bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of a version of scaled particle theory for nonadditive hard disks
Authors: Bearman R. J. , Mazo R. M.
Published: July 2002

The scaled particle theory for nonadditive hard disks developed by Tenne and Bergmann [J. Chem. Phys. 70, 1952 (1979)] has been studied for binary mixtures of equal size particles with various values of the nonadditivity parameter and mole fraction. Above a certain special density, we find that there are two branches of solutions of the equations determining the thermodynamic properties of the system. An argument is given that one of these branches is unphysical and should be discarded. Nevertheless, it explains a kink in the free energy vs density plot, and other anomalous features of the results of Tenne and Bergmann. We conclude that the existence of a phase transition has not yet been established for this approximation. It is pointed out that the theory does not give scaled particle theory for pure liquids in the limit of zero concentration of one component.
Symmetry-adapted discrete variable representations
Authors: Whitnell Robert M. , Light J. C.
Published: July 2002

A general approach for the construction of symmetry-adapted discrete variable representations (SADVR) is described. The method is shown to give DVRs of Gaussian quadrature accuracy. The SADVR is explicitly constructed for the nuclear motion of three atom systems with D3h symmetry (e.g., H3 or H+3 ) in hyperspherical coordinates. It is shown that H3 surface functions at constant hyperspherical radius can be calculated very accurately and efficiently using SADVRs based on either D3h or C2v symmetry.
Electronic emission spectroscopy of Group IV tetrachloro molecular ions
Authors: Lambert I. R. , Mason S. M. , Tuckett R. P. , Hopkirk A.
Published: July 2002

Two broad continuous bands are observed in the visible region following electron impact ionization of a He-seeded molecular beam of SiCl4 or GeCl4. By using tunable vacuum UV radiation from a synchrotron source to measure the threshold energy at which the fluorescence bands occur, it is shown that the bands are related to the initial formation of the C̃ 2T2 state of the parent ion SiCl+4/GeCl+4. By comparison with photoelectron data, the bands are assigned to bound–free transitions in SiCl+4/GeCl+4 C̃ 2T2–Ã 2T2 and X̃ 2T1. The C̃ state of CCl+4 does not fluoresce. The results are compared with the electronic emission spectra of the three tetrafluoro molecular ions.
Statistical fine structure in the inhomogeneously broadened electronic origin of pentacene in p-terphenyl
Authors: Carter T. P. , Manavi M. , Moerner W. E.
Published: July 2002

Recently, the first observation of statistical fine structure on an inhomogeneously broadened absorption profile was reported [W. E. Moerner and T. P. Carter, Phys. Rev. Lett. 59, 2705 (1987)] for mixed crystals of pentacene in p-terphenyl using laser frequency-modulation spectroscopy. Statistical fine structure is time-independent structure on the inhomogeneous line caused by statistical variations in the spectral density of absorbers in each frequency interval. In this work, a model and an analysis of statistical fine structure using autocorrelation techniques are presented, and the dependence of the effect for pentacene in p-terphenyl at 1.4 K on modulating frequency, detection phase, center concentration, and position in the sample is described. Statistical fine structure provides a new method for probing inhomogeneous systems that allows investigation of the underlying probability distribution function and the determination of the homogeneous linewidth. This fine structure also represents a fundamental limit on the detectability of shallow spectral features in inhomogeneously broadened lines.
The open-shell coupled-cluster method in general model space: Five states of LiH
Authors: Ben-Shlomo Sigalit , Kaldor Uzi
Published: July 2002

The open-shell coupled-cluster method is used to calculate the potential functions of the five lowest states of LiH, at internuclear separation of 2.0≤R≤10.0 bohr, with a 18σ9π2δ basis. The usual complete-model-space calculation diverges, and a general, incomplete model space, comprising the physically important 2σ 2, 2σ3σ, and 2σ1π configurations (but excluding the high-energy determinants 3σ 2 and 1π 2 from the P space) is used. Good agreement with experimental molecular constants is obtained.
Experimental and molecular dynamics study of the pressure dependence of Raman spectra of oxygen
Authors: Ladanyi B. M. , Geiger L. C. , Zerda T. W. , Song X. , Jonas J.
Published: July 2002

The pressure dependence of Raman spectra of gaseous O2 at 300 K has been studied experimentally and by molecular dynamics (MD) simulation. Experimental spectra are reported for the pressure range of 40–3000 bar and MD spectra for four thermodynamic states in the pressure range of 130–3000 bar. The MD trajectories are calculated using the Lennard-Jones atom–atom intermolecular potential. The interaction-induced effects on the system polarizability are evaluated using the first order dipole–induced dipole (DID) approximation. In the case of depolarized Raman scattering, the experimental line shapes and time correlation functions agree very well with the MD results. The density dependence of the experimental second spectral moment is also in excellent agreement with the MD predictions. The MD results indicate that the relative contribution of the interaction-induced polarizability to the depolarized spectrum increases with increasing density, but remains small within the density range considered, and that the spectrum is dominated by orientational relaxation of the molecular polarizability. The experimental depolarized Raman and MD orientational time correlation functions are compared to the results of J-diffusion and Steele models of relaxation. We find that neither of these models can account for single molecule reorientation in oxygen gas over the entire range of pressures. At high pressures, the experimental results for the frequency-dependent depolarization ratio η differ significantly from the ‘‘classical’’ value of 3/4 over most of the accessible frequency range. Similar behavior is found for the corresponding Rayleigh depolarization ratios. The MD calculations predict a much smaller deviation of η from the value of 3/4, suggesting that induction mechanisms other than DID are needed to explain the experimental data. The experimentally observed pressure dependence of the Q branch of the Raman spectrum is explained using the motional narrowing model of Brueck.
Structures and energetics of monovalent ion–water microclusters. II. Thermal phenomena
Authors: Lin Su , Jordan Peter C.
Published: July 2002

We have applied the polarizable electropole model of water and of water–ion interaction to molecular dynamics simulations of aqueous microclusters of alkali metal cations and halogen anions in the temperature range 150–400 K. This model effectively correlates observed enthalpies of formation; it is especially reliable for the larger alkali metal cations. Our simulations show unambiguous evidence of hydrogen atom inequivalency in the anion mono- and dihydrates over the temperature range studied. For the analogous cation hydrates, the structures are symmetric. We find two solvation shells in the larger microclusters; the largest ions have the fewest water molecules in their first solvation shells. The iodine and cesium inner solvation shells appear to contain only three and four water molecules, respectively. The structure of the anion hydrates strongly suggests that their stability arises in part from water–water binding.
A diabatic reaction path Hamiltonian
Authors: Miller William H. , Ruf Beverly A. , Chang Yan-Tyng
Published: July 2002

A reaction path Hamiltonian is constructed that is based on a straight-line, least motion path that interpolates linearly between equilibrium reactant and product geometries of the molecular system. Conservation of linear and angular momentum are correctly accounted for. The resulting Hamiltonian has a Cartesian-type kinetic energy, the Coriolis coupling terms originally present in the kinetic energy having been transformed to potential energy coupling (hence the term ‘‘diabatic’’ reaction path Hamiltonian). Curvature coupling terms that appear in the original reaction path Hamiltonian, which is based on the minimum energy reaction path, are absent here because the present reaction path is straight. This new, diabatic reaction path Hamiltonian should be especially useful for describing H-atom transfer reactions in polyatomic systems, a case for which the minimum energy reaction path provides a poor description.
Ultrafast dynamics of hydrogen bonds directly observed by time-resolved infrared spectroscopy
Authors: Graener H. , Ye T. Q. , Laubereau A.
Published: July 2002

Internal, hydrogen bonded OH groups of ethanol oligomers (solvent CCl4) are vibrationally excited by intense picosecond pulses at 3320 cm−1. The transient band shape observed in the OH stretching region (3000 to 3700 cm−1) is monitored by an independently tunable picosecond infrared pulse. The bands in this region are direct probes of hydrogen bridges. The time dependent growth and decay of these bands provides strong evidence for rapid bond breaking with a vibrational predissociation time of ≊5 ps, and for partial reassociation with a time constant of ≊20 ps.
Correlated studies of infrared intensities
Authors: Stanton John F. , Lipscomb William N. , Magers David H. , Bartlett Rodney J.
Published: July 2002

Coupled-cluster and many-body perturbation theories are applied to an investigation of infrared absorption intensities within the double-harmonic approximation. In agreement with previous studies, both electron correlation and basis set dependencies are found to be significant, particularly for stretching vibrations which involve hydrogen atoms. Intensities calculated at the highly correlated CCSD+T(CCSD) level with large Gaussian basis sets are in reasonable agreement with experiment for HF and the ν2 and ν3 modes of water, while the intensity of ν1 is significantly overestimated even with a relatively large 53-CGTO basis. In addition, intensities and harmonic frequencies calculated at the SCF and MBPT(2) levels with a double-zeta plus polarization (DZP) basis set are presented for a number of first row compounds, and are compared to recent experimental values. Although agreement between experimental and SCF intensities is poor, these discrepancies are moderated considerably when correlation is introduced at this simple level. The importance of including hydrogen polarization functions in the basis set is also discussed.
The argon hydrogen–fluoride differential scattering cross section
Authors: Vohralik P. F. , Miller R. E. , Watts R. O.
Published: July 2002

The total differential cross section for Ar–HF was measured at a collision energy of 1637 K (141 meV). Although diffraction oscillations were not resolved, a broad primary rainbow peak was observed. Scattering calculations were carried out for the Ar–HF interaction potentials developed by Douketis et al. and Hutson and Howard. The spherical potential and infinite order sudden approximations do not give a good description of the total differential scattering. The final-l labeled coupled states approximation, on the other hand, is in good agreement with the more accurate close coupled approximation. Neither potential predicts cross sections which are in good agreement with the measured scattering intensities. Contributions to the total scattering from elastic and inelastic processes were investigated using the coupled states approximation. The contributions to the total scattering from different initial HF rotational states were also studied. The total scattering for j initial=0 differs significantly from that for other values of j initial. It is therefore important to know the distribution of rotational states in the HF beam if one is to compare calculated total differential cross sections with measured ones. The largest inelastic cross section is for the j=0 to j′=1 transition. Collisions in which the Ar atom interacts strongly with the anisotropic potential well, rather than near head-on collisions off the repulsive wall of the potential, are responsible for the large j=0 to j′=1 cross section. The results of accurate SCF calculations for Ar–HF are also reported in this paper.
On the critical behavior of circular intensity differences of a nonpolar binary liquid mixture
Authors: Lee Dong J. , Kim Shoon K.
Published: July 2002

The theory of critical scattering for a nonpolar fluid is extended to a nonpolar binary liquid mixture, whose solute is chiral. Then, the effect of the double scattering on the circular intensity differences is discussed in detail in the critical region, where the Ornstein–Zernike theory holds.
Short-range order in glycerol. A molecular dynamics study
Authors: Root Leslie J. , Stillinger Frank H.
Published: July 2002

A molecular model has been constructed for glycerol and has been applied to a study of clusters and condensed phases by molecular dynamics simulation. Each molecule is regarded as a flexible polar complex of nine mass points (CH and CH2 groups are treated as ‘‘united atoms’’). Periodic arrays stabilize in a close approximation to the observed glycerol crystal structure, with identical extended molecules connected by infinite chains of hydrogen bonds. By contrast the stable and supercooled liquids display a variety of asymmetrical molecular conformations and an irregular network of hydrogen bonds. Detailed examination of the various atomic pair correlation functions reveals considerable overlap of intramolecular and intermolecular features, implying intrinsic ambiguity in the interpretation of x-ray and neutron diffraction measurements designed to determine the short-range order of liquid glycerol.
Explanation of anomalous mobility and birefringence measurements found in pulsed field electrophoresis
Authors: Deutsch J. M.
Published: July 2002

The study of DNA in a gel under the application of a time-dependent field has revealed some unexpected experimental features. These features are recovered by means of a detailed numerical simulation incorporating the many internal degrees of freedom of the DNA chain. These results can in turn be understood qualitatively by means of a model containing only four degrees of freedom.
Transition state structure, barrier height, and vibrational frequencies for the reaction Cl+CH4→CH3+HCl
Authors: Truong Thanh N. , Truhlar Donald G. , Baldridge Kim K. , Gordon Mark S. , Steckler Rozeanne
Published: July 2002

We have carried out ab initio calculations using second- and fourth-order Mo/ller–Plesset perturbation theory, scaled electron correlation, and several basis sets for the reaction Cl+CH4→CH3+HCl. We found that including electron correlation is essential for obtaining accurate barrier heights and vibrational frequencies. Furthermore, scaling the correlation energy further improves the barrier height predictions provided that the basis set being used is correlation balanced for both bonds involved in the reaction. Geometries and transition state frequencies calculated at the MP2 and MP-SAC2 levels with the most extensive and best balanced basis set are in good agreement with one another for all bound modes, but the unbound-mode frequency changes by 214i cm−1.
Vacuum ultraviolet laser spectra of ICl
Authors: Lipson R. H. , Hoy A. R.
Published: July 2002

Vacuum ultraviolet fluorescence excitation spectra of jet cooled ICl have been recorded between 158 and 166 nm using a VUV ‘‘laser’’ generated by four-wave sum-mixing in Mg vapor. An unambiguous vibrational analysis of the spectra has shown that transitions from v″=0 of the ground state probe high vibrational levels of the first tier EO+ ion-pair state between v′=245 and 360. Molecular constants obtained from both Dunham and near dissociation expansions are presented. Strong intensity modulation of the spectra was observed. Possible reasons for this effect are discussed.
Intramolecular energy transfer and mode-specific effects in unimolecular reactions of 1,2-difluoroethane
Authors: Raff Lionel M.
Published: July 2002

The unimolecular decomposition reactions of 1,2-difluoroethane upon mode-specific excitation to a total internal energy of 7.5 eV are investigated using classical trajectory methods and a previously formulated empirical potential-energy surface. The decomposition channels for 1,2-difluoroethane are, in order of importance, four-center HF elimination, C–C bond rupture, and hydrogen–atom dissociation. This order is found to be independent of the particular vibrational mode excited. Neither fluorine–atom nor F2 elimination reactions are ever observed even though these dissociation channels are energetically open. For four-center HF elimination, the average fraction of the total energy partitioned into internal HF motion varies between 0.115–0.181 depending upon the particular vibrational mode initially excited. The internal energy of the fluoroethylene product lies in the range 0.716–0.776. Comparison of the present results with those previously obtained for a random distribution of the initial 1,2-difluoroethane internal energy [J. Phys. Chem. 92, 5111 (1988)], shows that numerous mode-specific effects are present in these reactions in spite of the fact that intramolecular energy transfer rates for this system are 5.88–25.5 times faster than any of the unimolecular reaction rates. Mode-specific excitation always leads to a total decomposition rate significantly larger than that obtained for a random distribution of the internal energy. Excitation of different 1,2-difluoroethane vibrational modes is found to produce as much as a 51% change in the total decomposition rate. Mode-specific effects are also seen in the product energy partitioning. The rate coefficients for decomposition into the various channels are very sensitive to the particular mode excited. A comparison of the calculated mode-specific effects with the previously determined mode-to-mode energy transfer rate coefficients [J. Chem. Phys. 89, 5680 (1988)] shows that, to some extent, the presence of mode-specific chemistry is correlated with the magnitude of the energy transfer rate. However, the particular pathways for energy flow seem to be more important than the magnitude of the rate coefficients. It is suggested that the propensity for the energy to remain

isolated in small subset of modes, such as the CH2F deformation modes or the rocking modes, is primarily responsible for the observation of mode-specific chemistry. The results clearly demonstrate that an intramolecular energy transfer rate that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure the absence of mode-specific chemical

effects.
Rotationally mediated vibration–vibration and vibration–translation energy transfer in silane
Authors: Hetzler Joanne , Millot Guy , Steinfeld J. I.
Published: July 2002

Vibration–vibration (V–V) and vibration–translation (V–T) energy transfer efficiencies have been measured for the v4=1 mode of silane in collisions with He, Ar, Kr, H2, CH4, and itself, using the time-resolved infrared double-resonance technique. The V–V cross sections are approximately one-third to one-half of the Lennard-Jones cross sections, and show a variation with the nuclear–spin symmetry state (A, E, or F) of the molecule. The ν4 V–T deactivation efficiencies are in the range 0.0001–0.002, with the polyatomic molecules being about an order of magnitude more efficient than the noble-gas atoms. This can be quantitatively interpreted by the vibration–rotation (V–R) resonance transfer model of Poulsen et al. [J. Chem. Phys. 58, 3381 (1973)]. A simple breathing-sphere model does not, however, provide a good representation of the V–T collision efficiencies for rare gas–silane collisions.
Chemical equilibrium calculations on the molecular-to-nonmolecular transition of shock compressed liquid nitrogen
Authors: Hamilton Douglas C. , Ree Francis H.
Published: July 2002

Chemical equilibrium calculations are reported on the dissociation of shock-compressed liquid nitrogen to infer the monatomic nitrogen–nitrogen potential in the condensed dissociated phase at high temperature. The analysis suggests that two types of interatomic potentials with very different physical characteristics can about equally describe experimental shock-wave data. The two differ in the interatomic potential parameter α, characterizing the stiffness of the exponential repulsion, and the parameter ε which scales the repulsive and attractive interactions. The first type, with a large α and a small ε, produces a continuous shock dissociation without thermodynamic phase change, and the second, with a small α and a large ε, predicts that the shock dissociation can accompany a first-order phase change. The predicted first-order phase change is new and similar to a theoretically predicted plasma phase transition. The calculation suggests that the transition in nitrogen at high compression may be associated with a physical change at the atomistic level from a van der Waals-type to a much stronger (metallic, semimetallic, or covalentlike) interaction. The implications in interpreting shock-wave data and possible future experiments to distinguish the two potentials are also discussed.
A three dimensional quantum reactive scattering study of the I+HI reaction and of the IHI− photodetachment spectrum
Authors: Schatz George C.
Published: July 2002

In this paper, we present the results of coupled channel hyperspherical reactive scattering calculations on the reaction I+HI→IH+I using a semiempirical potential surface. Only the J=0 partial wave is considered. The bimolecular reaction probability is found to exhibit two sharp resonant peaks at energies below threshold for direct reaction. The resonances are associated with transition state quantum numbers (ν1ν2ν3)=(100) and (200), and their energies are in excellent agreement with the result of L2 calculations due to Clary and Connor, and to Bowman and Gazdy. At higher energy the reaction is dominated by rotational threshold effects, and then below the HI(ν=1) energetic threshold, additional resonances are found which correspond to quantum numbers (002), (102), (202), and (302). Franck–Condon factors associated with photodetachment of IHI− have also been calculated, and these show mainly direct scattering threshold behavior at low energies (E<0.30 eV), with the (100) and (200) resonances contributing only slightly. Resonant behavior is dominant at higher energies (0.3–0.4 eV) where the (002) resonance especially contributes. Agreement of our calculated photodetachment spectra with experimental results due to Neumark and co-workers is good provided that an 0.08 eV energy shift is made in the calculated spectra. This shift is probably due to an incorrect energy barrier on the semiempirical surface.
Product state distributions for the vibrational predissociation of NeCl2
Authors: Cline Joseph I. , Sivakumar N. , Evard Dwight D. , Bieler Craig R. , Reid Brian P. , Halberstadt Nadine , Hair Sally R. , Janda Kenneth C.
Published: July 2002

Product state distributions are reported for the vibrational predissociation of the NeCl2, B state, v′=6 through v′=13 levels. For the lower vibrational levels, Δv=−1 dissociation produces a bimodal Cl2 product rotational state distribution with the first maximum at j=4 and a secondary maximum at j=20. Surprisingly, the positions of these maxima are the same for v′=6, 7, 8, 9, and 10. For higher vibrational levels the limited available phase space constricts the observed rotational distribution allowing the Ne–Cl2 bond energy D0 to be determined. D0 is 54±2 cm−1 for the B electronic state, and 60±2 cm−1 for the ground electronic state. Δv=−2 dissociation produces a rotational distribution which, although not bimodal, is otherwise quite similar to that of the Δv=−1 channel, even though significantly more energy is released to product translation for Δv=−2. This behavior is quite different from what would be predicted by an impulsive half-collision model for the dynamics. Three dimensional quantum calculations on a simple atom–atom potential energy surface were able to reproduce most of the essential features of the experimental results. We conclude that the anisotropy of the initial wave function and that of the coupling between the covalent and van der Waals modes is more important in determining the product rotational distribution than is the kinematics of the dissociation trajectory. Since the rotational distribution produced by the Δv=−2 channel is similar to that of the Δv=−1 channel, the Δv=−2 dynamics probably occurs by a direct coupling between the quasibound state and the continuum rather than by a sequential mechanism with two Δv=−1 steps.
Intrinsic atomic-level stresses in polymer melts and networks
Authors: Gao J. , Weiner J. H.
Published: July 2002

By means of the virial stress formula, the macroscopic stress tensor in a polymer system may be expressed as a sum of atomic-level stress tensors, with each of the latter associated with a single atom and with all tensors, macroscopic and atomic, referred to the same fixed laboratory reference frame. In order to gain insight into the interplay between the covalent and noncovalent interactions in such systems, we refer the atomic-level stress tensor associated with a given atom to a moving local frame which maintains a fixed orientation with respect to the covalent structure attached to that atom; we term this the intrinsic atomic-level stress. We compute, by the method of molecular dynamics, the intrinsic stresses for model polymer melts and networks based on systems of freely rotating chains. The noncovalent interactions are through a truncated Lennard-Jones potential which is taken as either purely repulsive or has an attractive portion. We observe that (i) the intrinsic atomic-level stresses are highly anisotropic, even in an equilibrium melt in which the macroscopic stress is isotropic; (ii) the intrinsic atomic-level stress is the same in a polymer melt and in the corresponding polymer network model, if the latter is only moderately deformed; (iii) the intrinsic stresses in dense polymer melts are very different in the presence of noncovalent interactions from those in ideal systems; (iv) the addition of an attractive portion to the noncovalent potential has large effects on the deviatoric part of the intrinsic stress, thus casting doubt on the utility of the van der Waals’ picture for such systems.
Anisotropic intermolecular potentials. III. Rare-gas–hydrogen bromide systems
Authors: Hutson Jeremy M.
Published: July 2002

Intermolecular potentials for Ar and Kr interacting with HBr are obtained by least-squares fitting of potential parameters to data obtained from the molecular-beam microwave spectra of the Ar–HBr and Kr–HBr van der Waals complexes. The equilibrium geometry is linear Rg–H–Br in each case, but there are substantial secondary minima at the linear Rg–Br–H geometries; for Ar–HBr, the secondary minimum is only about 5 cm−1 shallower than the primary minimum. This potential feature is found to explain the anomalous H/D isotope effects in centrifugal distortion constants that have been observed for the Rg–HBr complexes. It is predicted that Ar–HBr will have a very low-energy bending state, only 11 cm−1 above the ground state, arising from the secondary minimum.
High-pressure equations of state of krypton and xenon by a statistical mechanical theory
Authors: Kim Jae Hyun , Ree Taikyue , Ree Francis H.
Published: July 2002

We present statistical mechanical calculations for krypton and xenon, employing accurate pair potentials with and without condensed-phase modifications. A unique feature of the present work is that solid- and fluid-phase thermodynamic properties are both computed within a single framework, using our recently developed hard-sphere perturbation theory. Results are applied to analyze experimental fluid, solid, and fluid–solid transition data, ranging up to 2×106 atmospheres at several temperatures. Effective pair potentials for both krypton and xenon, inferred from the analysis, contain short- and long-range modifications to the pair potential of Aziz and Slaman. The long-range correction is repulsive and originates from the well-known Axilrod–Teller three-body potential, while the short-range correction is attractive and is needed for describing high-compression data. Experimental isotherms above 50 GPa for xenon require a further softening of the short-range repulsion from Barker’s correction (obtained from experimental data below 50 GPa). Implications of the short-range correction and its possible relation to many-body forces are discussed. Additional tests of the present rare-gas calculations against available computer simulations and Monte Carlo and lattice-dynamics calculations carried out in this work show satisfactory agreement. Computation of solid–fluid transition properties shows that the Axilrod–Teller three-body potential must be included to obtain reliable agreement with experimental melting and freezing data.
Close-coupling and coupled states calculation of low energy He–CH4 and Ne–CH4 collisions
Authors: Phillips Timothy , Secrest Don
Published: July 2002

Exact close-coupling calculations are used to evaluate the effectiveness of the coupled states approximation in the interpretation of low energy, noble gas methane collisions. The effect of the higher order angle dependent terms of the potential on the inelastic rotational transition was investigated. Calculations using only open channel basis functions were clearly different from converged calculations, but the differences were small enough to be negligible in comparison with experiment. It was shown that open channel coupled states calculations are of sufficient accuracy to evaluate the agreement of the potential model with experiment. The use of the coupled states approximation does not fully account for discrepancies between theory and experiment observed earlier.
The collapse transition of self-avoiding walks on a square lattice: A computer simulation study
Authors: Meirovitch H. , Lim H. A.
Published: July 2002

Employing the scanning simulation method, we study the tricritical behavior (at the Flory θ point) of self-avoiding walks with nearest-neighbors attraction energy ε(−‖ε‖) on a square lattice. We obtain −ε/kBTt=0.658±0.004, where Tt is the tricritical temperature and kB is the Boltzmann constant. The radius of gyration G and the end-to-end distance R lead to νt(G)=0.5795±0.0030 and νt(R) =0.574±0.006, respectively. We also obtain γt=1.11±0.022 and μt =3.213±0.013, where γt is the free energy exponent and μt is the growth parameter. Three estimates are calculated for the crossover exponent φt , based, respectively, on G, R and the specific heat C: φt (G)=0.597±0.008, φt(R)=0.564±0.009, and φt(C)=0.66±0.02. Our values for νt and γt are close to the Duplantier and Saleur exact values for the θ′ point, νt =4/7=0.571... and γt=8/7=1.142 ... . However, our values of φt are significantly larger than the exact value φt=3/7=0.42... . This suggests that the θ and θ′ points belong to different universality classes.
Theoretical study of Ca(4s5p 1P)⇄Ca(4s5p 3P) transitions in collision with noble gases: Integral cross sections and alignment effects
Authors: Pouilly Brigitte , Robbe J. M. , Alexander Millard H.
Published: July 2002

The present article describes fully quantum calculations of spin changing collisions of Ca(4s5p 1P)⇄Ca(4s5p 3P) in full collisions with all the noble gases (He, Ne, Ar, Kr, and Xe). The approach and scattering formalism follow our earlier article [J. Chem. Phys. 86, 4790 (1987)]. Flexible Morese-Spline–van der Waals functions are used to describe the relevant 1,3Σ and 1,3Π Ca*+M potential curves. The interpretation of the dependence of the cross section on initial polarization will be guided by diatomic spectroscopy and a consideration of the crossings between thse potential curves. The interpretation of the dependence of the cross section on initial polarization will be guided by diatomic spectroscopy and a consideration of the crossings between these potential curves. A new simple statistical model, based on the consideration of the reflection symmetry of the initially prepared state, will provide the point of departure for the prediction of the polarization dependence. The exact quantum cross sections agree well both with the predictions of the statistical model and with the recent experimental results from Leone’s laboratory at JILA. In addition the calculated 1P→3P cross sections increase with decreasing collision energy, since the crucial 1Π→3Σ crossing point is reached by the attractive branch of the 1Π potential.
Raman emission as a probe for photodissociation dynamics
Authors: Jacon M. , Atabek O. , Leforestier C.
Published: July 2002

A time-dependent wave packet propagation method based on the split operator technique is used to describe the complete history of photodissociation. Three successive steps of the dynamics are discussed by setting the analogy with the energy dependent version of the quantum theory: namely the absorption, the Raman emission, and the fragments internal state analysis. Informations concerning early (absorption), intermediate (Raman emission), and long time dynamics (fragments internal distribution) are related to the parameters of the initial ground and final dissociative potential energy surfaces and to the excitation laser wavelength. The photodissociation of ICN(C̃ 1A′) is taken as a numerical illustration and comparisons are presented with previous calculations carried in the energy frame.
Autoionization mechanism of the NO molecule: Calculation of quantum defect and theoretical analysis of multiphoton ionization experiment
Authors: Nakashima Keiji , Nakamura Hiroki , Achiba Yohji , Kimura Katsumi
Published: July 2002

Quantum defects of lλ-Rydberg states (l=s,p,d, λ=σ,π,δ) of NO were calculated by SCF and CI methods at various internuclear distances. Employing the multichannel quantum defect theory (MQDT), the recently observed resonantly enhanced multiphoton ionization (REMPI) photoelectron spectra were analyzed with use of the calculated quantum defects. Analysis was made particularly to the autoionization of vibrationally excited Rydberg states of σ and δ symmetry. It is concluded that the autoionization is dominated by the two-step electronic mechanism in which dissociative superexcited state plays a role of intermediate state. Potential curves and electronic coupling strengths of the relevant dissociative states (B′ 2Δ and I 2∑) were estimated from this analysis. This provides us with useful information to investigate various dynamic processes involving these superexcited states. Importance of dissociative superexcited state is emphasized in reference to the various dynamic processes.
Infrared multiphoton excitation of CF3I. II. Collisional energy transfer of vibrationally highly excited CF3I
Authors: Abel B. , Herzog B. , Hippler H. , Troe J.
Published: July 2002

Energy transfer of vibrationally highly excited CF3I molecules (E≈18 000 cm−1) in collisions with argon, propane, and octane was studied using hot UV absorption spectroscopy of CF3I. the preparation of the excited CF3I was achieved by IR multiphoton absorption which, under the conditions applied, produces a narrow initial energy distribution of CF3I near to the dissociation energy. The average energies 〈ΔE〉 transferred per collision were found to be proportional to E for the bath gases propane and octane; they showed a stronger increase with energy at low excitation energies in the bath gas argon. The energy dependence of 〈ΔE〉, therefore, is not only governed by the properties of the excited molecule (e.g., its density of states) but also by features of the collision partner (e.g., the magnitude of 〈ΔE〉). At energies near to the dissociation energy for all colliders, 〈ΔE〉 was found to approach values similar to those obtained from single UV photon excitation experiments with highly excited triatomic and large polyatomic molecules.
Vibrational spectrum, dipole moment function, and potential energy surface of the CH chromophore in CHX3 molecules
Authors: Dübal Hans-Rolf , Ha Tae-Kyu , Lewerenz Marius , Quack Martin
Published: July 2002

The Fermi-resonance overtone spectra of the CH chromophore up to about 18 000 cm−1 are evaluated by variational vibrational calculations for the CHX3 molecules trideuteromethane (CHD3), trifluoromethane (CHF3), chloroform (CHCl3) and 1,1,1,3,3,3-hexafluoro-2-trifluoromethylpropane [(CF3)3CH]. Using appropriate model potential functions in a normal coordinate subspace, one can derive parameters for the CH chromophore potential and empirical dipole moment functions. For CHD3 and CHF3 ab initio (SCF-CI and vibrational variational) calculations are presented, the results of which compare well with the experiments and for CHD3 also with previous (MRD-CI) ab initio results. For all cases an accurate similarity transformation to the equivalent tridiagonal form of the effective hamiltonian can be made and the corresponding spectroscopic parameters agree with previous results. Comparison is also made with results from an internal coordinate model Hamiltonian.
Stochastics of rotational isomeric transitions in polymer chains
Authors: Bahar Ivet
Published: July 2002

The stochastic process of conformational transitions between isomeric states in polymer chains is considered. In analogy with the conventional treatments of chain statistics where equilibrium configurations are assigned statistical weights based on near neighbor intramolecular potential, stochastic weights are defined for the configurational transitions undergone by chains of pairwise interdependent bonds. The stochastic weights are expressed by vN×vN matrices for N mobile bonds with v isomeric states accessible to each bond. For a given time, serial multiplication of stochastic weight matrices yields the configurational transition partition function corresponding to the space of time-delayed probabilities of occurrence of two distinct configurations for a mobile segment. A matrix multiplication scheme is devised to determine the fraction of bonds and/or segments that undergo specific isomeric transitions in a given time interval.
Time development of geometric phases in the Longuet-Higgins model
Authors: Romero-Rochin Victor , Cina Jeffrey A.
Published: July 2002

Using a time-dependent variational method, we study the evolution of nonstationary states in Longuet-Higgin’s model of a Jahn–Teller molecule. Conditions are found for the nuclear motion to be adiabatic. The effects of wave-packet spreading are neglected upon specializing to the case of nearly harmonic motion. It is shown explicitly how the effective vector potential introduced by Mead and Truhlar gives rise to an electronic Berry phase. In a semiclassical approximation sufficient to produce the electronic adiabatic phase anticipated from the result for a given sequence of nuclear configurations, it is demonstrated that the effective vector potential has a negligible effect on the nuclear motion; the effective vector potential, the source of an effective field proportional to ℏ, is seen to affect the nuclear trajectory only in higher order. For the special case of periodic nuclear motion the electronic adiabatic phase is seen as a contribution to the geometric phase attending an arbitrary cyclic evolution. It is demonstrated that a molecular state prepared with identically pseudorotating nuclear wave packets in both electronic levels corresponds, in a weak coupling limit, to a spin 1/2 in a conically varying external field. Geometrical phase differences are shown to make discernible contributions to the frequencies of oscillation of the electronic charge and current densities, which may serve as classical sources for superradiant emission. Our results are shown to be gauge invariant.
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s-tunneling spectra of (HF)2 and (DF)2
Authors: Bunker P. R. , Carrington Tucker , Gomez P. C. , Marshall Mark D. , Kofranek Manfred , Lischka Hans , Karpfen Alfred
Published: July 2002

Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the K-type rotation and trans-tunneling motion for (HF)2 and (DF)2 are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B̄ values. The energy levels we obtain are in close agreement with experiment; for example, the K=0 tunneling splitting in (HF)2 is calculated as 0.65 cm−1 compared to the experimental value of 0.658 69 cm−1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual Kdependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK=0 and ±1 subbands in the tunneling spectra of (HF)2 and (DF)2, many of which have not yet been observed.
Photodissociation of CH3ONO in the first absorption band: A three-dimensional classical trajectory study
Authors: Nonella Marco , Huber J. Robert , Untch Agathe , Schinke Reinhard
Published: July 2002

The photodissociation of cis-CH3 ONO following excitation into the first absorption band near 350 nm is investigated by means of classical trajectories and an ab initio potential energy surface. The calculations include the O–N coordinate, the N=O coordinate, and the ONO bending angle as variables whilst the internal degrees of freedom of the CH3 O moiety are kept fixed. The calculated lifetimes range from 120 to 410 femtoseconds for excitation of the n*=4 to n*=0 vibrational states of the terminal NO group in the intermediate complex. They agree well with the lifetimes estimated from the anisotropy parameter β. The ONO bending degree of freedom has only a small effect on the lifetime of the complex. The final vibrational state (n) distribution of the NO fragment exhibits a systematic energy dependence which manifests itself in a propensity for the excitation of level n=n*−1 that is in excellent agreement with the measurement. Two-dimensional calculations for a fixed ONO bending angle cannot satisfactorily reproduce these experimental findings. The rotational state distributions are highly inverted with maxima around j∼30–35 depending slightly on the initial state (n*) and the final state (n) of NO. The overall agreement with the measured distributions is satisfactory. The results of this study emphasize the importance of the bending degree of freedom in the dissociation of CH3 ONO and by revealing the interplay of the three active vibrational modes they provide a detailed picture of the predissociation mechanism in a polyatomic molecule.
Reactions of O−+ N2O at 300 K: The totally labeled experiments
Authors: Barlow S. E. , Bierbaum Veronica M.
Published: July 2002

The tandem flowing afterglow-selected ion flow tube was employed to study the isotopically labeled reactions of O−+N2O→NO−+NO at 300 K. In the reactions of 16O−+15N14N16O, 16O−+14N15N16O and 18O−+14N14N16O, both of the possible NO− products are formed with equal probability, indicating that equilibration is achieved within the reaction complex before dissociation. In the totally labeled reactions of 18O−+15N14N16O and 18O−+14N15N16O all possible NO− products are observed which strongly supports the formation of both trigonal and linear N2O−2 intermediates along the reaction path. The reaction mechanism is discussed and these results are compared with those of other workers.
Dynamics of H2 elimination from 1,4-cyclohexadiene
Authors: Cromwell Evan F. , Liu Di-Jia , Vrakking Marcus J. J. , Kung A. H. , Lee Yuan T.
Published: July 2002

The dynamics of H2 elimination from 1,4-cyclohexadiene (CHDN), dissociated with 212 nm light, have been studied by state specific probing of the H2 product via (1+1) REMPI using a transform limited vacuum ultraviolet laser system. The quantum state distributions, the translational energy distribution and the Doppler profiles were measured giving a ‘‘rotational temperature’’ of ≂1600 K, a ‘‘vibrational temperature’’ of ≂4000 K, and a translational energy of ≂30 kcal/mol for H2. Anisotropy in the Doppler profiles indicates that the H2’s velocity vector is aligned parallel to its rotational angular momentum vector.
Resonance Raman quantum yields for CS2 in solution: Dynamics of solvent-induced spectral broadening
Authors: Myers Anne B. , Li Bulang
Published: July 2002

Electronic absorption spectra, resonance Raman spectra and quantum yields, and total emission yields have been measured for the S3←S0 transition of CS2 in cyclohexane, pentane, acetonitrile, hexadecane, and perfluorohexane solvents. The solution-phase absorption spectra are significantly broadened and redshifted relative to the vapor. The solution-phase S3 state lifetimes inferred from the total emission yields are 0.6–1.0 ps, close to the vapor-phase lifetime, while the resonance Raman quantum yields imply electronic dephasing times of 25–50 fs in solution. This rapid dephasing due to intermolecular (solvent–solute) interactions is sufficient to account for almost all of the increased electronic spectral breadth in solution. The data are analyzed quantitatively with the aid of a stochastic theory of line broadening that accounts for solvent memory effects, and evidence is found for non-Markovian (nonexponential in time) electronic dephasing. Possible physical origins for the rapid electronic dephasing in solution are suggested, and the relevance of these results to other dynamical processes in liquids such as activated barrier crossing and electron transfer reactions is discussed.
Density functional theory of freezing for quantum systems. I. Path integral formulation of general theory
Authors: McCoy John D. , Rick Steven W. , Haymet A. D. J.
Published: July 2002

We report the details of a theory which predicts the freezing of quantum liquids, such as helium [J. Chem. Phys. 90, 4622 (1989)]. The freezing of a wide variety of classical liquids has been described by the density functional (DF) theory of statistical mechanics. By choosing a new ideal system, we construct a new DF theory which addresses directly many of the unusual features of the freezing of liquid helium, such as the weakly modulated liquid pair correlation function g(r) at freezing densities. The theory combines DF techniques with the Feynman path integral formulation of quantum mechanics, to include correctly dispersion effects. In classical DF theories, the density and external field of the ideal system are connected by a Boltzmann relation ρ(r)∝exp[−βV(r)]. In our quantum DF theory, we relate the density and external field of the ideal system through the Feynman path integral representation, in which the quantum particle is represented by a classical ‘‘ring polymer’’ of P beads. In practical applications, the DF perturbation expansion is truncated at second order, and classical DF theories fail for helium because they employ an ideal system which is too far removed from the interacting system. For a certain simplified problem, the density path integral of the ideal system can be performed in closed form, leading to a compact, physically descriptive theory. In a companion paper, the full theory is applied to the freezing of helium-4 and yields good results.
Density functional theory of freezing for quantum systems. II. Application to helium
Authors: Rick Steven W. , McCoy John D. , Haymet A. D. J.
Published: July 2002

The path integral density functional theory of quantum freezing is used to study the freezing of 4He and 3He. The crystal–liquid phase diagram is examined over the temperature range 8 to 204.4 K. The isotopic shift in the liquid–crystal coexistence line is also studied, and we conclude that mass effects, rather than quantum statistics, are important in the freezing transition at these temperatures.