or compete with ground state contribution and the vibronic effects cannot be neglected in the MVCD analysis. The model is successfully applied to the MVCD spectrum of ν6 vibration in the W(CO)6 molecule. The estimation of the excited d 1Tlu state magnetic moment is made on the basis of this analysis.
The potential energy surface of (NH3)2
Authors: Latajka Zdzisl/aw , Scheiner Steve
Published: July 2002

A b initio calculations at the SCF and correlated levels are carried out to characterize the potential energy surface of the NH3 dimer. The two basis sets used are 4-31G* and a larger one containing two sets of d-functions on N centers, 6-31G** (1p, 2d). The only minimum occurring on the surface is a cyclic C2h structure in which the two H-bonding protons are displaced 42° from the N--N axis. The surface contains a very shallow valley along the direction leading from this geometry to a single linear H bond although there is no minimum corresponding to this arrangement. Despite the symmetrically nonpolar character of the cyclic geometry, the shallowness of the potential in the direction of the linear structure may allow zero point vibration effects to displace the minimum, thereby leading to the observed small dipole moment and its sensitivity to isotopic substitution.
Autodetachment study of the electronic spectroscopy of FeO−
Authors: Andersen T. , Lykke K. R. , Neumark D. M. , Lineberger W. C.
Published: July 2002

The anion FeO− was studied by autodetachment spectroscopy in a coaxial laser–ion beam photodetachment spectrometer. Transitions were observed between the ground electronic state of the ion and several excited electronic states near the electron detachment threshold. Rotational assignments were carried out for several bands, and the measured linewidths yielded autodetachment lifetimes as a function of rotational energy for these bands. The results indicate a 4Δ ground state of FeO−. The autodetachment lifetimes show that some of the excited electronic states are dipole bound, and that one weakly bound state may be a valence excited state.
Overtone NMR spectroscopy
Authors: Tycko R. , Opella S. J.
Published: July 2002

The theoretical background and experimental implementation of overtone NMR spectroscopy of quadrupolar nuclei, i.e., the direct observation of NMR signals at multiples of the Larmor frequency, are described. Perturbation theory is used to derive expressions for the dependence of overtone NMR frequencies, transition moments, and dipole–dipole splittings on molecular orientation for a nucleus with arbitrary spin. Experimental techniques for overtone cross-polarization and for two-dimensional spectroscopic measurements of transition moments and dipole–dipole splittings are demonstrated with 14N overtone spectra of single crystals. Overtone powder pattern line shapes are analyzed, both for spin S=1 and spin S=3/2 nuclei, and examples of 14N overtone powder patterns are presented. Overtone spectroscopy can be used to characterize quadrupole and dipole–dipole interactions in polycrystalline samples. The sensitivity, resolution, and spectral width characteristics of overtone NMR spectra are discussed and contrasted with those of conventional NMR spectra. The sensitivity and resolution of overtone spectra are shown to be comparable to those of conventional spectra, while the reduced spectral width of overtone spectra is a significant advantage in studies of single crystals and, in particular, polycrystalline materials.
Thermal energy reactions involving Ar+ monomer and dimer with N2, H2, Xe, and Kr
Authors: Shul R. J. , Passarella R. , Upschulte B. L. , Keesee R. G. , Castleman A. W.
Published: July 2002

Rates and products of reaction are reported for the thermal energy charge transfer of Ar+ and Ar+2 with N2, H2, Xe, and Kr using a selected ion flow tube (SIFT). New rate coefficients were measured for the reactions Ar++Xe→Xe++Ar, Ar+2+N2 →Ar+⋅N2+Ar, Ar+2+H2 →ArH++H+Ar, and Ar+2+H2 →ArH+2+Ar; the rates are 4.3×10−13, 2.2×10−10, 3.6×10−10, and 1.1×10−10 cm3/s, respectively. The finding of (Ar⋅N2)+ as the product of the reaction of Ar+2 with N2 shows the bond energy of Ar+⋅N2≥1.27 eV in agreement with other techniques. In the case of H2, two reaction products are observed for reaction with Ar+2: ArH+ (85±10%) and ArH+2, but not Ar2H+ as suggested by earlier studies. Lower limits for the proton affinity of Ar (3.69 eV) and the dissociation energy for H+2⋅Ar (0.97 eV) are determined from the present work. Considering all the reaction rates, the proximity of the recombination energy of Ar+ and Ar+2 to a band in the photoelectron spectrum of a specific neutral reactant offers an explanation for the observed ordering of the relative rates of the monomer and dimer reactions.
Analysis of 14N ESEEM patterns of randomly oriented solids
Authors: Flanagan Heather L. , Singel David J.
Published: July 2002

Simulations of 14N ESEEM patterns for an S=1/2, I=1 spin system with an isotropic hyperfine coupling, in a nonordered solid are reported. The dependence of the simulated patterns on the quadrupole and hyperfine coupling constants, and on the external field strength is examined and discussed. Three distinct types of powder patterns are identified, all of which exhibit discrete peaks. The dependence of the frequency, width, and amplitude of these peaks on the coupling constants and field strength is elucidated in simple terms. The results provide a basis for the interpretation of ESEEM patterns and for the development of experimental tactics: the variation of field strength (electron spin-excitation frequency) can be used to manipulate modulation depths, select specific nuclei, and ascertain accurate quadrupole coupling constants. The anisotropy of the amplitude of modulation components, its effect on ESEEM powder patterns, and its implications as regards orientation selection experiments are also considered.
C++H2( j)→CH++H: The effect of reagent rotation on the integral cross section in the threshold region
Authors: Gerlich D. , Disch R. , Scherbarth S.
Published: July 2002

Integral cross sections for the reaction of C+ with hydrogen have been determined as a function of translational and rotational energy. Measurements were made with a guided beam apparatus, using a temperature variable scattering cell or alternatively a nozzle beam and different ortho–para mixtures of H2. The detailed results are in good agreement with statistical phase space calculations provided that the nuclear spins are assumed to be completely decoupled (frozen spin approximation). One consequence of fundamental interest is that H2( j=1) is less reactive than H2( j=0) in the threshold region. Further, the theory predicts an oscillatory structure for the kinetic energy dependence of the reaction cross section which has been observed for the first time. The calculated cross sections are used to predict reliable rate coefficients for H2 in selected rotational ( j=0–7) states. An analytical approximation of these results allows a simple calculation of rate coefficients for nonequilibrium rotational populations as can be found, e.g., in interstellar space.
Density functional theory applied to the isotropic–nematic transition in model liquid crystals
Authors: Perera A. , Patey G. N. , Weis J. J.
Published: July 2002

The isotropic–nematic phase transition is investigated for several model liquid crystals using the density functional method. The models considered are hard ellipsoids of revolution (both prolate and oblate cases), hard spherocylinders, and two additional fluids characterized by pair potentials of a generalized Maier–Saupe type. The direct pair correlation functions for the isotropic phase are obtained by numerical solution of the hypernetted-chain (HNC) and Percus–Yevick (PY) integral equation theories. It is shown that second order density functional theory is strongly dependent upon the approximation used for the isotropic direct pair correlation function. In all cases the density functional results are qualitatively consistent with conclusions based upon orientational stability criteria.
Laser desorption jet-cooling spectroscopy of para-amino benzoic acid monomer, dimer, and clusters
Authors: Meijer Gerard , de Vries Mattanjah S. , Hunziker Heinrich E. , Wendt H. Russell
Published: July 2002

The technique of laser desorption followed by jet cooling allows wavelength-selective as well as mass-selective detection of molecules, desorbed from a surface without fragmentation. Resonance enhanced multi photon ionization (REMPI) spectra of the para-amino benzoic acid (PABA) molecule and its methyl and n-butyl ester were obtained in this way. The origin of the S1←S0 transition in PABA was found at 34173±2 cm−1. The adiabatic ionization potential of PABA was determined as 7.998±0.001 eV. In addition, jet-cooled REMPI spectra of the PABA dimer and its ring-deuterated isotopes were recorded. The dimer is formed by two identical hydrogen bonds between the carboxylic acid groups. The excitation in the dimer is found to be almost completely localized in one monomer unit. Clusters of PABA molecules with molecules seeded in the beam (argon, methanol, water) were resonantly detected as well, using PABA as the chromophore.
Equation of state for athermal lattice chains in a 3d fluctuating bond model
Authors: Deutsch Hans-Peter , Dickman Ronald
Published: July 2002

A generalization of the well-known Flory and Flory–Huggins mean-field approximations to the equation of state is derived for a three-dimensional lattice model in which a monomer occupies an entire unit cell, and many bond lengths and bond angles are possible. By measuring the probability for particles to be in contact with the walls of the system, the pressure is determined via computer simulation over the full density range from dilute solution to dense melt. The results are used to test the mean-field predictions. Comparing the equation of state of the present model to those of conventional lattice models and of hard-sphere chains in continuous space, it is seen that our method approximates the continuum limit far better than single site lattice models. Also the large n des Cloizeaux scaling behavior is approached more rapidly.
Rate of diffusion-limited reactions in dispersions of spherical traps via multipole scattering
Authors: Bonnecaze R. T. , Brady J. F.
Published: July 2002

The effective reaction rate is calculated for a random array of reactive, stationary spherical traps in a medium containing a highly mobile reactant. Multipole scattering up to the quadrupole level, properly accounting for the conditionally convergent long-range interactions, plus direct addition of exact two-body interactions is employed. It is found that the addition of two-body interactions has a negligible effect on the effective reaction rates computed, in contrast to the case of the effective conductivity. Our results closely match the random walker simulation results of Lee, Kim, Miller, and Torquato [Phys. Rev. B 39, 11833 (1989)] up to 30% trap volume fraction, after which they underpredict the effective reaction rate. To accurately compute the effective reaction rate at high volume fractions, higher order many-body multipole interactions are required.
Population transfer by multiple stimulated Raman scattering
Authors: Band Y. B. , Julienne P. S.
Published: July 2002

We present results of density matrix calculations describing population transfer in a four level system via stimulated Raman scattering with three laser pulses whose frequencies are in (or nearly in) resonance with the transitions between the levels. A high degree of transfer to the terminal level is possible via transitions through the two intermediate excited levels, despite the large decay rates of both levels compared with the inverse of the laser pulse durations, provided that the pulse timing sequences are correctly adjusted. Hence multiple stimulated Raman scattering can be an efficient method of moving population into a given molecular level.
The application of photoemission, molecular orbital calculations, and molecular mechanics to the silver–poly(p-phenylene sulfide) interface
Authors: Gerenser L. J. , Goppert-Berarducci K. E. , Baetzold R. C. , Pochan J. M.
Published: July 2002

The interfacial chemistry between evaporated Ag and poly(p-phenylene sulfide) (PPS) has been investigated with x-ray photoelectron spectroscopy (XPS). The initial stages of nucleation and growth of Ag in the subatomic-layer regime were monitored. The PPS core level spectra exhibit no detectable changes as a function of Ag coverage; however, the Ag 3d5/2 peak position shifts ∼0.6 eV to higher binding energy at low Ag coverages. At low Ag coverages, the valence band spectra consist of two main features centered at 2.5 and 5.9 eV. Unrestricted Hartree–Fock (UHF) molecular orbital calculations suggest that the 5.9 eV band consists of weakly perturbed Ag 4d orbitals and bonding Ag 4d–S 3p hybrid orbitals. The 2.5 eV band consists of antibonding Ag 4d–S 3p hybrid orbitals. Molecular mechanics results suggest that the sulfur atom transforms from a bent to either a trigonal pyramidal or trigonal planar configuration after bonding with Ag. Both the molecular orbital calculations and molecular mechanics results suggest that the trigonal pyramidal configuration is more favorable.
Stable and metastable phases of water adsorbed on Cu(111)
Authors: Hinch B. J. , Dubois L. H.
Published: July 2002

Metastable and stable phases of water adsorbed at low coverages on Cu(111) were studied using vibrational spectroscopy (both time-resolved and high-resolution electron energy-loss spectroscopies) and temperature programmed desorption. The two water species have distinct spectroscopic signatures and different desorption kinetics. The more stable species appears to be three-dimensional clusters of bulklike, polycrystalline ice while the metastable species was observed to have a higher desorption rate despite a larger activation enthalpy for desorption. A simple model is proposed to explain this intriguing result. The conversion from the metastable to the stable species, as well as the affects of preadsorbed carbon monoxide on the adsorption/desorption kinetics, are also discussed.
Calculation of the entropy of binary hard sphere mixtures from pair correlation functions
Authors: Laird Brian B. , Haymet A. D. J.
Published: July 2002

We evaluate the entropy of several binary hard sphere fluid mixtures using two approximate expressions that require as input only the pair correlation functions, gαβ(r). An approximation based on the incompressible limit is found to be accurate for medium to high density fluids.
A molecular dynamics study of doubly charged Ar2+n clusters
Authors: Goldberg Alexander , Last Isidore , George Thomas F.
Published: July 2002

Molecular dynamics modeling of doubly charged Ar2+n cluster destruction is performed, where analytical expressions are constructed for the interatomic potentials. It is shown that for the small cluster Ar2+13, Coulomb explosion leads mostly to the detachment of ionic dimers. In larger clusters (n≥55), Coulomb explosion leads to the fission of the doubly charged cluster to two singly charged clusters of similar size. After the fission, the singly charged clusters evaporate some number of neutral atoms. The critical size of the Ar2+n clusters’ stability is found to be nc=92–95, i.e., 92–95 atoms is the maximal size of a cluster undergoing Coulomb explosion.
Rotor-synchronized amplitude-modulated nuclear magnetic resonance spin-lock sequences for improved cross polarization under fast magic angle sample spinning
Authors: Hediger S. , Meier B. H. , Ernst R. R.
Published: July 2002

We demonstrate that in solid-state nuclear magnetic resonance the Hartmann–Hahn cross-polarization efficiency in the presence of fast magic angle sample spinning can be improved by the application of rotor-synchronized amplitude-modulated radio-frequency irradiation during the mixing time (AMCP pulse sequences). The modulation reintroduces time-independent coupling elements between dressed eigenstates of the Floquet Hamiltonian which lead to flip–flop transitions. By numerical optimization, an optimal AMCP pulse sequence is designed which provides a significant improvement over conventional cross polarization. The matching condition can be broadened by rotor-synchronized 180° phase shifts to render the transfer less sensitive to exact matching of the applied radio-frequency field strengths.
Electronic absorption spectra of linear carbon chains in neon matrices. II. C−2n, C2n, and C2nH
Authors: Freivogel Patrick , Fulara Jan , Jakobi Michael , Forney Daniel , Maier John P.
Published: July 2002

Absorption spectra observed between 400 and 2500 nm in 5 K neon matrices have been assigned to electronic transitions of linear C−2n (n=2–10): 2Π←X 2Π, C2n (n=3–7): 3Σ−u←X 3Σ−g, and C2nH (n=3–8): 2Π←X 2Π chains. The species have been produced by mass selected deposition of cations or anions produced in a hot cathode discharge source (C2n, C−2n, C2nH) and by laser vaporization of graphite (C2n, C−2n). In addition to experimental and chemical evidence, the dependence of the absorption wavelength on the number of carbon atoms was used to assign the electronic transitions. Infrared absorptions which were recorded around 2000 cm−1 are attributed to asymmetric stretching frequencies of C8, C10, and C12. This is based on correlation of their intensities with the identified electronic bands. The possible relevance of the electronic spectra of these carbon chains to astrophysical observations of diffuse interstellar bands is discussed.
The low frequency density of states and vibrational population dynamics of polyatomic molecules in liquids
Authors: Moore Preston , Tokmakoff A. , Keyes T. , Fayer M. D.
Published: July 2002

Instantaneous normal mode calculations of the low frequency solvent modes of carbon tetrachloride (CCl4) and chloroform (CHCl3), and experiments on the vibrational population dynamics of the T1u CO stretching mode (∼1980 cm−1) of tungsten hexacarbonyl in CCl4 and CHCl3 are used to understand factors affecting the temperature dependence of the vibrational lifetime. Picosecond infrared pump–probe experiments measuring the vibrational lifetime of the T1u mode from the melting points to the boiling points of the two solvents show a dramatic solvent dependence. In CCl4, the vibrational lifetime decreases as the temperature is increased; however, in CHCl3, the vibrational lifetime actually becomes longer as the temperature is increased. The change in thermal occupation numbers of the modes in the solute/solvent systems cannot account for this difference. Changes in the density of states of the instantaneous normal modes and changes in the magnitude of the anharmonic coupling matrix elements are considered. The calculated differences in the temperature dependences of the densities of states appear too small to account for the observed difference in trends of the temperature dependent lifetimes. This suggests that the temperature dependence of the liquid density causes significant changes in the magnitude of the anharmonic coupling matrix elements responsible for vibrational relaxation.
Equilibrium phase behavior of nematic mixtures
Authors: Chiu Hao-Wen , Kyu Thein
Published: July 2002

A phenomenological model for predicting phase diagrams of a binary nematic mixture containing side chain liquid crystalline polymers and/or low molar mass liquid crystals has been proposed by combining Flory–Huggins free energy of isotropic mixing and Maier–Saupe free energy for nematic ordering of the nematogens. Two orientational order parameters, s1 and s2, of the two components in the mixtures having two different clearing temperatures are taken into consideration in the calculation. The Flory–Huggins interaction parameter, χ, and the nematic interaction parameter of the Maier–Saupe theory, ν11 and ν22, are assumed to be functions of inverse absolute temperature. Further the cross-nematic interaction is assumed to be proportional to the square root of the product of the nematic interaction parameters of the two mesogens, i.e., ν12=c (ν11⋅ν22)1/2. The theory predicts a variety of phase diagrams depending on a single parameter, c, which is a measure of a relative strength of interaction between two dissimilar mesogens to that in the same species. The predicted phase diagrams have been tested rigorously with experimental phase diagrams of various nematic mixtures reported by others as well as by us.
Theoretical study on the photostimulated desorption of CO from a Pt surface
Authors: Nakatsuji H. , Morita H. , Nakai H. , Murata Y. , Fukutani K.
Published: July 2002

Photostimulated desorptions (PSD’s) of CO, CO+, and CO− from a Pt surface are studied theoretically using Pt2–CO model cluster including image force correction. Calculations are performed by the single excitation configuration interaction and the symmetry adapted cluster (SAC)/SAC-CI methods. The PSD’s of the ground state CO occur as the Menzel–Gomer–Redhead (MGR) process and those of CO+ (n cation) and excited (n→π*) CO* through the modified MGR process in which the upper repulsive potential curves are nonadiabatic; the process proceeds through a sequence of nonadiabatic transitions between the similar pertinent states embedded in the metal excited bands. The excited states as the desorption channels are characterized by the excitations from the Pt–CO bonding orbitals to the antibonding MO’s: metal-adsorbate chemical bond cleavage by photons which leads to a repulsive potential is essential for the PSD. The electrostatic image force interaction plays only a minor role and the present result does not support the Antoniewicz model. The calculated excitation-energy thresholds for the CO, CO+, and CO* desorptions are 1.6∼2.6, 11.3, and 11.3–12.7 eV, respectively, which explains the energy thresholds and the fluence dependencies of the incident laser in the PSD experiments. On the other hand, the PSD giving CO− would occur with the energy range of 6.2–8.2 eV, one to two photon energy of the 193 nm (6.4 eV) laser. Since the upper nonadiabatic potential curves have shallow minima, in this case, the lifetime of the CO− species would be larger than those of the CO+ and CO* species. The present study clarifies the electronic structures of the desorbed CO+, CO−, and CO* species, which have not been identified experimentally.
Observation of the ν1 OH(OD) stretch of HOI and DOI by Fourier transform infrared emission spectroscopy
Authors: Klaassen Jody J. , Lindner Jörg , Leone Stephen R.
Published: July 2002

The spectra of vibrationally hot HOI formed in the reaction of alkyl iodides with oxygen atoms are observed by Fourier transform infrared emission spectroscopy. The v=1–3 levels of the OH stretch are observed via the Δv=−1 and Δv=−2 sequence bands. The spectrum of DOI is observed by using 2,2,2-d3-iodoethane as the precursor in the oxygen atom reaction. The v=1–4 levels of the OD stretch are observed in the Δv=−1 sequence band, and the v=1–5 levels of the OD stretch are observed in Δv=−2. Medium resolution spectra (0.031 cm−1 apodized) are recorded and rotationally analyzed for the ν1 fundamental and 2ν1−ν1 hot band of HOI. An estimate of the HOI ground state structure is made by constraining the OH bond length to its value for HOCl and HOBr and calculating the HOI bond angle and the OI bond length by least squares fit to the ground state rotational constants.
Nonadiabatic molecular dynamics simulation of photodissociation and geminate recombination of I2 liquid xenon
Authors: Batista V. S. , Coker D. F.
Published: July 2002

In this paper we investigate the B state predissociation and subsequent geminate recombination of photoexcited iodine in liquid xenon using a coupled quantum-classical molecular dynamics method and a model Hamiltonian gained from the diatomics-in-molecules semiempirical approach to excited state electronic structure including spin-orbit coupling. We explore the capabilities of these techniques as applied to studying the dynamics of realistic condensed phase reactions by comparing with available experimental data from recent ultrafast spectroscopic studies and Raman scattering measurements. We present a microscopic understanding of how the solvent perturbs the electronic states of the chromophore and opens various channels for dissociation from the bound excited B state. We survey the different possible dissociative channels and determine their relative importance as a function of solvent density. We find that predissociation usually occurs during the first bond extension within about 50–100 fs. We follow our trajectories out to 2 ps and observe early solvent collisions which, at the highest solvent densities studied, often result in geminate recombination to the excited bound A′ state with in this time.
The zero static internal stress approximation in lattice dynamics, and the calculation of isotope effects on molar volumes
Authors: Allan N. L. , Barron T. H. K. , Bruno J. A. O.
Published: July 2002

For crystals in which relative positions of atoms within the unit cell are not wholly determined by symmetry, a complete application of the quasiharmonic approximation requires the minimization of the free energy with respect to both external (ηλ) and internal (εj) strains. The zero static internal stress approximation first minimizes the static lattice energy with respect to the εj for each state of external strain; the total free energy is then minimized only with respect to the ηλ. We show that although this gives an incorrect internal strain, to the first order it gives the correct external strain at each temperature; in principle, errors are thus of the same order as those due to the use of the quasiharmonic approximation. In particular, recent calculations by Lacks [D. J. Lacks, J. Chem. Phys. 103, 5085 (1995)] of the effect of deuteration on the molar volume of polyethylene are shown by the present analysis to include indirectly the effect of vibrational stretching of the C–H(D) bonds, and their reasonable agreement with room temperature measurements may after all not be fortuitous.
Atomic and electronic structures of neutral and charged boron and boron-rich clusters
Authors: Niu J. , Rao B. K. , Jena P.
Published: July 2002

Ab initio molecular orbital theory based on both density functional formalism and quantum chemical methods has been used to calculate the equilibrium geometries, binding energies, ionization potentials, fragmentation patterns, and electronic structures of neutral and charged boron clusters containing up to six atoms. Calculations have also been performed on restricted geometries for BnX (n=1,5,12; X=Be, B, C) and B20 clusters to see if clusters can be designed so as to increase their stability. Energetics of doubly charged Bn++ clusters have also been studied to find the critical size for Coulomb explosion. The results are compared with existing experimental and theoretical data.
Vibrational coherence in nonadiabatic dynamics
Authors: Bixon M. , Jortner Joshua
Published: July 2002

In this paper we explore temporal vibrational coherence effects in nonadiabatic radiationless transitions between two electronic states in a large molecule or in the condensed phase, accounting explicitly for the role of the (intramolecular and/or medium) vibrational quasicontinuum of the final states. Our treatment of the time evolution of the wave packet of states and of coherence effects in the nonradiative population probabilities of the reactants and the products rests on the diagonalization of the Hamiltonian of the entire multimode system, with supplementary information being inferred from the effective Hamiltonian formalism. New features of the vibrational Franck–Condon quasicontinuum, which originate from weak, but finite, correlations between off-diagonal coupling terms, were established. The state dependence of the off-diagonal couplings Vsα between the doorway states manifold {|s〉} and the quasicontinuum {|α〉} was quantified by the correlation parameters ηss=〈VsαVαs〉/[〈Vsα2〉〈Vsα2〉]1/2, where 〈 〉 denotes the average over the relevant energy range. Calculations were conducted for a Franck–Condon four-mode system consisting of ns=100 doorway states and nα=3000 quasicontinuum states. The correlation parameters for all pairs of doorway states are considerably lower than unity (|ηss|≲0.4), obeying propensity rules with the highest values of |ηss| corresponding to a single vibrational quantum difference, while for multimode changes between |s〉 and |s〉 very low values of |ηss| are established. Quantum beats in the population probabilities of products and reactants in nonadiabatic dynamics are characterized by an upper limit for their modulation amplitudes ξ≅(Γ/ΔE)η (for ΔE/2πΓ⩾1), where Γ is the decay width of the doorway states and ΔE is their energetic spacing. These low ξ values originate from a small (∼Γ/ΔE) contribution to the off-diagonal matrix elements of the nonradiative decay matrix in conjunction with low correlation parameters. The amplitudes of the quantum beats in nonradiative temporal dynamics provide dynamic information on the larger correlation parameters ηss. Our theoretical and numerical analysis was applied for temporal coherence effects in nonadiabatic electron transfer dynamics in a Franck–Condon quasicontinuum of Mulliken charge transfer complexes [K. Wynne, G. Reid, and R. M. Hochstrasser, J. Chem. Phys. 105, 2287 (1996)]. This accounts for the “preparation” (signature of coherent excitation), for the low amplitudes of coherent temporal modulation of reactants and products (ξ≅0.05–0.06 determined by the ηss parameters) and for the dominating contributions to temporal coherence (subjected to propensity rules).
Investigation of the pure rotational spectrum of magnesium monobromide by Fourier transform microwave spectroscopy
Authors: Walker Kaley A. , Gerry Michael C. L.
Published: July 2002

The pure rotational spectrum of the free radical MgBr has been measured in its Σ+2 ground electronic state by Fourier transform microwave spectroscopy. Transitions have been observed for both Mg24Br79 and Mg24Br81 in the v=0 and v=1 vibrational states. Rotational and centrifugal distortion constants have been determined for each isotopomer in each vibrational state. Equilibrium rotational constants have been calculated and an accurate equilibrium bond length has been determined. Spin-rotation constants, for both the unpaired electron and the bromine nuclei, have been calculated along with magnetic and nuclear quadrupole hyperfine constants for the bromine nuclei. From these constants, the electronic structure of MgBr has been investigated and comparisons have been made to similar compounds. The unpaired electron spin density on the bromine nucleus has been found to be very small, suggesting that this is a very ionic compound. However, the Mg–Br bond has been found to have more covalent character than the bond in other alkaline earth monobromides.
Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams
Authors: Olsen C. , Rowntree P. A.
Published: July 2002

We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ infrared spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C–H stretching modes has permitted the determination of the first dissociation cross sections for electron stimulated reactions in self-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0–15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous work that directly measured H2 desorption yields during irradiation. The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of ∼4 eV. Unlike previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the organic film, but is strongly localized near the methyl terminations of the film. The dissociation cross sections at this interface increase rapidly with increasing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via charge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consistent with a dipole-image dipole quenching model, whereby the excited state lifetimes are reduced from an estimated ∼26 fs (for a gas-phase electron-alkane collision) to ∼2–10 fs, depending on the chain length. These distance-dependent lifetimes cause the dissociation yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced dissociation cross sections for alkanethiol monolayers should show much stronger isotopic dependencies than found with the gas-phase alkane species.