bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of virtual cathode formation and electron density profile measurements in an intense ion beam diode
Authors: Noonan W. A. , Hammer D. A.
Published: July 2002

A recently developed high pulse rate intense ion beam diode has been exploited to measure the relative density profile of electrons in the ion diode’s acceleration gap as a function of time. This diode was magnetically insulated and it produced 100 keV Ar+ beams at up to 4 A/cm2 using an active anode plasma ion source. The diode also had a backfill of 10−2 Torr of He that was collisionally excited by energetic electrons in the acceleration gap. When the excited He atoms radiatively decayed, the emitted light was measured and used to deduce the relative electron density profile with space and time resolution. The profile was peaked toward the center of the gap and dropped off significantly toward both the anode and the cathode. The absence of electron density near the physical cathode is evidence for magnetic field profile modification from the diamagnetic drift of the electrons in the diode. With a macroscopic electric field of order 100 kV/cm, the electron current takes many tens of nanoseconds, from the moment the cathode begins to emit, to reach the Child–Langmuir current. The extent to which this limit is exceeded depends strongly on the profile of the insulating magnetic field and appears to be dominated by the dynamics of the electron flow in the diode gap. In contrast, the delay between when electrons are first observed in the gap spectroscopically and when the ion beam is first formed is independent of the magnetic field profile. This delay is about the time that it takes an argon ion to transit the acceleration gap, suggesting that ion inertia is the rate limiting factor.
Carbon self-organization in the ternary Si1−x−yGexCy alloy
Authors: Guedj C. , Portier X. , Hairie A. , Bouchier D. , Calvarin G. , Piriou B. , Gautier B. , Dupuy J. C.
Published: July 2002

This article demonstrates for the first time the possible self-ordering of carbon in Si1−x−yGexCy thin films pseudomorphically grown on silicon. Germanium and carbon atomic distributions have been studied for a C-rich Si0.9−yGe0.1Cy heterostructure using high-resolution transmission electron microscopy (HRTEM), high-resolution x-ray diffraction, Raman spectrometry, and secondary ion mass spectrometry (SIMS). HRTEM images show the spontaneous formation of carbon-rich tilted sublattices and local germanium fluctuations, despite constant growth parameters. X-ray diffraction confirms this thin sublayers formation. A complementary insight into local ordering effects around C is obtained by Raman spectroscopy. A new model for perpendicular lattice parameter reduction is proposed. It involves C atoms mostly in third-nearest-neighbor positions and the local formation of a distorted CSi3 graphitic arrangement. In these C-rich sublayers, the perpendicular lattice mismatch to silicon is as low as −0.014. This aperiodic structure remains highly distorted and a statistical description of these strain fluctuations is detailed. The atomistic configuration of these δ layers indicates the likely contribution of surface steps during the growth, while SIMS measurements hint at the probable involvement of carbon interstitials to explain this ordering. For technological applications, this self-organization of carbon is promising for the ultrashallow junction challenge. These carbon-rich embedded layers can be considered as quantum wells, etch stops or very thin barriers against transient enhanced diffusion.
A method for determining microinhomogeneities in semiconductors
Authors: Chaikovsky I. , Alperovich L.
Published: July 2002

A method is suggested for determining random irregularities in semiconductors. The method is based on the anomalous sensitivity of the effective electrical conductivity of semiconductors with weak random irregularities to an applied strong magnetic field. The primary advantage of this method is the possibility to control the sensitivity of measurements of small values of concentration of random weak inhomogeneities changing an external magnetic field. Preliminary results are presented here of experiments with Si:B thin layers at He temperatures under a broad range of magnetic fields (0–30 kOe). The experimentally determined measure of the sample inhomogeneity was found to be close to the theoretical predicted value.
The influence of the critical current spread on the frequency locking of Josephson junctions in two-dimensional arrays
Authors: Laub A. , Keck M. , Doderer T. , Huebener R. P. , Traeuble T. , Dolata R. , Weimann T. , Niemeyer J.
Published: July 2002

Two-dimensional arrays of overdamped Josephson junctions under microwave irradiation have been examined experimentally. We show that the shape of the Shapiro steps in the current–voltage curve of the array is closely related to the critical current spread of the junctions and that its dynamic resistance is closely related to the number of frequency locked junctions which are visualized by low-temperature scanning electron microscopy.
Excitons in near-surface quantum wells in magnetic fields: Experiment and theory
Authors: Gippius N. A. , Yablonskii A. L. , Dzyubenko A. B. , Tikhodeev S. G. , Kulik L. V. , Kulakovskii V. D. , Forchel A.
Published: July 2002

The exciton transition and binding energies have been investigated in near-surface InGaAs/GaAs quantum wells theoretically and experimentally (by photoluminescence and photoluminescence excitation spectroscopy at 4.2 K). The contribution induced by vacuum has been analyzed for the ground and excited exciton states in perpendicular magnetic fields up to 14 T. The vacuum potential barrier has been shown to increase the magnetoexciton transition energies, ℏωn, but nearly not to influence their binding energies, En. In contrast, the image charges (caused by the abrupt, one order of magnitude, decrease of the dielectric constant at the semiconductor-vacuum interface) modify the Coulomb interaction and lead to the increase of both ℏωn and En. The magnetic field has been found to enhance the contribution of the image charges to the exciton binding energy and to decrease their influence on the transition energy. The effect is due to the in-plane exciton wave function squeezing in a magnetic field.
Electromigration failure of integrated circuit metallizations subjected to high-frequency pulsed currents
Authors: Malone D. W. , Hummel R. E.
Published: July 2002

Electromigration life tests were performed on copper-alloyed aluminum test structures that were representative of modern integrated circuit metallization schemes. A total of 18 electrical stress treatments were investigated. One was a steady dc current, and the others were pulsed dc currents varied according to duty cycle and frequency. The duty cycle was varied from 33.3% to 80%, and the frequency was varied such that three distinct orders of magnitude were roughly represented—100 kHz, 1 MHz, and 100 MHz. The median time to failure, t50, was used as the primary basis of comparison between test groups of six–nine samples. There was no discernible dependence of t50 on the pulse frequency. In contrast, the duty cycle, d, played a strong role. Duty cycles greater than 50% produced median lifetimes that varied as 1/d2, while duty cycles equal to and less than 50% produced a shift toward (but not reaching) a 1/d dependence. This shift was most pronounced at the smallest duty cycle of 33.3%, for which lifetimes were midway between the predictions of the 1/d2 and 1/d relationships. Post-test optical micrographs were obtained for each test stripe, and these suggested that the location of electromigration damage was influenced by the pulse duty cycle. Most damage occurred near the cathode contact in all instances, but there was an increased incidence of damage farther downwind with decreasing duty cycle. The results are explained in terms of a Blech length effect that may operate as a result of the bamboo test stripe structure.
Dose- and annealing-induced changes in the microstructure of buried SiNx: An x-ray absorption study
Authors: Paloura E. C.
Published: July 2002

The microstructure and the annealing behavior of N-rich SiNx films are studied with x-ray absorption measurements at the N-K edge. The SiNx films were fabricated with ion implantation of 35 keV N+ ions into Si substrates, in the dose range 2×1017–2×1018 ions/cm2. The near-edge x-ray absorption fine structure (NEXAFS) spectra of the N-rich films are characterized by a strong resonance line (RL2) at 403.3±0.1 eV whose intensity increases with the implantation dose. RL2 is attributed to dipole electron transitions, from 1s to unoccupied p orbitals, at a defect site containing a N dangling bond defect. The defect related to RL2 has high thermal stability and is practically unaffected by vacuum annealing up to 1000 °C. One more defect-related resonance (RL1) is detected at 401.1±0.3 eV in the films fabricated with the lower and intermediate used implantation doses (2×1017 and 1×1018 cm−2, respectively). The activation energy for the annealing of RL1 is 0.50±0.05 eV. Extended x-ray absorption fine structure spectroscopy (EXAFS) on the as-implanted and annealed films reveals that subnitrides are formed upon implantation. These nitrides are characterized by a Si-N nearest-neighbor distance which is comparable to that of the reference nitride, while the coordination number in the first nearest-neighbor shell N1 depends on the implantation dose. The stoichiometry deviation, as measured from the value of N1 which takes values between 1.7 and 2.8 for the limiting doses of 2×1018 and 2×1017 cm−2, respectively, shows the correlation between the RL2 in the NEXAFS spectra and the presence of N dangling bonds in the as-implanted films.
Theoretical study of antisite arsenic incorporation in the low temperature molecular beam epitaxy of gallium arsenide
Authors: Muthuvenkatraman S. , Gorantla Suresh , Venkat Rama , Dorsey Donald L.
Published: July 2002

A stochastic model for simulating the surface growth processes in the low temperature molecular beam epitaxy of gallium arsenide is developed, including the presence and dynamics of a weakly bound physisorbed state for arsenic. The physisorbed arsenic is allowed to incorporate into the arsenic site or gallium site (antisite) and evaporate. Additionally, the antisite As is allowed to evaporate from the surface of the crystal. The arsenic flux, temperature and growth rate dependences of antisite arsenic (AsGa) concentration and the resultant % lattice mismatch obtained from our simulation are in excellent agreement with the experimental results. The activation energy of 1.16 eV for the evaporation of antisite arsenic from the crystal obtained from our model is in good agreement with theoretical estimates. At a constant substrate temperature and growth rate (Ga flux rate), the antisite arsenic concentration and hence, the % lattice mismatch increase with arsenic flux in the low flux regime and saturate for high flux regime. The critical arsenic flux at which the AsGa concentration and the % lattice mismatch saturate, increases with temperature. The AsGa concentration and % lattice mismatch saturate at lower values for higher temperatures. As the arsenic flux increases, the coverage of the physisorbed layer increases and at a critical flux dictated by the fixed temperature and growth rate, the coverage saturates at its maximum value of unity (a complete monolayer) and hence, the concentration of AsGa and % lattice mismatch saturate. Lower AsGa concentration and % lattice mismatch result at higher temperature due to more evaporation of AsGa from the surface of the growing crystal. Additionally, an analytical model is developed to predict the AsGa concentration and % lattice mismatch for various growth conditions.
Electrical deactivation of interstitial Zn in heteroepitaxial InP by hydrogen and its effect on electronic properties
Authors: Ringel S. A. , Chatterjee B.
Published: July 2002

Hydrogen passivation of InP layers grown on lattice-mismatched substrates can achieve thermally stable deactivation of dislocation-related deep levels, making this a promising process for improving the performance of heteroepitaxial InP space solar cells. However, in addition to dislocation-related defects, interstitial Zn (Zni) defects that are characteristic of Zn-doped InP and which form deep donor states within the InP band gap, are important considerations for optimizing the electronic quality of these layers. Here, we show that hydrogen forms complexes with and deactivates Zni donor states within Zn-doped InP grown by metalorganic chemical vapor deposition. A combination of photoluminescence (PL), electrochemical capacitance–voltage dopant profiling, secondary ion mass spectroscopy and current–voltage (I–V) measurements are applied to a set of samples receiving systematic hydrogenation and annealing treatments. We find that the deactivation of Zni deep donors, as detected by monitoring the evolution of the donor–acceptor transition using PL measurements, causes an increase of ∼50% in the net acceptor concentration of heavily Zn-doped heteroepitaxial InP by elimination of the acceptor compensation effect due to active Zni donors. Analysis of I–V characteristics indicates that Zni passivation sharply reduces depletion region recombination and shunt currents within heteroepitaxial diodes, causing an increase in the diode turn-on voltage from 680 to 960 mV. Subsequent annealing above 500 °C reactivates the Zni defects, resulting in a systematic increase in doping compensation as well as a decrease in VTO toward the original, as-grown value. A study of the reactivation kinetics for the H–Zni complex reveals a greater thermal stability than that of H–Zn acceptor complexes but less than that of H-dislocation complexes in InP, with an estimated dissociation energy for the H–Zni complex of 2.3 eV. While these effects are observed for both homoepitaxial and heteroepitaxial Zn-doped layers, the effect is far more pronounced for the heteroepitaxial layers due to the relatively high Zni concentration in the latter.
Magnetic relaxation in Sm2Fe14Ga3 and Sm2Fe14Ga3C
Authors: Mommer N. , Gerlach M. , van Lier J. , Kronmüller H.
Published: July 2002

Magnetic after-effect measurements of Sm2Fe14Ga3 and the carbide Sm2Fe14Ga3C were performed in the temperature range of 140 to 450 K. In the case of Sm2Fe14Ga3C, the relaxation spectrum is composed of two characteristic maxima at 220 and 320 K. The latter maximum is not present in Sm2Fe14Ga3 and therefore assigned to local jumps of interstitially dissolved carbon atoms while the former is attributed to hydrogen. Numerical evaluation yielded an activation enthalpy of (0.96 ± 0.02) eV and a pre-exponential factor τ0=3×10−14±0.5 s for the short-range diffusion of C atoms. The corresponding values for the short-range diffusion of hydrogen are (0.64±0.04) eV and τ0=3×10−14±1 s.
The field-space perspective on hysteresis in uniaxial ferromagnets
Authors: Millev Y. T. , Cullen J. R. , Oepen H. P.
Published: July 2002

A procedure for the analysis of hysteresis in the H space of a uniaxial ferromagnet with higher-order anisotropy is put forward. The formulation is valid to any order n in the anisotropy expansion. The critical boundaries separating stable from metastable states are cast in a formally decoupled parametric way as Hx=Hx(Mx), Hz=Hz(Mz). The analytic expressions provide the basis for the construction of generalized astroids to any order. For n>1, new features are found and interpreted in their relation to rotational hysteresis and possible spin-reorientation transitions in uniaxial materials. The shape and symmetry of the critical boundaries depend crucially on up to n−1 independent ratios of the anisotropy constants against a suitable normalizing quantity; the normalizer can be any from among the set of constants or any linear combination thereof. Self-crossing of an astroid indicates the existence of additional extrema and, hence, of complicated hystereses.
Influence of the system parameters on the non-Arrhenius magnetic relaxation of systems having distributed properties
Authors: Smirnov-Rueda R. , Chubykalo O. A. , González J. M. , González J.
Published: July 2002

We have investigated, in the framework of the micromagnetic approximation, the relaxation behavior of both simple systems and systems having distributed properties. In the case of single-particle-type systems, our study focused on the exchange constant dependence of the so-called “waiting time” for the onset of the relaxation to conclude that this parameter linearly increased with the increase of the exchange constant. Our results for the relaxation of polycrystalline-type systems having distributed anisotropy easy axes showed the occurrence in limited time ranges of a magnetization decrease which was adequately fitted by the M(t)−M(0)∝ ln(t+t0) law. The exchange constant dependence of the additive fitting parameter in the logarithmic law qualitatively reproduced that of the waiting time indicating that both parameters were linked to the same underlying characteristic of the demagnetization process: the coupling of the magnetic moments forming the domain-wall-like structures through which the system reversed its magnetization.
Electronic structure and magnetic properties of URhSi
Authors: Galli M. , Marabelli F. , Continenza A. , Monachesi P. , Canepa F. , Fornasini M. L.
Published: July 2002

Orthorhombic URhSi orders ferromagnetically below Tc=9.5 K, with an ordered magnetic moment μ=0.11μB per U atom and has a relatively enhanced value of the specific heat cp/T=186 mJ/mol K2. U-U atomic distances fall in a critical region around the Hill limit so that it is thought to be an itinerant magnet. Our present investigation includes: (i) preparation of high purity crystalline samples further analyzed by single crystal x-ray diffraction and magnetization measurements; (ii) band structure calculations performed with self-consistent full potential linearized Augmented plane wave method within the local density approximation; (iii) optical measurements at room and at low temperature over a large spectral range (2 meV–6 eV). As a result, the details of the crystal structure of URhSi have been refined with respect to previous measurements on powdered samples. The comparison of electronic structure calculations and optical results show: (a) a low mobility of free carriers and a large dielectric screening at the lowest frequencies, connected with excitation of f states hybridized with d states at the Fermi level; (b) smooth spectral features due to the huge number and density of states (the main contribution being due to occupied Rh d states). The whole amount of information obtained would suggest the presence of Kondo-like behavior in this compound, independently on the magnetic order.
Hydrogen absorption and desorption behavior in Pr–Fe–B type alloys
Authors: Kim Yoon B. , Jeung W. Y.
Published: July 2002

In this study, the hydrogen absorption and desorption behavior of Pr–Fe–B type alloys with addition of Co, Ga, and Zr was investigated focusing on the disproportionation and recombination reaction in HDDR process. In Pr13B6Febal. alloy, the second hydrogen absorption reaction which corresponds to the disproportionation reaction in HDDR process occurs at about 620 °C, and the recombination reaction was not observed below 1000 °C in hydrogen atmosphere. In alloys with Co content of 12 at.%, the second hydrogen desorption reaction, which corresponds to the recombination reaction in HDDR process, occurs at about 955 °C and the occurring temperature of the recombination reaction is further decreased with increasing Co content. The Ga addition to Pr13B6Febal. alloy also reduces the recombination reaction temperature and has no significant influence on the disproportionation reaction. Moreover, the lowering of the recombination reaction temperature is remarkable when Ga and Co are added simultaneously. On the contrary, Zr addition to Pr13B6Febal. alloy may not cause the recombination reaction but make the disproportionation reaction sluggish. The Pr–Fe–B type magnet powder which has a remanence of 9.8 kG and coercivity of 4.8 kOe, was obtained by HDDR treatment.
A comparative study of heterostructures InP/GaAs (001) and InP/GaAs (111) grown by metalorganic chemical vapor deposition
Authors: Derbali M. B. , Meddeb J. , Mâaref H. , Buttard D. , Abraham P. , Monteil Y.
Published: July 2002

Heteroepitaxial InP layers were grown under the same growth conditions by metalorganic chemical vapor deposition on (001), (111)A, and (111)B surfaces of GaAs substrates. The heteroepilayers were studied by transmission electron microscopy, high-resolution x-ray diffraction, low-temperature photoluminescence, and low-temperature photoluminescence excitation. It is demonstrated that good quality InP epitaxial layers can be grown on GaAs substrates. Since layers and substrates have the same crystal structure, but different lattice parameters (aGaAs=5.6535 Å, aInP=5.8687 Å), the accommodation at the interface may occur by the formation of misfit dislocations parallel to the heterointerface. A remarkable reduction of the threading dislocation density for the (111) orientation and a decrease in the full width at half maximum values of the x-ray diffraction peaks were obtained. These results signify a dramatic crystalline improvement due to the reduction of the dislocation density using (111)-oriented GaAs substrates. The efficient photoluminescence and the full width at half maximum of the exciton peak compared with that of InP homoepitaxy show that good quality InP epilayers can be obtained on (111)-oriented GaAs substrates. The strain relaxation was investigated by high-resolution x-ray diffraction, and low-temperature photoluminescence excitation. The difference between the optical and the x-ray diffraction results is attributed to the thermoelastic strain due to the difference in the thermal expansion coefficients between epilayers and substrates.
Postgrowth thermal treatment of CuIn(Ga)Se2: Characterization of doping levels in In-rich thin films
Authors: Dirnstorfer I. , Hofmann D. M. , Meister D. , Meyer B. K. , Riedl W. , Karg F.
Published: July 2002

Optical and electrical measurements were carried out on annealed CuIn(Ga)Se2 (CIGS) thin films with Ga content of 10%. Annealing of as-grown In-rich CIGS in air/oxygen at 400 °C changes the photoluminescence spectrum to a spectrum which is usually obtained from as-grown Cu-rich CIGS. The annealing step reduces the donor density and the high compensation. This effect allows the investigation of the defect levels of In-rich CIGS which is not possible in as-grown layers due to the dominating fluctuating potentials. The activation energies for the donors and acceptors in In-rich CIGS were found to be 10 and 75 meV, respectively. The densities are in the order of 1018 cm−3 each, with a compensation ratio of 0.99.
Magnetic properties and colossal magnetoresistance of the perovskites La2/3Ca1/3Mn1−xTixO3
Authors: Li Run-wei , Wang Zhi-hong , Chen Xin , Sun Ji-rong , Shen Bao-gen , Yan Chun-hua
Published: July 2002

Structure, magnetic, and transport properties of the compounds La2/3Ca1/3Mn1−xTixO3(0⩽x⩽0.3) have been investigated. The substitution of a Ti4+ ion for a Mn4+ ion suppresses the ferromagnetism and metallic conduction significantly. In the case of low doping (x⩽0.04), Curie temperature TC decreases linearly with the substitution at a rate of about 31 K for 1% Ti. The magnetic spontaneous state has become a cluster glass state for the sample where x=0.06. Corresponding to the decrease of TC, the metal–insulator transition temperature and the peak of magnetoresistance (MR) shift to low temperature, while the resistivity and the maximum of MR increase with increasing Ti concentration. The magnetic dilution should be responsible for the experimental results.
Microstructural characterization of an α-Fe/Nd2Fe14B nanocomposite magnet with a remaining amorphous phase
Authors: Wu Y. Q. , Ping D. H. , Hono K. , Hamano M. , Inoue A.
Published: July 2002

The microstructures of a Nd8Fe76.5Co8B6Nb1Cu0.5 nanocomposite magnet have been investigated by transmission electron microscopy and three-dimensional atom probe microanalysis. The ribbon prepared by the single roller melt-spinning process under an optimal cooling rate was partially crystallized, being composed of α-Fe, Nd2Fe14B, and the intergranular amorphous phase. The volume fraction of the Nd2Fe14B phase increases by annealing at 740 °C for 3 min, but a small amount of amorphous phase remains in the intergranular region. Atom probe results show Nb and B are significantly enriched in the remaining amorphous phase, which would be the reason for the high thermal stability of the remaining amorphous phase. Cu atoms are dissolved in the Nd2Fe14B phase during solidification, and they do not have much influence on the microstructural evolution.
Calculation of spectroscopic properties of the ground and excited states of Be2 according to the antisymmetrized geminal power (AGP) based excitation propagator
Authors: Weiner B. , Öhrn Y.
Published: July 2002

The ground and excited states of the beryllium dimer are calculated using the antisymmetrized geminal power-Tamm Dancoff approximation (AGP-TDA) that produces the excited states from a correlated reference ground state with excitation operators that diagonalize the excitation propagator, and the adjoints of which annihilate the reference state. The overall theoretical spectroscopy of the Be2 molecule is qualitatively in agreement with experiment.
Photodissociation processes of water vapor below and above the ionization potential
Authors: Dutuit O. , Tabche-Fouhaile A. , Nenner I. , Frohlich H. , Guyon P. M.
Published: July 2002

Fluorescence excitation spectra of OH(A), OH(B), OH(C), Lyman α, Balmer α, Balmer β, and H2O+(Ã) are recorded in H2O in the 9–35 eV photon excitation range. The OH(A) fluorescence quantum yield is measured from 9.2 to 13.6 eV incident photon energy. The pressure dependence of the H2O+(Ã) fluorescence excitation spectrum is investigated and the quenching rate constant of H2O+(Ã) by H2O is found to be equal to (4±2)×10−10 cm3 s−1. New Rydberg states are observed: a continuum-like state in the 10–12.6 eV range assigned to the (3a1)−1 (3pb2) 1B2 state and two broad bands lying at 27–29 and 30–32 eV, respectively, assigned to Rydberg states converging towards the (2a1)−1 innervalence state of H2O+. The valence character of the nsa1 Rydberg states is discussed. The decay processes of valence-type vs pure Rydberg states are investigated. All energetically accessible dissociation channels producing fluorescent fragments are observed and the fragments appear at their thermodynamical threshold. The competition between autoionization and dissociation into neutrals is shown to occur only for pure Rydberg states. Possible dissociation mechanisms are discussed.
On the electron affinity of the oxygen atom
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Partridge Harry , Taylor Peter R.
Published: July 2002

The electron affinity of oxygen is computed to be 1.287 eV, at the full CI level using a 6s5p3d2f Slater-type orbital basis and correlating only the 2p electrons. The best CASSCF–MRCI result including only 2p correlation is 1.263 eV. However, inclusion of 2s intrashell and 2s2p intershell correlation increases the computed EA to 1.290 eV at the CASSCF–MRCI level. At the full CI basis set limit, the 2s contribution to the electron affinity is estimated to be as large as 0.1 eV. The higher excitation contribution to the electron affinity is found to increase substantially with basis set completeness, especially when the 2s electrons are correlated. Relativistic effects are shown to make a small (less than 0.01 eV) change in the EA.
A coupled cluster polarization propagator method applied to CH+
Authors: Geertsen Jan , Oddershede Jens
Published: July 2002

A new approach to the direct evaluation of excitation energies and transition moments from the polarization propagator is presented. The method, which uses a coupled cluster doubles (CCD) reference state within the framework of perturbative propagator methods, is applied to the lowest singlet and triplet excitations in CH+. Comparison of the coupled cluster doubles polarization propagator approximation (CCDPPA) results with experiments and standard perturbative polarization propagator calculations shows that a significant improvement is obtained with a coupled cluster rather than a Rayleigh–Schrödinger reference state: the singlet excitation energy is improved by about 0.5 eV and the triplet instability of the standard second order approach is removed. The radiative lifetime of the v′=0 level of the A 1Π state is estimated to be very close to 800 ns. The improved performance of the coupled cluster propagator method over propagator calculations based on Rayleigh–Schrödinger expansion mainly stems from a enhancement of the correlation coefficients for the lower double excitations.
Atom-probe study of a conducting polymer: The oxidation of polypyrrole
Authors: Nishikawa Osamu , Kato Hitoshi
Published: July 2002

Oxidation of the ClO−4 and BF−4 doped conducting polypyrrole (PP+ClO−4 and PP+BF−4) was studied using the high mass resolution atom probe. The specimens were prepared by depositing a PP film on a Pt tip electrochemically. At the early stage of oxidation, O−2 ions diffused into the PP film and reacted with C atoms of the PP film. As oxidation proceeds, hydrogen of the PP film forms hydro-oxides and diffuses out of the PP film. The carbon–oxygen bonds also transform to the stable double bonds, C=O. Thus the fragment ions detected from the fully oxidized PP film were CO+, CNO+, C2O+2, C2N2O+2, C2O2+2, C2N2O2+2, and C3N2O2+3, indicating that nearly all N atoms were strongly interacting with C and O atoms. Oxidation progresses from the surface and the thickness of the fully oxidized section is approximately 66 nm in the specimen exposed to air for three months. The boundary between the oxidized surface layer and the unoxidized substrate PP section was found to be rather abrupt. The mass spectra of the PP+ClO−4 and PP+BF−4 films are identical but the oxidation of the PP+BF−4 advances remarkably faster than that of the PP+ClO−4 film. No satisfactory explanation is available at present. No doped elements were detected except for B atoms. Tentative explanations are given.
Time domain formulation of optical spectroscopy involving three potential energy surfaces
Authors: Coalson Rob D. , Kinsey James L.
Published: July 2002

We consider the computation of electronic absorption and Raman scattering spectra when three potential energy surfaces are involved; namely, a ground state surface coupled radiatively to one excited state which is in turn coupled nonadiabatically to a second excited surface. Frequency domain formulas are given for both kinds of spectra under the (standard) assumption of weak radiative coupling, but with no restriction on the nonadiabatic coupling strength. These formulas are transcribed into the time domain so that the computational burden is shifted to determination of the temporal evolution of wave packets simultaneously on both excited surfaces. We show how to implement the appropriate dynamics utilizing a perturbation expansion in the nonadiabatic coupling strength. Finally, extensive numerical results are presented for a one dimensional test system. Lowest order nonvanishing perturbation theory corrections are found to account accurately for nonadiabatic effects of ∼15%. Concentrating mainly on low resolution spectra such as arise in the case of direct photodissociation, the general effect of nonadiabatic coupling is observed to be broadening of absorption spectra and suppression of Raman scattering intensities.
Infrared fluorescence study on the threshold of intramolecular vibrational state mixing
Authors: Kim Hong L. , Kulp T. J. , McDonald J. D.
Published: July 2002

We have performed an infrared fluorescence study on the intramolecular vibrational relaxation (IVR) from C–H stretch fundamentals in isolated ground state polyatomic molecules. It was found that a density of about 70 combined rotational–vibrational states of a common J and symmetry species per cm−1 is needed to ensure that complex state mixing and IVR occur, although we observed a substantial range of couplings for initial states of the same molecule at higher state densities above the threshold. Propynal was studied as an example of a molecule with a low state density. Fluorescence from propynal shows that although there is substantial state mixing, it is best described as a complicated case of Fermi resonance and Coriolis coupling, as in smaller molecules rather than the multistate mixing seen in larger ones. Infrared fluorescence from terminal C–H stretches of some acetylene derivatives was also measured; these show a slight difference in the extent of IVR from other C–H stretches. Molecules were cooled and isolated in supersonic molecular beams and excited in the C–H stretch region with an optical parametric oscillator. Fluorescence spectra were recorded with either a circular variable filter machine or a cryogenic Michelson interferometer.
Diagrammatic theory of temperature-dependent resonance Raman scattering from polyatomic systems with general harmonic potential surfaces
Authors: Tonks D. L. , Page John B.
Published: July 2002

A detailed derivation and discussion is given of our diagrammatic theory of temperature-dependent resonance Raman (RR) scattering and the optical absorption for multimode systems having general quadratic plus linear electron-vibrational coupling. By combining the time-correlator reformulation of RR scattering with suitably developed nonzero temperature many-body diagrammatic techniques, we obtain the RR excitation profiles and the absorption as one-dimensional Fourier transforms of analytic expressions involving just the model parameters and the temperature. The expressions are very convenient for explicit multimode model calculations. In addition, the theory brings out in a natural way the relation between RR profiles and the absorption, such that within well-defined special cases useful ‘‘transform’’ techniques can

be developed for computing profiles directly from the observed temperature-dependent absorption. The many practical advantages of the theory for the analysis of experimental data have been demonstrated in earlier papers dealing with specific systems. In this paper we provide a comprehensive discussion of the theoretical details, which have not been given previously. The theory applies for any number of normal modes, and for arbitrary normal coordinate mixing, mode frequency shifts, and atomic equilibrium position shifts under electronic excitaiton. It involves products of phonon operators having both positive and negative time ordering, necessitating specialized combinatorial arguments. The use of an appropriate linked cluster expansion is shown to lead very naturally to a separation of the RR scattering into ‘‘orders,’’ which is the essential component producing the important general features listed above.Detailed derivations are given of the exact expressions for the first-order RR profiles in the most general model, and for first- , second- , and third-order profiles in the frequency-shift limit of no mode mixing. The latter formulas are recast into their absorption→profile ‘‘transform’’ versions, and these are simplified to more useful approximate forms for the practically important special case of small frequency shifts. Renormalizations of the linear electron-vibrational coupling parameters due both to mode mixing and to frequency shifts are also briefly discussed.
Spin-adapted multireference coupled-cluster approach: Linear approximation for two closed-shell-type reference configurations
Authors: Jeziorski B. , Paldus J.
Published: July 2002

An explicit form of the spin-adapted multireference coupled-cluster formalism in the linear approximation is developed for the special case of a two-dimensional model space involving only closed-shell-type configurations. The formalism is applicable to a number of quasidegenerate systems with two valence orbitals of distinct spatial symmetry and should serve as a convenient testing ground for the suitability of the multireference coupled-cluster theory. General problems of the multireference coupled-cluster approach and its relationship with the corresponding configuration interaction formalism are discussed as well as the problems pertaining to a practical implementation of this formalism.
Reactions of N+(3P) ions with normal, para, and deuterated hydrogens at low temperatures
Authors: Marquette J. B. , Rebrion C. , Rowe B. R.
Published: July 2002

The reactions of N+(3P) ions with normal, para, and deuterated hydrogens have been examined at temperatures below 163 K by using the CRESU technique with both helium and nitrogen buffer gases. All these reactions have decreasing rate coefficients with decreasing temperature. Comparison of the data for normal and para-H2 reveals the drastic importance of rotational energy in promoting these processes. Analysis of these results shows a better agreement between the n-H2, p-H2, and HD data if the spin–orbit energy of N+(3P) is also considered as efficient as kinetic and rotational energies in driving the reactions. It follows that the endothermicity of the reaction N+(3P0) +H2(X 1Σ+g,J=0) →NH+(X 2Π,J=0) +H(2S) is (18±2) meV. This yields a proton affinity of N equal to (3.524±0.003) eV and a dissociation energy of NH of (3.42±0.01) eV.
Fifth-order many-body perturbation theory for molecular correlation energies
Authors: Kucharski Stanislaw A. , Noga Jozef , Bartlett Rodney J.
Published: July 2002

Full fifth-order many-body perturbation theory (MBPT) correlation energy contributions have been calculated for the molecules: BH, CH2 , H2 O, HF, SiH2 , CO, C2 H2, and O3 at the double zeta plus polarization level. The connected T4 component of the fifth-order energy has been computed with only an N6 basis set dependence due to a new factorization introduced for the ‘‘connected’’ quadruple T4 diagrams. For the systems where MBPT is converging smoothly the fifth-order recovers 30%–50% of the difference between MBPT(4) and the full configuration interaction (FCI) values. The E5 based [2,1] Padé approximants generally provide a better approximation to the FCI energy. The absolute values of E5 corrections vary from 1–2 mhartrees for the hydrides at the equilibrium geometry up to 22 mhartrees for the O3 molecule, which is poorly described by a single self-consistend-field reference function.
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o determination of the potential-energy surfaces and rotation–vibration energy levels of methylene in the lowest triplet and singlet states and the singlet–triplet splitting
Authors: Comeau Donald C. , Shavitt Isaiah , Jensen Per , Bunker Philip R.
Published: July 2002

The potential-energy surfaces and rotation–vibration energy levels of the ground (X̃ 3B1) and first excited (ã 1A1) electronic states of the methylene radical have been determined by purely ab initio means. The potential-energy surfaces were determined by multireference configuration interaction calculations, using a full-valence complete-active-space reference space, with an atomic-natural-orbital basis set of size [5s4p3d2f1g/3s2p1d]. The configuration interaction (CI) calculations were carried out at 45 points on the triplet surface and 24 points on the singlet surface. The Morse oscillator rigid bender internal dynamics (MORBID) procedure was used to calculate vibrational and rotational energy levels for 12CH2, 12CD2, 13CH2, and 12CHD. Also calculated were the zero-point vibrational energies, the singlet–triplet splitting, and the dissociation energy. The zero-point energy of 12CH2 is found to be 127 cm−1 (0.363 kcal/mol) greater in the triplet state than in the singlet. The singlet–triplet splitting in 12CH2 is computed as T0=3116 cm−1 (8.909 kcal/mol), compared with the experimentally derived value of 3156±5 cm−1 (9.024±0.014 kcal/mol). The dissociation energy of the ground state is obtained as D0=179.06 kcal/mol, compared to an experimental value of 179.2±0.8 kcal/mol. The fundamental frequencies for the triplet state are obtained as ν1=3015, ν2=974, and ν3=3236 cm−1 (the experimental value of ν2 is 963.10 cm−1). The corresponding values for the singlet (experimental values in parentheses) are ν1=2787 (2806), ν2=1351 (1353), and ν3=2839 (2865) cm−1.
Theoretical study of the spectroscopy of Al2
Authors: Langhoff Stephen R. , Bauschlicher Charles W.
Published: July 2002

The singlet and triplet states of Al2 below about 30 000 cm−1 have been studied at the multireference configuration-interaction level in a [8s 7p 5d 2f] Gaussian basis. We attempt to identify and characterize the band systems in both the singlet and triplet manifolds that should be most amenable to experimental study. The spectroscopy of Al2 can be understood in terms of an X 3Πu ground state, and except for the well known (1)3Σ−u–A 3Σ−g emission system, all other transitions that we can unambiguously assign involve the X 3Πu ground state. Above about 27 000 cm−1 the spectrum of Al2 is complicated by the presence of several overlapping transitions. The calculations suggest that the assignments of the E and F systems observed recently in a jet-cooled beam are correct, although the considerable remaining differences between the experimental and theoretical spectroscopic constants and radiative lifetimes preclude a definitive assignment. The very intense E′ system observed by Morse is assigned to the (3)3Πg–X 3Πu band system. Several singlet–singlet transitions are studied to help determine whether metastable singlet states exist in jet-cooled beams of Al2.
Langevin dynamics study of NaCl electrolyte solutions at different concentrations
Authors: Trullàs J. , Giró A. , Padró J. A.
Published: July 2002

Langevin dynamics simulations of NaCl electrolyte solutions at 25° C and several concentrations (0.1–2 M) were performed. The solvent-averaged potentials of Pettitt and Rossky were assumed. These potential models show remarkable oscillatory shapes that are associated with the discrete structure of the solvent. The changes in the ionic structure and self-diffusion coefficients with the increase of the concentration are discussed. Special attention is paid to both the mean number of ionic pairs and the time that these ionic associations survive. The results are compared with the ones obtained from other potential models and with the available experimental data.
The infrared spectrum of difluorovinylidene, F2C=C:
Authors: Gallo Mary M. , Schaefer Henry F.
Published: July 2002

Single and double excitation coupled cluster theory was applied in conjunction with a double zeta plus polarization basis set to the prediction of the infrared spectrum of difluorovinylidene, a target molecule of considerable current experimental interest. The C–F asymmetric stretch estimated at 1260 cm−1 has a very strong IR intensity, about 300 km/mole. Parallel studies of difluoroacetylene are reported.
How important is electron correlation for the hyperpolarizability of ethyne?
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

The dipole polarizability(α) and second dipole hyperpolarizability(γ) tensors are calculated for the ground state of ethyne (HCCH). An extensive basis set study is carried out at the finite-field self-consistent-field level. Accurate results based on the finite-field coupled-cluster double excitation model (commonly known as CCD) are also calculated. Electron correlation lowers the isotropic averages ᾱ and γ̄ by only 3.4% and 1.8%, respectively, although the components of the γ tensor are affected by amounts up to 13%. The CCD values of ᾱ≂ 22.52 e2a20E−1h and the anisotropy Δα≂11.58 e2a20 E−1h agree well with experimental measurements. A CCD value of γ̄ ≂ 5.31×103 e4a40E−3h is obtained but a meaningful comparison with experiment is not possible. Further measurements of the second-dipole hyperpolarizability are definitely needed.
Photoion-pair formation and photoelectron-induced dissociative attachment in C2H2: D(HCC–H)
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The formation of C2H− is observed in two broad resonance bands when C2H2 is irradiated with vuv light. The higher-energy band has partially resolved structure, approximately linear pressure dependence, and a threshold at 16.335±0.021 eV. It is attributed to photoion-pair formation (C2H−+H+) consequent upon predissociation of one or more Rydberg states. This threshold, together with IP(H) and EA(C2H), gives D0(HCC–H)≤5.706±0.022 eV≡131.6±0.5 kcal/mol, or ΔH0f0 (C2H)≤134.5±0.5 kcal/mol. The lower-energy band has an approximately quadratic pressure dependence and curved step-like structure. It is attributed to photoelectron-induced dissociative attachment mediated by a πg shape resonance. The threshold, at 878.5±2.0 Å, corresponds to a photoelectron energy of 2.715±0.032 eV. This threshold combined with EA(C2H)=2.969±0.010 eV, yields D0(HCC–H)≤5.684±0.033 eV≡131.1±0.7 kcal/mol, or ΔH0f0 (C2H)=134.0±0.7 kcal/mol.
Use of scaled external correlation, a double many-body expansion, and variational transition state theory to calibrate a potential energy surface for FH2
Authors: Lynch Gillian C. , Steckler Rozeanne , Schwenke David W. , Varandas Antonio J. C. , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

A new potential energy surface is presented for the reaction F+H2→HF+H. The regions of the surface corresponding to collinear and bent geometries in the F–H–H and H–F–H barrier regions are based on scaled external correlation (SEC) electronic structure calculations, and the F–H⋅⋅⋅H exit channel region is based on the previously developed surface No. 5. The functional form of the new surface includes dispersion forces by a double many-body expansion (DMBE), and the surface was adjusted so that the van der Waals well in the F⋅⋅⋅H–H region agrees with available experimental predictions. We have calculated stationary point properties for the new surface as well as product–valley barrier maxima of vibrationally adiabatic potential curves for F+H2→HF(v′=3)+H,F+HD→HF(v′=3)+D, and F+D2→DF(v′=4)+D. The new surface should prove useful for studying the effect on dynamics of a low, early barrier with a wide, flat bend potential, as indicated by the best available electronic structure calculations.
Crystal fields and local lattice distortions in some tetragonal symmetry centers in fluorite crystals doped with trivalent rare-earth ions
Authors: Leśniak K.
Published: July 2002

A semiempirical procedure was developed, within the framework of a modified electrostatic effective charge model, that links crystal field parameters with microscopic local structures of tetragonal symmetry centers in fluorite crystals doped with trivalent rare-earth ions. The procedure was used to study tetragonal centers in Er3+:CaF2, Dy3+:CaF2, and Eu3+:SrF2 systems. It is shown that satisfactory a priori predictions of experimentally observed energy level schemes of the spectra of interest, as well as reconstructions of local microscopic structures of the emitting centers in accord with other relevant information collected up to date, are thus obtained. An examination is also made of a spectrum that in the past was linked to a tetragonal center in the Eu3+:CdF2 system, and the results obtained indicate that it appears to be associated with a center (or centers) that is structurally different from the tetragonal centers present in the other rare-earth: fluorite systems.