bluuebunny/crossref_metadata_2025 huggingface dataset:
The standard enthalpy of formation of CH2
Authors: Császár Attila G. , Leininger Matthew L. , Szalay Viktor
Published: May 2003

High-quality ab initio quantum chemical methods, including higher-order coupled cluster and full configuration interaction benchmarks, with basis sets ranging from [C/H] [4s3p1d/2s1p] to [9s8p7d5f4g3h2i/7s6p5d4f3g2h] have been employed to obtain the best technically possible value for the standard enthalpy of formation of X̃ 3B1 CH2 and ã 1A1 CH2. Careful extrapolations of finite basis MP2, CCSD, CCSD(T), and CCSDT energies to the complete basis set full configuration interaction limit plus inclusion of small corrections owing to relativistic effects, core correlation, and the diagonal Born–Oppenheimer correction results in the final extrapolated enthalpies of formation of this study, ΔfH0o(X̃ 3B1 CH2)=390.45−0.64+0.68 kJ mol−1 and ΔfH0o(ã 1A1 CH2)=428.10−0.64+0.68 kJ mol−1. The computed value for X̃ 3B1 CH2 is in between the best two experimental results of 389.87±0.86 and 390.73±0.66 kJ mol−1. The elaborate calculations leading to these enthalpies of formation also resulted in accurate estimates of the singlet-triplet splitting, T0(ã 1A1 CH2)=37.54−0.29+0.41 kJ mol−1, in excellent agreement with the best empirical value of 37.65±0.06 kJ mol−1, of the total atomization enthalpy, D0(X̃ 3B1 CH2)=753.03−0.62+0.43 kJ mol−1, in excellent agreement with the best experimental value of 753.3 kJ mol−1, of the bond dissociation energy, DU1o(0 K)(CH–H)=417.85±0.35 kJ mol−1, and of the quartic force field representations of the potentials of the two states around their respective minima.
Quantum reactive scattering calculations for the D+H2→HD+H reaction
Authors: Kendrick Brian K.
Published: May 2003

The results of accurate quantum scattering calculations in symmetrized hyperspherical coordinates are reported for the D+H2(v,j)→HD(v′,j′)+H reaction at 48 values of total energy in the range 0.4–2.32 eV. Integral and differential cross sections are computed using the Boothroyd–Keogh–Martin–Peterson (BKMP2) potential energy surface for all values of total angular momentum J⩽34. Two sets of calculations are reported: one set includes the effects of the geometric phase and the other does not. By comparing these two sets of calculations, the effects of the geometric phase on the scattering results are investigated both as a function of total energy and total angular momentum. Several transition state resonances survive the sum over J and are observed in many of the fully converged state-to-state integral and differential cross sections. In some cases a series of resonances is observed. The energy spacings between many of the resonances are consistent with the energy spacings between the even or odd bending modes of the quantized transition states of H3.
Radiative association of HeH2+
Authors: Mrugała Felicja , Špirko Vladimír , Kraemer Wolfgang P.
Published: May 2003

Rigorous state-to-state quantum calculations of the dynamics of the radiative association reaction He+H2+→HeH2++hν are performed. For this purpose the appropriate methodology is described in detail and computational aspects facilitating the actual calculations of the resonances and the free-bound phototransition amplitudes are discussed. Under the assumptions that the reaction is a single-state process proceeding entirely on the ground electronic state potential energy surface of HeH2+ and that higher dissociation channels of the ion complex can be neglected, all resonances contributing to the association are determined and the rate constant as a function of temperature is calculated for the low-temperature interval 2⩽T⩽100 K. Its maximum value is predicted to be small, 2.1×10−20 cm3s−1 at a temperature of about 20 K.
Termolecular collisions between benzene and Ar
Authors: Bernshtein V. , Oref I.
Published: May 2003

Termolecular collisions between a benzene molecule and two Ar atoms were studied by quasiclassical trajectory calculations. The calculations show that termolecular collisions form termolecular complexes and occur by three mechanisms: (a) the Chaperon mechanism, in which the first Ar in is the first Ar out of the termolecular complex, is the dominant one at high pressures. Two-thirds of all termolecular collisions go by this mechanism. (b) The energy transfer mechanism, in which the first Ar in is the last Ar out of the termolecular complex, comprises about a quarter of all termolecular collisions at high pressures. (c) The concerted channel, in which both argon atoms depart from the benzene simultaneously and does not lead to products in reactive systems, comprises about 10% of all termolecular collisions. Energy transfer quantities and collision complex lifetimes in binary and termolecular collisions are evaluated and their dependence on inter- and intramolecular harmonic and anharmonic potentials, temperature, pressure, and internal energy of the benzene molecule are reported.
Accurate quantum calculations on three-body collisions in recombination and collision-induced dissociation. II. The smooth variable discretization enhanced renormalized Numerov propagator
Authors: Colavecchia F. D. , Mrugała F. , Parker G. A. , T Pack R.
Published: May 2003

We introduce a novel solution of the coupled-channel Schrödinger equation. This new procedure dramatically improves on our previous paper on this subject. The method uses a truly adiabatic internal basis and combines a smooth variable discretization (SVD) with an enhanced renormalized Numerov (ERN) propagator. Although the basis is truly adiabatic, this method does not require derivative coupling terms, and it involves less numerical work than previous SVD approaches. Boundary conditions are applied using Jacobi coordinates for bound states and using hyperspherical coordinates for continuum states; that allows application of the boundary conditions at smaller distances. We apply this new algorithm to the model collision-induced dissociation process Ne2+H→Ne+Ne+H for zero total angular momentum. We study the convergence of the probabilities as a function of the number of channels, distance propagated, and step size in the propagation. The method is fast, reliable, and provides considerable savings over previous propagators.
Photodissociation of CH2Br2, 1,1- and 1,2-C2H4Br2 at 248 nm: A simple C–Br bond fission versus a concerted three-body formation
Authors: Lee Ya-Rong , Chen Cheng-Chu , Lin Shen-Maw
Published: May 2003

The photodissociation of CH2Br2 (DBM), 1,1- and 1,2-C2H4Br2 (DBE) at 248 nm was investigated using product translational spectroscopy. The results show that DBM and 1,1-DBE undergo a simple C–Br bond fission with fragments recoiling anisotropically. When the laser energy increased from 10 to 40 mJ/pulse, the nascent products, CH2Br and CH3CHBr, absorbed an additional photon to produce the secondary Br atom. In contrast, 1,2-DBE dissociates into the triple products Br (fast) + Br (slow) + C2H4 in a concerted reaction. This is confirmed from the measured anisotropy of the products, which indicates that the reaction is completed in a fraction of rotational period. Following an asynchronous concerted reaction, the simulation for the triple products was derived with the P(Et) distributions coupled by asymmetric angular distributions. The results are discussed in terms of the weakness of the C–Br bond strength in the β-bromoethyl radical that a rapid scission of the second C–Br bond occurred asynchronously with the cleavage of the first C–Br bond along the nσ* potential energy surface.
Experimental and theoretical investigation of the AlH b 3Σ−–a 3Π electronic transition
Authors: Tao Chong , Tan Xiaofeng , Dagdigian Paul J. , Alexander Millard H.
Published: May 2003

The laser fluorescence excitation spectrum of the b 3Σ−–a 3Π (0,0) band of AlH and AlD is reported. The AlH/AlD(a 3Π) state was prepared in a free-jet supersonic expansion by the reaction of photolyzed trimethylaluminum with hydrogen or deuterium. Spectroscopic constants for the upper and lower vibronic levels were derived from fits to the measured transition wave numbers of the rotational lines. Lifetimes of J′=1 rotational/fine-structure levels of electronically excited AlH/AlD(b 3Σ−,v′=0) were determined from fluorescence decay waveforms with laser excitation on isolated rotational lines. The measured lifetimes were compared with values obtained in a theoretical treatment of the excited-state decay dynamics, wherein both radiative decay to the a 3Π state and nonradiative decay through the repulsive 1 3Σ+ state were considered. The experimental and theoretical lifetimes are in good agreement. The theoretical treatment shows that the nonradiative excited-state decay dominates over radiative decay. The observed fine-structure dependence of the lifetimes results is due to the nature of the spin–orbit coupling of the b 3Σ− state with the 1 3Σ+ continuum.
Discrete breathers in classical ferromagnetic lattices with easy-plane anisotropy
Authors: Khalack J. M. , Zolotaryuk Y. , Christiansen P. L.
Published: May 2003

Discrete breathers (nonlinear localized modes) have been shown to exist in various nonlinear Hamiltonian lattice systems. This paper is devoted to the investigation of a classical d-dimensional ferromagnetic lattice with easy plane anisotropy. Its dynamics is described via the Heisenberg model. Discrete breathers exist in such a model and represent excitations with locally tilted magnetization. They possess energy thresholds and have no analogs in the continuum limit. We are going to review the previous results on such solutions and also to report new results. Among the new results we show the existence of a big variety of these breather solutions, depending on the respective orientation of the tilted spins. Floquet stability analysis has been used to classify the stable solutions depending on their spatial structure, their frequency, and other system parameters, such as exchange interaction and local (single-ion) anisotropy.
Monodromy in the spectrum of a rigid symmetric top molecule in an electric field
Authors: Kozin I. N. , Roberts R. M.
Published: May 2003

We show that for rigid symmetric top molecules in electric fields the phenomenon of monodromy arises naturally as a “defect” in the lattice of quantum states in the energy-momentum diagram. This makes it impossible to use either the total angular momentum or a pendular quantum number to label the states globally. The monodromy is created or destroyed by classical Hamiltonian Hopf bifurcations from relative equilibria. These phenomena are robust and should be observable in quasi-symmetric top molecules with field strengths ℰ satisfying μE/b>4.5, where μ is the dipole moment and b the rotational constant perpendicular to the symmetry axis of the molecule.
High-resolution jet-cooled and room temperature infrared spectra of the ν3 CH stretch of vinoxy radical
Authors: Utkin Yu. G. , Han Jia-Xiang , Sun Fuge , Chen Hong-Bing , Scott Graham , Curl R. F.
Published: May 2003

Rotationally resolved jet-cooled and room temperature spectra of the ν3 stretch of vinoxy radical (CH2CHO) have been obtained near 2800 cm−1 by means of tunable infrared laser spectroscopy. About 500 transitions corresponding to b-type selection rules have been assigned in the jet-cooled spectrum recorded in the region 2809–2860 cm−1. The band origin has been determined to be 2827.913 cm−1. A number of lines are split into doublets by the electron spin–rotation interaction. The jet-cooled spectrum has been supplemented by transitions with high N or K values observed in the room temperature spectrum, which was recorded over the spectral region 2754–2894 cm−1. Both local and global perturbations have been found in the upper state energy levels. Least-squares fitting of the apparently unperturbed levels yielded molecular constants as well as centrifugal distortion and spin-rotational constants for the upper state.
An improved multigain range linear current electrometer
Authors: Rajput S. S.
Published: May 2003

The JFET switch properties set the measurement range of a multiple gain range current electrometer and affect its performance at large input currents through the introduction of nonlinearities in the resultant current measurements. Drain saturation current (Idss) and leakage current (Io) of a JFET decide its current measuring range. An improved current electrometer, which is capable of measuring current beyond Idss(≈2 Idss), is presented. The measurement errors are within ±2%.
A new spectrometer using multiple gratings with a two-dimensional charge-coupled diode array detector
Authors: Han Tao , Ni Wei-Ming , Zhou Peng , You Hai-Yang , Jia Jian-Hu , Wang Song-You , Chen Liang-Yao , Lynch David W.
Published: May 2003

A new spectrometer with no moving parts uses a two-dimensional Si-based charge-coupled diode (CCD) array detector and an integrated grating consisting of three subgratings. The effective spectral range imaged on the detector is magnified threefold. The digitized spectral image in the 200–1000 nm wavelength range can be measured quickly. The nonlinear relationship between CCD pixel position and wavelength is corrected with multiple polynomial functions in the calibration procedure, which fits the data using a mathematical pattern-analysis method. The instrument can be applied for rapid spectroscopic data analyses in many types of photoelectronic experiments and routine testing.
Design, development, and performance of an adapter for simulation of ocular melanoma patients in supine position for proton beam therapy
Authors: Daftari I. , Phillips T. L.
Published: May 2003

A patient assembly adapter system for ocular melanoma patient simulation was developed and its performance evaluated. The aim for the construction of the apparatus was to simulate the patients in supine position using a commercial x-ray simulator. The apparatus consists of a base plate, head immobilization holder, patient assembly system that includes fixation light and collimator system. The reproducibility of the repeated fixation was initially tested with a head phantom. Simulation and verification films were studied for seven consecutive patients treated with proton beam therapy. Patient’s simulation was performed in a supine position using a dental fixation bite block and a thermoplastic head mask immobilization device with a patient adapter system. Two orthogonal x rays were used to obtain the x, y, and z coordinates of sutured tantalum rings for treatment planning with the EYEPLAN software. The verification films were obtained in treatment position with the fixation light along the central axis of the eye. The results indicate good agreement within 0.5 mm deviations. The results of this investigation showed that the same planning accuracy could be achieved by performing simulation using the adapter described above with a patient in the supine position as that obtained by performing simulation with the patient in the seated, treatment position. The adapter can also be attached to the head of the chair for simulating in the seated position using a fixed x-ray unit. This has three advantages: (1) this will save radiation therapists time; (2) it eliminates the need for arranging access to the treatment room, thus avoiding potential conflicts in treatment room usage; and (3) it allows the use of a commercial simulator.
High duty, long lifetime, cathodic arc plasma source
Authors: Wang L. P. , Wang X. F. , Tang B. Y. , Gan K. Y. , Peng X. Y.
Published: May 2003

We have developed a repetitively pulsed cathodic arc plasma source that can operate at high duty cycle and with long lifetime between downtimes for cathode maintenance. The arc discharge current can be up to 300 A, the arc pulse width can be varied from 0.1 to 4 ms, and the source can operate at a duty cycle of up to 30% for many hours. The cathode can be of a diameter from 20 to 40 mm, and can be easily and quickly changed. A 45° magnetic duct is used for macroparticle filtering, employing a pulsed magnetic field. We have operated the source over a wide parameter range, and we report here on the variation of plasma ion deposition current with arc current, duct magnetic field, and duct bias voltage. A momentary ion deposition current of 0.7 A was obtained with the source running stably for more than 8 h with an arc current of 100 A and duty cycle of 20%.
High-temperature and high-pressure cell for kinetic measurements of supercritical fluids reactions with the use of ultraviolet-visible spectroscopy
Authors: Gorbaty Yuri E. , Venardou Eleni , Garcia-Verdugo Eduardo , Poliakoff Martyn
Published: May 2003

A high-temperature high-pressure ultraviolet-visible (UV-Vis) cell is described. The cell has been designed specifically for use with the UV spectrophotometer Hewlett–Packard 8453 but it could work up to 780 K at 100 MPa with any other UV-Vis spectrophotometer, as well as for near infrared (NIR) experiments. Three features of the cell make it convenient for experiments with supercritical fluids: the possibility to choose an optimal path length, the presence of three interchangeable ports into the cell, and a movable thermocouple in the working zone of the cell. The cell has been used to study a range of chemical reactions in water under near-critical and supercritical conditions, as well as for measuring the kinetics of such reactions. Some examples illustrating the performance of the cell are given.
A combined heating cooling stage for cluster thermalization in the gas phase
Authors: Ievlev D. N. , Küster A. , Enders A. , Malinowski N. , Schaber H. , Kern K.
Published: May 2003

We report on the design and performance of a combined heating/cooling stage for the thermalization of clusters in a gas phase time-of-flight mass spectrometer. With this setup the cluster temperature can sensitively be adjusted within the range from 100 up to 800 K and higher. The unique combination of a heating stage with a subsequent cooling stage allows us to perform thermodynamic investigations on clusters at very high temperatures without quality losses in the spectra due to delayed fragmentation in the drift tube of the mass spectrometer. The performance of the setup is demonstrated by the example of (C60)n clusters.
Theoretical investigations on relative stabilities of fullerene C94
Authors: Zhao Xiang , Slanina Zdenek , Goto Hitoshi , Ōsawa Eiji
Published: May 2003

The complete set of 134 isolated-pentagon-rule isomers of C94 has been first investigated by various quantum-chemical approaches. Twenty low-energy cage structures are computed by the ab initio Hartree–Fock self-consistent field method and hybrid density functional theory treatment. All the methods point out a C2 species as the system ground state. The computed temperature-dependent relative concentrations under the interisomeric thermodynamic equilibrium evaluated by a combined quantum-chemical and statistical-mechanical treatment, predict two distinct structures (Cs,C2) as the most thermodynamically populated isomers in the system. This finding agrees quite well with the recently reported experimental observations.
New rovibrational kinetic energy operators using polyspherical coordinates for polyatomic molecules
Authors: Schwenke David W.
Published: May 2003

We illustrate how one can easily derive kinetic energy operators for polyatomic molecules using polyspherical coordinates with very general choices for z-axis embeddings and angles used to specify relative orientations of internal vectors. Computer algebra is not required.
Mixed quantum-classical Liouville molecular dynamics without momentum jump
Authors: Ando Koji , Santer Mark
Published: May 2003

An alternative Liouville formulation of mixed quantum-classical dynamics outlined recently [K. Ando, Chem. Phys. Lett. 360, 240 (2002)] is expanded in detail by taking an explicit account of the parametric dependence of the electronic (adiabatic) basis on the nuclear coordinates. As a consequence of the different operational order of the partial Wigner transformation for the nuclear coordinates and the calculation of the matrix elements in the adiabatic electronic basis, the present formula differs from the previously proposed one, slightly in the appearance but significantly in the treatment of nonadiabatic transitions in the trajectory implementation in that the former does not contain the “off-diagonal Hellmann–Feynman forces” representing the so-called “momentum-jump” associated with the nonadiabatic transitions. Because of this, the present formula is free from the numerical instability intrinsically coming from the momentum-jump operation at around the classical turning points of the nuclear motion. It is also shown that the density matrices from the two approaches coincide when the electronic basis is independent of the nuclear coordinates (R), and hence the momentum-jump approximation stems from the R-dependence of the adiabatic electronic basis. Improved stability and comparable to better reproduction of the quantum reference calculations are demonstrated by applications to one and three dimensional spin-boson models and a two-state three-mode model of the S2→S1 internal conversion of pyrazine. Also discussed is the importance of electronic coherence for the proper treatment of nonadiabatic transition rates which is naturally described by the Liouville methods compared to the conventional independent trajectory approaches.
A thin–thick transition in the surface-frozen layer of a binary alcohol mixture
Authors: Sloutskin E. , Kraack H. , Gang O. , Ocko B. M. , Sirota E. B. , Deutsch M.
Published: May 2003

A new quadrilayer surface phase, and its transition to bilayer and liquid surface phases, are observed by x-ray and surface tension measurements in the surface-frozen layer of a binary mixture of ∼20%C26OH:80%C18OH alcohols. The surface phase diagram is rationalized qualitatively based on the free energy landscape. A quantitative elucidation of the molecular-level origin of these effects is not currently available.
Quantitative prediction of gas-phase C13 nuclear magnetic shielding constants
Authors: Auer Alexander A. , Gauss Jürgen , Stanton John F.
Published: May 2003

Benchmark calculations of C13 nuclear magnetic shielding constants are performed for a set of 16 molecules. It is demonstrated that near-quantitative accuracy (∼1–2 ppm deviation from experiment) can be achieved if (1) electron correlation is adequately treated by employing the coupled-cluster singles and doubles (CCSD) model augmented by perturbative corrections for triple excitations [CCSD(T)], (2) large (uncontracted) basis sets are used, (3) calculations are performed at accurate equilibrium geometries (obtained from CCSD(T)/cc-pVTZ or CCSD(T)/cc-pVQZ calculations), and (4) vibrational averaging is included. In this way [CCSD(T)/13s9p4d3f calculations corrected for vibrational effects], the mean deviation and standard deviation from experiment are 1.6 and 0.8 ppm, respectively. Less complete theoretical treatments result in larger errors. Consideration of relative shifts might reduce the mean deviation (through an appropriately chosen reference compound), but cannot change the standard deviation. Density-functional theory calculations of nuclear magnetic shielding constants are found to be less accurate, intermediate between Hartree–Fock self-consistent-field and second-order Møller–Plesset perturbation theory.
Ab initio study of the two lowest triplet potential energy surfaces involved in the N(4S)+NO (X 2Π) reaction
Authors: Gamallo P. , González Miguel , Sayós R.
Published: May 2003

This work presents ab initio electronic structure calculations of the two possible N(4S)+NO(X 2Π ) abstraction reaction channels on the lowest A″3 and A'3 potential energy surfaces (PESs). Complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations (CASPT2), and multireference configuration interaction calculations (MR-CI) based on CASSCF wave functions, along with some coupled cluster (CC) calculations were carried out by using the standard correlation-consistent (cc-pVnZ and aug-cc-pVnZ, n=D,T,Q,5) Dunning’s basis sets. It was shown that there was no energy barrier along the minimum energy path in the A″3 PES for the N-abstraction reaction channel. However, an energy barrier (6.74 kcal/mol) was located in the A'3 PES. This energy barrier was considerably smaller than the previously reported MR-CCI value (14.4 kcal/mol). It was established that the N and O 2s electron correlation, neglected in previous studies of these authors, was the main source of this energy decrease. As a result, the present ab initio data will produce larger values of the thermal rate constants at high temperatures. High-energy barriers were found for the O-abstraction reaction channel in both PESs (41.13 and 30.77 kcal/mol for A″3 and A'3 , respectively), which agree with the accepted idea that this channel will be only important at high collision energies. Nonetheless, current ab initio results show that this channel will be open at not very high collision energies (e.g., over 30 kcal/mol could take place). Experimental studies on the O-abstraction reaction channel are missing and would be useful to confirm its ab initio expected importance.
Combustion and atmospheric oxidation of hydrocarbons: Theoretical study of the methyl peroxyl self-reaction
Authors: Ghigo Giovanni , Maranzana Andrea , Tonachini Glauco
Published: May 2003

Alkyl peroxyls form in the atmospheric oxidation of hydrocarbons and in their combustion. When NO concentration is low, they can appreciably react with themselves. This reaction has both propagation and termination channels. Multireference second-order perturbative energy calculations CAS(16,12)-PT2/6-311G(2df,p) have been carried out on the CAS(8,8)-MCSCF/6-311G(d,p) geometries pertaining to the reaction pathways explored. The tetroxide intermediate put forward first by Russell in 1957 is found as a stable energy minimum, but the calculations indicate that, as the system moves from atmospheric to combustion temperatures, its formation becomes problematic. A concerted synchronous transition structure, apt to connect it with the termination products, formaldehyde, methanol, and dioxygen, is not found. The concerted dissociation of the two external O–O bonds in the tetroxide leads to the (CH23O•)23⋯3O2 complex, with overall singlet spin multiplicity. Both termination via H transfer, to give H2CO, CH3OH, and O2, or dissociation to 2 CH3O•+O2 (possible propagation) are feasible. The former could occur in principle with production of either excited O21 or excited H23CO. However, if a sufficiently easy intersystem crossing (ISC) could take place in the complex, the process would end up with all ground-state molecules. The (possible) propagation channels are favored by higher temperatures, while lower temperatures favor the ISC mediated termination channel. A fairly good qualitative agreement with experimental T dependence of the relevant branching ratio is found. From the tetroxide over again, dissociation of a single external O–O bond leads to CH3O• and CH3O3•, or possibly to a (CH3O•⋯CH3O3•)1 complex, but further transformations along this line are not competitive.
On the performance of four-component relativistic density functional theory: Spectroscopic constants and dipole moments of the diatomics HX and XY (X,Y=F, Cl, Br, and I)
Authors: Fossgaard O. , Gropen O. , Valero M. Corral , Saue T.
Published: May 2003

We have tested the performance of four-component relativistic density functional theory (DFT) by calculating spectroscopic constants (re, ωe, and ωexe) and dipole moments μ0 in the vibrational ground state for a selected set of 14 molecules: the hydrogen halides HX, the dihalogens X2, as well as the interhalogens XY (X,Y=F, Cl, Br, and I). These molecules have previously been studied by four-component relativistic wave function based methods by Visscher and co-workers [J. Chem. Phys. 108, 5177 (1998); 104, 9040 (1996); 105, 1987 (1996)]. We have used four different nonrelativistic functionals at the DZ and TZ basis set level. What is perhaps the most striking result of our study is the overall good performance of the local density approximation functional SVWN5; at the triple zeta basis set level it predicts bond lengths re, harmonic frequencies ωe, anharmonicities ωexe, and dipole moments μ0 with relative errors of 0.46%, 0.39%, −16.3%, and −0.74%, respectively. The corresponding values for the B3LYP hybrid functional are 1.27%, −2.10%, −20.4%, and 4.71%. The two generalized gradient approximation functionals PW86 and BLYP show a less convincing performance, characterized by a systematic overestimation of bond lengths and underestimation of harmonic frequencies. We show that only the constant term is modified in second-order vibrational perturbation theory upon the inclusion of a linear term, corresponding to the choice of a nonstationary reference geometry. Upon shifting the reference geometry from the optimized to the experimental geometry the calculated harmonic frequencies are significantly improved, whereas the anharmonicities are basically unchanged. Dipole moments calculated at the experimental geometry at the B3LYP/TZ level appear to be remarkably accurate with a mean relative error of −1.1% and a standard deviation of less than 4%. Our study reveals that anharmonicities are quite sensitive to the numerical integration scheme employed in the DFT calculations, and for the interhalogens we had to modify the Becke partitioning scheme by using atomic adjustments along the lines of the atom in molecules approach of Bader.
Novel method for the simultaneous estimation of density and surface tension of liquids
Authors: Thirunavukkarasu G. , Srinivasan G. J.
Published: May 2003

The conventional Hare’s apparatus generally used for the determination of density of liquids has been modified by replacing its vertical arms (glass tubes) with capillary tubes of 30 cm length and 0.072 cm diameter. When the columns of liquids are drawn through the capillary tubes with reduced pressure at the top of the liquid columns and kept at equilibrium with the atmospheric pressure acting on the liquid surface outside the capillary tubes, the downward pressure due to gravity of the liquid columns has to be coupled with the pressure arising due to the effect of surface tension of the liquids. A fresh expression for the density and surface tension of liquids has been arrived at while equating the pressure balancing system for the two individual liquid columns of the modified Hare’s apparatus. The experimental results showed that the proposed method is precise and accurate in the simultaneous estimation of density and surface tension of liquids, with an error of less than 5%.
Combined techniques of Raman spectroscopy and synchrotron two-dimensional x-ray diffraction for in situ study of anisotropic system: Example of polymer fibers under deformation
Authors: Ran Shaofeng , Fang Dufei , Sics Igors , Toki Shigeyuki , Hsiao Benjamin S. , Chu Benjamin
Published: May 2003

Simultaneous measurements of Raman spectroscopy and synchrotron two-dimensional (2D) wide-angle x-ray diffraction (WAXD) have been successfully demonstrated for in situ study of an anisotropic system: isotactic polypropylene (iPP) fiber under tensile deformation. A fiber-optic probe was used to remotely deliver the incident laser beam on the sample as well as to collect the Raman signal based on the confocal arrangement, whereas high resolution 2D WAXD patterns were obtained simultaneously at the same position during deformation of polymers. The combined techniques yielded complementary information on the molecular structural evolution in both crystalline and amorphous phases. 2D WAXD results showed that the α-form iPP crystals were converted into the mesophase upon stretching at room temperature. Corresponding Raman spectra showed that characteristic bands from the crystal phase became weaker or disappeared during the transition from the crystal phase to the mesophase. However, the bands associated with the helical structure were still present, indicating the remainder of the helical conformation in the mesomorphic phase. The persistence of the Raman band at 812 cm−1 in the mesophase implied that the structural difference between the crystal phase and the mesophase in iPP is due to the packing defects, rather than the conformational deviations from the (⋯TGTG⋯) sequence in chains. The variations in the Raman intensity ratio (I812/I843) indicated that the overall orientation of the fiber increased during stretching. The demonstrated techniques will be particularly useful for in situ studies of anisotropic polymers, where orientational dependent structural information from both crystalline and amorphous phases needs to be collected simultaneously.
Time- and angle-resolved photoelectron spectrometer based on a picosecond Nd:glass laser system
Authors: Michelato P. , Monaco L. , Sertore D. , Pollini I.
Published: May 2003

A time- and angle-resolved photoelectron spectrometer based on an amplified picosecond Nd:glass laser system is described. The kinetic energy of photoemitted electrons, excited by 0.4 ps laser pulses at 264 nm, is measured with a time-of-flight electron energy analyzer. A theoretical model that takes into account the electron drift inside the μ-metal shielded tube as far as the electron analyzer has been introduced to calculate the energy distribution curves of photoelectrons. As a test of the experimental apparatus we present photoemission spectra of cesiated silver excited with 4.7 eV photons. Good agreement between the experimental and the calculated distribution curves of photoemitted electrons has been found. Finally, we have briefly discussed the electronic properties of silver in terms of free electron and d-band transitions, together with Auger processes for photon energies near 4 eV.
Structural investigation of amorphous materials at high pressures using the diamond anvil cell
Authors: Shen Guoyin , Prakapenka Vitali B. , Rivers Mark L. , Sutton Stephen R.
Published: May 2003

A modified diamond anvil cell (DAC) is used for structural studies of amorphous materials at high pressures using a monochromatic synchrotron x-ray beam. The DAC modification includes (1) the use of x-ray transparent seats for a large angular opening for x-ray scattering, and (2) the introduction of a boron gasket insert to increase the sample thickness and to minimize the gasket–hole deformation. A procedure for absorption correction and background subtraction in DAC experiments is described, together with an optimization process for obtaining accurate data of the structure factor and the corresponding pair distribution function. Data for amorphous iron at 67 GPa are presented for demonstration. It is shown that quantitative structural data can be determined for amorphous materials at very high pressures using the DAC. The apparatus should be also useful for structural studies of liquids at high pressures.
High-stability quartz-crystal microbalance for investigations in surface science
Authors: Bouzidi L. , Narine S. S. , Stefanov K. G. , Slavin A. J.
Published: May 2003

This article describes a high-stability quartz-crystal microbalance (QCM) and the methodology for measuring the change in mass during thin-film growth in deposition and sputter processes. Much lower noise and higher-frequency stability have been achieved than with conventional QCMs. A stability of ±0.1 Hz at 6 MHz has been obtained over 4 h, with a rms stability of 0.03 Hz. The adsorption of one atomic monolayer of oxygen produces a frequency shift of about 5 Hz, so this stability enables the QCM to be used to determine the stoichiometry of submonolayer oxide films, as well as for high-accuracy measurements of adsorbate sticking probability and ion-milling rate.
Measurement of force acting on a moving part of a pneumatic linear bearing
Authors: Fujii Yusaku
Published: May 2003

A method for evaluating the components of the force acting on a moving part of a pneumatic linear bearing is proposed. The total force acting on the moving part is accurately measured as the inertial force using an optical interferometer. Then, the components of the force, such as the force component depending on position, the force component depending on velocity, and the force component depending on tilt angle, are evaluated using the least-squares method. In the experiment, the total force acting on the moving part is measured with the standard uncertainty of approximately 0.001 N, which corresponds to approximately 0.003% (30 ppm) of the gravitational force acting on it (M=4.119 kg) of approximately 40 N (40 N). The experimentally evaluated values of these force components well coincide with the theoretically expected values.
Intense, hyperthermal source of organic radicals for matrix-isolation spectroscopy
Authors: Zhang Xu , Friderichsen Anders V. , Nandi Sreela , Ellison G. Barney , David Donald E. , McKinnon J. Thomas , Lindeman Theodore G. , Dayton David C. , Nimlos Mark R.
Published: May 2003

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, we have derived the residence time (τnozzle) of the gas pulse in the nozzle to be around 30 μs.
Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation
Authors: Alexander Andrew J. , Kim Zee Hwan , Zare Richard N.
Published: May 2003

Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 239 nm results in the production of open-shell photofragments O(3P)+O(3P). Product O(3Pj; j=0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization (2+1 REMPI) near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2. Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization.
Simple air-tight powder x-ray diffraction cell
Authors: Palchik O. , Gedanken A. , Rubinstein Y.
Published: May 2003

The design and a description of a very simple and inexpensive air-tight cell for powder x-ray diffraction analysis are discussed. The cell is designed for the Bruker AXS powder diffractometer. It is easily manipulated inside a glove box and reduces the sample preparation time to a fraction of the time required for other hermetic cells or environmental chambers.
Ultrahigh vacuum three-axis cryogenic sample manipulator for angle-resolved photoelectron spectroscopy
Authors: Aiura Y. , Bando H. , Miyamoto T. , Chiba A. , Kitagawa R. , Maruyama S. , Nishihara Y.
Published: May 2003

An ultrahigh vacuum three-axis cryogenic sample manipulator suitable for angle-resolved photoelectron spectroscopy experiments was developed. The sample manipulator is constructed by combining three modules with translation, polar rotation, and azimuthal-tilt rotation capabilities. Polar rotation and the azimuthal-tilt rotation are performed using a differentially pumped rotary stage and a sample goniometer, respectively. Continuous 360° rotation around the polar axis is possible. The sample goniometer is capable of azimuthal rotation of up to ±180° and tilt rotation from +95° to −5°, measured from the plane normal to the polar axis. Nonmagnetic materials are used near the sample holder of the goniometer. The sample holder can be cooled using a continuous-flow cryostat. To serve as a radiation shield, the lower portion of the goniometer surrounding the sample holder is cooled separately by another cell filled with liquid nitrogen. With liquid nitrogen or liquid helium for the cryostat, the sample holder can be cooled down to 90 or 12.5 K, respectively.
Wavelength-tunable surface plasmon resonance microscope
Authors: Fu Elain , Foley Jennifer , Yager Paul
Published: May 2003

We report the development of a simple surface plasmon resonance microscope that uses a method of varying the operating wavelength of the system. The instrument is based on the Kretschmann configuration and uses a single compact rotation element and an interference filter to vary the imaging wavelength of the system. The operation of the instrument is demonstrated.
Reusable ultrahigh vacuum viewport bakeable to 240 °C
Authors: Cox S. G. , Griffin P. F. , Adams C. S. , DeMille D. , Riis E.
Published: May 2003

We have developed a removable and bakeable viewport for UHV applications. The viewport consists of a modified blank Conflat flange; a ring of flux-free solder to form a vacuum seal; a viewport window; a clamping flange, and conical disk springs. We have cyclically baked viewports up to 240 °C and have achieved an ultimate pressure of 1.2×10−10 Torr (limited by our pumping station), with no leak detected at the 10−10 atm cm3/s level. Both BK7 and zinc selenide windows have been used successfully.
Versatile small volume closed-cycle flow cell system for transient spectroscopy at high repetition rates
Authors: Bredenbeck Jens , Hamm Peter
Published: May 2003

A closed-cycle flow cell system was designed and built for application in ultrafast laser spectroscopy. The system features small cycle volume of down to less than 200 μl, adjustable beam path length, and compatibility with a large variety of solvents. The construction principle is simple, reliable, and versatile. The thickness and material of the windows can be easily exchanged to meet different demands of femtosecond spectroscopy. The flow rate is sufficient to allow complete sample exchange for measurements with a kHz repetition rate.
Reduction of electronic noise in THz generation
Authors: Wetzels A. , Gürtler A. , Buijserd A. , Vijftigschild T. , ter Horst H. , van der Zande W. J.
Published: May 2003

The generation of half-cycle pulses, using laser illumination with femtosecond pulses of a biased GaAs wafer, induces strong electronic noise. We found that electronic noise can be significantly reduced by mounting an RC circuit directly on the semiconductor wafer.
Nonlinear oscillations in marine hydroids
Authors: Venslauskas Mindaugas S. , Asmantas Sarunas
Published: May 2003

Irregular oscillations in a colony of marine hydroids Podocoryne carnea were investigated. Quantitative characteristics were obtained as a result of long term (10–12 h) monitoring of oscillations at arbitrary sites. The sliding window spectra as well as the pulse-to-pulse dynamics argue the transient chaotic behavior of hydroid colony. The significant change of amplitudes and frequencies in intact colony oscillations after feeding and long sustained oscillations of stolons separated from colony suggest that the irregular activity could be determined by the network of pacemakers residing in stolon wall cells. These are influenced mechanically by the amount of digesting food and/or by chemical action of nutrients inside the stolon lumen. The possible correlation of these oscillations which can evoke Ca2+ waves in stolon wall cells is discussed.
Local atomic structural order in the supercooled liquid and glassy Al under normal and high pressures
Authors: Hui Li , Pederiva Francesco
Published: May 2003

Empirical tight binding potential has been used in molecular dynamics studies of the local atomic structural order and defects in liquid and glassy Al under high and normal pressures. Results are reported for some solidlike clusters and structural properties of liquid aluminum. The simulated pair correlation function of the liquid Al is in good agreement with the experimental results. Both a structure very close to an ideal icosahedron and a structure related to a defective icosahedron are found. The fivefold symmetric structure plays an important role in glass formation. The fcc structural unit is demonstrated in glass by the existence of 1421 bond pair, but only in small proportion. The effect of high pressure on the supercooled liquid and glass is also studied. High pressure causes the nearest neighbor distance to shorten. High pressure favors the icosahedral and defective icosahedral structures. Glassy Al is an entanglement of a very large number of icosahedra and defective icosahedra which interpenetrate and share faces with each other.
Kirkwood–Buff derived force field for mixtures of acetone and water
Authors: Weerasinghe Samantha , Smith Paul E.
Published: May 2003

A united atom nonpolarizable force field for the simulation of mixtures of acetone and water is described. The force field is designed to reproduce the thermodynamics and aggregation behavior of acetone–water mixtures over the full composition range at 300 K and 1 atm using the enhanced simple point charge water model. The Kirkwood–Buff theory of solutions is used to relate molecular distributions obtained from the simulations to the appropriate experimental thermodynamic data. The model provides a very good description of the solution behavior at low (xa<0.2) and high (xa>0.8) acetone concentrations. Intermediate compositions display a small systematic error in the region of highest water self-aggregation, which is removed on using larger system sizes. In either case, the activity of the solution is well reproduced over the full range of compositions.
Coarse molecular dynamics of a peptide fragment: Free energy, kinetics, and long-time dynamics computations
Authors: Hummer Gerhard , Kevrekidis Ioannis G.
Published: May 2003

We present a “coarse molecular dynamics” approach and apply it to studying the kinetics and thermodynamics of a peptide fragment dissolved in water. Short bursts of appropriately initialized simulations are used to infer the deterministic and stochastic components of the peptide motion parametrized by an appropriate set of coarse variables. Techniques from traditional numerical analysis (Newton–Raphson, coarse projective integration) are thus enabled; these techniques help analyze important features of the free-energy landscape (coarse transition states, eigenvalues and eigenvectors, transition rates, etc.). Reverse integration of coarse variables backward in time can assist escape from free energy minima and trace low-dimensional free energy surfaces. To illustrate the coarse molecular dynamics approach, we combine multiple short (0.5 ps) replica simulations to map the free energy surface of the “alanine dipeptide” in water, and to determine the ∼1/(1000 ps) rate of interconversion between the two stable configurational basins corresponding to the α-helical and extended minima.