bluuebunny/crossref_metadata_2025 huggingface dataset:
A multiple trajectory theory for curve crossing problems obtained by using a Gaussian wave packet representation of the nuclear motion
Authors: Sawada S. , Metiu H.
Published: July 2002

This article is concerned with the time dependent quantum theory of collision of heavy particles that result in the excitation of internal degrees of freedom, such as electronic excitations or charge transfer. Attempts to treat the motion of the heavy particle classically encounter difficulties typical to all problems in which the classical degrees of freedom are strongly coupled to quantum degrees of freedom: They lack a feedback mechanism that will force the classical degree of freedom to respond to the excitation of the quantum companion. To avoid such difficulties we propose a method in which the nuclear wave function associated with each electronic state is represented by a Gaussian wave packet. Each packet is propagated by the time dependent Schrödinger equation on a different electronic energy surface in a manner that resembles classical mechanics but perserves many quantum properties such as Heisenberg uncertainty principle, quantum interference, zero point motion, and the quantum mechanical rules for computing observables. Various limiting cases are discussed in detail and compared numerically.
Transient spectroscopy of pyridazine vapor: Photophysics of the S1 origin
Authors: Holt Patrick L. , Selco Jodye I. , Weisman R. Bruce
Published: July 2002

Nonradiative relaxation processes of pyridazine’s S1 origin level have been investigated using several varieties of gas phase time-resolved spectroscopy. Measurements of ground state repopulation kinetics under vibrationally relaxing conditions showed rapid and nearly complete return of thermalized S0, implying a quantum yield for S1 → S0 internal conversion of at least 0.95. The room temperature photochemical quantum yield was found to be higher in the low pressure gas than in solution by a factor of at least 600. From these results the photochemically active state was deduced to be vibrationally energized S0, rather than S1. The collision-free lifetime of the S1 origin level was measured as 3–4 ns on the basis of near ultraviolet Sn  ← S1 transient absorption and S1 → S0 stimulated emission kinetics. Hot S0 molecules formed as the product of S1 internal conversion showed a broad, structureless transient absorption spectrum. Apart from its collision-free radiationless decay, the S1 origin level was also found to undergo an unusual collisional quenching process induced by electronically inert collision partners at rates between 5% and 10% of gas kinetic. This collisional channel is suggested to involve promotion to the anomalous level 373 cm−1 above the S1 origin, which had been tentatively assigned as the origin of pyridazine’s S2 state.
Processes involved in the Cs(7P)+H2→CsH reaction and nascent rotational distribution
Authors: Visticot J. P. , Ferray M. , D’Oliveira P. , Sayer B.
Published: July 2002

The kinetics of CsH formation in the reaction of Cs(7P) and H2 is studied in the gas phase. We find that two processes coexist: a fast direct mechanism leading to a rotational distribution having the same temperature as the gas mixture and a slower process which most probably involves two collisions.
Chemical timing 4. The rovibronic level structure associated with intramolecular vibrational redistribution in S1 p-difluorobenzene
Authors: Dolson D. A. , Holtzclaw K. W. , Moss D. B. , Parmenter C. S.
Published: July 2002

The method of chemical timing for obtaining picosecond time-resolved fluorescence spectra has been applied to the measurement of intramolecular vibrational redistribution (IVR) in S1 p-difluorobenzene. In this report, a more detailed examination of the spectroscopy of some vibrationally mixed levels is used in conjunction with the previously measured IVR parameters to construct a generalized picture of the rovibronic level structure giving rise to IVR. Strong, off-resonance interactions are observed to mix levels separated by several cm−1, contributing additional discrete structure to the spectrum but not directly affecting the IVR dynamics. The dynamics, instead, reflect on-resonance interactions between levels separated only by a few tenths of a wave number that give rise to congested background emission. Evidence is seen in comparisons between bulb and jet spectra for the involvement of vibration-rotation coupling in both the off-resonance and the on-resonance interactions. A particularly important and subtle rotational effect is the change of the coupled vibrational field with rotational excitation due to the variation of rotational constants with vibrational excitation, leading to a smoother congested background.
Electronic spectroscopy of tryptophan analogs in supersonic jets: 3-Indole acetic acid, 3-indole propionic acid, tryptamine, and N-acetyl tryptophan ethyl ester
Authors: Park Young D. , Rizzo Thomas R. , Peteanu Linda A. , Levy Donald H.
Published: July 2002

The electronic spectroscopy of four different tryptophan analogs, 3-indole acetic acid, 3-indole propionic acid, tryptamine, and N-acetyltryptophan ethyl ester (NATE) has been studied in a supersonic molecular beam using laser-induced fluorescence and resonantly enhanced two-photon ionization. The electronic transition to the lowest excited singlet state occurs at 35 039, 34 965, 34 918, and 34 881±2 cm−1 for 3-indole acetic acid, 3-propionic acid, tryptamine, and NATE, respectively. The relatively small differences in the electronic origin transition frequencies suggests that the lowest excited singlet state for all of these moelcules is the 1Lb state. The spectra reveal that each of these molecules have stable conformers in the gas phase, analogous to our previously reported studies of tryptophan. A low frequency vibrational mode has been observed in 3-indole propionic acid, tryptamine, NATE, and tryptophan which involves motion of the side chain against the indole ring. We have observed that forming a van der Waals complex between tryptamine and a single methanol molecule causes the spectral features due to different conformers of the free molecule to collapse to a single line, suggesting that one particular conformer becomes the most stable species. This emphasizes the importance of including solvent interactions in any attempt to model the behavior of these molecules in solution.
Computer simulation of the entropy of continuum chain models: The two-dimensional freely jointed chain of hard disks
Authors: Meirovitch Hagai , Scheraga Harold A.
Published: July 2002

A method for estimating the entropy of polymer chains from a sample of conformations has recently been proposed by Meirovitch and has been applied, up to now, only to lattice chain models. In this paper, the method is extended to continuum chains and applied to a two-dimensional model with excluded volume, the freely jointed chain of hard disks. As in the case of lattice models, the method is based on scanning ‘‘future’’ partial chains of b bonds (steps); however, it is necesary to approximate the continuum of directions of a bond by a finite number of directions l selected at random. Linear chains with a different number of bonds N, from N=11 to 35, are generated by the direct Monte Carlo (DMC) procedure, and their entropy is calculated by the present method. The results for the entropy are found to be very accurate, as compared to those obtained from an asymptotically exact formula based on the DMC procedure; even for the worst approximation of b=1 and l=3, for N=35, the error is only 1%. This provides strong suggestive evidence that the method can be employed for estimating the entropy of longer chains as well as that of much more complex chain molecules such as polypeptides and proteins.
Dissociation rates for individual eigenstates of S formaldehyde: Fluctuations and barrier height
Authors: Guyer Dean R. , Polik William F. , Moore C. Bradley
Published: July 2002

Spectra of S0 D2CO rovibronic states near the dissociation threshold have been taken with a new method by monitoring S1 lifetime while Stark shifting an S1 level through the S0 manifold. An order of magnitude variation in the unimolecular dissociation rate and in the S1–S0 vibronic coupling matrix element is observed among S0 levels within a 0.2 cm−1 energy range. These individual molecular eigenstates exhibit distinctive chemical properties. Dissociation rates averaged over an energy range of a few cm−1 vary smoothly with energy and give a barrier height of 78.0–81.1 kcal/mol for H2CO→H2+CO.
The potential surface of X̃ 3B1 methylene (CH2) and the singlet–triplet splitting
Authors: Bunker P. R. , Jensen Per , Kraemer W. P. , Beardsworth R.
Published: July 2002

The data in the two immediately preceding papers, when combined with the extant microwave, infrared, and photodetachment spectroscopic data, provide 152 rotation and rotation-bending energy level separations in X̃ 3B1 methylene (involving 12CH2, 13CH2, and CD2). In the present paper we fit all this data using the two nonrigid bender Hamiltonians NRB1 and NRB2. The more refined model (NRB2) leads to the following results for triplet methylene: re=1.0766±0.0014 Å, αe=134.037°±0.045°, and the barrier height to linearity=1931±30 cm−1 (the uncertainties are three times the standard errors). Rotation-bending energy levels for CH2, CD2, and CHD are calculated for v2≤4 and N≤6. The determination of the rotation-bending energy levels in CH2 leads to an improved determination of the singlet–triplet splitting T0(ã1A1) in methylene as 3156±5 cm−1 (9.023±0.014 kcal/mol, 0.3913±0.0006 eV). Although the rotation-bending energy levels are accurately predicted it is not possible to predict the stretching frequencies of CH2 very accurately, since the data to hand are not very sensitive to the stretching force field.
Equation of state for athermal lattice chains
Authors: Dickman Ronald , Hall Carol K.
Published: July 2002

We report results of Monte Carlo simulations of athermal chains in the square lattice, and present a detailed comparison between Flory and Flory–Huggins theories and the numerical results.
Hyperspherical coordinates for molecular dynamics by the method of trees and the mapping of potential energy surfaces for triatomic systems
Authors: Aquilanti Vincenzo , Cavalli Simonetta , Grossi Gaia
Published: July 2002

Some results on hyperspherical coordinates and harmonics for the representation of the many-body problem are presented, extensive use being made of the method of trees. Properties of these trees are examined: a lemma on the simplification of trees possessing a particular symmetry is proven, and used to discuss the internal coordinates for a system of three particles and the mapping of potential energy surfaces. A framework is provided for relating different couplings of particles by rotations on hyperspheres and alternative hyperangular parametrizations by orthogonal basis transformations. Extensions to nonzero angular momentum or to more than three particles are shown not to be trivial, and the possible role of developments of the tree method, leading to more general hyperspherical coordinates, is briefly considered.
A theory for the structure and thermodynamics of molten Li/K Cl
Authors: Wiechen Jochen
Published: July 2002

We use an improved hypernetted chain (IHNC) approximation to calculate the total correlation functions hλν and some thermodynamic properties of the binary alkali halide Li/K Cl over the whole concentration range. The pair interactions are assumed to be of the Born–Huggins–Mayer form. The bridge functions incorporated in our scheme and neglected in ordinary HNC calculations are based on the first diagram of their hξ1ξ2-bond, density expansions. Comparison with simulation data shows that certain severe structural defects of the HNC results are removed by our procedure. We find semiquantitative agreement with experimental estimates for the long-wavelength limits of the partial structure factors which are connected to the mixing thermodynamics of the system. This is a significant improvement over the HNC approximation which yields a qualitatively wrong concentration dependence of these quantities. The significance of thermodynamic self-consistency is demonstrated.
The argon–hydrogen fluoride binary complex: An example of a long lived metastable system
Authors: Huang Z. S. , Jucks K. W. , Miller R. E.
Published: July 2002

The optothermal laser–molecular beam method has been used to measure the infrared spectrum of Ar–HF [10°0←00°0]. The results show that the vibrational predissociation lifetime of this complex is greater than the flight time of the molecules from the laser crossing region to the bolometer detector. This gives a lower limit on the lifetime of 3×10−4 s! The upper vibrational state dipole moment has also been obtained for the complex (μ1=1.495 D) by carrying out infrared stark spectroscopy. This corresponds to a 12% increase in the dipole moment upon vibrational excitation. This change can be related to a stiffening of the van der Waals bond, and hence a reduction in the amplitude of the bending motion, in the vibrationally excited state.
Theory of magnetic susceptibility in terms of atomic quantities
Authors: Zanasi Riccardo , Lazzeretti Paolo
Published: July 2002

A new theory rationalizing molecular magnetic susceptibility in terms of atomic contribution is presented. The proposed atomic susceptibilities are proved numerically to be near gauge independent. A first set of theoretical atomic magnetic susceptibilities for O, N, C, and H, obtained from calculations of magnetic properties of water, ammonia, and methane molecules which are among the best available so far, is given.
Absolute I* quantum yields for the ICN Ã state by diode laser gain-vs-absorption spectroscopy
Authors: Hess Wayne P. , Leone Stephen R.
Published: July 2002

Absolute I* quantum yields have been measured as a function of wavelength for room temperature photodissociation of the ICN Ã state continuum. The yields are obtained by the technique of time-resolved diode laser gain-vs-absorption spectroscopy. Quantum yields are evaluated at seven wavelengths from 248 to 284 nm. The yield at 266 nm is 66.0±2% and it falls off to 53.4±2% and 44.0±4% at 284 and 248 nm, respectively. The latter values are significantly higher than those obtained by previous workers using infrared fluorescence. Estimates of I* quantum yields obtained from analysis of CN photofragment rotational distributions, as discussed by other workers, are in good agreement with the I* yields reported here. The results are considered in conjunction with recent theoretical and experimental work on the CN rotational distributions and with previous I* quantum yield results.
Stochastic dynamic models of curve crossing phenomena in condensed phases
Authors: Cline Raymond E. , Wolynes Peter G.
Published: July 2002

Two stochastic dynamic models are used to study several aspects of curve crossing phenomena in dissipative systems. A surface hopping model is used to test the qualitative predictions of earlier theories. The simulation results agree well with the qualitative picture. Results are obtained for an alternate semiclassical model based on a vector spin representation which is derived via a variational principle. The vector model shows some differences in behavior as compared to the hopping model. In certain regimes the vector model shows chaotic behavior.
Close coupling-wave packet formalism for gas phase nonreactive atom–diatom collisions
Authors: Kouri D. J. , Mowrey R. C.
Published: July 2002

In this paper we discuss the adaptation of the close coupling-wave packet (CCWP) method for solving the time dependent Schrödinger equation for inelastic, nonreactive gas phase atom–diatom collisions. The approach is novel in that (a) it is an initial value rather than boundary value method, (b) it can be formulated to either avoid or include the partial wave expansion normally used for gas phase atom–diatom collisions, (c) it can be formulated to determine either a single column of the differential scattering amplitude matrix or S matrix rather than the full matrix, (d) the labor involved in a single calculation scales with the number of rotor states squared rather than cubed as in standard close coupling, (e) a single calculation yields numerically exact results over the full range of energies contained in the original wave packet, and (f) results for other initial states can be obtained by means of the energy sudden (ES) or energy corrected sudden (ECS) factorization relations. The analysis for extracting the differential scattering amplitude at fixed energies is given in detail because it differs markedly from that normally given in textbook treatments of the wave packet formulation of gas phase scattering. Finally, an example approximate version of the formalism (namely the energy sudden) is given.
Comparison of MBPT and coupled cluster methods with full CI. II. Polarized basis sets
Authors: Cole Samuel J. , Bartlett Rodney J.
Published: July 2002

Results from many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations relative to a single reference function are compared with recent full CI results for Ne, F, F−, HF, H2O, and NH2. MBPT results include second through fourth order, while CC results are reported for the CCSD, CCSD+T(CCSD), and CCSDT-1 methods. The basis sets used are of double-zeta plus polarization quality or better. HF, H2O, and NH2 are studied at several displaced geometries. Agreement to within 0.3 kcal/mol is found between infinite order CCSDT-1 results and full CI at the equilibrium geometries, while finite-order MBPT(4) is still quite good. For FH, CCSDT-1 relative to an incorrectly separating RHF reference is still accurately described all the way to the separated atom limit. At 2.0 Re, only the CCSDT-1 method obtains satisfactory agreement with full CI, having an average error of 3.6 kcal/mol, while MBPT(4) has an average error of 13.1 kcal/mol.
A quasiclassical trajectory study of final state distributions in collisions of fast H(D) atoms with HF(DF)
Authors: Schatz George C.
Published: July 2002

This paper presents a detailed theoretical study of the reactive and nonreactive final vibrational state distributions obtained in collisions of translationally hot H atoms with HF (and isotopic counterparts D+HF, H+DF, and D+DF). The potential surface used is surface No. 5 of Brown, Steckler, Schwenke, Truhlar, and Garrett, and it is characterized by a high barrier (1.9 eV) to F atom transfer. Cross sections and other dynamical information were generated using the quasiclassical trajectory (QCT) method, and we also did classical infinite-order-sudden (CIOS) calculations to characterize vibrational excitation mechanisms. Perhaps our most important results refer to the nonreactive final state distributions, where we find that collision of H with the F atom end of HF gives a broad vibrational distribution spread over many states while collision with the H atom end of HF gives a narrow distribution in which v′=1 is the only significant excited product. For D+HF, only the first collision mechanism is important, while for H+HF, H+DF, and D+DF, the second mechanism makes the dominant contribution to v′=1, and the first mechanism is the major contributor to v′>1. This leads to nonreactive vibrational distributions for H+HF, H+DF, and D+HF in which v′=1 is much larger relative to v′>1 than in D+HF. Comparison of these results with experiment for H+HF and D+HF indicates excellent agreement. Reactive distributions are also studied, and we find that the variation of these distributions with isotope can be explained in terms of a Franck–Condon overlap model. Comparison of the reactive final state distribution for D+HF with experiment indicates excellent agreement. Rotational excitation is examined for both reactive and nonreactive collisions, and we find that while the nonreactive rotational excitation is sensitive to which end of the molecule is struck, the reactive rotational distribution is controlled by kinematic propensities.
The effect of vibrational state mixing on the predissociation lifetime of ν1 excited OC–HF
Authors: Jucks K. W. , Miller R. E.
Published: July 2002

Sub-Doppler resolution infrared spectra have been obtained for the ν1 band of OC–HF. For most of the observed rovibrational transitions the linewidths are found to have a Lorentzian component of 190±10 MHz FWHM, presumably resulting from the vibrational predissociation of the complex. In several cases, however, perturbations, due to either anharmonic or Coriolis coupling between the vibrational state corresponding to the excited HF stretch and other vibrational states of the molecule, have been observed in the spectrum. Where these perturbations are present the width of the transitions vary with the relative contributions from the two states involved. This is explained in terms of a simple perturbative treatment of the coupling in conjunction with a Golden Rule treatment of the vibrational predissociation process. Stark measurements have also been performed in order to determine the dipole moment of the complex in the vibrationally excited state, namely μ1=2.545±0.008 D.
Completely ℒ2 Golden Rule method for resonance energies and widths
Authors: Tucker Susan C. , Truhlar Donald G.
Published: July 2002

We have calculated the resonance energies and widths for both one-dimensional scattering resonances and a two-dimensional model of van der Waals molecule predissociation by a general method involving only Hamiltonian and overlap integrals in a single square-integrable basis set containing a scale parameter. We use a stabilization method with a compactness criterion to find the resonance energies and a generalization of the Golden Rule formalism of Macías and Riera to calculate the widths. The results are compared to accurate resonance energies and widths obtained by Breit–Wigner fits. For the final method, as applied to four cases, the errors in the resonance energies are 10−3%, 0.8%, 0.5%, and 0.03%, and the errors in the widths are 2%, 3%, 6%, and 11%, respectively. The new method has particular advantages over the analytic continuation of stabilization graphs when the density of states is high.
Wide fluctuations in fluorescence lifetimes of individual rovibronic levels in SiH2 (Ã 1B1)
Authors: Thoman J. W. , Steinfeld J. I. , McKay R. I. , Knight A. E. W.
Published: July 2002

We have measured fluorescence lifetimes of individual rovibronic levels in SiH2 (Ã 1B1, 020). The lifetimes vary widely from one level to the next, ranging from ≲10 ns to >1 μs. Similar behavior is seen in the (000), (010), and (030) levels. This behavior is interpreted in terms of coupling of the Ã 1B1 rovibronic levels with background levels in the X̃ 1A1 (S0) and ã 3B1 (T1) states, and thence via predissociation to Si(3P)+H2. The irregular variation in the lifetimes reflects the quasirandom spacings of S0 and T1 levels in the vicinity of any particular Ã state level.
Observation of forbidden transitions of ammonium ion (NH+4) ν3 band and determination of ground state rotational constants. Observation of ν3 band allowed transitions of ND+4
Authors: Crofton Mark W. , Oka Takeshi
Published: July 2002

The high resolution infrared spectrum of forbidden transitions of the ν3 band of ammonium ion, NH+4, has been recorded using a difference frequency spectrometer. These are the first forbidden NH+4 lines to be observed. For 5≤R≤11, 24 forbidden transitions have been assigned. From combination difference relations involving the same excited state level, 13 independent energy differences between ground state levels with 4≤J≤11 have been determined with uncertainties of ≲0.004 cm−1. From this data, the following values (in cm−1) for ground state constants of NH+4 have been determined (±3σ):B=5.929 32 ±0.000 20, D=(−1.282±0.020)×10−4, and Dt=(4.80±0.35)×10−6. The value of r0 is calculated to be 1.028 73±0.000 02 Å, while re is estimated to be 1.0208±0.0020 Å. More than 100 transitions of the ν3 band of ND+4 were also observed (J≲10) by the same spectrometer. Nearly all of these have been assigned and fit together using a linear model. A determination of B0 for ND+4, while unsuccessful at this time, will allow a more accurate determination of re.
A picosecond photon echo study of a chromophore in an organic glass: Temperature dependence and comparison to nonphotochemical hole burning
Authors: Walsh C. A. , Berg M. , Narasimhan L. R. , Fayer M. D.
Published: July 2002

The first two-pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.
Absolute state-to-state total cross sections for the reactions N+2(X̃,v′=0–2) +Ar(1S)→N2(X,v)+Ar+(2P3/2,1/2)
Authors: Shao J.-D. , Li Y.-G. , Flesch G. D. , Ng C. Y.
Published: July 2002

Absolute state-selected total cross sections σv′, v′=0 and 1, for the reaction N+2(X̃,v′=0,1) +Ar(1S0)→N2(X,v)+Ar+(2P3/2,1/2) [reaction (1)] over the center-of-mass collisional energy (Ec.m.) range of 1.2–140 eV have been measured using the photoionization mass spectrometric and radio frequency ion guide methods. These measurements, together with the relative values for σv′, v′=0–2, and spin-orbit-state distributions of product Ar+ ions determined using the crossed ion-neutral beam photoionization apparatus, allow the determination of the absolute values for σ2 and partial state-to-state cross sections σv′→J, v′=0–2, for reaction (1). Absolute values for σv′, v′=0–2, at Ec.m.=8 and 20 eV are in good agreement with those determined previously by the threshold photoelectron secondary ion coincidence method. Absolute values for σv′→J, v′=0–2, at Ec.m.=8 and 20 eV are also found to be in satisfactory accord with the predictions of the semiclassical multistate calculation which uses the ab initio potential energy surfaces of the [N2+Ar]+ system. Experimental state-to-state cross sections obtained in this study are consistent with those for the reaction Ar+(2P3/2)+N2(X,v=0)→Ar(1S0)+N+2 (X̃,v′) from the consideration of microscopic reversibility.
A theoretical study of alignment effects in collisions of N2 with a Ag surface
Authors: Corey Gregory C. , Alexander Millard H.
Published: July 2002

We report rotationally inelastic transition probabilities for N2 scattered from a rigid, flat as well as a weakly corrugated Ag surface. For this system Sitz, Kummel, and Zare [J. Vac. Sci. Technol. A 5, 513 (1987)] have recently measured the population state distribution and the quadrupole and hexadecapole alignment of the rotational angular momentum vector with respect to the surface normal. These alignment ratios are directly proportional to the coefficients in a Legendre expansion of the classical angular momentum spatial distribution function. Quantum mechanical expressions for this distribution function in terms of state multipoles of the density operator are presented, and then the classical limit is obtained by introducing asymptotic expressions for the vector coupling coefficients which appear in them. The effect of lattice symmetry on the collisional reorientation of the incident rotational angular momentum vector is investigated through close-coupled scattering calculations based on a model interaction potential. Specular scattering is dominated by ΔM selection rules. The alignment of the rotational angular momentum vectors is analyzed in terms of these selection rules and the azimuthal dependence of the interaction potential.
Numerical studies of disordered-chain vibrations. I. Low-frequency modes of polymethylene-like skeletal chains
Authors: Snyder R. G. , Strauss H. L.
Published: July 2002

The vibrational density of states and isotropic Raman spectra of ensembles of conformationally disordered polymethylene-like skeletal chains have been computed numerically in the low-frequency CCC bending region below 600 cm−1. Chain lengths ranged from 8 to 25 atoms and the average trans-bond concentration varied from 0% to 100%. The calculated Raman spectra of the disordered chains are notable in that they display a progression of D-LAM-k bands. The existence of this progression indicates that the skeletal vibrations of the disordered-chain ensemble can, to some extent, be described in terms of a phase-angle model of the type normally associated with ordered chains. The applicability of a phase-angle model is also indicated by the frequency distribution of standing waves obtained from a harmonic analysis of the normal coordinates.
Three-dimensional quantum mechanical study of Ne⋅⋅⋅Cl2 vibrational predissociation
Authors: Halberstadt Nadine , Beswick J. Alberto , Janda Kenneth C.
Published: July 2002

Three-dimensional quantum mechanical calculations for vibrational predissociation of the Ne⋅⋅⋅Cl2 van der Waals complex are presented and compared with experiments. Lifetimes and final rotational state distributions were obtained for the two processes: (i) Ne⋅⋅⋅Cl2(X,v=1) →Ne+Cl2(X,v=0) and (ii) Ne⋅⋅⋅Cl2(B,v=11) →Ne +Cl2(B,v=10,9) where v denotes the vibrational quantum number of Cl2 and X and B specify electronic states of Ne⋅⋅⋅Cl2 which correlate with the X 1∑+0g and B 3∏+0u states of the free Cl2 molecule, respectively. The van der Waals interaction potential was taken to have the same form in the X and B states. At short distances, it is described by a sum of Morse pairwise potentials between the Ne atom and each of the Cl atoms, and between the neon atom and the center of mass of Cl2. At large distances the potential switches to an anisotropic van der Waals interaction with R−6 and R−8 dependence. The parameters were adjusted so that the T-shaped configuration the potential matched the one determined from scattering experiments. The initial quasibound state wave function of the complex was calculated variationally, while the final continuum wave functions were obtained by integration of the rotational close coupled Schrödinger equations. Finally, the lifetime and the final rotational state distribution were calculated using the Fermi golden rule. A line shape calculation verified the validity of the golden rule approximation for this system. The lifetimes obtained for the X and the B states differ by several orders of magnitude, the X state being the longest lived as observed experimentally. The calculated lifetimes and rotational distributions of the Cl2 fragments agree qualitatively with the experimental values. The rotational distribution is compared to that obtained from a decomposition of the initial quasibound state in terms of free rotor states.
SCF and electron correlation studies on structures and harmonic in-plane force fields of ethylene, t
r
a
n
s 1,3-butadiene, and all-t
r
a
n
s 1,3,5-hexatriene
Authors: Szalay P. G. , Karpfen A. , Lischka H.
Published: July 2002

Equilibrium structures and in-plane harmonic force fields of ethylene, trans 1,3-butadiene, and all-trans 1,3,5-hexatriene were investigated with the aid of ab initio Hartree–Fock (SCF) calculations applying basis sets of DZ+P and TZ+P quality. The most interesting parts of the force fields, namely those involving stretching vibrations of the conjugated carbon backbone, were subsequently reevaluated including electron correlation at the coupled pair functional (CPF) level. We demonstrate that electron correlation has a significant influence not only on the degree of bond alternation in the ground state equilibrium structure but simultaneously leads to an increase in absolute value of all off-diagonal stretch–stretch coupling constants within the carbon backbone.
Molecular beam photoelectron spectroscopy of SO2: Geometry, spectroscopy, and dynamics of SO+2
Authors: Wang Laisheng , Lee Y. T. , Shirley D. A.
Published: July 2002

We have reinvestigated the He i (584 Å) photoelectron spectroscopy of SO2 using a supersonic molecular beam. High resolution and rotational cooling allowed us to observe new features and to resolve explicitly the vibrational structure in the first six electronic states of SO+2 ion. Improved spectroscopic constants for the ionic states and the adiabatic ionization potentials (IPs) are reported. The vibrational structure of the transition to the X̃ 2A1 ground state was assigned to the ν2 mode exclusively. Irregularity of the vibrational progression on the high IP side was observed, which indicated a potential barrier (to linearity) along the direction of Q2 coordinate. The barrier height was estimated to be 0.42 eV (3400 cm−1). Unusual vibrational structure in the Ã 2A2 state was observed, due to the principal excitation of the ν3 mode and the presence of a potential barrier along the Q3 coordinate. The barrier height was estimated to be less than 220 cm−1. A new progression was resolved in the B̃ 2B2 state, which was assigned to be a combination of ν2 with 2ν3. The ν2 vibration was observed to be strongly coupled with the ν3 mode. The dynamics of the ion dissociations in the C̃, D̃, and Ẽ states was discussed in connection with previous works. A satellite band at about 17.5 eV with resolved vibrational structure which consists of two ν1 vibrational progressions was observed.
Impurity perturbed domains: Resonant enhancement of bulk mode CARS by coupling to the electronic states of dilute impurities
Authors: Wilson William L. , Wäckerle Gerhard , Fayer M. D.
Published: July 2002

A new experimental method for the direct observation of electronic excitation–phonon coupling is presented. The CARS signal from the 69 cm−1 optical phonon mode of naphthalene crystals is resonantly enhanced by coupling to the electronic states of dilute pentacene impurities. The resonantly enhanced signal decays over three times faster than the nonresonant decay in the same crystal. This is explained in terms of impurity perturbed domains.
Theoretical investigation of the dip in the far infrared absorption spectrum of dense rare gas mixtures
Authors: Guillot Bertrand
Published: July 2002

The far infrared absorption spectrum of dense rare gas mixtures is analyzed in terms of two-, three-, and four-body contributions. It is argued that pair, triplet, and quadruplet terms must be accurately known at short times, in order to describe the overall spectral shape, since the long time region, where diffusion processes can occur, is drastically reduced by the range of the interaction and the cancellation effect. A model calculation is presented where each piece of the total correlation function C(t) is expressed as a Gram–Charlier series. The parameters are then estimated with the help of a lattice gas model which includes a correction to the Kirkwood superposition approximation in the case of three-body correlations. The dip in the reduced spectral density is interpreted as a dynamical cancellation effect between two-, three-, and four-body correlation functions, the role played by the cage effect at liquid density being exacerbated by the cancellation. The width and the depth of the dip are found to be strongly dependent on the mass ratio, density, and concentrations, and some comparisons with experimental data are made for Ne–Ar and Kr–Ar liquid mixtures. Finally, the phenomenological interpretation of the dip given by Van Kranendonk and Lewis in the 1970’s (intercollisional interference effect at low density) is discussed in detail and the possible relationship with the present theory is examined.
A comparative study of potential energy surfaces for CH3+H2↔CH4+H
Authors: Steckler Rozeanne , Dykema Kenneth J. , Brown Franklin B. , Hancock Gene C. , Truhlar Donald G. , Valencich Trina
Published: July 2002

We analyze potential energy surfaces that have been proposed by one of the authors, Bunker, and Chapman and by Raff for the reaction CH3+H2↔CH4+H. The surfaces are modified to remove discontinuities and zero frequencies, where present, and the modified surfaces are compared to each other in terms of reaction-path properties and to ab initio calculations for stationary point properties. They are also used for rate constant calculations which are compared to experiment. The rate constants were calculated by improved canonical variational transition state theory with small-curvature semiclassical adiabatic ground-state transmission coefficients (ICVT/SCSAG) over a wide temperature range, 298–1340 K. Both surfaces yield rate constants in poor agreement with experimental values. The reaction-path analysis leads to a list of potential energy surface features that are important for the rate constants but inaccurate in the existing surfaces and that should be improved in subsequent work.
Dense-fluid Lyapunov spectra via constrained molecular dynamics
Authors: Hoover W. G. , Posch H. A. , Bestiale S.
Published: July 2002

A Lagrange-multiplier method for finding the complete spectrum of Lyapunov exponents, which describe the spreading and mixing of many-body phase-space trajectories, is developed and applied here to simple two- and three-dimensional equilibrium fluids with short-range repulsive forces. The numerical values of the Lyapunov exponents converge well, in computer simulations of 103 to 105 time steps, and are insensitive both to the initial conditions and to the numerical accuracy of the trajectory integration.
Computer simulation of long polymers adsorbed on a surface. II. Critical behavior of a single self-avoiding walk
Authors: Meirovitch Hagai , Livne Shelly
Published: July 2002

The scanning simulation method is applied to a model of polymer adsorption in which a single self-avoiding walk is terminally attached to an attracting impenetrable surface on a simple cubic lattice. Relatively long chains are studied, of up to 1000 steps, which enable us to obtain new estimates for the reciprocal transition temperature ‖ε‖/kBTa=θa =0.291±0.001 (ε is the interaction energy of a monomer with the surface), the crossover exponent φ=0.530±0.007 and the free energy exponents at Ta, γ1SB =1.304±0.006 and γ11SB =0.805±0.015. At T=∞ we obtain, γ1=0.687±0.005, γ11=−0.38±0.02, and the effective coordination number q=4.6839±0.0001, which are in good agreement with estimates obtained by other methods. At T>Ta we demonstrate the existence of strong correction to scaling for the perpendicular part of the mean-square end-to-end distance 〈R2〉⊥ and for the monomer concentration profile ρ(z) (z is the distance from the surface). At T=∞ the leading correction to scaling term for 〈R2〉⊥ is c/Nψ, where c≊−0.9 and ψ≊0.4 is close to 0.5 obtained for the random walk model in the preceding paper. This means that the asymptotic regime, in which these corrections become negligible, corresponds to a large polymer length that is not realized experimentally. Close enough to Ta we demonstrate for our lattice model the validity of various scaling forms predicted by Eisenriegler, Kremer, and Binder [J. Chem. Phys. 77, 6296 (1982)] for a continuum model on the basis of the n-vector model.
Measurement of collisional V→T, R energy transfer in vibrationally excited SF6 using Doppler broadening of the Hg(6 1S–6 3P1) multiplet
Authors: Braun Walter , Scheer Milton D. , Cvetanovic R. J.
Published: July 2002

A new method for measuring energy transfer between colliding molecules has been developed. It is based upon the temperature dependence of the isotopic and hyperfine line widths of the Hg multiplet in the neighborhood of 254 nm. The light source was a Hg resonance lamp whose line profiles could be precisely controlled by varying the Hg pressure. Multiphoton absorption at 944 cm−1 was used to excite SF6 vibrationally during a 250 ns CO2 laser pulse. The subsequent rate of equipartition of this excess vibrational energy with the translational and rotational degrees of freedom was determined by measuring the initial slope of the increase in the absorption of 254 nm radiation by a trace of Hg vapor. It was found that pressures above 2 Torr were required to involve all of the SF6 molecules in the absorption of the laser radiation. The V→T, R energy transferred per SF*6 –SF6 collision was found to be proportional to the 3/2 power of the excitation energy between 1500 and 5000 cm−1 and gradually changed to a first power dependence in the 300 to 1000 cm−1 range.
Rotational and state-resolved translational distributions of NO scattered from organized amphiphilic monolayers
Authors: Cohen Sidney R. , Naaman Ron , Sagiv Jacob
Published: July 2002

Two-photon ionization has been used to probe NO scattered from two different long chain organic amphiphiles. Rotational and state-resolved translational distributions were obtained. The results show that there is a large difference in the dynamics of scattering from an unsubstituted aliphatic chain as compared to a monolayer in which the exposed end has been perfluorinated. NO scattered from the latter is more energetic both rotationally, and translationally. This effect becomes particularly noticeable as the incident energy of the NO is raised. The results can be explained by a mechanism which ignores the weak NO–surface potential and treats only the differences in rigidity and phonon modes in the two monolayers.
Density functional theory of freezing for molecular liquids
Authors: Smithline S. J. , Rick Steven W. , Haymet A. D. J.
Published: July 2002

The density functional theory of freezing is extended to molecules with orientational degrees of freedom, using a spherical harmonic expansion of the correlation functions. A number of corrections to earlier theories are noted. We present preliminary numerical applications to the freezing of hard dumbbells, hard ellipsoids, and dipolar hard spheres.
Theoretical studies of the potential surface for the F+H2→HF+H reaction
Authors: Bauschlicher Charles W. , Walch Stephen P. , Langhoff Stephen R. , Taylor Peter R. , Jaffe Richard L.
Published: July 2002

The F+H2→HF+H potential energy hypersurface has been studied in the saddle-point and entrance channel regions. Using a large [5s 5p 3d 2f 1g/4s 3p 2d] atomic natural orbital basis set, we obtain a classical barrier height of 1.86 kcal/mol at the CASSCF/multireference CI level (MRCI) after correcting for basis set superposition error and including a Davidson correction (+Q) for higher excitations. Based upon an analysis of the computed results, the true classical barrier is estimated to be about 1.4 kcal/mol. We also compute the location of the bottleneck on the lowest vibrationally adiabatic potential curve, and determine the translational energy threshold from a one-dimensional tunneling calculation. Using the difference between the calculated and experimental threshold to adjust the classical barrier height on the computed surface yields a classical barrier in the range of 1.0–1.5 kcal/mol. Combining the results of our direct estimates of the classical barrier height with the empirical values obtained from our approximate calculations of the dynamical threshold, we predict that the true classical barrier height is 1.4±0.4 kcal/mol. Arguments are presented in favor of including the relatively large (≈1 kcal/mol)+Q correction obtained when nine electrons are correlated at the CASSCF/MRCI level.
Thermodynamic stability of the molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 by high temperature Knudsen effusion mass spectrometry
Authors: Pelino Mario , Haque R. , Bencivenni L. , Gingerich K. A.
Published: July 2002

The molecules YC2, YC3, YC4, YC5, YC6, YC7, and YC8 have been observed in a Knudsen effusion mass spectrometric investigation of the gas phase above the Y–Ir–Au–graphite system. The enthalpies ΔH○0 of the reactions Y(g)+n C(graph)=YCn(g)(n=2–8) were evaluated by the third law method; the second law method was employed for the gaseous molecules with up to six carbon atoms. The selected values of the reaction enthalpies ΔH○0 were combined with ancillary literature data to yield the atomization enthalpies ΔH○a,0 and the standard enthalpies of formation ΔH○f,298.15 in kJ mol−1 of the gaseous yttrium carbides. The values of ΔH○0 and ΔH○a,0 (in kJ mol−1) are 197±5 and 1225±8 for YC2; 377±10 and 1757±12 for YC3; 361±8 and 2484±10 for YC4; 499±10 and 3057±15 for YC5; 536±15 and 3731±20 for YC6; 684±35 and 4294±35 for YC7, and 693±35 and 4727±35 for YC8. The values of ΔH○f,298.15 (in kJ mol−1) are 623±8, 804±12, 790±10, 924±15, 966±20, 1105±35, and 1124±35 for YCn (n=2–8), respectively.
State-selected ion–molecule reactions: N+2(v)+H2→N2+H+2 and Ar+(2P
J) +H2→Ar+H+2
Authors: Henri G. , Lavollée M. , Dutuit O. , Ozenne J. B. , Guyon P. M. , Gislason E. A.
Published: July 2002

Charge transfer cross sections for state-selected N+2 (X,A;v) and Ar+(2PJ) ions colliding with H2 have been measured at laboratory energies of 10.5 and 14.5 eV by means of the TPEPICO (threshold-photoelectron/photoion coincidence) method using synchrotron radiation. The cross sections are put on an absolute basis by normalizing to earlier measurements for N+2+H2. For the purpose of comparison total cross sections have also been measured for state-selected H+2(v)+N2 at 10 eV lab. A simple model, which assumes that small impact parameter collisions lead to chemical reaction, gives a qualitative explanation of the N+2+H2 and Ar++H2 data. The work sheds light on previous problems with the application of the principle of microscopic reversibility to these systems.
Refinement of the autoneutralization lifetimes of short lived states of SF−6
Authors: Appelhans A. D. , Delmore J. E.
Published: July 2002

Previous measurements of the autoneutralization lifetime of SF−6 indicated there are multiple states (lifetimes) and that the distribution of states is controlled, at least in part, by the temperature of the SF6 molecules prior to electron capture. These measurements indicated the existence of a short lived state with a lifetime of the order of 2 μs. The experimental apparatus has been revised to confirm the existence of the short lived state and provide a more accurate measurement of the lifetime.
The vibrational assignment for the A 2Π–X 2Σ+ band system of the SiN radical: The 0–0 bands of 29SiN and 3SiN
Authors: Yamada Chikashi , Hirota Eizi , Yamamoto Satoshi , Saito Shuji
Published: July 2002

The vibrational assignment for the A–X system of the SiN radical was reexamined by observing the spectra in the 5 μm region of 29SiN and 30SiN in natural abundance. The original assignment of Mulliken for the A state was found to be correct, rather than the one currently accepted, i.e., the 5 μm spectrum was assigned to the 0–0 band of the A–X system. The term value of the A state was thus modified to Te(A)=2032.4(1) cm−1 and the equilibrium internuclear distance in the A state was recalculated to be 1.641 879(20) Å, with the uncertainty in parentheses which was primarily due to that in Planck’s constant. The vibrational and isotopic variations observed for the Λ-type doubling and the spin–orbit interaction constants of the A state and for the spin–rotation interaction constants in the X state were explained by treating the electronic matrix elements of the spin–orbit interaction and of the orbital angular momentum between the A/X, B/A, and D/X states as parameters.