bluuebunny/crossref_metadata_2025 huggingface dataset:
Index of refraction and mechanical behavior of soda lime glass under shock and release wave propagations
Authors: Dandekar Dattatraya P.
Published: July 2002

This article reports a set of experiments designed to measure change in the refractive index of transparent material under both planar shock and release wave propagations. Information about both mechanical and optical properties of transparent material were obtained simultaneously through the measurement of particle velocity at or near the impact surface and the free surface velocity. Data thus obtained is used to determine shock and release wave velocities and the Hugoniot elastic limit (HEL) of the material. Shock wave velocity in soda lime glass remains unchanged at 5.83±0.04 km/s, i.e., equal to the measured ultrasonic longitudinal wave velocity, when it is shock compressed to less than or equal to 4.3 GPa. The value of shock wave velocity begins to decline when the impact stress in the glass exceeds this value. The release wave velocity, however, remains equal to the measured ultrasonic longitudinal wave velocity to only 3 GPa, it begins to decline at higher stresses. The variation in the refractive index of the glass shows a cusp at 3.04–3.14 GPa. Additionally, up to and including the impact stress of 3.14 GPa, the refractive index after shock compression and release is within 1% of its ambient value, but at higher stresses it differs by larger percentage points. The HEL of the glass is determined to be 3.10±0.06 GPa although it lacks the classic well defined cusp in all the recorded wave profiles of soda lime glass.
Step structure of arsenic-terminated vicinal Ge (100)
Authors: Gan S. , Li L. , Begarney M. J. , Law D. , Han B.-K. , Hicks R. F.
Published: July 2002

Germanium (100) crystals, 9° off-axis towards the [011] were exposed to 2.0 Torr of tertiarybutylarsine and 99.0 Torr of hydrogen at 650 °C, then heated to between 450 and 600 °C in vacuum or H2. The resulting surfaces consist of narrow dimer-terminated terraces, with (1×2) and (2×1) domains, that are separated by steps between one and eight atomic layers in height. The distribution of (1×2) and (2×1) domains changes with temperature, exhibiting a pronounced maximum in the (1×2) fraction at 510 °C. These results suggest that the arsenic passivation of germanium is a critical step in gallium arsenide heteroepitaxy.
Optical constants of cubic GaN in the energy range of 1.5–3.7 eV
Authors: Köhler U. , As D. J. , Schöttker B. , Frey T. , Lischka K. , Scheiner J. , Shokhovets S. , Goldhahn R.
Published: July 2002

The refractive index and extinction coefficient of cubic GaN in the energy range of 1.5–3.7 eV were determined with high accuracy using combined reflectivity and spectroscopic ellipsometry studies of layers grown by molecular beam epitaxy on GaAs(001). A comparison of the experimental reflectivity data with theoretical calculations demonstrates that the data analysis has to be performed by taking into account both surface roughness and a nonabrupt substrate–film interface. In the transparent region the refractive index of cubic GaN was found to be slightly higher than that of the hexagonal modification.
Stochastic dynamics in quenched-in disorder and hysteresis
Authors: Bertotti Giorgio , Basso Vittorio , Magni Alessandro
Published: July 2002

The conditions under which relaxation dynamics in the presence of quenched-in disorder lead to rate-independent hysteresis are discussed. The calculation of average hysteresis branches is reduced to the solution of the level-crossing problem for the stochastic field describing quenched-in disorder. Closed analytical solutions are derived for the case where the disorder is characterized by Wiener–Lévy statistics. This case is shown to be equivalent to the Preisach model and the associated Preisach distribution is explicitly derived, as a function of the parameters describing the original dynamic problem.
Hall effect of La2/3(Ca,Pb)1/3MnO3 single crystals
Authors: Chun S. H. , Salamon M. B. , Han P. D.
Published: July 2002

The Hall resistivity ρxy of a La2/3(Ca,Pb)1/3MnO3 single crystal has been measured as a function of temperature and field. The overall behavior is similar to that observed previously in thin-film samples. At 5 K, ρxy is positive and linear in field, indicating that the anomalous contribution is negligible. However, the slope is small and, if a free carrier, single band model were used, the carrier density would be 2.4 holes per unit cell, even larger than the 0.85–1.9 holes per cell that have been reported using thin-film data and far larger than the 0.33 holes per cell expected from the doping level. As the temperature is increased, a strong, negative contribution to ρxy appears, due to the anomalous contribution to the Hall effect. Making use of a detailed measurement of the magnetization M(B,T), we separate the ordinary (∝B) and anomalous (∝M) contributions. The anomalous contribution is negative and proportional to the zero-field resistivity ρxx below TC, indicating that magnetic skew scattering is the dominant mechanism in the metallic ferromagnetic regime. Far above TC, ρxy shows a negative slope, and is to be associated with the hopping of small polarons. Above TC, the Hall mobility is field independent despite the changes in ρxx and nonlinear ρxy.
Biquadratic coupling in sputtered Fe/Cr/Fe still in need of a new mechanism
Authors: Rezende S. M. , Chesman C. , Lucena M. A. , de Moura M. C. , Azevedo A. , de Aguiar F. M. , Parkin S. S. P.
Published: July 2002

The bilinear (J1) and biquadratic (J2) exchange coupling constants were measured in sputtered trilayers of (100) Fe(40 Å)/Cr(s)/Fe(40 Å) for several Cr spacer layer thicknesses in the range s=8–35 Å and as a function of temperature T, using magneto-optical Kerr effect magnetometry, Brillouin light scattering, and ferromagnetic resonance. In the samples in the range s=8–13 Å, corresponding to the first antiferromagnetic peak of J1, J2 follows J1 with a room temperature ratio J2/J1≅0.1, while in the range 25–35 Å, corresponding to the second antiferromagnetic peak, J2 also follows J1 but with a much larger ratio J2/J1≅1. This result, as well as the temperature dependence of J2 in all samples but the one with s=15 Å, cannot be explained by any of the intrinsic or extrinsic mechanisms that have been proposed for the origin of the biquadratic exchange coupling in Fe/Cr/Fe.
Vacuum ultraviolet annealing of hydroxyapatite films grown by pulsed laser deposition
Authors: Craciun Valentin , Boyd Ian W. , Craciun Doina , Andreazza Pascal , Perriere Jacques
Published: July 2002

The effect of a post-deposition vacuum ultraviolet (VUV) radiation-assisted annealing treatment performed under 1 bar of oxygen at moderate temperatures (450 °C) upon thin hydroxyapatite (HA) films grown by the pulsed laser deposition technique was investigated. The HA layers were deposited at 650 °C under different partial oxidizing pressures without any water vapor and exhibited, besides the HA crystalline phase, tetracalcium phosphate and calcium oxide phases, more so for the films grown at lower oxidizing pressures. After the VUV-assisted anneal the layers were transformed into high quality crystalline HA films, exhibiting Ca/P ratio values closer to 1.67, the value for stoichiometric HA. The content of the other crystalline phases initially present was reduced significantly. Infrared spectroscopy also showed that the amount of OH− in the films increased after the treatment. The combination of these two low temperature techniques opens the possibility of growing high quality HA layers without significant oxidation of the substrate material.
Ordering kinetics in Cu–Al–Ni shape memory alloys
Authors: Recarte V. , Pérez-Sáez R. B. , Nó M. L. , San Juan J.
Published: July 2002

The atomic order has a strong influence on the martensitic transformation behavior in Cu-based shape memory alloys. In this work we have studied the influence of the quenching and postquench aging treatments on the atomic order of the metastable β phase obtained by quenching in Cu–Al–Ni shape memory alloys by means of thermoelectric power (TEP) measurements. The initial evolution of TEP during aging shows two clear stages where TEP value decreases. These two TEP stages are interpreted as ordering processes at nearest neighbors (B2 order), and ordering processes at next nearest neighbors (L21 order), respectively. The analysis of the isothermal kinetics during aging at different temperatures has allowed us to determine the activation energies for both ordering processes.
Fermi-level pinning and Schottky barrier heights on epitaxially grown fully strained and partially relaxed n-type Si1−xGex layers
Authors: Mamor M. , Nur O. , Karlsteen M. , Willander M. , Auret F. D.
Published: July 2002

The Schottky barrier height on n-type Si1−xGex films has been studied as a function of the composition and strain relaxation. We have used electrical I–V measurements complemented by high-resolution x-ray measurements for assessment of the relaxation in the epilayers. In addition, Schottky barrier height on n-Si1−xGex films has also been investigated as a function of the metal work function. Our results shows that the barrier height on n-type Si1−xGex does not depend on either the Ge content or strain relaxation, but is sensitive to the metal work function. The experimental results indicate that the Fermi level is pinned to the conduction band and provide also the evidence that the pinning position of the Fermi level is metal work function dependent. This pinning behavior in metal Si1−xGex is opposed to that observed in metal/Si contacts, were the Fermi level is pinned either to the valence or conduction band depending on the metal work function. These findings regarding the relaxation independent barrier height on n-type Si1−xGex are suggesting only the movement of the valence band of Si1−xGex/Si heterostructure upon relaxation as expected.
Photogeneration and transport of charge carriers in hybrid materials of conjugated polymers and dye-sensitized TiO2
Authors: Däubler T. K. , Glowacki I. , Scherf U. , Ulanski J. , Hörhold H.-H. , Neher D.
Published: July 2002

Steady state photoconductivity and current–voltage (I–V) experiments are performed on solid films of organic/inorganic composites of dye-sensitized TiO2 in combination with poly(N-vinylcarbazole) (PVK), a ladder-type PPP and a soluble PPV derivative. The I–V characteristics of the composites in the dark are explained by the formation of percolation networks of nanoparticles between the electrodes. Photoaction spectra of the devices prove that the photogeneration of charge carriers is significantly enhanced and spectrally broadened only if electron transfer from the polymer to the dye is possible. Increasing the concentration of the nanoparticles in the hybrid materials changes the spectral shape of the photoresponse. For high TiO2 contents signatures due to the absorption of the Ruthenium dye can be observed. The different electronic properties of anatase/brookite TiO2 and rutile TiO2 have only minor effects on the generation of charge carriers and on the shape of the photoaction spectra. Indeed, photocurrents are more related to the Brunauer—Emmett–Teller surface area of the nanoparticles and thus, to the amount of dye adsorbed. Charge collection efficiencies exceeding 100%, observed for both bias directions, can be explained by photoconductivity gain. It is proposed that recombination is reduced by transport of the oppositely charged carriers in two different phases.
Effects of as-quenched structures on the phase transformations and magnetic properties of melt-spun Pr7Fe88B5 ribbons
Authors: Wang Zuocheng , Zhou Shouzeng , Zhang Maocai , Qiao Yi , Wang Run
Published: July 2002

Phase transformations and magnetic properties of overquenched Pr7Fe88B5 ribbons during annealing have been investigated. X-ray diffraction and Mössbauer measurement indicate that melt spinning at different wheel velocities caused the as-quenched ribbons to have distinctive structures. Depending on their as-quenched structure, the phase transformation of the ribbons during annealing may take place in one of the following sequences: (1) amorphous phase (Am)+Pr2Fe14B+α-Fe→Pr2Fe14B+α-Fe; (2) Am+α-Fe→Am′+α-Fe→α-Fe+1:7 phase+Pr2Fe14B→Pr2Fe14B+α-Fe; and (3) Am→Am′+α-Fe→1:7 phase+α-Fe→Pr2Fe14B+α-Fe. In all cases, the microstructure of the ribbons after optimal annealing was found to only consist of two magnetic phases: Pr2Fe14B and α-Fe. However, with increasing initial quenching rate, the microstructure of optimally heat treated ribbons becomes coarser and more irregular, and the magnetic properties of them deteriorated drastically. The δM plots, irreversible susceptibility, and the temperature dependence of coercivity of the annealed ribbons were measured and analyzed. It was found that the coarser-grained and more irregular microstructures, as mentioned above, would lead to the weakening of the exchange coupling effect between the hard and soft phase, the inhomogeneous nucleation of reverse domain, a lower critical nucleation field, and a decrease in αex and Neff. These may be the reasons that the optimal magnetic properties of annealed ribbons reduce significantly with increasing initial quenching rate.
Canted exchange bias in antiparallel biased spin valves
Authors: Marrows C. H. , Stanley F. E. , Hickey B. J.
Published: July 2002

Conventional spin valves consist of two magnetic layers, one of which is pinned by exchange bias to act as a reference magnetic layer in giant magnetoresistance measurements. Antiparallel biased spin valves feature a Co/Ru/Co trilayer in place of this pinned layer, where the Co layers are strongly antiferromagnetically coupled. This leads to hugely enhanced pinning since an applied field will exert opposite torques on the Co layer moments. Spin valves are typically deposited in a magnetic field to define magnetic anisotropies, and the development of magnetic structure in the pinned AF couple is not trivial under these conditions. The resultant moment of the AF couple is found to slowly rotate as the final Co layer is deposited. This results in canted pinning once the exchange bias layer is added. There appears to be some magnetic friction as the Co layers rotate during growth. The rotation means that the pinning direction can take any angle with respect to the growth field, leading to positive or negative (or even orthogonal) exchange bias.
Trap characterization by gate-drain conductance and capacitance dispersion studies of an AlGaN/GaN heterostructure field-effect transistor
Authors: Miller E. J. , Dang X. Z. , Wieder H. H. , Asbeck P. M. , Yu E. T. , Sullivan G. J. , Redwing J. M.
Published: July 2002

Gate-drain capacitance and conductance measurements were performed on an Al0.15Ga0.85N/GaN heterostructure field-effect transistor to study the effects of trap states on frequency-dependent device characteristics. By varying the measurement frequency in addition to the bias applied to the gate, the density and time constants of the trap states have been determined as functions of gate bias. Detailed analysis of the frequency-dependent capacitance and conductance data was performed assuming models in which traps are present at the heterojunction (interface traps), in the AlGaN barrier layer (bulk traps), and at the gate contact (metal–semiconductor traps). Bias-dependent measurements were performed at voltages in the vicinity of the transistor threshold voltage, yielding time constants on the order of 1 μs and trap densities of approximately 1012 cm−2 eV−1.
Microwave spectrum of the boron monoxide radical, BO
Authors: Tanimoto Mitsutoshi , Saito Shuji , Hirota Eizi
Published: July 2002

The microwave spectrum of the boron monoxide radical in the ground electronic state X 2Σ+ has been observed by using a glow discharge cell spectrometer. The boron monoxide radical was generated in an absorption cell by a glow discharge in a gaseous mixture of boron trichloride, nitric oxide, and nitrogen. The observed transitions were N=1–0 and 3–2 in 100 and 300 GHz regions, respectively. The latter transition was observed by using a frequency multiplier working in the 300 GHz region as a microwave source. The spectra of two isotopic species 11BO and 10BO were analyzed to yield molecular parameters including the rotational constants, centrifugal distortion constants, spin-rotation interaction constants, magnetic hyperfine interaction constants, and nuclear quadrupole interaction constants. The spin density of the unpaired σ electron was calculated from the magnetic hyperfine coupling constants and was found to be in good accordance with the result of ab initio calculations.
Stochastic thermodynamics of nonequilibrium steady states in chemical reaction systems
Authors: Mou Chung Yuan , Luo Jiu-li , Nicolis Gregoire
Published: July 2002

We present a stochastic theory of entropy production for steady states in chemical reaction systems. Small scale internal fluctuations around steady states are considered in the Gaussian regime. It is shown that in addition to the usual Gibbsian form of entropy production, there is an entropy production due to fluctuation which is of order O(V0). This comes from the non-Poisson character of the probability distribution in a nonequilibrium system. Two approaches are considered: in the first, we use an entropy balance equation based on the master equation; in the second, we use a stochastic potential related to the probability distribution and built from the generalized Einstein relation. We show that both approaches give the same result for the entropy production of fluctuation (diS/dt) f . Next we consider a simple one-component nonequilibrium system under the perturbation of a macroscopically large external fluctuation as a power generator. We interpret (diS/dt) f  in terms of net power gain factor under random external fluctuations in the spirit of a generalized ‘‘fluctuation–dissipation’’ theorem. It is shown that positive power gain is possible only for system staying far enough from equilibrium.
Determination of NO (v=0–7) product distribution from the N(4S)+O2 reaction using two-photon ionization
Authors: Winkler I. , Stachnik R. A. , Steinfeld J. I. , Miller S. M.
Published: July 2002

The product vibrational state distribution for the reaction N(4S)+O2→NO(2Π,v)+O has been measured using saturated multiphoton ionization spectroscopy to determine NO electronic ground-state distributions. The fraction of the reaction exothermicity appearing in product vibration is 〈 f v〉=0.34; however, the even vibrational levels (v=0,2,4,6) are relatively overpopulated with respect to the odd vibrational levels (v=1,3,5). It is not possible to obtain a good linear-surprisal fit to all the data, but the even and odd subsets fit quite well to individual surprisal plots. The results are compared with previous measurements, including a reanalysis of laser-excited fluorescence data corrected for electronic transition moment and Franck–Condon factor variations. Collisional relaxation is observed at high O2 pressures for NO levels v=4 through 7, and is fit with a phenomenological relaxation model.
T–V energy transfer and the exchange reaction of H(D)+HF at 2.2(2.1) eV: Vibrational state distributions by time and wavelength resolved infrared fluorescence
Authors: Cousins Lisa M. , Leone Stephen R.
Published: July 2002

The product state distributions for hot atom collisions of H(D)+HF are measured by the laser photolysis–infrared emission technique. The vibrational distribution of the HF T–V transfer process and exchange reaction product at 2.2 eV is 0.81±0.08, 0.16±0.02, and 0.03±0.01 corresponding to v=1–3, respectively. The HF and DF distritubions resulting from D+HF collisions at 2.1 eV are 0.65±0.09, 0.25±0.05, and 0.10±0.02 for HF(v=1–3) and 0.55±0.09, 0.25±0.04, 0.14±0.02, and 0.06±0.01 for DF(v=1–4). It is found that H atoms are 3.0 times more efficient than D atoms at exciting HF vibrations for the same kinetic energy. Although the vibrational distributions are similar, the D+HF T–V channel deposits approximately two times as much energy in the HF molecules as the vibrational exchange channel leaves in the DF molecules. The results are compared to recent three-dimensional quasiclassical trajectory calculations and classical infinite-order-sudden calculations (accompanying paper) and are also considered in light of some simple models. The agreement between experiment and theory is excellent. The theoretical results show that significantly different mechanisms are resonsible for T–V energy transfer on the reactive and unreactive portions of the potential energy surface.
Calculated potential surfaces for the reactions: O+N2→NO+N and N+O2→NO+O
Authors: Walch Stephen P. , Jaffe Richard L.
Published: July 2002

Complete active space SCF/contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N+O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A′ surface and 18.0 kcal/mol for the 4A′ surface. The O+N2 reaction is endoergic by 75 kcal/mol. The 3A″ surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A′ surface is calculated to have a late barrier of 14.4 kcal/mol relative to NO+N.
Torsional dynamics of molecules on barrierless potentials in liquids. I. Temperature and wavelength dependent picosecond studies of triphenyl-methane dyes
Authors: Ben-Amotz Dor , Harris C. B.
Published: July 2002

The nonradiative decay dynamics of crystal violet, and other triphenyl-methane dyes, dissolved in a variety of solvents, are studied as a function of temperature. A linear viscosity dependence of the excited state absorption decay time in n-alcohol solvents is found at several constant temperatures. The temperature dependence at constant viscosity is anomalously negative over the entire viscosity range (0.6 to 8.0 cP) of these experiments. Various possible mechanisms for the observed behavior are critically discussed. Two color excite-and-probe studies reveal ground state bleach recovery times which are independent of excitation wavelength but strongly dependent on probe wavelength. The faster decay on the red side of the ground state absorption is shown to be the result of stimulated emission rather than the influence of a second state.
Self-consistent Monte Carlo simulations of the electron and ion distributions of inhomogeneous liquid alkali metals. I. Longitudinal and transverse density distributions in the liquid–vapor interface of a one-component system
Authors: Harris Jonathan G. , Gryko Jan , Rice Stuart A.
Published: July 2002

Previous Monte Carlo simulations of the liquid–vapor interfaces of simple metals, which predict stratification of the liquid–vapor transition zone extending three atomic diameters into the bulk, suffer from a limitation arising from the use of a local electroneutrality approximation. This approximation is embodied in the assumption that, for the purpose of calculating the change in potential energy accompanying a change in configuration, there is exact coincidence of the electronic and ionic density profiles. In this paper we describe a generalized Monte Carlo simulation which avoids the local electroneutrality approximation by directly incorporating the Lang–Kohn treatment of the jellium-vacuum interface into the simulations. We report the results of generalized Monte Carlo simulations of slabs of sodium at 100 and 200 °C and cesium at 100 °C. The new simulations predict structure in the liquid–vapor interface very similar to that predicted using the local-electroneutrality approximation. The geometry of the slabs used in the new simulations allows us to make more careful studies of the transverse pair correlation functions in the interface than does the geometry of the clusters used in previous simulations of simple alkali metals. We find that in the peaks of the oscillations of the longitudinal density profile, the transverse pair correlation function resembles that of a fluid which is less dense than that of the corresponding homogeneous liquid, except for a slight amplification of the height of the innermost peak. Thus the longitudinal density oscillations predicted by the model do not lead to transverse structure factors resembling those of a higher density liquid, as had previously been expected. Despite the stratification, outer layers of the interface are not packed in a crystalline lattice, as is indicated by the isotropy of the transverse structure factor.
State specific electronic quenching rates for 14N+2 and 15N+2
Authors: Katayama D. H. , Dentamaro A. V. , Welsh J. A.
Published: July 2002

We have conducted laser double resonance experiments to measure the branching ratio for the rates of electronic transfer from the A 2Πui (v=4) state to the X 2∑+g (v=8 and 7) levels for 14N+2 and 15N+2. Because of the propensity for ΔJ∼0 in these electronic transfer processes, the energy gaps can be determined and although they are similar in magnitude for the two isotopes, the branching ratios are significantly different. For 14N+2 the rate from A(v=4) to X(v=8) is about twice as great as to the X(v=7) level whereas for 15N+2, this ratio is approximately 6. This difference is reflected in the dramatic change in shape from 14N+2 to 15N+2 of the radiative decay curve from the A 2Πui (v=4) level. The 14N+2 curve appears to be single exponential in contrast to the double exponential for 15N+2. We use these branching ratios in an electronic relaxation model that describes the observed decay curves well, to deduce the state specific electronic quenching rates for these two isotopes.
Kinetic oscillations in the catalytic CO oxidation on Pt(100): Periodic perturbations
Authors: Schwankner R. J. , Eiswirth M. , Möller P. , Wetzl K. , Ertl G.
Published: July 2002

Periodic modulations of oxygen pressure or temperature were applied in the catalytic oxidation of CO on a Pt(100) surface under isothermal, low pressure conditions. Transitions from aperiodic autonomous oscillations to regular phase-locked behavior could be observed. Computer simulations using a stochastic cellular automaton model yielded qualitatively similar results. The spatial distribution in both experiment and simulation varies essentially in phase over the whole surface area under the influence of the periodic perturbation, while wave propagation in the autonomous system causes more irregular overall behavior.
Infrared flash kinetic spectroscopy of HCO
Authors: Dane C. Brent , Lander D. R. , Curl R. F. , Tittel F. K. , Guo Yili , Ochsner Martin I. F. , Moore C. Bradley
Published: July 2002

The high resolution infrared spectrum of the CH stretching fundamental of the formyl radical (HCO) has been observed by means of infrared kinetic spectroscopy using 308 nm (XeCl) excimer laser flash photolysis of formaldehyde or acetaldehyde followed by diode or difference frequency laser probing of the transient absorption. The high resolution spectra obtained were assigned and fitted with rotational, spin–rotational, and centrifugal distortion constants. The ν1 band origin is 2434.48 cm−1. New ground-state constants are reported from a least-squares fit combining this ν1 data with previous microwave and FIR LMR measurements.
Vibrational relaxation and vibrational cooling in low temperature molecular crystals
Authors: Hill Jeffrey R. , Chronister Eric L. , Chang Ta-Chau , Kim Hackjin , Postlewaite Jay C. , Dlott Dana D.
Published: July 2002

The processes of vibrational relaxation (VR) and vibrational cooling (VC) are investigated in low temperature crystals of complex molecules, specifically benzene, naphthalene, anthracene, and durene. In the VR process, a vibration is deexcited, while VC consists of many sequential and parallel VR steps which return the crystal to thermal equilibrium. A theoretical model is developed which relates the VR rate to the excess vibrational energy, the molecular structure, and the crystal structure. Specific relations are derived for the vibrational lifetime T1 in each of three regimes of excess vibrational energy. The regimes are the following: Low frequency regime I where VR occurs by emission of two phonons, intermediate frequency regime II where VR occurs by emission of one phonon and one vibration, and high frequency regime III where VR occurs by evolution into a dense bath of vibrational combinations. The VR rate in each regime depends on a particular multiphonon density of states and a few averaged anharmonic coefficients. The appropriate densities of states are calculated from spectroscopic data, and together with available VR data and new infrared and ps Raman data, the values of the anharmonic coefficients are determined for each material. The relationship between these parameters and the material properties is discussed. We then describe VC in a master equation formalism. The transition rate matrix for naphthalene is found using the empirically determined parameters of the above model, and the time dependent redistribution in each mode is calculated.
Small-angle x-ray scattering study of the fractal morphology of porous silicas
Authors: Schmidt Paul W. , Höhr Axel , Neumann Hans-Bernd , Kaiser Helmut , Avnir David , Lin Jar Shyong
Published: July 2002

Small-angle x-ray and neutron scattering measurements have shown that on a length scale smaller than the average pore diameter but larger than the diameters of atoms or small molecules, the pore surfaces in four commercial porous silica gels with average pore diameters ranging from approximately 200 to 2500 Å are fractal and have a fractal dimension D=2.15±0.10. When these gels were manufactured, the nonequilibrium micropore structure was relaxed by thermal methods. The scattering data indicate that in the gels with average pore diameters of about 200 and 500 Å, and perhaps also in the two gels with larger average pore diameters, the relaxation process leads to a pore structure nearly identical in form but on a larger scale than the structure in a gel with an average pore diameter of 60 Å that was the material from which the other four gels were produced.
Molecular order at the surface of an organic monolayer studied by low energy helium diffraction
Authors: Chidsey Christopher E. D. , Liu Gang-Yu , Rowntree Paul , Scoles Giacinto
Published: July 2002

We demonstrate that the surface structure of organic monolayers can be determined by low energy helium diffraction at low surface temperatures. This uniquely surface-sensitive and nondestructive technique shows that the CH3-terminated surface of a monolayer of docosane thiol (CH3(CH2)21SH) on Au(111) is composed of small, ordered domains (lattice constant 5.01±0.02 Å), a large fraction of which share a common orientation. The helium diffraction intensities decrease monotonically with increasing temperature and vanish around 100 K, due to thermal motion of the CH3 groups. Surface order is observed for chains as short as ten carbons (CH3(CH2)9SH) but a shorter chain, (CH3(CH2)5SH), gave no diffraction.
Measure of clustering in continuum percolation: Computer-simulation of the two-point cluster function
Authors: Lee Sang B. , Torquato S.
Published: July 2002

The two-point cluster function C2(r1,r2) is determined for a D-dimensional interpenetrable-sphere continuum model from Monte Carlo simulations. C2(r1,r2) gives the probability of finding two points, at positions r1 and r2, in the same cluster of particles, and thus provides a measure of clustering in continuum-percolation systems. A pair of particles are said to be ‘‘connected’’ when they overlap. Results are reported for D=1,2, and 3 at selected values of the sphere number density ρ and of the impenetrability index λ, 0≤λ≤1. The extreme limits λ=0 and 1 correspond, respectively, to the cases of fully penetrable spheres (‘‘Swiss-cheese’’ model) and totally impenetrable spheres.
Analysis of the potential energy surface of Ar–NH3
Authors: Chal/asiński G. , Cybulski S. M. , Szczȩśniak M. M. , Scheiner S.
Published: July 2002

The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of Ar–NH3. Anisotropy of the self-consistent field (SCF) potential is determined by the first-order exchange repulsion. Second-order dispersion energy, the dominating attractive contribution, is anisotropic in the reciprocal sense to the first-order exchange, i.e., minima in one nearly coincide with maxima in the other. The estimated second-order correlation correction to the exchange effect is nearly as large as a half ΔESCF in the minimum and has a ‘‘smoothing’’ effect on the anisotropy of ε(20)disp. The model which combines ΔESCF with dispersion energy (SCF+D) is not accurate enough to quantitatively describe both radial and angular dependence of interaction energy. Comparison is also made between Ar–NH3 and Ar–PH3, as well as with the Ar dimer.
The statistical mechanics of inhomogeneous hard rod mixtures
Authors: Vanderlick T. K. , Davis H. T. , Percus J. K.
Published: July 2002

The exact statistical mechanical solution to the problem of an equilibrium inhomogeneous classical one-dimensional mixture of hard rods is presented. From the solution, thermodynamic properties, density profiles, and correlation functions of hard rod fluids confined to small regions (micropores) can be calculated. The theory is applied to investigate microstructure, pore pressures, and pore adsorption selectivity of micropores in equilibrium with binary hard rod mixtures. A prescription is suggested for generalizing the one-dimensional results to higher dimensions.
The ethyl radical: Photoionization and theoretical studies
Authors: Ruscic B. , Berkowitz J. , Curtiss L. A. , Pople J. A.
Published: July 2002

The ethyl radical has been produced by the F+C2H6 reaction, and studied by photoionization mass spectrometry. The adiabatic ionization potential (IP) is found to be 8.117±0.008 eV. The radical and cation have been studied by the G1 level of molecular orbital theory. The calculated adiabatic ionization potential is 8.06 eV. On the basis of both experiment and theory, it is concluded that the cation in its ground state has a bridged structure. The adiabatic value resolves a previous inconsistency involving ΔH○f(C2H5), ΔH○f(C2H+5) and IP (C2H5). Broad autoionizing structures are observed at higher energy. These features can be rationalized as Rydberg states converging to the first excited state 3A″ calculated to be at 12.05 eV.
Stark level-crossing spectroscopy of S formaldehyde eigenstates at the dissociation threshold
Authors: Polik William F. , Guyer Dean R. , Moore C. Bradley
Published: July 2002

Spectra of S0 D2CO rovibrational eigenstates with 28 300 cm−1 of vibrational excitation are measured by Stark level-crossing spectroscopy. In this new method, the lifetime of a single J, K, M-resolved S1 state is monitored as a function of electric field. Enhanced nonradiative decay causes the S1 lifetime to decrease as S0 states are Stark tuned into resonance. Analysis of the resulting resonance lineshapes yields complete distributions of S0 decay rates (linewidths) and S1-S0 coupling matrix elements. The S0 decay rates represent the first measurements of unimolecular dissociation rates of a polyatomic molecule at the eigenstate-resolved level. S0 decay widths vary from 6.4×10−5 to 3.8×10−3 cm−1 and S1-S0 coupling matrix elements vary from 3.5×10−7 to 4.7×10−5 cm−1, demonstrating that chemical properties of neighboring eigenstates fluctuate by over two orders of magnitude. The observed density of S0 vibrational states is ∼400 per cm−1, six times greater than an estimate including first-order anharmonic corrections. The small increase of level density with J indicates that Ka is nearly a good quantum number for J≤4. The barrier height to unimolecular dissociation on the S0D2CO surface is determined to be 80.6±0.8 kcal/mol, corresponding to 79.2±0.8 kcal/mol for H2CO, in good agreement with ab initio predictions. Quantitative agreement between the magnitude of experimentally determined decay rates and an RRKM rate calculation with all parameters set by ab initio calculation is found.
Theory of ultrahot molecular solids: Vibrational cooling and shock-induced multiphonon up pumping in crystalline naphthalene
Authors: Kim Hackjin , Dlott Dana D.
Published: July 2002

A new method is presented for calculating ultrafast vibrational energy redistribution in anharmonic solids composed of large molecules. It is an improvement over the previous weak coupling model of Hill and Dlott [J. Chem. Phys. 89, 842 (1988)] because the emitted phonons are now allowed to act back on the excited vibrations. The model is used to investigate the dynamics of ‘‘ultrahot’’ molecular solids, materials with enormous levels of vibrational or phonon excitation. Ultrahot solids are produced in laser ablation and shock-induced detonation. Using model parameters for crystalline naphthalene, we investigate multiphonon up pumping after a 40 kbar shock and vibrational cooling after strong excitation of a high frequency vibrational fundamental. In both processes, the phonons attain a state of internal equilibrium characterized by a time-dependent phonon quasitemperature θp(t) within a few ps. Energy redistribution among the phonons is efficient because phonons are more anharmonic than molecular vibrations. In up pumping, there is a large excess of phonons at t=0, which decreases as vibrations are pumped by phonons. Under these conditions, the rates of anharmonic scattering processes are maximum at t=0 and the lower levels of the ladder of molecular vibrations are pumped before the higher levels. The vibrational population distribution then rapidly attains an approximate state of quasiequilibrium, characterized by a vibrational quasitemperature θv(t). Thermal equilibrium where θp(t) = θv(t) is achieved in ∼100 ps. In vibrational cooling, there is initially a large excess of high frequency vibrations and few phonons. Because phonons accumulate as the vibrations cool, the rates of anharmonic scattering processes are a minimum at t=0. Under these conditions, the vibrations are far from a state of quasiequilibrium until thermal equilibrium is attained at ∼1 ns.
Unimolecular decomposition of methylsubstituted benzenes into benzyl radicals and hydrogen atoms
Authors: Park Jeunghee , Bersohn Richard , Oref Izhack
Published: July 2002

A homologous series of methylsubstituted benzenes and pyrazines are excited with an energy E by a short laser pulse following which, typically on a time scale of several hundred nanoseconds, hydrogen atoms are released from the methyl groups. The rate constants k(E) for the formation of the H or D atoms were determined from the time dependence of the laser-induced fluorescence (LIF) signals, and their velocity distributions were measured from the shapes of their LIF excitation curves. The velocity distributions were Maxwellian with translational temperatures remarkably close to the vibrational temperatures, typically 2000–3000 K, of the excited molecules assuming internal equilibration of the large energy E among the large number of internal modes. The logarithm of the rate constants obtained with 193 nm excitation plotted against 1/TV yields a straight line with an activation energy of 97 kcal/mol. A Rice–Ramsberger–Kassel–Marcus model invoking a transition state with several soft modes accounts very well for the rates of dissociation and for most but not all of the average kinetic energy of the released atoms.
Potential energy surfaces of ozone. I
Authors: Xantheas Sotiris S. , Atchity Gregory J. , Elbert Stephen T. , Ruedenberg Klaus
Published: July 2002

The cross section through the ground-state potential energy surface of ozone which contains the open minimum, the ring minimum, and the ring-opening reaction path, including the ring-opening transition state, is determined through full-valence-space multiconfiguration self-consistent-field calculations. It is shown that, at a point on the ridge separating the open-structure basin from the ring-structure basin in C2v symmetry, very close to the transition state, the ground-state surface connects with the lowest excited state surface of the same symmetry (1A1). This point is part of an intersection seam between these two 1A′ surfaces in Cs symmetry. It is furthermore found that the upper state has its equilibrium structure very close to the transition state of the lower state. The quantitative data of all critical points are calculated. In addition, a ground-state potential energy surface cross section describing the detachment of an oxygen atom is determined. For several other states, C2v constrained and bond-length-optimized energy curves E(φ) are also reported.
Continuous wave multiquantum electron paramagnetic resonance spectroscopy. II. Spin-system generated intermodulation sidebands
Authors: Sczaniecki P. B. , Hyde James S. , Froncisz W.
Published: July 2002

Irradiation of a single homogeneous electron paramagnetic resonance (EPR) transition by two microwave sources separated by Δf, where Δf≪ the linewidth, results in the production of intermodulation sidebands at f0±(k+1/2)Δf, where f0 is the mean of the two irradiating frequencies and k is an integer ≥1. These sidebands can be seen directly with a microwave spectrum analyzer. Any one of the sidebands can be observed by suitable phase sensitive detection and displayed as a function of the polarizing magnetic field H0, resulting in a multiquantum EPR spectrum. The widths of the EPR lines decrease as k increases, resulting in improved resolution. The two k=1 transitions can be superimposed, resulting in a signal of double intensity. At saturating microwave power, this double-intensity line is two times less intense than an ordinary EPR line. No magnetic field modulation needs to be used when detecting multiquantum transitions, and pure absorption or pure dispersion signals are obtained with good baseline stability. Considerable attention is paid to the technical problem of irradiating the sample with just two microwave frequencies. In the apparatus described, spurious instrumental sidebands are reduced by 60 dB or more.
Activation barriers for series of exothermic homologous reactions. I. Metal atom reactions with N2O
Authors: Futerko Peter M. , Fontijn Arthur
Published: July 2002

We recently observed that the activation barriers of O-atom abstraction reactions between metal atoms and N2O, in which both reactants are in their ground electronic states and the atoms contain no valence p electrons, vary systematically with the sums of the metal atom ionization potential and the energy required to promote a valence s electron to the lowest p orbital. It is shown here that this observation can be explained by the assumption that the activated complex results from the resonance interactions of ionic and covalent structures. Activation barriers for 43 reactions are calculated and where experimental measurements are available, are shown to be in good agreement with those. New interpretations are offered for literature data on the Ca and Cr reactions. The resonance treatment leads to a more general relationship in which activation barriers depend simultaneously on ionization potentials, electron affinities, promotion energies, and bond energies of the reactants. A number of further series of reactions, where activation barrier or rate coefficient trends with some of these parameters have been observed, are discussed and seen to be special cases of this formalism. Good agreement is found between the height of the barrier for the N2O+H→N2+OH reaction calculated by the present resonance treatment and an ab initio method.
Continuum channel coupling of shape resonances in N2
Authors: Poliakoff E. D. , Kakar Sandeep , Rosenberg R. A.
Published: July 2002

We have measured vibrational branching ratios for 2σ−1u photoionization of N2 in an effort to elucidate fundamental aspects of continuum channel coupling. Calculations have shown that photoejection of a 2σu electron from N2 should be influenced by a shape resonance in the 3σg →εσu photoionization channel and that this continuum channel coupling can result in deviations from Franck–Condon behavior for the resulting N+2(B 2Σ+u) ion. In the present study, the N2 molecules are ionized by monochromatic synchrotron radiation (25<hν<55 eV) and dispersed fluorescence is measured to determine the vibrational branching ratios v′=1/v′=0 and v′=2/v′=0 for the N+2(B 2Σ+u) state. The observed branching ratios are enhanced at hν≊30 eV and we attribute this Franck–Condon breakdown to continuum coupling between the 2σ−1u and 3σ−1g ionization channels. However, our results exhibit significant discrepancies with theory. The areas of agreement and disagreement suggest useful avenues of further study to clarify the nature of continuum channel coupling in molecular photoionization.
Validity of additivity approximations used in GAUSSIAN-2 theory
Authors: Curtiss Larry A. , Carpenter John E. , Raghavachari Krishnan , Pople John A.
Published: July 2002

Three additivity approximations used in GAUSSIAN-2 (G2) theory to compute effective QCISD(T)/6-311+G(3df,2p) energies for molecular systems are investigated by carrying out full QCISD(T)/6-311+G(3df,2p) calculations on the set of 125 systems used for validation [J. Chem. Phys. 94, 7221 (1991)]. The results indicate that the approximations work very well. The average absolute deviation of the full results from experiment is 1.17 kcal/mol which is close to that of G2 theory (1.21 kcal/mol). The average absolute deviation with G2 theory is only 0.30 kcal/mol. However, the cpu cost increases by a factor of 2–3.
The discrete variable representation of a triatomic Hamiltonian in bond length–bond angle coordinates
Authors: Wei Hua , Carrington Tucker
Published: July 2002

The discrete variable representation (DVR) is used to calculate vibrational energy levels of H2O and SO2. The Hamiltonian is written in terms of bond length–bond angle coordinates and their conjugate momenta. It is shown that although these coordinates are not orthogonal and the appropriate kinetic energy operator is complicated, the discrete variable representation is quite simple and facilitates the calculation of vibrational energy levels. The DVR enables one to use an internal coordinate Hamiltonian without expanding the coordinate dependence of the kinetic energy or evaluating matrix elements numerically. The accuracy of previous internal coordinate calculations is assessed.
The classical Boltzmann equation of a molecular gas
Authors: Curtiss C. F.
Published: July 2002

A classical Boltzmann equation which describes the time evolution of the distribution function in the phase space of a single molecule is developed for a low-density gas made up of nonreacting molecules of arbitrary structure. This development is based on the Liouville equation, the equation describing the time evolution of the distribution function in the phase space of the entire system through the Bogoliubov–Born–Green–Kirkwood–Yvon set of equations, the equations which describe the time evolution of the contracted distribution functions. The set of equations is truncated by use of the molecular chaos assumption, the assumption that the dynamical states of a pair of molecules are uncorrelated before a collision. From the general result the form of the collision integral arising from collisions between two diatomic molecules is considered as a special case. This result is then further reduced to the idealized rigid-rotor, harmonic-oscillator model. This result is then shown to be a generalization of an earlier development.