bluuebunny/crossref_metadata_2025 huggingface dataset:
Discrete Bessel transform and beam propagation method for modeling of vertical-cavity surface-emitting lasers
Authors: Seurin Jean-François P. , Chuang Shun Lien
Published: July 2002

An efficient matrix formulation combining a discrete Bessel transform and a beam propagation method for the modeling of vertical-cavity surface-emitting lasers is presented. Devices with circular symmetry are considered. The theory generalizes the concepts of field reflection and transmission coefficients, and generates a round-trip matrix that characterizes the entire structure. The model yields, among other things, the threshold current and intensity distribution at threshold and higher-order mode profiles. We apply the model to the threshold analysis of a gain-guided device. Our theoretical results agree very well with experimental observations on the temperature-dependent threshold characteristics. The optical modal reflection behavior of distributed Bragg reflectors based on our discrete Bessel transform approach is also analyzed and compared to other models. The importance of coupling loss for devices with oxide confinement layers is also discussed.
Electrical properties of buried B/Si surface phases
Authors: Zotov A. V. , Lifshits V. G. , Rupp T. , Eisele I.
Published: July 2002

Conductivity and Hall-effect measurements have been carried out at 24 K for a set of various layered structures with buried B/Si surface phases used as delta-doped layers. Evidence is found for hole mobility enhancement as a consequence of boron dopant ordering. The electrical measurements reveal a basic difference in room temperature adsorption for boron on Si(100) and Si(111) surfaces. The characterization of the samples containing buried B/Si(111) interfaces and extra-thin Ge layers suggests promise for improving the structure and electrical properties of the buried surface phases. The hole mobility in epi-Si/Ge/B/Si(111) structures is found to be about 2.5 times higher than in epi-Si/B/Si(111) samples.
Role of intermediate temperature molecular beam epitaxy grown GaAs defects in tunneling and diffusion
Authors: Youtz Andrew E. , Nabet Bahram
Published: July 2002

Current transport in GaAs grown by molecular beam epitaxy in the intermediate temperature (IT-GaAs) range of about 400 °C is investigated. A model is proposed which explains the direct role of deep defects in assisting tunneling from Schottky contacts to semiconductor, where, due to rapidly changing potential, the Wentzel–Kramers–Brillouin approximation cannot be used. The indirect role of defects in diffusion process in the IT-GaAs semiconductor is also investigated. The model is used to explain the dark I–V behavior of metal–semiconductor–metal photodetectors made on unannealed, i.e., as-grown, IT-GaAs. dc responses of annealed and unannealed IT-GaAs are compared showing much smaller values of dark current in low biases in the unannealed device but sharp increase due to defect assisted tunneling at medium bias levels. Since the defect that best fits the model has an activation energy of about 0.5 eV, the annealing behavior suggests that the As interstitial has all the requisite properties of the defect which determines conduction behavior in this material. The observed reduction of dark response by light at high biases, the negative photoresponse, can then be explained based on occupancy of these defects. Device applications include ohmic contacts based on tunneling through IT-GaAs, and, conversely, reduction of tunneling through growth of a barrier layer between Schottky metal and semiconductor.
Anisotropy of thermal conductivity and energy-flux relaxation time of hot electrons in semiconductors
Authors: Brunetti R. , Golinelli P. , Rudan M. , Reggiani L.
Published: July 2002

In this article a theoretical and computational analysis of the hot-electron thermal conductivity and related quantities in semiconductors is given. Two types of anisotropy are analyzed: the first is related to the dependence of the thermal conductivity on the direction of an externally applied electric field; the second is associated to the difference between the longitudinal and transverse thermal conductivity (i.e., along the field direction and along a direction perpendicular to the field, respectively). Two theoretical approaches based on a set of generalized relaxation times or on a set of microscopic correlation functions are considered and compared. Numerical results are obtained using a Monte Carlo simulator for electrons in silicon at 77 and 300 K. This approach can be extended to other semiconductors of interest within a semiclassical approach where two-particle interactions are neglected.
Dynamical behavior of electron hop transport over insulating surfaces
Authors: Hendriks B. H. W. , van Gorkom G. G. P. , van Keersop A. T. M. H. , Lambert N. , de Zwart S. T.
Published: July 2002

The dynamical behavior of electron hop transport over insulators is studied. The electron hop transport is based on a self-stabilizing secondary electron emission process. It is reported that, in the steady state, the electron–current propagation is governed by a diffusion equation. Analytical calculations and Monte Carlo simulations of this type of electron propagation are presented. In particular, the effect of the backscattering process on the hop transport properties is investigated. Finally, the results are qualitatively compared with the results of experiments.
Laser-frequency mixing in a scanning tunneling microscope at 1.3 μm
Authors: Gutjahr-Löser Th. , Hornsteiner A. , Krieger W. , Walther H.
Published: July 2002

The radiation of two single-mode diode lasers at 1.3 μm is focused into the tunneling junction of a scanning tunneling microscope, and gigahertz difference-frequency signals radiated from the tip are detected. Simultaneous measurements of the bias-voltage dependence of the mixing signal and the tunneling current for different surface samples show that the mixing process is due to the nonlinearity of the static current–voltage characteristic of the tunneling junction. The coupling of the laser radiation into the junction conforms to antenna theory. The experimental results are compared with previous measurements at a laser wavelength of 9.3 μm. Surface images produced by means of the difference-frequency signal show the chemical contrast between micron-sized Au islands and a graphite substrate.
Detailed defect study in proton irradiated InP/Si solar cells
Authors: Walters R. J. , Romero M. J. , Araújo D. , Garcı́a R. , Messenger S. R. , Summers G. P.
Published: July 2002

A detailed study of the effects of proton irradiation-induced defects in heteroepitaxially grown InP/Si solar cells has been made through a combination of cathodoluminescence (CL), electron beam induced current (EBIC), and electrochemical capacitance versus voltage (ECV) carrier profiling measurements. The CL data indicate the distribution of nonradiative recombination centers both before and after proton irradiation, and temperature dependent and spectroscopic analysis of the CL signal give an estimate of the energies of the dominant defect levels. The EBIC data yield an estimate of the magnitude and spatial variation of the minority carrier diffusion length (L) in the base region. Values of L determined from EBIC measurements made on solar cells irradiated by protons ranging in energy from 0.1 up to 4.5 MeV follow a single curve when plotted versus displacement dose, Dd, allowing a single proton damage coefficient to be determined. The ECV measurements show the evolution of the carrier concentration profile in the cell under irradiation, as carrier removal first depletes and eventually type converts the base region. From an in-depth analysis of the combined data, the physical defects that give rise the radiation-induced energy levels are suggested, and a detailed understanding of the physical mechanisms causing the radiation response of InP/Si solar cells is developed.
Laser-induced fluorescence from collisionally excited Si atoms in laser ablation plume
Authors: Okano Akiko , Takayanagi Kunio
Published: July 2002

Laser-induced fluorescence (LIF) from neutral Si atoms in a laser ablation plume is investigated using a probe laser beam at 251.6 nm. Fluorescence at 288 nm from the 4s(1P1) state is observed, aside from the deexcited fluorescence at 251.6 nm from the 4s(3P2) state. The coincidence of the 288 nm fluorescence and the 251 nm fluorescence strongly indicates that the Si atoms in the 4s(3P2) state are responsible for the 288 nm fluorescence. The 288 nm LIF signal is detectable only when the probe laser beam passes near the Si surface, and has maximum intensity for a time delay of 20 ns. The 288 nm LIF could be emitted when the Si atoms in the 4s(3P2) state, pumped by the probe laser, collide with other Si atoms in the gas phase, since a high-density gas phase of ejected particles exists near the surface. The LIF intensities from the ablated Si atoms decrease for large time delays of the probe laser (0.2 μs<td<100 μs), and the 288 nm fluorescence originating from the droplets (probably microparticles) is observed instead. Since droplets moving at ∼100 m/s are fragmented by the probe beam, the collisional excitation among these fragmented atoms can generate Si atoms in the 4s(1P1) state.
Magnetic properties of ultrathin NiFe and CoFe films
Authors: Hung C.-Y. , Mao M. , Funada S. , Schneider T. , Miloslavsky L. , Miller M. , Qian C. , Tong H. C.
Published: July 2002

In this article, we report the magnetic properties of ultrathin (15–200 Å) NiFe and CoFe films deposited using ion beam deposition techniques. They are symmetrically sandwiched between Ta, Cu, or Ta/Cu under and capping layers. NiFe and CoFe films grown between Ta/Cu and Cu/Ta bilayers exhibit the smallest magnetic thickness loss of about 1 Å. This interfacial magnetic dead layer thickness, t0, is about 5 Å for Cu-sandwiched films and about 15 Å for Ta-sandwiched films. As the film thickness becomes thinner than 100 Å, the magnetic properties are found to be more sensitive to the choice of material and growth environment. CoFe films show an interfacial contribution, λi, about ten times larger than that for NiFe films. Among others, NiFe and CoFe films sandwiched by Ta/Cu and Cu/Ta bilayers exhibit the smallest values of λi. The magnetic anisotropy in Ta-sandwiched CoFe films appears to be predominantly magnetoelastic in nature.
X-ray absorption fine structure study of short-range order of iron in Fe/Al multilayers
Authors: Meyer Dirk C. , Richter Kurt , Paufler Peter , Gawlitza Peter , Holz Thomas
Published: July 2002

Results of quantitative analysis of fluorescence x-ray absorption fine structure (XAFS) experiments at the Fe–K absorption edge are presented for a 6*(4.3 nm Fe/10.4 nm Al) multilayer prepared by pulsed laser deposition. Fluorescence XAFS experiments have been combined with excitation of x-ray standing waves. This combination in one experiment allowed for a depth-controlled excitation of Fe fluorescence and hence for a depth-resolved analysis of short-range order. Depth-resolved analysis showed that instead of sharp Fe/Al or Al/Fe interfaces extended interlayer regions exist. The structure retained that of bulk α-Fe. In the upper half of the Fe layer 37 at. % Al as nearest and next nearest neighbors of Fe were found, whereas in the lower half 80 at. % Al atoms occur. Thus the Fe/Al interface (deposition of Fe on Al) should be characterized by an intermixing zone significantly larger in comparison to that of the Al/Fe interface (deposition of Al on Fe). By conventional XAFS measurement carried out at a fixed angle of incidence of the exciting beam an average Fe neighborhood for the entire Fe layer of 58 at. % Al atoms was found. This value agrees with the average obtained when the analysis of the layer was performed separately for two sublayers by shifting the wave field through the layer.
Simulation and growth of gold on silicon oxide in one-dimensional and quasi-one-dimensional arrays
Authors: Boero M. , Vincent J. K. , Inkson J. C. , Mejias M. , Vieu C. , Launois H. , Mulheran P. A.
Published: July 2002

The granular film deposition of gold atoms by means of thermal evaporation on silicon oxide surfaces is known to produce nanometer-scale islands which show electrical characteristics dominated by Coulomb blockade. Here we report on the growth and the simulation of structures produced by a combination of evaporation and lithography. A lithographic technique is used to define a region of a width of several nm that is surrounded by polymethylmethacrylate (PMMA). The PMMA confines the diffusion of the atoms, and the growth process is characterized by different boundary conditions compared with the case where atoms are deposited on a macroscopic surface. This method enables us to create quasi-one-dimensional (quasi-1D) and 1D structures in which the gold islands are arranged in a single row where the lateral size is only a few nanometers. Such structures offer the possibility of studying Coulomb blockade in 1D arrays and a signature of self-organized growth is observed in such structures.
Electrical and electrochemical properties of fly ash and effect of pyrolysis
Authors: Take H. , Kajii H. , Yoshino K.
Published: July 2002

The electrical and electrochemical characteristics of pyrolized fly ash which has a porous structure have been investigated for use as an electrode active material in rechargeable batteries. The electrodes of pyrolized fly ash show good stability and high Coulombic efficiency in charge–discharge cycles in LiClO4/PC. The charge–discharge capacity of pyrolized fly ash with a heat treatment temperature of 1100 °C is as high as 205 mA h/g. From the viewpoint of recycling of industrial waste, we propose to use fly ash as a negative electrode in a lithium ion secondary battery.
Density variations in thermoelectrics
Authors: Mahan G. D.
Published: July 2002

Equations are solved to give the variations in temperature, density, and potential, when a solid has electrical currents, heat currents, and particle diffusion. Solutions are presented in one dimension for currents down a bar. These solutions are used to calculate the efficiency of a thermoelectric refrigerator, which is optimized to give the coefficient of performance (COP). The COP depends upon the temperature difference ΔT, but does not depend upon the density difference Δn between the two ends of the bar.
Optical properties of reflective bistable twisted nematic liquid crystal display
Authors: Cheng Hongfei , Gao Hongjin
Published: July 2002

A model of a reflective bistable twisted nematic liquid crystal display was studied by detailed numerical simulation. The optimized modes of a 5 μm reflective bistable twisted nematic liquid crystal were determined by the parameter space method. We have shown that large wavelength dispersion in the reflection coefficient of such displays cause the optimal mode to have a small value of dΔn, around 0.2 μm. By using a quarter-wave plate compensation film the optimized modes can have a dΔn value of around 0.4 μm. Numerical simulations show a wide viewing angle of up to ±60° for both types of reflective bistable twisted nematic liquid crystal display.
Decrease of electron paramagnetic defect density and enhancement of electron field emission in annealed carbon films
Authors: Lee Yun-Hi , Kim Dong-Ho , Jang Yoon-Taek , Ju Byeong-Kwon , Oh Myung-Hwan
Published: July 2002

We have studied the effect of the paramagnetic defects in carbon films on the field-emission properties. The paramagnetic defects in carbon films originating from the carbon dangling bonds were measured using electron spin resonance (ESR). We found a reduction of the dangling bond density in the annealed carbon films in N2/H2 atmosphere from ESR spectroscopy. The annealed films with lower defect densities, i.e., dangling bonds, showed an improved characteristic of higher emission current density as well as a lower turn-on electric field in the Fowler–Nordheim type tunneling. Also in the annealed carbon films a correlation is observed between the decrease of band gap estimated from the electrical conductivity and the reduction of dangling bond density. The earlier results indicate that the enhancement of the emission current level in the annealed carbon films is related to the decrease of the ESR centers.
Response to “Comment on ‘Electron microprobe quantification: A new model based on electrons rather than on mass’ ” [J. Appl. Phys. 87, 8220 (2000)]
Authors: Pingitore Nicholas E. , Donovan John J. , Jeanloz Raymond
Published: July 2002

Fundamental physical law informs that neutrons play no role in electron probe microanalysis, and therefore, mass-based models follow an improper route to data reduction. Castaing’s mass-based first approximation inappropriately remains the starting point—see any microprobe textbook—for development of quantitative electron probe microanalysis. By incorrectly relating intensity of characteristic x rays to mass rather than to charge, such conventional models demand an isotope (mass) effect; our robust experiments detected none.
Mesoscopic kinetics of nonequilibrium solidification
Authors: Cai Yingwen , Zhang Guilin , Fu Hengzhi
Published: July 2002

Disadvantages of Aziz interface partition model [J. Appl. Phys. 53, 1158 (1982)] are summarized in this article. Using transition state theory and Maxwell–Boltzmann distribution law, a mesoscopic solute partition model (MPM) including multilayer interface for dilute solution is put forward. It is shown that the static interface structure and roughening behavior have significant effects on solute partitioning. Both the partition processes of semiconductors and metals can be explained by MPM rationally. The Aziz model is just a particular case of MPM for monolayer interface without roughening before complete trapping occurs. Comparisons of MPM with experimental results in Si (As, Ge, Bi, Sn) and Al(Cu, Sn, Ge, In) show pretty good agreement, provided an educated speculation of interface roughening behavior is given. Also in MPM, it is pointed out that interface diffusivity Di correlates with the interface scale. For semiconductors with an atomic sharp solid/liquid interface, Di is close to DL, the diffusivity of bulk liquid. On the other hand, for metals with a diffuse interface, Di can be orders of magnitude lower than DL, to some extent estimated by DSDL, the geometric mean of the diffusivity of bulk solid and liquid.
Determination of the optical constants of the γ-phase GdH3 thin films
Authors: Lee M. W. , Lin C. H.
Published: July 2002

We have measured the optical transmittance of thin films of gadolinium trihydride GdH3 over a broad spectral range, 0.05–6 eV. The analysis indicates that GdH3 is a wide-gap semiconductor with an energy gap of 2.55 eV. The fundamental absorption and impurity-related transitions dominate the spectra in the high-energy region, 1–6 eV. Hydrogen vibration modes dominate the spectra in the low-energy region, 0.05–0.2 eV. A strong free-carrier absorption effect is observed. In addition, the frequency-dependent refractive index, extinction coefficient, dielectric function, and optical conductivity are determined from the transmission spectra. This work also evaluates the plasma frequency associated with free carriers.
Photoreflectance study of phosphorus passivation of GaAs (001)
Authors: Beaudry Richard , Watkins Simon P. , Xu Xiangang , Yeo Philip
Published: July 2002

We present a study of the effect of phosphorus passivation on the surface electric field of undoped GaAs using photoreflectance spectroscopy. Surface electric fields were determined in surface/intrinsic/n-type (s-i-n+) samples prepared with various surface phosphorus treatments. A comparison with H2S passivated films and InGaP capped layers prepared in situ in the same growth equipment is also presented. Phosphorus surface passivation was achieved by (1) exchange reaction of the GaAs surface under tertiarybutylphosphine vapor or (2) direct growth of GaP thin epitaxial layers. The total coverage of phosphorus has been estimated by x-ray diffraction techniques. We observe a maximum reduction of the surface Fermi level using epitaxially grown GaP with an effective coverage of two monolayers. Atomic force microscopy images of all passivation treatments exhibit excellent surface morphology without roughening. The passivation effects decreased somewhat over time but reductions in the Fermi level and density of surface states are still quite large after several months of air exposure.
Vibrational electron energy loss spectroscopy of the Si(111)(7×7)–H2O(D2O) system
Authors: Nishijima M. , Edamoto K. , Kubota Y. , Tanaka S. , Onchi M.
Published: July 2002

High-resolution electron energy loss spectroscopy (EELS) has been applied to the study of the Si(111)(7×7)–H2O(D2O) system. At 300 K, H2O(D2O) is partially dissociated on the Si(111) surface to form the SiOH(SiOD) and SiH(SiD) species. Angle and primary-electron-energy dependences of the vibrational loss intensities were measured. Relative contributions to the vibrational excitations of the dipole, impact, and resonance mechanisms were estimated. The O–H(O–D) stretching and Si–O–H (Si–O–D) bending vibrations are partly excited by the resonance mechanism in the primary energy region of Ep ≂2–7 eV. EELS spectra of the Si(111) surface exposed to H2O(D2O) at 300 K and of the same surface heated to ∼700–900 K are presented, and surface reaction mechanisms are discussed.
Addendum to the paper ‘‘On the dynamics of polyelectrolyte solutions’’
Authors: Benmouna M. , Akcasu A. Z. , Hammouda B.
Published: July 2002

Hydrodynamic effect on the relaxation modes for a system of charged particles is reexamined. This effect is shown to produce a decrease in the diffusion coefficient due to the cloud of particles surrounding each charge.
Theoretical investigation of many dicationic states and the Auger spectrum of benzene
Authors: Tarantelli F. , Sgamellotti A. , Cederbaum L. S. , Schirmer J.
Published: July 2002

The outer valence double ionization transitions in the benzene molecule have been computed using Green’s functions and the results are discussed in connection with the Auger spectrum of this molecule. It is found that already at low energy the double ionization transitions are characterized by strong correlation effects and the appearance of a very large number of satellite states. The 226 computed dicationic states are analyzed in terms of their energy distribution weighted by their two-hole components. It is shown that the relative energies of the maxima in this distribution agree with the experimental Auger peaks to within 0.3 eV. These results emphasize the extreme usefulness of the method in the investigation of double ionization spectra of large molecules, which are practically beyond the reach of conventional ab initio approaches.
Generation of NH(a 1Δ) in the 193 nm photolysis of ammonia
Authors: Kenner R. D. , Rohrer F. , Stuhl F.
Published: July 2002

We report the observation of NH(a 1Δ) formation in a one-photon process during the ArF laser (193 nm) photolysis of ammonia. We have used laser-induced fluorescence to detect NH(a) and ND(a) and report the rotational and vibrational populations generated. We have also reinvestigated the generation of NH(a) in the 248 nm photolysis of HN3 and find substantial vibrational excitation in contrast to the results of previous workers. We give estimates for the quantum yields for NH(a) and ND(a) in the photolysis of ammonia based on comparison with the yield in 248 nm photolysis of HN3.
Rotational distributions from resonant and direct scattering in H+CO and tests of statistical theories
Authors: Lee Ki-Tung , Bowman Joel M.
Published: July 2002

Coupled-channel scattering calculations are reported for H+CO using an ab initio potential energy surface. Rotational state-to-state transition probabilities are presented for resonant and direct scattering for J=0 and treating CO as a rigid rotor. The presence of a barrier to complex formation in this system is found to have a crucial role in determining which asymptotic rotational states couple strongly to the resonances. In addition, a propensity for even-Δj transitions is seen for direct scattering and also for rotational states which do not couple to the resonances for resonant scattering. The features of the potential surface which are responsible for these results are uncovered by using distorted wave Born and Franck–Condon approximations. Tests of phase space theory and the statistical adiabatic channel model are made for resonant scattering. One important conclusion of these tests is that the bending motion of the HCO transition state is apparently not adiabatic.
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a high-resolution fluorescence excitation spectrum of 1B1 pyrimidine in a molecular beam. Structural assignments, analysis of singlet–triplet perturbations, and implications for intersystem crossing in the isolated molecule
Authors: Konings J. A. , Majewski W. A. , Matsumoto Y. , Pratt D. W. , Meerts W. Leo
Published: July 2002

We have observed, and assigned, the fluorescence excitation spectrum of the 000 band in the 1B1←1A1 electronic transition of pyrimidine, at a resolution of ∼10 MHz. The rotational constants of the 1B1 state, the lowest excited singlet state, are A′=6352±3, B′=5853±3, and C′=3042.0±0.5 MHz. The magnitudes of these constants are not very different from those of the ground (1A1) state. However, the in-plane a and b inertial axes in the 1B1 state are rotated by 90° with respect to those of the 1A1 state. The spectrum also exhibits numerous perturbations, evidenced by the presence of extra lines, anomalous intensities and lifetimes, and shifts of the main lines from their expected positions. The perturbations are strongly magnetic-field dependent, demonstrating that they arise from an intramolecular coupling of the 1B1 state with nearly isoenergetic rovibronic levels of a lower triplet (3B1) state. Models are proposed to account for this behavior based on a deconvolution of the experimental spectrum and simulations of the observed Zeeman effects. The most satisfactory interpretation of the data (in the language of the zero-order states) is obtained if it is assumed that a single rovibronic 1B1 level is spin–orbit coupled to one or a few 3B1 levels, which in turn are coupled via rotationally dependent Coriolis interactions to a dense manifold of background levels, probably those of the 1A1 state. Because the latter coupling is small, typically less than the linewidths in the spectra, it is manifested only in a K′+1 dependence of the lifetimes of selected molecular eigenstates and the reduced g values required to fit the magnetic-field dependence of their spectra.
Emission spectra from single rovibronic quantum states in S1 benzene after Doppler-free two-photon excitation
Authors: Schubert U. , Riedle E. , Neusser H. J.
Published: July 2002

Dispersed emission from single rovibronic quantum states in S1 benzene is measured after Doppler-free two-photon excitation under low pressure conditions (0.3 Torr). This was made possible by a long-term stabilization of the single-mode dye laser yielding a stability of better than 1 MHz/h. The emission spectra of unperturbed rotational levels in the 141 and the 14111 vibronic states reveal a great number of detailed results on Duschinsky rotation and long-range Fermi resonances in the electronic ground state. By contrast, it is seen that the emission spectra from perturbed rovibronic states are contaminated by additional bands. The analysis of these bands leads in most cases to an identification of the coupled dark background state and the responsible rotation–vibration coupling process (H42 resonances). The emission spectra clearly demonstrate that even for a density of states of 60 1/cm−1, coupling in S1 benzene is still selective and far from the statistical limit. It is further demonstrated that the dark and the light states are more efficiently mixed by short-range couplings with coupling matrix elements of some GHz than by long-range Fermi resonances.
Application of unimolecular reaction rate theory for highly flexible transition states to the dissociation of CH2CO into CH2 and CO
Authors: Klippenstein Stephen J. , Marcus R. A.
Published: July 2002

A previously described implementation of Rice–Ramsberger–Kassel–Marcus (RRKM) theory for unimolecular dissociation processes involving a highly flexible transition state is applied to the dissociation of CH2CO into CH2 and CO. Results of theoretical calculations for the energy and angular momentum resolved rate constants are presented. Using an added dynamical approximation, the product vibrational–rotational distributions are also calculated. The calculated rate constants are compared with the corresponding experimentally determined quantities where possible. Comparison is also made with phase space theory (PST). The RRKM-based calculations are in good agreement with both the experimentally determined rate constants of Zewail and co-workers and the experimentally determined photofragment excitation spectra of Moore and co-workers. The results on rates are in contrast to the corresponding results from PST calculations. The RRKM-based theory for the product vibrational–rotational distributions predicts a moderately greater probability for vibrational excitations than does PST (particularly for excess energies just above the threshold for excitation of a particular vibrational mode of the products). In other respects the RRKM-based predictions of the ro-vibrational product state distributions are quite similar to those of PST.
Computer simulation of ammonia on graphite. II. Monolayer melting
Authors: Cheng Ailan , Steele W. A.
Published: July 2002

Molecular dynamics simulations of monolayers of ammonia adsorbed on graphite are reported. The interaction potentials used were discussed in paper I. Temperatures were chosen to include melting and the orientational disordering. It is shown that orientational disordering in the systems studied is a gradual process which is nearly complete at temperatures below the layer melting point. Melting occurs at 145±5 K for a submonolayer coverage and at 180±5 K for a complete layer. These results are in moderately good agreement with the available experimental data. Possible reasons for the observed discrepancies are discussed.
The bending-corrected-rotating-linear-model calculations of the rate constants for the H+H2 reaction and its isotopic variants at low temperatures: The effect of van der Waals well
Authors: Takayanagi Toshiyuki , Sato Shin
Published: July 2002

The reactive cross sections for the H+H2 reaction and its isotopic variants at low collision energies have been calculated using the bending-corrected-rotating-linear model on the Liu–Siegbahn–Truhlar–Horowitz and double-many-body-expansion potential energy surfaces. Van der Waals wells included in both potential surfaces significantly affected the calculated rate constants at the temperatures lower than 10 K. The difference in the effect between two potential surfaces resulted from whether or not the bound state of H3 is formed and how close the bound state is located to the dissociation threshold.
Magnetization transfer modes in scalar-coupled spin systems investigated by selective two-dimensional nuclear magnetic resonance exchange experiments
Authors: Di Bari Lorenzo , Kowalewski Jozef , Bodenhausen Geoffrey
Published: July 2002

Longitudinal nuclear magnetic relaxation in coupled two-spin systems is discussed in terms of magnetization transfer modes, a generalization of the magnetization modes commonly used in the discussion of one-dimensional NMR relaxation experiments. The symmetry properties of the transfer modes and their relationship to the elements of the usual Redfield relaxation matrix are discussed. Experimental strategies for measuring the amplitudes of the transfer modes are discussed and various selective two-dimensional exchange (‘‘soft-NOESY’’) methods are proposed. Experimental data are presented for Exifone, a small organic molecule, and the auto- and cross-correlation spectral densities, the magnitudes of the proton chemical shift anisotropies, and the strengths of the dipolar interaction are derived from the spectra.
Quadratic response theory of frequency-dependent first hyperpolarizability. Calculations in the dipole length and mixed-velocity formalisms
Authors: Parkinson William A. , Oddershede Jens
Published: July 2002

The quadratic response function (QRF) is evaluated within the random phase approximation (RPA), to compute frequency-dependent first hyperpolarizabilities β(ω,ω). The method treats electron correlation consistent through first order, so the computed values are equivalent to coupled-perturbed Hartree–Fock (CPHF) results. The QRF is obtained by solving systems of linear equations, thus circumventing the RPA eigenvalue problem. The QRF equation of motion is used to develop hyperpolarizability identities in the dipole length and mixed-velocity representations. The two forms of β are equivalent at the RPA level, and provide a useful measure of completeness of basis. The method is applied to the hyperpolarizability of HF and H2O. It is found that basis sets used in previous studies were not saturated for all β components, and that basis sets which satisfy length–velocity sum rules for linear response properties are not sufficient for agreement of quadratic response properties. The calculated dispersion ratios are in good agreement with experimental measurement, indicating that dispersion effects are properly described by frequency-dependent calculations in the RPA at field energies which are small compared to vertical excitation energies.
Rotational predissociation, vibrational mixing, and van der Waals intermolecular potentials of NeDF
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

The near-infrared spectrum of NeDF formed in a slit free jet expansion is recorded with a high resolution, tunable laser spectrometer. Four bands, consisting of the DF stretching fundamental and three internal rotation and van der Waals stretch combination bands, are observed and analyzed for both the 20Ne and 22Ne isotopomers. All three combination bands reveal a sudden onset of rotational predissociation at modest J, which is modeled with effective one-dimensional potentials to determine the binding energy D0=34.7±0.8 cm−1 for 20NeDF (v=0) and D0=35.1±0.8 cm−1 for 20 NeDF (v=1). The experimental results are compared with predictions of a recently published ab initio anisotropic potential surface, and an improved potential is developed and tested. This refined potential has an absolute minimum of −86 cm−1 in the linear Ne–D–F geometry, a secondary minimum at −55 cm−1 in the inverted linear Ne–F–D geometry, and an intervening saddle point at −39 cm−1 near the perpendicular geometry. The lowest bound state lies ≊4 cm−1 above the saddle so internal DF rotation is only slightly hindered in this complex.
Double layer interactions in mono- and divalent electrolytes: A comparison of the anisotropic HNC theory and Monte Carlo simulations
Authors: Kjellander Roland , Åkesson Torbjörn , Jönsson Bo , Marčelja Stjepan
Published: July 2002

The interaction between charged surfaces in 1:1, 1:2, and 2:2 electrolyte solutions at various concentrations have been calculated using the anisotropic hypernetted chain (HNC) theory and Monte Carlo (MC) simulations. For divalent counterions, the surface interaction has an attractive minimum at short separations. This minimum turns more attractive at increasing electrolyte concentration, while the interaction at somewhat larger separations becomes oscillatory. The agreement between the HNC and the MC results is excellent for the 1:2 and 2:2 electrolyte systems. For 1:1 electrolytes the surface interaction is repulsive except at high concentrations at which it shows a weak attractive minimum. The HNC and the MC results agree quantitatively except in a few angstroms wide region at short separations, where the agreement is only qualitative due to a slight difference in the contributions from the hard core interactions. This is a consequence of the neglect of the short-range bridge function in the HNC approximation.
Transition state structures and energetics using Gaussian-2 theory
Authors: Durant Joseph L. , Rohlfing Celeste McMichael
Published: July 2002

The availability of the easily implemented Gaussian-2 (G2) methodology has made it possible for the nonspecialist to calculate accurate heats of formation for many molecules on workstations. In order to quantify its performance for transition state structures, we have used G2 and a modified G2 on several transition states whose structures and energies have been well characterized either by experiment or multireference configuration interaction studies. The G2 method performs well in predicting energies of transition states (even for nonisogyric reactions), with an absolute average deviation of 1.5 kcal/mole in the classical barrier height for the cases studied, while it is less successful in predicting geometries and frequencies. We investigated modifying the G2 method for use with transition states by using QCISD/6-311G(d,p) geometries and frequencies instead of MP2/6-31G(d) geometries and scaled HF/6-31G(d) frequencies. The QCISD geometries and frequencies agree well with values from the literature, and this modified G2 procedure offers improved performance in predicting transition state energies.
Ultrafast temperature jump in polymers: Phonons and vibrations heat up at different rates
Authors: Wen Xiaoning , Tolbert William A. , Dlott Dana D.
Published: July 2002

Optical calorimetry is used to study the dynamics of a polymer, poly-(methyl methylacrylate), (PMMA), subjected to a temperature jump which is faster than the time required for Boltzmann equilibrium. The temperature jump is produced by exciting a near-infrared dye embedded in the polymer with a 23 ps duration optical pulse. The magnitude of the temperature jump ΔT was as large as 125 degrees. To attain such a large temperature jump with good spatial uniformity requires optical heating pulses which strongly saturate the heater dye absorption. A formalism is developed to quantitatively treat optical heating with saturation. The heat capacity of the polymer, Cpol, can be determined to an accuracy of 8% using this method. The temperature jump data could not be fit by assuming the polymer heats up in a single stage. A quasitemperature model with two-stage heating, where the dye first excites polymer phonons and then the phonons excite polymer vibrations by multiphonon up pumping, gave quantitative agreement. The data at several values of ΔT were simultaneously fit using three adjustable parameters: κvc, the molecular thermal conductivity for vibrational cooling of the heater dye; κup, the molecular thermal conductivity for multiphonon up pumping; and Cpol. The value of κ vc was the same magnitude as κth, the thermal conductivity of the polymer, despite the fact that the vibrational cooling process occurs on the 1 nm length scale. The value of κup was 2 orders of magnitude smaller than κth.
The heats of formation of some C2H5O+ isomers, relevant bond energies in ethanol and PA(CH3CHO)
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The ionization potentials of the transient species CH3CH2O, CH3CHOH, and CH2CH2OH (generated by the F+ethanol reactions) are measured by photoionization mass spectrometry: I.P.(CD3CD2O)=10.29±0.08 eV (tentative), I.P.(CH3CHOH)<6.85 eV, and I.P.(CD2CH2OH) ≤8.35±0.06 eV. The latter results in a cation of uncertain structure. These reactions also generate vinyl alcohol (adiabatic I.P.=9.33±0.01 eV) and acetaldehyde. A redetermined appearance potential of CH3CHOH+ from ethanol enables one to infer the proton affinity of acetaldehyde to be ≥183.8±0.2 kcal/mol and an α (C–H) bond energy in ethanol ≳91.1 kcal/mol (0 K). The appearance potential of m/e=45 ion from bromoethanol is interpreted as formation of a C2H5O+ isomer having the oxirane structure, with ΔHf00 of ∼173.9 kcal/mol, consistent with earlier alternative measurements. A second increase in the m/e=45 ion yield curve from ethanol is interpreted as formation of this same isomer. This interpretation, and an alternative cycle, lead to a β (C–H) bond energy in ethanol of 98±2 kcal/mol. The implication of the current results to the dynamics of dissociation of ethanol cations is discussed.
van der Waals complexes between COCl2, COFCl, COF2, and chlorine molecule: An infrared matrix isolation and ab initio study
Authors: Bouteiller Y. , Abdelaoui O. , Schriver A. , Schriver-Mazzuoli L.
Published: July 2002

Due to their importance in stratosphere, spectroscopic studies of halogenated carbonyls (COF2, COFCl, COCl2) and their complexes with Cl2 are reviewed, including previously unpublished data. In particular, quantitative analysis of νCO Fermi resonance has been made for free and complexed halogenated carbonyls. Ab initio calculations have been undertaken on all these structures. The structure, complexation energy and vibrational analysis at the harmonic approximations have been calculated at the self-consistent field and Mo/ller–Plesset 2 levels. The calculated frequency shifts are in overall agreement with the experimental data. This result was not obvious owing to the weakness of the complexes.
Characterization of the minimum energy path for the reaction of singlet methylene with N2: The role of singlet methylene in prompt NO
Authors: Walch Stephen P.
Published: July 2002

We report calculations of the minimum energy pathways connecting 1CH2+N2 to diazomethane and diazirine, for the rearrangement of diazirine to diazomethane, for the dissociation of diazirine to HCN2+H, and of diazomethane to CH2N+N. The calculations use complete active space self-consistent field (CASSCF) derivative methods to characterize the stationary points and internally contracted configuration interaction (ICCI) to determine the energetics. The calculations suggest a potential new source of prompt NO from the reaction of 1CH2 with N2 to give diazirine, and subsequent reaction of diazirine with hydrogen abstracters to form doublet HCN2, which leads to HCN+N(4S) on the previously studied CH+N2 surface. The calculations also predict accurate 0 K heats of formation of 77.7 kcal/mol and 68.0 kcal/mol for diazirine and diazomethane, respectively.
High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2
Authors: Fulle D. , Hamann H. F. , Hippler H. , Troe J.
Published: July 2002

Thermal rate constants of the complex-forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser-induced fluorescence. The derived second-order rate coefficients showed a pronounced pressure and complicated non-Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy- and angular-momentum-dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high-pressure rate constants; with the limiting low-pressure rate constants, activated complex properties for the dissociation HOCO→H+CO2 could be fitted as well. The observed transitions between low- and high-pressure limiting rate constants were well reproduced with these molecular parameters and collisional contributions; some evidence for rotational effects in collisional energy transfer was found. The surprisingly successful theoretical modeling of all available experimental data (80–2800 K, 0.0001–700 bar) allows for a satisfactory data representation of the rate coefficients over very wide ranges of conditions.
A class of exact dynamical solutions in the elastic rod model of DNA with implications for the theory of fluctuations in the torsional motion of plasmids
Authors: Tobias Irwin , Coleman Bernard D. , Lembo Marzio
Published: July 2002

New explicit solutions are obtained for the nonlinear equations of Kirchhoff’s theory of the dynamics of inextensible elastic rods without neglect of rotatory inertia. These exact solutions describe a class of motions possible in closed circular rings possessing a uniform distribution of intrinsic curvature ku and intrinsic torsion. When ku≠0, the motions in this class are such that the axial curve of the ring remains stationary while the cross sections rotate about their centers in such a way that the angle ψ of rotation is independent of axial location and is governed by the nonlinear pendulum equation. When ku=0, such uniform rotation of cross sections can occur at an arbitrary steady rate. The methods of classical equilibrium statistical mechanics yield the following conclusions for canonical ensembles of rings for which the motion is this type of pure homogeneous torsion. When 1/ku=11.85 nm (i.e., when the intrinsic curvature ku is among the highest observed in naturally occurring, approximately uniformly curved, stress-free DNA segments), if the flexural rigidity is assigned a value usually accepted for duplex DNA, at T=298 K the root-mean-square value, 〈ψ2〉1/2, of the angle ψ is 11.2°. For motions in this class, the heat capacity per ring, as a function of T/ku, shows a maximum which, when T=298 K, occurs where 1/ku=127 nm and corresponds to an ensemble of rings of which approximately 1% have sufficient energy for escape over the barrier associated with the separatrix between periodic and monotone solutions of the nonlinear pendulum equation; for that ensemble of rings, 〈ψ2〉1/2=43.3°.
Solvent dynamics: Modified Rice–Ramsperger–Kassel–Marcus theory. II. Vibrationally assisted case
Authors: Marcus R. A.
Published: July 2002

Expressions are given for a solvent dynamics-modified Rice–Ramsperger–Kassel–Marcus (RRKM) theory for clusters. The role of vibrational assistance across the transition state region is included. The usual differential equation for motion along the slow coordinate X in constant temperature systems is modified so as to apply to microcanonical systems. A negative entropy term, −Sv(X), replaces the (1/T)∂U/∂X or (1/T)∂G/∂X which appears in canonical systems. Expressions are obtained for the RRKM-type rate constant k(X) and for the Sv(X) which appear in the differential equation. An approximate solution for steady-state conditions is given for the case that the ‘‘reaction window’’ is narrow. The solution then takes on a simple functional form. The validity of the assumption can be checked a posteriori. Recrossings of the transition state are included and the condition under which the treatment approaches that in Part I is described.
A mean-field modeling study of the interaction between hydrogen and a palladium (110) single crystal
Authors: Mavrikakis M. , Schwank J. W. , Gland J. L.
Published: July 2002

A new interpretation of the temperature programmed desorption (TPD) spectra of hydrogen on a Pd(110) single crystal surface is presented. The transient mean-field model developed accounts for both adsorption on the surface and diffusion of hydrogen into the lattice of the crystal. A new approach for modeling TPD spectra is developed, in which both the isothermal exposure as well as the temperature ramp of the TPD experiment are modeled in a consistent way. We demonstrate that for systems with substantial diffusion of the adsorbate into the substrate’s lattice it is not sufficient to report exposure values. For such systems, exposure time has a far more pronounced effect than the exposure pressure has on the TPD spectra, because diffusion is a strongly time-dependent process. The modeling and experimental results for the H2/Pd(110) system are in good agreement and suggest the existence of a distinct subsurface state, in addition to the surface and bulk states of hydrogen. Low frequency factors derived from the model for the processes connecting the subsurface with the surface and bulk state emphasize the restricted nature of the corresponding transition states.