bluuebunny/crossref_metadata_2025 huggingface dataset:
Short time behavior of the dipole autocorrelation function and molecular gases absorption spectrum
Authors: Boulet C. , Robert D.
Published: February 2003

A quasistatic theory of the dipole autocorrelation function for isolated molecular spectral lines has been developed and applied to linear molecules perturbed by atoms and molecules. The matrix elements of the evolution operator associated with the molecular dipole moment are treated to all orders in the interaction without any cutoff procedure. An analytical expression of the intensity distribution in terms of the intermolecular potential has been obtained which is easily tractable for involved molecular systems. Numerical tests of this theory have been performed for rotation–vibration lines of HCl–Ar, HCl–Xe, and CO2–Ar gas mixtures by comparing the present results with that of a unified theory of the line shape for molecule–atom couples, valid from resonances to the far wings and including all orders in the interaction. These tests justify the present approach and show a considerable reduction of the computation time required (by more than two orders of magnitude), allowing now the study of the polyatomic preturbers case. Application to infrared absorption lines of HCl self-perturbed and perturbed by CO2 leads to a good agreement with the experimentally determined super-Lorentzian profile.
Deuterium quadrupole coupling in BD3CO
Authors: Murray Alice M. , Kukolich Stephen G.
Published: February 2003

The hyperfine structure of the J=1→0 rotational transitions of 10BH3CO, 11BH3CO, and 11BD3CO was observed with a molecular beam maser spectrometer. Rotational constants, quadrupole coupling, spin rotation, and spin-spin interaction constants were determined from the data analysis. The deuterium quadrupole coupling constant in 11BD3CO, was determined to be eQqaa(D)=−48.5±2.3 kHz and eQqzz(D)=116.9±5.4 kHz. The procedure for determining the matrix elements of the spin-spin interaction between the equivalent deuterium nuclei is presented.
Detection of the a 1Πg (v′=0, 1)←X 1Σ+g (v″=0) transition in N2 by laser-induced fluorescence
Authors: van Veen N. , Brewer P. , Das P. , Bersohn R.
Published: February 2003

The a1Πg(v′=0.1)←X1Σ+g(v″=0) transitions in N2, part of the Lyman–Birge–Hopfield bands, have been seen by two-photon absorption detected by VUV fluorescence. Fluorescence was observed over a pressure range from 5 m Torr to 10 Torr. From the lifetime decrease with pressure at low pressure, quenching rate constants were determined, which are equivalent to cross sections in the range of 2.6×10−16 cm2. At higher pressures a slow decay was observed that is interpreted as being due to the a′ state which acts as an energy reservoir with a pressure dependent decay constant. We anticipate that laser-induced fluorescence will be a useful scheme for measuring ground state rotational and vibrational populations for the nitrogen molecule.
Deuterium quadrupole coupling in methanol, salicyclic acid, catechol, resorcinol, and hydroquinone
Authors: Clymer J. W. , Ragle J. L.
Published: February 2003

Deuteron NQR spectra of several model systems involving alcoholic of phenolic-OD groups are discussed. The spectra of alpha hydroquinone and its two isomers resorcinol and catechol show complex structure due to the presence of inequivalent O–D⋅⋅⋅O hydrogen bonds. In the case of hydroquinone, this structure collapses to that characteristic of a single type of hydrogen bond in the beta- or clathrate-forming phase. An attempt is made to place the data in theoretical perspective by calculations of the deuterium field gradient in hydroxide ion, hydroxyl radical, methanol, and methanol dimer and by comparison with precise Hartree–Fock computations from the literature.
Isolated ultracold porphyrins in supersonic expansions. I. Free-base tetraphenylporphyrin and Zn-tetraphenylporphyrin
Authors: Even Uzi , Magen Jacob , Jortner Joshua , Friedman Joel , Levanon Haim
Published: February 2003

In this paper we report the results of an experimental study of the fluorescence excitation spectra and of the time-resolved emission of Zn-tetraphenylporphyrin (ZnTPP) and of free-base tetraphenylporphyrin (H2TPP) seeded in pulsed supersonic expansions of He. We have studied the S0→S1 transition (the Q band) and the S0→S2 transition (the Soret, B band) of ZnTPP, as well as the S0→S1x transition (the Qx band), the S0→S1y transition (the Qy band), and the S0→S2x transition (the Bx band) of H2TPP. Information was obtained on the electronic energy levels, the vibrational level structure, the details of low-frequency nuclear motion, and some characteristics of electronic relaxation.
Isolated ultracold porphyrins in supersonic expansions. II. Zn-tetrabenzoporphyrin
Authors: Even Uzi , Magen Jacob , Jortner Joshua , Friedman Joel
Published: February 2003

Laser-induced fluorescence excitation spectra of zinc tetrabenzoporphyrin in seeded, pulsed, supersonic expansions of He reveal well-resolved vibrational excitations for the S0→S1 and S0→S2 electronic transitions. Electronic relaxation in the S1 manifold was interrogated by time-resolved spectroscopy, exhibiting a practical independence of the lifetimes on the excess vibrational energies in the range Ev=0–10 000 cm−1. Spectroscopic line broadening data provided semiquantitative information on interstate electronic relaxation from the electronic origin of the S2 state, whose lifetime is τ(S2)≳4 ps.
Isolated ultracold porphyrins in supersonic expansions. III. Free-base porphine
Authors: Even Uzi , Jortner Joshua
Published: February 2003

The vibrational level structure of the S0→S1x transition (the Qx band) and of the S0→S1y transition (the Qy band) of free-base porphine in supersonic expansions of He was interrogated by laser-induced fluorescence excitation spectroscopy. Electronic relaxation in the S1x manifold was explored by time-resolved spectroscopy, revealing a constant value of the decay lifetime τ=9.5±1.0 ns for excess vibrational energies in the range Ev=0–5000 cm−1. The line broadening (FWHM) Δ=1.0–1.5 cm−1 of the electronic origin and of low-lying vibrational excitations in the S1x manifold originates from inhomogeneous unresolved rotational structure, while the large linewidth Δ=11.9 cm−1 of the electronic origin of the S1y state is due to homogeneous electronic relaxation broadening in the statistical limit. The line shape of the electronic origin of S1y was found to be Lorentzian, providing a quantitative determination of the lifetime τ=5×10−13 s for interstate S1y–S1x electronic relaxation within a bound level structure of a large isolated molecule.
Solution of vibrational excitation problems with constant step size Magnus propagators: Convergence, perturbation analysis, and approximate decoupling
Authors: Anderson Roger W.
Published: February 2003

Constant step size Magnus propagators are used to integrate the Secrest–Johnson vibrational excitation problem with parameters given by Stechel, Walker, and Light. Calculations are done for a six channel basis for E=6 and 8 and for a 30 channel basis for E=60. For all the calculations the error scales as the fourth power of the step size for small steps. This indicates a significant cancellation between the errors in the Magnus propagation and the assumed diagonalization of the potential over each interval. The error grows rapidly when the step size is such that at least one channel is propagated by near half a wavelength. The errors in the E=6 calculation are compared with those for the log derivative, renormalized Numerov, R matrix propagation methods, and a quadratic approximate potential method. The constant step size Magnus method is superior to the other methods for this vibrational excitation problem. A perturbation analysis is presented to show why accurate calculations are possible with large nonclassical steps, since we have used steps much larger than those usually recommended for use with the Magnus propagator. Finally, a perturbative calculation of the transform that diagonlizes the potential matrix is described. The perturbative transform is rapidly calculated, and gives excellent results for diagonalizations at large distances.
A search for mode-selective chemistry: The unimolecular dissociation of t-butyl hydroperoxide induced by vibrational overtone excitation
Authors: Chandler David W. , Farneth William E. , Zare Richard N.
Published: February 2003

The use of optoacoustic spectroscopy permits both the monitoring of the overtone excitation of t-butylhydroperoxide (t-BuOOH) and the in situ detection of the resulting reaction product t-butanol (t-BuOH). The sample is contained in a reaction cell, equipped with a microphone, in which all surfaces have been specially passivated. The cell is placed inside the cavity of a dye laser tuned to excite the 5–0 O–H stretch of the t-BuOOH at 619.0 nm. The dissociation process yields directly ⋅OH and t-BuO⋅, and the latter readily abstracts a hydrogen atom from a parent molecule to form t-butanol (t-BuOH). The appearance rate of t-BuOH is obtained by ratioing the area under the 5–0 O–H stretch of t-BuOH to that of a combination band of t-BuOOH. At low pressures, below 40 Torr, a plot of the reciprocal of the t-BuOH appearance rate versus total pressure shows near linear behavior. This linearlity can be well described by a statistical model (RRKM) when careful averaging of the dissociation rate over the thermal energy distribution of the photoactivated molecules is included. At pressures above 40 Torr, a marked deviation from linearity appears. This deviation is fit to a kinetic model in which the dissociation rate of an energy nonrandomized molecule competes with the rate of intramolecular energy relaxation. This places a lower bound of ≥5.0×1011 s−1 on the rate of energy randomization. A discussion of this model in the context of other possible kinetic schemes as well as other photoactivated and chemically activated systems is presented.
Photoelectron studies of resonant multiphoton ionization of CO via the A 1Π state
Authors: Pratt S. T. , Poliakoff E. D. , Dehmer P. M. , Dehmer J. L.
Published: February 2003

We report measurements of multiphoton ionization of CO involving a three photon resonance to the v=1–3 levels of the CO A 1Π state followed by the absorption of either two (v=3) or three (v=1,2) additional photons to reach the ionization continua. In one measurement, the CO+ ion intensity is measured as a function of wavelength, yielding the rotational structure of the intermediate resonant state. In a second measurement, the laser frequency is set at the v=1–3 bandheads and the kinetic energy spectra of the ejected electrons are measured. The observed vibrational branching ratios in these photoelectron spectra do not follow the pattern predicted by the Franck–Condon overlap between the intermediate A 1Π state and the ionization continua. Several possible causes for this (not unexpected) deviation from Franck–Condon behavior are discussed.
Rotational modulation of fluorescence decay and quantum beats in expansion-cooled azabenzenes: Small molecule behavior of rotationally cold s-triazine in the lower vibronic levels of the Ã 1E″ state
Authors: Saigusa H. , Lim E. C.
Published: February 2003

The measurements of the fluorescence decay and quantum beats as a function of photoselected rovibronic levels in the 1E″(nπ*) state of jet-cooled s-triazine indicate that the intramolecular dynamics of the molecules excited to low rotational states of the 610 level, and still lower rotational states of the 620 level, fall into the small molecule limit. For the 620 (E″) level, changes from the small molecule decay behavior to the intermediate case decay behavior can be induced by rotational excitation of the molecule. Comparison of the fluorescence decay characteristics of s-triazine with those of pyrimidine and pyrazine indicates that s-triazine has much stronger singlet–triplet coupling and much smaller triplet level density as compared to the other azabenzenes. The strong singlet–triplet coupling in s-triazine can be attributed to the efficient spin-orbit coupling between the 1E″(nπ*) and 3A′(ππ*) states, while the small density of triplet levels is consistent with its small singlet–triplet energy gap, smaller vibrational degrees of freedom and higher symmetry of the molecule, relative to those of the other azabenzenes. The theory of intermediate case radiationless decay provides satisfactory qualitative accounts of all the experimental results.
Photoelectron spectroscopy of carbon dioxide
Authors: Kumar Vijay , Krishnakumar E.
Published: February 2003

Photoelectron spectra of carbon dioxide have been studied at photon wavelengths of 736, 744, 876, and 584 Å, respectively. Many previously unknown vibrational levels of the X 2Πg state of the molecular ion have been observed. Some new levels corresponding to (ν1, 0, 0) with ν1=0 to 10 and (0, 0, ν3) with ν3=2, 4, 6, 8 have been measured along with some levels of the (ν1, 0, ν3) series. Level (0, ν2, 0) with ν2=1, has been observed in the photoelectron spectrum obtained at 876 Å. The values of 1309±40, 605±60, and 1476±40 cm−1 have been assigned to ν1, ν2, and ν3, respectively. The effect of autoionization on the vibrational intensities of the (ν1, 0, 0) states with ν1=0, 1, 2 has also been studied at 736 Å and the vibrational populations have been compared with those at 744 and 584 Å, respectively.
The conformation of hydrocarbon chains in disordered layer systems
Authors: Vacatello Michele , Busico Vincenzo , Corradini Paolo
Published: February 2003

A Monte Carlo computer model of a monolayer of n-hexadecane molecules has been built at a surface density corresponding to 0.27 nm2 per molecule. Properties calculated from the model are in good agreement with available data on bilayer systems at a comparable surface density. The average conformation of the alkyl chains is examined in some detail and compared with recently proposed models of the chain conformation in the ‘‘fluid’’ phase of lipid bilayers.
Stirling numbers and hard-core particles
Authors: Datoussaīd H. , DeConinck J. , de Gottal Ph.
Published: February 2003

A system of N non−interacting hard−core particles with volume v0 in the (T,p,N)-ensemble is considered. Laplace transformation of the canonical partition junction is performed and an equation of state is derived based on properties of stirling numbers. (AIP)
Coincidence detection of dissociative fragments from CH3Cl
Authors: Monce M. N. , Edwards A. K. , Wood R. M. , Steuer M. F. , Shah A. V.
Published: February 2003

In this letter, we describe the results of the detection, in coincidence, of CHn+ and Cl+ ions which result from the dissociation of CH3Cl after bombardment by 1 MeV He+ projectiles. The results confirm the existence of four dissociation channels, each with a kinetic energy release 0f 6.6 eV. The results suggest the channels could be due to a process in which 0, 1, 2,, or 3 hydrogens are dissociated from the methyl chloride and the remaining CH+n and Cl+ dissociate by a breaking of the carbon–chlorine bond. (AIP)
Relaxation in liquid chloroform
Authors: Cardamone M. J.
Published: February 2003

Spontaneous Brillouin scattering has been used to determine the character of the vibrational relaxation present in liquid chloroform at 20 °C. The results indicated that the observed relaxation is explainable on the basis of a single relaxation theory. (AIP)
Reply to ‘Comment on hole burning into velocity distribution of thermal electrons due to resonance attachment to O2’
Authors: Koura Katsuhisa
Published: February 2003

The comments of McMahon and Crompton1 on a previous paper 2by the author regarding the attachment of thermal electrons to oxygen are answered. (AIP)
A bound-state perturbation expansion with superior convergence properties
Authors: Allen Clinton W.
Published: February 2003

A perturbation method is presented which, while using the same Born elements as the Rayleigh–Schrödinger (RS) theory, does not use an expansion in powers of the perturbation parameter. Its use will give a convergent sequence of approximations in problems where the RS series may be asymptotic or divergent. By using RS basis set the physical interpretation of the RS theory is retained, unlike some alternate methods. Computationally, in the first few orders, it is only slightly more difficult than the RS method. Some simple examples are presented which illustrates the convergence properties in relation to the RS series.
Cancellation of low order electric moments of the unit cell: An efficient way of calculating crystal field effects in Hartree–Fock computations on periodic systems
Authors: Piela L. , Brédas J. L. , André J. M.
Published: February 2003

In the framework of a real quantum mechanical problem, we investigate the modified unit cell concept in order to calculate the crystal field effects in periodic systems. We make use of a recently proposed procedure based on the cancellation of the unit cell low order electric moments. Applied to a one-dimensional periodic chain of lithium hydride molecules, this procedure compares in an excellent way with the recently developed multipole expansion technique and offers a dramatic improvement of the results with respect to the usual cluster-type approaches.
Solution of the Schrödinger equation by a spectral method II: Vibrational energy levels of triatomic molecules
Authors: Feit M. D. , Fleck J. A.
Published: February 2003

The spectral method utilizes numerical solutions to the time-dependent Schrödinger equation to generate the energy eigenvalues and eigenfunctions of the time-independent Schrödinger equation. Accurate time-dependent wave functions ψ(r, t) are generated by the split operator FFT method, and the correlation function 〈ψ(r, 0) ‖ ψ(r, t)〉 is computed by numerical integration. Fourier analysis of this correlation function reveals a set of resonant peaks that correspond to the stationary states of the system. Analysis of the location of these peaks reveals the eigenvalues with high accuracy. Additional Fourier transforms of ψ(r, t) with respect to time generate the eigenfunctions. Previous applications of the method were to two-dimensional potentials. In this paper energy eigenvalues and wave functions obtained with the spectral method are presented for vibrational states of three-dimensional Born–Oppenheimer potentials applicable to SO2, O3, and H2O. The energy eigenvalues are compared with results obtained with the variational method. It is concluded that the spectral method is an accurate tool for treating a variety of practical three-dimensional potentials.
Theoretical study of NH2: Potential curves, transition moments, and photodissociation cross sections
Authors: Saxon Roberta P. , Lengsfield Byron H. , Liu B.
Published: February 2003

Photodissociation cross sections from the ground state of NH2 have been calculated using a pseudodiatomic model. The potential curves needed in these calculations, functions of one NH bond length, were obtained by ab initio MCSCF calculations on the five lowest doublet states of NH2 using a contracted Gaussian basis set of double zeta quality augmented by polarization and Rydberg functions. Transition dipole moments between the ground and excited states were evaluated using MCSCF wave functions.
Terminal vs bridge bonding of methylene to metal systems: Al2CH2 as a model system
Authors: Fox Douglas J. , Schaefer Henry F.
Published: February 2003

The metal dimer-methylene system M2CH2 is the simplest which can in principle display both terminal M–M–CH2 and bridging M CH2 M geometrical structures. Having previously studied the terminal Al–CH, Al–CH2, and Al–CH3 metal-organic fragment species, the Al2CH2 system was chosen to allow a competition between the terminal and bridged structures. Nonempirical molecular electronic structure theory was used, with double zeta (DZ) and DZ + polarization basis sets in conjunction with both self-consistent field (SCF) and configuration interaction (CI) methods. Among structures considered, the bridging arrangement, with the Al C Al and CH2 planes perpendicular to each other, lies lowest energetically. For this structure the Al–Al distance is 3.61 Å, the Al–C distance 2.00 Å, and the methylene bond angle 105.5°. The completely planar structure, found by twisting the methylene group by 90°, is predicted to lie 31 kcal higher, but has a much shorter Al–Al distance 3.03 Å. The terminal structure lies 46 kcal above the absolute minimum on the energy surface and has re(Al–Al)=2.87 Å, re(Al–C)=1.81 Å, and a methylene bond angle of 112.2°. All of the above structures are closed-shell singlets in their lowest electronic states, but the energies of several triplet species are also discussed. The Al–C bond energy for the terminal structure is 81 kcal, in good agreement with that predicted (77 kcal) earlier by comparable methods for AlCH2. However, for the bridging Al2CH2, the Al2 ⋅⋅⋅ CH2 dissociation energy is much larger, 127 kcal. Vibrational frequencies for the bridging and terminal Al2CH2 species are presented and discussed.
Configuration interaction studies on low-lying valence and Rydberg states of PO
Authors: Grein Friedrich , Kapur Anil
Published: February 2003

CI studies with a double-zeta plus polarization plus diffuse basis set were performed on the X 2Π, a4Π, B 2Σ+, B′ 2Π, b 4Σ−, D′ 2Π, A 2Σ+, and C′ 2Δ states of PO over a wide range of internuclear separations. Spectroscopic constants, as well as adiabatic excitation energies and dissociation energies were calculated, and are in good agreement with experimental values. Allowing for crossing of some 2Π and 2Σ+ states improves their calculated spectroscopic constants and identifies short sections of the D 2Π, F 2Σ+, and A′ 2Σ+ (repulsive) states. Particular emphasis was placed on the nature of the B 2Σ+ state, which is shown to require diffuse orbitals for a proper description, although it is not a Rydberg state. The valence-isoelectronic molecule NO does not have an equivalent state.
Guaranteed convergence in ground state multiconfigurational self-consistent field calculations
Authors: Jo/rgensen Poul , Swanstro/m Peter , Yeager Danny L.
Published: February 2003

We show how an optimization constraint algorithm of Fletcher that guarantees convergence to the lowest state of a given symmetry may be practically implemented in a multiconfigurational self-consistent field (MCSCF) calculation. Other MCSCF procedures in current use have not been proven mathematically to guarantee convergence. Calculations on the ground states of N2 and CO show that rapid and efficient convergence is obtained with the Fletcher restricted step size algorithm.
Simple mixing rule for mixtures with exp-6 interactions
Authors: Ree Francis H.
Published: February 2003

Monte Carlo simulations on mixture systems composed of hydrogen molecules and helium atoms have been performed to develop an improved one-fluid van der Waals model. This model is applicable to three-parameter potentials having an exponential-6 form. The proposed model has been shown to be as accurate as the conventional one-fluid van der Waals model, but it can be used over an extended density and temperature range, where the conventional model is physically less appropriate.
The application of the modified Gouy–Chapman theory to an electrical double layer containing asymmetric ions
Authors: Bhuiyan L. B. , Blum Lesser , Henderson Douglas
Published: February 2003

The modified Gouy–Chapman theory of the electrical double layer in which ion-core effects among the ions are neglected, but are taken into account in the ion–electrode interaction, is applied to the case of 1:1, 2:2, 1:2, and 2:1 electrolytes for both equal and unequal ionic diameters. At large charge densities on the electrode, the counterions dominate and the results approach those of a symmetric electrolyte. However, at small charge densities on the electrode, the effects of asymmetry are evident. In particular, for unequal ionic diameters, a nonzero potential can occur at zero charge on the electrode, even in the absence of specific adsorption.
Two-photon photoselection in rigid solutions: A study of the B2u←A1g transition in benzene
Authors: Scott Thomas W. , Haber K. S. , Albrecht A. C.
Published: February 2003

The first detailed study of simultaneous two-photon photoselection is reported, using the benzene B2u←A1g two-photon forbidden transition to illustrate this new technique. Polarized emission from the B2u fluorescent state is used to probe the symmetries of excited molecular states reached by two-photon absorption in an initially isotropic solution. Three independent polarization measurements are reported for each of the e1u, e2u, and b2u false origins. These transitions span all possible D6h molecular symmetry species which are two-photon allowed from the ground state in a single color experiment. It will be seen how the photoselection method complements the conventional circular/linear polarization measurements in fluid solutions by providing empirical symmetry assignments for the nontotally symmetric transitions. The partial randomization parameter ε, commonly used in one-photon photoselection studies to rationalize nonideal polarization values, is introduced into the two-photon theory. This considerably improves the agreement between experimental and theoretical two-photon polarization ratios.
The d→p spectra of matrix-isolated copper atoms: An MCD and synchrotron radiation absorption study
Authors: Hormes J. , Grinter R. , Breithaupt B. , Kolb D. M.
Published: February 2003

The electronic absorption spectra of copper atoms isolated in argon, krypton, and xenon matrices have been measured in the range 3.6–8.6 eV using a synchrotron radiation source. By correlating the results with gas-phase data, and with the help of magnetic circular dichroism measurements and calculations of energies and intensities, a detailed assignment of the 3d→4p region of the spectra has been proposed. A band at 6.357 eV in the xenon matrix is assigned to the Rydberg 4s→5p transition with a fair degree of confidence, and two high-energy bands in the krypton matrix are tentatively assigned to Rydberg 3d→5p transitions. The matrix shifts confirm the view that the principal influence of the matrix is upon the upper orbital involved in the transition. Attention is drawn to the fact that matrix perturbations may increase the number of observable spectral bands, either by lifting degeneracies or by mixing allowed character into previously forbidden transitions.
NO2 Ã 2B2 state properties from Zeeman quantum beats
Authors: Brucat P. J. , Zare R. N.
Published: February 2003

We have observed the temporal modulation of fluorescence induced by a static magnetic field after selective optical excitation of a single fine structure level via the Ã 2B2–X̃ 2A1 electronic transition in NO2. The rotationally resolved population and alignment decay rates, fluorescence polarization anisotropy, and magnitude and sign of the excited Landé g factor are determined simultaneously from measurement of the Zeeman quantum beats. The importance of the these properties to the description of the Ã state of NO2 is discussed.
The microwave spectrum of the CF radical
Authors: Saito Shuji , Endo Yasuki , Takami Michio , Hirota Eizi
Published: February 2003

Rotational transitions of the CF radical in both the 2Π1/2 and 2Π3/2 ground electronic states were observed in the region up to 300 GHz by using a source-frequency modulation microwave spectrometer. The CF radical was generated directly in a free-space absorption cell by a dc discharge in carbon tetrafluoride containing a small amount of methyl fluoride. The rotational constant, the centrifugal distortion constant, and the Λ-type doubling constants, as well as all four hyperfine coupling constants a, b, c, and d of the fluorine nucleus, were determined with good precision. From the observed hyperfine coupling constants the unpaired-electron spin and spatial (orbital) distributions 〈1/r3〉s, 〈(3 cos2 χ−1)/r3〉s, 〈sin2 χ/r3〉s, and 〈1/r3〉0 were calculated and discussed in terms of a simple molecular-orbital picture.
Rotational R branch Raman spectroscopy in CO2
Authors: Godfried H. P. , Silvera Isaac F.
Published: February 2003

Intensity measurements of the low energy rotational Raman spectrum of CO2 are reported. In these measurements the rotational R and S branch transitions in the excited state bending mode have been resolved for the first time through the use of a supersonic expansion. Their intensities are compared to the predictions of the Placzek–Teller polarizability theory for the Raman effect. Excellent agreement is obtained between theory and experiment if account is taken of the presence of isotopic impurities in the CO2 sample.
Simultaneous observation of the vibrational and electronic-vibrational resonance Raman spectra of the hexachloroiridate(IV) ion at 15 K
Authors: Hamaguchi Hiro-o , Tasumi Mitsuo
Published: February 2003

The spin-orbit split Eg″ and Ug′ states of 2T2g of d5 octahedral IrCl62− at 15 K have been studied by resonance Raman scattering. The electronic Raman spectrum with Ug′←Eg″ electronic excitation was observed at about 5200 cm−1 with extensive well-resolved vibrational structures (hence called electronic-vibrational spectrum). It is found that the Ug′ state is split by 89 cm−1 due to a lower symmetry of the Ir4+ site. The vibrational frequencies in the Ug′ were found to be very close to those in the Eg″ ground state. No evidence has been found for the Jahn–Teller effect in Ug′ state. By a qualitative analysis of the resonance Raman excitation profiles of the vibrational and electronic-vibrational spectra, it is suggested that an optically forbidden Eu′ state is located at around 470 nm and that this state is coupled to a nearby Uu′ through the ν5 bending mode. The usefulness of the simultaneous observation of the vibrational and electronic-vibrational Raman spectra in studying excited electronic states is demonstrated.
ESR of Mn2 and Mn5 molecules in rare-gas matricesa)
Authors: Baumann C. A. , Van Zee R. J. , Bhat S. V. , Weltner W.
Published: February 2003

Mn2 and Mn5 have been isolated in argon, krypton, and xenon matrices and their X-band ESR spectra observed at 4 and up to 70 K, depending upon the matrix. As predicted by Nesbet, the lowest state of Mn2 is 1Σ, and the two atoms are exchange coupled (antiferromagnetically). The temperature behavior of the ESR bands in the higher spin states (S=1,2,3) was approximately in accord with a Lande interval rule, and a value of J=−9±3 cm−1 was obtained from the S=2 intensity variations. Each fine structure line appears with a superimposed 11-line hyperfine pattern with splitting one-half that of isolated 55Mn atoms (30 G). The anisotropic exchange interactions fit the Judd–Owen relationship with De=−0.043(2) and Dc=−0.001(4) cm−1. Assuming De arises solely from magnetic dipole interaction, the interatomic distance in Mn2 is calculated to be 3.4 Å. Mn5 appears in more concentrated matrices as a highly oriented axial molecule with its axis perpendicular to the flat sapphire substrate surface. The observed fine structure indicates that the molecule contains 25 unpaired electrons (S=25/2) and has magnetic parameters g⊥=1.997(1), g∥=1.979(2), D=−0.013(1) cm−1. All 55Mn hyperfine structure lies within the linewidths and is therefore unresolved. It was concluded that the molecule most probably contains five Mn atoms and is a plane pentagon. With this model an attempt was made to rationalize the observed g shifts.
Stieltjes-imaging calculations of photodissociation
Authors: Asaro C. , Dalgarno A.
Published: February 2003

The applicability of Stieltjes-imaging techniques to calculations of molecular photodissociation cross sections is explored. Detailed comparisons for the diatomic molecules LiH and H2 are made which suggest that reasonable accuracy can be obtained with moderate basis sets. The methods are used to predict the photodissociation cross sections for absorption from low-lying vibrational levels of H3+.
Off-energy-shell effects in energy sudden factorization relations: Computational study of He+H2 rotational–vibrational scattering
Authors: Beard L. H. , Kouri D. J.
Published: February 2003

We apply an approximate off-shell version of the ES scaling developed for molecule–surface scattering by Gerber, Beard, and Kouri to the He+H2 CC cross section results of Flower and Kirkpatrick. The predicted results are in much better agreement with CC values than are those obtained using earlier on-shell versions of the ES scaling.
Effective exchange potentials for electronically inelastic scattering
Authors: Schwenke David W. , Staszewska Grazyna , Truhlar Donald G.
Published: February 2003

We propose new methods for solving the electron scattering close coupling equations employing equivalent local exchange potentials in place of the continuum-multiconfiguration-Hartree–Fock-type exchange kernels. The local exchange potentials are Hermitian. They have the correct symmetry for any symmetries of excited electronic states included in the close coupling expansion, and they have the same limit at very high energy as previously employed exchange potentials. Comparison of numerical calculations employing the new exchange potentials with the results obtained with the standard nonlocal exchange kernels shows that the new exchange potentials are more accurate than the local exchange approximations previously available for electronically inelastic scattering. We anticipate that the new approximations will be most useful for intermediate-energy electronically inelastic electron–molecule scattering.
Three-dimensional vibrational predissociation of the van der Waals complex He⋅⋅⋅I2(B). A quasiclassical study
Authors: Delgado-Barrio G. , Villarreal P. , Mareca P. , Albelda G.
Published: February 2003

We apply a quasiclassical trajectory method to treat vibrational predissociation of the van der Waals molecule He⋅⋅⋅I2(B 3Π). In order to compare with quantum mechanical calculations and the experimental data, we have introduced the rotational degree of freedom by using an approximate treatment. The potential energy surface used was a sum of pairwise atom–atom potentials. The initial conditions were selected at random taking into account previous quantal results. The final rotational distribution for the I2 fragment and the total rate for vibrational predissociation, as functions of vibrational excitation, are in good agreement with the quantum mechanical values and with the experimental measurements.
On the extension of the Kramers theory of chemical relaxation to the case of nonwhite noise
Authors: Marchesoni Fabio , Grigolini Paolo
Published: February 2003

The first step of our approach consists of relating the generalized Brownian motion in a double-well potential to a suitable time-independent Fokker–Planck operator implying that an arbitrary large number of ‘‘virtual’’ variables be used. Then, to simplify the solution of this multidimensional Fokker–Planck equation, we develop a procedure of adiabatic elimination of the fastly relaxing variables. As a significant feature of this reduction scheme, we point out that no limitation on the number of the virtual variables is implied. The explicit form of the first correction term to the Smoluchowski equation is also shown to depend on whether or not the stochastic force is white. Via a comparison with the analytical results of Grote and Hynes’ theory [J. Chem. Phys. 73, 2715 (1980)] it is argued that the ‘‘exact’’ approach and the ‘‘reduction’’ procedure can be regarded as being complementary to one another.
Brillouin scattering in polyphenyls. II. Through the incommensurate phases of biphenyl
Authors: Ecolivet C. , Sanquer M. , Pellegrin J. , DeWitte J.
Published: February 2003

Brillouin scattering experiments from room temperature down to liquid helium temperature are reported. Two phase transitions occur in this temperature range at TI=40 K and TII=17 K, but elastic anomalies only happen at TI. Several attempts for the direct phason detection have been found unsuccessful. The elastic anomalies are qualitatively interpreted inside the frame of the Landau theory with simple group theory arguments.
Study of anisotropic diffusion of oriented molecules by multiple quantum spin echoes
Authors: Zax D. , Pines A.
Published: February 2003

Experiments have been completed which demonstrate the greatly enhanced sensitivity of the multiple quantum version of the NMR spin echo diffusion experiment. These experiments are particularly well-suited to studies of anisotropic diffusion where the presence of dipolar couplings may make the standard experiment technically infeasible. (AIP)
Angular distributions from photodissociations. V. The bent triatomic molecule
Authors: Morse M. D. , Band Yehuda B. , Freed Karl F.
Published: February 2003

The generalized Franck–Condon theory of bent triatomic molecule dissociations is extended to permit the calculation of state-to-state photofragment angular distributions. The axial recoil limiting forms of Zare and Bersohn are obtained for the state-averaged angular anisotropies β̄ with corrections for the effects of bending vibrations. For certain orientations of the transition dipole within the molecule, βfi is found to be nearly independent of fragment rotational angular momentum j, while for other orientations, it is found to be a strongly oscillatory function of j. In some cases, calculated photofragment rotational distributions at fixed detection angles are strong functions of the detection angle, suggesting the possibility of partial state selection by spatial resolution. Approximate analytical expressions for βfi are presented for initially bound molecular states that are either nonrotating or are well described by a prolate symmetric top wave function with k=0. The distributions are linear combinations of parallel and perpendicular-type distributions. Numerical applications are provided for direct photodissociations from the bent X̃ HOC1 and C̃ HCN states.
Correlation effects in momentum space for the electronic shells in Be
Authors: Mobbs R. J. , Banyard K. E.
Published: February 2003

Our previous partitioning technique for analyzing correlation effects within the individual electronic shells of Be in position space has been applied here to a corresponding examination in momentum space. Comparability with the results in position space was ensured by considering the same configuration-interaction (CI) wave functions and Hartree–Fock (HF) description as before; the functions were transformed into momentum space by applying the standard Dirac procedure. By analogy with position space, Coloumb shifts Δf(p12) vs p12 were derived for the K(1S), L(1S), KL(1S), and KL(3S) shells in Be. Selected one- and two-particle momentum expectation values are also reported along with various radial and angular correlation coefficients. The opposing effects of radial and angular correlation in momentum space for Be gave a K-shell Coulomb shift which, being small, suggested a rough balance between these correlation components. For the L shell, however, the large excess of angular correlation produced a ‘‘shift’’ which, relative to the K shell, was of considerable magnitude. Thus, when forming the sum total Coulomb shift for the whole atom, the L-shell component virtually masked all contributions arising from the other shells. This is in direct contrast with position space where the intershell effects in Be produced some detailed structure in the total Coulomb hole Δf(r12) vs r12. The characteristics of the momentum curves for the various shells are rationalized and their relationship with the corresponding results in position space is discussed.
The quantum mechanical linearly damped forced oscillator
Authors: Landovitz L. F. , Levine A. M. , Ozizmir E. , Schreiber W. M.
Published: February 2003

The solution is obtained to the quantum mechanical linearly damped harmonic oscillator subjected to a time dependent driving force. This solution allows the calculation of interesting quantum quantities such as the Green’s function and the transition amplitudes between harmonic oscillator states. The specific results for a harmonic driving force are given.
Nonlocal polarizability densities and van der Waals interactions
Authors: Hunt K. L. C.
Published: February 2003

The van der Waals interaction energy of a molecular pair is derived within a simple physical model that represents the distribution of polarizable matter in each molecule by the use of nonlocal polarizability densities. In the model the field due to the instantaneous polarization of one molecule polarizes the second molecule nonlocally, thereby producing a reaction field that acts on the first molecule. The resulting energy change depends upon the averaged product of the fluctuating polarization at different points in the first molecule; by the fluctuation–dissipation theorem, this product is related to the imaginary part of the nonlocal polarizability density. The model gives the van der Waals energy as an integral of a contracted tensor product of two dipole propagators and the imaginary-frequency nonlocal polarizabilities of the interacting molecules. The validity of the model is established by comparison with results from second-order perturbation theory.
The X 1Σ+g state vibration-rotational energies of the H2, HD, and D2 molecules
Authors: Wolniewicz L.
Published: February 2003

The energies of all the bound vibrational states of H2, HD, and D2 with rotational quantum numbers up to J=5 are calculated. Nonadiabatic, relativistic, and radiative corrections are taken into account with an accuracy of the order of 0.01 cm−1. The resulting theoretical ground state dissociation energies are 36 118.01, 36 405.73, and 36 748.32 cm−1 for H2, HD, and D2, respectively.
Classification schemes of nuclear geometries and the concept of chemical structure. Metric spaces of chemical structure sets over potential energy hypersurfaces
Authors: Mezey Paul G.
Published: February 2003

A general concept of chemical structure based on topologies of the nuclear configuration space is proposed. A metric, describing chemically significant structural differences, is introduced and two recently proposed definitions of molecular structure are compared within the general topological framework.
Theoretical investigation of the a 3Σ+u, A 1Σ+u, c 3Σ+g, and C 1Σ+g potential energy curves of He2 and of He*(2 1S, 2 3S)+He scattering
Authors: Sunil K. K. , Lin J. , Siddiqui H. , Siska P. E. , Jordan K. D. , Shepard Ron
Published: February 2003

Potential energy curves for the a 3Σ+u, A 1Σ+u, c 3Σ+g, and C 1Σ+g states of He2 are calculated using the MCSCF method. Comparison of the calculated dissociation energies and spectroscopic constants with the experimental values indicates that these are the most accurate theoretical curves reported to date for these states. The resulting curves are utilized to calculate the vibrational and rotational energy levels and to carry out He*(2 1S)+He(1 1S) and He*(2 3S)+He(1 1S) scattering calculations.
Mean field approach to ferroelastic transitions in molecular crystals
Authors: Raich J. C. , Yasuda H. , Bernstein E. R.
Published: February 2003

A mean field approach to the dynamics of structural phase transitions in molecular crystals is presented. The approach is based on a description of the rotational and translational molecular motions, and the coupling between them, in terms of generalized susceptibilities. Two models for the orientational susceptibility are used. One is a classical description in terms of two-dimensional rotors, the other a two-dimensional anharmonic oscillator model. The specific example considered is sym-triazine. In this crystal molecules experience a very strong orienting field which restricts the molecular rotational motion to libration. The coupling between the molecular rotations and translations is shown to lead to a softening of acoustic phonons. This softening has considerable anisotropy in reciprocal space. An approximate solution for the high temperature phase is shown to be in good agreement with experiments.
Thermal expansion of cubic ammonium compounds
Authors: Smith David
Published: February 2003

The contribution to the thermal expansion due to the hindered rotation of the ammonium ion has been computed for the disordered bcc phases of NH4Cl and NH4Br and for the ammonium pervoskites NH4ZnF3 and NH4CoF3. The thermal expansion due to the motion of the ammonium ion is computed from the hindered rotational energy levels. It is assumed that the potential function V(ω)∝r−n. The value of ‘‘n’’ gives an indication of the type of bonding, whether due to electrostatic or valence forces, that are present in the solid. The thermal expansions computed for the disordered bcc phases of NH4Cl and NH4Br, for NH4CoF3, and for NH4ZnF3 are in good agreement with the observed results, when electrostatic forces are assumed to predominate in these solids. The contribution of the thermal expansion from the hindered rotation of the ammonium ion amounts to about 10% of the observed thermal expansion for the ammonium pervoskites and about 25% for the disordered bcc phases of the ammonium halides. These percentages are nearly independent of temperature.
Integral equation theory for the equilibrium properties of the semiclassical fluid with hard-core potential
Authors: Sainger Y. S. , Sinha S. K. , Singh Y.
Published: February 2003

Quantum corrections to the radial distribution function (RDF) and thermodynamic properties for semiclassical fluids whose molecules interact via pair potentials which are nonanalytic functions of the intermolecular separation are calculated by means of statistical mechanical perturbation theory using the extended Percus–Yevick (PY) and hypernetted-chain (HNC) theories. Expressions are obtained in terms of effective two-body potential which includes quantum mechanical corrections. One attractive feature of this method is that the higher order quantum corrections, in principle, are as easily calculated as the first order term. Numerical results are given for the first- and second-order corrections to the free energy and to the pressure, and for the first-order correction to the RDF. Significant features are the large increase in quantum corrections with increasing density, and the qualitative difference that quantum mechanics makes to the RDF near the hard core.
Microscopic foundation for the hydrodynamic description of flow near a macroparticle
Authors: Sung Wokyung , Dahler John S.
Published: February 2003

The two-fluid mean-field theory of the preceding paper is applied to the special case of a macroscopic test particle immersed in a quasicontinuous background fluid. Here, as in the preceding investigation, the particle interactions are taken to be pair-additive sums of long-ranged continuous and short-ranged, rigid-core contributions. The kinetic equation for the conditional singlet distribution function of untagged solvent particles is recast in terms of a related smoothly varying function which satisfies a boundary condition at the interface where this fluid comes into contact with the test particle. This formulation of the theory is then used to examine the case when the test particle is much larger and more massive than a solvent particle and when the free path length of the background medium is significantly smaller than the test particle diameter. In this limit the moments of the background fluid kinetic equation reduce to the linearized Navier–Stokes equations for pure solvent and the velocity moment of the boundary condition becomes identical with the macroscopic slip-flow condition of hydrodynamics. From these, one can obtain the usual Stokes’ formula for the frictional force on the test particle. The theory also can be described by a kinetic equation for the background fluid which incorporates singular source terms arising from the rigid-core interactions between the solvent and the test particle. In the case of a large, very massive test particle the moments of this equation are found to be generalizations to high density of the ‘‘source hydrodynamic equations’’ of Cukier, Kapral, Lebenhaft, and Mehaffey.