bluuebunny/crossref_metadata_2025 huggingface dataset:
Triplet distribution functions for hard spheres and hard disksa)
Authors: Ueharab) Yosei , Lee Y.-T. , Ree Taikyue , Ree Francis H.
Published: February 2003

The hard-sphere and hard-disk triplet correlation functions are computed from the Born–Green–Yvon 2 (BGY2) theory at several different densities for triplet geometries in which two particles are held fixed at contact. The same quantities are generated for hard spheres by means of a Monte Carlo method. Both the BGY2 and the Monte Carlo data agree satisfactorily. Next, the Kirkwood superposition approximation and the ’’one-dimensional’’ approximation introduced earlier are examined to determine the regions of their applicability.
Theoretical determination of molecular structure and conformation. IV. Electronic effects influencing the stability of methyl substituted primary ozonidesa),b)
Authors: Cremer Dieter
Published: February 2003

The conformational surfaces of three methyl substituted primary ozonides (PO), namely, 4-methyl-1,2,3-trioxolane (propene PO), cis-4,5-dimethyl-1,2,3-trioxolane (cis-2-butene PO) and the corresponding trans PO (trans-2-butene PO), are studied with restricted Hartree–Fock (RHF) theory employing a split valence [3s2p/2s] and an augmented split valence [3s2p1d/2s] basis set. An analysis of the computed RHF energies reveals that only with polarization functions in the basis set is a reliable theoretical description of the three PO’s guaranteed. In the case of propene PO and trans-2-butene PO, reasonable results are obtained by means of a rigid pseudorotor model based on the theoretically determined structures of various 1,2,3-trioxolane conformations. However, a flexible pseudorotor model is necessary in the case of cis-2-butene PO in order to achieve a realistic account of substituent–substituent interactions. The conformational surfaces of all three PO’s closely resemble that of the parent PO, 1,2,3,-trioxolane. Thus, the preferential interconversional process of a methyl substituted PO is pseudorotation hindered by barriers between 2.3 and 3.5 kcal/mole. In marked contrast to previous semiempirical results, the oxygen envelope (E) conformations are calculated to be the most stable puckered PO forms. This finding is most important with regard to the stereochemical aspects of the ozonolysis mechanism. The methyl groups either stabilize or destabilize the puckered PO ring depending on the position of the substituent. With the aid of the pseudorotation phase angle and a well-defined substituent orientation angle, reliable predictions with regard to the electronic effect of a methyl group can be made. It is shown that only an equatorially placed methyl group leads to a favorable orbital mixing between the π-type MO’s of the ring and the substituent and a resultant lowering of the two highest occupied MO’s of the PO. The puckered ring is stabilized by back-donation of electron charge to the substituent which is not possible if this is axially positioned.
Resonant Raman scattering by adsorbed molecules
Authors: Efrima Shlomo , Metiua) Horia
Published: February 2003

We extend the classical model used in our previous work, to discuss the resonant Raman scattering by a molecule located near a metallic surface. We use the Drude model, modified to incorporate the electrodynamic effects of the surface. The frequency dependence of the polarizability is substantially modified and this lowers the frequency at which the resonant Raman effect appears. We derive the intensity of the fundamental and that of the overtones, for resonant scattering, and exemplify numerically their behavior.
Photodissociation and photodetachment of molecular negative ions. VI. Ions in O2/CH4/H2O mixtures from 3500 to 8600 Å
Authors: Lee L. C. , Smith G. P.
Published: February 2003

The photodestruction cross sections for O−, O2−, O2−⋅H2O, O3−, O3−⋅H2O, O4−, OH−, and OD− have been measured in the 3500–5400 Å and 6300–8600 Å wavelength regions. The ions were produced in a drift tube mass spectrometer and interacted with a dye laser or ion laser inside the laser cavity. The photodetachment cross sections for O− and OH−(OD−) have sharp onsets at wavelengths near 8480 and 6795 Å, respectively, and at shorter wavelengths their values are nearly constant. The photodestruction cross sections for O2−, O4−, and O2−⋅H2O increase monotonically with increasing photon energy. In contrast, in the photodestruction cross section for O3−, structure is observed over this wavelength region. The processes for creation and photodestruction of the various negative ions are discussed. Comparison is made with other measurements.
Numeric MCSCF methods for molecules. I. Theory of unicenter expansions
Authors: Tobin Frank , Hinze Juergen
Published: February 2003

A method is presented for developing MCSCF numeric methods for molecules. By the use of a single center expansion technique and the use of symmetry, powerful formalisms suited to particular problems can be developed. The explicit cases of azimuthally symmetric atoms and linear polyatomic molecules are fully developed.
Infrared spectra of liquid and solid DT and T2 a)
Authors: Souers P. C. , Fearon D. , Garza R. , Kelly E. M. , Roberts P. E. , Sanborn R. H. , Tsugawa R. T. , Huntb) J. L. , Pollb) J. D.
Published: February 2003

The fundamental vibration–rotation spectra of condensed DT and T2 were measured. The 24 separate spectral lines identified consist mainly of QQ, QR, QQ+So(0), and QQ+So(1) transitions. The last two appear as triad sets in D2–DT–T2 solutions because of double transitions between different isotopes. The QQ frequencies are 2458 cm−1 for T2 and 2736 cm−1 for DT. Some approximate kinetic half-times have been measured. In the 21 K liquid, these include: 18 h for the reaction of D2+T2→2DT and 25 h for the T2 rotational J=1 to J=0 conversion. In solid T2 at 20 K, the J=1 to J=0 conversion time is also about one day, but it is only 1 h at 4 K. Catalysis by atomic tritium is considered to be the cause of the fast 4 K-conversion time.
Criterion of minimum state density in the transition state theory of bimolecular reactions
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

We discuss two minimum-density-of-states criteria for the location of generalized transition states for chemical reactions. One is due to Bunker and Pattengill; the other is due to Wong and Marcus. We prove that both provide upper bounds on the exact classical equilibrium rate constant. In addition, we show that for several-dimensional systems both methods are exact at threshold, and in the limit of an infinite number of dimensions they agree with the variational theory of reactions of Wigner, Horiuti, and Keck. However, it is also shown that for a finite number of degrees of freedom both methods yield rate constants which are only as accurate as or less accurate than rate constants given by the variational theory of reactions. We note that, where tested by others for actual systems, the differences of the results obtained with the variational and Bunker–Pattengill criteria have been minor.
Organic metals. Molecular coordination along the stacking direction of a conducting organic compound: TTF–TCNQ (tetrathiafulvalene–tetracyanoquinodimethane)
Authors: Silverman B. D.
Published: February 2003

CNDO/2 molecular orbital calculations have been performed on ionized TCNQ and TTF dimers for different values of relative displacement of the monomers. The intermolecular separation is held fixed. The total energy of the TCNQ dimer goes through two minima, one when the monomers are exactly over each other (eclipsed geometry) and one for which the monomers are slipped with respect to each other (slipped geometry) as observed in TTF–TCNQ. The total energy of the TTF dimer exhibits a minimum for the eclipsed geometry only. The qualitative agreement with previous extended Huckel results highlights the close screening of the core–core interaction by the electrons. The implications of the different geometries with respect to electronic banding of the affinity and ionization levels in segregated stack structures is discussed.
Stochastic theory for molecular collisions: Application to the CO–He system
Authors: Ramaswamy Ramakrishna , Augustin Stuart D. , Rabitz Herschel
Published: February 2003

Vibration–rotation inelasticity in the CO–He collision system is studied within the stochastic formulation. Cross sections are obtained for purely rotational transitions using a modified electron gas potential. Vibration–rotation cross sections have been calculated in the energy range 2200 cm−1<E<4000 cm−1. At the higher energy, a total of 76 molecular states are energetically accessible. A comparison with earlier results is made, and coarse graining techniques for the treatment of large problems are utilized.
The absorbing boundary method for the calculation of quantum state decay rates. I. Numerical implementation and verification
Authors: Nordholm Sture , Bacskay George
Published: February 2003

The aim of this work is to provide a new conceptually and practically useful method of obtaining the time development and particularly the decay of quantum states initially localized on a domain with dynamically open boundaries. A projection operator derivation and a Markoffian approximation is used to obtain an effective Hamiltonian capable of describing the dynamics on the open domain. The basic algebra is implemented for one dimensional systems and a series of test calculations presented which serve to verify the validity of the approximations and illustrate the accuracy of the new procedure. The absorbing boundary method developed here bears some resemblance to the optical potential methods and the Feshbach projection scheme used in scattering theory and can be applied to dissociation and ionization processes as well as to scattering processes.
Interpretation of the optical absorption spectrum and of the paramagnetic susceptibility of neodymium A-type sesquioxide
Authors: Caro P. , Derouet J. , Beaury L. , Soulie E.
Published: February 2003

A-Nd2O3 is a layer structure built on the (NdO)n+n cation. Data for the 4.2 K optical spectrum and the strongly anisotropic parallel and perpendicular paramagnetic susceptibilities between 4 and 1000 K measured on single crystals, are reproduced using a Hamiltonian, that involves 12 free atom parameters (three Racah’s parameters, the spin–orbit coupling constant, Trees’ parameters α and β, and Judd’s six Tk operators) and six C3v crystal field parameters. The Hamiltonian operates on the 364 kets in 4f3, that is including J mixing. 88 Stark levels (from 0 to 34 000 cm−1) were fitted to a mean square deviation of 17 cm−1. The crystal field parameters (Wybourne’s formalism) are B20=−836 cm−1, B40=634 cm−1, B43=−1606 cm−1, B60=752 cm−1, B63=237 cm−1, B66=672 cm−1. The spectrum is strongly displaced to the red with respect to Nd3+ in LaCl3 and this is due to a lowering of Slater’s integrals F2 and F4 by 2% to 3%. The F6 integral is unchanged. The spin–orbit coupling constant is also lowered by 2%. The interaction parameters are slightly affected.
Determination of neon metastable beam composition
Authors: Coggiola M. J. , Gaily T. D. , Gillen K. T. , Peterson J. R.
Published: February 2003

A laser induced-fluorescence technique was used to determine the 3P2:3P0:1S0 ratio of a fast Ne beam. The beam was produced by Ne+–Na charge exchange collisions.(AIP)
Experimental verification of the spinodal curve
Authors: Abraham Farid F. , Mruzik Michael R. , Pound G. M.
Published: February 2003

Using structure factor measurements from molecular dynamics experiments on fluid phase stability, we establish the existence of the spinodal curve.(AIP)
The ’’noncrossing’’ rule for electronic potential energy surfaces: The role of time-reversal invariance
Authors: Mead C. Alden
Published: February 2003

Two new proofs are presented for the well-known noncrossing rule. It is believed that these proofs are not subject to the objections recently raised against other proofs in the literature. In both proofs, we calculate the number of independent conditions that must be satisfied for an arbitrary number n of potential energy surfaces to intersect. The answer is shown to depend on the behavior of the system under the operation of time reversal. One must distinguish three cases: In case (T+), the system is invariant under time reversal, and the overall spin is an integer. In case (T−), there is time-reversal invariance, but the total spin is half-odd integer, and the relevant question concerns the intersection of n Kramers doublets. Finally, in case (NT), there is no time-reversal invariance.
Mean molecular size distributions and the sol–gel transition in finite, polycondensing systems
Authors: Donoghue Edward , Gibbs Julian H.
Published: February 2003

A new theory is presented for the molecular size distributions and the sol–gel transition in a model, nonlinear, polycondensing system. The theory differs from the earlier theories of Flory and of Stockmayer in that the molecular size distributions are calculated as averages over the ensemble of all states of the finite system rather than as the ’’most probable’’ distribution in the limit as the size of the system goes to infinity. This modification is necessary because the mean molecular size distribution for a finite system is not described adequately over the range of large molecular sizes, at any extent of reaction, by the correctly normalized most probable distribution. This is clearly indicated by the failure of the most probable distribution to describe the gel explicitly. Numerical evaluations of the mean and the most probable distributions are shown to agree over the range of smallest molecular sizes at all extents of reaction. Beyond the gel point a second peak, clearly descriptive of the gel, is present over the range of larger molecular sizes in the mean molecular weight distribution. This peak is well-separated from the one describing the sol, and reflects thereby the disparate molecular sizes distinctive of the coexisting sol and gel. It is noted that in view of the analogy between gelation theory and Mayer vapor condensation theory, which was originally observed by Mayer and by Stockmayer and was recently established more firmly by Cohen et al., the difficulties in condensation theory can reasonably be expected to disappear if that theory is similarily reformulated.
Resonant CARS spectra of NO2
Authors: Guthals Dennis M. , Gross Kenneth P. , Nibler Joseph W.
Published: February 2003

Resonant CARS spectra are reported for nitrogen dioxide gas for frequency shifts of 1200–3400 cm−1 from a 532 nm ω1 pump source. The spectra change dramatically with slight changes in ω1 frequency. Much vibrational–rotational structure is observed but the analysis is complicated by the contributions of more than one resonant process. Various possible resonances are considered and absorption spectra and intensity measurements are used to assess the importance of some of these.
Franck–Condon factors in studies of dynamics of chemical reactions. V. Simple construction of quasiadiabatic potential energy surfaces and numerical evaluation of Franck–Condon integrals
Authors: Vilá Carlos L. , Kinsey James L. , Ross John , Schatz G. C.
Published: February 2003

We test the Franck–Condon (FC) approximation for chemical reactions by prescribing a simple construction of quasiadiabatic potential energy surfaces and evaluating numerically FC overlap integrals for the collinear case of the chemical reactions H2(D2)+F and H+Cl2. The FC model is derivable from the exact transition matrix by the use of four basic approximations: neglect of virtual transitions to excited electronic states; Born–Oppenheimer approximation; neglect of nuclear–electronic couplings; and the Franck–Condon approximation. The wave functions involved in the FC overlap are determined from quasiadiabatic potential surfaces, which were chosen to be constructed from the corresponding LEPS and anti-LEPS adiabatic surfaces for the chemical reaction in question. A coupling function which involves a single free parameter is needed to connect the quasiadiabatic surfaces. Our calculations show that the results are insensitive to this free parameter. We calculate vibrational distributions of reaction products for various initial kinetic energies of reactants and find the results to be in good qualitative agreement with both exact quantal calculations and FC models which include further approximations (with compensatory ease of calculation). Our results agree with the maximum in the vibrational distribution predicted by the other calculations and show similar trends with variation in initial relative kinetic energy and the masses (including isotopic substitution) as well as certain features of the potential surface.
Infrared spectra of (SN)X and brominated (SN)x
Authors: Macklin J. W. , Street G. B. , Gill W. D.
Published: February 2003

The infrared spectra of (SN)x and brominated (SN)x have been measured down to 60 cm−1 and compared with the previously reported Raman spectra. Specific vibrational assignments are suggested based on these measurements. The vibrational spectra indicate that in brominated (SN)x the bromine is located between the (SN)x chains and also in the interfibrillar regions. Both Br2 and Br3 species appear to be present.
Photophysics of pyrazine and its methylated derivatives at high pressure
Authors: Mitchell D. J. , Schuster G. B. , Drickamer H. G.
Published: February 2003

Fluorescence quantum yields and phosphorescence quantum yields and lifetimes were measured for pyrazine, 2, 6, dimethylpyrazine (DMP) and tetramethylpyrazine (TMP) in polymethyl methacrylate (PMMA) over the pressure range 20–120 kilobar. An additional low energy emission appeared at high pressure in these samples and in crystalline pyrazine. This is attributed to excimer fluorescence. The fluorescence yield increased with pressure at low pressures for all three materials, but exhibited a distinct maximum in pyrazine, a weak one in DMP and a monotonic increase in TMP. The behavior is attributed to increasing π–π* character of the fluorescence balanced by increased excimer formation. The pressure effect on excimer emission is strongest in pyrazine and essentially nonexistent in TMP. For pyrazine the quantum yield for intersystem crossing (at 120 K) decreases with increasing pressure, while for TMP it remains near 1.0 at all pressures. This difference in behavior may be associated with different pressure effects on excimer formation noted above or to differences in the excimer peak location vis a vis the phosphorescence. In pyrazine it lies some 4–5000 cm−1 below the phosphorescence peak so that excimer to T1 transfer is difficult, while in TMP the two peaks are nearly isoenergetic so that the transfer is possible.
The solvent effect on di-t
e
r
t-butyl nitroxide. A dipole–dipole model for polar solutes in polar solvents
Authors: Reddoch Allan H. , Konishi Shiro
Published: February 2003

New measurements of the solvent effect on the nitrogen hyperfine coupling constant of di-tert-butyl nitroxide are reported. These, together with literature data, are used to test various models for the solvent effect. At the Hückel level of approximation, aN is a linear function of the applied electric field. Thus various reaction field theories may be considered. The widely used Onsager reaction field does not account for the effects of the more polar solvents or for the differences between polar and nonpolar solvents. The Wertheim and Block–Walker reaction fields are better, especially for very polar solvents. However none of these continuum reaction fields is entirely satisfactory theoretically or experimentally. We propose a dipole–dipole model for polar solvents which is superior to the continuum models. From the dipole–dipole model, we suggest that the quantity μρ/M is a convenient linear parameter for polar solvent effects, the factors being solvent dipole moment, density, and molecular weight. The dipole–dipole model should apply to a wide range of polar solutes. Some special situations are not explained by the model, including hydrogen-bonding solvents, halogenated aromatics, and solvents with more than one conformation. The temperature dependence of the solvent effect is also considered.
Longitudinal nuclear relaxation of an ABX spin system application to a 13C–13C–1H fragment with and without proton irradiation
Authors: Canet D. , Nery H. , Brondeau J.
Published: February 2003

The longitudinal relaxation and the nuclear Overhauser enhancement, relative to the A and B transitions of an ABX spin system, are studied theoretically. A complete treatment based on density tion of X transitions. Assuming a doubly enriched fragment of the type O=13CA–13CB–1Hχ, numerical simulations have been performed with the aim to study the effects of cross-correlation spectral densities and motional anistropy. For the example considered, the initial slopes of carbonyl lines recovery curves yield a wealth of information. This interesting behavior arises from mixing effects produced by the observing pulse. The other data (recovery curves and NOE factors) can be, to a good approximation, interpreted in terms of generalized Solomon equations. These latter conclusions are however valid regarding the geometry of the considered fragment and, in the general case, cross-correlation terms must be taken into account.
LEED analysis of acetylene and ethylene chemisorption on the Pt(111) surface: Evidence for ethylidyne formation
Authors: Kesmodel L. L. , Dubois L. H. , Somorjai G. A.
Published: February 2003

The stable surface species formed from the chemisorption of acetylene (C2H2) or ethylene (C2H4) on the Pt(111) surface (T∼300–350 K) has been studied by a low-energy electron diffraction intensity analysis. High resolution electron energy loss spectra reported by Ibach et al. have been interpreted by comparison to infrared data on relevant model compounds. The surface species most consistent with these studies is ethylidyne ( C–CH3). The species is coordinated to a threefold surface site with the C–C axis normal to the surface within an uncertainty of ∼15°. A saturated C–C bond length of 1.50±0.05 Å and three equivalent Pt–C bond lengths of 2.00±0.05 Å are determined by the LEED analysis and are consistent with the reported structures of ethylidyne in organometallic clusters. The ethylidyne group forms readily upon exposure of C2H4 to the Pt(111) surface at T∼300 K with the loss of one hydrogen atom per ethylene. The complete conversion of C2H2 to ethylidyne requires the presence of hydrogen atoms and proceeds rapidly at T∼350 K. By comparison to reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene ( C=CH2) is an intermediate in both reactions.
A photoacoustic study of intramolecular energy transfer in CO2 deactivated by monatomic gases between 153 and 393 K
Authors: Lepoutre F. , Louis G. , Taine J.
Published: February 2003

Rates of (V–V) vibrational energy transfer from the CO2 (0001) level to the levels of the bending (b) and symmetric (s) stretching modes and from these two modes to translation–rotation (V–T) are measured for CO2 deactivated by monatomic gases between 153 and 393 K by a photoacoustic method. This technique is complementary to experiments involving fluorescence quenching or ultrasonic dispersion and makes possible a better analysis of the details of vibrational exchange processes. In the temperature range under study the variation in the (V–T) rate constants appears to be considerably less than semiclassical theories might lead us to believe even if attractive forces are accounted for. The hypothesis in favor of transfer to the rotation of CO2 at low temperatures permits a better approach to experimental results, but nevertheless remains insufficient. For the cases of deactivation by all the inert gases except helium, the experimental results lead to retaining three dominant (V–V) transfer processes: (0001) → (2000), (0001) → (1110) and (0001) → (1000). In the case of neon the last of these three processes is the most important above 350 K. Shin’s theory shows that this result is consistent with fluorescence measurements which go up to temperatures as high as 800 K. Below 250 K the measured rates of both (V–V) and (V–T) transfers change more slowly with temperature than those calculated.
Analysis of the electron spin echo decay envelope for Nd3+:ATP complexes
Authors: Shimizu Toru , Mims W. B. , Peisach J. , Davis J. L.
Published: February 2003

We have studied the nuclear modulation effect in electron spin echoes for complexes of Nd3+ and adenosine triphosphate (ATP) in frozen solution. Our aim was to find out whether this effect might be used to investigate metal–ion ATP complexes in biological systems. Experimental results have been compared with computer simulations based on NMR data for the Nd3+:ATP complex. The data has also been subjected to Fourier analysis. From the Fourier transforms it was found that the Nd3+:ATP complex tended to incorporate Na+ ions which were present in the starting material from which the samples were prepared, thus forming a coordination compound containing two different metal ions.
Collision-induced atomic dipole moments
Authors: Bentley John
Published: February 2003

A multipole expansion procedure is given for obtaining collision-induced atomic dipole moments from the molecular moments of a pair of atoms in a collision governed by the Pauli exclusion principle. Atomic moments are reported for He, Ne, Ar, and H, as determined from homonuclear diatomic wave functions. These are combined, via the distortion model of Smith [Phys. Rev. A 5, 1708 (1972)], to obtain molecular dipole moments for the heteronuclear collision pairs. The molecular dipole moments are in fair agreement with results of molecular orbital calculations. Limitations and extensions of the procedure are discussed.
A re-examination of the intensity distribution in the electron energy-loss spectrum of ethylenea)
Authors: Johnsonb) Kenneth E. , Johnston David B. , Lipsky Sanford
Published: February 2003

The ϑ=0° electron spectrum of ethylene was determined for the energy range 20–100 eV and the results compared with the predictions of the Born Approximation. (AIP)
Thermochemical properties of gaseous EuPO2 and EUPO3 molecules
Authors: Balducci G. , Gigli G. , Guido M.
Published: February 2003

Two gaseous ternary rare-earth element–phosphorus–oxygen compounds, EuPO2 and EnPO3 have been observed and identified. Their atomization energies and other thermochemical properties have also been determined.
Thermal hysteresis and reversibility of gel–sol transition in agarose–water systemsa)
Authors: Indovina P. L. , Tettamanti E. , Micciancio-Giammarinaro M. S. , Palma M. U.
Published: February 2003

Thermal hysteresis and stability of agarose-water gelling systems were studied by techniques providing information at different microscopic levels. Results are discussed in terms of the two distinct orders (molecular and supramolecular) predicted in the current model of gels of this type, i.e.,branched double helices of polysaccharide chains and bundles of associated double helices. The two types of order appear to be lost through two distinguishable transitions. Supramolecular order disappear gradually over a temperature span of about 50° when the temperature of the gel is increased. Over the same temperature span, it appears reversibly restorable without hysteresis although with long kinetics. In agreement with other data available in the literature, molecular order instead appears and disappears through much sharper, largely hysteretical transitions occuring with considerably quicker kinetics. Mimicking of thermal hysteresis in supramolecular ordering appears to be only a consequence of the fact that this ordering requires the existence of molecularly ordered ’’building blocks’’ to occur. The wider interest of systems of this type lies in their being recognized model systems of a variety of biologically organized structures.
The vibrationally resolved electronic spectrum of the s
y
m-trichlorobenzene radical cation in gas and condensed phases
Authors: Miller Terry A. , Bondybey V. E. , English J. H.
Published: February 2003

Laser induced fluorescence excitation spectra of 1,3,5-C6H3Cl3+ have been observed in Ar and Ne matrices and in the gas phase. Resolved emission spectra while laser pumping a single vibronic level have also been observed in the matrices. Combination of the spectra in the three media show that the excitation is little perturbed by the matrix environment. Most of the upper state a′1 and e′ vibrational modes are assigned and determined with a smaller number of ground state frequencies being similarly obtained. The strong appearance of the e′ modes in the spectrum shows that the symmetry of one of the states is lowered from D3h. This is most likely caused by a Jahn–Teller distortion of the ?2E′′ state. An estimate for the barrier between the Jahn–Teller minima for mode 13 is obtained. Comparison of the upper ?2A2′′ state vibrational frequencies with those of the ground state parent molecule supports the proposal that the ? state is formed by removal of an electron from an orbital centered predominately on the Cl atoms.
Atomic coreless Hartree–Fock pseudopotential for atoms Li through Nea)
Authors: Topiolb) Sid , Moskowitzc) Jules W. , Melius Carl F.
Published: February 2003

The coreless Hartree–Fock (CHF) pseudopotential method, previously developed and applied to atoms K through Zn, is used in this paper to generate Hartree–Fock quality pseudopotentials for atoms of the first row of the Periodic Table (Li–Ne). These potentials are tested for various electronic states of the atoms. Also presented are small optimized basis sets designed for use with these potentials. These CHF pseudopotentials and basis sets satisfactorily replace the effects of the core orbitals thereby allowing for savings in computational effort when used in molecular calculations.
The variational method and the stochastic–Liouville equation. I. A finite element solution to the CIDN(E)P problema)
Authors: Zientara Gary P. , Freed Jack H.
Published: February 2003

A variational formulation is developed for the stochastic–Liouville equation (SLE). It is shown how this formulation may be used as a general basis for the study of numerical and approximate methods of solution of the SLE. The finite element method is developed for the approximate solution of the spin–density matrix elements using the variational formulation. The method is illustrated by employing it to obtain a compact computer-oriented solution to the (high-field) chemically-induced spin polarization problem. This solution is both more efficient as well as more accurate than the previous treatment by Pedersen and Freed using finite difference methods. Various features of finite element and finite difference methods are compared from the viewpoint of this solution. The great flexibility of finite element methods for solution of the SLE is discussed.
Ultraviolet preresonance Raman scattering of benzene derivatives. I. Excitation profiles for fundamentals
Authors: Ziegler L. D. , Albrecht A. C.
Published: February 2003

The excitation frequency dependence of the Raman scattered intensities of several fundamentals of three benzene derivatives (toluene, m-xylene, and fluorobenzene) has been measured in the ultraviolet spectral region approaching the lowest lying absorption band in each of these molecules. As in benzene, the most preresonance active fundamentals are the C–H out-of-plane bending modes. The scattering cross section of these modes is found to result from the vibronic coupling of the 1Ba,b states (∼190 nm) and weak nearby out-of-plane polarized transitions. Most of the preresonance scattered intensity for the other modes reported here (the ring-breathing, ring-substituent, and internal methyl vibrations) have Raman active electronic origins in the far UV. With excitation very close to resonance with the lowest lying 1Lb transition, this source of scattering cross section becomes conspicuous for those modes seen prominently in this transition (the ring breathing and ring substituent vibrations). In addition the ring-substituent fundamentals are found to derive significant scattering cross section from the region of the 1Ba,b transitions.
Ultraviolet preresonance Raman scattering of benzene derivatives. II. Interference effects in the excitation profiles of the vibronically active fundamentals
Authors: Ziegler L. D. , Albrecht A. C.
Published: February 2003

Raman resonance and antiresonance behavior is observed in the ultraviolet preresonance excitation region for the scattering of those vibrations known to be vibronically active in the 1La and 1Lb electronic transitions of toluene, m-xylene, and fluorobenzene. These effects are found to be due to the constructive and destructive interferences between scattering contributions derived from the vibronic coupling of the low lying ππ transitions and transitions in the far-UV region. This behavior is seen to reflect the varying sign of the allowed 1Lb and 1La transition moment among these derivatives. Calculations are performed which show how the allowed transition moment in m-xylene is of opposite sign to that of these two monosubstituted benzenes.
Electric dipole moment of SO2 in ground and excited vibrational states
Authors: Patel D. , Margolese D. , Dykea) T. R.
Published: February 2003

Radiofrequency electric resonance spectra of SO2 molecular beams have been observed for a number of rotational levels of the (000), (100), (010), (001), (200), (110), (200), and (011) vibrational states. Heated supersonic nozzle beams of SO2 in argon were found to give enhanced signal-to-noise ratios. Stark effect measurements for the vibrational ground state give μ000=1.63305(4) D and polarizability anisotropies of αaa−αbb=1.62(40) Å3 and αaa−αcc=0.1(7) Å3. The dipole moment data for excited vibrational states can be expressed as μ=1.62673+0.00017(v1+1/2)  − 0.00684(v2+1/2)  +  0.01943(v3+1/2)  − 0.00002(v1+1/2)2 + 0.00002(v1+1/2)   (v2+1/2)  − 0.00003(v2+1/2)2 − 0.00012(v2+1/2)   (v3+1/2). By combining the Stark effect data with infrared intensity measurements, the electric dipole moment function for SO2 through second derivative terms has been determined.
Ground- and lower excited-state discrete a
b i
n
i
t
i
o electronic potential-energy surfaces for doublet HeH2+a)
Authors: McLaughlinb) Donald R. , Thompson Donald L.
Published: February 2003

A b initio electronic energy calculations are reported for 596 nuclear configurations of HeH2+ (in Cs symmetry). The lowest four doublet spin-state eigenfunctions for HeH2+ were computed by partially diagonalizing a subset of the full-configuration interaction Hamiltonian matrix, selected by perturbation theory estimation, relative to a reference set of configurations including the Hartree–Fock configuration and all appropriate single excitation occupations. Trial wave functions corresponding to ground and excited states were constructed from 30 molecular orbitals expanded in a twice-double-zeta-plus polarization contracted Gaussian basis. Two basis sets were employed: one constructed to produce greater accuracy for the ground-state (principal quantum number n equal to one) surface, and a second more contracted set in the n=1 space and augmented with n=2 basis functions to describe low-lying excited states. Absolute accuracy estimates of the ground- and excited-state surfaces are within 5 and 10 millihartree, respectively; relative errors are estimated to be less than 2–3 millihartree.
Theoretical study of the 400 eV core-excited valence states of N2
Authors: Rescigno T. N. , Orel A. E.
Published: February 2003

We show that accurate transition energies for the 1s core-valence excited 1Πu and 1Πg states of N2 can be obtained from limited configuration–interaction calculations using localized 1σ orbitals. Generalized oscillator strengths are also obtained for these states and comparisons with recent experimental findings are made.
Exciton–surface plasmon interactions
Authors: Pockrand I. , Swalen J. D. , Gordon J. G. , Philpott M. R.
Published: February 2003

A thin film possessing excitonic states within the range of energies of a plasmon surface polariton of a metallic substrate can strongly influence this surface electromagnetic wave either by a resonance interaction or a quenching interaction. We have analyzed this coupling and present our results showing the effects of an anisotropic orientation in the molecular film compared to an isotropic orientation and the effects of damping of the molecular electronic transition in the film. In a typical attenuated total reflection (ATR) experiment either an angular scan or a wavelength scan is usually used. A comparison of these two situations is given and sample calculations are displayed with perspective figures of reflectivity versus both angle and wavelength. The results show that a combination of both types of scans, i.e., angular scans at a series of wavelengths or wavelength scans at a series of angles or some combination to follow along the dispersion curve, are necessary to map the dispersion. Further isotropic excitons interact with the plasmon surface polariton at two wavelengths and anisotropic excitons interact at either one or the other depending on the orientation.
Stochastic Liouville equation formalism for translational and rotational diffusiona)
Authors: Bootsb) Henk , Deutch J. M.
Published: February 2003

Stochastic Liouville equations are used to study the relaxation behavior of position and orientation correlation functions of a fluid particle, if the second-order velocity or angular-velocity correlation is given. Two assumptions for reducing higher-order (angular) velocity correlation functions to two-point correlation functions are compared, the Gaussian assumption and ’’the assumption of vanishing higher-order G-cumulants.’’ In the case of translation both assumptions predict the same diffusion constant; only the latter one yields a Burnett coefficient that diverges in accordance with microscopic results. For rotational diffusion the second assumption is studied and compared with results obtained by Pomeau and Weber on the basis of the first assumption.
A classical analog for electronic degrees of freedom in nonadiabatic collision processes
Authors: Meyera) Hans-Dieter , Miller William H.
Published: February 2003

It is shown how a formally exact classical analog can be defined for a finite dimensional (in Hilbert space) quantum mechanical system. This approach is then used to obtain a classical model for the electronic degrees of freedom in a molecular collision system, and the combination of this with the usual classical description of the heavy particle (i.e., nuclear) motion provides a completely classical model for the electronic and heavy particle degrees of freedom. The resulting equations of motion are shown to be equivalent to describing the electronic degrees of freedom by the time-dependent Schrödinger equation, the time dependence arising from the classical motion of the nuclei, the trajectory of which is determined by the quantum mechanical average (i.e., Ehrenfest) force on the nuclei. Quantizing the system via classical S-matrix theory is shown to provide a dynamically consistent description of nonadiabatic collision processes; i.e., different electronic transitions have different heavy particle trajectories and, for example, the total energy of the electronic and heavy particle degrees of freedom is conserved. Application of this classical model for the electronic degrees of freedom (plus classical S-matrix theory) to the two-state model problem shows that the approach provides a good description of the electronic dynamics.
A carbon reaction studied by crossed molecular beamsa)
Authors: Krause H. F.
Published: February 2003

The chemiluminescent reaction C2+NO→CN(B 2Σ+) +CO+2.89 eV has been studied in a crossed molecular beam configuration. The carbon beam was obtained from a graphite sublimation source operating at 2550° K. The beam (principally C, C2, C3) was pulsed by a rotating slotted disk before it crossed the NO beam. Electronically excited molecules formed in reaction, CN(B 2Σ+), were detected as they rapidly decayed to the ground state. Photon time-of-arrival measurements identified the reactant as C2. The vibrational energy disposal within CN(B 2Σ+) was derived from wavelength resolved measurements. Reaction energetics and the signal intensity indicated the companion product, CO(X1Σ). This study indicates that energy deposition into vibrational degrees of CN(B 2Σ+) disagrees mildly with ’’prior expectation’’ (Levine, Bernstein, and co-workers). The reaction probably proceeds on a potential energy surface that correlates to C2(X′ 3Πu)+NO(X 2Π).
Note on the products’ translational energy for the reactions of CH3Br and CH3I with Rb and Ka)
Authors: Pollakb) Eli , Bernstein R. B.
Published: February 2003

A joining of the impulsive ’’photodissociation’’ model with the impulsive kinetic model is proposed.
Zeeman–quadrupole cross relaxation between two nuclear spin species
Authors: Stokesa) Harold T. , Ailion David C.
Published: February 2003

We derive an expression for the rate τ−1IS of the cross relaxation between the Zeeman splitting of one nuclear spin species (I spins, I=1/2) and the quadrupole splitting of another spin species (S spins, S≳1/2) via the I–S dipolar interaction. We calculate τIS for the case of CFCL2–CFCL2 (I spins are 19F, and S spins are 35Cl and 37Cl) and compare the results with experimental data.
Laser photodetachment of electrons from O2− in nonpolar liquidsa)
Authors: Sowada Ulrich , Holroyd Richard A.
Published: February 2003

The wavelength dependence of the photodetachment cross section of electrons from O2− in liquid tetramethylsilane, 2,2-dimethlypropane, 2,2,4-trimethylpentane and argon was measured from the conductivity change induced by a short laser pulse. The O2− was generated by exposing a solution of O2 to a pulse of 2 MeV x rays just prior to the laser pulse. Comparison with gas phase results from the literature shows that the threshold law is the same in the two phases, and that the size of the cross section is similar at comparable excess energies. However, the thresholds are shifted from the infrared in the gas phase to the visible region in solution. The observed values are: Eth=2.08, 2.02, 2.55 eV in tetramethylsilane, 2,2-dimethylpropane, and 2,2,4-trimethylpentane at 296 K, respectively, and 2.32 eV in tetramethylsilane at 200 K. The major energy term causing this shift is the solvation energy of O2−. If this is described by Born’s equation the effective radius of O2− is 1.54±0.02 Å in the hydrocarbons and tetramethylsilane. The threshold observed in argon is 2.43 eV which is attributed to photodetachment from O4−. From the observed threshold law it is concluded that the density of states of the conduction band in these liquids has the same (parabolic) shape as in the gas phase. This technique is shown to be useful to measure rate constants for electron transfer in solution.
Semiclassical statistical mechanics of fluids: Nonperturbative incorporation of quantum effects in classical many body models
Authors: Stratt Richard M.
Published: February 2003

Some of the basic equations of the equilibrium statistical mechanics of fluids involving the radial distribution function are derived in a nonperturbative semiclassical form by employing the phase space sampling approach in conjunction with a semiclassical pairwise interaction approximation. Analysis of the approximation suggests two practical models for effective potentials which automatically introduce the desired quantum effects into the classical equations.
High-energy approximation for atom–surface elastic scatteringa)
Authors: Lee H. W. , Georgeb) Thomas F.
Published: February 2003

A simple, approximate treatment of atom–surface elastic scattering is given. The treatment requires the incident beam energy to be high, such that all the atoms leading to different channels which contribute significantly to diffraction may be described by the same normal motion. This requirement can be realized in experiments with supersonic beams for a (near-)normal incidence. Diffraction intensities are given in an analytic form that requires little computational effort. Applications to the systems He/LiF and Ne/W are shown, and the results are compared with other calculations and experimental data.
A scaled Boltzmann theory for k-dependent test particle correlation functions in moderately dense gasesa)
Authors: Chen Sow-Hsin , Rahman Aneesur
Published: February 2003

Applicability of the hard sphere Boltzmann–Lorentz–Enskog test particle kinetic equation to the study of self-diffusion in moderately dense gases is assessed by comparing the numerical solutions of the equation for the velocity autocorrelation function, the kinetic energy autocorrelation function and the k-dependent test particle position correlation function with data generated from computer molecular dynamics and also with an incoherent quasielastic neutron scattering experiment. In order to adapt the hard sphere theory to explain data generated from systems in which particles interact via continuous potentials, the following procedure has been tested: We fix the effective collision frequency in the hard sphere kinetic equation by using the experimentally measured value of the self-diffusion coefficient. The results of the comparison show that this ’’scaled Boltzmann theory’’ compares well with the data for a system with a purely repulsive potential up to fairly high densities; however, increasing deviations are noticeable for a system in which the attractive part of the potential is included. This is so, especially at lower temperatures.
The electronic and vibrational absorption spectrum of tantalum nitride (TaN) isolated in an argon matrix
Authors: Bates John K. , Gruen Dieter M.
Published: February 2003

TaN was produced in a hollow cathode sputtering source and trapped in argon matrices at 14 °K. The absorption spectra was found to be most intense near 19 200 and 24 000 cm−1.
A simple method for the potential in ionic crystals
Authors: Hajj Fuad Y.
Published: February 2003

During the past sixty years many methods for finding the potential inside ionic crystals were proposed. Recently, the author developed a simple and accurate method based on summing Coulomb terms. Most of these methods succeeded in some cases but failed in other cases with no obvious reason. Some methods have procedures which could not be accounted for. Here, a basis theory is found which explains the failures, the successes, and the strange procedures of these methods. The theory is based on the fact that the finite block which represents the ’’infinite’’ crystal and which is used for computing the potential must be free from electric moments, the most important of which are the dipole, quadrupole, and octapole moments. It is shown that for a crystal of regular structure (with no defects) such a block does not exist and the potential is not defined. However, a satisfactory definition is found based on this theory. Starting with these facts, more simplifications were introduced into the computations and more insight was introduced into the problem of the general oblique structure.
A solution of the Debye–Smoluchowski equation for the rate of reaction of ions in dilute solution
Authors: Rice Stephen A. , Butler P. Robin , Pilling Michael J. , Baird James K.
Published: February 2003

Reactions of isolated ion pairs in solution have been modelled using the Debye–Smoluchowski equation for diffusion and conduction. An activation step was incorporated using a partially reflecting boundary condition. The method of matched expansions and the Abelian theorem of Laplace transforms was used to give an approximate solution of the Debye–Smoluchowski equation. Numerical integrations based on the finite-difference method confirmed these approximate analytic formulas.