bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermodynamic and magnetic properties of metastable FexCu100−x solid solutions formed by mechanical alloying
Authors: Ma E. , Atzmon M. , Pinkerton F. E.
Published: July 2002

Metastable solid solutions of Fe and Cu, which are immiscible in equilibrium, have been formed using high-energy ball milling of elemental powder mixtures. Single-phase face-centered-cubic (fcc) solid solution was obtained for 0<x≤60, and body-entered-cubic (bcc) solid solution for 75≤x<100. The transition from fcc to bcc occurred near x=70, where a mixture of fcc and bcc phases was obtained. The enthalpy of transformation to equilibrium was measured using differential scanning calorimetry. The average atomic volume of the phases exhibits a positive deviation from Vegard’s law, in qualitative agreement with the large positive enthalpy of mixing in this system. The magnetic moments and Curie temperatures for the metastable solid solutions have been determined and compared with those reported for Fe-Cu alloys formed by vapor deposition. Calculations of the formation enthalpy (ΔH) and free energy (ΔG) have been performed based on calphad data, with corrections based on our magnetization measurements. The calculated ΔG results are used to explain the observed fcc-bcc transition under polymorphous constraints.
Determination of the interaction coordinate in drift detectors through the timing of induced signals
Authors: Gatti E. , Sampietro M. , Zocchi F. E.
Published: July 2002

The time centroid of the current pulse induced at the anode of a semiconductor drift detector is calculated as a function of the interaction coordinate for arbitrary drifting fields. The effects of its behavior on the determination of the absolute position of the interaction point are studied. For a typical bias condition of the detector, the paper shows that an error up to few hundred micrometers is made in the position reconstruction of the event if the described effects of the induction are not taken into account. The paper also shows that the time shift due to the perturbation of the drifting field caused by the discreteness of the field electrodes, is only of less than 1 ns, and therefore negligible in most applications.
Transverse susceptibility and ferromagnetic resonance of Hi-8 metal-evaporated tape
Authors: Sürig C. , Zimmermann G. , Hempel K. A.
Published: July 2002

Transverse susceptibility (TS) and ferromagnetic resonance absorption (FMR) are measured on commercially available metal-evaporated video tape manufactured by the continuous varying incidence method, so that the columns are inclined at an angle ϑint with respect to the recording direction. Due to the demagnetization of the tape the magnetization vector is at an extrinsic inclination angle ϑext<ϑint with respect to the recording direction, which is determined to be 24° by analyzing angular remanence curves. The results of TS and FMR measurements on the dependence of the angle ϑN between applied field and tape normal are symmetrical to ϑN=ϑext. In-plane TS measurements parallel and perpendicular to the recording direction lead to HA=440 kA/m and an intrinsic inclination angle ϑint=44°. Using these values, measured and calculated resonance field values Hres are in good agreement except for 0°<ϑN<50°. The analysis of Hres in the vicinity of ϑN=24° leads to HA=640 kA/m and ϑint=37°. Thus, Stoner–Wohlfarth behavior with lower anisotropy field strength as determined perpendicular to the preferential axis of magnetization is suggested. In contrast to TS measurements, no results can be found that fit the resonance field value both parallel and perpendicular to the film normal.
Light transmission characteristics of telluride-based chalcogenide glass for infrared fiber application
Authors: Katsuyama Toshio , Matsumura Hiroyoshi
Published: July 2002

Optical properties of Ge-Se-Te chalcogenide glasses for CO2 laser light transmission are investigated. It is found that the Te content of more than about 60 mol % is essential in reducing fundamental absorption caused by lattice vibration at 10.6 μm (CO2 laser light wavelength). It is also found that the Se content above 5 mol % is required to eliminate wavelength-independent scattering loss due to glass imperfections. As a result, the transmission loss of the optical fiber drawn from the Ge22Se20Te58 glass block is reduced to 1.5 dB/m at 10.6 μm. Further loss reduction is expected by suppressing the creation of the Te crystallites during the drawing process.
Crystal growth and the nonlinear optical properties of 4-nitrophenol sodium salt dihydrate and its deuterated material
Authors: Minemoto Hisashi , Ozaki Yusuke , Sonoda Nobuo , Sasaki Takatomo
Published: July 2002

Large organic ionic crystals on the order of a few centimeters of 4-nitrophenol sodium(:Na) salt dihydrate (NPNa) and DNPNa, in which the water of crystallization of NPNa was deuterated, were grown for second-harmonic generation (SHG). The Vickers hardness and thermal conductivities of NPNa were about two times larger than those of conventional organic molecular crystals. The effective nonlinear optical constants of NPNa and DNPNa were estimated to be 5.0 and 5.5 pm/V, respectively. The optical loss coefficients, measured by spectrophotometry, of NPNa and DNPNa were 1.8 and 0.6 dB/cm at 1064 nm, respectively. The substitution of D2O for H2O in NPNa crystal is very effective in reducing the optical loss coefficient. High power of 4.4 mW green SH light was obtained using a 1.5-mm-thick DNPNa crystal as the intracavity SHG device of a diode-pumped Nd:YVO4 laser.
Control and analysis of leakage currents in poly-Si thin-film transistors
Authors: Brotherton S. D. , Ayres J. R. , Trainor M. J.
Published: July 2002

Control of leakage current in autoregistered columnar and a solid phase crystallized poly-Si thin-film transistors (TFTs) is discussed. For n-channel TFTs, two parasitic leakage current paths, due to bulk conduction and back interface conduction, have been identified. It is demonstrated that these can be controlled by using sufficiently thin films and by low dose boron back channel implants, respectively. By these means, generation limited leakage currents, with values of <4×10−14 A/μm of channel width, have been obtained. The minimum leakage currents, for n- and p-channel TFTs, display the well-known field enhancement which we confirm can be described by phonon assisted tunneling. In well-engineered TFTs, with subthreshold slopes of <1 V/dec, we show that the drain fields required to promote the tunneling process are independent of the trap state density and result entirely from two-dimensional gate-drain coupling effects. Therefore, improving the quality of the poly-Si will not reduce the exponential dependence of the leakage current on gate and drain bias, although the absolute value of leakage current will be reduced.
Femtosecond laser excitation dynamics of the semiconductor-metal phase transition in VO2
Authors: Becker Michael F. , Buckman A. Bruce , Walser Rodger M. , Lépine Thierry , Georges Patrick , Brun Alain
Published: July 2002

We have measured the subpicosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of polycrystalline VO2 thin-films by making pump-probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties very close to the high-temperature metallic state being attained within about 5 ps. The equilibration of the metallic state after femtosecond excitation was modeled by non-exponentially decaying perturbations in the metallic state electron density and collision frequency. The decay of both these plasma parameters was well fit by a 1/√t time dependence. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2.
Model for noncollisional heating in inductively coupled plasma processing sources
Authors: Rauf Shahid , Kushner Mark J.
Published: July 2002

Low pressure (<10 mTorr) inductively coupled plasma sources are being developed for etching and deposition of semiconductors and metals. In models for these devices, plasma dynamics are typically coupled to the electromagnetic fields through Ohm’s law, which implies that collisional heating is the dominant power transfer mechanism. In this article, we describe an algorithm to couple plasma dynamics to electromagnetic field propagation which self-consistently includes noncollisional heating effects as well. The algorithm makes use of kinetic information available from an electron Monte Carlo simulation to compute plasma currents that are then used in computation of the electromagnetic field. Results for plasma density and electric field amplitude are presented as a function of power and pressure, and are compared to results from models that consider only collisional heating. We find that noncollisional heating effects are important at pressures of less than 10–20 mTorr, a range that depends both on gas mixture and geometry. Noncollisional heating effects allow the wave to couple more efficiently to the plasma. As a result, the electric field amplitude required to deposit a given amount of power in the plasma is smaller than that needed when only collisional heating is considered. For a constant power deposition, this generally leads to lower plasma densities.
Distribution of internal strains in nanocrystalline Pd studied by x-ray diffraction
Authors: Reimann K. , Würschum R.
Published: July 2002

Internal strains in cluster-compacted nanocrystalline Pd (crystallite size 10–20 nm) were studied by means of wide-angle x-ray scattering. The strain- and size-induced broadening of the Bragg diffraction peaks was determined by Hall or Warren–Averbach analyses. The Warren–Averbach analysis indicates a spatial confinement of internal strain fields. This result supports theoretical models according to which the sources of the stresses are located in the interfaces between the crystallites. Indirect further evidence for a stress localization is derived from recent findings on the variation of internal strains and atomic displacements with the crystallite size. In addition, an anisotropy of the line broadening was observed which cannot entirely be attributed to planar defects (maximum concentration 0.04) but indicates enhanced microstrains in the crystallographic 〈100〉 directions compared to the 〈111〉 directions. Based on this strain anisotropy, various distributions of microstresses are discussed taking into account the substantial elastic anisotropy of Pd.
Modeling and simulation of tunneling through ultra-thin gate dielectrics
Authors: Schenk Andreas , Heiser Gernot
Published: July 2002

Direct and Fowler-Nordheim tunneling through ultra-thin gate dielectrics is modeled based on an approach for the transmission coefficient (TC) of a potential barrier that is modified by the image force. Under the constraint of equal actions the true barrier is mapped to a trapezoidal pseudobarrier resulting in a TC very close to the numerical solution of the Schrödinger equation for all insulator thicknesses and for all energies of the tunneling electron. The barrier height of the pseudopotential is used as a free parameter and becomes a function of energy in balancing the actions. This function can be approximated by a parabolic relation which makes the TC of arbitrary barriers fully analytical with little loss of accuracy. The model was implemented into a multidimensional device simulator and applied to the self-consistent simulation of gate currents in metal-oxide-semiconductor (MOS) capacitors with gate oxides in the thickness range 15 Å–42 Å. Excellent agreement with experimental data was obtained using a thickness-independent tunnel mass mox=0.42 m0. Thanks to the CPU-time efficiency of the method the simulation of a complete MOS-field-effect-transistor with dominating gate current becomes possible and shows the potential for further applications.
Ultrasonic investigations of PbO–V2O5–P2O5 glass
Authors: Paul A. , Chattopadhyay A. K. , Basu C.
Published: July 2002

The velocity and attenuation of longitudinal and transverse ultrasonic waves in (PbO)x–(V2O5)y–(P2O5)1−x−y glass systems of different compositions have been measured at temperatures between 80 and 300 K using the ultrasonic pulse echo overlap technique at 10 MHz. For each sample the velocity data have been used to find the elastic moduli and Debye temperature at different temperatures. The results indicate that for each sample, the velocities decrease slowly and steadily with increasing temperature. The temperature dependence of ultrasonic attenuation has been explained in terms of a thermally activated relaxation process. On the other hand, the combined effect of relaxation, anharmonicity, and the effect due to frozen-in fluctuation is taken into consideration to explain the variation of velocity with temperature.
Light-scattering study of the glass transition in silica, with practical implications
Authors: Saito K. , Kakiuchida H. , Ikushima A. J.
Published: July 2002

The glass transition in silica glass was investigated by light-scattering measurements, and relations between the glass transition temperature, fictive temperature, cooling rate, and viscosity in silica glass were clarified. Furthermore, the effect of OH ions on the structural relaxation was found to work in two ways: The OH ions reduce the activation energy of viscoelastic relaxation, and they also generate a new relaxation due to local structure change. Application of such a relaxational process due to impurities should be effective for controlling the performances of silica glass.
Recombination processes in doubly capped antimonide-based quaternary thin films
Authors: Saroop Sudesh , Borrego Jose M. , Gutmann Ronald J. , Charache Greg W. , Wang Christine A.
Published: July 2002

Recombination processes in antimonide-based materials for thermophotovoltaic (TPV) devices have been investigated using a radio-frequency (rf) photoreflectance technique, in which a Nd–YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively coupled noncontacting rf probe. Both lattice-matched AlGaAsSb and GaSb have been used to double cap InGaAsSb active layers to evaluate bulk lifetime and surface recombination velocity with different active layer thicknesses. With an active layer doping of 2×1017 cm−3, effective bulk lifetimes of 95 ns and surface recombination velocities of 1900 cm/s have been obtained. As the laser intensity is increased the lifetime decreases, which is attributed to radiative recombination under these high-level injection conditions. Similar measurements have been taken on both TPV device structures and starting substrate materials for comparison purposes.
Explosive compaction of Li-ion battery components
Authors: Jak M. J. G. , Kelder E. M. , Schoonman J. , Lashkov V. N. , Strikanov A. V. , Selezenev A. A. , Potanin A. A.
Published: July 2002

The compaction behavior of Li-ion battery components was studied by using explosive dynamic compaction. Dynamic compaction experiments were conducted on the cathode material LiMn2O4 and the ceramic electrolyte Li-doped BPO4. The compaction caused by the shock wave was studied by varying the compaction pressure and the compaction assembly. Densities up to 93.4% of the theoretical mass density were obtained without any heat treatment. A comparison between explosive compaction and magnetic pulse compaction as well as static pressing has been made.
Yield properties, phase transition, and equation of state of aluminum nitride (AlN) under shock compression up to 150 GPa
Authors: Mashimo Tsutomu , Uchino Masakazu , Nakamura Akira , Kobayashi Takamichi , Takasawa Eiichi , Sekine Toshimori , Noguchi Yuichi , Hikosaka Hideaki , Fukuoka Kiyoto , Syono Yasuhiko
Published: July 2002

Inclined-mirror Hugoniot measurements were performed on pure AlN polycrystals in the pressure range up to 150 GPa to study the yield properties, phase transition, and equation of state. The Hugoniot-elastic limit (HEL) stress was approximately 9.4 GPa. Above the HEL, the Hugoniot data converged to a static compression curve despite the high thermal conductivity, which indicated that the thermal property is not an important factor in determining the shock yield property. The phase transformation from wurtzite-type (B4) to rock salt-type (B1) structure took place at approximately 19.4 GPa, and was completed by about 75 GPa. The corrected transition pressure at 298 K was 19.2 GPa. Shock velocity (Us) versus particle velocity (Up) relation of the final phase was given by Us=3.27+1.81Up km/s. The Birch–Murnaghan fitting curve of the calculated isothermal compression curve of the B1-type phase roughly coincided with the recent static x-ray diffraction data up to over 100 GPa. The Grüneisen parameter, bulk moduli (K0), and the pressure derivative (K0′) at zero pressure of the B1-type phase were estimated to be 1.51±0.03, 304±4 GPa, and 3.9±0.2 (K0″=−0.02), respectively.
Electronic properties of CuGaSe2-based heterojunction solar cells. Part II. Defect spectroscopy
Authors: Jasenek A. , Rau U. , Nadenau V. , Schock H. W.
Published: July 2002

CuGaSe 2 /CdS/ZnO heterostructures with different CuGaSe2 stoichiometry deviations, glass substrates with different Na content and varying CdS buffer deposition procedures are analyzed with admittance spectroscopy, deep level transient spectroscopy, and capacitance–voltage measurements. Cu-rich CuGaSe2 exhibits two acceptor-like bulk traps with activation energies of 240 and 375 meV. The density of both defect states is reduced by air annealing at 200 °C. Ga-rich CuGaSe2 material displays a tail-like energetic distribution of acceptor defects. The maximum of this distribution is at an energy of 250 meV. Defect densities and doping concentrations of Ga-rich material are considerably lower than in Cu-rich material. The different defect and doping densities found in the present investigation fully explain the efficiency gain which has recently been made by changing the material stoichiometry, the glass substrate and the CdS-deposition method for CuGaSe2-based thin film solar cells.
Stress-dependent surface reactions and implications for a stress measurement technique
Authors: Yu H. H. , Suo Z.
Published: July 2002

In contact with an environment, a solid may gain or lose mass due to, for example, deposition or etching. As the reaction proceeds, the surface of the solid moves, either extending or receding. If the solid is under stress, the elastic energy adds to the driving force of the reaction, and may cause the surface to roughen. This phenomenon has recently led to a novel experimental technique to determine the stress state in a solid by using an atomic force microscope to scan the surface profiles before and after etching. Stress is also known to change the mobility of a reaction. By this mechanism, the stress may either roughen or stabilize a flat surface. This article describes a linear perturbation analysis of a three-dimensional solid surface evolving under stress, using a general kinetic law. It is found that when the reaction is near equilibrium, the stress effect on driving force dominates; when the reaction is far from equilibrium, the stress effect on mobility dominates. Under these two conditions, the surface profile spectra have different patterns and length scales. The implications for the stress measurement technique are discussed. It is suggested that the same experimental procedure be used to measure surface energy and activation strains.
Sellmeier parameters for ZnGaP2 and GaP
Authors: Madarasz F. L. , Dimmock J. O. , Dietz N. , Bachmann K. J.
Published: July 2002

Sellmeier parameters are determined for the birefringent material ZnGaP2 and the singly refractive material GaP by the minimization of chi-square employing the Levenberg–Marquardt method. The distinguishing feature of the present work is that all five Sellmeier parameters are treated as adjustable. In previous work the Sellmeier parameter related to the restrahlen frequency was fixed and set equal for both ordinary and extraordinary waves in the birefringent material. The fitted results show there is approximately an 8% difference between the two. Taking this parameter as adjustable allows for a better fit on all other parameters.
Enhancement of tunnel magnetoresistance in ferromagnet/granular/ferromagnet junction related to the Coulomb blockade effect
Authors: Kubota Hitoshi , Fukumoto Yoshiyuki , Thamrongsing Siripongsakul , Ando Yasuo , Miyazaki Terunobu , Yu Chung
Published: July 2002

We study tunnel magnetoresistance effect in ferromagnet/granular/ferromagnet tunnel junctions in the Coulomb blockade regime. The granular consists of a Al–O/Co/Al–O/Co/Al–O multilayer, which was sputtered sequentially. The magnetoresistance (MR) ratio and the resistance (Rs) of the junction increased with decreasing bias voltage at low temperatures. The MR ratio and Rs were calculated based on the theory proposed by Takahashi and Maekawa [S. Takahashi and S. Mackawa, Phys. Rev. B 90, 1758 (1998)]. Although the variation of these values on bias voltage was different from experimental results, the calculated MR ratio in the Coulomb blockade regime agreed well with the maximum value measured.
Statistical theory of multiphoton excitation of polyatomic molecules based on the Wigner function
Authors: Bo-Min Xie , Xie Wei
Published: July 2002

This paper is aimed at establishing a statistical theory of rotational and vibrational excitation of polyatomic molecules by an intense IR laser. Starting from the Wigner function of quantum statistical mechanics, we treat the rotational motion in the classical approximation; the vibrational modes are classified into active ones which are coupled directly with the laser and the background modes which are not coupled with the laser. The reduced Wigner function, i.e., the Wigner function integrated over all background coordinates should satisfy an integro-differential equation. We introduce the idea of ‘‘viscous damping’’ to handle the interaction between the active modes and the background. The damping coefficient can be calculated with the aid of the well-known Schwartz–Slawsky–Herzfeld theory. The resulting equation is solved by the method of moment equations. There is only one adjustable parameter in our scheme; it is introduced due to the lack of precise knowledge about the molecular potential. The theory developed in this paper explains satisfactorily the recent absorption experiments of SF6 irradiated by a short pulse CO2 laser, which are in sharp contradiction with the prevailing quasi-continuum theory. We also refined the density of energy levels which is responsible for the muliphoton excitation of polyatomic molecules.
On the nature of the tensile instability in metastable liquids and its relationship to density anomalies
Authors: Debenedetti Pablo G. , D’Antonio Michael C.
Published: July 2002

Tensile instability is a hitherto unexplained phenomenon whereby a metastable liquid loses tensile strength as the temperature is reduced in the vicinity of its triple point. The thermodynamically consistent behavior which must be exhibited by any liquid in the vicinity of a tensile instability displays a variety of unusual phenomena: nonanalytic density maxima, spinodal retracing, and density anomalies (negative thermal expansion coefficient) in the vicinity of the spinodal curve. Loss of tensile strength implies (and is inseparable from) density anomalies in the vicinity of the spinodal curve. This important conclusion is derived here from first principles.
High-resolution total differential cross sections for scattering of helium by O2, N2, and NO
Authors: Beneventi Laura , Casavecchia Piergiorgio , Volpi Gian Gualberto
Published: July 2002

High-resolution crossed molecular beam measurements of the total differential cross sections (DCS) for the scattering of He by O2, N2, and NO in the thermal energy range are reported. The data show well resolved diffraction oscillations which appear damped with respect to the corresponding isotropic He–Ar case. Information on the anisotropy of the interaction is indirectly obtained from the damping of the diffraction oscillations within the framework of the infinite-order-sudden (IOS) approximation. While large discrepancies, within the same scheme of analysis, are found with respect to anisotropy estimates from similar experiments, but performed at a lower resolution, good agreement is observed with respect to those obtained for He–O2 and He–N2 from state-to-state rotationally inelastic DCS measurements. Potential energy surfaces (PES) are derived for all systems by also simultaneously fitting absolute total integral cross sections and second virial coefficient data. Comparisons are made with previous both empirical and theoretical potentials, which also have been obtained from or tested against experimental bulk and scattering properties within the IOS approximation. The derived He–O2 surface is found to be in good agreement with recent results. For He–N2 we propose an empirical PES which predicts several independent scattering and bulk data. For He–NO the surface obtained is an improvement with respect to that previously proposed.
Adiabatic approach to the coupling between local- and normal-like modes
Authors: Carrington Tucker
Published: July 2002

An adiabatic method of incorporating harmonic bending and stretching modes into the local mode model is developed. This is achieved by defining local normal mode coordinates which enable one to treat some of the stretches as local modes and the other vibrational degrees of freedom as normal modes in a perfectly systematic fashion. An energy level expression for a system of local modes coupled to local normal modes (harmonic oscillators) is derived. The expression has the same form as the standard normal mode formula. A simple formula for the anharmonicity constant xbs is obtained. The formulas are shown to reproduce well the experimental results for H2O, H2S, and H2Se and are tested against previous calculations.
Collision time-correlation functions in the semiclassical limit. II. Vibrational–rotational energy transfer in molecule–molecule collisions
Authors: Vilallonga Eduardo , Micha David A.
Published: July 2002

A semiclassical limit of collisional time-correlation functions is applied to energy transfer in collisions between two molecules. The internal degrees of freedom (vibration and rotation) are treated quantum mechanically while the relative motion is described by classical trajectories for fixed molecular orientations. Hence the theory is well-suited to the analysis of energy and angular distributions in collisions governed by general intermolecular potentials, including highly anisotropic short-range forces. Cross sections are given as Fourier transforms of correlation functions of time-evolution operators for the internal degrees of freedom. These operators are determined by action integrals of the intermolecular forces evaluated along the classical trajectories. Correlation functions for coupled harmonic vibrations are obtained using an operator algebra. Rotational correlation functions are calculated by means of an asymptotic expansion appropriate to large transfers of angular momentum. Compact analytical expressions are derived for cross sections in collisions involving symmetric-top, spherically symmetric, and linear molecules. Expressions for average rotational energy transfer, readily obtainable from time-of-flight experiments, are also derived for symmetric top molecules.
Association of N2 with NH+4 and H3O+(H2O)n, n=1,2,3
Authors: Gheno F. , Fitaire M.
Published: July 2002

We used a plasma chromatograph-mass spectrometer to study association reactions in high purity nitrogen at atmospheric pressure between 27 and 200 °C. The thermodynamic data for the formation of N2⋅NO+, H2O⋅H3O+(H2O), and H2O⋅H3O+(H2O)2 are found to be in agreement with previous results obtained under very different conditions. The enthalpies and entropies of formation of the N2 clusters N2⋅NH+4 and (N2)nH3O+(H2O)p, were determined for (p=1, n=1–3), (p=2, n=1–3), (p=3, n=2). The values of −ΔH° and −ΔS° were estimated for (p=2, n=4) and (p=3, n=3). The binding energies are found to be low, except for N2⋅NH+4. The bond strength of the latter is certainly due to chemical bonding.
Comparison of experiment and theory for the resonance Raman spectrum of I2 in solution. II. The Raman excitation and depolarization profiles inn-hexane
Authors: Sension Roseanne J. , Kobayashi Takamichi , Strauss Herbert L.
Published: July 2002

The calculations for I2 in n-hexane, which were started in paper I [J. Chem. Phys. 85, 3791 (1986)] are improved by considering the effect of the D state and of charge–tranfer states. Changes in intensities and positions of the B states and the effect of the rotation of I2 in the excited states are also considered. The experimental excitation profiles are recalibrated and compared to calculations. The depolarization profiles are measured and are also compared to calculations. The measured Raman cross sections are higher than calculated in the high frequency wing of the profile—a discrepancy for which we have not found an explanation. The depolarization ratios of the overtones may be explained by shifting the B and B″ states about 200 cm−1 further apart than in the gas phase and by taking into account the effect of rotation of the I2. The excited state relaxation time, T2, is still determined as about 0.3 ps.
Large local energy fluctuations in water
Authors: Tanaka Hideki , Ohmine Iwao
Published: July 2002

A detailed analysis is made for the dynamical behavior of an individual water molecule in liquid water by using a classical molecular dynamics (MD) calculation. It is found that there exist very large potential energy fluctuations in water; a single water molecule can exhibit a fluctuation of the order of 10∼20 kcal/mol. These potential energy fluctuations can be classified into two categories; the fast component (10−14–10−13 s) associated with librational motions of water molecules and the slow component (10−12–10−11 s) associated with water binding structure changes. Both amplitudes can be reached up to 10 kcal/mol. Due to strong Coulomb (dipole–dipole) interaction, small mutual geometrical changes, caused by the libration motions, induce large fast potential energy fluctuations. Due to large cohesion energy of the hydrogen bond and the nature of the water binding structure, there exist many water pair interactions which are unattactive or even repulsive; the water molecule potential (binding) energy distribution is inhomogeneous in space. This inhomogeneity alters as the water structure changes. Each water molecule then makes a transition from a stable binding to an unstable one with surrounding molecules and vice versa, inducing large potential energy changes in the picosecond order. A detailed analysis is made concerning how the hydrogen bonding pattern changes with water molecular motions. Significance of the existence of large water potential energy fluctuations on chemical processes is discussed.
Regular/irregular phase space structure of HCN/HNC
Authors: Founargiotakis M. , Farantos S. C. , Tennyson J.
Published: July 2002

A detailed 3D classical nonlinear mechanical analysis of HCN/HNC system is carried out and the results are compared with 2D quantum mechanical vibrational calculations as well as with the recent 3D quantum calculations of Bacic and Light. HCN is marked by regular behavior which persists at high energies when the stretching modes are excited. Chaotic trajectories located in the HNC well are those which lead to isomerization of HNC. The regular/irregular phase space structure agrees well with the corresponding assignment of quantum states. A multiple resonance among the vibrational modes of HCN, located at the top of the effective barrier of isomerization, is considered as the classical cause of the lack of delocalization of the wave functions found in the 3D quantum calculations.
Theoretical studies of AlF, AlCl, and AlBr
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Taylor Peter R.
Published: July 2002

Accurate spectroscopic constants are presented for the lowest six singlet and lowest five triplet states of AlF and AlCl. We propose that the correct ordering of the triplet manifold in both molecules should be a 3Π, b 3Σ+, c 3Σ+, d 3Π, and e 3Δ. This supports the assignment of the triplet state observed at Te=63 203 cm−1 to a 3Δ state but changes the designation from d to e. Radiative lifetimes are computed for the excited states. In addition, we present a more detailed study of the A 1Π–X 1Σ+ transition in AlF, AlCl, and AlBr. For AlF and AlCl, potential curves for the X 1Σ+ and A 1Π states, and A–X transition moment functions, are presented, together with Einstein coefficients and A 1Π vibrational lifetimes. The respective lifetimes for the v=0 level of the A 1Π states are observed to increase from about 1.9 ns for AlF to 5.2 ns for AlCl to about 8.5 ns for AlBr. The value for AlCl agrees with a recent laser-induced fluorescence study to within the experimental error bars.
Dynamics of intermolecular electronic energy transfer at an air/liquid interface
Authors: Sitzmann E. V. , Eisenthal K. B.
Published: July 2002

Electronic energy transfer dynamics at the air/liquid interface have been studied by surface second harmonic techniques. The effects of dimensionality on the dynamics are discussed.
Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid nitrogen
Authors: Moore D. S. , Schmidt S. C. , Shaw M. S. , Johnson J. D.
Published: July 2002

Vibrational spectra of liquid nitrogen shock compressed to several high pressure/high temperature states were recorded using single-pulse multiplex coherent anti-Stokes Raman scattering. Vibrational frequencies, third-order susceptibility ratios, and linewidths are presented for the fundamental and several excited-state transitions. Vibrational frequencies were found to increase monotonically up to ≈17.5 GPa single shock and ≈30 GPa double shock. Above these pressures, the vibrational frequencies were observed to decrease with further increases in pressure. The decrease in vibrational frequency occurs in a pressure regime where the shocked nitrogen is becoming optically opaque. The consequence of the decrease in vibrational frequency on the Grüneisen mode gamma and its effect on the N2 equation of state is discussed. The transition intensity and linewidth data suggest that thermal equilibrium of the vibrational levels is attained in less than 10 ns at these high pressures and temperatures. Finally, the measured linewidths exhibit an almost linear dependence on shock temperature, and also suggest that the vibrational dephasing time has decreased to less than 1 ps at the highest pressures.
Microwave detection of direct g
a
u
c
h
e to t
r
a
n
s transitions in CHD2OH
Authors: Su Chun Fu , Quade C. Richard
Published: July 2002

A portion of the microwave torsional–rotational spectrum of CHD2OH has been assigned between 12.4 and 41.3 GHz. Of particular interest is the detection and identification of direct gauche to trans transitions from both gauche states as well as gauche to gauche transitions. The energy level system has three substates in the torsional motion with a rotational manifold for each torsional substate. In terms of torsional substate, ten Q-type and three R-type transitions have been assigned for e0 (gauche) to o1(gauche); 13 Q-type for o1(gauche) to e1(trans); and four R-type for e0(gauche) to e1(trans). Additional lines have been assigned within each torsional substate. Spectral analysis has been made using a phenomenological rotational Hamiltonian plus an internal axis torsion–rotation Hamiltonian in zeroth order. The approximate torsional energy level differences are gauche–gauche=184.7 GHz, gauche(upper)–trans=202.5 GHz, and gauche(lower)–trans=387.2 GHz. Two sets of potential energy coefficients, one with positive V2 and the other with negative V2, have been determined that give essentially equivalent fit in the spectral analysis.
Semiclassical calculation of the vibrational structure of theB̃ 1BuRydberg state oftrans-di-imide fromab initioconfiguration interaction potential energy surfaces
Authors: Groenenboom Gerrit C. , van Lenthe Joop H. , Buck Henk M.
Published: July 2002

The applicability of the SDCI and CEPA methods to the excited state of the title molecule is investigated. Two basis sets are used, one of triple zeta quality extended with diffuse functions and another which also contained polarization functions. For the subsequent CI calculations closed shell as well as open shell, molecular orbitals are used. We investigate the Pople correction as a way to obtain size consistent results from the SDCI calculation. For each method, a two-dimensional potential energy surface of the 1(4ag,5bu) Rydberg state of trans-di-imide (HNNH) is calculated. The vibrational fine structure in the corresponding B̃←X̃ UV absorption spectrum is derived from these surfaces and the result is compared to the spectrum measured by Neudorfl et al. [P. S. Neudorfl, R. A. Back, and A. E. Douglas, Can. J. Chem. 59, 506 (1981)]. A semiclassical method [K. S. Sorbie, Mol. Phys. 32, 1577 (1976)] is used to obtain the vibrational frequencies. A slightly modified version of the Heller frozen Gaussian approximation [E. J. Heller, J. Chem. Phys. 75, 2923 (1981)] is proposed and used to obtain the intensities of the vibrational bands. We conclude that it is important to use the open shell molecular orbital basis and the SDCI plus Pople correction, or even better, the CEPA. Both methods give good results for the vertical transition energy and excited state geometry. The error in the vibrational frequencies is in the order of 10%, but the NN-stretch mode is best described by the CEPA method.
Three-dimensional quantum theory of the H+H2 transition-state spectrum
Authors: Seideman T. , Shapiro M.
Published: July 2002

Three-dimensional quantal transition-state spectra due to optical transition between the reactive H+H2 system on the ground electronic surface and the lowest excited H3 Rydberg state are computed. The computations are based on a three-dimensional (3D) extension of the reactive-WKB [J. Chem. Phys. 88, 5525 (1988)] method, which is also shown to be a reliable tool for studying reactive processes in the tunneling regime. The three-dimensional quantum transition-state spectrum is found to be substantially different from both its classical 3D and its quantum-collinear analogs. Due to the prominence of high partial waves for most transitions, the quantum undulations which reflect in the collinear domain the initial scattering states, are masked (save for the far red end of the spectrum) by the rotational line contours. The latter are shown to reflect the quantum oscillations of the final bound vibrational states in the excited electronic state. The role of the stretching vs bending overlaps in determining the band shapes is discussed. Contrary to ordinary linear-to-bend spectra, the greater latitude for bending motion in the initial scattering states results in only a moderate progression of bending states. The possibility of extracting individual partial scattering amplitudes and the use of transition-state spectroscopy as a first step in a laser catalysis scheme is discussed.
Predissociation of the acetyleneÃ 1Austate and its mechanism
Authors: Haijima Akimitsu , Fujii Masaaki , Ito Mitsuo
Published: July 2002

The fluorescence excitation spectrum, the MPI spectrum, and the absorption spectrum of acetylene were observed for the Ã 1Au ←X̃ 1∑+g band system in a static gas cell and in a supersonic jet. A sudden and drastic decrease in fluorescence quantum yield, Φf , was found between the V 4 K2 (46 339 cm−1 ) and 21 V 3 K0 (46 673 cm−1 ) sublevels. The decrease in Φf is concluded to be due to the predissociation into C2 H+H. J dependence of Φf was found for the V 4 K1 vibronic sublevel, while it is absent for the level lying above V4 K1 . From the observed J dependence, the predissociation mechanism was concluded to be vibrational predissociation by tunneling through a potential barrier.
Infrared, resonance enhanced multiphoton ionization double resonance detection of energy transfer in NH3
Authors: Shultz Mary Jane , Wei Jian
Published: July 2002

2+1 resonance enhanced multiphoton ionization (REMPI) spectroscopy has been utilized to probe the temporal evolution of the rovibrational distribution of vibrationally excited NH3 following infrared excitation. High resolution, 2+1 REMPI spectra of the origin of the C̃′←X̃ transition of NH3 are presented and the rotational assignments are discussed. In addition, high resolution spectra of the hot band origin are obtained for the first time and rotational assignments are made. The time evolution of some specific JK states of the X̃(21+ ) and X̃(21− ) levels are reported, and collisional energy transfer between them is discussed. The rate constant, k1−←1+, for the energy transfer process: X̃(21+ )→X̃(21− ) is determined to be 0.21 μs−1  Torr−1 : 50 times slower than the hard-sphere, single-collision rate and about 100 times slower than in the ground vibrational state.
Laser-induced fluorescence of Rb2: The (1)1Σ+g(X), (2)1Σ+g, (1)1Πu(B), (1)1Πg, and (2)1Πu(C) electronic states
Authors: Amiot C.
Published: July 2002

More than 10 000 resolved rotational lines in the (2) 1Πu(C)–(1) 1Σ+g(X), (2) 1Πu(C)–(1) 1Πg, (2) 1Πu(C)–(2) 1Σ+g, (1) 1Πu(B)–(1) 1Σ+g(X), and (1) 1Σ+u(A)–(1) 1Σ+g(X) transitions of Rb2 have been accurately measured with the technique of Fourier-transform spectroscopy. The wave numbers of all the rotational lines were determined with an accuracy better than 5×10−3 cm−1. A thorough and simultaneous analysis of all the measured data yields molecular constants, potential-energy curves, and dissociation energies for five different excited electronic states. The observation of highly excited vibrational levels in the fundamental state (v″=112) leads to the value of the energy of dissociation: 3994.4±0.4 cm−1.
Nonstatistical CO product distributions from the hot H-atom reaction, H+CO2→OH+CO
Authors: Rice Jane K. , Baronavski A. P.
Published: July 2002

The hot H-atom reaction, H+CO2→OH+CO is investigated under several initial conditions designed to vary the angular momentum of the CO2 reactant. The translationally hot H atoms are produced by photodissociating H2S at 193 nm, resulting in a reaction exoergicity of ∼120 kJ mol−1. The internal energy in the CO product is monitored by laser-induced fluorescence in the VUV spectral range. Under near-nascent conditions we report the rotational distributions of CO in v″=0 under CO2 reactant ‘‘temperatures’’ of 300, 70 and 40 K. Also reported are the rotational distributions of CO in v″=1 at 300 K and 70 K and the population ratios of [v″=1]/[v″=0] at both initial CO2 temperatures. Three distinctively ‘‘dynamical’’ aspects of the potential energy (PE) surface are exhibited in this set of experiments: (i) As the CO2 reactant is cooled, a cooling of the CO distribution is seen which suggests the reaction intermediate does not live long enough to randomize its internal energy. (ii) We report a coupling of vibrational and rotational excitation in the CO product indicated by a ‘‘hotter’’ rotational energy in v″=1 than v″=0 at 300 K. (iii) In addition, we report different [v″=1]/[v″=0] ratios at the two CO2 initial temperatures. Contrary to nearly all indications from measurements on the OH product, the CO product is rich in ‘‘nonstatistical’’ behavior.
Theoretical study of HCl+ : Potential curves, radiative lifetimes, and photodissociation cross sections
Authors: Pradhan Atul D. , Kirby Kate P. , Dalgarno A.
Published: July 2002

Configuration interaction wave functions and potential energy curves have been calculated for the four lowest states of 2Π and 2Σ+ symmetry and the lowest state of 4Σ−, 2Σ−, 2Δ, and 4Π symmetry for the molecular ion HCl+. Dipole moment functions of the X 2Π and A 2Σ+ states are presented as well as dipole moments for transitions from the X state to dipole-allowed excited states. The electronic wave functions were constructed to give a balanced description of Rydberg–valence interactions. The computed radiative lifetime of the X 2Π(v=1) is found to be in good agreement with previous theoretical and experimental values. Oscillator strengths, transition probabilities, and radiative lifetimes are calculated for the A 2Σ+–X 2Π transition for vibrational levels v′≤6 and compared to previous theoretical and experimental results. Vibrational levels v′≥7 of the A 2Σ+ state are predissociated by the 4Π, 4Σ−, and 2Σ− states. Theoretical photodissociation cross sections are calculated showing that photodissociation occurs primarily through absorption into the (3) 2Π and (3) 2Σ+ states in the wavelength region λ<100 nm and also the 2Σ−, 2Δ, and (2) 2Π states for wavelengths λ>100 nm.
Calculation of nuclear magnetic shieldings. VIII. Gauge invariant many-body perturbation method
Authors: Fukui H. , Miura K. , Matsuda H.
Published: July 2002

The calculation of nuclear magnetic shieldings σ was performed for four first-row hydrides, i.e., HF, H2O, NH3, and CH4 using the finite-field many-body perturbation theory through second order in the electron correlation correction with London’s gauge invariant atomic orbitals. The calculated isotropic shieldings σ̄ for the heavy atoms were larger than the experimental values. The rotational and vibrational correction for the σ̄ of the F atom in HF at 300 K was estimated to be −4.0 ppm.
Theoretical studies of H2 desorption from Si(100)–2×1H
Authors: Jing Z. , Whitten J. L.
Published: July 2002

Theoretical studies of H2 desorption from a cluster model of the Si(100)–2×1H surface show that the desorption pathway is symmetrical and has a desorption energy barrier of 3.75 eV and a corresponding adsorption energy barrier of 1.15 eV. The proper treatment of electron correlation lowers the desorption energy barrier considerably. The present results suggest that the desorption of two hydrogen atoms from different Si atoms of a surface dimer is not the desorption pathway observed experimentally at activation energies in the range 2.0–2.9 eV.
Pulsed-field ionization threshold photoelectron spectroscopy with coherent extreme ultraviolet radiation: A comparison of CO and N2
Authors: Kong W. , Rodgers D. , Hepburn J. W. , Wang Kwanghsi , McKoy V.
Published: July 2002

Single-photon zero-kinetic-energy pulsed-field-ionization spectra have been measured for the v+=0 and 1 levels of CO+ (X 2Σ+) and the v+=0 level of N2+ (X 2Σg+) by coherent XUV radiation. In spite of similarities in the electronic structure of CO and N2, the measured ion spectra show dramatically different intensities for the Q branches. These threshold spectra are interpreted on the basis of ab initio calculations of the ion rotational distributions. Agreement between the calculated and measured spectra is very encouraging. Improved values for the ionization potentials of CO (113 025.6 and 115 211.2±1.5 cm−1 for v+=0 and 1, respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed. The CO spectra show quite different behavior for the ΔN<0 transitions for v+=0 and v+=1 bands, which is interpreted in terms of the relative importance of rotational autoionization in the two bands.
The inducible multipole solvation model: A new model for solvation effects on solute electrostatics
Authors: Davis Malcolm E.
Published: July 2002

A new approach to modeling the electrostatics of molecules in solution is presented, which is comparable in accuracy to finite-difference continuum models, yet is comparable in speed to polarizable in vacuo models. This is achieved by modeling the polarization charge resulting from solvation as inducible multipoles at the solute’s atom centers. Pilot studies of small molecule solvation energies, and of pKa shifts and molecular fields in proteins are presented. The results compare favorably with the finite-difference results while requiring significantly shorter execution time. Bearing in mind that no parameter optimization was performed in this study and that nonelectrostatic energy contributions were not considered, the results also compare reasonably with experimental results where those are available.