bluuebunny/crossref_metadata_2025 huggingface dataset:
Potential new high energy density materials: Cyclooctaoxygen O8, including comparisons with the well-known cyclo-S8 molecule
Authors: Kim Kwang S. , Jang Jee Hwan , Kim Sungwook , Mhin Byung-Jin , Schaefer Henry F.
Published: July 2002

The cyclic O8 molecule has been studied using ab initio quantum mechanical methods. Molecular structures were fully optimized at levels of theory up to and including second-order perturbation theory (MP2) using a double zeta plus polarization basis set. Parallel theoretical studies were carried out for the valance isoelectronic S8 molecule, for which much experimental data exists. With double zeta plus polarization (DZ+P) self-consistent-field (SCF) theory vibrational frequencies and infrared and Raman intensities have been predicted. Cyclo-O8 is considerably more stable than experimental O–O bond energies would suggest and is predicted to lie only 94 kcal/mol above four infinitely separated O2 molecules.
Differential anisotropy of polarizability measured by picosecond transient dichroism and birefringence
Authors: Myslinski Piotr , Wieczorek Dariusz
Published: July 2002

The difference in the anisotropy of the polarizability between ground and excited states for two dyes in ethanol was first directly measured by Waldeck et al. using transient molecular dichroism and the birefringence method. In this paper, we present the development of the theoretical description of this method and we measure the differential polarizability for the dyes DODCI, sulforhodamine 101, and nile blue in various alcohols. The following values were obtained: <+200 Å3, +590 Å3, and +720 Å3, respectively, independent of the solvent. The proposed theory was used in a computer simulation and good agreement of the observed and calculated values was obtained. In the experiment, the rotational relaxation times were also determined and compared to the results obtained for the same molecules from the measurements using the up-conversion technique. The difference in measured rotational times for dyes in more viscous alcohols using both experimental techniques suggests that the rotational diffusion is different in the ground and excited states.
Effect of pressure on the conformation of p-terphenyl in carbon disulfide
Authors: Katō Minoru , Taniguchi Yoshihiro
Published: July 2002

The effect of pressure on the conformation of p-terphenyl in carbon disulfide has been measured by using high pressure infrared spectroscopy with a diamond anvil cell up to 7.1 kbar. Pressure effects on the nonplanar vibrational modes indicate that p-terphenyl molecule in the solution gets more planar when compressed. The intensities at atmospheric pressure indicate that the conformation in the solution is predominantly in D2 symmetry. Assuming that the two dihedral angles between the adjacent phenyl rings are equal to each other at all pressures, we estimate the rate of decrease of the dihedral angles by compression to be 2° kbar−1, which is equal to that of biphenyl in previous work.
On the bilinear approximation for time-dependent Hamiltonians
Authors: Echave Julián , Fernández Francisco M. , Castro Eduardo A.
Published: July 2002

The transition probabilities for a one-dimensional harmonic oscillator with a time-dependent interaction potential are obtained. Two different bilinear approximations to the interaction potential are obtained by expanding it in Taylor series around two reference coordinate values, namely, the equilibrium position q0=0 and the expectation value of the coordinate q0=〈q〉, keeping only first- and second-order terms. The effect of the choice of q0 on the accuracy of the results is investigated for several values of the interaction potential parameters. The model posed by the semiclassical approach to the collision between a structureless particle and a harmonic oscillator via an exponential interaction potential is chosen as an illustrative example. The expansion around 〈q〉 is found to be related to an iterative procedure, based on Lie algebraic methods, recently proposed to treat time-dependent quantum-mechanical problems.
Photodesorption of NO from chemically modified Ni(111) surfaces
Authors: Yoshinobu Jun , Guo Xingcai , Yates John T.
Published: July 2002

NO photodesorption from the oxygen-covered, oxidized, and sulfur-saturated Ni(111) surfaces was studied. Temperature programmed desorption effectively discriminates between NO species adsorbed on Ni metal sites and on Ni sites modified by O or S, making the study of photodesorption from each site possible. Oxygen- and sulfur-modified Ni(111) surfaces both exhibit enhanced photodesorption cross sections compared to a clean Ni surface. Both the direct observation of NO(g) photodesorption and postirradiation thermal desorption measurements of NO coverage have been carried out. The photodesorption process is first order in photon flux and in NO coverage within a given chemisorption state. The photon-energy dependence of the NO photodesorption cross section from oxidized and S-saturated Ni(111) surfaces was measured in detail over a photon energy range of 1.5–3.8 eV. For both oxidized and sulfur-saturated Ni(111), the threshold for NO photodesorption is ∼1.5 eV. In addition, the photodesorption cross sections are dependent on the adsorption states of NO on the modified Ni(111) surfaces. The mechanisms of NO photodesorption are discussed and it is proposed that NO photodesorption is probably induced by the valence electron excitation from the occupied NO 2π*-d state to the unoccupied NO 2π*-d state. However, a hot electron model originating from the substrate cannot be excluded. The quenching of electronically excited NO on the clean Ni(111) surface is efficient.
On the use of combination/overtone band resonance Raman excitation profiles for understanding the vibronic coupling mechanism in the 700 nm absorption band of azulene
Authors: Lu H. M. , Page J. B.
Published: July 2002

Finite temperature model expressions for the excitation profiles of second-order Stokes resonance Raman (RR) scattering are derived within the framework of the time correlator theory for systems with simultaneous linear electron–phonon coupling and linear non-Condon coupling. These expressions have all the advantages common to solutions from the time correlator theory, allowing convenient and efficient multimode modeling procedures. These results, together with those obtained elsewhere for the second-order profiles of systems within the Condon approximation, but having mode mixing and frequency shifts, are applied to the RR scattering originating from the 700 nm absorption band of azulene. For this system, it was shown previously that the measured optical absorption and fundamental RR profiles cannot discriminate between a mode mixing model, invoked to account for the measured relative intensities of the fundamental profiles of two modes, and an equally successful non-Condon model; nor can recent quantum-chemistry calculations by others for this system. With our new model expressions for the second-order profiles, we calculate here the profiles of the combination/overtone bands involving these two modes, using either the same mode mixing model parameters, or the same non-Condon model parameters which were previously successful for the absorption and fundamental profiles. Our results show that the second-order profiles predict a clear discrimination between the two models.
Calculation of desorption rates for Xe/Pt(111) using a realistic gas–surface potential
Authors: Bethune D. S. , Barker J. A. , Rettner C. T.
Published: July 2002

We review the classical transition state theory (TST) of desorption and employ it to predict the desorption rate kTST for the Xe/Pt(111) system using a realistic gas–surface interaction potential. The Xe/Pt(111) potential surface is derived from a Xe–Pt pair potential with parameters suitably adjusted to give agreement with experimental data on the Xe/Pt(111) vibrational frequency, well corrugation and well depth. The calculated rates agree well with earlier measurements which span 7 orders of magnitude in rate, corresponding to temperatures in the range 80<T<160 K. However, we find that the calculated preexponentials vary by a factor of 5 over this range in T, implying that the actual potential well depth for Xe/Pt(111) is 10 meV greater than the energy obtained directly from the experimental Arrhenius plot slope. The effective preexponential given by the 1/T=0 intercept of an Arrhenius plot of kTST is found to be 1.6×1012 s−1, in excellent agreement with the measured value. We then extend this treatment to calculate desorption rates when surface defects are present. Our pair potential is used to calculate the potential in the vicinity of the close-packed step edge chosen as a model defect. This potential and the measured defect site density are used with TST, generalized to include the effect of defects, to predict desorption rates in the defect-dominated regime. The desorption preexponential factor found in this case is >103 larger than the value describing the ideal Pt(111) surface, consistent with the striking increase found experimentally, while the well depth obtained at step sites is 355 meV, 100 meV deeper than for Xe on terraces, again in reasonable agreement with experiment.
Rotationally inelastic scattering of glyoxal by H2 at E=80 meV
Authors: Kroes G.-J. , Rettschnick R. P. H.
Published: July 2002

Using the Monte Carlo classical trajectory (CT) method and the azimuthal close-coupled, infinite-order sudden (ACC-IOS) method, we have calculated cross sections for rotational excitation of S1 trans-glyoxal by H2 at E=80 meV. The cross sections σ(k=0, j→k′) calculated with the CT method are nearly independent of j. The classical values of σ(k=0, j=5→k′) are in good agreement with the quantum values of σ(k=0→k′) for 2≤k′≤12, although the quantum calculations show a slight preference for odd Δk transitions which is not found in the CT calculations. Both the CT results and the ACC-IOS results are in good agreement with results obtained in a recent crossed beam experiment. Rotational excitation to high k′ (k′=11,12) occurs by collisions of H2 with one of the H atoms of glyoxal, and the initial value of the orbital angular momentum approximately equals the final value of k in such collisions. Since backward scattering is dominant in collisions leading to high k′, angular momentum constraints alone cannot explain the maximum observed in Δk experimentally (Δk=14).
General features of lattices of point dipoles against a charged wall
Authors: Guidelli Rolando
Published: July 2002

Some general properties of monolayers and multilayers of point dipoles against a charged wall, which do not depend on the discrete or infinite number of allowed orientations nor on the statistical-mechanical procedure employed, are derived. These properties stem directly from purely electrostatic considerations on the regular lattice structure of the layer and from the very definition of a statistical-mechanical average. The ‘‘effective coordination number’’ ce, frequently employed in monolayer models, is shown not to be an adjustable parameter, but rather a well-defined lattice sum. The implications of the use of erroneously high ce values are pointed out. Finally, the general features of monolayer models of point dipoles with an infinite number of orientations in the framework of the mean-field approximation are stressed, together with the limitations of this widely employed approximate statistical-mechanical procedure.
Role of dispersion potential anisotropy in the theory of nematic liquid crystals. I. The mean-field potential
Authors: Eldredge Clark P. , Heath Holly T. , Linder Bruno , Kromhout Robert A.
Published: July 2002

We reexamine the effect of the anisotropy in the intermolecular attractive (dispersion) interaction on the attractive mean-field potential used in the generalized van der Waals theory of the nematic–isotropic transition. Contrary to results in the literature, we show (1) that the anisotropic part of the dispersion potential reduces the anisotropy of the mean field contributed by the isotropic part of the dispersion potential; (2) that the form of anisotropy in the attractive intermolecular interaction usually chosen for simplicity in thermodynamic calculations corresponds to a preaveraged form of the dispersion interaction. The anisotropy of both the dispersion and of the mean included angle potential (which corresponds to the mean-field potential in a perfectly ordered nematic) are shown graphically, and the expansion coefficients for the latter in terms of even-ordered Legendre polynomials are tabulated.
Calculations on the van der Waals spectrum of Ar–tetrazine
Authors: Tiller A. R. , Clary D. C.
Published: July 2002

The van der Waals bound states of Ar–tetrazine are calculated by a method which treats the tetrazine fragment as a rigid rotor, but which is otherwise exact within the Born–Oppenheimer approximation. The results are used to obtain frequencies and intensities for transitions between van der Waals states in the S1←S0 electronic spectrum of the complex. Selection rules for such transitions are derived using permutation–inversion symmetry arguments. The normal rigid molecule selection rules are relaxed due to coupling between the low frequency van der Waals vibrations and the overall rotation of the complex, leading to the possibility of observing single excitation of the nontotally symmetric van der Waals bending modes. Transition intensities for van der Waals stretching excitation are predicted much smaller than experiment and various possible explanations for this disagreement are discussed.
Light-induced shifts in chemical equilibria: Optical orientation-induced shift in 2Li(g)↔Li2(g) and o
r
t
h
o/p
a
r
a enrichment
Authors: Bernheim R. A. , He Chun , Alzetta G.
Published: July 2002

The shift in the equilibrium distribution between reactants and products in the chemical reaction 2Li⇋Li2 that accompanies an optical orientation of the atomic Li spin angular momentum has been observed, and its dependence and degree of atomic orientation is reported. An examination of the resolved rotational structure in the Li2 electronic spectrum reveals the production of molecular nuclear spin polarization that accompanies the equilibrium shift, resulting in a change in the ortho/para ratio for Li2. Orientation of the spin angular momentum of the 2 2S1/2 ground state Li atoms was produced in a vapor cell by optical pumping with a tunable, single mode cw dye laser, and the molecular density in different rotational states of the 7Li2 X 1Σ+g (v=0) molecular state was monitored using a second cw dye laser.
Multichannel analysis of He*(21S)+Ne elastic and inelastic scattering in crossed atomic beams
Authors: Martin D. W. , Fukuyama T. , Siska P. E.
Published: July 2002

State-to-state elastic and inelastic angular distribution and time-of-flight measurements are reported for the scattering of He*(21S) by Ne in crossed supersonic atom beams at four collision energies in the range 0.6–2.8 kcal/mol. The inelastic collision products He+Ne*(nl), where nl=3d′, 4p, 4p′, 5s, 5s′, and 4d, are scattered predominantly forward with respect to the direction of incidence, except for endothermic states near threshold. The data are analyzed with a numerically exact multichannel curve-crossing model that yields good agreement with experimental cross section branching fractions and total quenching and state-to-state rate constants as well as the angular measurements. The model suggests the importance of intermediate ‘‘chaperone’’ states, in which the excited electron is temporarily trapped in a d or f Rydberg Ne orbital, in channeling flux into the 4s′ and 5s′ upper laser states of Ne by energy transfer from He*(2s1,3S).
A new quantum isotope effect: Extreme local mode selectivity in unimolecular dissociations imposed by antagonism between dynamic propensities of educts and zero point energies of products
Authors: Hartke B. , Manz J.
Published: July 2002

We predict a new quantum isotope effect for unimolecular dissociations of molecules with two equivalent but isotopically substituted bonds l (light isotope) and h (heavy isotope), e.g., HOT where l=HO and h=OT. Consider two near-degenerate local vibrational excitations of bonds l or h, with energies between the gap of product zero point energies. Dynamically, these excitations should induce preferential fissions of bonds l or h, but energetically, these decay channels are open and closed, respectively. Therefore, local excitation of bond h must be followed by extremely slow internal vibrational energy redistribution to bond l before dissociation, whereas local excitation of bond l induces direct, rapid decay. The resulting decay rates differ by many orders of magnitudes. The effect is demonstrated by fast Fourier transform propagation of representative wavepackets for a model system, HOT→H+OT. Extended applications to more excited educts HOT also confirm an effect discovered previously for HOD, i.e., local mode selective control of competing bond fissions H+OT←HOT→HO+T.
Vibrational relaxation of carbon dioxide at LiF(100)
Authors: Hamers J. S. , Houston P. L. , Merrill R. P.
Published: July 2002

The vibrational relaxation of CO2 at LiF(100) has been investigated by monitoring infrared fluorescence from vibrationally excited molecules under conditions where they are relaxed primarily by collisions with the solid surface. The relaxation probabilities are found to be 0.65±0.10 at room temperature and 0.35±0.10 at 450 K. In order to understand better the vibrational relaxation results, angular distributions of CO2 scattered from LiF(100) were measured with a molecular beam scattering apparatus. At slow incident beam velocities, the trapping probability of CO2 at LiF(100) is essentially unity. Thus, in the vibrational relaxation measurements, where the incident velocity is even slower than in the scattering experiments, vibrational relaxation is preceded by trapping. Possible mechanisms for relaxation are discussed. Excitation of phonons in the solid and transfer of energy to other degrees of freedom of the molecule (i.e., translation, rotation, and other vibrational modes) are both plausible relaxation channels.
Time-of-flight spectra of a particle scattering from a collinear harmonic lattice at finite temperature
Authors: Messina Michael , Coalson Rob D.
Published: July 2002

A new formalism is developed for computing the time-of-flight spectrum of a particle scattering from a collinear harmonic lattice prepared at finite temperature. We use a time-domain transcription to construct an S-matrix formalism that can be easily implemented via Gaussian wave packet dynamics. Numerical results are presented for a particle scattered from a lattice containing 100 oscillators at several temperature values.
The equation of state for hard-sphere systems in solid phase in arbitrary dimensions
Authors: Goldman Joseph I. , White John A.
Published: July 2002

Analytic equations of state for the solid phase of hard-sphere systems of dimensionality d=2,3,4,5 are suggested and compared with computer simulation data. Analytic expression for the heat capacity Cp is also given.
Silicon dimer formation by three-body recombination
Authors: Martin David L. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The rates and dynamics of three-body thermal recombination of silicon atoms to form dimers is investigated at temperatures of 800, 1000, and 1200 K with Ar and Si atoms acting as the third body. A previously reported global potential-energy surface fitted to the results of ab initio calculations at the MP4/6-31G* level and experimental data are employed for the [Si,Si,Si] system. A simple, pairwise potential is used for the [Ar,Si,Si] system. The calculated rate coefficients for the [Ar,Si,Si] system all lie in the range of 1.34–1.46×1016 cm6/mol2 s. If rotationally trapped dimers are included, the results are in the range of 2.51–2.68×1016 cm6/mol2 s. The weak temperature dependence is characterized by an activation energy of 1.2 kcal/mol. When silicon is the third body, the rates are more than an order of magnitude larger due to the increased interaction and the opening of a complex formation channel for recombination. Four mechanistic pathways leading to recombination are identified. These are direct energy exchange, direct atom exchange, complex formation, and metastable formation due to a rotational barrier. For the [Si,Si,Si] system at 800 K, the contributions of these pathways to the total recombination rate are: direct energy and atom exchange (65.5%), complex formation (6.5%), and metastable formation (28%). Internal energy distributions for product Si2 dimers are reported. In every case, these distributions exhibit a prominent maximum at the Si2 dissociation threshold. The falloff at energies below the maximum reflects the expected exponential distribution of translational energies in unimolecular dissociation processes. The distributions for the [Si,Si,Si] system are broader than those obtained when Ar is the third body. This increased breadth is interpreted to be due to the increased interaction and complex formation that is not present for the [Ar,Si,Si] system.
On the performance of atomic natural orbital basis sets: A full configuration interaction study
Authors: Illas F. , Ricart J. M. , Rubio J. , Bagus P. S.
Published: July 2002

The performance of atomic natural orbital (ANO) basis sets has been studied by comparing self-consistant field (SCF) and full configuration interaction (CI) results obtained for the first row atoms and hydrides. The ANO results have been compared with those obtained using a segmented basis set containing the same number of contracted basis functions. The total energies obtained with the ANO basis sets are always lower than the one obtained by using the segmented one. However, for the hydrides, differential electronic correlation energy obtained with the ANO basis set may be smaller than the one recovered with the segmented set. We relate this poorer differential correlation energy for the ANO basis set to the fact that only one contracted d function is used for the ANO and segmented basis sets.
Scaling the reaction path potential
Authors: Collins Michael A. , Ischtwan Josef
Published: July 2002

A molecular potential energy surface may be approximately described in terms of the energy, energy gradients, and second derivatives at a discrete set of points along a reaction path. The accuracy of this approach is limited, in part, by the level of ab initio theory that can be employed to obtain energy gradients and second derivatives. A method is derived and demonstrated whereby the energy surface can be scaled to achieve a greater degree of accuracy, using a set of energies calculated at a higher level of ab initio theory.
Franck–Condon enhancement of χ(3) in a conjugated polymer under double resonance conditions
Authors: Blanchard G. J. , Heritage J. P.
Published: July 2002

Stimulated inverse Raman spectroscopy is used to investigate exciton–phonon coupling in the polydiacetylene PTS. The Raman resonances used the v=0 and one v=1 singlet exciton as intermediate states, and the consequent Stark shiftings and splittings of the excitons are measured. We use these optical Stark effects to determine the coupling between the ground state phonons and the v=0 exciton. This coupling is as strong for the combination modes as for the fundamental modes. The coupling between several combinations modes and the v=1 exciton is 4 to 10 times greater than it is for the v=0 exciton, in agreement with calculations of the overlap integrals for each transition.
Photochemistry of adsorbed molecules. VI. Ultraviolet photoreaction of OCS on LiF(001)
Authors: Dixon-Warren St. J. , Leggett K. , Matyjaszczyk M. S. , Polanyi J. C. , Young P. A.
Published: July 2002

The surface-aligned photoreaction (PRXN) of OCS physisorbed on LiF(001) was investigated at 222 nm. Ultraviolet PRXN within the adsorbate was detected over a wide range of coverages. Diatomic sulphur was produced by reaction of photolytically generated sulphur atoms with a coadsorbed OCS molecule: S*, S+OCS(ad)→S*2, S2(g)+CO(ad/g). Translational energy and angular distributions of the S2 reaction product were obtained by angle-resolved time-of-flight mass spectrometry. The dynamics of PRXN were found to vary with adsorbate coverage. At low coverages (<10−2 ML), reaction was attributed to a ‘‘direct’’ abstraction mechanism, leading to energetic S2. As the coverage was increased above 10−2 ML, both ‘‘direct’’ and ‘‘indirect’’ PRXN dynamics were observed. Possible origins of these differing photoreaction dynamics are discussed.
Picosecond measurements of relaxation of internal modes in crystalline benzene as a function of temperature
Authors: Torre Renato , Righini Roberto , Angeloni Leonardo , Califano Salvatore
Published: July 2002

The relaxation rates of four internal vibrational levels of benzene crystal have been measured by time-resolved coherent anti-Stokes Raman scattering (CARS) at different temperatures. The important information on the dephasing mechanisms provided by the experiment has been supported by anharmonic calculations, which include the full contribution of the density of phonon states, by utilizing average coupling coefficients. Different behaviors have been observed for the different modes considered. Three of the four vibrations (v1,v6, and v10) show linewidths that increase linearly in the classical regime with temperature; the experimental evidence for the important role played by three-phonon processes (driven by cubic anharmonicity) is confirmed by the calculations, which give a quantitative agreement with the observed linewidths. For the above-mentioned vibrations the role of pure dephasing results in a minor contribution, while the effect of isotopic impurities is important in determining the low temperature relaxation rate. On the other hand, the linewidth of v9 increases quadratically with T: Both decay processes of high order and pure dephasing may be responsible for such a behavior. This ambiguity cannot be overcome by our calculations, since these do not include the effect of high order (mainly quartic) anharmonic terms. Finally, the analysis of the decay mechanisms as predicted by the calculation shows that the anharmonic coefficients may differ from mode to mode: the relaxation mechanism is highly mode selective, and its efficiency depends greatly on the nature of the molecular normal coordinates involved in the process.
Phase transitions in sterically stabilized silica colloids studied by adiabatic calorimetry
Authors: de Kruif C. G. , van Miltenburg J. C.
Published: July 2002

Adiabatic calorimetry measurements were made on colloidal silica particles (designated SJ4) coated with a dense layer of octadecyl chains. A dry powder consisting of these particles shows a phase transition at 284 K. This is attributed to an order–disorder transition in the stabilizing chains. Dispersing the particles in toluene shifts this order–disorder transition to lower temperatures. We propose a model in which the organic layer dissolves in the solvent at high temperatures and solidifies at low temperatures. This eutectic model predicts qualitatively correct the phase behavior of such colloidal dispersions. Particularly the pressure dependence of the phase transition is consistent with the model.
Kinetics of irreversible multilayer adsorption: One-dimensional models
Authors: Bartelt M. C. , Privman V.
Published: July 2002

We study microscopic lattice models of the kinetics of irreversible multilayer adsorption on uniform surfaces. Analytical results are derived for the deposition of multilayers of dimers on a one-dimensional lattice within the rate equation approach. The resulting expressions are compared with the mean-field theory.
Observation of the 2ν2(l=2)←0 overtone band of H+3
Authors: Xu Li-Wei , Gabrys Charles , Oka Takeshi
Published: July 2002

The overtone band of 2ν2(l=2)←0 of H+3 in the 2 μm infrared region has been observed in absorption by using a difference frequency laser spectrometer. The LiIO3 nonlinear optical system assembled by Bawendi and the bidirectional multiple path optical arrangement with velocity modulation have enabled us to observe 34 vibrational–rotation transitions from 5094 to 4557 cm−1. Liquid-N2 cooled plasmas were used for transitions involving low rotational levels, while gaseous-N2 cooled plasmas were used for high rotational levels, up to J,K=(9,9). From the relative intensities of the transitions, rotational temperature has been estimated to be ∼270 K for the former and ∼1300 K for the latter. The Δ‖k−l‖=±3 selection rule of the overtone band allows us to determine the absolute energy level values by combining the frequencies of the overtone band with those of the fundamental and hot bands.
An a
b i
n
i
t
i
o study of the 24 electron radicals PF, SO, NCl, SF+, ClO+, SiF−, PO−, NS−, and CCl− in their X 3∑− electronic states
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

The potential energy functions (PEFs) of the X 3∑− states of PF, SO, NCl, SF+, ClO+, SiF−, PO−, NS−, and CCl− have been calculated by several ab initio methods using a basis set of 101 contracted Gaussian-type orbitals. The methods used included fourth order Mo/ller–Plesset perturbation theory with single, double, and quadruple substitutions (MP4SDQ), singles and doubles configuration interaction (CI-SD), version 1 of the coupled electron pair approximation (CEPA-1), and complete active space self-consistent field (CASSCF). The CI-SD PEFs were very similar to those calculated by CEPA-1 when a size-consistency correction was added to the CI-SD total energies, and these results were designated CI-SD(s). The results for the neutral radicals are compared to the available experimental data, and predictions of the rotational and vibrational spectra of the ions have been made. Dissociation energies have been calculated for all nine species at the MP4SDTQ (MP4SDQ including triple substitutions) level of approximation, which also yielded MP4SDTQ ionization potentials for SF and ClO and electron affinities of SiF, PO, NS, and CCl. All of the negative ions are predicted to be stable with respect to loss of an electron by at least 20 kcal/mol. Electric dipole moment functions have also been computed for each species by CI-SD, CEPA-1, and CASSCF. Molar integrated IR absorption intensities for the fundamental transitions are predicted to be (in cm2mol−1) 15 860 (PF, CEPA-1), 1890 (SO, CASSCF), 5995 (NCl, CEPA-1), 7235 (SF+, CEPA-1), 1205 (ClO+, CASSCF), 20 070 (SiF−, CEPA-1), 20 390 (PO−, CASSCF), 90.0 (NS−, CASSCF), and 22 600 (CCl−, CEPA-1).
Electric field effects in the photoionization of N2 near threshold
Authors: Berkowitz J. , Ruscic B.
Published: July 2002

Structural features well below the usual Stark field depression of the ionization threshold are observed in the photoionization of N2 in the presence of weak (10–15 V/cm) fields. The energies of the structural features can be simulated by assuming rotational autoionization (ΔJ = −2). They are interpreted as originating from Q (J″) transitions, forbidden to spontaneously autoionize in the absence of a field. The field relaxes the rigorous autoionization selection rules by l spoilage of the Rydberg electron. The effect appears accentuated for high J, low n* states (J ∼27, 28; n* ∼ 23) and quenches at J∼11, n* ∼ 37. The quenching is attributed to the onset of the Inglis–Teller effect.
Optimum muffin-tin sizes and corrected potential energy curves in the MSXα method. Application to the a1 and t2 curves for TiF4
Authors: Huke Jeremy P. , Parsons Ian W. , Till Stephen J.
Published: July 2002

The usefulness of the muffin-tin (MT) Xα method for computing potential energy curves of molecules containing transition metal atoms is investigated using TiF4 as a test case. Two previously suggested improvements to the MT calculation of the total energy—optimization of the MT sphere radii and the ‘‘nonmuffin-tin’’ (NMT) correction of Connolly and Danese—are studied. To facilitate the optimization of sphere radii, a formula for the derivative of the total energy with respect to these radii is derived, and its accuracy tested. Procedures for choosing sphere radii are discussed and calculations of the a1 symmetric stretch potential energy surface are performed to compare the results of optimizing the sphere radii with those of the commonly used Norman method of choosing radii. It is found that optimization does not greatly improve the calculated equilibrium bond length, vibrational frequency, or dissociation energy. The inclusion of the NMT correction, however, does lead to a bond length and vibrational frequency in good agreement with experiment. The frequency of the t2 stretching vibration is also calculated using the NMT correction, but here the agreement with experiment is less good.
Observation and spectroscopy of metallic free radicals produced by reactive collisions during a supersonic expansion
Authors: Whitham C. J. , Soep B. , Visticot J-P. , Keller A.
Published: July 2002

Using the techniques of laser vaporization, supersonic jet, and laser-induced fluorescence spectroscopy, we have studied a number of small calcium containing free radicals, formed by the reaction of calcium atoms with the appropriate precursor molecule. We present spectra for CaCCH (from HCCH), CaNH2 (from NH3 ), and CaCN (from CH3CN ) in the region of the first electronically excited states. The cooling achieved in the jet has revealed structure hidden in previous high temperature studies. In the case of CaCN, we report the first rotationally resolved spectra of this molecule.
State selective doubly enhanced sum-frequency mixing in CO
Authors: Merkt F. , Softley T. P.
Published: July 2002

Doubly enhanced sum-frequency mixing (DESMI) of two collinear laser beams is studied using a pulsed jet of carbon monoxide as the nonlinear medium. The pump laser frequency (ω1) is in two-photon resonance with a selected rotational component of the A1Π–X1Σ+ (5,0) transition. Tuning the probe laser frequency (ω2) to a sequential rotational component of the B1Σ+–A1Π (1,5) transition leads to strong enhancement of VUV generation at frequency 2ω1+ω2. The double enhancement is shown to be subject to firm overall selection rules. It is demonstrated that DESMI can be accurate and efficient for the study of molecular states in the VUV and XUV provided that low laser powers are used. The high selectivity of the four-wave mixing processes investigated allows a better understanding of the competition between four-wave mixing, reabsorption of the sum frequency, and multiphoton absorption. In particular, it is shown that resonance-enhanced multiphoton absorption is an important saturation mechanism at high laser powers.
Extension of the Pack–Parker quantum reactive scattering method to include direct calculation of time delays
Authors: Darakjian Zareh , Hayes Edward F.
Published: July 2002

A direct method for determining time delays for quantum reactive scattering is developed for three atoms scattering in three physical dimensions. The method is a simple extension of the Pack–Parker approach to reactive scattering. In their formulation, adiabatically adjusted-principle axis-hyperspherical (APH) coordinates are used to generate coupled equations for the exchange region. These solutions are then projected onto Delves coordinate wave functions to generate the corresponding set of coupled equations that must be propagated out into the asymptotic region. The Delves wave functions are then mapped onto the Jacobi coordinate wave functions from which the reactive scattering S matrix is obtained. The extension of this method to include the direct calculation of the time delays for state-to-state reactive scattering processes involves three essential steps: (1) Modification of the log-derivative method for propagating accurate solutions to the coupled equations so that the log-derivative and its energy derivative are propagated simultaneously; (2) establishing that the APH to Delves projection is independent of the energy; and (3) extension of the energy dependent Delves to Jacobi mapping to include the global R matrix and its energy derivative. The necessary mathematical expressions for accomplishing each of these steps are developed in sufficient detail so that the power and simplicity of the method can be understood and so that the method can be efficiently implemented.
Triple-resonance spectroscopy of the higher excited states of NO2. II. Vibrational mode selectivity in the competition between predissociation and autoionization
Authors: Campos Francis X. , Jiang Yanan , Grant Edward R.
Published: July 2002

Three-color, triple-resonance ionization-detected absorption spectra of the autoionizing (010) s and d states of NO2 are presented and compared with similar spectra of the (100) and (110) manifolds. The (010) states are well described by Hund’s case (d), and exhibit evidence of vibronic coupling with other vibrational manifolds. The laser limited linewidths of the (010) resonances show autoionization by ν2 to be slower than autoionization via ν1 by a factor of at least 30. The mode dependence of the autoionization rate can be explained by a model established some time ago for diatomics that connects the Rydberg-electron-cation-core coupling dynamics with the time-dependent core charge distribution associated with each vibrationally excited state. Simple calculations, which focus on the monopolar components of the core-vibrational dynamic multipole potentials, agree qualitatively with the experimental results. A large signal increase at the (010) vertical threshold evidences significant decay through predissociation. Comparison with the (100) and (110) vertical thresholds suggests that, in addition to coupling less effectively to the ionization continuum, excitation in ν2 promotes dissociation.
Quantum mechanical analysis of impulsive stimulated light scattering from phonons
Authors: Walsh Alan M. , Loring Roger F.
Published: July 2002

We analyze two strategies for calculating the signal in a time-resolved, coherent light scattering experiment in which intermolecular motions, such as phonons in a molecular crystal, are excited. In the first strategy, the detected radiation is quantized, and the signal intensity is calculated to eighth order in perturbation theory. In the second method, this radiation is treated classically, and the signal amplitude is calculated to fourth order in perturbation theory. Each of these approaches has been used in the literature of nonlinear spectroscopy. We establish that for a general model of molecules whose electronic transitions are coupled to intermolecular motions, the two approaches are not formally equivalent. We present calculations of the experimental observable for a model of a molecular crystal with linear exciton–phonon coupling. These calculations allow us to determine the relative importance of contributions to the signal from phonon dynamics of the crystal in its electronic ground state and from dynamics of the electronically excited crystal. A similar analysis is performed for coherent light scattering from intramolecular vibrations.
The vibration and orientation bands of the NH+4 ion in ammonium tetraphenylborate. IV. Raman studies
Authors: Roberts Malcolm P. , Cavagnat Raymond M. , Lucazeau Guy , Bocquet Jean-François , Strauss Herbert L.
Published: July 2002

The N–H stretching and low frequency regions of the Raman spectrum of the title compound [ammonium tetraphenylborate (ATPB) and ATPB-d20 ] are examined at low temperatures. In the low frequency Raman region, we observe the predicted orientational transitions directly. The Raman N–H spectrum shows features similar to those in the infrared spectrum of this region, interpreted as showing nearly free rotation of the NH+4 ions. Many of the features of both the low frequency and N–H stretching regions arise from the effect of D2d site symmetry on the otherwise tetrahedral NH+4 ion. The picture of some of the NH+4 ions in this crystal as perturbed, but very nearly free to rotate is confirmed.
Orientational dependence of the translational energy transfer in the scattering of oriented fluoroform and t
e
r
t-butyl chloride molecules by a graphite(0001) surface
Authors: Ionov Stanislav I. , LaVilla Michael E. , Bernstein Richard B.
Published: July 2002

Time-of-flight distributions of beams of hexapole-oriented CHF3 and t-BuCl molecules scattered from a graphite (0001) surface have been measured for parallel vs antiparallel incident orientations of the molecular dipole with respect to the surface normal, over a range of surface temperatures 170≤Ts≤730 K. The observed difference in arrival times, Δtexp, for opposite initial orientations depends strongly on the degree of orientation of the incident molecules. In the analysis of the Δtexp data, we make use of the two-component model, which assumes that the scattered beams are composed of directly scattered and trapped/desorbed molecules. It is shown that in the common case of short residence times for the trapped molecules, the difference in arrival times for the directly scattered molecules, Δtdir, can be ascertained from the measured Δtexp. The magnitudes of the calculated Δtdir correspond to a strong orientation dependence in the translational energy transfer accompanying the direct scattering of CHF3 and t-BuCl by graphite (0001). The final translational energy of directly scattered molecules E′ is found to be smaller for the collision of the H ‘‘end’’ of fluoroform with the graphite surface; for t-BuCl, E′ is smaller for the Cl ‘‘end’’ collision. These are the orientations that also give rise to higher trapping probability. In the course of the present study, the residence times of t-BuCl on graphite (0001) have been measured over the surface temperature range 170<Ts<240 K.
Periodic pseudopotential Hartree–Fock study of α-quartz structure SiO2 and GeO2
Authors: Silvi Bernard , D’Arco Philippe , Causà Mauro
Published: July 2002

Periodic pseudopotential Hartree–Fock calculations have been performed on the quartz structure of silica and germanium dioxide. It is shown that the tilt angle is the most important structural parameter. This parameter emphasizes the role of the oxygen polarization function which appear to be necessary to recover the correct geometry of low quartz. The structure of β-quartz is rationalized on the basis of the energy dependence upon the tilt angle and the absence of β-structure in GeO2 is explained. Finally the behavior of both oxides under pressure is discussed.
Coupled Hartree–Fock dipole polarizabilities of the smaller n-alkanes CnH2n+2(n≤7)
Authors: Beck Donald R. , Gay David H.
Published: July 2002

We have calculated the dipole polarizabilities of the ‘‘chain’’ alkanes CnH2n+2 (n ≤7) using the coupled Hartree–Fock method. Our average values are 97%–98% of gas phase experimental values. For the larger alkanes, each CH2 unit adds 12.3 a.u. to the average value (within 2%). The anisotropic polarizability also agrees well with gas phase experimental values, but it continues to increase with the length of the chain. We have also obtained dipole moments for the odd alkanes−propane, pentane, and heptane. For propane, there is a gas phase experimental value with which we agree quite well.
Equation of state of athermal lattice chains: Effects of polydispersity
Authors: Hertanto Agung , Dickman Ronald
Published: July 2002

The repulsive-wall simulation method is applied to a polydisperse system of athermal chains on the simple cubic lattice. The pressure is determined over the full range of densities for two chain length distributions with average length 〈n〉=40. Our results indicate that the compressibility factor is insensitive to polydispersity, in accord with the predictions of simple mean-field theories. The density dependence of the mean-square end-to-end distance is also investigated.
Molecular arrangements and conformations of liquid n-tridecane chains confined between two hard walls
Authors: Vacatello Michele , Yoon Do Y. , Laskowski Bernard C.
Published: July 2002

Liquid n-tridecane (C13H28) chains confined between two parallel hard walls separated by 5.0 nm have been studied by Monte Carlo simulations, employing united CH2 atoms linked with fixed bond lengths and angles and continuously varying torsional angles subject to appropriate interaction energies. Molecules located in the central region are in remarkable agreement with ideal unperturbed chains. Significant influences of walls persist over a distance of ∼1.5 nm, exhibiting progressively less-dense and less-pronounced segmental layers of ∼0.4 nm thickness. Segmental orientations preferentially aligned along the surface are found only in the first layer, accompanied by little perturbations in the fraction of trans-conformations. Furthermore, nearly all the chain units in the first segmental layer adjacent to the walls belong to two-dimensional chains, and exhibit considerable orientational correlations between neighboring segments of different chains. The terminal portions of some of the molecules in the succeeding layer are also located in the first layer, thus increasing the fraction of methyl chain ends in contact with the walls. These findings match very closely the experimental results of surface forces and viscosities of liquid n-alkanes confined between two mica plates, measured as a function of the plate separation.
The structure of NH3⋅H2S and free internal rotation effects
Authors: Herbine Peter , Hu T. A. , Johnson G. , Dyke T. R.
Published: July 2002

The structure of NH3⋅H2S has been determined from microwave and radiofrequency spectroscopy of this complex and its deuterated isotopomers, using molecular beam electric resonance techniques. Rotational constants, electric dipole moments and nitrogen quadrupole coupling constants were obtained from the spectra. The molecule was found to have a linear, hydrogen bonded structure with the ammonia as the proton acceptor. The N⋅⋅⋅S distance is 3.639 Å, the root-mean-square angular deviation of the NH3 axis from the N–S axis is 24.6° and the H2S C2 axis is 40.5° from the N–S axis. Although the molecule is an asymmetric rotor, first-order Stark effects were observed for K=1 rotational levels. These Stark effects are caused by torsional oscillations which are essentially ammonia monomer free internal rotation. Similar effects were observed for NH3⋅H2O and are reported here.
Thermal dissociation of diatomics in inert gases: A Nosé equation approach
Authors: Hamilton I. P. , Liu L.
Published: July 2002

The thermal dissociation of diatomics in inert gases has been the subject of numerous experimental and theoretical studies. There is excellent agreement between the measured and calculated bimolecular rate constants for H2, and this has become a test case for master-equation and other approaches. In this paper we consider a Nosé equation approach, which is appropriate if the inert gas simply acts as a heat bath, as may be the case in the limit of infinite dilution of H2. We examine the extent to which the Nosé equation can thermalize the H2 dynamics for the temperature range 4000–10 000 K. We show that we can calculate meaningful pseudo-unimolecular rate constants for the temperature range 7000–10 000 K and, for this temperature range, we obtain an activation energy of 3.7±0.5 eV, which is compatible with the experimental value.
On the structure of the alkaline earth dihalides dimers
Authors: Gigli G.
Published: July 2002

A simple polarizable ion model of the Rittner type has been applied to the magnesium, calcium, strontium and barium dihalide dimers. Various energetically stable isomers were found. Structures, binding energy, vibrational frequencies and dipole moments were evaluated. At the typical temperatures of interest for these high temperature gaseous molecules the planar D2h structure has been found to be either at an absolute minimum of energy or entropy stabilized for most of the sixteen species studied. The relaxing, which could occur in inert gas matrix spectroscopy or in cluster sources, of the high temperature configurations to lower energy structures, mainly of C3v or Cs symmetry, is qualitatively discussed.