bluuebunny/crossref_metadata_2025 huggingface dataset:
Line interference effects in the rotational Raman spectrum of C2H4
Authors: Hays J. T. , Armstrong R. L.
Published: February 2003

Line interference effects have been observed in the rotational Raman spectrum of the nearly prolate, asymmetric top C2H4 at pressures ∼1 atm. An interferometric detection scheme is used which enhances the contribution of quasiperiodic, symmetric-top-allowed lines to the spectrum. A simple, cluster model provides an adequate description of interference effects in these lines. Optimum values of the cluster spectral window and width parameters are determined from a least-χ2 fitting between experimental and theoretical Raman interferogram spectra. The value obtained for the cluster linewidth is in good agreement with the results of previous experiments on C2H4, and provides a physically meaningful alternative to the ad hoc line shape introduced to account for the presence of non-Lorentzian line shapes in this latter spectra. The cluster model is also compared with the results of a model calculation of interference effects in a simple spectrum.
Fluorescence depolarization by anisotropic rotational diffusion of a luminophore and its carrier molecule
Authors: Burghardt Thomas P.
Published: February 2003

The analytical expression for the polarization anisotropy is derived for a luminophore undergoing rotational diffusion about a single axis while attached to a nonluminescing, rotationally diffusing, symmetrical carrier molecule. In contrast to previous related calculations, the rotation axis of the luminophore is assumed to have an arbitrary orientation relative to the carrier. Additionally, the polarization anisotropy is measured for bovine serum albumin (BSA) labeled with dansyl, NBD, rhodamine, or eosin that is: (a) surface adsorbed to a glass/buffer interface, using a variation of the technique of total internal reflection fluorescence spectroscopy (TIRFS), or (b) bulk dissolved, using conventional transmitted illumination fluorescence spectroscopy. With this theory, using previously published values for the rotational diffusion constants of BSA and the fluorescence lifetimes of the fluorophores, the rotational diffusion constant of the covalently bound probes is estimated from the measured anisotropy values. The results indicate a wide variability in the rotational diffusion constant of the probes (from ∼107 s−1 for dansyl to ∼109 s−1 for eosin) attached to both the surface adsorbed and bulk dissolved forms of BSA. Contrasting the rotational diffusion constant for each probe for surface adsorbed BSA vs bulk dissolved BSA indicates surface adsorption of the BSA molecule inhibits the rotational motion of the probe. These results have important implications in the application of other fluorescence techniques, such as singlet–singlet energy transfer, where the rotational mobility of the probe is important.
Experimental evidence of collective vibrations in DNA double helix (Raman spectroscopy)
Authors: Urabe Hisako , Tominaga Yasunori , Kubota Kenji
Published: February 2003

The collective vibrational motions in the solid DNA fiber have been found from the wave vector dependence of the low frequency Raman spectra. The main part of the low frequency modes have been assigned to the αxz(αyz) component of the Raman tensor from the polarized Raman spectroscopy.
Matrix ESR spectra of polyatomic alkali metal clusters
Authors: Thompson G. A. , Tischler Frances , Lindsay D. M.
Published: February 2003

ESR spectra have been obtained for polyatomic sodium and potassium clusters in argon and nitrogen matrices. The spectra, tentatively assigned to alkali septemer molecules, show a large isotropic hf (a1) from two equivalent alkali nuclei plus a much smaller splitting (a2) from an additional (at least) five equivalent nuclei. For an argon matrix, the measured hf constants and g values are a1=118.16(4) G, a2=6.66(12), g0=1.9994 (1) and a1=30.55(6) G, a2=1.79(2) G, g0=2.0017(1) for Na7 and K7, respectively. The ESR spin populations, ρi=ai/a(atom), are compared with the orbital symmetries predicted by the simple Huckel approximation for a range of M7, M9, M11, and M13 structures. Aside from the relatively unstable ‘‘T-shaped’’ arrangement, only a few three-dimensional structures give a satisfactory agreement with the ESR spectra. For M7, a likely geometry is the pentagonal bipyramid. The ESR spin populations for Na7 are almost identical in magnitude to those of K7. If a2<0, the total isotropic spin population is Σiρi≂0.64, which implies an ∼ 36% p character for the unpaired electron.
Infrared photoacoustic spectroscopy of undoped polyacetylene
Authors: Will Fritz G. , McDonald Robert S. , Gleim Robert D. , Winkle Mark R.
Published: February 2003

Photoacoustic infrared spectroscopy has been applied to undoped polyacetylene powders and films and the results are compared to transmission infrared spectra obtained on both types of samples. the photoacoustic technique has been found to be particularly suitable for the study of polyacetylene powders which are more sensitive to air exposure than films, due to their higher surface area. The method yields spectra whose major features are essentially identical to transmission spectra, but allows more flexibility with respect to sample thickness, size, and shape. A band near 1920 cm−1, also observed but not discussed in previous studies, is assigned to the presence of cumulative carbon double bonds. Bands observed near 1375 and 890 cm−1 on films and, more prominently, on powders are assigned to the incorporation of small amounts of oxygen during preparation and handling, resulting in inadvertent doping. The large surface area of powders also results in more pronounced oxidation of the polymer backbone.
EPR study of the structure and spin distribution at the binding site in human nitrosylhemoglobin single crystalsa)
Authors: Utterback S. G. , Doetschman D. C. , Szumowski J. , Rizos A. K.
Published: February 2003

The anisotropic EPR spectrum was measured in single crystals of human nitrosylhemoglobin. The principal values and axis directions of the g factor and of the N hyperfine interaction of the NO ligand were determined for an alpha and a beta subunit. A model for the geometry, orientation and orbital spin distribution at NO bound to the Fe atom in the subunits is derived from the measured g values and hyperfine interaction. The relationship between the NO bond, the Fe crystal field and the g and A principal axis directions are discussed.
CO2 laser-induced multiphoton absorption of fluoroform-d: The effects of collisionsa)
Authors: McAlpine Robert D. , Evans D. K. , Adams H. M.
Published: February 2003

Studies of the multiphoton absorption of the 10 μm R(26) CO2 laser line by fluoroform-d have been extended to laser pulse widths (Δτ) of 60 ns over the range of pressures P=0.33 to 2.00 kPa. Comparison with earlier studies with Δτ=2 and 6 ns shows that for all fluences Φ the cross section σ=σ(Φ, PΔτ), which means that σ scales with the number of collisions during the pulse. Log–log plots of σ(Φ) vs Φ (P, Δτ fixed) show a bend at a fluence Φc. For Φ<Φc collisions appear mainly to induce rotational relaxation to fill the hole ‘‘burned’’ by the laser in the ground state rotational distribution. For Φ≳Φc the role of collisions is more complex and likely involves intramolecular relaxation processes.
The influence of diffusion on surface reaction kinetics
Authors: Freeman David L. , Doll J. D.
Published: February 2003

An analysis is given of diffusion-influenced surface reactions using models similar to those used in solution kinetics. It is shown that a pure two-dimensional model of surface reactions yields no steady state rate constant. By incorporation of adsorption and desorption processes the deficiencies in the two-dimensional results are eliminated. Expressions are derived for diffusion-controlled and diffusion-influenced rate constants for surface reactions. Expressions are also derived for the activation energies of these surface reactions. It is shown that the activation energy for diffusion-controlled reactions will approximately be given by the activation energy for surface diffusion. Bounding expressions are developed for the activation energy for diffusion-influenced reactions. Comparisons are made betweeen Langmuir–Hinshelwood and Eley–Rideal mechanisms, and it is found that Langmuir–Hinshelwood mechanisms should be more important than Eley–Rideal processes for many surface reactions.
Nonadiabatic semiclassical scattering: Atom–diatom collisions in self-consistent matrix propagator formalism
Authors: Herman Michael F. , Freed Karl F.
Published: February 2003

The self-consistent matrix propagator method of Laing and Freed is extended to treat semiclassical nonadiabatic scattering in the collinear atom–diatom system. Applications are made to a model system in which diabatic surfaces are parallel, so the nonadiabatic transitions are not well localized in space, thereby introducing difficulties in some previous nonadiabatic semiclassical methods. In the self-consistent matrix propagator method nonadiabatic transitions occur at the boundaries of Magnus regions, and the relative phases, associated with trajectories undergoing transitions at different boundaries, must accurately be determined. This necessitates the determination of the absolute phases of the uniformized classical S matrix, a phase which is unnecessary in single potential surface semiclassical scattering. Semiclassical calculations are compared with full close coupled quantum calculations of Schmalz. The agreement is very good even at relatively low energies. The largest errors enter, as anticipated, for highly classically forbidden transitions whose overall probabilities are, however, rather small. The self-consistent matrix propagator method becomes simpler to apply and more accurate as the total energy increases, i.e., as the fully quantum calculations become prohibitively large. The method has the physical appeal that the self-consistent trajectories follow essentially adiabatic surfaces in strongly interacting regions and diabatic surfaces in weakly interacting regions, with a self-consistent interpolation between these regions.
Infrared multiphoton induced isomerization of cis-3,4- dichlorocyclobutene. I. Experimental results
Authors: Presser Nathan , Mao Chung-Rei , Moriarty Robert M. , Gordon Robert J.
Published: February 2003

The yield and product distribution in the laser-induced isomerization of cis-3,4-dichlorocyclobutene (DCCB) have been measured as functions of laser fluence, DCCB pressure, and buffer gas pressure. Both Woodward–Hoffman allowed and ‘‘forbidden’’ isomers were observed. The fraction of ‘‘forbidden’’ products was found to increase linearly with fluence above a threshold of 3.5 J/cm2. At high fluence the yield increased quadratically with neat DCCB pressure. Both the yield and fraction of ‘‘forbidden’’ products declined monotonically with buffer gas pressure. These observations are explained in terms of a two-channel mechanism, in which the allowed isomer is formed in a concerted path and the forbidden isomers are produced from a diradical intermediate in a nonconcerted reaction.
Fall-off behavior of the separable exponential Kassel model for thermal unimolecular reactions
Authors: Snider Neil
Published: February 2003

For thermal unimolecular reactions a fairly realistic model can be constructed by combining the separable exponential model for the collisional activation and deactivation steps with the Kassel model for the reactive steps. The resulting model admits of interesting results which can be expressed analytically. It is argued that the model has a well defined strong collision limit, and an equation for the efficiency factor β̄ for weak colliders is then derived. This factor is a function of the ratio of the rate coefficient κ to its high pressure limiting value κ∞. The model qualitatively, albeit not quantitatively, reproduces the experimental behavior of β̄(κ/κ∞) for methyl isocyanide isomerization in helium at 518 K. Experiments and previous computations both show β̄(κ/κ∞) to decrease at a decreasing rate. The separable exponential-Kassel model gives β̄’s which either increase or decrease with κ/κ∞, but increasing β̄’s occur only for physically unrealistic values of one of the model’s parameters. The model shows that the decreasing rate of decrease of β̄ is related to the small value of the Kassel parameter s for methyl isocyanide isomerization. It predicts that β̄ decreases at an increasing rate for reactions with large s.
General quantitative expression of the positive ionization efficiency in dissociative self-surface ionization of ionic crystals
Authors: Kawano Hiroyuki
Published: February 2003

The dissociative self−surface ionization (DSSI) is discussed for thick polycrystalline layers of alkali−metal halides deposied on metal surfaces which are heated in vacuum. The positive ionization efficiency for the above process is discussed in some detail. (AIP)
Singlet molecular oxygen pumping of IF B 3Π(0+)a)
Authors: Whitefield P. D. , Shea R. F. , Davis S. J.
Published: February 2003

Chemiluminescence from the reactions: I2+F2; I2+F2+O2(1Δ); and I2+F+O2(1Δ) has been studied. All three reactions produced emission from the B 3Π(0+)→X 1Σ+ system of IF. The two reactions utilizing O2(1Δ) produced much higher chemiluminescent intensities than did the I2+F2 reaction. The photon yield of the I2+F+O2(1Δ) reaction was measured to be 0.3%. Rotational and vibrational temperatures in IF(B) were determined. Rotational temperatures were approximately 300 K while nonthermal vibrational distributions were observed. Possible mechanisms of IF(B) production are discussed.
Isoelectronic molecules: The 13 and 22 electron sequences of diatomics
Authors: Laurenzi B. J. , Litto C.
Published: February 2003

A theory of isoelectronic molecules [J. Chem. Phys. 74, 1840 (1981)] has been applied to the ground state of an even (22) electron and an odd (13) electron sequence of diatomics. Each of these is compared to the 14 electron sequence which had been studied previously. In the first instance, a striking correspondence in the dissociation energy De(Z,Z′), bond distance R(Z,Z′), and force constant E(2)(Z,Z′) surfaces for isovalent molecules from the 14 and 22 electron systems has become apparent. For the second, gross differences which arise in these surfaces when one electron is removed from 14 electron molecules are explicable in terms of the nature of the electron-pair bond as opposed to the one-electron bond. A stability rule which excludes ions with charges greater than plus or minus two units is found to hold for these sequences as well as the 14 electron one.
The constrained variational formulation of energy functionals
Authors: Westhaus Paul
Published: February 2003

We present a generalization of the density functional formalism based upon constrained variations of the expectation value ℰ=〈Ψ‖Ĥ‖Ψ〉. The essential idea is to find the minimum value of E with respect to a set of state vectors Sa={‖Ψ〉a}, each member of which yields the specified expectation values ai≡a〈Ψ‖Ai‖Ψ〉a of set of observables Âi, i=1...M. a denotes the collection of M expectation values ai, i=1...M. If one or more N-electron vectors exist yielding the given M expectation values, a is said to be N representable. One thus obtains this minimum expectation value E(a)=min E over Sa as a function of the assigned expectation values. In terms of the function E(a), the exact ground state energy may be obtained by seeking its minimum value over all possible (N-representable) expectation values a. The traditional density functional formalism of Hohenberg and Kohn is seen to be a special case where the constraints are expressed by the expectation values of an indenumerable set of charge density operators, the members of which are indexed by the continuum of eigenvalues of the single particle position operator. The general ideas are illustrated by computing a function based upon constraining 〈1/r〉 and 〈1/r2〉 for a one-electron atom. In this example, the formalism is carried out by solving a modified Schrödinger equation which includes Lagrange multipliers in order to take into account the constraints. We conclude, however, with an outline of how the unitary transformation formalism can be used to obtain the function ℰ(a) in a systematic fashion.
On the theory of nonlinear dielectrics
Authors: Fulton Robert L.
Published: February 2003

A simple yet general prescription for finding the connections between phenomenological coefficients and susceptibilities for nonlinear dielectrics is found. In the linear realm the standard connections are recovered. The connections found between the nonlinear susceptibilities and phenomenological coefficients for isotropic polar media are found to differ from previous proposals, all of which rely on extensions of either the procedure of Onsager or of Kirkwood and Fröhlich. Application is made to two simple models; the first that of polar fluids, the second of nonpolar, polarizable fluids.
Oxygen self-diffusion in Fe-doped MgO single crystals
Authors: Ando Ken , Kurokawa Y. , Oishi Y.
Published: February 2003

Oxygen self-diffusion coefficients in Fe-doped MgO single crystals were determined by an isotope exchange technique. The results indicated dopant-insensitive diffusion, which was interpreted as due to the vacancy pair mechanism.
Effect of gravity on density distributions and orthopositronium annihilation rates in ethane and methane near the critical point
Authors: Sharma Suresh C. , Kafle Surendra R. S.
Published: February 2003

The effect of gravity on density distributions has been studied in ethane and methane near their critical points using the linear-model parametric equation of state. The results obtained from this study are used to further understand the sensitivity of orthopositronium annihilation rates to density fluctuations in molecular gases. It is shown that the influence of gravity is too small to account for the recently observed dependence of orthopositronium annihilation rates on the density of ethane gas at 306.4 K. However, a significant variation in local density vs height is calculated at temperatures closer to the gas–liquid critical point. The density and temperature dependencies of the annihilation rates of orthopositronium atoms, recently observed in ethane and methane gases, are discussed in terms of the findings of this study.
Effects of topological solitons on autocorrelation functions for chains of coupled torsional oscillators
Authors: Perchak Dennis , Yaris Robert , Skolnick Jeffrey
Published: February 2003

Brownian dynamics computer simulations were performed on chains of coupled torsional oscillators. The purpose was to observe the changes in autocorrelation functions, related to typical experimental measurements, caused by the introduction of topological solitons or kinks into the system. We considered three model systems: a chain of coupled torsional oscillators, a chain of coupled torsional oscillators with additional onefold rotational potentials acting on each oscillator, and a chain of coupled torsional oscillators with additional threefold rotational potentials. These models are of interest because of their application to torsional motions in polymeric systems, and, in particular, the system with onefold rotational potentials has been studied extensively as the sine–Gordon chain. We present simulation results for three autocorrelation functions of these three systems both with and without topological solitons.
A study on the orientation of imidazoles on copper as corrosion inhibitor and possible adhesion promoter for electric devices
Authors: Yoshida Shuji , Ishida Hatsuo
Published: February 2003

The structure of various imidazoles on the surfaces of copper and gold mirrors has been studied by Fourier transform infrared reflection–absorption spectroscopy in order to elucidate the molecular mechanisms of corrosion inhibition of metal surfaces by imidazoles. Heptadecylimidazole, undecylimidazole, phenylimidazole, and imidazole were used to investigate the orientation of the imidazole ring and the accompanying side groups. When a gold mirror was withdrawn slowly from an ethanol solution of undecylimidazole, the imidazole ring adsorbed onto the surface flatly and the C–C bond of the hydrocarbon chain became parallel to the surface. A similar trend was observed on all imidazole derivatives studied. The orientation of the outer layers was disturbed when the thickness was increased. With a copper mirror, the structure of the imidazole at room temperature depended on the method of adsorption. A slowly withdrawn sample showed a predominant complex formation whereby the solution placed dropwise on the copper surface showed a combination of complexes, randomly oriented layers, and oriented layers similar to gold. The heat treatment at 80 °C eliminated the difference caused by the adsorption methods. Upon complex formation, the orientation of the imidazole became random. This complex is tentatively assigned to diundecylimidazolato copper(II), an inner complex with copper. Further heat treatment at 150 °C yielded another complex. The structure of undecylimidazole adsorbed from an acidic aqueous solution on a copper mirror has also been studied. The imidazole adsorbed very rapidly to form a thick layer due to the acceleration of complex formation by the activated imidazole ring. A similar structure in the imidazole layers was obtained both from an ethanol solution and an acidic aqueous solution. However, coadsorption of the small amount of acid used remains to be a question. Again, the heat treatment at 150 °C yielded the infrared spectrum similar to the complex obtained from the ethanol solution.
Relationship of the virial series to vapor condensation and the liquid state
Authors: Yang A. J. , Pavlin I. , Gibbs J. H.
Published: February 2003

A complete set of at least approximate virial coefficients is constructed for the Lennard-Jones fluid by supplementing the four known ones with a remainder set taken from the complete set known for the hard sphere potential. When used in the virial series for the pressure, this set gives excellent agreement with the available Monte Carlo results, even in the region of vapor condensation, where both give a loop of the van der Waals type, and in the region of pure liquid. When the canonical-ensemble partition function given by integration of this virial series is used in the standard expression for the grand-canonical-ensemble partition function and pressures are calculated from this, the van der Waals loop is replaced by a constant pressure in the region of coexistence of gas and liquid, but the isotherms are unchanged in the pure gas and pure liquid regions.
A formula for calculating molecular energy differences from electrostatic potentials at nuclei
Authors: Politzer Peter , Sjöberg Per
Published: February 2003

A formula for calculating molecular energy differences from electrostatic potentials at nuclei is derived. The calculated energy differences using this formula are very close to both the Hartree−Fock and the experimental values molecular electron affinities are also calculated for several diatomic molecules. (AIP)
The 2H thermal diffusion isotope effect in benzene and methanol
Authors: Ma Ning-Yuan Richard , Beyerlein Adolph L.
Published: February 2003

The 2H thermal diffusion isotope effect is investigated for mixtures benzene−benzeed benzene−o dideuterobenzene, benzene−p didouterobenzen, mathanol d−methanold4, methanol−melhanol d. The thermal diffusion measurements wre made using a cylindrical thermo gravitational diffusion apparatus. (AIP)
Theoretical studies of H2–H2 collisions. V. Ab initio calculations of relaxation phenomena in parahydrogen gas
Authors: Köhler Walter E. , Schaefer J.
Published: February 2003

The temperature dependence of effective Waldmann–Snider cross sections determining relaxation and line broadening phenomena has been studied for p-H2 between 20 and 200 K. In particular, the rotational relaxation cross section and the relaxation cross sections of the rotational angular momentum vector and tensor polarizations and their respective fluxes have been calculated in an entirely ab initio treatment and close coupling formalism. As far as experimental results were available, quantitative agreement has been obtained. The rotational relaxation cross section has been proven to be quite sensitive to the potential anisotropy. Furthermore, the validity of some approximate relations between effective cross sections has been tested.
Energy dependence of the O− transfer reactions of O3− and CO3− with NO and SO2
Authors: Albritton D. L. , Dotan I. , Streit G. E. , Fahey D. W. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2003

The O− transfer reactions of O3− and CO3− with NO and SO2 were measured as a function of temperature in the range 170–605 K in a variable-temperature flowing afterglow system and as a function of center-of-mass kinetic energy in the range 0.04–2 eV in a flow-drift tube system. The rate constant in each case showed a marked energy dependence. The rate constants measured as a function of temperature were essentially the same as those measured as a function of kinetic energy in He and Ar buffer gases when the comparison is made at the same mean center-of-mass kinetic energy. The results imply that the reactions have negligible vibrational or rotational energy dependence.
Collisional ionization of high-Rydberg lithium atoms in thermal-energy encounters with rapidly rotating polar molecules
Authors: Shepard Chester L. , Kocher Carl A.
Published: February 2003

Ionization cross sections for high-Ryberg lithium atoms in collisions with the polar molecules NH3, H2O, and H2S have been determined by analysis of time-of-flight distributions for a thermal beam of lithium atoms excited by pulsed electron impact. States with principal quantum numbers n in the range 20≲n≲75 are excited, with a peak in the distribution at n≂35. The cross sections are of order 10−12 cm2 for all three molecules. Theoretical calculations are also described, based on the first Born approximation, in which the outermost Rydberg electron is assumed to be essentially free. This theory, as well as the calculation of Matsuzawa, slightly underestimates the experimental results.
Collisional quenching of indium (5p2 4P1/2) and indium (6p 2P) by Ar, H2, D2, N2, and CH4
Authors: Hovis F. E. , Gelbwachs J. A.
Published: February 2003

Collisional quenching of In(5p2 4P1/2) and In(6p 2P) by Ar, H2, D2, N2, and CH4 has been measured by laser pumping of the 5p2 4P1/2 level. The quenching cross sections for 5p2 4P1/2 range from 1.6×10−16 cm2 for Ar to 37×10−16 cm2 for H2. The cross sections for quenching the 6p 2P state range from 0.96×10−16 cm2 for Ar to 18×10−16 cm2 for H2. The radiative lifetimes of the 5p2 4P1/2 and 6p 2P state were found to be 950 and 104 ns. Population ratios measured under steady-state conditions show that much of the excited-state population resides in the 6s 2S1/2 level after excitation of the 5p2 4P1/2 or 5d 2D levels.
Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene
Authors: Hippler H. , Troe J. , Wendelken H. J.
Published: February 2003

Vibrationally highly excited toluene molecules with 52 000 cm−1 of vibrational energy have been prepared by UV laser excitation of the isomer cycloheptatriene and subsequent isomerization. The collisional loss of energy from the excited toluene molecules has been observed directly by monitoring hot UV absorption spectra. Direct measurements of the average energies 〈ΔE〉 transferred per collision have been made for about 60 different bath gases. For complex bath gases, 〈ΔE〉 values appear to be considerably smaller than those derived from earlier indirect measurements. 〈ΔE〉 was found not (or only slightly) to depend on the excitation energy.
Determination of product population and alignment using laser-induced fluorescence
Authors: Greene Chris H. , Zare R. N.
Published: February 2003

For collision systems having axial symmetry, the anisotropy of the laser-induced fluorescence is given explicitly in terms of the zeroth, second, and fourth rank moments of the angular momentum distribution, which, respectively, are proportional to the population, the quadrupole alignment, and hexadecapole alignment of the product internal state under study. Expressions are presented for determining these three quantities from the dependence of the fluorescence intensity on the polarizations of the absorbed and detected photons. Results are presented for an arbitrary excitation-detection geometry which is then specialized to the commonly occurring cases where the direction of fluorescence detection is at right angles to the axis of cylindrical symmetry and the direction of the incoming light beam is either along the axis of cylindrical symmetry or at right angles to it and to the fluorescence detection direction. The approach of these expressions to the high-J limit is considered. The effect of nuclear spin on the fluorescence intensity is analyzed and the extent of depolarization is shown to be unimportant for large J. The use of angular momentum recoupling algebra permits the geometrical and dynamical aspects of this problem to be completely disentangled.
Wave packet formulation of the boomerang model for resonant electron–molecule scattering
Authors: McCurdy C. William , Turner Julia L.
Published: February 2003

A time-dependent formulation of the boomerang model for resonant electron–molecule scattering is presented in terms of a wave packet propagating on the complex potential surface of the metastable anion. The results of calculations using efficient semiclassical techniques for propagating the wave packet are found to be in excellent agreement with full quantum-mechanical calculations of vibrational excitation cross sections in e−–N2 scattering. The application of the wave packet formulation as a computational and conceptual approach to the problem of resonant collisions with polyatomic molecules is discussed in the light of recent wave packet calculations on polyatomic photodissociation and Raman spectra.
Stark-tuned Lamb-dip spectroscopy of the ν4 and 2ν2 bands of 14NH3
Authors: Weber W. H. , Terhune R. W.
Published: February 2003

Results are presented of Stark-tuned Lamb-dip measurements in the 6 μm ν4 and 2ν2 bands of 14NH3 obtained using an isotopic CO laser with an intracavity Stark cell. Some 230 lines from the isotopes 12C 16O, 13C 16O, 12C 18O, and 13C 18O were investigated with fields up to 30 kV/cm. 71 coincidences with Stark-tuned NH3 lines, involving several hundred Lamb dips, were assigned and analyzed. The positions of these lines relative to the CO laser lines are reported with a typical accuracy of 5–20 MHz. Model calculations of Lamb-dip spectra are also given that include power and pressure broadening and the collisionally transferred resonances. These are directly compared with observed spectra, from which we deduce values for the pressure broadening coefficients and the collisional transfer efficiency. Nuclear hyperfine structure is resolved on several lines, and from fitting one of these we obtained the value eqQ=−4.83±0.3 MHz for the nuclear quadrupole coupling constant in the s2ν2 (J=10, K=9) state. We see numerous examples of weak ‘‘forbidden’’ transitions ΔMJ=0, ±2. These arise from the nuclear quadrupole coupling, which mixes MJ states.
A theory of visible/UV multiphoton processes in molecules. I. Resonance effect on three-photon absorption
Authors: Fujimura Y. , Lin S. H.
Published: February 2003

Using a transition operator formalism, an expression for three-photon transitions is derived. The transition operator formalism makes it possible to take into account the effects of population decays and pure dephasings occurring in the relevent vibronic states on the cross sections of the resonance multiphoton processes. It is shown that for a two-photon resonant (2+1) process, the cross section can approximately be divided into two terms, one is called sequential term which cannot be taken into account by using the lowest order time-dependent perturbation theory, and the other which can be derived from simultaneous three-photon cross section under constraint of a resonant condition for the second photon absorption. Assuming the adiabatic approximation for the molecular states, analytical expressions for the cross sections for the three-photon absorptions are derived in the displaced harmonic oscillator model of the molecules. An analytical expression taken into account the temperature effect is given for the simultaneous three-photon absorption cross section.
Thermally stimulated luminescence studies of x-irradiated L-alanine:Cr3+ single crystals
Authors: Cooke D. W. , Jahan M. S.
Published: February 2003

Thermally stimulated luminescence studies of x-irradiated L-alanine:Cr3+ have been conducted in the interval 10–300 K. Glow peaks were observed at 42, 60, 72, 148, and 208 K. The 148 K peak has been previously reported, while the one at 208 K was of insufficient intensity to study. Also, the 60 K peak was difficult to analyze due to overlap with the neighboring peaks. Detailed analyses of the 42 and 72 K peaks yielded, respectively, the following parameters: E=24 meV, s=5.3 s−1, and l=1; E=180 meV, s=2.4×1010 s−1, and l=2.8, where l is the kinetics order. Identical emission was observed from each peak, characterized by a maximum at 445 nm with FWHM equal to 0.605 eV. A tentative model is presented to explain these results in terms of detrapping and deexcitation via the excited singlet and triplet states.
Quantum interference phenomena in the radiative decay of the C̃(1B2) state of SO2
Authors: Ivanco Michael , Hager James , Sharfin Wayne , Wallace Stephen C.
Published: February 2003

Quantum beat effects in the fluorescence decay of the C̃(1B2) state of SO2 have been analyzed in detail and the coupling mechanism, which gives rise to the quantum interference phenomena identified as vibronic mixing with the ground state. The use of transform-limited dye laser pulses for excitation permitted both the sensitive detection of weakly modulated quantum beats and a direct quantitative analysis of the magnitude of the interstate coupling. Small Zeeman shifts were also observed to be present, but because these are less than 1/20 of those observed for singlet–triplet interactions in SO2 and other molecules, they are attributed to indirect coupling effects via the ground state. The onset for predissociation at 218 nm is marked by the disappearance of the quantum beats and both confirms the vibronic coupling model and yields a spectroscopic measure of the ground state dissociation energy. Thus the dynamical behavior of the C̃(1B2) state of SO2 has been uniquely characterized through analysis of the quantum interference effects arising through interstate coupling and is the first such example in the limit of a sparse density of states in a polyatomic molecule.
Operator and algebraic methods for NMR spectroscopy. I. Generation of NMR spin species
Authors: Balasubramanian K.
Published: February 2003

Algebraic methods are developed for generating NMR spin species and irreducible representations spanned by spin functions. These methods use generalized character cycle indices (S function when NMR groups are symmetric groups). A correspondence between the unitary group approach and the permutation group method for NMR is established by generating NMR Gel’fand states using Schur functions. The generalized character cycle indices of molecules whose NMR groups are expressible as generalized wreath products are shown to be generalized NMR plethysms of Schur functions. These techniques enable generation of spin species without the knowledge of the character tables of NMR groups. We illustrate the methods developed here with several examples. The use of these techniques in generating symmetry-adapted NMR spin functions can be found in the accompanying paper.
The far-infrared spectrum of ice Ih in the range 8–25 cm−1. Sound waves and difference bands, with application to Saturn’s rings
Authors: Mishima O. , Klug D. D. , Whalley E.
Published: February 2003

The absorption spectrum of single crystals of ice Ih with the light traveling perpendicular or at 5° to the c axis has been measured in the frequency range 8–25 cm−1 at 80, 100, 150, and 202 K. At high frequencies, the absorptivity is proportional to the fourth power of the frequency, and is caused by fundamental hypersonic waves having wavelengths of about 100 Å. At low frequencies, it is probably caused by difference bands from the translational lattice vibrations. The theory of the intensity of binary combination bands of ionic crystals has been specialized to the low-frequency difference bands. Ice Ih has been assumed to have the same absorptivity as ice Ic, and ice Ic has been represented by an ordered crystal that resembles sphalerite. The model fits the measured absorptivity of ice Ih assuming that the difference bands arise mostly from the strong first-order band centered at ∼230 cm−1. The acoustic branches do not contribute significantly. The absorptivity, extrapolated to 3.0 cm−1 and interpolated to 85 K by means of the theory, was used to determine the mean thickness of ice in Saturn’s rings from the measured thermal component of the brightness temperature of the rings. If the slab model is used and scattering is neglected, the mean thickness is 30±10 cm.
A kinetic model for hydrogen–metal reactions controlled by a phase transformation step
Authors: Mintz M. H. , Bloch J.
Published: February 2003

A probabilistic kinetic model for gas–solid (particularly hydrogen–metal) reactions, controlled by a phase transformation step, is presented. The transformation is assumed to occur in small aggregates of atoms (structural transformations units) with a transition probability which depends on the occupation of the structural unit by the gas (e.g., hydrogen) atoms. The weighed contributions of all possible occupancies are considered. The model predicts the pressure–temperature dependence of the product–reactant (hydrogen–metal) interface growth rate. The general properties of the growth rate functions that are derived and the evaluation of the various parameters which appear in these functions are discussed.
Comment on ‘‘Temperature dependence of thermal vibrations in cubic ZnS: A comparison of anharmonic models’’
Authors: Moss Barbara , Roberts R. B. , McMullan R. K. , Koetzle T. F.
Published: February 2003

The temperature scale used in the analysis of neutron diffraction study of cubic ZnS1, is corrected. The revised temperature scale2 is used to re-examine the thermal parameters. (AIP)
Nonadiabatic investigation of the V–N spectrum of ethylene in a new diabatic representation
Authors: Petrongolo Carlo , Buenker Robert J. , Peyerimhoff Sigrid D.
Published: February 2003

A nonadiabatic investigation of the V–N spectrum of ethylene has been carried out in a new diabatic representation which is characterized by constant (different from zero, in general) single-differentiated vibronic couplings which are obtained by imposing symmetry conditions on the adiabatic–diabatic transformation matrix. The present results agree very well with those recently reported in the adiabatic representation [C. Petrongolo, R. J. Buenker, and S. D. Peyerimhoff, J. Chem. Phys. 76, 3655 (1982)] and show that the adiabatic basis is better than the diabatic for a nonadiabatic study of this transition. In the Appendix, the present treatment is extended to a many-state, many-coordinate problem.
Adsorption, structure, and stress in binary interfaces
Authors: Falls A. H. , Scriven L. E. , Davis H. T.
Published: February 2003

The physics of fluid–fluid interfaces is investigated with the gradient theory of inhomogeneous fluid. The theory is used to predict component density and principal pressure (negative of stress) profiles in flat and curved interfaces formed in carbon dioxide and decane mixtures. In some cases the component density (and total density) profiles are not monotonic, with CO2 being prominently surface active. In composition regions near where a perfect-wetting third phase splits out the composition and pressure profiles are especially structured, a thin-film appearing which is akin to the perfect-wetting phase and which almost separates two distinct interfaces. This precursor to the third phase gives rise to Antonov’s rule for the interfacial tension between the two phases that are nearly in equilibrium with the third, perfect-wetting phase. Below a perfect-wetting transition temperature Tcw, such thin-films do not arise near compositions of the three phase region. Curvature of sufficiently small drops or bubbles significantly affects adsorption at interfaces. The theory predicts that the Young–Laplace equation as traditionally applied overestimates the pressure drop across the interface in some cases (as found earlier in the case of one-component drops), but underestimates the pressure drop across interfaces related to perfect-wetting structures. The minimum work of forming critical nuclei is predicted.
Introduction of Andersen’s demon in the molecular dynamics of systems with constraints
Authors: Ryckaert J. P. , Ciccotti G.
Published: February 2003

The method of constant pressure molecular dynamics (MD), developed by Andersen for monoatomic fluids is extended to the MD, in Cartesian coordinates, of molecular systems with constraints. Andersen’s proof is easily generalized after decoupling internal degrees of freedom from the center-of-mass. Only these last degrees are directly affected by Andersen’s transformation (demon). The Cartesian equations of motion of individual atoms are derived from a generalized Andersen’s Lagrangian. The equations are quite similar to those of the usual MD simulation at constant volume apart from an additional term coupling the molecular center-of-mass and the volume of the sample. The volume appears now as a dynamical variable evolving from the imbalance between imposed external pressure and instantaneous values of the molecular stress tensor. Some numerical aspects are discussed and the technique is briefly illustrated for the case of rigid diatomic molecules.
Theory of electronic relaxation in solution in the absence of an activation barrier
Authors: Bagchi Biman , Fleming Graham R. , Oxtoby David W.
Published: February 2003

We present a theory which describes the effects of viscosity on those electronic relaxation processes in solution in which the intramolecular potential surface does not present a barrier to the motion leading to the decay of the initially formed excited state. We model the reactive motion as the motion of a solute particle on the excited state potential surface with a position dependent sink which gives rise to the decay of the excited state population. Three different types of sinks are considered: (A) a pinhole sink at the minimum of the potential surface; this models the situation when the molecule decays to ground state as soon as it reaches the potential minimum; (B) a Gaussian sink with probability of decay maximum at the potential minimum; (C) a Lorentzian sink with maximum decay at the potential minimum. For case (A) an explicit analytic solution is obtained for the decay rate, but for cases (B) and (C) we obtained the decay rate numerically. Model (A) predicts nonexponential decay at all viscosities except at long times when the decay is single exponential. For cases (B) and (C) the decay is single exponential at low viscosities but becomes multiexponential at high viscosities. We show that the experimentally observed fractional viscosity dependence of fluorescence quantum yield arises naturally in this theory due to the position dependence of the sink as well as due to the competition between radiative and nonradiative relaxation. Our model also predicts a crossover from an apparent negative (constant viscosity) activation energy at low viscosities to a positive activation energy at high viscosity. The physical significance of these results is discussed in light of the available experimental results on TPM dye relaxation. Some possible generalizations of our theory to more realistic cases are indicated.
The effect of underpotential deposition on the surface enhanced Raman effect
Authors: Kester J. J.
Published: February 2003

The first experimental observation of surface enhanced Raman scattering (SERS) from benzotriazole (BTA) on Ag as a function of the underpotential deposition of Tl is reported. The complete quenching of SERS before completion of the submonolayer deposition of Tl is attributed primarily to displacement of BTA from the Ag surface. The decrease in intensity of SERS is found to be approximately linear with the number of Tl ions deposited. But the rate of quenching of SERS is dependent on the initial surface topography. Increased electron scattering is proposed as an additional quenching mechanism. Upon Tl deposition, frequency shifts for one of the BTA lines are determined to be three times larger than that expected based on a surface charging mechanism. Mechanisms for SERS based on electronic interactions of the adsorbate and substrate are indicated.
Nuclear magnetic resonance study of Li-exchanged zeolites. I. Cation spin dynamics: 7Li in hydrated A, X, and Y zeolites
Authors: Tokuhiro Tadashi , Iton Lennox E. , Peterson E. Mark
Published: February 2003

7Li spin-lattice relaxation times (T1) were measured at 25 MHz in hydrated Li-exchanged A, X, and Y zeolites over the temperature range between 140 and 330 K. By comparing 7Li 1/T1 in hydrated (H2O) zeolites with those in hydrated (D2O) analogs, the contributions from the 7Li quadrupolar relaxation [(1/T1)QLi] and the 7Li–1H dipolar relaxation [(1/T1)DLi–H] were determined separately as a function of temperature. The 7Li quadrupole coupling constants (e2Qq) determined from (1/T1)QLi are 61.86, 72.35, and 202.8 kHz in A, X, and Y zeolites, respectively. The temperature dependence of the correlation times for 7Li quadrupolar relaxation (τQ) reveals that: (1) the dynamic state of Li cations in hydrated zeolites resembles that of the cations present in associated liquids at high concentrations; and (2) the mobility of cations increases in the order of A, X, and Y zeolites. Variations of 7Li e2Qq and τQ were explained in terms of a composite picture of the structural state of Li+ ions in the hydrated forms of three zeolites.
Electron spin-lattice relaxation in the phosphorescent state of xanthione
Authors: Taherian M. R. , Maki A. H.
Published: February 2003

Spin-lattice relaxation in the phosphorescent T1 state of xanthione has been measured at 1.0 K in a dilute n-hexane matrix. Direct excitation of the T1z←S0 transition with a laser pulse of 10−8 s width was followed by observation of the phosphorescence decay monitoring either a T1z or T1y emission band. These bands are optically resolved in the n-hexane Shpol’skii matrix because of the large zero-field splitting, D=−15.5 cm−1. Depopulation of T1z by the spin-lattice relaxation processes, T1z∼≳T1x, T1y was found to be complete within 10−7 s, the time resolution of the measurement. The relaxation process, which at 1.0 K requires the spontaneous creation of a lattice phonon of 15.5 cm−1, is unusually efficient. Thus, when the T1 state is populated by intersystem crossing using either S1←S0, or S2←S0 optical pumping, lack of observed T1z phosphorescence at 1.0 K does not imply that the lower T1x and T1y sublevels are selectively populated. Observation of T1z emission at higher bath temperatures, ∼3.3 K, following a T1z←S0 pulse reveals a peculiar initial rise in intensity followed by an eventual decay. This behavior is explained in terms of lattice heating which accompanies the radiationless T1∼≳S0 decay. The initial increase in intensity results from a thermal pumping rate from the lower energy sublevels which exceeds the overall decay rate of the T1 state. Nonexponential decay of the T1y emission is explained by the same model. We find that the T1z emission intensity vs time cannot be explained quantitatively by a uniform lattice temperature during the decay, but that temperature gradients must be present.