bluuebunny/crossref_metadata_2025 huggingface dataset:
On the absorption spectrum of the 3s Rydberg state of acetone
Authors: McDiarmid Ruth
Published: July 2002

The optical absorption spectra of acetone and acetone-d6 in the 3s Rydberg region have been remeasured and reanalyzed. Activity is observed in the a1 CH3 deformation, CH3 rock, and CC stretching vibrations, as before. The two low frequency displacement subbands of the transition, +250 and +309 cm−1 have been reassigned; the former is twice the a2 CH3 torsion, the latter cannot be assigned. Possible photophysical implications of the large frequency shifts in the methyl-centered modes are discussed.
Potential energy surfaces for the insertion of Ta and Ta+ into H2
Authors: Dai D. G. , Cheng W. , Balasubramanian K.
Published: July 2002

We compute the bending potential energy surfaces of 12 electronic states of TaH2 and TaH+2 using the complete active space multiconfiguration self-consistent field (CAS-MCSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations. Spin–orbit effects are also included using the relativistic configuration interaction (RCI) approach. We find that the 4F ground state of Ta atom requires a barrier of ∼24 kcal/mol for insertion into H2 while the 5F ground state of Ta+ does not insert into H2. The low-spin excited states of Ta and Ta+ are considerably more reactive with H2. We find three nearly-degenerate bent electronic states of 4B1, 4A2, and 4B2 symmetries as the candidates for the ground state of TaH2. Likewise 3B1 and 3A1 electronic states of TaH+2 are nearly-degenerate candidates for the ground state. The spin–orbit coupling strongly mixes some of these states leading to bond angle changes of up to 10°.
The structure and dynamics of silica polymorphs using a two-body effective potential model
Authors: Tse John S. , Klug Dennis D.
Published: July 2002

The performance of a recently proposed two-body potential model for SiO2 [van Beest et al., Phys. Rev. Lett. 64, 1995 (1990)] was critically evaluated through the calculation of the static and dynamical properties of several polymorphs of SiO2 using molecular dynamics methods. It was found that the calculated static structures are in excellent agreement with experiments. In particular, the pressure–volume equations of state for α-quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in α-quartz and thermally induced the α→β transformation in cristobalite are well reproduced by this model. The calculated vibrational spectra are in fair agreement with experiments. The strengths and the weaknesses of the potential will be presented and discussed.
Molecular-dynamics studies of adsorption and transport in micropores of different geometries
Authors: Demi T.
Published: July 2002

We report molecular-dynamics simulation results on adsorption and self-diffusion of Lennard-Jones fluids confined in cylindrical and ‘‘sphere–cylindrical’’ pore geometries. The results include local-density profiles for a wide range of temperatures and densities for both models, dependence of the self-diffusion coefficient parallel to the pore walls D*∥ on temperature, density, and in the case of the sphere–cylinder pore geometry on the ratio of the radius of the sphere to the radius of the cylinder Rs/Rc.
Can the independent binary collision theory describe the nonlinear solvent density dependence of the vibrational energy relaxation rate?
Authors: Dardi Peter S. , Cukier R. I.
Published: July 2002

Serious doubt has been cast on the validity of the independent binary collision (IBC) theory for describing nonlinear solvent density effects on the rate of vibrational to translational and rotational energy transfer [P. S. Dardi and R. I. Cukier, J. Chem. Phys. 89, 4145 (1988)]. Here, we compare the IBC theory predictions with experiment. Inconsistencies in previous applications of the IBC theory are discussed. Correcting these inconsistencies, we find that the agreement between the IBC theory and experiment is not good. Furthermore, we find that the collision diameter needed to obtain the best fit with experiment is significantly temperature dependent. We conclude that the IBC theory is not a valid theory to describe solvent density effects on vibrational relaxation.
Confined geometry effects on reorientational dynamics of molecular liquids in porous silica glasses
Authors: Liu G. , Li Y. , Jonas J.
Published: July 2002

This work investigates the relative role of the pure geometrical confinement and the strength of the surface effect on the dynamics of liquids in porous silica glasses prepared by the sol-gel process. The deuteron NMR spin-lattice relaxation times T1 of several molecular liquids in porous silica glasses are reported as function of pore size in the range from 18 to 143 Å over the temperature range from 260 to 310 K. Molecular liquids studied include strongly interacting polar liquids such as pyridine-d5, aniline-d5, and nitrobenzene-d5, whereas the saturated cyclic hydrocarbon liquids of cyclohexane-d12 and cis-decalin-d18 represent the weakly interacting liquids. In a first approximation, toluene-d1 and dioxane-d18 are chosen as examples of liquids with intermediate interactions with the silica surface. The experimental relaxation data are analyzed by using the two-state, fast-exchange model which is found to be valid for the strongly interacting liquids and liquids with intermediate interactions. In terms of this model, the viscosity of the surface layer for pyridine-d5 is about 30 times higher than that for bulk liquid pyridine.

The importance of the two-dimensional approach to describe motional dynamics of liquids confined to pores smaller than 30 Å is illustrated in the case of weakly interacting liquid of cyclohexane-d12. Additional information on the relative role of surface interactions and the pure topological effects on the dynamics of liquids in confined geometries was obtained by using surface-modified glasses in which the surface hydroxyl groups were replaced by OSi(CH3)2OC2H5 groups. Indeed, the effects of surface modifications on the 2H T−11 are most pronounced for strongly interacting liquids whereas they are absent for cyclohexane. In agreement with the concept of two-dimensional behavior of liquids in small pores, one finds that the low-frequency relaxation times, namely, the spin–spin relaxation time T2, and the spin-lattice relaxation time in the rotating coordinate frame, T1ρ, remain unchanged by surface modification. In fact, this is a consequence of logarithmic enhancement of the spectral density at low frequencies so that the effect of pure geometrical confinement on the T−12 and T−11ρ relaxation rates is much larger than any relaxation rate changes arising from surface modification. Several selected NMR T1 experiments on pyridine-d5 confined to anopore and zeolites are also presented.
Fluids in micropores. III. Self-diffusion in a slit-pore with rough hard walls
Authors: Diestler D. J. , Schoen Martin , Hertzner Alfred W. , Cushman John H.
Published: July 2002

Self-diffusion coefficients for a model slit-pore consisting of Lennard–Jones (12,6) fluid constrained between two plane-parallel rough hard walls have been computed by standard (microcanonical) molecular dynamics. Fluid molecules collide impulsively with the walls, their speeds remaining unchanged, but the directions in which they rebound from the walls are chosen probabilistically so that the walls behave as Maxwell’s ‘‘perfectly adsorbing’’ walls. For comparison, fluid contained in the same thermodynamic state by a pore with smooth walls has also been simulated. The local density profiles, which are the same in both rough and smooth-walled pores, show that the pore fluid forms distinct layers parallel with the walls. The rate of transverse diffusion (parallel with walls) within a given fluid layer depends markedly on its distance from the walls and on the nature of the walls. Transverse diffusion within the layer in contact with the rough wall is slower than that within the layer in contact with the smooth wall.
Cooperative diffusion in weakly charged polyelectrolyte solutions
Authors: Ajdari Armand , Leibler Ludwile , Joanny Jean-François
Published: July 2002

We study the dynamics of concentration fluctuations of a weakly charged polyelectrolyte solution in the limit where they are controlled by hydrodynamic interactions. We find three relaxation modes: a plasmon mode corresponding to the fast relaxation of charge fluctuations; a diffusive mode involving mainly the motion of the small ions (salt and counterions) to restore the ‘‘Donnan equilibrium,’’ and a slower cooperative diffusion mode corresponding to the breathing motion of the polyelectrolyte chains and ions. In the limit where the electrostatic effects are dominant, at zero wave vector, the cooperative diffusion constant has a nonmonotonic variation with the monomer concentration. Contrary to what happens in neutral polymer solutions, in the limit of low ionic strength, it decreases with the monomer concentration in agreement with recent experimental observations of Schosseler et al. on gels.The cooperative diffusion constant as a function of wave vector shows a minimum for a wave vector corresponding approximately to the peak in the structure factor.
Dissociative charge-transfer reactions of Ar+ with CFnCl4−n at thermal energy
Authors: Tsuji Masaharu , Funatsu Tsuyoshi , Kouno Hiroyuki , Nishimura Yukio , Obase Hiroshi
Published: July 2002

A flowing-afterglow apparatus coupled with a low pressure chamber was used for studying charge-transfer reactions of Ar+ with CFnCl4−n(n=0–4) at thermal energy. The product ion distributions and rate constants were determined. Near-resonant charge transfer followed by successive loss of F or Cl atom were major product channels observed. Slower rate constants for CFCl3 and CCl4 than those of CF4, CF3Cl, and CF2Cl2 were qualitatively explained by the absence of energy-resonant ionic state with favorable Franck–Condon factors. However, the relation kobsd>0.1kcalcd (Langevin or ADO) holds for all the reactions, indicating that the existence of favorable Franck–Condon factors is not a significant factor for assessing the magnitude of the Ar+/CFnCl4−n charge-transfer reactions.
Theoretical studies of the reactivity and spectroscopy of H+CO=HCO. I. Stabilization and scattering studies of resonances for J=0 on the Harding a
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Authors: Cho Seon-Woog , Wagner Albert F. , Gazdy Bela , Bowman Joel M.
Published: July 2002

We report stabilization and coupled-channel scattering calculations of isolated resonances for a triatomic molecule HCO using a global, ab initio potential energy surface. The lowest nine resonances are identified for total angular momentum J=0. The two set of calculations agree closely with each other on the resonance locations which are then compared to experimentally determined locations. These resonances show widths varying 10−6 to 100 cm−1. Pictures of resonance wave functions are presented for the proper assignment of quantum numbers of resonance states. A simple technique for locating very narrow resonances in scattering calculations is presented and a worked example shows large savings of computational efforts for scattering calculations. Partial widths and resonance lifetimes are obtained by analysis of the S matrices. Final state distribution probabilities are presented for resonance and direct scattering and the characteristics of this scattering are discussed.
Rotational transitions in O(3P)+H2(HD,DH,D2) reaction: Roles of coordinate transformation and rotationally inelastic half collisions for product distributions
Authors: Nakamura Masato
Published: July 2002

Rotational transitions in the O(3P)+H2 reaction and its isotopic variations are studied. Product rotational-state distributions are calculated using an independent-event approximation together with a simplified distorted-wave cross section for collinear-favored reactions, both of which have been proposed recently. The calculation quite well reproduces the product rotational distribution obtained by Schatz performing an accurate CSDW (coupled-state–distorted-wave) calculation. It is found that not only the coordinate transformation but also the rotationally inelastic half collisions play significant roles in determining the product distribution. The contributions of each mechanism depend strongly on the system; the first mechanism is much more important than the second one for the O+DH reaction, while the second mechanism dominates the O+HD reaction. The rotational rainbow feature in a half collision is found to appear in the product distribution.
Calculation of high order resonance interactions in C2HF using the cubic force field
Authors: Lehmann Kevin K.
Published: July 2002

Using ab initio cubic force constants, estimates are made for four higher order interaction matrix elements recently determined recently from the infrared spectrum. The predicted (observed) values for the matrix elements are as follows: 〈ν2‖Heff‖ν3 + 2ν±24Δe〉 = 6.4(5.4) × 10−5 cm−1 ×√J(J+1)[J(J+1)−2], 〈2ν3‖Heff‖3ν4 + ν5(∑+)〉 = −0.14(0.24) cm−1, 〈3ν4 + ν5(Δε)‖Heff‖2ν3〉 = 3.1(3.2) × 10−5 cm−1 × J(J + 1), 〈2ν2‖Heff‖ν1 + 2ν04〉 = 4.0(3.8) cm−1. The level of agreement between predicted and observed interaction matrix elements is excellent. This results supports the hope that even slow IVR rates may be predictable in terms of the low order anharmonic force constants which can be estimated by ab initio calculations.
Collisional dynamics of the BrF B 3Π(0+) state. I. Electronic quenching
Authors: Melton David W. , Perram Glen P. , Roh Won B.
Published: July 2002

Pulsed laser induced fluorescence techniques have been used to study electronic quenching of the B 3Π(0+) state of bromine monofluoride. BrF(B) deactivation rate coefficients were determined at room temperature for vibrational levels v′=2, 5, and 6 and for the following collision partners: Ar, SF6, CF4, N2, O2, CO2, and Br2. The quenching rate coefficients ranged from <6×10−14 cm3/molecule s for Ar to 6.9×10−11 cm3/molecule s for Br2. The radiative lifetime for v′=5 was determined as τr=59.6±0.4 μs. Double exponential decays were observed with molecular oxygen, indicating two distinct mechanisms for collisional deactivation. The BrF(B–X) total fluorescence increases at early times when the collision partner SF6 is added. This result is attributed to efficient vibrational relaxation to the states v′=1 and 0 which are detected most efficiently.
Emission spectrum of the CN(B 2Σ+–X 2Σ+) tail band system: B 2Σ+∼4Π perturbations in the v
B=9, 12, and 17 levels
Authors: Ito Haruhiko , Ozaki Yasushi , Suzuki Kaoru , Kondow Tamotsu , Kuchitsu Kozo
Published: July 2002

The CN(B 2Σ+–X 2Σ+) tail band system was observed in the reaction of argon metastable atoms with BrCN. The observed transitions in the 17-14 and 17-16 bands were analyzed in terms of the B 2Σ+,v=17∼A 2Πinv,v=34 and 4Π,v=p+11 perturbations, and the molecular constants for these levels and the interaction constants were determined. Intensity anomalies were also observed in the 9-8, 9-9, and 12-12 bands. The quantum numbers of the total angular momentum, J, of the perturbed rotational levels of B 2Σ+ were assigned as (vB,N;J)=(9,56;55.5) and (12,10;10.5). The vibrational quantum number was determined as p=0, and the crossing spin sublevels at the anomalies of vB=9 and 12 were determined as 4Π5/2 and 4Π1/2 by examination of the electronic part of the B 2Σ+∼4Π perturbation constant, HSOel, for the (vB,vΠ)=(14,6) and (17,11) perturbations estimated by use of the Rydberg–Klein–Rees (RKR) potentials for B 2Σ+ and 4Π.
Simulations of solvation dynamics in simple polar solvents
Authors: Neria Eyal , Nitzan Abraham
Published: July 2002

This paper describes the results of computer simulations of charge solvation dynamics in a Stockmayer solvent (Lennard-Jones spheres with point dipoles at their centers). The solvent molecules are characterized by mass and moment of inertia which can be varied independently, thus providing the possibility to study the separate effects of the rotational and translational solvent motions on the solvation process. We focus on the role played by these degrees of freedom, and on the contributions of different solvation shells around the solute to the solvation process in order to check the validity of recently proposed theories of solvation dynamics. We find that even in this structureless solvent, as in the more structured solvents studied earlier, inertial effects dominate the solvation process, and dielectric solvation theories which do not take into account these effects cannot describe the observed dynamics. The dynamic mean spherical approximation and generalized diffusion theories cannot account for the observed dynamics even when solvent translations are frozen.
State-to-state rotational energy-transfer measurements in the v2=1 state of ammonia by infrared–infrared double resonance
Authors: Abel Bernd , Coy Stephen L. , Klaassen Jody J. , Steinfeld Jeffrey I.
Published: July 2002

An infrared double-resonance laser spectroscopic technique is used to study state-resolved rotational (R–R, R–T) energy transfer in ammonia (14NH3) (self-collisions and between ammonia and foreign gases). NH3 molecules are prepared in selected rovibrational states of the v2=1 level using coincidences between CO2 -laser lines and ν2 fundamental transitions. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v,J,K) have been carried out using time-resolved tunable diode laser absorption spectroscopy. For NH3–NH3 collisions, measurements of total depopulation rates of selected JK states in v2=1 and ground-state recovery rates are found to be three and eight times larger, respectively, than the Lennard-Jones collision rate, in accord with theoretical expectations for polar molecules.

A kinetic master-equation analysis of time-resolved level populations yields state-to-state rate constants and propensity rules for NH3–NH3 and NH3–Ar collisions. Individual rotational energy-transfer rates in v2=1 are slower than in the vibrational ground state, but still comparable to the Lennard-Jones collision frequency. Our experiments show that rotational energy transfer in v2=1 is not governed by simple ‘‘dipolelike’’ selection rules. They show fast rotational energy transfer, which can be related to long-range interaction potentials, but at the same time considerable amounts of ΔJ=2 and 3, ΔK=0, and ΔJ=1–4, ΔK=3, transitions, which may be attributed to higher-order terms in the multipole expansion of the intermolecular potential. No pronounced symmetry-state correlation and no preferred pathways were found except the preference for relaxation within a K stack and the expected separate relaxation of different nuclear-spin species, which can be labeled by their K-quantum number. Rates of collision-induced symmetry change (a↔s) in v2=1 are on the order of kas=4 μs−1 torr−1, smaller than kas in the ground state, but over an order of magnitude larger than that recently reported in the literature. Depopulation rates for other collision partners (Ar, H2, N2, and He) can be understood in terms of the intermolecular potentials. Comparisons are made between the relaxation rates measured in this work and infrared pressure-broadening coefficients reported in the literature.
Product energy partitioning in the unimolecular decomposition of vibrationally and rotationally state-selected hydrogen peroxide
Authors: Luo X. , Rizzo T. R.
Published: July 2002

Infrared-optical double resonance prepares HOOH molecules in single rotational levels of the 6νOH, 5νOH+νOOH, 5νOH+νOO, and 4νOH+νOH′ vibrational states which range from 3 to 2287 cm−1 of excess energy above the unimolecular dissociation threshold. Laser-induced fluorescence probes the nascent OH rotational state distributions from the decomposition of rovibrationally selected reactants. The nascent rotational state distributions reveal that both OH spin–orbit states can be populated by the decomposition of a single molecule and hence that electronic angular momentum is not conserved throughout the dissociation process. The product state distributions from reactants excited to the 6νOH and 4νOH+νOH′ vibrational levels are generally in good agreement with the predictions of phase-space theory provided electronic angular momentum is treated statistically. Reactants decomposing from single rotational states in the 5νOH+νOOH combination level (and to a lesser extent the 5νOH+νOO level) show product state distributions which are systematically colder than phase-space theory predictions. This observation indicates that energy redistribution in vibrationally excited HOOH is not complete on the time scale of unimolecular decomposition.
Analysis of the dipole polarizabilities of polyenes
Authors: Grant A. J. , Pickup B. T.
Published: July 2002

We present an analysis of ab initio self-consistent-field level linear-response properties in terms of one-electron and inductive effects. The population analysis of the perturbed density is used to discuss mechanisms of response. Polyene oligomers are used as an example.
Ab initio adiabatic and diabatic potential-energy curves of the LiH molecule
Authors: Boutalib A. , Gadéa F. X.
Published: July 2002

For nearly all states below the ionic limit [i.e., Li(2s, 2p, 3s, 3p, 3d, 4s, and 4p)+H] we perform the first adiabatic and diabatic studies. This treatment involves a nonempirical pseudopotential for Li and a full configuration-interaction treatment of the valence-electron system. Core–valence correlation is taken into account according to a core-polarization-potential method. We present an analysis of the diabatic curves and introduce appropriate small corrections accounting for basis-set limitations. For the low-lying states, our vibrational level spacings and spectroscopic constants are in excellent agreement with the available experimental data and with the best all-electron results. Experimental suggestions are given for the higher states.
Neighborship partition of the radial distribution function for simple liquids
Authors: Mazur Stephen
Published: July 2002

In a disordered system of N particles, the probability of finding any neighbor at a given distance from a reference particle is described by the radial distribution function G(r). In many instances it would also be useful to know the distribution functions P(1,r),P(2,r),...,P(n,r) corresponding to each subset of ‘‘neighborship;’’ namely, the nearest, next nearest, ...nth nearest neighbor. While G(r) is a two-particle correlation function, each P(n,r) is formally dependent upon all levels of multiparticle correlations. By means of suitable approximations relating higher-order correlations to two-particle correlations, it becomes possible to calculate P(n,r) from G(r). This approach takes advantage of the fact that G(r) is often accurately known from either experiment or theory. For the case of hard spheres, the superposition approximation and a first-order correction to it are found to give P(1,r) in good agreement with simulations and earlier analytic results, even at high densities. P(n,r) for neighborships n≥1 are obtained for the first time. These reveal some interesting features of structure within and beyond the first shell.
Rotational–translational coupling and asymmetric line shapes in the high resolution stimulated Brillouin gain spectra of liquid carbon disulfide
Authors: Grubbs W. T. , MacPhail Richard A.
Published: July 2002

We have measured high resolution stimulated Brillouin gain spectra of liquid carbon disulfide at a number of temperatures between 158 and 301 K. As the temperature decreases the shape of the Brillouin peak becomes increasingly asymmetric, with the intensity skewed away from the center of the spectrum. We show that this unusual asymmetry results from the coupling between rotational and translational motions. Rotational–translational coupling is well known to give rise to line shape distortions in the depolarized light scattering spectra of nonspherical molecules (e.g., the ‘‘Rytov dip’’), but the observation of line shape distortions (the asymmetry) in the polarized Brillouin lines has not been noted before. In the latter case, the asymmetry in the Brillouin peak can be traced to a cross correlation between the collective molecular orientation variable and the number density fluctuation and can be viewed as an acoustically induced birefringence. A calculation using the theory of rotational–translational coupling and experimental values for the relevant parameters reproduces quantitatively the observed asymmetry in the Brillouin lines over the entire temperature range.
Quenching, Λ-doublet mixing and fluorescence lifetimes of single excited NH(A 3Π,v′=0,N′,J′,e/f ) states
Authors: Kaes A. , Stuhl F.
Published: July 2002

Eleven isolated states of NH(A 3Π,v′=0) with selected Λ-doublet, spin and rotational properties were prepared by pulsed laser excitation of ground state NH(X 3Σ−,v″=0,J″) radicals. The average population in both Λ-doublet components was investigated by time resolved fluorescence. Kinetic data for the quenching and collisional Λ-doublet mixing, and fluorescence lifetimes were derived using a simple model. The fluorescence lifetimes for N′=3–8 are found to be independent of rotational excitation in agreement with a recent calculation. The quenching rate constants for collisions with NH3 support the mechanism of complex formation. The mixing of the Λ-doublets induced by collisions with NH3 occurs by dipolar interaction. The kinetics of the mixing by He is in agreement with a recent quantum calculation of inelastic cross sections based on ab initio potential surfaces. Several experiments were performed to characterize the system, particularly the NH(X) precursor.
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Authors: Jameson Cynthia J. , de Dios Angel C.
Published: July 2002

The chemical shifts observed in nuclear magnetic resonance experiments are the differences in shielding of the nuclear spin in different electronic environments. These are known to depend on intermolecular interactions as evidenced by density-dependent chemical shifts in the gas phase, gas-to-liquid shifts, and adsorption shifts on surfaces. We present the results of the first ab initio intermolecular chemical shielding function calculated for a pair of interacting atoms for a wide range of internuclear separations. We used the localized orbital local origin (LORG) approach of Hansen and Bouman and also investigated the second-order electron correlation contributions using second-order LORG (SOLO). The 39Ar shielding in Ar2 passes through zero at some very short distance, going through a minimum, and asymptotically approaches zero at larger separations. The 21Ne shielding function in Ne2 has a similar shape. The Drude model suggests a method of scaling that portion of the shielding function that is weighted most heavily by exp[−V(R)/kT]. The scaling factors, which have been verified in the comparison of 21Ne in Ne2 against 39Ar in Ar2 ab initio results, allows us to project out from the same 39Ar in Ar2 ab initio values the appropriate 129Xe shielding functions in the Xe–Ar, Xe–Kr, and Xe–Xe interacting pairs. These functions lead to temperature-dependent second virial coefficients of chemical shielding which agree with experiments in the gas phase. Ab initio calculations of 39Ar shielding in clusters of argon are used to model the observed 129Xe chemical shifts of Xe, Xe2,...,Xe8 trapped in the cages of zeolite NaA.
Theoretical studies of spin-forbidden radiationless decay in polyatomic systems. II. Radiationless decay of a-N2O2
Authors: Nguyen Kiet A. , Gordon Mark S. , Montgomery John A. , Michels H. Harvey , Yarkony David R.
Published: July 2002

The stability with respect to spin-forbidden radiationless decay of the previously reported [J. Chem. Phys. 88, 7248 (1988)] asymmetric dimer of NO, N–N–O–O (a-N2O2) is considered. The spin-allowed decay channel a-N2O2(1A′)→N2O(X 1Σ+)+O(1D) is endoergic. However, the spin-forbidden decay channel a-N2O2(1A′)→N2O(X 1Σ+)+O(3P) is exoergic. Large scale multireference configuration interaction wave functions, approximately 300 000–1 400 000 configuration state functions, based on double zeta polarization and triplet zeta polarization bases are used to study this process. The minimum energy crossing of the ground singlet 1A′ state and the lowest excited triplet 3A″ state was determined as was the interstate spin–orbit coupling. This electronic structure data was used in the context of a simple one-dimensional model to show that a-N2O2 is rapidly predissociated to N2O(X 1Σ+) and O(3P).
Effect of sequence and intermolecular interactions on the number and nature of low-energy states for simple model proteins
Authors: O’Toole Eamonn M. , Panagiotopoulos Athanassios Z.
Published: July 2002

We have studied the thermodynamically significant low-energy conformations of 200 random 24-residue model proteins on a square lattice with the Rosenbluth and Rosenbluth (1955) chain growth algorithm combined with multilink additions and Boltzmann weighting. We use a model proposed by Dill (1985) that represents the protein as a connected sequence of hydrophobic and hydrophilic beads on the lattice with nearest-neighbor interactions between the constituent beads. Two interaction sets were investigated—attraction between just the hydrophobic beads and attraction between hydrophobic residues and also between hydrophilic residues. The distribution of energies found with attraction only between hydrophobic beads is broad and consistent with the previous results of Lau and Dill (1989). However, the low-energy states are highly degenerate and noncompact. When attraction between hydrophilic beads is included with the attraction between hydrophobic beads, the energy distribution is sharp. Also, the low-energy configurations are reasonably nondegenerate and compact. This indicates that even with this simple model, important characteristics of the low-energy states of the model proteins are sensitive to the details of the interaction set used.
Molecules excited to regions of high state density are not deflected by an inhomogeneous electric field
Authors: Fraser Gerald T. , Pate Brooks H.
Published: July 2002

Conclusive experimental evidence is presented that molecules excited to a single ro-vibrational eigenstate in a high density-of-states region are not deflected by an inhomogeneous electric field. The onset of nondeflection behavior occurs in the same density of states region as has been found for the onset of intramolecular vibrational energy redistribution IVR (about 10 vibrational states/cm−1). By using large polyatomic molecules, where the hydride stretch fundamentals occur in this range of state density, the nondeflection behavior can be studied. However, the results are generally applicable to the dynamics of smaller molecules at higher levels of excitation. Thus, the observed nondeflection in our experiments is expected to be a general phenomenon associated with highly vibrationally excited molecules. In addition, it is shown how the nondeflection characteristics of these systems can be exploited experimentally to obtain spectra with increased sensitivity.
Fourier transform infrared isotopic study of the ν4 and ν5 stretching modes of linear C6 in Ar at 10 K
Authors: Kranze Richard H. , Graham W. R. M.
Published: July 2002

The vibrational spectrum of the linear C6 cluster produced by trapping the products of the high temperature evaporation of graphite in Ar at ∼10 K has yielded a complete set of measurements of the ν4(σu) and ν5(σu) stretching modes for all possible 13C-substituted isotopomers. The observed isotopomer frequencies are in excellent agreement with the predictions of recent second-order, Mo/ller–Plesset, ab initio calculations.
The dependence of laser-induced refractive index changes in glass on the chemical composition
Authors: Munoz F. A. , Reeves Roger J. , Taheri B. , Powell Richard C. , Blackburn Douglas H. , Cranmer David C.
Published: July 2002

The properties of permanent, laser induced refractive index gratings in rare-earth doped glasses have been characterized in a variety of materials including lithium borates, lead and magnesium silicates, lead borate-germanates and lead germanates. This work is combined with the previous results obtained on phosphate and silicate glasses to develop an understanding of the effects of glass composition (both network former and modifier ions) on the ability of a material to produce gratings with high scattering efficiencies. The grating efficiencies and the writing/erasing dynamics were studied, and the two-level-system-model developed previously was employed to explain the results. The results show the importance of having flexible glass networks with low polarizing power of the network former ions, and a high content of nonbridging oxygen ions. In addition, it is important to have light, highly polarizable modifier ions with weak chemical bond strengths to the oxygen ions. This type of glass combined with rare-earth ions that like to exist in more than one local coordination environment and that couple strongly to high energy vibrational modes of the host, are found to be very effective in producing local structural changes that lead to room-temperature-stable refractive index gratings.
Icosahedral, decahedral, fcc, and defect-fcc structural models for ArN clusters, N≳500: How plausible are they?
Authors: van de Waal Benjamin W.
Published: July 2002

Different structural models for solid argon clusters with more than 500 atoms, that have been proposed previously on the basis of energy considerations, but whose origin and growth histories are not well understood, are compared with other models that have been devised mainly to overcome these difficulties, at the expense, however, of a smaller binding energy. Diffraction functions have been calculated for a variety of structures and sizes, and are compared with the observed electron-diffraction patterns obtained by Farges et al. [Adv. Chem. Phys. 70, 45 (1988)]. Since the nonsplitting of the first peak in these patterns up to N∼3000 suggests the absence of the— energetically most favorable— decahedra, and other features reflect an increasing fraction of clusters with— at that size unfavorable— bulklike fcc structure, it seems appropriate to relax the energy criterion and to consider other properties as well, notably the rate of growth. Defect-fcc clusters, obtained from perfect fcc crystallites by introducing at least two crossing stacking faults, derive this property from a modified surface structure, involving immobile, nonvanishing, stacking fault resisting steps. Calculated diffraction functions compare favorably with observed patterns for N∼600 and N∼3000. A growth mechanism is proposed which can explain the disappearance of multishell icosahedra and the appearance of (defect-) fcc clusters at N∼500, with no structural transition involved.
Simulation of the intermolecular vibrational spectra of liquid water and water clusters
Authors: Bosma Wayne B. , Fried Laurence E. , Mukamel Shaul
Published: July 2002

We report simulated Raman and infrared spectra of liquid water and water clusters in the frequency range 0–1000 cm−1. The librational peak in the Raman spectrum of the liquid, which has a strong dependence on the anisotropy of the assumed gas-phase polarizability tensor, allows us to choose between various models for that tensor. Most of the spectroscopically probed dynamics of the liquid are present in the small clusters, with N as low as 5. The librational peaks in the pentamer spectra are shown to redshift with increasing temperature.
Infrared external reflection spectroscopic studies of phase transitions in Langmuir monolayers of heneicosanol
Authors: Buontempo Joseph T. , Rice Stuart A.
Published: July 2002

We report the results of an infrared external reflection spectroscopic study of phase transitions in Langmuir monolayers of heneicosanol. One of our main goals was to examine monolayers which were pure and at equilibrium. The monolayers we have studied were fully relaxed, judging from the exceptionally good long term stability demonstrated by each state on the isotherms. In addition, our fully relaxed systems displayed good reversibility, though each compression–expansion cycle showed a small hysteresis, which may possibly be a real physical property of these systems. The phase boundaries displayed in these fully relaxed systems differed in number and location from those in the partially relaxed systems usually studied. Our second goal was to advance the utility of infrared external reflection spectroscopy for the study of Langmuir monolayer structure, especially molecular conformation and orientation, by examining the hydrocarbon CH2 symmetric and antisymmetric stretching modes as a function of surface pressure, temperature, etc. In pursuit of this goal we have exploited nonimaging optics to improve the signal to noise in these difficult experiments.

Our studies of the relaxed structure of Langmuir monolayers of heneicosanol have uncovered new and interesting phase behavior. In general, we find a clear trend showing that at high surface pressure and low temperature the hydrocarbon chains are rather highly ordered, perhaps all-trans; at low surface pressure and high temperature the hydrocarbon chains become somewhat disordered, containing a higher concentration of gauche defects, though that concentration is still lower than that found in the fluid phase. In all of the cases studied the region of the isotherm immediately prior to the highest density phase can be associated with a distinct ordering transition of the hydrocarbon chain. Regarding the molecular orientation, we have obtained data which are often in disagreement with the current x-ray data, the latter taken on what appear to be partially relaxed monolayers. For example, as the surface pressure increases from 0 to 13 dyn/cm for our 20 °C monolayer the hydrocarbon chains continuously become more ordered, and the molecules retain a collective tilt of perhaps 30°–50°. However, the current view is that along this isotherm there is a continuous tilting transition as the surface pressure increases in the range mentioned. We have rationalized this difference by assuming a kinetic model in which the collective tilting of the molecules can respond to a change in area much more quickly than can a reduction in gauche conformation concentration. Finally, we have found that there are no liquid phases present when the surface pressure is nonzero. In this way fatty alcohol monolayers are quite different from phospholipid monolayers. In fact, even at nearly zero surface pressure and fairly low density the fatty alcohol monolayer still exists as a mesophase.
A solution to the infrared paradox
Authors: Riera A.
Published: July 2002

The infrared paradox is an inconsistency in the current derivation of the Einstein coefficient for vibrational excitations in molecular spectroscopy. A solution to the paradox is proposed by extending well established developments in the theory of atomic collisions to the case of vibrating molecules. When delay effects are unimportant, the new method does not require any change in the standard calculation of frequencies or intensities—only a reinterpretation of the nuclear wave functions. The improved formalism eliminates the problem of asymptotic couplings in molecular spectroscopy, as well as a basic difficulty in the usual picture of molecular structure.
High resolution photoelectron spectroscopy and femtosecond intramolecular dynamics of H2CCO+ and D2CCO+
Authors: Niu Baohua , Bai Ying , Shirley David A.
Published: July 2002

High resolution helium Iα (584 Å) photoelectron spectra of H2CCO and D2CCO are reported. The present spectra of the ground states of ketene cations show more vibrational fine structure than previously reported. The adiabatic ionization energies (AIEs) of the cations’ first, second, and fifth excited states are determined unambiguously. The doubletlike fine structures present in the first excited states of ketene cations imply the excitation of a ‘‘soft’’ mode that was not observed before. It was assigned to the ν5 mode, which is characterized by the CH2 (CD2) group out-of-plane wagging motion. The complexity of the photoelectron spectra obtained for the ionic first excited states is attributed to the possible dissociation and predissociation of this state. Strong isotope effects are observed in the vibronic (vibrational) couplings in most of the ionic states. Vibrational autocorrelation functions are calculated from the high-resolution photoelectron spectra for four of the six ionic states observed. The dynamics of the ground states of the cations are characterized by a wave packet oscillating with small amplitude around the minimum of the upper potential energy surfaces (PES). The decay dynamics of the ionic first and fifth excited states of ketene are characterized by ultrafast intramolecular processes such as dissociation and predissociation.
Accurate quantum probabilities and threshold behavior of the H+O2 combustion reaction
Authors: Pack Russell T , Butcher Eric A. , Parker Gregory A.
Published: July 2002

We report accurate 3D quantum reaction probabilities for the H+O2 combustion reaction and find that they are resonance dominated and rise linearly above threshold. The reaction probability is not enhanced by vibrational or rotational excitation of the reactants but does increase above the threshold for vibrationally excited products.
Hartree–Fock and second-order Mo/ller–Plesset perturbation theory calculations of the 31P nuclear magnetic resonance shielding tensor in PH3
Authors: Wolinski Krzysztof , Hsu Chih-Li , Hinton James F. , Pulay Peter
Published: July 2002

The 31P shielding tensor for PH3 has been calculated without electron correlation (self-consistent field) and with electron correlation (second-order Mo/llet–Plesset) using the gauge including atomic orbital method. The best results obtained for the absolute isotropic chemical shielding constant (608.09 ppm) and chemical shielding anisotropy (64.54 ppm) compare quite favorably with the experimental values of 600 and 64.5 ppm. The contribution of electron correlation to the absolute shielding value has been found to be significant. The convergence of the calculated shielding tensor with respect to the quality of the basis set employed and the importance of d and f polarization orbitals are discussed.
Fast responses from ‘‘slowly relaxing’’ liquids: A comparative study of the femtosecond dynamics of triacetin, ethylene glycol, and water
Authors: Chang Yong Joon , Castner Edward W.
Published: July 2002

We have measured the ultrafast solvent relaxation of liquid ethylene glycol, triacetin, and water by means of femtosecond polarization spectroscopy, using optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy. In the viscous liquids triacetin and ethylene glycol, femtosecond relaxation processes were resolved. Not surprisingly, the femtosecond nonlinear optical response of ethylene glycol is quite similar to that of water. Using the theory of Maroncelli, Kumar, and Papazyan, we transform the pure-nuclear solvent response into a dipolar-solvation correlation function for comparison with ultrafast electron-transfer reaction rates.
Vibrationally resolved translational energy release spectra from the ultraviolet photodissociation of methyl mercaptan
Authors: Segall J. , Wen Y. , Singer R. , Dulligan M. , Wittig C.
Published: July 2002

Product translational energy release spectra resulting from 248 and 193 nm photodissociation of methyl mercaptan are obtained for the hydrogen atom channels (CH3SH+hν→CH3S+H) by using the high-n Rydberg time-of-flight technique. The spectra exhibit vibrational structure that is assigned to a CH3–S stretch progression. At 248 nm, the progression extends only to v=2, while at 193 nm levels up to approximately v=17 are populated. The progression observed at 193 nm is bimodal, with the higher kinetic energy component showing greater spatial anisotropy than the lower energy component, suggesting that two different processes occurring on different time scales are responsible for the two components. The results at 248 nm are consistent with excitation to a repulsive electronic surface. For 193 nm excitation, the high kinetic energy component is consistent with direct photoexcitation to a repulsive surface and/or rapid intramolecular access to a repulsive surface. The lower kinetic energy component presumably derives from the molecule spending more time on an excited surface. A simple model is applied to estimate the extent of C–S bond extension for the various processes.
Pressure tuning spectroscopy of the low-frequency Raman spectrum of liquid amides
Authors: Goossens K. , Smeller L. , Heremans K.
Published: July 2002

The effect of hydrostatic pressure up to 4 kbar on the low frequency Raman spectrum of hydrogen bonded (formamide, N-methylformamide, N-ethylformamide, and N-methylpropionamide) and nonhydrogen bonded (N,N′-dimethylformamide and N,N′-dimethylacetamide) liquid amides is reported. A shift of ca. 4.0 cm−1/kbar is observed for the nonhydrogen bonded amides. For the hydrogen bonded amides the shift is ca. 2.0–3.0 cm−1/kbar. The difference may be explained by the compression of the hydrogen bond which affects the librational motion of the molecules.
Polar order and relaxation of second order nonlinear optical susceptibility in an electric field polarized amorphous polymer
Authors: Wang C. H. , Gu S. H. , Guan H. W.
Published: July 2002

The dependence of the second harmonic generation (SHG) signal of an amorphous polymer system containing nonlinear optical (NLO) chromophores on the polarizing electric field is investigated. It is shown that under a weak poling field condition, the SHG intensity is proportional to Ep2, provided that the polarizing electric field Ep is greater than a threshold field Es. Above Es, the macroscopic polar orientational order is established. The lengthening of the relaxation time for polar orientational order with increasing Ep is also observed. Theoretical analysis shows that the relaxation time is proportional to the polar order initially induced by the polarizing field. Experiments have been carried out to confirm the theoretical prediction.
Photodissociation measurements of bond dissociation energies: Ti+2, V+2, Co+2, and Co+3
Authors: Russon Larry M. , Heidecke Scott A. , Birke Michelle K. , Conceicao J. , Morse Michael D. , Armentrout P. B.
Published: July 2002

The bond dissociation energies of Ti+2, V+2, Co+2, and Co+3 have been measured from the sudden onset of predissociation in the photodissociation spectra of these molecules, yielding values of D○0(Ti+2)=2.435±0.002 eV, D○0(V+2)=3.140±0.002 eV, D○0(Co+2)=2.765±0.001 eV, and D○0(Co+3)=2.086±0.002 eV. These values are in good agreement with values previously determined from collision-induced dissociation experiments. General criteria for the interpretation of predissociation thresholds as bond dissociation energies and periodic trends in the bonding of the 3d transition metal diatomic neutrals and monocations are discussed.
Reptation as a dynamic mean-field theory: Self and tracer diffusion in a simple model of rodlike polymers
Authors: Szamel Grzegorz , Schweizer Kenneth S.
Published: July 2002

The simplest model of entangling polymers, a semidilute solution of hard infinitely thin rods that perform only translational Brownian motion, is studied. An approximate microscopic theory of self and tracer diffusion is presented. Within this theory independent binary collisions are modified to account for the influence of the surrounding rods on the two-particle dynamics. The interaction with the other rods is taken into account in an average self-consistent way. In the semidilute regime the theory leads to the same scaling law for the transversal self-diffusion constant as that derived from a reptation-tube theory. The relaxation time and the localization length asymptotically follow the same scaling law as the disentanglement time and the tube radius, respectively. For the tracer diffusion problem, if the length of the matrix rods is larger or equal to the length of the test rod the reptation prediction for the transversal diffusion constant is asymptotically recovered. For matrix rods much shorter than the test rod the transversal diffusion constant follows a different scaling law.