bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonclassical behavior of energy transfer from molecules to metal surfaces: Biacetyl(3nπ*)/Ag(111)
Authors: Alivisatos A. P. , Waldeck D. H. , Harris C. B.
Published: July 2002

The distance dependent lifetime of biacetyl separated from a Ag(111) crystal by NH3 spacer layers ranging in thickness from 28 to 457 Å has been measured. We extended previous work, where the molecular emission was resonant with the silver interband/plasmon transition, to the case where the emission is below the interband transition. The modulation of the radiative rate is described inadequately by the classical theory for our experimental geometry. At short distances where nonradiative energy transfer to the metal is important, the classical prediction deviates from the data as well. These observations are consistent with a model in which energy is transferred to electrons localized at the metal surface but might also be explained by an inability of the classical theory to model the radiative rate properly.
Magnetic linear dichroism of matrix-isolated iron atoms
Authors: Vala Martin , Rivoal Jean-Claude , Grisolia Christian , Pyka Jan
Published: July 2002

The first observation of the magnetic linear dichroism (MLD) of a matrix-isolated sample is reported. Iron atoms isolated in a krypton matrix at 4.2 K display a magnetic linear dichroism spectrum which is complementary to its magnetic circular dichroism (MCD) spectrum. The combination of MLD and MCD is shown to be a powerful method for the assignment of the spin-orbit multiplet levels of atomic excited electronic states.
Monte Carlo simulation of the crystal structure of the rotator phase of n-paraffins
Authors: Yamamoto Takashi
Published: July 2002

Structures of two rotator phases of n-paraffin, orthorhombic and hexagonal rotator phases recently found by Doucet et al., are simulated by use of the Monte Carlo method. Only subcell structures are considered; a layer structure and effects of chain ends are neglected. The molecules of n-paraffin are assumed to take a rigid trans-planar structure, and they are assumed to be packed in an orthorhombic or orthohexagonal lattice, with each molecule having 36 possible orientations around the chain axis. Interactions between nearest-neighbor chains are calculated from van der Waals interactions between nonbonded C–C, C–H, and H–H atom pairs. A Monte Carlo calculation, similar to that used in our previous work on the structure of the low temperature phase of PTFE, is applied to disordered packings of the molecules in the rotator phases. It is found that the crystals in the rotator phases are composed of many ordered domains within which the chains tend to parallel their zigzag planes. The average size of the ordered domains is calculated to be about 30 Å at 400 K. It is shown that this structure explains various experimental results which are not yet well understood.
On the empirical equation for electron energy-loss rate in rare gases
Authors: Koura Katsuhisa
Published: July 2002

The validity of the empirical equation for the electron energy-loss rate in rare gases is quantitatively examined by making comparisons of the electron energy degradation and thermalization time with the rigorous one obtained by the discrete ordinate method for the Fokker–Planck equation and by the Monte Carlo simulation. It is found that the empirical equation is superior to the approximate theory based on the Maxwellian electron velocity distribution in the easy estimation of the electron thermalization time in rare gases.
Orientational opacity function for CH3I+Rb reactive backscattering: The steric cone of nonreaction
Authors: Choi Seung E. , Bernstein Richard B.
Published: July 2002

An improved orientational opacity function is obtained for the reaction of methyl iodide with Rb. This opacity function (i.e., the reaction probability as a function of the ‘‘angle of attack’’ γ) results from an improved deconvolution (for the precessional effect) of the reactive backscattering data of Parker et al. (1981) at a collision energy of 13 kJ mol−1. From this opacity function, the cut-off angle of the steric cone blocked out by the methyl group in the course of a reactive encounter is found to be 53±2°. This is larger than the best previous estimate, and quite comparable with that predicted from a simple steric model. The shape of the orientational opacity function is well represented by the nonspherical rigid-body approximation formula of Evans et al. (1985), with a value of the colliding-pair anisotropy parameter of λ=0.41±0.01 and a cut-off angle of 50±2°.
MRD-CI potential surfaces using balanced basis sets. III. HCl and N2
Authors: Wright James S. , Buenker Robert J.
Published: July 2002

Multireference configuration interaction methods are used to calculate potential curves, dissociation energies, and vibration-rotation energy levels for HCl and N2. These calculations use basis sets optimized in a previous study. The potential curves show very good agreement with experimental (RKR) data. Both CI and RKR data are accurately fitted by five-parameter generalized Morse functions. Spectroscopic parameters calculated for HCl are De=4.62 eV, Re=1.277 Å, ωe=3023 cm−1, ωexe=52.9 cm−1, Be=10.67 cm−1, and αe=0.338 cm−1 (experimental v, J values give 4.62 eV, 1.275 Å, 2989 cm−1, 51.2 cm−1, 10.71 cm−1, and 0.341 cm−1, respectively) and for N2 are De=9.96 eV, Re=1.106 Å, ωe=2344 cm−1, ωexe=13.9 cm−1, Be=1.934 cm−1, αe=0.014 cm−1 (experimental v, J values give 9.91 eV, 1.098 Å, 2363 cm−1, 14.7 cm−1, 1.960 cm−1, and 0.015 cm−1, respectively). This good agreement with experiment is obtained using small but carefully balanced basis sets containing bond functions, an approach which we suggest should be adopted in calculations involving molecular dissociation.
Solid–liquid coexistence in the adhesive hard sphere system
Authors: Smithline S. J. , Haymet A. D. J.
Published: July 2002

We have calculated the phase diagram of coexisting liquid and solid adhesive hard spheres, using the modern density functional theory. The predicted phase diagram displays many of the observed experimental trends in the freezing of simple substances. It also has unusual features, such as a ‘‘reentrant’’ liquid phase at certain densities. We have also identified distinct metastable solids at low temperature.
Investigation of the spatially anisotropic component of the laterally averaged molecular hydrogen/Ag(111) physisorption potential
Authors: Whaley K. Birgitta , Yu Chien-fan , Hogg C. S. , Light John C. , Sibener S. J.
Published: July 2002

A detailed investigation of the spatially anisotropic component of the laterally averaged molecular hydrogen/Ag(111) physisorption potential is presented. Experimentally derived rotationally inelastic transition probabilities for H2, D2, and HD, taken as a function of collision energy, are compared with those resulting from close-coupled quantum scattering calculations. These calculations utilize exponential-3 and variable exponent parametrizations of the laterally averaged isotropic potential which reproduce the experimental bound state resonance spectra for p-H2 and o-D2 on Ag(111). Complementary information is obtained by analyzing the magnetic sublevel splittings for physisorbed J=1 n-H2, using diffractive selective adsorption resonance energies calculated with first order perturbation theory. Theoretical predictions for HD/Ag(111) rotationally mediated selective adsorption resonances are also compared with previously reported experimental results, which show well resolved J-dependent energy shifts resulting in part from the orientational anisotropy of the potential. The results obtained in this study indicate that both the attractive and repulsive parts of the anisotropic potential exhibit only a weak orientation dependence, in agreement with recent theoretical predictions for this system.
Time resolved fluorescence from parity mixed rotational energy levels: Collisions vs electric field effects
Authors: Mandich Mary L. , Gaebe Carl E. , Gottscho Richard A.
Published: July 2002

We describe time-resolved and spectrally resolved laser-induced fluorescence measurements from the parity or Λ doublet levels of the A 1Π state of BCl radicals formed by CO2 laser multiphoton dissociation of BCl3 molecules. We show how the effects of collisional mixing can be discerned from the effects of electric field mixing. A set of rate equations for population transfer which include the effects of both field and collisional mixing under conditions of incoherent excitation are derived and compared to the more general theory of Alexander. Zero-field measurements of collisional mixing rates yield parity-changing rate constants kef whose rotational quantum number J dependence is consistent with a dipole–induced-dipole collision model: kef=3.4, 2.6, 1.9, and 1.4×106 s−1 Torr−1 for J=3, 5, 11, and 18, respectively. Measurement of time-resolved fluorescence from Stark mixed parity levels in the presence of a strong electric field indicates that the levels are prepared coherently. Time integrated measurements as a function of the applied field are used in generating empirical calibration curves for determining local field magnitudes in discharges containing BCl radicals.
A test of the rotational infinite order sudden approximation in molecular fragmentation
Authors: Atabek O. , Beswick J. Alberto , Delgado-Barrio G.
Published: July 2002

A quantum mechanical close-coupling calculation is presented for predissociation of a triatomic molecule and the results are compared with the infinite order sudden approximation for rotational motion (RIOSA). The calculations are performed for a model system which mimics the predissociation of the zero-point level of N2O+(Ã). It is shown that for the case treated here the infinite order sudden approximation obtained by setting all the rotational energies equal to zero gives poor results. Good agreement between RIOSA and exact calculations is obtained by setting them equal to the average rotational energy in the fragments. The relationship between RIOSA as applied to full collisions and half-collisions is also discussed.
The structure and spectrum of SiC2
Authors: Oddershede Jens , Sabin John R. , Diercksen Geerd H. F. , Grüner N. E.
Published: July 2002

Second order polarization propagator calculations indicate that the blue/green bands attributed to SiC2 in the atmospheres of certain carbon stars arise from a triangular (C2v) form of the molecule. It appears that this form is separated from a linear (C∞v) form by only a very small barrier, and that spectra from both forms might be observable. The potential for the movement of Si around the C2 fragment, the spectral lines, oscillator strengths, and lifetimes are discussed. We find, as observed previously, that only in a correlated calculation is the triangular geometry the preferred ground state conformation. Also the excitation properties change markedly with correlation.
Nonequilibrium phenomena in dense polyatomic fluids. I. Transport coefficients for axially symmetric (polar) molecules
Authors: Jagannathan S. , Dahler J. S. , Sung Wokyung
Published: July 2002

Formulas are derived for the viscosity coefficients and thermal conductivity of a dense fluid of rigid, axially symmetric molecules. The only quantities appearing in these formulas which have not been evaluated explicitly are integrals over the orientational coordinates of two interacting molecules. The theory is based on a kinetic equation which incorporates a Vlasov term, an Enskog operator specific to the short-ranged noncentral impulsive forces, and Fokker–Planck operators associated with the longer ranged interactions. The method of moments is used to construct an approximate solution of this kinetic equation. The theory and the transport coefficients it produces are compared with those of earlier investigations.
Deuterium relaxation and vibrationally averaged quadrupole coupling in an alkane/urea clathrate
Authors: Greenfield Michael S. , Vold Robert L. , Vold Regitze R.
Published: July 2002

Measurements of the temperature dependence of deuterium relaxation rates for the methylene groups of perdeuterated n-nonadecane in a urea clathrate single crystal are reported. Individual spectral densities J1(ω0) and J2(2ω0) were determined using Jeener–Broekaert experiments, and data obtained at two orientations of the main crystal axis were fit to a motional model which combines unrestricted rotational diffusion about the alkane long axis with restricted wobbling of the methylene group planes. Both motions are fast on the NMR time scale and both affect the observed, time averaged quadrupole coupling constant (e2qQ/h)eff=163 kHz. However, quadrupole tensor components appearing in the relaxation Hamiltonian should not be averaged over that part of the motion which is in fact responsible for the relaxation, and by fitting the relaxation data together with the observed quadrupolar splitting to the motional model described above we obtain a value ≥ 185 kHz for the methylene quadrupole coupling constant. This value of e2qQ/h has been averaged only over those vibrational motions which are too fast to contribute to observable relaxation.
The exact thermal rotational spectrum of a two-dimensional rigid rotor obtained using Gaussian wave packet dynamics
Authors: Reimers Jeffrey R. , Heller Eric J.
Published: July 2002

The exact thermal rotational spectrum of a two-dimensional rigid rotor is obtained using Gaussian wave packet dynamics. The spectrum is obtained by propagating, without approximation, infinite sets of Gaussian wave packets. These sets are constructed so that collectively they have the correct periodicity, and indeed, are coherent states appropriate to this problem. Also, simple, almost classical, approximations to full wave packet dynamics are shown to give results which are either exact or very nearly exact. Advantages of the use of Gaussian wave packet dynamics over conventional linear response theory are discussed.
Structure and predissociation dynamics of electronically excited nitrogen dioxide: A resonance Raman study
Authors: Rohlfing Eric A. , Valentini James J.
Published: July 2002

Resonance Raman spectra of NO2 have been recorded as a function of excitation frequency in the range 0–8000 cm−1 above the predissociation threshold. These spectra are interpreted using the time-dependent formulation of resonant Raman scattering, to elucidate the shape of the excited electronic state potential energy surface, and the dynamics which occur on it immediately following photoexcitation. Our results show that the dominant optical transition in this spectral range is 2B2–2A1. Our Raman spectra reveal a marked decrease in the dynamical time scale of the predissociation of the 2B2 state as excitation frequency increases. At excitation frequencies approximately 4850 cm−1 above the dissociation threshold, the 2B2 predissociative lifetime becomes comparable to a vibrational period. We have analyzed the resonance Raman spectra, along with the absorption spectrum, to determine the magnitudes of the slopes of the potential surface along the symmetric stretch and bending normal coordinates for the 2B2 excited state in the Frank–Condon region.
On the perturbative analysis of the dynamics of reactive collisions
Authors: Dillon Ian P. , Eno Larry
Published: July 2002

This paper explores the application of quantum mechanical perturbation theory to the determination of reactive transition probabilities. The very simple problem of particle reflection and transmission at a one-dimensional potential barrier is examined over a wide range of energies and for alternate perturbative schemes. The first scheme is conventional; the second is based on a quasi-adiabatic (QA) description of the dynamics. We find that for a choice of model parameter values, loosely based on the H+H2 system, the first (conventional) scheme provides quantitatively accurate results. However, at higher system masses, the first scheme results are in only moderate agreement with exact results and are highly sensitive to the variation of distortion potential parameters. For the high system mass case, the QA results are in much better agreement with the exact results. However, the QA results are also highly sensitive to the variation of QA potential parameters. We discuss finally a number of avenues for further investigation.
Study of electron solvation in liquid ammonia using quantum path integral Monte Carlo calculations
Authors: Sprik Michiel , Impey Roger W. , Klein Michael L.
Published: July 2002

The solvation of an electron in liquid ammonia has been studied using quantum path integral Monte Carlo calculations. In agreement with previous experimental and theoretical deductions the charge distribution of the electron is compact. Various distribution functions characterizing the structure around the solvated electron are presented and the surrounding solvent structure is compared to that around a classical atomic anion. A qualitative discussion is given of the absorption spectrum based upon the form of the complex time dependence of the electron mean squared displacement correlation function.
Complete basis set correlation energies. IV. The total correlation energy of the water molecule
Authors: Petersson G. A. , Braunstein M.
Published: July 2002

The second-order and total correlation energy of the water molecule are calculated with a basis set of 45 atomic pair natural orbitals. The asymptotic forms for the convergence of pair natural orbital expansions are then used to extrapolate to the values of the pair energies and pair-coupling terms that would be obtained with a complete basis set. The extrapolated complete basis set second-order correlation energy is equal to −361.1 mhartree and the extrapolated complete basis set total correlation energy is equal to −370.6 mhartree in agreement with the experimental value (−370±3 mhartree). The error in the extrapolated total correlation energy is more than an order-of-magnitude smaller than the error in any previous calculation.
Theory of nematic systems of semiflexible polymers. III. Phase equilibria in solutions
Authors: Ronca G. , Yoon D. Y.
Published: July 2002

Biphasic equilibria of lyotropic solutions of semiflexible polymers are calculated using the worm-like chain model with limiting curvature. The threshold concentration at incipient phase separation varies gradually with molecular weight to reach a finite value at infinite chain length. This limiting concentration is found to depend on the axial ratio of the persistence length and on the temperature. The theory is then applied specifically to poly(n-hexyl isocyanate) (PHIC) solutions. Deriving the configurational parameters from measurements on chain dimensions in dilute solutions, theoretical calculations are found to satisfactorily match recent experimental results on the molecular weight dependence of the threshold concentration of PHIC in toluene.
Oxidation of the Si(111) (7×7) surface: Electron energy loss spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy studies
Authors: Edamoto K. , Kubota Y. , Kobayashi H. , Onchi M. , Nishijima M.
Published: July 2002

High-resolution vibrational/electronic-transition electron energy loss spectroscopy, low-energy electron diffraction, and Auger electron spectroscopy have been used to study the oxidation of the Si(111) (7×7) surface at 300 K. From the initial stage (O2 exposure ∼1 L, fractional oxygen coverage θ∼0.2), an Si–Si bond breaking occurs, and atomic oxygen is adsorbed in the bridge site (between the first and second layers of Si substrate) as well as in the on-top site. Some molecular species (superoxide-like species) are also existent. With the increase in O2 exposure up to 100 L where θ∼1.5 is reached, the number of the Si–O–Si species in the selvedge region of Si substrate is greatly increased. By heating the surface pre-exposed to 1–100 L O2 at 850 K, the superoxide-like species are removed and the number of oxygen atoms in the on-top sites is decreased. With the increase in O2 exposure (0→100 L) and by heating (300→850 K), the bond angle of the Si–O–Si species is increased towards that of the vitreous SiO2. The Si oxide which is totally characteristic of vitreous SiO2 is not formed by exposing the Si(111) surface kept at 700 K to 10 000 L (10−5 Torr×103 s) oxygen. The effect of residual water–vapor adsorption has been carefully examined and minimized.
Molecular reorientation from NMR relaxation times in planar molecules
Authors: Díez Ernesto , Bermejo Francisco J. , Guilleme Joaquín
Published: July 2002

The spectral density J(0) for the autocorrelation function referred to an arbitrarily chosen body-fixed coordinate system has been rederived for a planar asymmetric rotor undergoing rotational diffusive motions. The angular dependence of J(0) was found to be more complicated than in the case of coincidence of the body-fixed frame with the principal axes of rotation. Therefore, the orientation of the principal axes of the rotational diffusion tensor can be determined from four different J(0) or 1/T1 values. This result disagrees with previous conclusions based upon the assumption that the same angular dependence should be expected irrespective of the relative orientation of the diffusion tensor axes. Likewise, the values of the three diffusion coefficients can be in principle determined from more than two (or three) different J(0) values for molecules with (or without) a binary symmetry axis. In practice, however, only two relations between them can usually be obtained due to the uncertainty in T1 measurements and to the relative insensitivity of J(0) to some correlated changes in the diffusion coefficients for moderate anisotropic molecular rotations.
Dipole properties, dispersion energy coefficients, and integrated oscillator strengths for SF6
Authors: Kumar Ashok , Fairley G. R. G. , Meath William J.
Published: July 2002

Dipole oscillator strength distributions (DOSDs) have been constructed for ground SF6 including the IR, UV, and higher energy parts of the absorption spectrum. An adopted DOSD is used to evaluate the dipole oscillator strength sums Sk, k=2,1, 1/2 (− 1/2 )−3,−4,−6,..., mean excitation energies Ik, k=2(−1)−2, dipole–dipole dispersion energies for the interaction of SF6 with itself and with Li, N, O, H2, N2, O2, NO, H2O, NH3, CO2, CO, H, He, Ne Ar, Kr, and Xe, and the triple–dipole dispersion energy for (SF6)3. Pseudo-DOSDs have been constructed for SF6 which facilitate the evaluation of the dispersion energies involving SF6. In most cases uncertainties in our results for the dipole properties and dispersion energy coefficients are ≲1–3%.
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o study of cyanogen: The X̃ 1Σ+g, ã 3Σ+u, B̃ 1Δu, and C̃ 1Πu states
Authors: Dateo C. E. , Dupuis M. , Lester W. A.
Published: July 2002

An ab initio multiconfiguration Hartree–Fock (MCHF) study of the X̃ 1Σ+g, ã 3Σ+u, B̃ 1Δu, and C̃ 1Πu states of cyanogen (C2N2) is presented. Equilibrium structures, harmonic vibrational frequencies, excitation energies, and dissociation energies have been calculated and found to be in semiquantitative agreement with available experimental data. This study confirms the π → π* character of C2N2 (ã 3Σ+u) and C2N2 (B̃ 1Δu), and the n → π* character of C2N2 (C̃ 1Πu) near their equilibrium structures. Configuration mixing among the n and π electrons is needed to correct the broken symmetry description given by the Hartree–Fock (HF) method, and leads to a qualitatively correct representation of these states. For C2N2 (ã 3Σ+u) and C2N2 (B̃ 1Δu), the harmonic force constants show a strong through-bond interaction which is due to the dominance of the charge transfer configuration +NCCN−. For C2N2 (C̃ 1Πu) the trans-bending vibrational mode and the cis-bending mode lead to different Renner–Teller splittings: the lower 1A′ state most likely has a slightly trans-bent minimum energy structure. The calculated harmonic stretching frequencies of C2N2 (C̃ 1Πu) are ν1 (sym. CN str.)=2145 cm−1, ν2 (CC str.)=989 cm−1, and ν3 (asym. CN str.)=1789 cm−1.
Coexistence curve of polystyrene in methylcyclohexane. V. Critical behavior of the three-phase coexistence curve
Authors: Nakata Mitsuo , Dobashi Toshiaki
Published: July 2002

The three-phase coexistence curve for the ternary system polystyrene (Mw=1.73×104)+polystyrene (Mw=7.19×105)+methylcyclohexane was analyzed from the viewpoint of the critical phenomena. The critical exponent at the upper critical end point was determined as βf=0.381±0.012, which is close to 0.384±0.004 at the plait point for the same ternary system and compared with the fully renormalized critical exponent. The critical exponent at the lower critical end point was found to be smaller as βf=0.342±0.031. According to the numerical analyses of the empirical Gibbs free energy, this small value was ascribed to an improper behavior of the tie lines in a composition triangle. The tie lines near the upper critical end point were parallel with each other and contained the critical end point. The observed three-phase coexistence curve in a temperature-total polystyrene volume fraction diagram was much more symmetrical than the calculated one and suggested a tricritical phenomenon.
An electron–gas plus damped-dispersion calculation of the N2–N2 interaction
Authors: LeSar R. , Shaw M. S.
Published: July 2002

We present the results of a calculation of the N2–N2 intermolecular potential using the modified Gordon–Kim (electron–gas) model with damped-dispersion terms (MGKD potential). The calculated potential agrees well in the well region with other proposed potentials. An analytical form that fits the potential with an average error of 0.2% from the well region to 30 kK on the repulsive wall is given. Solid-state properties, such as the 0 K phase diagram and the pressure–volume curve, are calculated and are in good agreement with experiment. As a test of the repulsive region, the shock Hugoniot calculated with this potential is compared with experiment and also shows good agreement.
Effect of kinetic and electronic energy on the reactions of Mn+ with H2, HD, and D2
Authors: Elkind J. L. , Armentrout P. B.
Published: July 2002

Reactions of several electronic states of Mn+ with H2, HD, and D2 have been examined using guided ion beam mass spectroscopy. The excitation function for the ground state of Mn+(7S) has two regions: one of very low reactivity at threshold and another more efficient pathway at higher energies. In contrast, the 5S and 5D states react efficiently at their thermodynamic thresholds. In reaction with HD, the 5S and 5D states produce ≊3 times as much MnH+ as MnD+ in the threshold region. This isotope effect is similar to that seen in previous studies of transition metal ion reactions. Reaction of Mn+(7S) with HD, on the other hand, exhibits an extreme isotope effect such that MnD+ is formed almost exclusively. The state dependence of the reactivity and reaction mechanisms is explained using simple molecular orbital concepts. The results are analyzed to yield a bond dissociation energy at 0 K for MnH+ of 2.06±0.15 eV (47.5±3.4 kcal/mol).
Cluster concentrations and virial coefficients for adhesive particles
Authors: Post Albert J. , Glandt Eduardo D.
Published: July 2002

The cluster concentrations and virial coefficients for systems of two- and three-dimensional particles obeying Baxter’s adhesive potential are reported, up to the (exact) fourth virial term. The exact radial distribution function displays an infinite series of Dirac-delta singularities which are absent in the Percus–Yevick approximation. The coefficient of the first of these singular diagrams is calculated.
Collision-induced absorption in gaseous N2
Authors: Moon Molly , Oxtoby David W.
Published: July 2002

We compute binary spectral integrals and the far-infrared absorption spectrum for N2 with our nonasymptotic theory of the pair dipole moment. Cancellation between two types of overlap corrections is responsible for the success of the asymptotic approximation in these calculations. We contrast this cancellation with the behavior of the contributions to the Coulomb interaction energy. We also evaluate the contributions to the second-order multipole model, including the anisotropy of the polarizability, the hexadecapole moment, and the dipole–octupole tensor.
Diode laser probe of CO2 vibrational excitation produced by collisions with hot deuterium atoms from the 193 nm excimer laser photolysis D2S
Authors: O’Neill J. A. , Cai Ji Ye , Flynn G. W. , Weston Ralph E.
Published: July 2002

The 193 nm excimer laser photolysis of D2S in D2S/CO2 mixtures produces fast deuterium atoms (ETR∼2.2 eV) which vibrationally excite CO2 molecules via inelastic translation–vibration/rotation (T–V/R) energy exchange processes. A high resolution (10−3 cm−1) cw diode laser probe was used to monitor the excitation of ν3 (antisymmetric stretch) and ν2 (bend) vibrations in CO2. The present results are compared with previous experiments involving hot hydrogen atom excitation of CO2 in H2S/CO2 mixtures as well as with theoretical calculations of the excitation probability. The probability for excitation of a ν3 quantum in CO2 is about 1%–2% per gas kinetic D/CO2 collision. Bending (ν2) quanta are produced about eight times more efficiently than antisymmetric stretching (ν3) quanta. The thermalization rate for cooling hot D atoms below the threshold for production of a ν3 vibrational quantum corresponds to less than 2 D*/D2S collisions or 15 D*/CO2 collisions.
Low-lying states of thiozone, S3
Authors: Jones R. O.
Published: July 2002

Density functional calculations are described for the energy surfaces of low-lying 1A1, 3A2, 3B1, and 3B2 states of S3 and for vertical excitation energies (from the energy minimum of the open 1A1 state) for these states and the corresponding singlets. The calculations predict a near-degeneracy between the energies of the open (C2v,αSSS=114°) and cyclic (D3h) forms. The calculated energy surfaces of the excited states are in good agreement with predictions based on the energy surfaces and single-particle eigenvalues of the 1A1 states. Calculations for S2 reproduce satisfactorily the equilibrium internuclear separation and vibration frequency, but the dissociation energies of both S2 and S3 are overestimated by ∼1.5 eV. These discrepancies are due to the use of the local spin density approximation for the exchange-correlation energy.
Photoionization mass spectrometric study and a
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o calculations of ionization and bonding in P–H compounds; heats of formation, bond energies, and the 3B1–1A1 separation in PH+2
Authors: Berkowitz J. , Curtiss L. A. , Gibson S. T. , Greene J. P. , Hillhouse G. L. , Pople J. A.
Published: July 2002

The ion yield curves of PH+3, PH+2, and PH+ from photoionization of PH3 have been measured. The free radical PH2 has been generated by pyrolysis, and the ion yield curve of PH+2 (PH2) determined. These measurements yield directly I.P. (PH3)=9.870±0.002 eV, and I.P. (PH2)=9.824±0.002 eV. In addition, we deduce D0 (H2P–H)=82.46±0.46 kcal/mol, D0 (HP–H)=74.2±2 kcal/mol, D0 (P–H)=70.5±2 kcal/mol, I.P. (PH)=10.18±0.1 eV, and other thermochemically related quantities. The ionization energies of PH, PH2, and PH3 are computed by ab initio molecular orbital methods to fourth order in Mo/ller–Plesset theory, and are found to be in good agreement with experiment. The ground state of PH+2 is inferred to be 1A1, with the 3B1 state higher by ≥0.71 eV. This ordering is the reverse of that in CH2 and NH+2.
The orientational-correlation time intercept in liquids
Authors: Evans Glenn T. , Kivelson Daniel
Published: July 2002

We propose a theory for the zero viscosity (η) or infinite temperature (T) intercept (τ0) in the much used quasihydrodynamic expression τ2=τ0+(const)(η/T) for the second rank orientational correlation time (τ2). Whereas we associate the (η/T) term with the integral over the torque–torque correlation function, we attribute the origin of τ0 to the integral over the cross-correlation function between torques and kinetic or convective terms. We are unable to evaluate the time dependence of this cross-correlation function exactly, but if a Gaussian form is assumed, a negative value is obtained for τ0. Experimentally, τ0 is sometimes positive and sometimes negative, which suggests that a monotonic form for the time dependence of the cross-correlation function might be inadequate. Because we have not been able to evaluate this cross-correlation function our explanation must still be considered a conjecture.
A direct determination of the role of vibration-to-vibration energy transfer in HF(v=3,4) self-relaxation
Authors: Robinson J. M. , Rensberger K. J. , Crim F. F.
Published: July 2002

Overtone vibration-laser double resonance directly measures the relative importance of vibration-to-vibration and vibration-to-translation-and-rotation energy transfer for HF(v=3 and v=4) at room temperature. The fraction of HF(v) molecules relaxing by V–V energy transfer is 0.44±0.05 and 0.16±0.05 for v=3 and v=4, respectively, compared to 0.59±0.10 for v=2. These measurements show that V–T,R energy transfer is the dominant relaxation mechanism for HF(v≥3) and the observed decreased amount of V–V energy transfer for higher initially excited vibrational levels is in good agreement with a chemiluminescence measurement and several theoretical calculations. The data demonstrate that the magnitude of the energy defects for the component pathways primarily determines the energy transfer mechanism for HF(v=2–4).
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o study of the locations and intensities of the lowest-lying electronic transitions of the C3 and C2O molecules
Authors: Chabalowski Cary F. , Buenker Robert J. , Peyerimhoff Sigrid D.
Published: July 2002

A series of ab initio SCF and CI calculations is reported for the C3 and C2O molecules in their lowest excited states. A large AO basis containing two primitive d functions on each atom and s- and p-type bond functions at the center of each bond is employed for C3 and CI secular equations of order 10 000 are solved based on a well-known configuration selection procedure. The location of the A 1Πu–X 1Σ+g transition is computed within an error of 0.07 eV relative to experiment and an oscillator strength of 0.052 is obtained for absorption, corresponding to a radiative lifetime for the A 1Πu state of 95 ns. The C 1Πg is found to lie only 0.838 eV above the A 1Πu state with a computed lifetime of 0.938 μs. Analogous calculations for the C2O molecule are carried out employing only a single primitive d function on each atom and geometrical optimization is undertaken for the (linear) X 3Σ− ground state. The A 3Πi, 1Σ+, 1Δ, and lowest two 1Π states are treated at the ground state equilibrium conformation, with the A–X transition energy again found to agree well with available experimental data; the lifetime of the A 3Πi state is calculated to be 0.773 μs. The relationship of the calculated results to problems of astrophysical interest is also discussed.
van der Waals induced dipoles
Authors: Linder Bruno , Kromhout Robert A.
Published: July 2002

Expressions are derived for calculating the induced dipole moment of an arbitrary molecule A interacting with an arbitrary system B through first- and second-order Coulomb interaction. The theory is formulated in terms of linear and quadratic charge-density susceptibilities and takes account of charge penetration but not exchange between the systems. The theory is specialized to the interaction of two nonoverlapping molecules and to a molecule interacting with a nonferroelectric solid, metallic, or crystalline. In the case of two interacting molecules, the induced moment is developed in inverse powers of R, the distance between the centers of the molecules, up to and including R−7; the coefficients of the series are given in terms of the total charges, permanent moments, polarizabilities, and hyperpolarizabilities. In the case of the solid, the results are given in terms of the molecule–solid distance z0, the dielectric function of the solid, and the permanent moments, polarizabilities, and hyperpolarizabilities of the molecule. The expressions for the dispersion and induction dipoles derived here in terms of the charge-density (hyper) susceptibilities are the most general to date in that ionic contributions are included, and the expansion in terms of the (hyper)polarizabilities is the most explicit for nonoverlapping systems. Our results are compared with the work in the literature.
Spectroscopic and kinetic studies of a dye laser pumped Br2 B 3Π(0+u) → X 1Σ+g laser
Authors: Perram G. P. , Davis S. J.
Published: July 2002

A dye laser pumped Br2 B 3Π(0+u) → X 1Σ+g laser has been studied. Spectroscopic assignments have shown that lasing occurs from 10≤J′≤63 in 12≤v′≤17 using Rhodamine 590 dye. The output appeared limited to the 79–81 isotope of Br2. By utilizing stimulated emission as a monitor for laser excitation spectra, dramatic increases in the resolution were observed that exceeded the normal resolution of the dye laser. The Br2 laser operated at Br2 pressures of up to 60 Torr. A simple model to explain the characteristics of the Br2 laser is described.
Large amplitude bending vibrations of the triatomic molecule: Low angular momentum states
Authors: Sage Martin L.
Published: July 2002

The large amplitude motions of the triatomic molecule are discussed in terms of two bond stretches and coupled rotations of two bonds. The same approach is used for linear and nonlinear equilibrium geometries. A model potential for bending is introduced which has reasonable behavior over the full range of bond angles. This model is applied to finding the energy levels of a linear and a nonlinear vibrating and rotating triatomic molecule with fixed bond lengths for total angular momentum quantum numbers 0 and 1.
Laser photoelectron spectroscopic determination of electronic states of Fe atoms produced in multiphoton dissociation of Fe(CO)5 in the gas phase
Authors: Nagano Yatsuhisa , Achiba Yohji , Kimura Katsumi
Published: July 2002

Multiphoton ionization (MPI) ion–current and photoelectron measurements were carried out for Fe(CO)5 in the gas phase with a visible dye laser in the wavelength region 446–452 nm. This wavelength region was chosen for the reason that the MPI ion–current spectrum shows a typical complicated pattern consisting of many atomic lines. At least 35 sharp ion–current peaks have been detected under moderate laser power conditions in the present work. From the observed photoelectron spectra, all these ion–current peaks have been successfully assigned to three-photon ionizations of Fe atoms which are produced in the five low-lying electronic states in the photodissociation of Fe(CO)5. In this paper, we want to report photoelectron spectroscopic evidences for the production of the excited state Fe atoms in the laser photolysis of Fe(CO)5. Furthermore we want to emphasize the potentiality of laser photoelectron spectroscopy for identification of various nonradiative electronic states of transient species.
Chemical timing 3. The picosecond dynamics of intramolecular vibrational redistribution from 11 levels in S1 p-difluorobenzene vapor
Authors: Holtzclaw Karl W. , Parmenter Charles S.
Published: July 2002

Collisional quenching of S1–S0 fluorescence (chemical timing) has been used to study the dynamics of intramolecular vibrational redistribution (IVR) from 11 initial vibrational levels in S1 p-difluorobenzene (pDFB). The levels range in energy from 1615 to 3310 cm−1 with total state densities ranging from about 10 to 2000 per cm−1. The IVR follows intermediate case kinetics of radiationless transition theory for levels below about 2500 cm−1 and statistical case kinetics for higher levels. The IVR involves coupled states spanning an energy width of 0.02–0.3 cm−1. The IVR lifetimes vary from about 15 to 300 ps with much variability among initial levels. A low frequency out-of-plane mode appears to be an accelerating mode for IVR. Average matrix elements for the level coupling associated with the IVR are found by analysis of the kinetic data to be in the range 0.01–0.1 cm−1. Vibration-rotation coupling appears to be an important contributor to IVR, at least for the lower levels.
Hydrogen bond energies of the HF and HCl dimers from absolute infrared intensities
Authors: Pine A. S. , Howard B. J.
Published: July 2002

The dissociation energies of the hydrogen-bonded complexes, (HF)2 and (HCl)2, have been obtained from high-resolution measurements of absolute infrared line strengths at a single temperature and pressure under the assumption of minimally perturbed local-mode behavior of the outer hydrogen stretch. The zero-point dissociation energies for the HF and HCl dimers are D0=1038(+43, −34) and 431(±22) cm−1, respectively. Estimates of the zero-point energies of the low frequency intermolecular vibrations enable us to obtain the well depths and equilibrium dissociation energies for comparison to ab initio calculations and empirical models.
The pressure variation of the glass transition temperature in atactic polystyrene
Authors: Stevens J. R. , Coakley R. W. , Chau K. W. , Hunt J. L.
Published: July 2002

Brillouin frequency shifts from light scattering are used to measure the variation with pressure of the zero-frequency glass transition temperature Tg in atactic polystyrene (PS) at pressures up to 3 kbar. These results are extrapolated to high pressures and compared with theoretical predictions. Experimentally it is found that Tg asymptotically approaches a critical value at high pressures. The equilibrium lattice model of DiMarzio, Gibbs, Fleming, and Sanchez which approximates the residual entropy of a polymeric glass correctly predicts the experimentally observed trends. The molecular theory of Bengtzelius, Götze, and Sjölander is also qualitatively correct at high pressures even though it deals with the glass transition of a classical monatomic fluid. Neither theory is quantitatively in agreement with our results.
An a
b i
n
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t
i
o investigation of possible intermediates in the reaction of the hydroxyl and hydroperoxyl radicals
Authors: Jackels Charles F. , Phillips Donald H.
Published: July 2002

A b initio quantum chemical techniques have been used to investigate covalently-bonded and hydrogen-bonded species that may be important intermediates in the reaction of hydroxyl and hydroperoxyl radicals. Stable structures of both types were identified. Basis sets of polarized double-zeta quality and large scale configuration interaction wave functions have been utilized. Based upon electronic energies, the covalently-bonded HOOOH species is found to be 26.4 kcal/mol more stable than the OH and HO2 radicals. Similarly, the hydrogen-bonded HO---HO2 species is found to have an electronic energy 4.7 kcal/mol below that of the component radicals, after correction is made for the basis set superposition error. The hydrogen-bonded form is found to be planar, to possess one relatively ‘‘normal’’ hydrogen bond, and to have lowest energy 3A′ and 1A′ states that are essentially degenerate. The 1A″ and 3A″ excited states produced by rotation of the unpaired OH electron into the molecular plane are found to be very slightly bound.
The structure of n-alkanes: High precision a
b i
n
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t
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o calculation and relation to vibrational spectra
Authors: Aljibury A. L. , Snyder R. G. , Strauss Herbert L. , Raghavachari Krishnan
Published: July 2002

Detailed geometries and deuterium-isolated CH stretching frequencies are calculated for several different conformers of small n-alkanes at a number of levels of ab initio theory. The results of the calculations are carefully compared to experiment. The calculated frequencies fit a linear correlation to the calculated bond lengths with a mean absolute error of about 3 cm−1. The experimental frequencies fit this correlation more closely. Notable deviations from the correlation occur for the crowded C–H bonds that exist in gauche conformers. The changes in geometry on rotating a methyl group in propane are calculated and compared with experimental results on frequency shifts for n-alkane methyl groups.
Field-induced fluctuation correlations and the effects of van der Waals interactions on molecular polarizabilities
Authors: Hunt K. L. C. , Bohr J. E.
Published: July 2002

New expressions for the van der Waals contribution to the collision-induced, static polarizability of a molecular pair (ΔαvdW) are derived within a reaction-field theory. For molecules interacting at long range, multipole expansions are used to determine the reaction field; at shorter range, where overlap is small but nonnegligible, the derivation is based on a nonlocal polarizability density model. In both cases, we obtain ΔαvdW in terms of integrals over imaginary frequencies, each involving the product of a hyperpolarizability for one molecule and a polarizability or hyperpolarizability for the other molecule. In addition, we show that the polarizability changes induced by van der Waals interactions between two molecules stem from two distinct physical effects. First, in an applied field F, each molecule is polarized nonlinearly by the simultaneous action of the field due to the fluctuating charge distribution of its neighbor and the field F. Second, the applied field F alters the correlations between the spontaneous, quantum mechanical fluctuations in the charge density of each molecule, thus affecting its interaction with the neighboring molecule. Effects of field-induced fluctuation correlations have not been included in earlier models for the van der Waals contributions to pair polarizabilities.
Complex coordinate calculation on an ethylene shape resonance
Authors: Donnelly Robert A.
Published: July 2002

This paper reports results of calculations of the energy and width of a 2B2g shape resonance in the ethylene molecule. A second-order treatment utilizing the coordinate-rotated electron propagator yields results in good agreement with experiment. Variation in basis set composition is shown to lead to varying estimates of the computed energy and width; the latter is found to be especially sensitive to basis composition. Fourier transforms of the Dyson orbital on resonance produce the expected quasiparticle momentum distribution, with numerically computed energy and width consistent with the coordinate rotation calculation. Off resonance, this momentum distribution is multimodal, and therefore not representative of a localized state.
A theoretical study of the infrared transition matrix elements for the X 1Σ+ state of the CH+ and CD+ molecules
Authors: Ornellas Fernando R. , Machado Francisco B. C.
Published: July 2002

Theoretical results for the absorption matrix elements, Einstein A spontaneous emission coefficients, and radiative lifetimes for all vibrational levels and selected rotational levels are reported for the CH+ and CD+ molecules in the X 1Σ+ state.
Molecular dynamics simulation of hydrocarbon molecules in condensed phases. I. Liquid n-butane
Authors: Ullo J. J. , Yip Sidney
Published: July 2002

A study is made of the feasibility of predicting the dynamical properties of liquid and solid hydrocarbons by means of molecular dynamics simulations using an empirical force field CFF developed for an entire family of alkanes. Results for mean square displacements and velocity autocorrelation functions are presented for liquid n-butane (C4H10). Vibrational frequencies are examined through a generalized density of states and compared with optical data. The van Hove correlation function for the protons is calculated and the result leads to a semiquantitative prediction of the energy distribution of neutron inelastic scattering. A certain deficiency of the force field is indicated by the present results.