bluuebunny/crossref_metadata_2025 huggingface dataset:
Test of variational transition state theory and the least-action approximation for multidimensional tunneling probabilities against accurate quantal rate constants for a collinear reaction involving tunneling into an excited state
Authors: Garrett Bruce C. , Abusalbi Najib , Kouri Donald J. , Truhlar Donald G.
Published: July 2002

We report and compare two sets of thermal rate constants for the collinear reaction Cl+HBr → HCl+Br, which is dominated by the n=0 vibrational state of reactants and the n=2 vibrational state of products. One set is based on converged multichannel quantal calculations carried out using hyperspherical coordinates. The other set is based on variational transition state theory (VTST) with a least-action (LA) ground-state (G) transmission coefficient (VTST/LAG). The two sets of rate constants agree within a factor of 2.2 over the whole factor-of-12 temperature range (200–2400 K) studied. The error is comparable to what was found previously for the symmetric reaction Cl+HCl′ → HCl+Cl′, which is dominated by n=0 states of reactants and products. This indicates that the VTST/LAG method is as applicable to reactions that proceed by tunneling into excited states as it is for ground-state-to-ground-state tunneling reactions. We also show that the VTST/LA approximation provides useful accuracy for the rate constant of the collinear excited-state reaction Cl+HBr(n=1) → HCl+Br.
Time-of-flight spectroscopy of high-overtone mode-selective vibrational excitation of CF4 and SF6 in collisions with H+(D+) at energies between 10 and 28 eV
Authors: Gierz U. , Noll M. , Toennies J. Peter
Published: July 2002

Time-of-flight spectra for H+(D+)–CF4 and SF6 collisions have been measured with an improved resolution and at higher collision energies (10≤Elab ≤28 eV) compared to earlier work. In the low energy region (≤13 eV) new distinct peaks are resolved for both molecules corresponding to small contributions from the second infrared active ν4 mode in addition to the dominant ν3 mode observed previously. Reexamination of experimental vibrational transition probabilities reveals an almost perfect agreement with a Poisson distribution for both modes up to the n=6 overtone transition of ν3. A simple straight line theory is used to calculate the energy transfer in small angle scattering from the long-range potential in good agreement with a full classical trajectory calculation. With this theory dipole moment derivatives can be determined directly from the observed energy transfers and are found to agree well with previous infrared measurements. At larger collision energies (≥16 eV) an additional low intensity vibrational distribution is identified in the high energy loss tail of the spectra which can be attributed to small impact parameter collisions which probe the repulsive region of the potential. The observed energy transfers are also in good agreement with trajectory calculations indicating that the forced oscillator model is also applicable in the repulsive potential region for the present systems. A closer examination of the high energy loss tail reveals resolved structure which has been assigned to discrete states of the ν3 mode in CF4 up to the n=14 overtone. These new results demonstrate that H+(D+) energy loss scattering can provide spectroscopic information not readily available from other experiments.
The exact eigenfunctions and eigenvalues of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics
Authors: Reimers Jeffrey R. , Heller Eric J.
Published: July 2002

Exact eigenfunctions for a two-dimensional rigid rotor are obtained using Gaussian wave packet dynamics. The wave functions are obtained by propagating, without approximation, an infinite set of Gaussian wave packets that collectively have the correct periodicity, being coherent states appropriate to this rotational problem. This result leads to a numerical method for the semiclassical calculation of rovibrational, molecular eignestates. Also, a simple, almost classical, approximation to full wave packet dynamics is shown to give exact results: this leads to an a posteriori justification of the De Leon–Heller spectral quantization method.
Electron energy loss spectroscopy (EELS) of CH3NH2 adsorbed on Ni(100), Ni(111), Cr(100), and Cr(111)
Authors: Baca A. G. , Schulz M. A. , Shirley D. A.
Published: July 2002

Adsorbed CH3NH2 has been studied on Ni(100), Ni(111), Cr(100), and Cr(111) at 300 K using electron energy loss spectroscopy. The vibrational spectra indicate that molecular CH3NH2 exists on all four surfaces with bonding through the nitrogen lone pair, although a substantial amount of dissociation also occurs on the chromium surfaces. Approximately one monolayer of surface species is the stable coverage on each surface at 300 K. The possible existence of coadsorbed dissociation products CHx and NHx is discussed. The CN stretch is anomalously broad on Ni (100) and Ni(111) but not detectably broadened on Cr(100), indicating a strong sensitivity of this bond to interactions with the Ni surfaces. A multiplicity of sites is indicated on Cr(111) by the breadth of all the peaks. The loss spectra exhibit striking intensity differences, which can be attributed partly to impact scattering and partly to intrinsic differences in the interaction of CH3NH2 with the different surfaces. A model explaining the linewidth and intensity differences is proposed.
A multiproperty empirical potential energy surface for the reaction H+Br2→HBr+Br
Authors: Blais Normand C. , Truhlar Donald G.
Published: July 2002

We have made systematic variations in our previously reported potential energy surface for the reaction H+Br2, and we have tested the new potential energy surfaces by extensive trajectory calculations. We have attempted to find a surface that simultaneously displays as many as possible of the attributes of this reaction in agreement with experiment and that has a bent transition state with a weak bending force constant for consistency with recent ab initio calculations. The best surfaces overall, called surfaces 18M and 19M, are in reasonable agreement with experiment for the angular scattering distribution, the reaction cross section as a function of translational energy, the average vibrational and rotational energies of the product HBr, and the thermal rate constant at room temperature. They are less accurate for the deuterium isotope effect and the breadth of the product vibrational distribution at room temperature, and for the average product translation energy at high collision energy.
The mean bond displacements and the derivatives of 19F shielding in CF2=CFX and CF2=CH2
Authors: Jameson Cynthia J. , Osten H. Jörg
Published: July 2002

The mean bond displacements 〈Δr〉 for molecules CF2=CFX (X=H, F, Cl, Br, I) and CF2=CH2 have been calculated as a function of temperature using a Urey–Bradley force field augmented by stretching anharmonic terms and cubic nonbonded interactions. The temperature dependence of the 19F nuclear shielding in the isolated molecule limit is interpreted in terms of 〈ΔrCF〉T and an electronic factor (∂σF/∂ΔrCF)e which is found by fitting the experimental data with σ0(T)−σ0(300 K)=(∂σF/∂ΔrCF)e[〈ΔrCF〉T −〈ΔrCF〉300] . The derivatives for each of the three F sites in the CF2=CFX molecules vary systematically with the absolute shielding of the F site. The 19F nucleus gem to the X substituent exhibits the greatest change of shielding with bond displacement. For X=Cl, Br, I, the derivatives (∂σF/∂ΔrCF)e are (−1940, −2000, −1950 ppm Å−1) for the F nucleus trans to X; (−2190, −2290, −2300 ppm Å−1) for the F nucleus cis to X; and (−2550, −2890, −3160 ppm Å−1) for the F nucleus gem to X. In CF2=CF2 and CF2=CH2 the derivatives are −2080 and −990 ppm Å−1, respectively. For CF2=CFH they are −1250, −1190, and −2260 ppm Å−1 for the F nucleus trans, cis, and gem, respectively, to the H substituent.
High resolution Raman spectra of near-critical CO2
Authors: Deacon C. G. , Clouter M. J. , Kiefte H.
Published: July 2002

The pure vibrational (Q branch) components of both the ν1 and 2ν2 Raman bands of supercritical CO2 have been studied using interferometric techniques. A significant broadening of the ν1 spectrum was observed as the critical point (Tc=304.15 K) was approached along the critical isochore.
Simulation of the cubic to orthorhombic phase transition in potassium cyanide
Authors: Impey Roger W. , Sprik Michiel , Klein Michael L.
Published: July 2002

Constant pressure (NPH) ensemble molecular dynamics calculations have been used to study the cubic to orthorhombic phase transition that occurs upon cooling potassium cyanide at low pressures. A rigid ion model consisting of interionic electrostatic terms plus nonbonded atom–atom interactions has been found to yield an almost quantitative account of the transition. In particular, the extreme softening of the shear elastic constant C44 and the anomalous dispersion of the transverse acoustic phonons propagating along the crystal [100] direction in the cubic phase are well reproduced.
Multiphoton ionization spectroscopy and vibrational analysis of a 3p Rydberg state of the hydroxymethyl radical
Authors: Dulcey Charles S. , Hudgens Jeffrey W.
Published: July 2002

The resonance enhanced multiphoton ionization (REMPI) spectra of CH2OH, CH2OD, CD2OH, and CD2OD between 420–495 nm are reported. Analysis of the excited state vibrational band progressions shows that the spectrum originates from simultaneous two photon absorption to form a 3p Rydberg state (ν00=40 064 cm−1) of the radical. Absorption of a third photon ionized the radical. A normal mode analysis of the REMPI spectra enabled assignments of six active vibrational modes in the excited state. The no isomerization of methoxy radicals into hydroxymethyl radicals was observed. The experimental measurement uncertainty limits this isomerization rate to less than 2.9 s−1 at 300 K.
Microwave spectrum and molecular structure of g
a
u
c
h
e-1,2-difluoroethane
Authors: Takeo Harutoshi , Matsumura Chi , Morino Yonezo
Published: July 2002

The microwave spectra of four isotopic species of gauche-1,2-difluoroethane have been observed. Having combined the rotational constants obtained with those of the normal species, all the structural parameters have been determined as follows; r(C–C)=1.493(8) Å, r(C–F) =1.390(3) Å, r(C–Hg)=1.099(2) Å, r(C–Ht)=1.093(5) Å, ∠CCF=110.6(5)°, ∠CCHg=108.4(6)°, ∠CCHt=111.3(6)°, ∠HCH=109.1(5)°, ∠FCHg=109.6(3)°, ∠FCHt=107.8(6)°, and FCCF (dihedral angle)=71.0(3)°. The spectra of normal species in the excited torsional states have also been observed up to vt=4, and the results of the analysis indicate that the gauche-gauche barrier should be higher than the value, 468.6 cm−1, reported by the analysis of vibrational spectra.
The randomly broken chain as a semiflexible macromolecular model. Computer simulation of statistical properties
Authors: García Molina Jesús J. , García de la Torre José
Published: July 2002

At any repeating unit of the randomly broken chain model, the direction of the chain can either follow that of the preceding bond, or change at random. The contour-length dependence of the mean-squared end-to-end distance and radius of gyration, when expressed in terms of a persistance length, is found to be the same as that for worm-like chains. However, other moments of the end-to-end distance have a clearly distinct length dependence, as found by computer simulation. Due to the finite probability of a fully extended, straight conformation, the distribution function of the end-to-end distance has a peculiar sharp peak. The similarities and differences between this model and the worm-like chains are discussed.
Reaction path and variational transition state theory rate constant for Li++H2O→Li+(H2O) association
Authors: Mondro Sandra L. , Vande Linde Scott , Hase William L.
Published: July 2002

Canonical variational transition state theory rate constants are calculated for Li++H2O→Li+(H2O) recombination. Temperature dependence transition states are determined by finding the maxima in the free energy along the reaction path. Only one maximum is found at each temperature. The transition state theory rate constants are larger than those determined in a previous quasiclassical trajectory study of Li++H2O recombination. This results from a dynamical recrossing of the transition state dividing surface in the trajectory calculations.
Optically detected magnetic resonance at high pressures: Crystalline and molecular compression in organic solids
Authors: Sandroff C. J. , Chan I. Y. , Chung W. M.
Published: July 2002

We report on optically detected magnetic resonance experiments conducted at high pressures on an organic mixed crystal system. Changes in triplet state zero-field splittings (ZFS) of the guest molecule monitored as a function of host crystal compression reveal the existence of three compressional regimes. Up to 7 kbar most contraction is intermolecular with little change in intramolecular distances. From 7 to 25 kbar shrinkage of the lattice is proportional to the decrease in molecular volume. Above 25 kbar this linearity no longer holds, and the ZFS suggests a distortion in molecular geometry.
A rotational-dependent analytical solution to the dissociative state: Application to b 3Σ+u state of H2
Authors: LaFleur Edward E. , Chiu Lue-Yung Chow
Published: July 2002

The rotational-dependent potential for a dissociative state is represented by U(r)=U0+B1/r +B2/r2+[N(N+1)−Λ2]/2Mr2. An analytical solution ψE(r) of the Schrödinger radial equation, valid for all regions of internuclear distance r and energy E, is obtained in terms of confluent hypergeometric function of the complex arguments. The solution is evaluated by expanding the confluent hypergeometric function onto a basis set of shifted Chebyshev polynomials. The expansion coefficients are recovered by a backward recursion technique. The summation process of Chebyshev polynomials converts a slowly convergent series or a divergent asymptotic series into a rapidly convergent one. The solution thus obtained is applied to calculate the vibrational wave function of the dissociative b 3Σ+u state of H2 to compare with the previous semiclassical WKB wave function. The solution of the rotational-corrected Morse potential is used for the upper bound c 3Πu state. The bound-continuum Frank–Condon overlap amplitude is computed as a function of energy E for different rotational quantum numbers N. Its dependence on N is found to be significant for large N. The decay rate of the metastable c 3Π+u (v=0), via perturbative mixing with b 3Σ+u, computed here with exact wave functions, is an order of magnitude smaller than the previous semiclassical value. However, the decay rate via forbidden radiative transitions to b 3Σ+u is close to the previous value. Radiative transition to b 3Σ+u is now believed to be the predominant decay mode of the metastable c 3Π+u state (at v=0). Lifetimes of the fine structure levels of N=1 and N=2 obtained are 1.00 ms for J=N and 1.31–1.32 ms for J=N±1. The lifetimes of the predissociative c 3Π−u (v=0) state are 2.33×10−8 s for N=1 and 7.65×10−9 s for N=2.
Fine structure effect on the charge transfer reaction of Ar+(2P3/2,1/2)+N2 (X̃ 1Σ+g, v=0)
Authors: Liao C. L. , Xu R. , Ng C. Y.
Published: July 2002

The relative spin-orbit-state-selected total charge transfer cross sections, σ3/2 and σ1/2, for the reactions, Ar+ (2P3/2,1/2)+N2 (X̃ 1∑+g ,v=0) → Ar+N+2 at the center-of-mass collision energy (Ec.m.) range of 0.41–164.7 eV, have been determined using the photoionization and crossed ion–neutral beam methods. The kinetic energy dependences for σ1/2/σ3/2 is found to exhibit a minimum at Ec.m.≊3.3 eV, an observation consistent with the prediction of a recent semiclassical theoretical calculation of Spalburg and Gislason.
Propagation of a chemical pulse in an illuminated thermochemical bistable system
Authors: Kramer Jesse , Reiter Johannes , Ross John
Published: July 2002

Propagating chemical pulses are observed in the illuminated dimerization reaction of S2O6F2 when this bistable system is suitably perturbed. The dependence of the speed of propagation on incident laser power and the temporal evolution of the absorption profile along the laser beam are measured. The results compare well with calculations based on a previously developed model. The propagating chemical pulse is compared to chemical waves in an excitable medium.
Accurate numerical orbital MBPT/CC study of the electron affinity of fluorine and the dissociation energy of hydrogen fluoride
Authors: Adamowicz Ludwik , Bartlett Rodney J.
Published: July 2002

The classic problem of the dissociation energy of FH and the electron affinity of F have been studied using our recent proposed numerical orbital coupled-cluster (CC) method. With 105 orbitals, 96.6% of the correlation energy of FH is obtained, and exact agreement with the experimental dissociation energy. Similarly, the electron affinity of F is found to be 3.37 eV compared to an experimental value of 3.399 eV.
On the dissociation dynamics of energy-selected nitrobenzene ion
Authors: Nishimura Toshihide , Das Pradip R. , Meisels G. G.
Published: July 2002

Fragmentation mechanisms of nitrobenzene ions have been investigated using threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The detailed breakdown curves were obtained at internal energies of the molecular ion from 0 to 8 eV, and fragmentation pathways were elucidated. The precursor of C4H+3 has been identified clearly as the C6H+5 ion from the breakdown graph. Decay rates of metastable decompositions were measured from the asymmetric peak shapes of their coincidence time-of-flight (TOF) peaks for the C6H5O+, C6H+5, C4H+3, and NO+ ions. The activation energies for NO+ and C6H5O+ formation were estimated as 1.02 eV on the basis of RRKM/QET calculations. Average kinetic energy release for the formation of phenyl cation by direct C–N bond cleavage showed that about 4.2% of the internal energy is converted to translation; this is in good quantitative agreement with the calculations based on the statistical phase space theory. On the other hand, the dependence of the fragmentation rate constant on internal energy is in good agreement with the RRKM/QET calculation assuming a slightly tight transition state. However, the decay of C6H+5 to C4H+3 and C2H2 was one to two orders of magnitude faster than predicted by RRKM/QET assuming direct fragmentation. This result is ascribed to isomerization of phenyl cation to a linear structure before dissociation.
Importance of linked diagrams in the recursive residue generation method
Authors: Schek Israel , Wyatt Robert E.
Published: July 2002

Previous studies on the use of diagrammatic moment methods in the recursive residue generation method [I. Schek and R. E. Wyatt, J. Chem. Phys. 83, 3028, 4650 (1985)] are extended, with emphasis upon the role played by linked diagrams. Equivalence between the Green function for the source state (which initiates the recursion method) in the zero order and the corresponding tridiagonal (Lanczos) representation is shown. Chain parameters in the tridiagonal representation are constructed by comparing linked diagrams in the two representations and the dynamics of the source state is introduced in terms of the linked moments. The equivalence between Lanczos states and generalized doorway states is shown. Finally, a closed expression for the level shift of the source state (due to its interaction with the rest of the states) is given, using a diagrammatic approach in the tridiagonal representation.
Single quantum state molecular beam scattering of vibrationally excited NO from Ag(111) and Ag(110)
Authors: Misewich J. , Loy M. M. T.
Published: July 2002

Using an infrared laser excitation/ultraviolet laser detection technique, we have studied the internal energy and state-specific angular and velocity distributions for a molecular beam of NO(v=1) scattered from single crystal Ag(111) and Ag(110) surfaces in ultrahigh vacuum (UHV). The vibrational energy survival probability is large in both cases, ∼0.9, although the details of the scattering are different for the two surfaces.
Radiative decay and radiationless relaxation of NH/ND(a 1Δ) isolated in rare gas matrices
Authors: Ramsthaler-Sommer A. , Eberhardt K. E. , Schurath Ulrich
Published: July 2002

Lifetimes of matrix-isolated NH/ND(a 1Δ) radicals have been measured as function of temperature and rare gas host. The metastable species were generated directly by in situ photolysis of hydrazoic or isocyanic acid, or by pulsed excitation of the b 1Σ+ state with a dye laser, which subsequently decays to the a state on a μs time scale. Rotation of NH/ND in the electronic ground state is perturbed or inhibited by the second photofragment in the photolysis systems. The lifetimes of NH(a 1Δ) in Ne, Ar, and Kr show little temperature dependence. This is consistent with a radiationless contribution to the overall relaxation in which the energy gap to the next lower vibrational level of the ground state is accepted by guest rotation and/or other local modes. The strong temperature dependent decay of ND(a 1Δ) in Ar and Kr is due to endothermic near resonant relaxation to the sixth vibrational level of the ground state, with an activation energy in the order of the energy gap, and a frequency factor of 20±5 s−1. The relaxation mechanism of NH/ND(a 1Δ) in solid Xe is of a different nature, the data being consistent with a temperature dependent external heavy atom effect. A lower limit of 1.9 s has been deduced for the radiative lifetime of the (a 1Δ) state in vacuo, in reasonable agreement with a recent ab initio calculation.
Polarized Raman spectra of thin films. II. Apparent anomaly of polarization with uniaxially oriented samples
Authors: Harrand Monique
Published: July 2002

With uniaxially oriented thin films it happens that the two polarized Raman spectra given by the E∥ excitation are not alike as could be expected. It is shown that because the exciting beam is slanted inside the film, two components of the electric field are to be considered which give the two ‘‘complex’’ polarized Raman spectra: I∥X=(α2XX cos2 r +α2XZ sin2 r)E20 and I∥Y=(α2YX cos2 r +α2YZ sin2 r)E20. With uniaxially oriented samples the intensities can be added, as shown in part I. The values of α2XZ=α2YZ, obtained from the spectra are small in agreement with the calculated values. As these terms decrease when the tilting angle θ of the chains decreases in the case of properly oriented samples, the second terms of I∥X and I∥Y are negligible compared with the first ones, even if cos2 r is small (when the exciting beam is nearly perpendicular to the scattered beam). Therefore, contrary to expectations, the observed spectra are more similar to the (XX) and (XY) spectra than to the (ZX) and (ZY) spectra. The discussion has been carried out with respect to the ν(CH2) vibrations of the acyl chains in phospholipid films and completed with the comparison of the ν(C–C) relative intensities.
Fluorescence of cyclohexane vapor: Pressure dependence and quenching studies
Authors: Wickramaaratchi M. A. , Preses J. M. , Weston Ralph E.
Published: July 2002

The pressure dependence of the fluorescence lifetime of cyclohexane vapor excited at wavelengths between 168.5 and 175.0 nm has been investigated. Due to the unusual dependence of the lifetime on cyclohexane pressure, a simple Stern–Volmer type of extrapolation cannot be used to obtain the collisionless lifetime at any of the excitation energies employed. Also, fluorescence decay curves fit a single-exponential function only under certain conditions of pressure and excitation wavelength. The rate constants for quenching of the excited state(s) by O2 at excitation wavelengths of 172.0 and 169.5 nm are (6.1±0.1)×10−10 and (7.9±1.4)×10−10 cm3 molecule−1 s−1, respectively. The difference between these rate constants is not significantly larger than the experimental uncertainty. At 172.0 nm, the rate constant for quenching by Xe is (7.1±0.3)×10−12 cm3 molecule−1 s−1. At this same excitation wavelength, quenching by He, Ar, and N2 is extremely small.
Electronic spectrum of Hg(3P)–Ar complex in a supersonic jet
Authors: Fuke Kiyokazu , Nonose Shinji , Kaya Koji
Published: July 2002

The ã excited state of HgAr, which correlates asymptotically with Hg (3P0)+Ar(1S0), was examined for the first time under supersonic jet conditions. Laser double resonance experiments on the ã–c̃, ã–D̃, and Ã–C̃ transitions of the complex have revealed the potential energy curve of this hitherto unknown state. The state was found to have a double minimum potential; D0=97 and ∼16 cm−1.
Order–disorder transitions of π-conjugated polymers in condensed phases. II. Model calculations
Authors: Schweizer Kenneth S.
Published: July 2002

Numerical calculations of the phase behavior and spectroscopic properties of π-conjugated polymers are presented based on the theory developed in the preceding paper. The specific model studied includes the single chain statistics and the configuration-dependent polymer–solvent interactions. The detailed dependence of the transition temperature and coexistence region on solvent, defect formation energy, chain length, and polydispersity is established. Relevance of the equilibrium calculations to electronic absorption and light scattering measurements is discussed. Experimental implications of the results for the polydiacetylenes, and the effects of system-specific generalizations of the theory, are briefly considered.
Studies of chain conformational kinetics in poly(di-n-alkylsilanes) by spectroscopic methods 2. Conformation and packing of poly(di-n-hexylsilane)
Authors: Kuzmany H. , Rabolt J. F. , Farmer B. L. , Miller R. D.
Published: July 2002

Unoriented and highly oriented films of poly(di-n-hexylsilane) have been studied by Raman scattering, wide angle x-ray diffraction and optical absorption measurements. From a comparison of group theoretical predictions with those bands observed in the Raman experiments and from the x-ray layer line spacing from an oriented sample, a planar zig–zag conformation for the silicon backbone was deduced. X-ray reflections were suitably indexed by a monoclinic unit cell containing two molecules. Polarized Raman studies on uniaxially oriented samples also revealed that the hexyl side chains are not orthogonal to the silicon backbone but may be slightly tilted in order to minimize intramolecular steric interactions.
The solution of the Percus–Yevick approximation for fluids with angle-dependent pair interactions. A general method with results for dipolar hard spheres
Authors: Fries P. H. , Patey G. N.
Published: July 2002

A general method is presented which allows the Percus–Yevick (PY) and reference Percus–Yevick (RPY) integral equation theories to be solved for fluids of particles interacting with anisotropic pair potentials. The numerical solution is based upon a fast quasianalytic method of expanding the closure relationships in a basis set of rotational invariants. The RPY equations are solved explicitly for dense dipolar hard sphere fluids and the results obtained are compared with Monte Carlo data and with other integral equation approximations.
Raman isosbestic points from liquid water
Authors: Walrafen G. E. , Hokmabadi M. S. , Yang W.-H.
Published: July 2002

Precise isosbestic points occur in the Raman OH-stretching spectra from liquid water between 3 and 85 °C if cell alignment is accomplished with Newton’s rings. Isosbestic frequencies measured for the orientations X(Y,X+Z)Y=6β2, X(ZX)Y=3β2, X(Y+Z,X+Z)Y=45α2+13β2, X(Z,X+Z)Y=45α2+7β2, and X(ZZ)Y=45α2+4β2 are 3524, 3522 (note β2 agreement), 3468, 3425, and 3403 cm−1, respectively. Isosbestic points from two different measurements calculated by the relations, X(ZZ)Y-(4/3)X(ZX)Y and X(Z,X+Z)Y-(7/6)X(Y,X+Z)Y agree exactly for 45α2, 3370 cm−1. (α and β2 correspond to the mean polarizability and square of the anisotropy.) The pure α2 isosbestic frequency, 3370 cm−1, coincides with the peak of the highest frequency hydrogen-bonded (HB) Gaussian OH-stretching component. The pure β2 isosbestic point, 3522–3524 cm−1, coincides with the peak of the nonhydrogen-bonded (NHB) Gaussian OH-stretching component, next above in frequency. The α2 and β2 isosbestic points are thus thought to provide an experimental distinction between, and a clear definition of, the HB and NHB OH-oscillator classes for water. Moreover, the various OH-stretching combinations of α2 and β2 simply provide different measures of the

HB→NHB equilibrium—no special information concerning the temperature dependence of this equilibrium results from use of any one linear polarizability combination over any other, including pure α2 or pure β2. The present results agree with mercury-excited data [Walrafen, J. Chem. Phys. 47, 114 (1967)] for X(Y+Z,X+Z)Y and with the corrected α2 data of d’Arrigo et al. [J. Chem. Phys. 75, 4264 (1981)]. Furthermore, the new data are in accord with the spectroscopic mixture model, but the continuum model conflicts with the observation of exact points. The isosbestic frequencies are also found to be strongly nonlinear in the amount of α2 or β2 involved in the spectra.
Two-Hilbert space formulations of the quantum statistical mechanics of reactive fluids: Dimer formation and decay
Authors: Evans J. W.
Published: July 2002

A two-Hilbert space formalism is first used to develop a general class of representations for the quantum mechanics of N-particle reactive systems. Here the N-particle Hilbert space ℋN is supplemented by a larger arrangement channel space 𝒞N of vectors with Hilbert space valued components for each N-particle clustering, and an injection mapping of ℋN vectors into ‘‘physical’’ 𝒞N vectors. Such representations, for which components of the latter vectors carry an appropriate physical clustering interpretation, provide a rigorous and flexible basis for describing the statistical mechanics of reactive fluids, where atoms and molecules are treated on an equal footing (the molecular picture). Corresponding equilibrium multispecies fugacity or virial expansions follow immediately. Here we focus on analysis of the (previously derived) arrangement channel BBGKY hierarchy for a system where recombination and dissociation, as well as exchange reactions, occur. This formulation (coupled with a corresponding scattering theory) automatically suggests a reactive Boltzmann ansatz which incorporates (standard) noninteracting asymptotic dynamics only for two-molecule nonreactive and reactive exchange collisions. In contrast, e.g., with three molecule recombination, two-molecule dynamics for all three pairs is included (as required for a description of recombination via gradual stabilization of metastables). Finally we compare the resulting reduced form of appropriate channel space hierarchy equations, for a process involving dimer formation and decay, with the corresponding kinetic equations of Lowry and Snider.
C–H local modes in cyclobutene. II. Laser photoacoustic studies 10 000–17 000 cm−1. Vibrational structure and C–H local mode dynamics
Authors: Baggott J. E. , Law D. W. , Lightfoot P. D. , Mills I. M.
Published: July 2002

In part I of this study [Baggott, Clase, and Mills, Spectrochim. Acta Part A 42, 319 (1986)] we presented FTIR spectra of gas phase cyclobutene and modeled the v=1–3 stretching states of both olefinic and methylenic C–H bonds in terms of a local mode model. In this paper we present some improvements to our original model and make use of recently derived ‘‘x,K relations’’ to find the equivalent normal mode descriptions. The use of both the local mode and normal mode approaches to modeling the vibrational structure is described in some detail. We present evidence for Fermi resonance interactions between the methylenic C–H stretch overtones and ring C–C stretch vibrations, revealed in laser photoacoustic spectra in the v=4–6 region. An approximate model vibrational Hamiltonian is proposed to explain the observed structure and is used to calculate the dynamics of the C–H stretch local mode decay resulting from interaction with lower frequency ring modes. The implications of our experimental and theoretical studies for mode-selective photochemistry are discussed briefly.
Product quantum state distributions in unimolecular reactions involving highly flexible transition states
Authors: Marcus R. A.
Published: July 2002

An expression for the distribution of quantum states of the reaction products of unimolecular dissociations is obtained, based on statistical theory. A recently formulated RRKM-type treatment of unimolecular reactions with highly flexible transition states is used to obtain a distribution of quantum states of the products, by introducing an adiabatic approximation for motion from transition state to products. Any impulsive (nonadiabatic) exit channel effects are neglected thereby. Both the final yields of the quantum states of the products and the time evolution of these states are considered. The time evolution of the yield of the products can permit a direct test of non-RRKM effects and, additionally via the long-time component, of other aspects of RRKM theory. The long-time component of the yield of individual quantum states of the products then provides a test of the additional (here, adiabatic) approximation. Such tests are the more definitive the narrower the distribution of initial E’s and J’s of the dissociating molecule.
The singlet–triplet energy separation in silylene
Authors: Balasubramanian K. , McLean A. D.
Published: July 2002

MCSCF calculations followed by large scale configuration interaction, comparable with the best done for isovalent methylene, give a singlet–triplet splitting in silylene of 21.0±1 kcal/mol. The singlet is the lower energy state. Structural parameters for the as yet unobserved triplet are re =1.48±0.005 Å and θe =118.5±1°.
Quantitative spectroscopy study of HBr in the 105–235 nm region
Authors: Nee J. B. , Suto Masako , Lee L. C.
Published: July 2002

The photoabsorption and fluorescence cross sections of HBr were measured in the 105–235 nm region using synchrotron radiation as a light source. Sharp absorption bands were observed in the 105–120 nm region. These bands were classified into Rydberg series, and their oscillator strengths were measured. Fluorescence in the ultraviolet region was observed and used to identify the V 1∑+ state.
Fine-structure-dependent product branching in the ionization of Ar by Ne*(3s 3P2, 3P)
Authors: Weiser C. , Siska P. E.
Published: July 2002

Using laser-state-selected metastable neon combined with mass spectrometric product detection in a crossed supersonic beam experiment, we have found a marked dependence of the fraction of associative ionization in Ne*+Ar Penning ionization on the initial fine-structure state of Ne*. Ne*(3P0) produces more NeAr+ than 3P2 relative to the total ionization cross section by amounts ranging from 33% more at E=1.5 kcal/mol to 72% at E=4.5.
On the choice of phase in the CS approximation: Integral equation approach
Authors: Snider R. F.
Published: July 2002

As usually presented, the centrifugal sudden approximation examines how each individual angular momentum component of the scattering wave function and scattering matrix element is to be estimated. Here the three-dimensional structure of the scattering wave function is emphasized and any decisions about associating exact and approximate partial waves with correcting phase factors are put off till the implications on the three-dimensional wave function have been made clear. It is found that no correcting phase factor needs to be applied to the CS S matrix when estimating the exact S matrix, while at the same time, the asymptotic behavior of the exact and approximating wave functions has the same form. Direct and operator methods of estimating the transition matrix confirm this conclusion. A suggested modification of how the sudden approximations are to be implemented allows the weak potential limit of the resulting scattering amplitudes to reduce to the Born approximation.
A lattice model for self- and mutually avoiding semiflexible polymer chains
Authors: Bawendi M. G. , Freed Karl F.
Published: July 2002

We introduce a spin field theory for many self- and mutually avoiding polymers with arbitrary stiffness on a regular lattice. The model allows for the complete crossover between flexible polymers and rods. The model also includes arbitrary polymer length distributions and arbitrary volume fractions from the highly dilute regime to the melt. The mean field approximation to the full theory reproduces Flory theory, but our model permits a rigorous and systematic evaluation of corrections to the mean field approximation. The corrections are in the form of a double expansion in powers of the volume fraction ψ and, formally, in powers of the inverse lattice coordination number z−1. We present the correction to first order in z−1 and discuss its relevance to the entropic contribution to the Flory χ parameter for semiflexible polymers.
Interpretation of the Auger electron spectra of nitrous oxide
Authors: Larkins F. P.
Published: July 2002

The nitrogen and oxygen Auger spectra associated with the nitrous oxide molecule have been calculated using a semiempirical model analogous to that used previously to explain atomic Auger spectra. The role of the central and terminal nitrogen atoms is elucidated. Good agreement with the experimental spectrum is obtained especially for the outer valence region.
Criteria for the rapid diffusion limit of fluorescence energy transfer
Authors: Kinosita Kazuhiko , Kouyama Tsutomu , Ikegami Akira
Published: July 2002

Excitation energy transfer in a system where the spatial relationship between the donor and acceptor is not fixed generally leads to a nonexponential decay of donor fluorescence. A single-exponential decay, however, is expected as a limiting form when the diffusion of the donors and/or acceptors is sufficiently rapid. The exponential character at the rapid diffusion limit will greatly facilitate the analysis of experimental data. In this paper a theoretical framework is presented that allows the calculation of the criterion for the rapid diffusion limit. Explicit criteria are given for various donor–acceptor geometries, all for the case of energy transfer via the resonance interaction of the Förster type. The criteria, except for the cases of densely distributed acceptors under a wide surface, have a common form DτDa4/R0≫λ, where τD is the lifetime of donor fluorescence in the absence of acceptors, D is the sum of diffusion coefficients of the donor and acceptor, a is the distance of closest approach between the donor and acceptor, R0 is the critical distance for energy transfer, and λ is a geometrical constant with a value less than one. Exponential decays are not easily obtained when acceptors are densely distributed under a wide surface. The results are compared with the criterion given earlier by Thomas et al. [D. D. Thomas, W. F. Carlsen, and L. Stryer, Proc. Natl. Acad. Sci. U.S.A. 75, 5746 (1978)]. Experimental aspects, such as the effect of heterogeneity in a sample, are also discussed.
Statistical phase space theory of ion–polar molecule systems: Application to the reaction H2O⋅H3O+→H2O+H3O+
Authors: Bass Lewis M. , Bowers Michael T.
Published: July 2002

The formalism is presented for incorporating the charge–permanent dipole interaction into the statistical phase space theory model (as developed by Chesnavich and Bowers for polyatomic ion–nonpolar molecule systems) of kinetic energy release distributions. The theory is applied to the dissociation of H2O⋅H3O+ clusters for which the measured kinetic energy release distribution is in the range of a few meV. Excellent agreement between theory and experiment is obtained when the ion–dipole interaction is treated according to ADO theory. It is suggested that the success of the ADO treatment in modeling experimental results in this energy regime is an indirect indication that ADO collision rates may be accurate down to the very low temperatures of interstellar clouds (e.g., 3/2 kT at 10 K is roughly 1.3 meV). This point is discussed with regard to recent work which has led to the opposite conclusion.
Reaction dynamics for O(3P)+H2, D2, and HD. VI. Comparison of TST and reduced dimensionality quantum and quasiclassical isotope effects with experiment
Authors: Wagner Albert F. , Bowman Joel M.
Published: July 2002

Reduced dimensionality exact quantum and quasiclassical trajectory isotope effects are presented for the O(3P)+H2, D2, and HD reactions. Two potential energy surfaces are used: the ab initio MODPOLCI and the semiempirical LEPS surfaces studied in previous papers in this series. Isotope effects are also calculated by conventional transition state theory with a Wigner tunneling correction. All the calculated results are compared to recent experimental measurements of the isotope effects. The measured values show that H atom abstraction is essentially the same from HH or HD, as is the D atom abstraction from either DD or DH. Only the reduced dimensionality quantum calculations on the MODPOLCI surface are in agreement with these results.
Theoretical and experimental estimates of the acetylenic deuteron quadrupole coupling constant
Authors: Asdjodi M. R. , Gregory R. V. , Lickfield G. C. , Spencer H. G. , Huffman J. W. , Savitsky G. B.
Published: July 2002

The acetylenic deuteron quadrupole coupling constant in 1-D-3,3-dimethyl-1-butyne was determined in neat liquid and in 20 mol % solution in an inert solvent (C6H12), by combining NMR relaxation data with Raman line shape analysis of the totally symmetric A1 mode at 1983 cm−1. The value in the inert solvent was found to be 233±4 Hz, which is significantly higher than previously found value for acetylenic deuteron in acetylene and nonyne, but is in excellent agreement with ab initio calculation which was performed using ‘‘double zeta plus polarization’’ basis functions on smaller model molecules.
Photoabsorption and photofragmentation of CS2 in the energy regions of the C 1s and S 2p thresholds
Authors: Hayes Robert G.
Published: July 2002

We have measured the production of various prominent ionic species from carbon disulfide by photoionization using radiation in the energy region of the C 1s edge and of the S 2p edge. We find that the species which are produced by core excitation depend on the level which is excited and also on the mode of excitation. That is, the products from excitation of resonances are different from those produced by core ionization.
Semidilute polymer solutions in the theta domain: A renormalization group study
Authors: Cherayil Binny J. , Kholodenko A. L. , Freed Karl F.
Published: July 2002

Renormalization group methods are developed for describing the osmotic pressure of semidilute polymers in the theta region, where large binary polymer interactions may become screened. The analysis requires a combination of renormalization group methods as they are applied, on the one hand, to semidilute polymer solutions in marginal-to-good solvents, and, on the other hand, to single polymers in theta solutions. Ternary interactions are shown to contribute to the screening of binary interactions in these semidilute theta solutions, and an effective binary interaction is introduced that is defined to vanish at the theta point along with the second virial coefficient, leaving residual ternary interactions. Screening of the effective binary interactions is described through an Edwards decoupling scheme, and perturbation expansions are developed in the presumed small screened effective binary and the residual ternary interactions. Formal divergences in the osmotic pressure are absorbed into phenomenologically defined second and third virial coefficients. The renormalized osmotic pressure, which is written solely in terms of direct observables, is found to be a simple generalization of the results obtained for semidilute polymer solutions in good-to-marginal solvents. It is also found to agree with experimental results on semidilute polymer solutions that cover the temperature domain between theta and good solvents. The present work is carried to lowest order in both the effective binary interaction and the residual ternary interaction, but the general method of treating higher order corrections is outlined. A recent result for the special limit of semidilute solutions at the theta point with three-body fixed point interactions is recovered as a limit of our more general formulation.
The far infrared spectra of IBr charge–transfer complexes
Authors: Sension Roseanne J. , Strauss Herbert L.
Published: July 2002

The IBr infrared band is examined as a function of temperature both in neat decane and in decane containing benzene or other complexing solutes. It is concluded that the spectra provide no evidence for a fast complexing reaction, contrary to previous reports.
Heat capacity of liquid equiatomic potassium–lead alloy: Anomalous temperature dependence
Authors: Johnson Gerald K. , Saboungi Marie-Louise
Published: July 2002

The temperature dependence of the enthalpy function for the equiatomic K0.5Pb0.5 alloy has been determined using high-temperature drop calorimetry over the temperature range 751–1124 K. Unusually large values for the heat capacity of the liquid equiatomic alloy have been derived in the vicinity of the melting point, which was determined as Tm=862±2 K; also, the temperature coefficient of the heat capacity was found to be anomalously large and negative. Such behavior is consistent with the previously reported compositional dependence of the heat capacity in liquid K–Pb alloys deduced from electromotive force measurements. At relatively high temperatures, the heat capacity drops to values typical of metallic solutions. Possible interpretations of this striking behavior include an order–disorder transition in the liquid and/or the formation of lead clusters, such as Pbm−m.
Classical resonances, Fermi resonances, and canonical transformations for three nonlinearly coupled oscillators
Authors: Neshyba S. P. , De Leon N.
Published: July 2002

Classical resonances arising from the interaction of three nonlinearly coupled oscillators are studied from both a theoretical and numerical perspective. In particular, our study focuses on ternary classical resonances defined by n1ω1 +n2ω2 −n3ω3 =0. We discuss some of the experimental and quantum mechanical consequences of binary and ternary classical resonances (e.g., Fermi resonances and vibration–rotation coupling). Numerically we show that it is possible to construct a three-dimensional map such that ternary classical resonances can be systematically found. Theoretically, we show that canonical transformations exist between resonant and nonresonant motion. These transformations predict various structural features of the three-dimensional numerical maps which are subsequently observed in a model numerical calculation. Finally we argue that the methods and ideas presented in this paper are generic and can be used for more general systems.
Structure of hard particle fluids near a hard wall. I. y
w(z) for hard disks
Authors: Smith William R. , Speedy Robin J.
Published: July 2002

The wall-cavity correlation function yw(z) for hard disks against a hard wall is predicted accurately by yw(z)=yw(0)exp[−ΔV(z)(a1+ a2z)]; 0≤z≤σ, where ΔV(z) is the area excluded to the disk centers by a cavity at a distance z from the wall. yw(0), a1, and a2 are expressed in terms of the thermodynamic properties of the bulk fluid, for which accurate semiempirical expressions are available. To test this relation, nwo(z) and yw(z) in the range 0≤z≤σ were calculated for the first time in a computer simulation experiment.
Comment on: ‘‘Magnetic birefringence measurement in hydrogen and deuterium gases’’
Authors: Buckingham A. D. , Williams J. H.
Published: July 2002

Comments are presented on a recent measurement (ref.1) of the Cotton−Mouton effect in gaseous hydrogen and deuterium.The role of magnetic hyperpolarizability in explaining the experimental results is discussed.(AIP)