bluuebunny/crossref_metadata_2025 huggingface dataset:
Shift of first-order phase transitions in thin films due to boundary fields: A computer simulation
Authors: Albano E. V. , Binder K. , Heermann D. W. , Paul W.
Published: July 2002

The first-order phase transition of the ferromagnetic Ising model driven by the magnetic field at temperatures below criticality is studied by Monte Carlo methods for a two-dimensional thin film geometry, L×M with two free boundaries of length M≫L, at which boundary fields act. This model study is relevant, in particular, for phase transitions in monolayers adsorbed at stepped surfaces. While in the bulk geometry (L→∞) this transition occurs for zero field in the present model, with the system ‘‘jumping’’ from a state with uniformly positive magnetization to a state with uniformly negative magnetization, in the thin film geometry the transition occurs at a critical field H*∼L−1, and the two states between which the transition occurs are characterized by strongly nonuniform magnetization profiles across the film. These findings are in agreement with the scaling theory of Fisher and Nakanishi.
The rate coefficient for quenching N(2D) by O(3P)
Authors: Piper Lawrence G.
Published: July 2002

We have studied the quenching of N(2D) by atomic oxygen in a discharge-flow reactor. Vacuum ultraviolet resonance fluorescence is used to detect the metastables. Three different discharge-based sources provide the atomic oxygen: discharged O2/Ar, discharged N2O/Ar, and the reaction of N with NO. Results from the three different approaches are congruent and indicate the lack of significant systematic errors in the study. The rate coefficient at 300 K is (1.06±0.26)×10−12 cm3 molecule−1 s−1.
Mean and Gaussian curvatures of randomly decorated Voronoi and cubic tesselations
Authors: Anderson David M. , Davis H. T. , Scriven L. E.
Published: July 2002

It is now generally accepted that some midrange microemulsions are bicontinuous, i.e., continuous in both oil and water simultaneously. The first model of the microstructure of microemulsion that could account for a progression from discrete to bicontinuous was the Talmon–Prager or ‘‘randomly decorated Voronoi’’ model. Space is tessellated into Voronoi polyhedra and the polyhedra are randomly decorated with oil and water. In variations of the model DeGennes and Taupin and Widom decorate a cubic tesselation of space. At first glance it might appear that the decorated Voronoi and cubic tessellations are zero-mean-curvature models, since they are constructed from polyhedra with planar faces. However, the edges of the polyhedra are concentrations of mean curvature, and the vertices are concentrations of Gaussian curvature. The area-averaged mean and Gaussian curvatures of the oil–water interface in the randomly decorated Voronoi and cubic models are calculated. The area-averaged mean curvatures of the two models are linear functions of oil volume fraction, change sign at a volume fraction of 0.5, and are within 0.2% of one another in magnitude. The area-averaged Gaussian curvature of the Voronoi model varies quadratically with volume fraction, and is negative for oil volume fractions from 0.18 to 0.82 (oil and water are bicontinuous for volume fractions ranging from 0.135 to 0.865). The area-averaged Gaussian curvature of the randomly decorated cubic model is a sixth-order polynomial function of oil volume fraction and is negative for volume fractions ranging from 0.23 to 0.77 (oil and water are bicontinuous over the volume fraction range 0.25 to 0.75). As an additional application, the model results are used to interpret curvature aspects of the bilayer theory of the L3 phase of surfactant solutions presented recently by Cates et al. [Europhys. Lett. 5, 733 (1988)].
Reduced potential curves of the excited states of alkali diatomic molecules
Authors: Jenč F. , Brandt B. A.
Published: July 2002

The reduced potential curve (RPC) method has so far been successfully employed for the discussion of the ground states of diatomic molecules, however, its efficacy in the complicated world of the excited states has been doubted by some spectroscopists. The present paper should prove that such doubts are in general unjustified. It is shown that, for an excited state of definite symmetry and order (e.g., 1 1Πu, 2 1Πu, etc.) in a group of affiliated molecules, e.g., the group of homo- or heteronuclear alkali diatomic molecules, the same rules hold in the reduced potential curve (RPC) scheme as have been shown before to hold for the ground state. Small deviations and also anomalies with respect to this rule exist for some excited states as must, of course, be expected. The RPC method just seems most suited to visualize such anomalies. Rydberg–Klein–Rees (RKR) and theoretical ab initio calculated potentials are studied in reduced form. The RPC scheme makes possible a systematic comparative study of excited states of diatomic molecules. The RPC method may be also used for detection of errors (inaccuracies) in the analysis of the spectrum or of deficiencies in the theoretical calculation, and for estimation of the potentials of excited states.
Coupling of hindered internal rotation with van der Waals vibration in aniline–H2(D2 ) complexes
Authors: Coutant Bruno , Brechignac Philippe
Published: July 2002

The laser-induced fluorescence excitation spectra from the van der Waals complexes of aniline with hydrogen and deuterium, formed in a free jet expansion, have been recorded in the spectral region associated to the electronic band origin S0 –S1 (000) of the free aniline molecule. For each complex the spectrum exhibits a vibrational progression involving a van der Waals vibration in the excited state. Each vibrational band of this progression is made of a doublet, the components of which are assigned to the ‘‘j=0’’ (para-H2 , ortho-D2 ) and ‘‘j=1’’ ortho-H2 , para-D2 ) Δj=0 transitions, as confirmed by nuclear spin statistics. The internal rotation of the hydrogen molecule in the complex is hindered by a barrier, the height of which is different in the ground X̃ 1 A1 state and the excited Ã 1B2 state. The doublet splitting (going from 3.1 to 6.2 cm−1 ) shows a systematic variation with the van der Waals vibrational quantum number, which is monotonic for D2 and presents a minimum for H2 , indicative of a strong coupling between internal rotation and the involved van der Waals motion. A quantum-mechanical treatment using the ‘‘nearly free rotor’’ picture has been done, which is able to reproduce the two spectra with very good accuracy, both for the positions and the intensities of the various bands. A detailed description of the full potential energy surface V(R, θ), including the coupling terms between the van der Waals bond length R and the hydrogen rotation angle θ in the excited state, is derived.
Quantum simulations and a
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o electronic structure studies of (H2O)−2
Authors: Barnett R. N. , Landman Uzi , Dhar S. , Kestner N. R. , Jortner Joshua , Nitzan Abraham
Published: July 2002

The energetics of the negatively charged water dimer (H2O)−2, is studied using quantum-simulation techniques and ab initio electronic structure calculations. Using the RWK2-M potentials for water and a pseudopotential for the interaction of an electron with a water molecule in the ground state, consisting of Coulomb, adiabatic polarization, exclusion, and exchange contributions, it was found via the quantum path-integral molecular dynamics and the coupled quantum-classical time-dependent self-consistent field methods that while the minimum energy of (H2O)−2 corresponds to a nuclear configuration similar to that found for the neutral (H2O)2 cluster, other nuclear configurations are also exhibited at finite temperature, characterized by a higher total molecular cluster dipole moment and a larger magnitude of the excess electron binding energy. Quantitative agreement is found between the results obtained by the quantum simulations, employing the excess electron–molecule pseudopotential, and those derived, for selected nuclear configurations, via ab initio calculations, employing the Gaussian 86 code with the basis set for the water molecules supplemented by a large diffuse set located at the midpoint of the two oxygens and in addition by a diffuse set for the excess electron.
Photoelectron spectroscopic studies of multiphoton processes in molecular chlorine involving the 2 1∏g Rydberg state
Authors: Koenders B. G. , Koeckhoven S. M. , Kuik G. J. , Drabe Karel E. , de Lange C. A.
Published: July 2002

Kinetic energy resolved electron detection is used to study resonance enhanced multiphoton ionization and dissociation of molecular chlorine via the 2 1∏g Rydberg state. In the two-photon energy region between 63 000 and 73 600 cm−1, a long vibrational progression up to v′=15 associated with this intermediate gerade Rydberg state is observed. The regularity and magnitude of the vibrational spacing indicate that the 2 1∏g state, converging to the ionic ground state, has virtually unperturbed Rydberg character. However, the molecular signals in the photoelectron spectra show strong deviations from Franck–Condon behavior. In addition, intense electron signals arising from one-photon ionization of excited chlorine atoms are observed. An interpretation in terms of a competition between electronic autoionization and dissociation from core-excited molecular Rydberg states situated above the lowest ionization energy is presented. Some of the excited atomic states observed are indicative for Rydberg–Rydberg interactions at large internuclear distances in the dissociation channel.
Improved semiclassical propagation of wave packets
Authors: Kay Kenneth G.
Published: July 2002

The time-dependent variational method is applied to develop a potentially exact wave packet propagation technique that systematically improves the semiclassical Gaussian treatment of Heller. Difficulties encountered in previous applications of the variational method are avoided. Numerical examples are presented which demonstrate the accuracy and convergence of the present approach.
Fermi resonances in overtone spectra of bromoform, CHBr3
Authors: Halonen Lauri , Kauppi Esa
Published: July 2002

A curvilinear internal coordinate Hamiltonian is used to analyze Fermi resonances between CH stretching and bending vibrations in bromoform. Besides the usual Fermi resonance terms, cubic and quartic kinetic and potential energy terms in bending variables are included in the Hamiltonian. The eigenvalues are obtained variationally with a Morse oscillator basis set for the stretch and a two-dimensional harmonic oscillator basis set for the bend. The potential energy parameters are optimized with the nonlinear least squares method. A bond dipole model is used to calculate infrared absorption intensities. The model reproduces well the observed vibrational term values and infrared absorption intensities.
Fluorescence excitation spectrum of the Si–Ar van der Waals complex
Authors: Dedonder-Lardeux C. , Jouvet C. , Richard-Viard M. , Solgadi D.
Published: July 2002

We report here the fluorescence excitations spectrum of the Si–Ar van der Waals complex in the region of the (3p4s)3P–(3p2)3P atomic transition. Long progressions are observed, which have been assigned to a Π–Σ transition. The potential curves derived from the analysis of these progressions are discussed in terms of effects of spin–orbit coupling on van der Waals interactions.
Quantum theory of the photodissociation of IBr adsorbed on an MgO(001) surface
Authors: McCarthy M. I. , Gerber R. B. , Shapiro M.
Published: July 2002

The photodissociation of an isolated IBr molecule adsorbed on an MgO(001) surface is studied theoretically. The calculations correspond to an excitation into the repulsive (Y0+) /quasibound (B 3Π0+) electronic state manifold, which may lead to the production of excited state or ground state bromine atoms. Using a quantum scattering method, we calculate the photoabsorption line shape for this process and the [Br]/[Br*] branching ratio as a function of photoexcitation wavelength. In the quantum calculations the IBr stretching was treated exactly, the in-plane librational mode was treated in the sudden approximation, and all other adsorbate and crystal modes were frozen. In addition, we studied the photodissociation process classically in order to explore the validity of freezing most of the modes. In the quantum calculations it was found that the width and intensities of the structured part of the absorption profile were greatly increased compared with the gas-phase photodissociation process. This was attributed to the stabilization of both electronic states by the molecule/surface interactions. The classical results showed that at least semiquantitatively, the crystal modes are unlikely to affect the process on the timescale pertinent to the calculated line shape.
Reduced dimensionality diatom–diatom reactive scattering: Application to a model H2+A2→H+HA2 reaction
Authors: Sun Qiyan , Bowman Joel M.
Published: July 2002

We apply a recently formulated quantum theory of diatom–diatom reactions [Q. Sun and J. M. Bowman, Int. J. Quant. Chem., Quant. Chem. Symp. 23, 9 (1989)] to a model collinear H2+A2→H+HA2 reaction, where A has the mass of a hydrogen atom. The theory assumes one diatom bond is nonreactive, and the reactive scattering Hamiltonian is written in terms of hyperspherical and cylindrical coordinates. The potential-energy surface used is the PK2 H+H2 surface augmented by a harmonic degree of freedom describing the nonreactive A2. Details of the formulation and solution of the coupled-channel equations are given, along with convergence tests, and a discussion of the new state-to-state transition probabilities. In particular, the partial quenching of the well-known collinear H+H2 resonances is noted.
Studies of alkaline earth and transition metal M++ gas phase ion chemistry
Authors: Blades Arthur T. , Jayaweera Palitha , Ikonomou Michael G. , Kebarle Paul
Published: July 2002

A breakthrough into the hitherto inaccessible alkaline earth and transition metal M++ gas phase ion chemistry is reported. Ions M++(L)n, where M++(Mg++, Ca++, Sr++, Ba++, Mn++, Fe++, Co++, Ni++, and Zn++) and L=H2O could be produced. The hydrate equilibria M++(H2O)n−1+H2O=M++(H2O)n (n−1, n), were determined for Mg++, Ca++, Sr++, Mn++, and Co++. These lead to successive ion–H2O binding energies for high n, i.e., n=8–13 which are in the 15 kcal/mol range. The above hydrates and many other ion–ligand complexes could be produced by transferring the ions from liquid solution into the gas phase by means of electrospray. The ions were detected with a triple quadrupole mass spectrometer. The much stronger inner shell ion–ligand interactions can be studied by collision-induced dissociation in the triple quadrupole. Single ligand loss gives way to charge reduction at low n. Thus the M++(H2O)n give MOH+(H2O)k+H3O+ at a low n. The n for which reduction occurs decreases as the second ionization energy of M decreases. Ligands such as DMSO and DMF lead to charge reduction at a lower n than that observed for H2O. For these ligands, the charge reduction occurs via simple charge transfer, i.e., Cu++(DMSO)3=Cu+(DMSO)2+DMSO+.
X-ray and thermal studies on the rotator phases of normal higher alcohols C17H35OH, C18H37OH, and their mixtures
Authors: Yamamoto Takashi , Nozaki Koji , Hara Tetsuhiko
Published: July 2002

The structure of the rotator phase and the phase transitions in n-alcohols C17, C18, and their mixtures are studied by differential scanning calorimetry (DSC) and by x-ray diffraction. Both the heating and cooling phase diagrams of the binary system are determined. During cooling, a new intermediate phase α′ is observed under a temperature region of the usual rotator phase α. These α and α′ phases are found to correspond to the rotator-II phase and the rotator-I phase, respectively, of n-paraffins. The dynamical crystal structure in the α phase is revealed by investigation of the diffuse scattering from the single crystal. The characteristic diffuse scattering observed originates from molecular fluctuations due to the translation along and the rotation around the molecular axes. Both modes of the molecular motions are studied separately from the diffuse scattering on the first and on the second layer lines. The translational disorder is described by the averaged relative translation between the nearest neighbor molecules; it is found to be as much as 20% of the repeat period 2.55 A of the polymethylene chain. On the other hand, it is found that the rotational disorder of the molecules is much smaller than the translational. The molecules are found to have a tendency to make their molecular planes parallel. It is suggested that the crystal in the rotator phase consists of domains, within which the molecules are ordered with respect to the orientations of their molecular planes but are making fairly active translations. Such structural model of the rotator phase is found to be quite consistent with that obtained in our recent computer simulation.
A nonlocal free-energy density-functional approximation for the electrical double layer
Authors: Mier-y-Teran L. , Suh S. H. , White H. S. , Davis H. T.
Published: July 2002

We construct a free-energy density-functional approximation for the primitive model of the electrical double layer. The hard-sphere term of the free-energy functional is based on a nonlocal generic model functional proposed by Percus. This latter model functional, which is a generalization of the exact solution for the nonuniform hard-rod model, requires as input the free energy of a homogeneous hard-sphere mixture. We choose the extension of the Carnahan–Starling equation of state to mixtures. The electrostatic part of the nonuniform fluid ion–ion correlations present in the interface is approximated by that of a homogeneous bulk electrolyte. Using the mean spherical approximation for a neutral electrolyte, we apply the theory to symmetrical 1:1 and 2:2 salts in the restricted primitive model. We present comparisons of density profiles and diffuse layer potentials with Gouy–Chapman theory and Monte Carlo data. We also compare our results with data from other recent theories of the double layer. For highly charged surfaces, the profiles show the layering of counterions and charge inversion effects, in agreement with Monte Carlo data.
A molecular beam study of the H+N3 reaction. Product NH internal state distribution and electronic state branching ratio
Authors: Chen Jing , Quiñones Edwin , Dagdigian Paul J.
Published: July 2002

The H+N3→NH(X 3∑−,a 1Δ, b 1∑+)+N2 reaction has been studied in a molecular beam-gas scattering arrangement in order to determine the nascent product state distribution. The NH product in specific rovibronic/fine-structure states has been detected by laser fluorescence excitation. The relative cross sections for formation of various vibrational levels in the a 1Δ electronic state were determined to equal 1:1.0±0.3:1.4±0.3:≤1.5 for v=0 through 3, inclusive, while the v=0 to v=1 population ratio in the X 3∑− state was found to be 1:0.015±0.003. The rotational distributions in all vibronic levels were found to be characterized by temperatures near 300 K, suggestive of relaxation of the nascent rotational distributions. By comparison of the populations of a specific pair of X 3∑− and a 1Δ state levels and with summation over the derived rovibrational distributions, an electronic state branching ratio of 3.2±1.3 was obtained for the X 3∑− to a 1Δ electronic state branching ratio. An upper limit of ≤0.02 was also derived for the ratio of the b 1∑+ v=0 to a 1Δ v=0 populations. These results are compared with NH fragment distributions observed in the photodissociation of HN3(X̃ 1A′) and with our expectations based on our fragmentary knowledge of HN3 potential energy surfaces.
Raman scattering study of LuFeO3
Authors: Venugopalan S. , Becker Margaret M.
Published: July 2002

The zone-center magnon modes of LuFeO3 have been studied by first-order Raman scattering. At room temperature, the two magnon modes (M1 and M2) associated with the canted, antiferromagnetically ordered Fe3+ moments are seen at 18.5 and 22 cm−1, respectively. Their corresponding symmetries, determined through polarization measurements on single crystals, are consistent with the magnetic point group (m′m′m) of the crystal. The distinct scattering observed from phonons is also discussed in the context of the dependence of the phonon spectra on the mass of the rare-earth atom.
Nonradiative decay pathways of electronic states of group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation
Authors: Creasey J. C. , Lambert I. R. , Tuckett R. P. , Codling K. , Frasinski L. J. , Hatherly P. A. , Stankiewicz M. , Holland D. M. P.
Published: July 2002

The nonradiative decay channels of the valence electronic states of the gas-phase tetrahedral ions CF+4, SiF+4, CCl+4, SiCl+4, and GeCl+4 have been studied in the range 35–100 nm by a novel form of photoionization mass spectrometry. Tunable vacuum UV radiation from a synchrotron source ionizes the parent neutral molecule, and electrons and ions are detected by the photoelectron–photoion coincidence technique. The experiment is repeated continuously as a function of photon energy, and a three-dimensional histogram of photon energy versus ion time of flight versus coincidence count rate is produced. By taking cuts through this histogram, photoionization curves for the different fragment ions can be extracted. The appearance energies of the fragment ions (e.g., CF+2 from CF4, CCl+ from CCl4) occur at the adiabatic ionization potential of an electronic state of the parent ion, and not at the thermodynamic appearance energy of that ion. Attempts to measure the kinetic-energy releases in the fragmentation pathways have only been partially successful. The results are complementary to those of recent experiments to probe the radiative decay of these electronic states of MX+4 [M=C, Si, Ge; X=F, Cl] [J. Chem. Phys. 89, 2675 (1988); 89, 2683 (1988)], where the C̃ 2T2 and D̃ 2A1 third and fourth excited electronic states can show radiative decay to a surprising degree. The decay dynamics of the C̃ and D̃ states of MX+4 are reviewed. In general, the fluorides show different behavior to the analogous chlorides, and the carbon species behaves differently to the corresponding silicon or germanium species.
Calculation of van der Waals spectra for H2HF, D2HF, and H2DF
Authors: Clary D. C. , Knowles P. J.
Published: July 2002

Calculations of spectra for the excitation of the van der Waals modes in the weakly bound complexes H2HF, D2HF, and H2DF are reported. An ab initio potential energy surface has been computed using the coupled electron pair approximation with a large basis set. The rovibrational bound states for the complexes are calculated using a variational method. A self-consistent field approach is used to optimize the basis sets for bending and stretching motion in the van der Waals complex. The calculated spectra compare very well with those measured in near-infrared experiments. The computations provide a systematic analysis of the relative stability and rigidity of the different complexes, the very large zero-point energy effects, and the unusual nature of the rovibrational wave functions in these van der Waals molecules. The predicted spectra contain some new bands with surprisingly large intensities that might be detectable in experiments.
Kinetics and mechanism studies of switching and association reactions involving Na+–ligand complexes
Authors: Yang X. , Castleman A. W.
Published: July 2002

The kinetics and mechanisms of the reactions of Na+⋅(X)n=0–3, X=water, ammonia, and methanol with CH3CN, CH3COCH3, CH3CHO, CH3COOH, CH3COOCH3, NH3, CH3OH, and CH3–O–C2H4–O–CH3(DMOE) were studied at ambient temperature under different pressures. All of the switching (substitution) reactions proceed at near-collision rate and show little dependence on the flow tube pressure, the nature and size of the ligand, or the type of core ions. Interestingly, all of the measured rate constants agree well with predictions based on the parametrized trajectory calculations of Su and Chesnavich [J. Chem. Phys. 76, 5183 (1982)]. The reactions of the bare sodium ion with all neutrals proceed via a three-body association mechanism and the measured rate constants cover a large range from a slow association reaction with NH3, to a near-collision rate with DMOE. The lifetimes and the dissociation rate constants of the intermediate complexes deduced using the parametrized trajectory results, combined with the experimentally determined rates, compare fairly well with predictions based on RRKM theory. The calculations also account for the large isotope effect observed for the clustering of ND3 and NH3 to Na+.
Photodissociation dynamics of jet-cooled ClNO on S1(1 1A″): An experimental study
Authors: Ogai A. , Qian C. X. W. , Reisler H.
Published: July 2002

We report measurements of photofragment yield (PHOFRY) spectra and NO E, V, R distributions following dissociation of jet-cooled ClNO on the S1(1 1A″) electronic surface. The dissociative S1(1 1A″)←S0(1 1A′) transition shows diffuse vibrational structure with a progression in ν1, the NO stretch. The absorption and PHOFRY spectra consist of two bands, corresponding to excitations into S1(000) and S1(100), whose widths are 1300±100 and 1000±70 cm−1, respectively. The relative partial absorption cross sections are S1(000):S1(100)=2.3:1.0. The narrowing of the absorption bands with increasing ν1 quanta is a consequence of the mismatch between ν1 and the free NO vibrational frequency. Dissociations on S1(000) and S1(100) yield NO in v″=0 and 1, respectively. The NO(X2∏) rotational distributions in v″=0 and 1 are inverted, peaking at J″∼30.5 with widths of 10±1 J″, and they do not vary significantly when the photolysis laser is scanned across the absorption band. The evolution of NO vibrational and rotational excitations appear to be largely uncoupled. In NO v″=0 and 1, the upper spin–orbit state 2∏3/2 is more populated than the lower state 2∏1/2. For both v″=0 and 1, the Λ-doublet ∏(A″) component of NO(2∏1/2) is more populated than the ∏(A′) component by a ratio of ∼3:1, as expected for excitation to a π* orbital of a″ symmetry, but this propensity is much lower for NO(2∏3/2), possibly due to perturbations with another surface. The absorption spectra and NO V, R distributions are in good agreement with recent dynamical calculations on a three-dimensional (3-D) potential-energy surface (PES) calculated ab initio. The vibrational distribution appears to be determined near the Franck–Condon (FC) region, while final-state interactions affect the rotational distributions at larger Cl–NO separations.
Quantum mechanics in phase space: New approaches to the correspondence principle
Authors: Torres-Vega Go. , Frederick John H.
Published: July 2002

We present a time evolution equation that provides a novel basis for the treatment of quantum systems in phase space and for the investigation of the quantum-classical correspondence. Through the use of a generalized Husimi transform, we obtain a phase space representation of the time-dependent Schrödinger equation directly from the coordinate representation. Such an equation governs the time evolution of densities such as the Husimi density entirely in phase space, without recourse to a coordinate or momentum representation. As an application of the phase-space Schrödinger equation, we compute the eigenfunctions of the harmonic oscillator in phase space, relate these to the Husimi transform of coordinate representation eigenstates, and investigate the coherent state, its time evolution, and classical limit (ℏ→0) for the probability density generated by this state. Finally, we discuss our results as they relate to the quantum-classical correspondence, and quasiclassical trajectory simulations of quantum dynamics.
Fourier transform far infrared spectroscopy of the ν′3 vibration of SiC2 in Ar at 10 K
Authors: Presilla-Márquez J. D. , Graham W. R. M. , Shepherd Richard A.
Published: July 2002

A Fourier transform study of the vibrational spectrum of SiC2 produced by vaporizing mixtures of silicon and carbon-12 or carbon-13 at 2900 K and quenching the products in argon at 10 K, has enabled the identification for the first time of the ν″3(b2) vibrational mode, which the results of an earlier matrix study had suggested should lie in the far infrared. The assignment of a frequency observed at 160.4 cm−1 to the ν″3 mode is confirmed by isotopic data and supported by the predictions of ab initio calculations. Optimized force constants have been derived using the frequencies of the newly assigned mode, the previously reported, ν″1(a1)=1741.3 and ν″2(a1)=824.3 cm−1 vibrations, and their values on single and double carbon-13 substitution. Two models, cyclic and T-shaped, are discussed for the molecule, which is of C2V symmetry.
Vibronic coupling and other many-body effects in the 4σ−1g photoionization channel of CO2
Authors: Roy P. , Bartlett R. J. , Trela W. J. , Ferrett T. A. , Parr A. C. , Southworth S. H. , Hardis J. E. , Schmidt V. , Dehmer J. L.
Published: July 2002

Vibrational branching ratios and photoelectron angular distributions were measured for 4σ−1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C̃ 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B̃ 2Σ+u state. This behavior suggests that vibronic coupling to the B̃ 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C̃ 2Σ+g state photoelectron spectrum. We also observe evidence for other many-body effects—shape-resonance-induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data.
An efficient microcanonical sampling procedure for molecular systems
Authors: Schranz Harold W. , Nordholm Sture , Nyman Gunnar
Published: July 2002

A general and efficient microcanonical sampling (EMS) procedure based on that of Severin et al. [Chem. Phys. Lett. 57, 117 (1978)] is outlined. The relevant statistical weights are derived. The EMS method can be used for the generation of initial microcanonical states for trajectories or directly in the efficient calculation of microcanonical statistical averages and rate constants. The validity and usefulness of the EMS method is demonstrated in applications to 1D linear chains and 3D molecules such as H2O, SiH2, and Si2H6.
Reaction path study of helix formation in tetrapeptides: Effect of side chains
Authors: Choi Chyung , Elber Ron
Published: July 2002

A computational protocol to calculate steepest descent paths in flexible molecules is discussed in detail. The algorithm does not use second derivatives and related matrices, and is therefore suitable for large systems. The shortest reaction coordinate from the helix to the extended chain conformation is calculated for a series of different tetrapeptides. The formation of a helical turn is investigated as a function of side chain properties. The known reaction coordinate for isobutyryl-ala3 -NH-methyl is employed as a starting guess in path calculations for six different tetrapeptides. The main results are: (i) χ1 (the side chain orientation angle) does not change significantly along the reaction coordinate. Alternative static values of χ1 (60,−60) significantly affect the size of the energy barrier (≊3 kcal/mol for valine). (ii) The mechanism for the transition is similar in all the peptides examined and is based on sequential flips of backbone torsions (Ψ dihedral angles). (iii) The energy barrier which is associated with the flip of Ψi is primarily determined by the side chain of residue i. Since the barriers for χ1 transitions are comparable to the barriers for conformational transitions in the backbone, the energetics of the helix formation is strongly affected by the side chain orientation angle. As a general conclusion, the side chain orientation angle affects more strongly the transition states than energy minima.
Manifestations of latent space group symmetry in the vibrational spectra of MSO4; where M=Ba, Sr, and Pb
Authors: Jayasooriya Upali A. , Kettle Sidney F. A. , Mahasuverachai Sujit , Al-Jowder Osama
Published: July 2002

The vibrational spectra of crystalline MSO4 where M=Ba, Sr, and Pb are reinterpreted in terms of a vibrational unit cell (Imma) which is one-half the size of the crystallographic (Pnma) unit cell.
Investigation of internal rotor dynamics of NeDCl and ArDCl via infrared absorption spectroscopy
Authors: Schuder Michael D. , Nelson David D. , Nesbitt David J.
Published: July 2002

The van der Waals complexes, NeDCl and ArDCl, are produced in a slit jet supersonic expansion and observed via direct absorption of tunable mid-infrared Pb-salt diode laser radiation. For the NeDCl complex, the DCl stretch fundamental [ν0=2091.3717(4) cm−1 ] and the DCl Σ and Πe, f bend combination bands [ν0=2099.5760(4) and 2104.9465(4) cm−1, respectively] are observed. The DCl stretch fundamental and Πe, f combination band are observed for ArDCl at 2089.4180(2) and 2117.4443(3) cm−1, respectively. The relative fundamental vs bend combination band intensity distributions are very different for the two complexes. The ArDCl fundamental to Π bend combination band intensity ratio is 4:1, whereas for NeDCl the corresponding ratio is 1:8. This anomalous intensity pattern for NeDCl and the proximity of the bend combination bands to the DCl R(0) line indicate that the DCl diatomic is exhibiting nearly free rotation within this complex, compared to more restricted librational motion of DCl in ArDCl. Strong Coriolis interactions between Πe and Σ bend states are observed for both complexes and analyzed quantitatively for NeDCl. The observed NeDCl and ArDCl absorption linewidths are only slightly larger than the instrumental limit determined from nearby OCS monomer absorptions in the slit jet, but the differences are not of high statistical significance. This FWHM of the observed transitions dictates a rigorous lower limit to the vibrational predissociation lifetime of 3 ns. Experimentally determined rotational constants, vibrational frequencies, and relative intensities are compared to predictions based on existing empirical potential surfaces.
Statistical and nonstatistical effects in bond fission reactions of SiH2 and Si2H6
Authors: Schranz Harold W. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

An efficient implementation of microcanonical, classical variational transition-state theory based on the use of the efficient microcanonical sampling (EMS) procedure is applied to simple bond fissions in SiH2 and Si2H6 using recently constructed global potential-energy surfaces. Comparison is made with results of trajectory calculations performed on the same potential-energy surfaces. The predictions of the statistical theory agree well with and provide an upper bound to the trajectory derived rate constants for SiH2→SiH+H. In the case of Si2H6, agreement between the statistical theory and trajectory results for Si–Si and Si–H bond fission is poor with differences as large as a factor of 72. Moreover, at the lower energies studied, the statistical calculations predict considerably slower rates of bond fission than those calculated from trajectories. These results indicate that the statistical assumptions inherent in the transition-state theory method are not valid for disilane in spite of the fact that many of the mode-to-mode rate constants for intramolecular energy transfer in this molecule are large relative to the Si–Si and Si–H bond fission rates. There are indications that such behavior may be widespread among large, polyatomic molecules.
Theory of spin relaxation by diffusion on curved surfaces
Authors: Halle Bertil
Published: July 2002

A comprehensive theoretical analysis is presented which accounts for nuclear spin relaxation due to molecular surface diffusion on spheroidal aggregates undergoing restricted rotational diffusion in a uniaxial potential of mean torque. The model is formulated primarily with surfactant systems in mind, and should provide a realistic description of spin relaxation in nematic and smectic liquid crystals as well as in isotropic micellar solutions. Two efficient numerical schemes are developed to compute the joint time correlation functions and spectral density functions. In addition, a variety of useful analytical results are derived for special cases. Numerical calculations suggest that nuclear spin relaxation may be a unique method for monitoring small deviations from spherical aggregate shape.
Electron-paramagnetic-resonance study of the structure of C4 in solid Ne
Authors: Jiang Qian , Graham W. R. M.
Published: July 2002

The electron-paramagnetic-resonance spectrum of C4 in a neon matrix at ∼4 K formed by the vacuum-ultraviolet photolysis of 1,3-butadiene has been investigated to determine if there is any evidence of the bending of the molecule, which had previously been indicated by the results of EPR and Fourier-transform infrared measurements made on C4 trapped in argon at 10–12 K. The observed spectra, which include 13C hyperfine splitting and zero-field splitting data, confirm the splitting of the perpendicular xy1 and xy2 lines of triplet C4 into separate x and y components, which is indicative of the inequivalent axes of a bent molecule.
Picosecond measurements of phenol excited-state proton transfer in clusters. I. Solvent basicity and cluster size effects
Authors: Syage J. A. , Steadman J.
Published: July 2002

Excited-state proton transfer (ESPT) rates in molecular clusters were measured as a function of cluster size using picosecond spectroscopy in a molecular beam mass spectrometer. ESPT from the S1 state of phenol to base solvent clusters (NH3)n occurs for a critical solvent cluster size n≥5, with a rate constant of k=(60±10 ps)−1 for n=5–7. ESPT showing critical cluster-size dependencies was also observed in the basic solvent N(CH3)3(n≂3). Proton transfer was not observed in the less-basic solvent clusters (CH3OH)n and (H2O)n. Mixed-solvent studies indicate that the addition of a dissimilar molecule to an otherwise neat solvent cluster impedes ESPT, presumably due to a disruption of the hydrogen bonding network. Evidence is also presented for the direct measure of solvent reorganization following ESPT. For (NH3)n solvation, the solvent reorganization appears as a long-time-scale component (0.3 ns) on the protonated solvent formation traces.
The longitudinal-optic–tranverse-optic mode splitting in ice Ih
Authors: Klug D. D. , Tse J. S. , Whalley E.
Published: July 2002

The magnitude of the longitudinal–transverse optic mode splitting on the 229 cm−1 infrared active band in the translational region of ice Ih is about 10 cm−1 as determined from an analysis of experimental infrared data. This agrees well with a recent calculation, and these results are used to give a precise assignment of the features in the vibrational density of states of ice Ih in the region 200–320 cm−1.
Rydberg series of charge-transfer excitations: Cl and H in rare gas crystals
Authors: Kunz H. , McCaffrey J. G. , Chergui M. , Schriever R. , Ünal Ö. , Stepanenko V. , Schwentner N.
Published: July 2002

Rydberg series observed in the excitation spectra of Cl and H atoms isolated in the rare gas (Rg) crystals Kr and Xe are associated with the charge transfer species Cl−Rg+ and H−Rg+. The progressions originate from hole states of Rg+ which converge to the top of the rare gas valence band in an analogous manner to the convergence of conventional exciton states to the bottom of the conduction band. A model based on the effective mass approximation and a quantum defect concept predicted such progressions in Xe and Kr crystals and it is supported by the present results.
Photochemistry of adsorbed molecules. IX. Ultraviolet photodissociation and photoreaction of HBr on LiF(001)
Authors: Bourdon E. B. D. , Cho C. -C. , Das P. , Polanyi J. C. , Stanners C. D. , Xu G. -Q.
Published: July 2002

The dynamics of the UV photochemistry of HBr on LiF(001) has been studied by angle-resolved time-of-flight mass spectrometry in ultra-high vacuum. Single-photon photodissociation of adsorbed HBr at 193 nm resulted in photofragment translational energy distributions that differed from those produced in the gas-phase photolysis. Angular distributions of the fast H-atom photofragments peaked at 55±5° to the surface normal, consistent with a preferentially oriented adsorbate geometry. The angular distribution of atomic H obtained from photodissociation of HBr(ad) using polarized light indicated that a substantial fraction of the H atoms collided with the surface before leaving it. Two types of photoreactions in the adsorbed state were observed. Molecular hydrogen was formed in the photoinitiated abstraction reaction, H+HBr(ad)→H2(g)+Br, and its markedly non-Boltzmann translational energy distribution was found to have less energy than would be consistent with gas-phase experiments (performed elsewhere). Photoproducts from the bimolecular reaction 2HX(ad)→H2+X2, X=Cl, Br were also observed in the present study. This photoreaction, which did not depend on prior photodissociation of HX(ad), is thought to proceed through electronic excitation of an HX dimer in the adsorbed state.
Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of HCl via the F 1Δ2 Rydberg state
Authors: Wang Kwanghsi , McKoy V.
Published: July 2002

Results of studies of rotational ion distributions in the X 2Π3/2 and X 2Π1/2 spin-orbit states of HCl+ resulting from (2+1′) resonance enhanced multiphoton ionization (REMPI) via the S(0) branch of the F 1Δ2 Rydberg state are reported and compared with measured threshold-field-ionization zero-kinetic-energy spectra reported recently [K. S. Haber, Y. Jiang, G. Bryant, H. Lefebvre-Brion, and E. R. Grant, Phys. Rev. A (in press)]. These results show comparable intensities for J+=3/2 of the X 2Π3/2 ion and J+=1/2 of the X 2Π1/2 ion. Both transitions require an angular momentum change of ΔN=−1 upon photoionization. To provide further insight into the near-threshold dynamics of this process, we also show rotationally resolved photoelectron angular distributions, alignment of the ion rotational levels, and rotational distributions for the parity components of the ion rotational levels. About 18% population is predicted to occur in the (+) parity component, which would arise from odd partial-wave contributions to the photoelectron matrix element. This behavior is similar to that in (2+1) REMPI via the S(2) branch of the F 1Δ2 state of HBr and was shown to arise from significant l mixing in the electronic continuum due to the nonspherical molecular ion potential. Rotational ion distributions resulting from (2+1) REMPI via the S(10) branch of the F 1Δ2 state are also shown.
Photodissociation of alkyl iodides in solution: Substituent effects on the early-time dynamics
Authors: Phillips David L. , Myers Anne B.
Published: July 2002

Resonance Raman spectra, including absolute scattering cross sections, have been measured for ethyl, isopropyl, and tert-butyl iodides in cyclohexane solution at seven to ten wavelengths between 303 and 200 nm. Spectra of fully deuterated ethyl iodide have also been obtained at five wavelengths. Spectra excited in the 300–250 nm region, on resonance with the directly dissociative A state, are dominated by long overtone progressions in the nominal C–I stretching mode near 500 cm−1. In all three molecules the fundamental of the C–I stretch is unexpectedly weak relative to the overtones when excited near the peak of the A band. This is shown to arise from interference between the A-state resonant part of the fundamental Raman amplitude and preresonant contributions from higher electronic states. In addition to the C–I stretching activity, A-state excitation generates significant intensity in fundamentals, overtones, and combination bands of modes nominally assigned as bending and CC stretching vibrations, suggesting a multidimensional character to the reaction coordinate. The absorption spectra and A-state resonant Raman intensities are modeled successfully through wave-packet propagation on a multidimensional locally harmonic potential with a preresonant contribution to the fundamental intensities included. The short-time photodissociation dynamics are then examined by using the normal-mode coefficients to convert the wave-packet motion from dimensionless normal coordinates into internal coordinates. It is found that while the dominant motion during the first 10 fs involves stretching of the C–I bond, other stretching and bending motions are also involved, although the precision of these conclusions for isopropyl and tert-butyl iodides is limited by the indeterminacy in the signs of the normal-mode displacements obtained from the intensity analysis. Comparison of the results for normal and perdeuterated ethyl iodide is used to resolve most of the sign indeterminacies for this molecule. The present results are compared and contrasted to conclusions of previous studies of energy partitioning in the vapor-phase photodissociation.
Fourier transform vibrational spectroscopy of Si2C in solid Ar
Authors: Presilla-Márquez J. D. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared study of the vibrational spectrum of Si2C produced by vaporizing mixtures of silicon and carbon-12 or carbon-13 and quenching the products in Ar at 13 K has confirmed a previously observed vibration at 1188.4 cm−1 as the ν″3(b2), antisymmetric Si–C stretching mode, and resulted in the identification of a new vibration at 839.5 cm−1 as the ν1(a1), symmetric Si–Si stretching fundamental. No bending mode has been observed; however, the identification of an absorption at 1354.8 cm−1 as the ν″2(a1)+ν3(b2) combination band is proposed. The results are confirmed by 13C, 29Si, and 30Si isotopic data and are in excellent agreement with the predictions of ab initio calculations carried out by Rittby in collaboration with this work. Force constant adjustment calculations confirm the ground state geometry as a floppy, bent symmetrical structure.
Model calculations for halogen-etching of GaAs and InSb. Potential-energy surfaces for GaAs+Cl, GaAs+Br, InSb+Cl, and InSb+Br reactions
Authors: Balasubramanian K. , Tao J. X. , Liao D. W.
Published: July 2002

We have investigated the potential-energy surfaces of GaAs+Cl, GaAs+Br, InSb+Cl, and InSb+Br reactions as a function of electronic state and orientation. We employed a complete active space multiconfiguration self-consistent field followed by multireference configuration interaction calculations to study the potential-energy surfaces of these species. We find that the collinear mode of approach in which the halogen atom approaches the group-III atom (Ga, In) is the most favorable approach. The loosely bound Cl–Ga...As (Br–Ga...As) complex is shown to be formed which would undergo ready desorption to emit mostly GaCl (GaBr). Our computed potential-energy surfaces explain the observed laser-induced desorption products in halogen-etching of a GaAs surface as well as HCl-etching of GaxAs−y. We also computed the spectroscopic constants of GaX, InX, AsX, and SbX (X=Cl, Br).
Measurement of the quantum yield of triplet formation and short triplet lifetimes by the transient grating technique
Authors: Terazima Masahide , Hirota Noboru
Published: July 2002

A transient grating method for measuring the quantum yield of triplet formation (Φisc) and lifetimes of short-lived triplet states is proposed. The method is based on a measurement of time dependence of a thermal grating signal with a nanosecond pulsed laser and a cw probe laser. The validity of the method is demonstrated by determining Φisc of a number of molecules whose Φisc are known. Advantages and limitations of this method are discussed in detail.