bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum and semiclassical vibrational transition frequencies for a Hamiltonian with stretch–bend potential energy coupling
Authors: Swamy Kandadai N. , Hase William L.
Published: July 2002

Vibrational eigenvalues are calculated for a hydrocarbon fragment which consists of a coupled HC stretch and CCH bend. The particular coupling investigated is the attenuation of the CCH bending force as the HC bond is extended. Eigenvalues are calculated quantum mechanically and with the approximate Sorbie–Handy and DeLeon–Heller–Miller semiclassical methods. For many of the eigenstates large differences are found between the quantum mechanical and approximate semiclassical eigenvalues. Attenuating the CCH bending force is found to affect the HC stretch to CCH bend frequency ratio for a particular HC stretch excitation.
Statistical thermodynamics of polyalkane-like chains in a uniaxial environment
Authors: Creamer Dennis B. , Pathria Raj K. , Rabin Yitzhak
Published: July 2002

We have studied the statistical thermodynamics of polyalkane-like chains in a uniaxial environment, assuming quadrupolar coupling between the monomers and the external field. Using the rotational isomeric state model, we have investigated the temperature, the coupling strength, and the conformational energy dependence of the uniaxial ordering of monomers and chain stretching as well as the entropy and the specific heat of the system. In the case of polymers for which the collapsed conformation is energetically favored over the extended one, we have demonstrated the existence of a ‘‘critical’’ coupling strength at which this tendency is balanced by the stretching influence of the external field and have shown that the system becomes truly critical in the low temperature limit. The relevance of this work to possible experimental situations is also discussed.
Coriolis-induced intensity perturbations in the rotationally resolved fluorescence spectra from the 51 and 1141 eigenstates of H2CO(Ã 1A2): Vibrational mixing near E′vib =3000 cm−1
Authors: Garland Nancy L. , Lee Edward K. C.
Published: July 2002

It has been found in the rotationally resolved fluorescence emission of the 51 and 1141 levels of S1 H2CO that the rotational line intensity distributions of various vibronic bands are very unusual. This is due to the Coriolis interaction between the zero-order 51 and 1141 rotational levels and also the interaction between these and other neighboring levels. The theory of the rotational intensity bias and borrowing based on the rotation-induced vibrational mixing is used to explain the intensity anomaly as well as the variation in the mixing coefficients with the variation in the rotational quantum numbers, J′ and Ka. For most of the rotational levels populated at room temperature, the Coriolis-induced vibrational mixing of 51 and 1141 appears nearly complete.
Analysis of the absorption spectra of complex pentavalent actinide halides: LiUF6, α-NaUF6, and CsUF6
Authors: Hecht H. G. , Malm J. G. , Foropoulos J. , Carnall W. T.
Published: July 2002

Absorption spectra of polycrystalline samples of LiUF6, α-NaUF6, and CsUF6 have been recorded at 4, 77, and 298 K. This group of compounds has a common sixfold U–F coordination that approaches an octahedral site symmetry for LiUF6 but exhibits increasing trigonal distortion (D3d symmetry) along the indicated series. Spectra have been systematically interpreted as consisting of sequences of vibronic transitions coupled to well-defined electronic excited states. Crystal-field calculations have been performed.
Mode–mode coupling theory of translations and rotations in orientationally disordered crystals
Authors: Michel K. H.
Published: July 2002

Dynamic equations of coupled translational and rotational motions in orientationally disordered crystals are derived within a microscopic theory. Beside a bilinear coupling, cubic terms are taken into account in the interaction potential of translation–rotation coupled modes. Transport coefficients for rotational and translation motion are calculated by mode–mode coupling theory. The orientational motion is not restricted to relaxation behavior.
Cubic lattice model simulation of an entangled polymer system
Authors: Naghizadeh J. , Kovac J.
Published: July 2002

The motion of nonintersecting chains on a cubic lattice with a super lattice of obstacles is simulated. Center-of-mass diffusion coefficients as well as the end-to-end vector relaxation times are measured both in ordered and random obstacle fields. Severe differences in dynamical quantities appear as a result of simultaneous obstacle motion and randomization of the obstacle field. The differences indicate a shift of the system to lower obstacle densities. This is interpreted to be due to preferential motion of the end group (snake head) into large holes, rendering the effective tube diameter larger than the average neighbor–neighbor obstacle distance.
Collisional time-correlation functions for energy transfer: The semiclassical limit
Authors: Micha David A. , Vilallonga Eduardo F.
Published: July 2002

We derive double differential cross sections for energy transfer into complex targets where both slow and fast internal motions may exist, such as polyatomics or adsorbates, in terms of collisional time-correlation functions (TCF’s). Starting with the formally exact relation between cross sections and TCF’s of transition operators, we express the later in terms of path, or functional, integrals and then carry out a stationary-phase approximation to relate the transition operators to time-evolution operators. Their TCF’s provide the desired cross sections. The formalism is developed for cases where: (1) the relative motion is uncoupled from the internal ones; (2) the internal motions are slow compared with collision times; and (3) the internal motions can be separated into slow and fast motions. The cross sections are determined by the TCF of the fast motions averaged over distributions of the slow variables and weighted by elastic cross sections and by the TCF of the slow motions.
BiF(A+, v′) radiative lifetimes and rate coefficients for V↔T transfer and electronic quenching
Authors: Heidner R. F. , Helvajian H. , Holloway J. S. , Koffend J. B.
Published: July 2002

The radiative lifetime for several vibrational states (v′=0–3) was measured for electronically excited BiF(A0+). The measured radiative lifetime is 1.4 μs and does not vary for the v′ levels investigated. Also measured for those v′ levels are the electronic quenching and V–T rate coefficients for the rare gases He and Ar at an experimental temperature of ∼485 K. The results indicate Ar is slightly more efficient in electronic quenching than is He. For both gases the quenching rate constants increases with v′, with these rates becoming competitive with the radiative decay rate only for pressures in excess of 40 Torr. The vibrational relaxation (V–T) rate constant for all levels studied are found to be at least an order of magnitude greater than those for electronic quenching. The reported V–T rate coefficients scale as V1.7 for both He and Ar for v′=0, to v′=3, a result which is contrary to the behavior predicted by the classical harmonic oscillator model.
Energy transfer collisions between vibrationally excited molecular nitrogen and atoms
Authors: Schearer Laird D.
Published: July 2002

Chemiluminescent reactions between metastable states of molecular nitrogen and atoms are used to determine the vibrational temperatures of these nonradiating molecular systems. The excitation rates of Na, Mg, Cd, and Zn are used to obtain the vibrational temperature of N2(X 1∑) and N2(A 3∑) states in a fast flowing nitrogen afterglow. The exponential dependence of the intensities of the emission from atoms excited by the energy transfer collisions on the energy of the level yields vibrational temperatures on the order of 3100 K. We also report the presence of apparent strong selection rules on the total spin of the system which effectively suppresses energy transfer to the singlet states of magnesium.
On the use of adiabatic switching to locate quantized, periodic orbits: Application to bound and reactive multidimensional problems
Authors: Skodje Rex T. , Borondo F.
Published: July 2002

We show that the adiabatic switching method can be used to find periodic orbits in nonseparable multidimensional Hamiltonian systems. It is shown that the stable periodic orbits for the fully coupled system can be found by adiabatically propagating periodic orbits from a soluble zero order reference system. A general validity criterion for the technique is presented. The method is successfully tested on several two-dimensional coupled Morse oscillator problems. The most exciting applications are to chemical reactions. We successfully locate trapped, quantized orbits for collinear and three-dimensional models of the H+MuH and H+FH reactions.
Perturbation theory of liquids
Authors: Williams C. P. , Gupta S.
Published: July 2002

Recently, Abascal and co-workers have developed an extended perturbation theory for diatomic liquids which combines the Weeks–Chandler–Andersen choice of reference potential with the Barker–Henderson choice of hard sphere diameter. Here we report results for the case of atomic liquids. This theory is shown to work well for atomic liquids at moderate densities, for ρ*<0.75. At higher densities, this theory is less accurate than the first order Barker–Henderson theory. Implications for perturbation theories involving molecular liquids are discussed.
Thermotropic ionic liquid crystals. V. Deuterium NMR study of sodium n-alkanoates
Authors: Phillips M. L. , Barbara T. M. , Plesko S. , Jonas J.
Published: July 2002

Deuterium NMR spectra and spin-lattice relaxation times T1 were obtained as functions of temperature for α-deuterated sodium n-alkanoates (nc=4–7) in the smectic A and ‘‘isotropic’’ liquid phases. The temperature dependence of the orientational order parameter is found to disagree with the predictions of McMillan’s and Woo’s theories for the smectic A phase. Reorientational models are presented to explain the experimental order parameters in terms of the structure of the anhydrous solid or the lyotropic gel phase of these compounds. The same models are used in the analysis of reorientational contributions to the 2D T1 in the mesophase; order director fluctuations are also estimated to contribute significantly to the relaxation times. The 2D T1 temperature dependence in the mesophase resembles that of the 23Na T1 measured previously, implying shared relaxation mechanisms for the two nuclei. Estimated correlation times for 2D relaxation in the isotropic phase provide evidence for the persistence of local structure characterized by motion on a time scale of tens of picoseconds.
Probing the dynamics of a polymer with paramagnetic end groups by magnetic fields
Authors: Schulten Klaus , Bittl Robert
Published: July 2002

We show that magnetic field effects observed on reactions between two paramagnetic end groups of a polymer can be explained through the folding motion of the polymer. This folding stochastically alters the exchange interaction between the paramagnetic end groups and, thereby, affects their spin dynamics. We describe the resulting stochastic quantum system by a (Monte Carlo) computer simulation as well as by a new approximation which involves the spectrum of the stochastic interaction. This approximation can be of general use for the description of stochastic quantum systems when a diagonal stochastic interaction originates from an arbitrary, i.e., nonlinear and non-Gaussian, stochastic process.
Tests of the extension of variational transition state theory to calculate reaction rates for molecules in selected excited vibrational states
Authors: Steckler Rozeanne , Truhlar Donald G. , Garrett Bruce C.
Published: July 2002

Variational transition state theory (VTST) with an adiabatic approximation for excited vibrational modes and semiclassical transmission coefficients to account for tunneling is used to calculate state-specific reaction rates for the collinear reaction H+F2(n=1)→HF+H and the three-dimensional reaction Cl+H2(n=1–4,7)→HCl+H, where n is the initial vibrational quantum number and the rate constants are summed over final vibrational states. We also consider deuterated and tritiated analogs for both reactions for n=1. The results are compared to previous state-selected rate constants calculated without transition state assumptions. Comparisons to accurate quantal results for the H+F2 series test both the semiclassical tunneling methods and the validity of the adiabatic approximation for excited-state dynamical-bottleneck locations. Comparison to quasiclassical trajectory calculations for the Cl+H2 series provide further tests of the adiabatic approximation. In general the VTST/adiabatic-plus-tunneling calculations are quite successful.
Analyses of the ‘‘allowed’’ inversion barriers of H2O and NH3: Incompleteness of the Woodward–Hoffmann HOMO–LUMO symmetry ideas due to neglect of 〈φi‖ΔN
A‖φi〉 molecular orbital terms
Authors: Edmiston C. , Jarvie J. , Bartleson J.
Published: July 2002

Walsh’s rules correctly attribute the ‘‘bent’’ structures of H2O and NH3 to the occupation of the 1πz→3a1 HOMO not occupied in linear BeH2 and planar BH3. In Walsh’s molecular orbital (MO) diagram E(3a1) decreases sharply with bending angle S. This has always been attributed incorrectly to changes in the 3a1 MO, mainly due to symmetry-allowed mixing with the LUMO, 4a*1. The forbidden bending of BeH2 and BH3 has been similarly ‘‘explained.’’ Using large-basis-set self-consistent field molecular orbital (SCF MO) ψs, we show that the integral Hellmann–Feynman theorem ΔEIHF≂ΔESCF much better than does the analogous second-order perturbation theory λE′′(SE′=0 and λ=S2/2, ΔH≂SH′+λH′′). ΔEIHF=〈ψ0‖ΔNA‖ψ0〉+〈ψ0‖ ΔNA‖Δψ̃〉+ΔNR≂Σni2Δ EIHFi+ΔNR, Δψ̃=(ψ/η)−ψ0, η=〈ψ0‖ψ〉, ΔEIHFi=〈φ0i‖ ΔNA‖φ0i〉+〈φ0i‖ ΔNA‖Δφ̃i〉, Δφ̃i=(φi/ηi)−φ0i, ηi=〈φ0i‖φi〉, ΔNA=ΔH−ΔNR. Both theories show a large negative 〈1πz‖ΔNA‖1πz〉 term and small 〈1πz‖ΔNA‖Δ1π̃z〉 HOMO–UMO mixing term, which is positive in ΔEIHF. The 〈1πz‖SH′‖3σ*g〉 HOMO–LUMO mixing term is small even when 3σ*g is optimized for the excited state. The ΔEIHFis and

λE″is give the usual Walsh diagrams for bending of H2O and NH3, with or without MO partitioning of the nuclear repulsion change (ΔNR). However ‘‘decoupling’’ of the φ′is in ψ′ makes the λE″is unreliable. The 〈1πz‖ΔNA‖1πz〉 term acts to create a large allowed barrier to inversion for H2O and CH4, but a strong ΔNR nearly destroys an otherwise large barrier for NH3. 〈1πz‖ΔNA‖1πz〉 acts to bend the linear H2O, planar NH3, and planar CH4, with HOMO–LUMO mixing being ‘‘antibending.’’ We show that understanding of MO correlation diagrams demands consideration of the ‘‘static’’ 〈φ0i‖ΔNA‖φ0i〉 terms as well as the OMO–UMO mixing terms, which has not been appreciated by earlier workers so far as we are aware.
Anharmonic vibration of n-paraffin molecules in urea adducts
Authors: Fukao Koji , Miyaji Hideki , Asai Kenjiro
Published: July 2002

X-ray integrated intensities of meridional Bragg reflections due to paraffin molecules in urea adducts are measured from 173 to 351 K. The B factor in the Debye–Waller factor, exp[−2B(T)(sin θ/λ)2], becomes extraordinarily large with increasing temperature (11.4 nm2 at 351 K for the second-order meridional reflection) and depends on the reflection index l for the meridional reflections; this l dependence cannot be explained by the harmonic approximation. The experimental results are analyzed with a model where a paraffin molecule in urea adducts vibrates in an anharmonic potential field with cubic and quartic terms in addition to a harmonic one. The origin of the anharmonic potential field is discussed.
Theory of initial yields of ions generated by electrons in binary mixtures
Authors: Eggarter Eugenia
Published: July 2002

Interactions of charged particles with matter concerns most often mixtures or chemically impure substances. This paper presents the first rigorous treatment of the ionization yields in a binary mixture of gases A and B in the absence of energy transfer between molecules. The Fowler equation was used, and as a first example the system Ar–H2 was studied. Results for the number of all ions N and for the number of ions of types A or B are presented for incident electron energies T up to 2000 eV. The calculations cover the complete range of the mixture composition. Comparison with available experimental data shows excellent agreement. The mean numbers of Ar and H2 ions present a very interesting dependence on the concentration of the species; their ratio is approximately proportional to the ratio of their partial concentrations.
The a p
r
i
o
r
i calculation of collisional energy transfer in highly vibrationally excited molecules: The biased random walk model
Authors: Lim Kieran F. , Gilbert Robert G.
Published: July 2002

An a priori calculation of collisional energy transfer has been carried out, based on an extension of Gilbert’s ‘‘biased random walk’’ model [J. Chem. Phys. 80, 5501 (1984)]. The model assumes that energy migration during the collision is random except for certain physical and statistical constraints. It is shown that the probability of energy transfer can be obtained accurately from a relatively small number (10–50) of trajectories using a Smoluchowski equation and generalized Langevin equation approach. Calculations for the azulene/argon system, employing realistic inter- and intramolecular potentials, show excellent agreement with the experimental results of Rossi, Pladziewicz, and Barker [J. Chem. Phys. 78, 6695 (1983)] and Hippler, Lindemann, and Troe [J. Chem. Phys. 83, 3906 (1985)]. This suggests that the extended model may be reliably and economically used to calculate appropriate energy transfer quantities. Moreover, a number of general trends seen in experimental results can be rationalized with the model.
On the vibrational Born–Oppenheimer separation scheme for molecules with regular and chaotic states
Authors: Farantos Stavros C. , Tennyson Jonathan
Published: July 2002

Quantum mechanical calculations on KCN, LiCN, and ArHCl with two-dimensional (2D) potentials show that the Born–Oppenheimer separation scheme with the slow coordinate as the adiabatic variable is reliable for regular states but not for chaotic states. Three-dimensional (3D) calculations on KCN and ArHCl, where the coupling of CN or HCl motion with the other coordinates is introduced through the kinetic part of the Hamiltonian, demonstrate the existence of an extra constant of motion even for high excitations of the diatom. In 3D, cuts through the wave function show regular nodal patterns across the regular and chaotic coordinates. However, the number of nodes displaced by the chaotic coordinate varies for different values of the third constant coordinate. It is suggested that these plots provide a useful diagnostic aid for multidimensional irregular states. The adiabatic separation of regular motion gives results in excellent agreement with fully coupled 3D calculations.
The sudden approximation for scattering from noncrystalline surfaces: Applications to models of adsorbed impurities and to mixed overlayers
Authors: Yinnon A. T. , Gerber R. B. , Dacol D. K. , Rabitz H.
Published: July 2002

It was recently proposed that the sudden approximation should be a powerful tool for the calculation of the angular intensity distribution in high-energy atom scattering from disordered surfaces. In the present study the sudden approximation is applied to scattering from one- and two-dimensional models of: (1) Isolated adsorbed impurities on crystalline surfaces (Ar on Cu); (2) Mixed overlayers on an underlying surface (Xe+Ar mixtures on a smooth surface). The results are tested against numerically exact quantum-mechanical wave packet calculations. Except for very low collision energies, the sudden approximation gives results of excellent quantitative accuracy for both types of noncrystalline surfaces. At low energies, several features of the intensity distribution are not produced correctly by the sudden: These are found to be due mainly to double collision effects. The accuracy and validity range of the method are discussed in the light of the results obtained in the test calculations.
Collision energy dependence of the cross sections for the electronic excitation transfer reactions: Rg(3P0,2)+N2(X 1Σg)→Rg(1S) +N2(C 3Πu) (Rg=Ar, Kr)
Authors: Tabayashi Kiyohiko , Shobatake Kosuke
Published: July 2002

The collision energy dependence of the integral cross sections for the title reactions were determined using crossed molecular beams and time-of-flight energy selection techniques. Applying arc-heated rare gas atom beams, the relative collision energies of metastable Ar(3P0,2) and Kr(3P0,2) atoms were selected between 0.4 and 2.5 eV for Ar(3P0,2)+N2, and 0.6 and 1.7 eV for Kr(3P0,2)+N2 systems. The negative energy dependence of the cross sections for the prototype reaction Ar(3P0,2)+N2 agrees well with the results of Parr and Martin in the overlapped energy range (0.4–0.8 eV). The absolute cross sections were determined by normalizing our cross sections to the ones of Parr and Martin. As to the endoergic Kr(3P0,2)+N2 reaction, the product fluorescence from N2(C 3Πu–B 3Πg) was also observed. The total cross section for this reaction exhibits a steep increase near the threshold for each component state [ΔH(3P0)=0.47 and ΔH(3P2)=1.12 eV] and then tends to level off. Assuming the component ratio Kr(3P2)/Kr(3P0) to be statistical in the arc-heated beams, the cross section for each reaction was evaluated by convolution analysis. The post-threshold cross sections for these processes are one to two orders of magnitude smaller than those obtained for Ar(3P0,2)+N2 reaction. The higher reactivity found for Kr(3P2) than for Kr(3P0) is consistent with the quenching rates of Ar(3P0) and Ar(3P2) by N2 obtained by Piper, Velazco, and Setser [J. Chem. Phys. 59, 3323 (1973)]. The present findings suggest that the same type of interaction is also effective for these collision-induced nonadiabatic Kr(3P0,2)+N2 processes as the one, such as spin-orbit interaction, which is applicable between Ar(3P0,2) and N2.
Electronic structure of SF6
Authors: Tang Rongqi , Callaway Joseph
Published: July 2002

We have performed self-consistent calculations for the SF6 molecule in the local density approximation to density functional theory. The minimum energy was calculated to occur at an S–F distance of 1.584 Å. The binding energy was found to be 25.06 eV. Convergent ionization potentials were obtained by a series of transition state calculations with different sizes of basis sets. The electron affinity was determined to be 1.19 eV by calculating the energy of SF−6 as a function of internuclear distance. Some excitation energies were also calculated.
Deduction of low density limit to rate of ter-molecular ion–molecule association from measurements
Authors: Bates David R.
Published: July 2002

The low density limit k0 to the ter-molecular ion–molecular association coefficient is shown to be related through a double integral to X≡k(exp) [M] ( μ/α)1/2, where k (exp) is the measured association coefficient at ambient gas density [M], α is the polarizability of the neutral reactant, and μ is the reduced mass of the pair. The results obtained from evaluations of the double integral are parametrized so that k0 may easily be found from laboratory data.
Theoretical calculation of the transport properties of monatomic lithium vapor
Authors: Holland Paul M. , Biolsi Louis , Rainwater James C.
Published: July 2002

Transport properties of dilute monatomic gases depend on two body atom–atom interaction potentials. When two ground state (2S) lithium atoms interact, they can follow either of two potential energy curves corresponding to the Li2 molecule in the X 1Σ+g or 3Σ+u state. Transport collision integrals for these states have been calculated by accurately representing quantum mechanical potential energy curves with the Hulburt–Hirschfelder potential. The excellent agreement of calculated viscosities with experimental results provides further evidence that this potential can be used to estimate accurately transport properties under conditions where experimental data are sparse or unavailable.
A multiple Gaussian wave packet theory of H2 diffraction and rotational excitation by collision with solid surfaces
Authors: Jackson Bret , Metiu Horia
Published: July 2002

We combine Gaussian wave packets and the coupled channel method to develop a theory of H2 diffraction and rotational excitation by collision with surfaces. This improves our previous work on H2 diffraction since it eliminates the mean trajectory approximation; it also extends Heller’s work to problems in which the dynamics require the creation of new packets which must be coupled to each other as they are propagated through the interaction region. The approximations involved in the above Gaussian wave packet can be removed by extending a method proposed by Fleck, Morris, and Feit, which propagates the Gaussian wave function exactly and efficiently.
Associative ionization in K(n
d)–H2O collisions at intermediate n
Authors: Zollars B. G. , Walter C. W. , Johnson C. B. , Smith K. A. , Dunning F. B.
Published: July 2002

Associative ionization in K(nd)–H2O collisions has been investigated for values of n in the range 9 to 13. The measured rate constants are very small, ∼6×10−12 cm3 s−1, and are much less than those reported for associative ionization in Na(np)–Na collisions at similar n. Possible reasons for this difference are discussed.
An accurate and efficient decoupling approximation for temperature-dependent multimode resonance Raman spectra
Authors: Jean John M. , Friesner Richard A.
Published: July 2002

We present two advances in the use of the matrix method to generate the time-dependent kernel for resonance Raman scattering. The first of these, a collocation method, allows a substantial decrease in computation time in addition to greater resolution of scattered profiles. Second, we have developed a decoupling approximation in which the exact method of Friesner et al. is used to generate the kernel at a relatively small number of time points with the remaining points obtained by fitting to the exact values a factorized expression containing both finite temperature and dephasing corrections. The approximation is exact in the zero temperature limit and is based on a sum rule which ensures exact reproduction of integrated intensities. Calculations with a wide variety of parameter values are presented and show that our approximate method is highly accurate in reproducing the exact results for a wide variety of physically realistic systems. In addition, our results for single mode systems are shown to compare favorably to those obtained using a transform-based method.
Dynamic surface tension and classical nucleation theory
Authors: Rasmussen D. H.
Published: July 2002

Regular solution theory is used to develop the concentration dependence for both the equilibrium and dynamic surface tension of water–alcohol binary solutions. The equilibrium surface tensions of methanol–water, ethanol–water, and n-propanol–water binary solutions are used to fit the parameters of the theory and the dynamic surface tension is calculated. The dynamic surface tension compares favorably with the surface tension which fits classical nucleation theory vs composition at a nucleation rate of one nuclei per cubic centimeter per second.
The temperature dependence of the EPR spectrum of Mn2+ in ZnSiF6 ⋅ 6D2O
Authors: Drumheller John E. , Rubins R. S.
Published: July 2002

The EPR spectrum of Mn2+ in ZnSiF6 ⋅ 6D2O was measured at 9.2 GHz between 25 and 300 K. The g tensor was essentially isotropic and temperature independent with all measured values in the range 2.000 to 2.001. The zero-field splitting parameters D and (a−F) were −(170.4±0.3)×10−4 and +(7.9±0.2)×10−4 cm−1, respectively, at 300 K and −(132.2±0.5)×10−4 cm−1 and +(9.3±0.3)×10−4 cm−1, respectively, at 50 K. The hyperfine tensor A, which was isotropic to within the experimental errors, varied from −(89.6±0.2)×10−4 cm−1 at 300 K to −(90.1±0.2)×10−4 cm−1 at 50 K. The temperature dependence of D was qualitatively similar to that measured for Ni2+ in the same crystal.
The rotationally resolved electronic spectrum of indole in the gas phase
Authors: Philips Laura A. , Levy Donald H.
Published: July 2002

The high resolution ultraviolet fluorescence excitation spectrum of indole in a supersonic jet has been measured. It was possible to resolve the rotational structure with an experimental resolution of 180 MHz. The details of the technique employed to achieve this resolution in the ultraviolet are described. Analysis of the high resolution spectrum yielded a transition moment direction which is compared to previous theoretical predictions. Application of high resolution spectroscopy to the problem of distinguishing the 1La and 1Lb electronic states in indole and indole derivatives is discussed. From the measured rotational constants possible geometries for the ground and excited states were also determined.
The Yukawa potential in momentum space: Analytic behavior of the eigenfunctions
Authors: Lombardi John R.
Published: July 2002

It is a commonplace that eigenfunctions in momentum space are the Fourier transform of eigenfunctions in position space. However, if we choose that space for which the relevant variables are chosen canonically conjugate to the position space variables, then the transform is not Fourier, but must involve the Jacobian of transformation between Cartesian coordinates and the curvilinear position space coordinates. (For Cartesian space the transform is Fourier.) In this work we find the correct form of the Hamiltonian in momentum space for the Yukawa potential. It is shown that the potential function in momentum space becomes an operator. The first term is shown to be the Coulomb operator in momentum space, while the remaining terms just mix other hydrogen atom momentum space functions. The resulting eigenvalues are in agreement with the comparable position space results. The analytic behavior of the eigenfunctions is examined. It is shown that all the singularities are poles (the function is meromorphic) and that all the poles lie along the imaginary axis no farther from the origin than p0 (the inverse of the Bohr radius). This result should be of considerable value in simplifying calculations with these functions.
Highly excited vibrational states of CHF3 and CHD3 in the range of the v
s=5 CH chromophore
Authors: Campargue A. , Stoeckel F.
Published: July 2002

The absorption spectra of CHF3 and CHD3 have been recorded between 12 900 and 14 300 cm−1 by the intracavity laser absorption spectroscopy (ICLAS) technique with a spectral resolution of 0.02 cm−1. The rotational structure of the four observed bands of CHD3 has been resolved while only the PQR contours of the five rotational bands of CHF3 were observed at this resolution. The observed vibrational levels of CHF3 are assigned as the vs =5 CH chromophore band system of the recent model developed by Quack et al. A simple anharmonic development of the energies vs vs and vb is sufficient to predict the energies of the observed bands of CHD3.
Corrections to the quasiharmonic approximation for evaluating molecular entropies
Authors: Rojas Olivia L. , Levy Ronald M. , Szabo Attila
Published: July 2002

Molecular dynamics and Monte Carlo simulations are now being used to analyze thermodynamic problems in chemical and biochemical systems. The evaluation of entropies requires, in principle, the calculation of partition functions which are difficult to compute accurately from simulations. One approach involves the approximation of the partition function by a multivariate Gaussian with coefficients evaluted from the simulation—the so-called quasiharmonic approximation. In this paper we show that the quasiharmonic approximation is the leading term in a series expansion of the configurational distribution function in its moments. We derive the formula for the cubic correction to the quasiharmonic entropy and apply it to two model systems. The exact entropies are calculated for comparison with the approximate results. We also show how the quasiharmonic model can be used as a reference system for the evaluation of molecular entropies using importance sampling methods. Importance sampling with the quasiharmonic reference appears to be a very promising approach for evaluating the entropy of macromolecules from computer simulations.
Degenerate four wave mixing study of conformational transition of a polydiacetylene, poly-4-BCMU, in solution
Authors: Cong Peijun , Pang Yang , Prasad Paras N.
Published: July 2002

The conformational change in the poly-4-BCMU polydiacetylene was investigated by using nanosecond time-resolution degenerate four wave mixing. Polarization data along with the time response of the signal indicated the signal to be from a thermal grating. The degenerate four wave mixing signal changes dramatically during the rod-to-coil transition and fits a theoretical model of single chain conformational transition.
Mean field theory of nematic order in gels
Authors: Halperin A.
Published: July 2002

We analyze the phase diagram of a gel of rod-like macromolecules undergoing a nematic transition using the Landau–de Gennes approach. We consider the effect of applied stress and of the sample history. We find that orienting the polymer solution prior to gelation and applying stress to the prepared gel influences the phase diagram in a similar fashion. For strong enough preordering or for sufficiently high stress a critical point is reached.
Spontaneous alignment in charged-particle systems: Fixed-length vs micellar situations
Authors: McMullen William , Gelbart William M. , Rosenfeld Yaakov
Published: July 2002

We use a recently developed version of the soft-mean-spherical approximation (SMSA) to treat spontaneous alignment in suspensions of ionic, rod-like particles. In particular our theory derives from the charge-smearing idea of Onsager in which a lower bound is established for the free energy of the interacting system. First we treat the usual case of fixed-particle lengths and establish the regimes in which the charged lines behave like hard spherocylinders (i.e., the ‘‘smearing’’ length plays the role of a hard-core diameter). In general, the nematic phase is superceded by the solid whenever the rod length is too small or too large. Then we consider explicitly the micellar situation in which the particle lengths are determined by the thermodynamics (and vice versa). Here we find the possibility of a reentrant isotropic phase. At low temperatures the nematic becomes unstable because the alignment-induced growth leads to too large a charging (‘‘self’’) energy for the micelles. We also predict an ideal solution-like behavior for the growth of micelles in sufficiently concentrated isotropic phases. Finally we comment on the differences between our charge-smearing description of spontaneous alignment in ionic systems and a collective-coordinate approach due to Deutsch and Goldenfeld.
Quantum dynamics of vibrational overtone induced photodissociation of a model polyatomic
Authors: Hutchinson John S.
Published: July 2002

The detailed processes, including state preparation, intramolecular energy transfer, and unimolecular reaction, are studied for the dissociation of a model molecule by tuning an intense laser to transitions in the vicinity of local mode overtone states. Photochemical yields are calculated quantum mechanically for a range of frequencies and pulse times, by combining the method of complex coordinates with the time-dependent interaction representation. Yields for long pulse times are dominated by the absorption cross section at each frequency, which is in turn dominated by the transition moment for the eigenstate to which the laser is tuned. As a consequence, we observe the result that more reactive states actually can produce smaller photochemical yields. With shorter pulses (0.1 to 10 ps), the yield is a sensitive function of both frequency and pulse time. By mapping out the photochemical yield as a function of both pulse time and excitation frequency, the details of the photodecay process are revealed.
A deuterium NMR study of ammonium thiocyanate-d4
Authors: Gruwel Marco L. H. , Facey Glenn , Wasylishen Roderick E.
Published: July 2002

The results of a deuterium NMR study of ND4SCN between 125 and 420 K are reported. The deuterium T1 data indicate phase transitions at 352 and 383 K, however no evidence was found for a recently reported reversible phase transition at 203 K in NH4SCN.
Energetic consideration of the vibrational potential function in the effective nuclear charge model. VI
Authors: Ohwada Ken
Published: July 2002

From detailed considerations of two homogeneity postulates of the total molecular and electronic energies proposed by Parr and Gadre, a new homogeneity hypothesis of the total molecular energy W is presented: ∑iZi(∂W/∂Zi)N =k0W0+kRWR, where Zi is the atomic number, W0 and WR are the R-independent (R: internuclear distance) and R-dependent molecular energies, and k0 and kR are the local and nonlocal homogeneity parameters. Such a hypothesis is examined through derivation of potential constants for diatomic molecules and is shown to lead to exact formulas relating quadratic, cubic, quartic, and higher potential constants. Inhomogeneous linear first- and second-order differential equations, derived on the basis of the newly proposed homogeneity hypothesis, for diatomic molecules are solved to obtain some information on general potential forms of molecules. Approximate Hartree–Fock methods with the new homogeneity constraint are developed and discussed in some detail.
Neon matrix ESR and CI theoretical investigation of 1BF+ and 11BF+: Photoionization of BF from reactive laser sputtering and high temperature sources
Authors: Knight Lon B. , Ligon A. , Cobranchi S. T. , Cobranchi D. P. , Earl Edward , Feller David , Davidson E. R.
Published: July 2002

The 11BF+ and 10BF+ radical cations have been generated and trapped in neon matrices at 4 K using the combined techniques of pulsed reactive laser sputtering and photoionization at 16.8 eV. An independent high temperature source of BF(g) in conjunction with photoionization was also employed and a comparison between these two different generation methods reveals no significant differences in their ESR spectra. The magnetic parameters for 11BF+ measured in neon matrices are g∥=2.0012(3), g⊥=2.0004(3), (11B) A∥=1784(1), and A⊥=1727(1) MHz, (19F)A∥=410(1), and A⊥=152(1) MHz. Extensive ab initio theoretical calculations have been conducted for BF+ and the nuclear hyperfine properties obtained from various types of CI wave functions show excellent agreement with the experimental measurements. Valence orbital occupancies obtained from a Mulliken-type population analysis performed on the CI wave functions are compared with the conventional free atom comparison method for obtaining electronic structure information from ESR A tensor measurements. The distribution of the unpaired electron in BF+ is compared to six other isoelectronic first row radicals.
Intermolecular interaction in Sn(CH3)4: An inelastic neutron scattering study of matrix isolated molecules
Authors: Prager M. , Langel W.
Published: July 2002

Rotational tunneling of methyl groups in Sn(CH3)4 molecules isolated in solid argon has been observed by inelastic neutron scattering at ℏωt =72 μeV. In contrast to the bulk material all CH3 groups are equivalent, and the hindrance potentials is strongly reduced. The tunneling lines show an inhomogeneous broadening which decreases with decreasing concentration of the guest molecules. A discontinuity in the temperature dependence of the tunnel splitting could only be interpreted as due to a spontaneous rearrangement of the guest molecules in the argon lattice. Transitions at 2.1 and 20.7 meV are assigned to a methyl torsion on the basis of the temperature dependence of the tunneling lines and to a scelettal bending mode, respectively. A mode at 11.0 meV (88 cm−1) had not been found by other techniques. It is tentatively assigned to a libration of the Sn(CH3)4 against it cage. An alternative interpretation in a model of coupled methyl groups is also possible. The spectra become similar to the bulk material if the sample is annealed at T∼45 K indicating a clustering of the Sn(CH3)4 molecules to a not completely ordered phase.
The infrared spectrum of silaethylene
Authors: Colvin Michael E. , Kobayashi Joy , Bicerano Jozef , Schaefer Henry F.
Published: July 2002

The harmonic vibrational frequencies and infrared intensities of the H2Si=CH2 molecule have been predicted by ab initio molecular quantum mechanics. Self-consistent-field (SCF) and configuration interaction (CI) methods have been used in conjunction with a double zeta plus polarization (DZ+P) basis. Comparison with pertinent experimental data is made.
Activation of dihydrogen by scandium ions
Authors: Rappé A. K. , Upton T. H.
Published: July 2002

The interaction of 3D, 1D, 3F, and 1D states of Sc+ with H2 has been explored through the generation of ab initio potential energy surfaces for two different interaction geometries. We find that all components of the ground 3D and lowest 1D react in an endothermic process to form ScH+ (ΔH0=48.6 kcal/mol for the lowest product state from 3D Sc+) from a collinear interaction, and that four of the five components of each state are likely to proceed to this product from any interaction geometry. The remaining component of 1D Sc+ will form ScH+2 in an activated process (ΔH0=−4.4 kcal/mol; E° =12.8 kcal/mol). Inclusion of the effects of spin-orbit interactions reveals that the last 3D component may also proceed adiabatically to ScH+2 (ΔH0=+1.8 kcal/mol; E° =19.0 kcal/mol) within a very narrow energy window. At least eight H2 adduct states are also formed in the interaction with excited singlet and triplet Sc+ states. We explore in detail the means by which Sc+ activates the H2 bond, and relate our results to other metal–H2 systems.
Trapping of atoms on disordered corrugated surfaces: A statistical wave function approach
Authors: Buch V.
Published: July 2002

Trapping of low energy atoms on disordered corrugated surfaces is studied in the framework of the statistical wave function approximation. Trapping probability Tr increases monotonically towards low temperatures; the process is mediated by temporary elastic adsorption. On stiff surfaces, on which trapping is inefficient in the flat surface limit, disordered corrugation induces Tr∼1 at low temperatures. This effect may be important astrophysically, inducing efficient trapping of hydrogen atoms on surfaces of interstellar silicate grains.
Molecular beam photoelectron spectroscopy: The C2D4+(X̃ 2B3) ground state
Authors: Wang Laisheng , Pollard J. E. , Lee Y. T. , Shirley D. A.
Published: July 2002

The He i (584 Å) photoelectron spectrum of C2D+4 in its ground electronic state has been measured, using a supersonic molecular beam. The combination of rotational cooling and improved resolution permits new vibrational fine structure to be observed and assigned. In particular, the ν3 value is accurately determined. A systematic increase in the ν4 torsional frequency with increasing excitation of the ν2 C=C stretching vibration is observed, indicating significant coupling between these modes.
Intramolecular vibrational energy relaxation induced by van der Waals molecule fragmentation: The systems C6H6−nDn⋅He1,2
Authors: Rosman Ron L. , Rice Stuart A.
Published: July 2002

We report the results of experiments which probe the vibrational energy redistribution which accompanies fragmentation of deuterobenzene–helium van der Waals molecules. Branching ratios for vibrational relaxation to various final states were obtained for initially prepared levels 61-He1,2, 61162-He1, 111171-He1, and 172-He1 in several deuterobenzene compounds. The probabilities of transitions to final states in the relaxation from 61-He1 show general agreement with an energy gap law, and relaxation from 61162-He1, 111171-He1, and 172-He1 favors small vibrational quantum number changes. The experiments were undertaken to illuminate how molecular symmetry influences the choice of pathways for vibrational energy redistribution associated with van der Waals molecule predissociation. The data for the various C6H6−nDn⋅He1,2 systems, however, show very different relaxation patterns. These patterns cannot readily be explained by the usual intramolecular vibrational coupling mechanisms.
Manifestations of latent space group symmetry in the vibrational spectra of three alkali metal perchlorates
Authors: Jayasooriya U. A. , Kettle S. F. A. , Mahasuverachai S.
Published: July 2002

The vibrational spectra of crystalline MClO4 where M=K, Rb, and Cs are reinterpreted in terms of a vibrational unit cell (Imma) which is one-half the size of the crystallographic (Pnma). Single crystal Raman data on these species are reported.
Intermolecular dynamics and exciplex formation of the 1-cyanonaphthalene/triethylamine van der Waals complex
Authors: Saigusa Hiroyuki , Itoh Michiya , Baba Masaaki , Hanazaki Ichiro
Published: July 2002

The exciplex formation dynamics of the 1-cyanonaphthalene/triethylamine van der Waals complex upon excitation has been examined under collision free conditions. Mode-specificity has been found in the excitation of a combination band between intra- and intermolecular vibrational modes in the S1 state of the vdW complex to promote the exciplex formation reaction. The specificity is lost at higher excess energies since randomization of energy among intermolecular modes is more extensive than the reaction rate. The fluorescence decays of the S1 and exciplex states have been measured as a function of excess vibrational energy above the S1 origin. The lifetime of the S1 state at the excess energy where the mode-specific reaction is observed is different from that of the exciplex fluorescence. As the excess energy increases, the decay rate of the S1 state shows a sharp increase due to the exciplex formation while the exciplex fluorescence rate remains constant. The observed decay rate constants have been discussed using a simple kinetic model which accounts for the mode-specific exciplex formation.