bluuebunny/crossref_metadata_2025 huggingface dataset:
Self-trapped exciton luminescence induced in alpha quartz by swift heavy ion irradiations
Authors: Costantini J. M. , Brisard F. , Biotteau G. , Balanzat E. , Gervais B.
Published: July 2002

Natural quartz single crystals (α-SiO2) have been exposed to pulsed heavy ion beams (C, 12F, 19S32) with energies of 1 MeV amu−1 in the electronic slowing down regime. The simultaneous recording of the ion fluence and emitted photons with time-resolved spectroscopy experiments allowed the measurement of the “blue luminescence” time decay at 85 K as a function of the fluence at the various electronic stopping power, Se=(−dE/dx)e, of the ions. For all ions, regardless of fluence, the spectra are similar and have two broad bands centered at 1.60 and 2.75 eV with full widths at half maximum around 0.30 and 0.75 eV, respectively. Single-exponential time decay curves are found regardless of Se increasing from 1.4 keV nm−1 (12 MeV C12) to 5.2 keV nm−1 (32 MeV S32) across the amorphous track-formation threshold at 2.5±0.5 keV nm−1. At low damaged fractions (⩽22%), the decay-time constant ranges between 1.0 and 1.6 ms. The luminescence intensities at zero delay time approximately decrease in an exponential fashion versus fluence with a decay cross section increasing by around one order-of-magnitude at the track-formation threshold, as found in the previous experiments with continuous beams. We analyze to which extent the luminescence decay versus fluence could be due to the quenching of the self-trapped exciton (STE) radiative recombinations by interactions with the ion-induced defects. For this, a STE diffusion model is devised where the STEs recombine nonradiatively at the neighboring cylindrical tracks. The model gives luminescence decay curves versus fluence in good agreement with the experimental data by varying the STE diffusion constant and the amorphous track-core radius in a reasonable range of values.
Phase formation and strain relaxation during thermal reaction of Zr and Ti with strained Si1−x−yGexCy epilayers
Authors: Aubry-Fortuna V. , Tremblay G. , Meyer F. , Miron Y. , Roichman Y. , Eizenberg M. , Fortuna F. , Hörmann U. , Strunk H.
Published: July 2002

Silicides are often used in Si technology for both their ohmic and rectifying properties. In this work, we have compared Zr and Ti germanosilicides as possible metallic contacts on SiGeC alloys in terms of phase formation and stability of the unreacted SiGeC alloy. The germanosilicides are obtained after rapid thermal annealings of Zr or Ti with strained SiGeC layers. The interactions of the metal films with these alloys have been investigated by sheet resistance measurements, x-ray diffraction (XRD), cross-sectional transmission electron microscopy (TEM), and energy dispersive spectroscopy in situ in the TEM. Four crystal x-ray diffraction was performed to measure the residual strain of the unreacted SiGeC epilayer after reaction. The analyses indicate that the final compounds are the C49–Zr(SiGe)2 and C54–Ti(SiGe)2 phases, respectively: In both cases, the compound is formed by monocrystalline grains with various orientations. Nevertheless, neither XRD, nor sheet resistance measurements give any clear information about the C incorporation in the phase, when the reaction occurs with a SiGeC layer. We have observed that the use of Zr completely avoids Ge segregation with an uniform layer formed, while in the case of the reaction with Ti, the grains do not form a continuous layer and Ge-segregation is evidenced: A Ge-rich Si1−z−yGez(Cy) alloy is detected in between the metallic grains. In addition, an early strain relaxation of the unreacted SiGe layer is observed after reaction, and it is much more important after reaction with Ti. During the reaction with nearly compensated SiGeC layers, Zr totally prevents the initial state of strain, while Ti strongly affects the unreacted SiGeC alloy and destroys its initial state. All these results indicate that Zr may be an interesting candidate for realizing germanosilicide contacts on IV–IV alloys, due to its good thermal stability.
Simple semiempirical potential energy surfaces for the reaction of alkali metal atoms with the bromine molecule
Authors: Goldfield E. M. , Gislason E. A. , Sabelli N. H.
Published: July 2002

Potential energy surfaces for the reactions of the alkali metals M (Li, Na, K, Rb, and Cs) with Br2 are computed using a simple semiempirical procedure. The calculations show that there is a vibrational barrier between M+Br2 and M++Br2− along the Br–Br coordinate which plays an important role in the reaction. A potential well exists for all angles of approach of the metal to the molecule; this M+Br2− species is lower in energy than any product channel. The calculated ionic/covalent coupling matrix elements between M+Br2 and M++Br2− agree well with the experimental values.
Electronic structure of polystyrene: A valence effective Hamiltonian theoretical study
Authors: Brédas J. L. , Street G. B.
Published: July 2002

We describe valence effective Hamiltonian (VEH) calculations of the band structure and density of valence states of polystyrene, the prototype of pendant-group polymers. The upper two occupied bands are found to derive directly from the upper two molecular orbitals of the benzene moieties, in agreement with the molecular-ion model developed by Duke et al. For a perfectly stereoregular chain, the highest occupied band is calculated to have a width of the order of 1 eV. Taking into account solid-state effects, we simulate the theoretical UPS spectrum of polystyrene and compare it to the experimental spectrum. Provided the influence of disorder is properly considered, we obtain an excellent overall accord which permits us to easily interpret the origin of the peaks occurring in the experimental spectrum on the basis of the VEH band structure. These results indicate that the VEH technique can be applied reliably to polymers which have both saturated and unsaturated components.
Rotational band contour analysis of symmetries and interactions of vibrational levels in anthracene S1
Authors: Keelan Brian W. , Zewail Ahmed H.
Published: July 2002

Over 50 of the more intense transitions in the S1 excitation spectrum (0–2000 cm−1 of vibrational energy) of supersonic jet-cooled anthracene are investigated using rotational band contour analysis, including computer simulations. This allows unambiguous assignment of the vibrational symmetries of all levels below 1100 cm−1 and many levels at higher energies, some of which were unassigned previously. Using complementary narrow-band excitation and energy-resolved detection techniques, a Fermi resonance at 755 cm−1 is thoroughly investigated to obtain the energy separation of the unperturbed states and the magnitude of the coupling matrix element. Levels which showed restricted or dissipative intramolecular vibrational-energy redistribution in time-resolved experiments (1380, 1420, and 1792 cm−1) are also studied.
The internal state distribution of CN radicals produced in the photolysis of HC2CN and CH3CN
Authors: Cody R. J. , Dzvonik M. J. , Glicker S.
Published: July 2002

Laser-induced fluorescence of the CN(B 2Σ+–X 2Σ+) electronic system was used to study the internal energy partitioning of CN radicals produced by the photolysis of HC2CN and CH3CN at λ≳130 nm. Experiments were performed in two collision regimes: (1) in the low collision number regime, the rotational and vibrational level distributions of CN(X) produced by the photolysis were measured; (2) in the high collision number experiments, the formation of CN(A) was monitored. Photolysis of HC2CN produced CN predominantly in the X state; Table I contains the vibrational population ratios and the temperatures computed from a Boltzmann fit of the rotational levels. Photolysis of CH3CN produces CN predominantly in the A 2Πi state; a lower limit of 0.3 was measured for the v′=1/v′=0 vibrational population ratio.
Time-correlation functions for a one-dimensional polymer model
Authors: Cook Robert , Helfand Eugene
Published: July 2002

As a simple model of a polymer we consider a series of vectors, situated in a sequence of planes perpendicular to a common axis. The vectors may rotate in these planes subject to a nearest neighbor interaction potential which has r stable states, circularly distributed. Transitions from well to well are the analog of conformational transitions in polymers. The time correlation function for the conformational state is shown to decay as a single exponential function with a rate related to single and cooperative transition processes. Simulation results for this model compare very well with the theory.
Thermal expansion coefficients of NH4ReO4 and NH4IO4 down to 58 K
Authors: Segel Stanley L. , Karlsson H. , Gustavson T. , Edstrom K.
Published: July 2002

The linear coefficients of thermal expansion have been determined for NH4ReO4 and NH4IO4 down to 58 K. Below 100 K the values are normal with αa=1×10−5/K and αc=6×10−5/K. Between 100 and 300 K the coefficients are anamalous with αa=−10×10−5/K and αc=+20×10−5/K. The use of these materials as secondary temperature standards in x-ray and neutron diffraction is discussed.
Double modulation techniques in ESR and slow rotations of nitroxide spin labels
Authors: Perić M. , Rakvin B. , Dulčić A.
Published: July 2002

We have examined the potential of double modulation electron spin resonance in the studies of slow rotation and tumbling of nitroxide radicals. Contributions to the homogeneous broadening of electron spin packets, due to several distinct processes, could be analyzed separately. The system used throughout the present work was TANOL in glycerol–water matrix. In a radical concentration study at 77 K, we could analyze the homogeneous broadening due to electron spin flip–flop processes. It has been found that the rate of this process increases linearly with the radical concentration. The temperature dependence of spin packet linewidths has shown to be a good indicator of slow rotations and tumbling of nitroxide radicals. Assuming the jump diffusional model, we have determined that the rotational correlation time varied from 2×10−4 s at 150 K to about 4×10−5 s at 200 K. Electron dipolar interactions were found to contribute to the spin packet broadening when the tumbling rate was very low, while, at higher tumbling rates, these interactions were averaged out gradually. The broadening of spin packets was also found to depend on the water content in the matrix. OFF
Sudden change of shape of time-dependent molecular fluorescence intensity as a nonequilibrium phase transition
Authors: Lendi K.
Published: July 2002

It is the aim of this note to show that, even under fixed excitation wavelength and bandwidth, the fluorescence time profile of a given molecule may exhibit a sudden and drastic qualitative change between smooth, purely exponential, and periodically structured quantum beat decay if appropriate external conditions are continuously varied. This abrupt change is analogous to a nonequilibrium phase transition which occurs at a bifurcation point of a suitable order parameter, identified to be the beat frequency, whose branching is governed by a tunable level width which turns out to be the appropriate control parameter for the transition.
Role of Feshbach resonances in the infrared multiphoton dissociation of small molecules
Authors: Leforestier Claude , Wyatt Robert E.
Published: July 2002

Calculations on the infrared multiphoton dissociation of a model linear triatomic molecule are reported. The molecule has 51 bound states and dissociation occurs after absorption of at least ten photons. The dissociation probability is computed within the quantum framework of the optical potential method [C. Leforestier and R. E. Wyatt, J. Chem. Phys. 78, 2334 (1983)] in conjunction with the complex extension of Floquet theory. For any given frequency, an analysis of the molecule in interaction with the laser field is presented, which permits us to follow the entire excitation path up to the continuum. This analysis shows that excitation into the continuum proceeds primarily through Feshbach resonant states. The role of these resonant states in the dissociation of polyatomic systems is discussed.
Discretized propagators, Hartree, and Hartree–Fock equations, and the Hohenberg–Kohn theorem
Authors: Harris Robert A. , Pratt Lawrence R.
Published: July 2002

The question of how electron exchange can be realistically included in discretized propagator treatments of simple quantum mechanical systems is investigated by showing how the many-body Hartree and Hartree–Fock approximations can be formulated in terms of discretized propagators. The Hartree approximation takes a surprisingly simple form suggestive of Thomas–Fermi theories in high dimensional spaces. For the Hartree–Fock approximation, the effect of nonlocal potential energy operators on the short-time propagators must be addressed. These nonlocal operators make the Hartree–Fock results considerably more complicated. However, in each case we indicate how the universal energy functional of the density implied by the Hohenberg–Kohn theorem may be obtained.
Energetic consideration of the vibrational potential function in the effective nuclear charge model. IV
Authors: Ohwada Ken
Published: July 2002

Molecular energy components are analyzed on the basis of the vibrational potential function in the effective nuclear charge (ENC) model. The analytic formulas for the electronic kinetic energy and the potential energies (electron–nucleus and electron–electron interaction) of polyatomic molecules are derived from such a ENC model potential, and applied tentatively to homonuclear and heteronuclear diatoic molecules. Also, the analytic expression for the eigenvalue sum (sums of Hartree–Fock orbital energies) is similarly derived and tested on a number of diatomic molecules. The homogeneity characteristics of the total molecular and electronic energies are evaluated through derivation of the harmonic force constant expressions and their consequences are discussed in detail.
The 2–0 band of the A 2Πi ← X 2Σ+ system of SiN near 3.3 μm
Authors: Foster Stephen C. , Lubic Karen G. , Amano T.
Published: July 2002

The observation and analysis of 187 lines in the 2–0 band of the A 2Πi ← X 2Σ+ system of the SiN radical are described. The 2–0 band, which lies near 3.3 μm, has been recorded with a tunable difference frequency system and discharge amplitude modulation. The analysis gives improved molecular constants for the ground and first excited states, including the first lambda doubling parameters for the A 2Π state.
The effect of colloidal forces on the self-diffusion coefficients in strongly interacting dispersions
Authors: Venkatesan M. , Hirtzel C. S. , Rajagopalan Raj
Published: July 2002

The effects of colloidal forces on the self-diffusion coefficients in dispersions of uniform spherical particles have been examined. The short-time coefficients (for times smaller than the structural relaxation time) are computed using the equilibrium mobility-averaging procedure. The long-time values are based on the solution of the two-particle Smoluchowski equation (with proper ‘‘zero-flux’’ boundary condition at contact). Results are presented for hard-sphere dispersions as well as for dilute-gas dispersions (the latter being applicable in the semidilute regime). The dilute-gas results are presented for Yukawa potentials with repulsive and attractive tails so that the individual roles of repulsion and attraction and their interplay with hydrodynamic interactions can be demonstrated. In addition to the above limiting cases, both short-time and long-time coefficients are presented for a sample case of a repulsive Yukawa dispersion with strong positional correlations (e.g., charged micelles or macroions), for which the potential of mean force can be obtained through the mean spherical or the rescaled mean spherical approximation.
Exact analysis of polycrystalline electron spin echo envelope modulation including mutual nuclear arrangements and quadrupole interactions and its application to methyl radicals in irradiated crystals of lithium acetate dihydrate
Authors: Iwasaki Machio , Toriyama Kazumi
Published: July 2002

The method of exact analysis including mutual nuclear arrangements and quadrupole interactions is developed for electron spin echo envelope modulation (ESEM) from polycrystalline samples using the formulation in an arbitrary laboratory coordinate system, which makes it possible to perform an exact angular integration. In order to assess polycrystalline ESEM by comparing with electron nuclear double resonance (ENDOR) data, the method is applied to analyze deuterium hyperfine modulation on the two-pulse electron spin echo (ESE) envelope decay observed for methyl radicals radiolytically produced in CD3COOLi ⋅ 2H2O and CH3COOLi ⋅ 2D2O. The results are totally consistent with our previous study by single crystal 1H ENDOR. Through this application, importance of the nuclear quadrupole term is also shown based on the exact method. The rigid deuterons in the water of crystallization show a marked quadrupole effect as compared with those in the freely rotating CD3 group. The contributions from the distant nuclei to the modulation pattern are also studied by the exact method using the crystallographic data. We have further clarified that the origin of the disagreement of a previous Li ESEM with our ENDOR is mainly in the use of a unit spin density on the spin probe despite its reduction by delocalization to the environment.
Classical scaling functions for moments of the order parameter profile in critical adsorption of a binary liquid mixture
Authors: Franck Carl
Published: July 2002

Because knowledge of the moments of the order parameter profile is required to understand reflected light studies of critical adsorption of a liquid mixture, the scaling functions in classical Landau theory for the two lowest order moments have been calculated. A contact substrate-liquid interaction of strength h1 is presumed. The first moment is found to saturate at large scaled h1. Finally, the connection with a recent experiment is discussed.
Electron correlations in doubly excited helium
Authors: van der Merwe P. du T.
Published: July 2002

Electron correlations in doubly excited s-state they are discussed, in accordance with the results reported by Herrick, Kellman and Polak.
Variation of inverse Raman spectrum near resonance
Authors: Saikan S. , Hashimoto N. , Kushida T. , Namba K.
Published: July 2002

The inverse Raman band shape in the vicinity of single photon resonance is interpreted in terms of both a double quantum transition and a dynamical Stark effect. It is pointed out that what is measured in the inverse Raman spectroscopy is not identical with what is measured in the corresponding Raman gain spectroscopy. The absorption change due to the dynamical Stark effect is shown to explain well the band shape as well as the position of the inverse Raman spectrum.
Computer simulation studies of soliton models for dielectric relaxation in crystalline polyethylene and related polymers. I. Continuum and pinned limits
Authors: Wahlstrand Karna J.
Published: July 2002

We study the sine-Gordon soliton model of the dielectric α relaxation in crystalline polyethylene and similar polymers. These systems are characterized by a coupling constant which is essentially the ratio of intramolecular to intermolecular interaction strengths. We perform a stochastic molecular dynamics computer simulation for a wide range of coupling constants. We investigate analytically the high coupling (continuum) limit, modeled as free particle Brownian motion of the soliton, and the low coupling (pinned) limit, treated as hopping diffusion of the soliton over barriers at the sites on the polymer chain. We compare the simulation with these analytical theories and with experimental results for polyethylene. Agreement between simulation and theory for nonpolar polymers is excellent in the continuum limit down to relatively small coupling constants, and we also find quite reasonable agreement with experiments in that limit, considering the neglect of defects on the chains. The best fits of the pinned limit theory with the simulation are identical to those of the continuum limit in the lower coupling regime, and are not very successful. The continuum limit analytical theory is extended to polar polymers and qualitative agreement with other theories is shown, in the absence of experimental data for comparison. It is suggested that a more specific theoretical treatment in the intermediate to low coupling regime might improve agreement with the simulation and that the inclusion of defect effects is necessary for more successful agreement with experimental results.
Theory of viscoelasticity of fluids
Authors: Eu Byung Chan
Published: July 2002

By analytically continuing the stress tensor evolution equation used in our previous studies of static phenomena in fluids and solving the equation, we have derived some analytic formulas for dynamic viscosities as functions of the amplitude and frequency of oscillating shear rate. They show markedly nonlinear rate dependencies different from those predicted by the linear theory. A set of approximate dynamic viscosity formulas (the first approximation) is used to derive a rule similar to the Cox–Merz rule, which is seen to be valid in the low frequency regime in the present approach. It is shown that the real and imaginary parts of the dynamic complex viscosity can be put into forms which depend on a reduced shear rate and a reduced frequency only, and therefore there exist corresponding states for the material functions. The formulas predict power laws in the high shear rate and frequency regimes which are quite reminiscent of those holding for some polymer solutions.
Numerical comparison of generalized surface hopping, classical analog, and self-consistent eikonal approximations for nonadiabatic scattering
Authors: Currier Robert , Herman Michael F.
Published: July 2002

Three recently proposed semiclassical methods for nonadiabatic scattering are numerically compared for a one-dimensional curve crossing model. The generalized surface hopping approach has been found to be in excellent agreement with quantum results over a wide range of energies. The simpler variant of the self-consistent eikonal method follows the quantum results well at low energies. At higher energies it still follows the correct overall trend of decreasing transition probability, but it does not quantitatively reproduce the oscillations in the transition probability. The classical analog technique provides relatively good agreement with the quantum results over the entire range of energies examined. It is found in the application of this last method that there are typically four to eight stationary phase contributions at each energy and it is crucial to correctly describe the interference between these terms. It is also necessary to include uniform and analytically continued contributions at many energies, complicating the application of the method.
Translational and rotational symmetries in integral derivatives
Authors: Banerjee Ajit , Jensen James O. , Simons Jack
Published: July 2002

Based upon the invariance under translations and rotations of quantum chemical one- and two-electron integrals, a method for obtaining a complete set of independent relations among integral derivatives is presented. Due to the unitary form of the operators corresponding to finite translations and rotations, this analysis is generally applicable to all orders of integral derivatives. It is shown that the number of dependent integral derivatives is equal to the number of such independent relations. These dependent integral derivatives can thus be straightforwardly determined in terms of the remaining derivatives which must be explicitly calculated. For example, out of a total of 21, 45, and 78 second-derivative integrals for the two- , three- , and four-center cases, respectively, only 1, 6, and 21 such integral derivatives need be explicitly calculated. The set of such independent and dependent integral derivatives can be chosen in a manner which imposes no restrictions on the allowable geometries of the nuclear positions. The special case of colinear nuclear centers is also separately analyzed.
The distribution of relaxation frequencies from photon correlation spectroscopy near the glass transition
Authors: Carroll P. J. , Patterson G. D.
Published: July 2002

The relaxation function obtained by photon correlation spectroscopy of a liquid near the glass transition has a highly nonexponential decay. This nonexponential behavior can be represented by a distribution of exponential relaxation processes. Using a real-time, 4-bit, 4-sample time, 256-channel correlator we have obtained very high quality depolarized spectra of polystyrene over seven decades in time. The relaxation function was calculated from the data and a least squares fit to the weighting factors of a sum of exponentials was performed using techniques described by Ostrowsky et al. A least squares fit was obtained using a powerful program in the port© mathematical subroutine library called IQP which finds a minimum for the quadratic programming problem subject to certain constraints. This yields the distribution of relaxation frequencies characteristic of a liquid near the glass transition. The shape of the distribution is very similar to the distribution of relaxation frequencies obtained from the empirical Williams–Watts function.
Angular correlations in dense hot diatomic fluids
Authors: MacGowan David , Johnson J. D. , Shaw M. S.
Published: July 2002

Molecular dynamics (MD) simulation data for rigid diatomic models of N2 and CO2 under conditions of extremely high density and temperature are analyzed for static correlation functions. The results show some significant qualitative differences from those for diatomic fluids at normal densities and temperatures (i.e., near the triple point). For a single thermodynamic state of N2, the radial distribution functions (RDFs) of the (spherical) RAM and median potentials are found, also by MD. Whereas the median gives good thermodynamic results and poor centers correlation functions, RAM produces just the opposite. Thus no explanation in terms of distribution functions is found for the success of the median for thermodynamics although an empirical correlation is found between the breakdown of median thermodynamics for CO2 and a distinctive feature of the molecular correlation functions.
Rotationally selective mode-to-mode vibrational energy transfer in D2CO/D2CO and D2CO/Ar collisions
Authors: Bewick C. P. , Duval A. B. , Orr B. J.
Published: July 2002

Infrared–ultraviolet double resonance is used to investigate fast (approximately gas–kinetic) vibrational energy transfer between the ν6 and ν4 modes of D2CO, arising from collisions with either D2CO or Ar. Relaxation channels specific to particular rotational states (J, Ka) are characterized and rationalized in terms of Coriolis coupling and quasielastic collisional interactions.
Translational energy distributions of the excited nitrogen atom produced by electron-impact dissociative excitation of nitrogen molecules
Authors: Ogawa Teiichiro , Ishibashi Shigeki , Kurawaki Junichi , Kawazumi Hirofumi
Published: July 2002

The Doppler line shapes of atomic nitrogen emission (4p 2S1/2–3s 2P3/2: 4935.12 Å) were measured precisely with a Fabry–Perot interferometer at a resolution of 0.015 Å. The translational energy distributions of N* were calculated and found to be similar with those of N+ and N(HR). There are three major components of N*; their peaks of the translational energy distribution and threshold energies are (1) 0.1–1.3 and 23.0 eV, (2) 2.8–3.0 and 29 eV, and (3) ∼ 5 and 38–44 eV, respectively. The first component is produced by dissociative excitation through Rydberg states converging to the C state of N2+.
Analysis of ultrasonic absorption amplitudes for multiple coupled chemical equilibria
Authors: Tamura Kiyoshi , Schelly Z. A.
Published: July 2002

A simple method is presented for the analysis of the ultrasonic relaxation amplitudes for an unrestricted number of coupled chemical equilibria. The method results in an explicit, general expression for the amplitudes and it offers significant reduction in mathematical manipulations compared with that of normal mode analysis. A general expression is presented for immediate computation of the ultrasonic relaxation amplitudes from an inspection of the elementary reactions of the mechanism. The reactions in the mechanism may be of any order and also parallel reaction paths may be included.
Convergent renormalized perturbation series for the Stark rotational energies of diatomic molecules
Authors: Maluendes Sergio A. , Fernández Francisco M. , Castro Eduardo A.
Published: July 2002

Convergent renormalized perturbation series for the Stark rotational energies of rigid polar diatomic molecules are presented. The small- and large-field expansions are considered and convergence for all field intensities is proved in both cases. Results are in excellent agreement with accurate numerical non- perturbative calculations.
Nuclear quadrupole effects on two and three pulse electron spin echo patterns: A first order perturbation approach for nuclear quadrupole interaction only
Authors: Romanelli M. , Narayana M. , Kevan Larry
Published: July 2002

The nuclear modulation of electron spin echo signals is evaluated by an approach which handles only the nuclear quadrupole interaction as a perturbation of the eigenvectors and eigenvalues of the nuclear Hamiltonian including nuclear Zeeman and hyperfine terms. The theory is shown in detail for a first order perturbation treatment for both two and three pulse methods. Final formulas are obtained which are valid for any value of the electron–nuclear distance r, thus removing the limit of previous approaches. The modulation envelopes for I=1(2D) and I=5/2 (27A1) nuclear spin systems are critically analyzed at different electron–nuclear distances including possible misalignment between quadrupole and dipolar hyperfine tensors. Comparison with a first order perturbation approach for both hyperfine and nuclear quadrupole interactions as perturbations of the nuclear Zeeman Hamiltonian is made and shows that the exact treatment of the hyperfine interaction considerably reduces the apparent nuclear quadrupole effects on the overall echo modulation envelopes, particularly at small electron–nuclear distances.
Dynamical constraints on the angular momentum transfer in rotationally inelastic molecular collisions
Authors: Agrawal P. M. , Agrawal N. C. , Garg Vinod
Published: July 2002

The rapid decrease in the integral inelastic cross sections, when rotational energy transfer ‖ΔE‖ is greater than a critical value (‖ΔE‖*), has been explained quantitatively for the systems CO2–H2, CO2–He, Na2–He, Na2–Ne, LiH–He, and N2–Ar on the basis of the model proposed by Dexheimer et al. An expression for ‖ΔE‖* has been obtained that shows the increase in ‖ΔE‖* with the increase in the reduced mass μ, initial rotational quantum number ji, and initial relative energy of the translational motion Ki. The predictions of the model for CO2–X (X=Ne, Ar, Kr, Xe) are also reported.
Measurement of Hönl–London factors on 2Σ+–2Π transitions in NO
Authors: Scheingraber H. , Vidal C. R.
Published: July 2002

Hönl–London factors for 2Σ–2Π transitions in the NO molecule have been measured for the first time, using single photon vacuum UV excitation of the A 2Σ+ and of the D 2Σ+ state. The measured and the calculated line intensities in the subsequent fluorescence are found to be in very good agreement for all 12 branches. The J dependence of the relative line intensities clearly reveals the transition from Hund’s coupling case (a) to case (b) for the 2Π ground state.
Nuclear modulation effect in electron spin echoes resulting from nuclear hyperfine and quadrupole interactions with a nucleus of an arbitrary spin
Authors: Ichikawa Tsuneki
Published: July 2002

Approximate algebraic expressions for the nuclear modulation effect in electron spin echoes resulting from nuclear hyperfine and quadrupole interactions with a nucleus of an arbitrary spin are derived by using a third order perturbation method. The contribution of the quadrupole interaction on both the modulation amplitude and the modulation frequencies is included in the expressions. The validity of the expressions is examined by comparing the modulation spectra with exact ones obtained by the numerical calculation of the Hamiltonian for the spin echoes. It is shown that the expressions are applicable even for relatively short electron–nuclear distance and strong quadrupole interaction (r>0 and e2qQ/h<0.6 MHz for D nucleus).
The effect of Wigner singularities on low-temperature vibrational relaxation rates
Authors: Schwenke David W. , Truhlar Donald G.
Published: July 2002

We have applied the vibrational-close-coupling, rotational-infinite-order-sudden (VCC-RIOS) and vibrational-rotational close coupling (CC) methods to collisions between He and I2(X 1Σ+g) to study the threshold dependence of vibrational deexcitation cross sections and rates on energy and temperature, respectively, for the v=1→0 and v=16→15 processes, where v is the vibrational quantum number. The long-range attraction causes the cross sections to be decreasing functions of energy below about 10 cm−1 and causes the thermally averaged deexcitation rate constants to have a minimum at about 1 K, but the Wigner threshold law holds only within 10−3 cm−1 of threshold and the Wigner singularity causes the rate constant to assume a nonzero temperature-independent value only below about 10−4 K. We also observe low-energy resonances in the CC calculations and these lead to additional structure in the rate constants but do not change the important conclusions.
Reaction of F atoms with the methyl halides. Vibrational spectra of CH3XF and of H2CX⋅⋅⋅HF trapped in solid argon
Authors: Jacox Marilyn E.
Published: July 2002

When the products of the reaction between F atoms formed in a microwave discharge and CH3Cl, CH3Br, or CH3I were frozen in a large excess of argon at 14 K, the infrared spectra of the primary reaction products were obtained. Isotopic substitution experiments have provided evidence for two major reaction channels in each of these three reaction systems. Attack of the F atom at the halogen position results in the formation of the CH3XF addition product, which has a moderately strong X–F bond and is photochemically stable at wavelengths as short as 250 nm. F-atom reaction with a hydrogen atom of the methyl halide results in the stabilization of a weakly bound F⋅⋅⋅HCH2X complex, intermediate to the formation of H2CX+HF. For all of the species studied except CH3Cl, the barrier to the decomposition of this complex is sufficiently great to require exposure of the solid deposit to visible light for the production of H2CX and HF. The infrared spectra of the H2CX⋅⋅⋅HF hydrogen-bonded complexes isolated in solid argon are discussed.
Pressure effects on the infrared spectrum of 1,2-dipalmitoyl phosphatidylcholine bilayers in water
Authors: Wong P. T. T. , Mantsch H. H.
Published: July 2002

Infrared spectra of dipalmitoyl phosphatidylcholine bilayers in water were measured over the pressure range 0 to 35 kbar. Discontinuities in the pressure dependences of various infrared spectral parameters indicate that structural phase transitions occur at 1.7, 4.8, and 15 kbar. More than two correlation field components of the CH2 scissoring mode are observed in the spectra of the high-pressure phases, suggesting that more than two acyl chains are present in the unit cell of these phases. Pressure-induced changes in the CH2 bond angle and in the chain orientation are evident from the pressure effect on the intensities of the CH2 stretching and CH2 rocking infrared bands. Pressure also affects the dispersion of various vibrational frequencies resulting in drastic changes of the Fermi resonance interaction between νsCH2 and the 2δCH2 continuum.
Generation and interpretation of chain parameters in the recursive residue generation method
Authors: Schek Israel , Wyatt Robert E.
Published: July 2002

In the recursive residue generation method [A. Nauts and R. E. Wyatt, Phys. Rev. Lett. 51, 2238 (1983)] molecular multiphoton excitation is treated by first converting the molecule-field Hamiltonian matrix into tridiagonal form, using the Lanczos equations. A procedure is developed here for construction of generating operators for successive recursive basis functions in the chain (tridiagonal) representation. These operators are used to construct the self-energies and the linking terms from successive moments of the Hamiltonian with respect to a source vector. These self-energies and linking terms are interpreted in terms of masses and stiffness of members of a mechanical chain and the chain dynamics is explained. The self-energies and coupling terms are expressed in terms of linked diagrams and explicit connections to the Hamiltonian moments are developed.
A simple model study of reactive collisions in an intense nonresonant laser field
Authors: Peploski James C. , Eno Larry
Published: July 2002

A simple model study of laser induced transitions between electronic surfaces in reactive molecular collisions has been undertaken. The investigation is characterized by laser and nonadiabatic couplings which are turned on during the course of a collision. Transition probabilities are determined within an exact quantum-mechanical framework, for switching between the model one-dimensional potential curves as a function of various system parameters. Such parameters include the photon energy, the reactant collision energy, and the coordinate separation between the positions of potential barrier maxima. The processes studied involve not only laser switching but, also, cooperative laser and nonadiabatic effects. A number of features of the results are emphasized.
Fast orientational-motional correlation time measurements from ESR hyperfine spectra of perdeuterated nitroxides with 15N (I=1/2)
Authors: Lee Sook , Shetty Anil
Published: July 2002

A simple, direct approach is presented for determining fast orientational-motional correlation times τ associated with motionally narrowed electron-spin-resonance (ESR) hyperfine spectra of perdeuterated nitroxide radicals enriched with 15N (I=1/2). This approach is based on (1) the experimental determination of net dynamic line broadening by taking the difference in linewidth between the M=+1/2 and M=−1/2 nitroxide hyperfine components, (2) the Kivelson expression for fast tumbling motional ESR hyperfine linewidth, (3) a simple, effective method of treating the problem of experimental nitroxide ESR hyperfine line shapes consisting of Gaussian and Lorentzian components, and (4) the spin-Hamiltonian constants adjusted for fast tumbling nitroxide radicals at high temperature from their rigid limit values at low temperatures. It is shown that the present method yields more reliable and accurate experimental τ values than those determined by conventional methods for fast tumbling nitroxide radicals. Detailed experimental procedures are described for the case of poly(ethylene oxide) polymer containing perdeuterated and 15N-enriched TANOL (TANOLD-15) spin probes.
Non-Markovian reaction sites and trapping
Authors: Weiss George H. , Havlin Shlomo
Published: July 2002

Most models of absorption sites for random walks or diffusion processes fall into one of two categories: (1) Perfect absorption, in which every encounter of a random walker with a trap produces a trapping event, and (2) imperfect absorption in which an encounter leads to a trapping event with probability α<1. We introduce the notion of a non-Markovian trap characterized by a set of probabilities {f j}, where f j is the probability that the jth encounter leads to a trapping event. Some consequences of this assumption are examined in the context of a one-dimensional trapping problem. It is shown that when the f j have an associated finite first moment the asymptotic survivial probability goes like n1/2 exp(−an1/3) where n is the step number and a is a constant. This is equivalent to the results one would obtain with a Markovian model. However, when f j is asymptotically proportional to 1/j1+α where 0<α<1 the survival probability falls off as 1/nα.
ENDOR evidence for inhomogeneous radical distribution in irradiated succinic acid: Sign and intensity of the signal
Authors: Khalaf Fawzi , Rakvin Boris , Shibata Kaoru , Miyagawa Ichiro
Published: July 2002

It was found that the ENDOR signal from a weakly interacting radical was negative while that from a relatively strongly interacting radical was positive; in crystals with very low doses, intensity of a strong negative ENDOR line was comparable with that of the original ESR line.
Laser microwave double resonance using high-field LMR spectra for the ν3 band of NF2
Authors: Uehara Hiromichi , Horiai Koui
Published: July 2002

Laser microwave double resonance signals using high-field LMR spectra were observed for the ν3 band of NF2. The least-squares fit of these data has revealed accurate electron spin g values for NF2. Zero-field constants were fixed to the diode laser or the microwave values. Rotational g values were also obtained with small standard deviations, though the accuracy depended on that of the spin-rotation constants fixed in the least-squares fit.
Molecular (Feshbach) treatment of charge exchange Li3++He collisions. I. Energies and couplings
Authors: Martín F. , Riera A. , Yáñez M.
Published: July 2002

We point out a fundamental difference between the molecular treatment of charge exchange Xn++H(1s) and Xn++He(1s2) collisions, which is that the latter process involves molecular states that are formally autoionizing. Then standard ab initio methods do not, in general, yield the relevant wave functions that are needed in the collision treatment, irrespective of whether quasimolecular autoionization be significant or not during the collision. We implement a particularly simple and useful form of the Feshbach formalism to calculate the energies of those two electron systems, and a method to evaluate the corresponding dynamical couplings is presented for the first time. Our implementation of this formalism together with the new computational techniques involved are presented in detail.
On the nature of high CF4 vibrational levels excited by proton energy loss spectroscopy
Authors: Levene Harold B. , Perry David S.
Published: July 2002

The forced harmonic oscillator model, which has been applied to proton energy loss spectroscopy (PELS), is extended here to probe the nature of CF4 vibrational levels excited by the proton beam. At each overtone of the triply degenerate ν3 vibration, there is a collection of vibrational sublevels which differ in the way amplitude is distributed among the four C–F bonds. The model predicts that PELS populates a broad distribution of these sublevels with a propensity for the highest energy, most delocalized vibrations in each overtone band. Comparison is made with infrared multiphoton absorption which excites delocalized vibrations in SF6 and with single photon overtone absorption which selects the most localized states. Peak positions in a simulated spectrum match the experiment up to 8ν3 but for higher overtones the anharmonic shift is up to 0.05 eV less than observed. A demanding test of the model must wait for higher resolution experiments and better spectroscopic constants.
Theoretical studies of termolecular thermal recombination of silicon atoms
Authors: Martin David L. , Thompson Donald L. , Raff Lionel M.
Published: July 2002

Termolecular recombination of silicon atoms in the presence of an atomic third body M is studied for thermal reactions on the ground-state 3Σ−g potential curve for Si2 dimers. The total potential surface is assumed to have pairwise form with the Si2 term being a Morse potential fitted to multireferenced double-excitation, configuration interaction (MRDCI) results. Lennard-Jones (LJ) (12,6) potentials are used for the two M–Si interactions. The termolecular rate coefficients for the temperature range 200≤T≤300 K are found to be in the range 1.69×1015≤kr(T) ≤2.56×1015 cm6/mol s if a statistical factor of unity is employed. It is argued that the most appropriate statistical factor is 1/9 for which the rate coefficients fall in the range 1.88×1014≤kr(T) ≤2.85×1014 cm6/mol s. The statistical error in the calculation precludes an accurate calculation of the small activation energy for the recombination. The limiting values are −1.24≤Ea ≤2.32 kcal/mol with a least-squares result of Ea =−0.028 kcal/mol.
ESR study of the complexes [ClO, O2] and [BrO, O2] formed by decomposition of ClO−3 and BrO−3 in solid KClO4
Authors: Byberg J. R.
Published: July 2002

The ESR spectra of defects arising from photolysis at 214 or 229 nm of ClO−3 and BrO−3 embedded in KClO4 crystals and subsequent radiolysis at 26 K are interpreted in terms of the complexes [XO, O2], X=Cl, Br, formed in the reactions XO−3 →hν [XO−,O2], [XO−,O2] +(ClO4)−2 →[XO,O2] +2ClO−4, where (ClO4)−2 represents the mobile holes produced during radiolysis of the host crystal. The main features of the spin Hamiltonian of [XO,O2] correspond to XO(2Π) in a crystal field, coupled through an isotropic exchange interaction to O2(3Σ−g ) to form a spin-doublet ground state, but the observed halogen and 17O hyperfine splittings are considerably smaller than expected for [XO,O2]. Moreover, the splitting of the 2Π states of XO inferred from the g tensor is an order of magnitude larger than that observed earlier in a similar system. Both findings indicate an interaction, which splits the degenerate pairs of antibonding π orbitals on XO and O2 by ∼104 cm−1. In O2 this may lead to near degeneracy of the lowest singlet state with 3Σ−g , thus allowing a significant singlet–triplet mixing which reduces the net spin density in the complex. The spin Hamiltonian indicates that the axes of XO and O2 are perpendicular. The strong interaction between XO and O2 furthermore suggests a planar T configuration of the complex.
Laser photoelectron spectroscopy of MnH−2, FeH−2, CoH−2, and NiH−2: Determination of the electron affinities for the metal dihydrides
Authors: Miller Amy E. S. , Feigerle C. S. , Lineberger W. C.
Published: July 2002

The laser photoelectron spectra of MnH−2, FeH−2, CoH−2, and NiH−2 and the analogous deuterides are reported. Lack of vibrational structure in the spectra suggests that all of the dihydrides and their negative ions have linear geometries, and that the transitions observed in the spectra are due to the loss of nonbonding d electrons. The electron affinities for the metal dihydrides are determined to be 0.444±0.016 eV for MnH2, 1.049±0.014 eV for FeH2, 1.450±0.014 eV for CoH2, and 1.934±0.008 eV for NiH2. Electronic excitation energies are provided for excited states of FeH2, CoH2, and NiH2. Electron affinities and electronic excitation energies for the dideuterides are also reported. A limit on the electron affinity of CrH2 of ≥2.5 eV is determined. The electron affinities of the dihydrides directly correlate with the electron affinities of the high-spin states of the monohydrides, and with the electron affinities of the metal atoms. These results are in agreement with a qualitative model developed for bonding in the monohydrides.
Rotational energy accommodation in OH radicals desorbing from a polycrystalline Pt foil and a Pt(111) single crystal
Authors: Hoffbauer M. A. , Hsu D. S. Y. , Lin M. C.
Published: July 2002

Boltzmann-like rotational energy distributions were obtained for desorbing OH radials formed in the oxidation of hydrogen on a Pt(111) single crystal and polycrystalline Pt foil by using the laser-induced fluorescence method in conjunction with an UHV apparatus. For an O/H=1 reactant gas mixture, relatively high rotational energy accommodation was indicated by the rotational to surface temperature ratios Tr/Ts of 0.85±0.03 over the temperature range 1227–1479 K for the Pt(111) single crystal, and of 0.86±0.04 over 1283–1475 K for the polycrystalline Pt foil. Tr seemed to be proportional to Ts over these temperature ranges. The lack of noticeable dependence of Tr/Ts on the O/H reactant gas mixture ratio suggests that the rotational energy accommodation is not sensitive to the apparent Pt–OH bond strength.