bluuebunny/crossref_metadata_2025 huggingface dataset:
The photoelectron spectra of the chloramines NH2Cl, NHCl2, NCl3 and the methyl chloramines CH3NHCl, CH3NCl2, and (CH3)2NCl
Authors: Colbourne D. , Frost D. C. , McDowell C. A. , Westwood N. P. C.
Published: February 2003

The complete series of chlorinated and methylated derivatives of ammonia have been prepared in the pure state and studied by Hei ultraviolet photoelectron spectroscopy. The nonmethyl derivatives were also studied using NeI radiation. Assignments are made on the basis of observed substitution effects, and the spectra are compared with the analogous phosphorus compounds. The first band in all the spectra (arising from ionization of the nitrogen lone pair), shows interesting trends both in position and in bandwidth.
One-phonon scattering of atoms in three dimensions by a simplified continuum model of a surface: Thermal desorption
Authors: Goodman Frank O. , Romero Isidro
Published: February 2003

A model of thermal desorption of atoms from surfaces is presented. The treatment is quantum mechanical, and calculations are made using the first-order distorted-wave Born approximation, within the one-phonon approximation. The atom–surface interaction potential is a Morse potential, and the modal properties of the surface are those of a bulk continuum, the work being a natural continuation of some earlier atom–surface scattering work of Goodman. Our conclusions are inconsistent with some of those of Bendow and Ying and Ying and Bendow, who reported pre-exponential factors τ0 of order 10−5 s in the standard Frenkel formula for desorption; our values of τ0 are more in accord with the previously accepted values, of order 10−12 s. We apply the model to the astrophysical system H–C, and to interpretation of some experimental data on the systems He–Ar, He–Kr, and He–Xe; we find satisfactory agreement with experiment.
Studies of molecular reorientation of 1,2-difluorotetrachloroethane in the orientationally disordered state by depolarized Rayleigh scattering
Authors: Satija S. K. , Wang C. H.
Published: February 2003

The reorientational motion of 1,2-difluorotetrachloroethane in the orientationally disordered state has been investigated using the depolarized Rayleigh scattering technique. Excellent optical quality single crystals have been prepared for the experiment. The Rayleigh relaxation time has been measured as a function of temperature. The activation energy is shown to be associated with molecular reorientation and not with the potential barrier for the trans–gauche conformer transition. A discontinuous decrease in the Rayleigh relaxation time is observed at the melting temperature, but maintaining the same activation energy in the solid phase, in contrast to the results so far observed in other plastic crystals. This discontinuous decrease is attributed to a large change of molecular volume upon freezing. The pair orientational correlation in the liquid state has also been investigated by using dilution experiments. The pair orientational correlation is found negligible in the liquid state. The reorientational motion of CFCl2–CFCl2 in the liquid state is found to agree better with the slip reorientational model of hydrodynamics.
The effects of electron correlation on the adiabatic correction and on equilibrium constants for isotopic exchange reactions
Authors: Bardo R. D. , Kleinman L. I. , Raczkowski A. W. , Wolfsberg Max
Published: February 2003

The adiabatic correction C to the Born–Oppenheimer approximation is calculated for H2, HD, LiH, LiD, HCl, DCl with self-consistent-field and configuration interaction wavefunctions of varying qualities. The effect of correlation on the adiabatic correction, on the isotope effect on the adiabatic correction, and on the values of equilibrium constants of isotopic exchange reactions is discussed.
HTFFR kinetics studies of Sn/N2O, a highly efficient chemiluminescent reaction
Authors: Felder William , Fontijn Arthur
Published: February 2003

High temperature fast-flow reactors (HTFFR) were used to study the Sn/N2O reaction from 300–950 K at pressures from 4 to 110 Torr. The observed emissions are SnO[a 3Σ+(1) –X 1Σ+] and (b 3Π–X 1Σ+). The photon yield of the former system is 0.53±0.26 independent of T, that of the latter (5.9±2.9) ×10−1 exp[−(1200±200)/T]. Comparison of the photon yields of N2O- in-excess experiments, where [Sn] is measured in absorption, to experiments where Sn is in excess allows determination of oscillator strengths for the ground electronic states of Sn: f[Sn(3P0) (286.4 nm)]=0.20±0.10 and f[Sn(3P1) (300.9 nm)]=0.052±0.026, in good agreement with literature values. At T≳950 K, emission from SnO(c–X 1Σ+) and (A 1Π–X 1Σ+) is observed, apparently due to N2O decomposition followed by Sn/O2 reaction. Quenching rate coefficients at ≈900 K for SnO (a) are determined to be kQN2(a)⩽2.3×10−16; kQAr(a)⩽4.0×10−16; kQN2O(a)⩽4.0×10−14; kQSn(a)⩽4.0×10−12 ml molecule−1 s−1 based on τrad(a)⩾2.5×10−4 s. For SnO (b) the data yield τbkQN2(b) =4.8×10−20; τbkQAr(b)⩽2.0×10−20; τbkQN2O(b)⩽1.0×10−17; τbkQSn(b)⩽1.0×10−15 ml molecule−1. The overall Sn(3P0)/N2O reaction (production of all states) proceeds with a rate coefficient (8.9±4.0)×10−13 exp [−(2260±180)/T] ml molecule−1s−1. Approximate overall rate coefficients are reported for Sn(3P1)/N2O, Sn(3P0)/NO2, and Sn(3P0,1)/O2.
Field emission into nonpolar organic liquids
Authors: Dotoku Kazunari , Yamada Hiroshi , Sakamoto Saburo , Noda Shoji , Yoshida Hiroshi
Published: February 2003

Field emission into nonpolar liquids was studied for n-hexane, 2,2,4-trimethylpentane, and tetramethylsilane at 275–334 K. The current–voltage dependence was found to vary depending on the liquids and the temperature, and was reproduced qualitatively on the basis of the Fowler–Nordheim theory modified by taking account of the space charge effect. The quasifree electron energy (the electron energy in a liquid with reference to that in vacuum) was estimated from the current–voltage dependence in a space charge-free region, i.e., from the apparent work functions of the metal emitter (tungsten tip) in the liquids, to be 0, −0.6, and −1.3 eV for n-hexane (as a standard), trimethylpentane, and tetramethylsilane, at 293 K. These values lie in the same order as determined previously by the photoelectric emission from a metal surface in the liquids, though the dependence on the liquids is more pronounced in the present field emission study.
Monte Carlo calculations on critical behavior in two-state, steady-state Ising systems
Authors: Hill Terrell L. , Chen Yi-der
Published: February 2003

This is the first part of a study, by the Monte Carlo method, of one-dimensional and two-dimensional steady-state Ising systems. We examine phase transitions and critical behavior at equilibrium (for reference) and steady state. The primary model is a 10×10 or 100×1 periodic square lattice of M=100 interacting, two-state, cycling enzyme molecules. We use a flat top on P (N2) (the probability N2 out of M molecules are in state 2) as a criterion to locate the approximate ’’critical point’’ in these finite systems. At equilibrium, cases M=10×10, 5×5, and 12 have been examined. Reducing M increases the nearest-neighbor correlation function c1 at fixed T and increases the critical temperature. At steady state, with M=100 in either one or two dimensions, the cooperativity of the system may be either enhanced or reduced compared to equilibrium, depending on the choice of certain kinetic parameters. With the rather conservative choices of parameters used here, phase transition behavior at steady state is qualitatively similar to that at equilibrium except, of course, the enzyme flux is involved in the steady-state cases but not at equilibrium. We confirm a number of qualitative predictions of the Bragg–Williams (mean field) approximation. Hysteresis and nucleation kinetics can be studied by the Monte Carlo method, but we do not pursue these topics here. We plan to study the noise power spectrum in N2 in future work.
Critical behavior of two-state, steady-state Ising systems, according to the Bragg–Williams approximation
Authors: Hill Terrell L. , Stein Leonard
Published: February 2003

The Bragg–Williams (mean field) approximation is applied to an infinite arbitrary lattice of two-state enzymes, with nearest-neighbor interactions, cycling at a stable steady-state arbitrarily far from equilibrium. General equations are given for the fraction of enzymes in state 2 and for the flux. A simple numerical procedure is introduced for the determination of critical constants. A considerable sampling of results, especially on critical properties, is given. Because ’’van der Waals loops’’ are often obtained, some quite complicated, hysteresis is of course possible in the conventional way. The equations are simple enough so that the interested reader can easily generate further examples of his own, if desired. Of particular interest are special cases (a) in which the phase transition occurs in two steps and (b) in which either attractive or repulsive interactions will produce a phase transition.
Friction on a rotating dipole
Authors: Hubbard J. B.
Published: February 2003

A formula is derived for the viscous and dielectric friction on a steadily rotating spherical dipole for the case of an arbitrary ’’slipping coefficient’’. In the perfect slip limit, I recover the Nee–Zwanzig formula for dielectric friction at zero frequency, given that a simple Debye dispersion describes the dielectric response of the medium.
Optical oscillator strengths for argon
Authors: Ganas P. S.
Published: February 2003

An analytic atomic independent particle model is used to calculate optical oscillator strengths for 3p–ns and 3p–nd transitions in Ar. The results are compared to experiment and Hartree–Fock calculations.
Relaxation of the squeezing mode in free liquid films, measured by laser light scattering
Authors: Fijnaut H. M. , Joosten J. G. H.
Published: February 2003

The experimental setup to prepare free liquid films (from solutions of hexadecyltrimethylammoniumbromide) and to perform light scattering experiments on these films is described. Dynamic light scattering experiments concerning the relaxation of the squeezing mode (i.e., periodic thickness fluctuations) are reported for films of thickness between 35 and 100 nm and for mode wavelengths between 2.3 and 17.9 μm. In part of the wavelength regime good agreement is found between the experiments and existing hydrodynamic and classical colloidal interaction theories.
Electric and magnetic field spectra of the T1←S0 transition in crystalline 1,4-naphthoquinone
Authors: Galaup J. P. , Megel J. , Trommsdorff H. P.
Published: February 2003

The pure electronic T1←S0 transition of naphthoquinone single crystals has been investigated in electric and magnetic fields. The factor group states have been assigned ; their energies with respect to their mean value at 20 238 cm−1 are as follows: Bu=−1.20 cm−1; Ag=−0.54 cm−1; Au=+0.45 cm−1; Bg=+1.29 cm−1. The molecular spin splitting parameters have been evaluated from the measured crystals spin splittings as follows: X=0 cm−1 Y=−0.16 cm−1 Z=+0.16 cm−1. The molecular triplet state in the crystal is assigned as a superposition of the two n–π* states of the free molecule: ‖T1≳ =‖n–π*3B2≳−0.08‖n–π*3A2≳. This assignment is in agreement with the observed polarization and intensity distribution among the spin sublevels; it also explains the direction of the change of dipole moment upon excitation which—in the crystal—does not coincide with the polar axis of the molecule and makes it possible to determine its sign along the polar axis of the free molecule.
Evolution of the spectrum of the solvated electron in BeF2 aqueous glasses at 76 K
Authors: Nguyen Tuan Q. , Walker David C. , Gillis Hugh A.
Published: February 2003

Evidence is presented for a spontaneous shift from infrared-to- red-to-green in the overall spectrum of the electron trapped in an aqueous glass at 76 K following nanosecond pulse radiolysis. As with the alcohols, after the pulse the infrared absorption decays, there is a growth-then-decay of absorbance at intermediate wavelengths and net growth of absorption due to the equilibrated trapped electron state, e−vis. This progressive transition was not characterised by a single-step process and was discernible in aqueous glasses containing 7.5 or 10.7M BeF2 in D2O. At higher BeF2 concentrations, or with LiBr as glass-forming electrolyte, no growth of e−vis was observed, probably because of concurrent loss of e−vis in those glasses, as in LiCl and other glasses. BeF2 glasses of trapped electrons showed several other features of interest: (i) wavelength- selective bleaching of the e−vis band indicates that it is composed of a broad ’’green’’ band (λmax, 530–590 nm) which bleaches homogeneously, and a broad ’’red’’ region which bleaches inhomogeneously. (ii) λmax for e−vis changes with BeF2 concentration, being blue shifted with increasing concentration , with both green and red underlying structures shifting slightly. (iii) No absorption due to F−2 was found following pulse radiolysis in these glasses at 76 K. (iv) Several recent findings with LiCl are mimicked by F−2 glasses. For instance, G (e−vis) increases and G (e−ir) decreases as the electrolyte concentration is increased, while the combined yield remains constant at 1.8±0.1.
The far infrared absorption spectrum of ethylene in the liquid phase
Authors: Chapados C. , Bose T. K.
Published: February 2003

The far infrared absorption spectra of liquid ethylene have been measured at −145, −130, and −70°C. We attempt to account for the absorption on the basis of the quadrupole induced dipole interaction. We point out certain discrepancies and anomalies regarding the cancellation effect in the liquid phase.
Variational treatment of the proton–phonon system in KDP
Authors: Blumen Alexander , Silbey Robert
Published: February 2003

Ferroelectric KDP is best described through an Ising Hamiltonian in a transverse field, coupled linearly to the phonon system. The soft optical mode is coupled to the order parameter, whereas the dependence of the transition temperature on deuteration follows from the coupling of the tunneling frequency to another optical branch. After unitary transformations of the Hamiltonian, in which parts of the phonon coupling are dealt with, an analytic upper bound for the free energy of the KDP system is found using Bogoliubov’s inequality. This allows a choice from the class of different canonical transformations of that one leading to the lowest free energy bound. Then, in a mean field approximation, the effective coupling constant is found to be a solution of an implicit analytic function, which is solved numerically for different coupling strengths. In the ferroelectric region a strong temperature dependence of the effective coupling is found; crossing into the paraelectric region, its value changes smoothly for weak and abruptly for medium coupling strengths. These results parallel the behavior found in phonon coupled exciton systems.
On the calculation of electron binding energies
Authors: Kurtz Henry A. , Öhrn Yngve
Published: February 2003

Theoretical determination of atomic and molecular electron binding energies (ionization potentials and electron affinities) is discussed and expressions derived which apply for the core as well as the valence region. Applications to Ne, H2O, LiH, and Li2 are used as indicators of the quality of the results.
Rare gas interactions using an improved statistical method
Authors: Brual Gregorio , Rothstein Stuart M.
Published: February 2003

New density functionals are employed to represent the correlation and exchange energies (per electron) in the calculation of rare gas interactions from the electron gas model. The correlation energy density functional is a rational function involving two parameters which were optimized to reproduce the correlation energy of He atom. Our results indicate that these parameters are ’’universal’’, i.e., they are accurate for all rare gas atoms. The exchange energy density functional is that recommended by Handler. Rare gas systems X2 and XY are investigated, where X and Y are He, Ne, Ar, and Kr. The results are a significant improvement over those available from competing electron gas models.
Investigation of the multiple-photon dissociation of SF5Cl with a real time chlorine-atom diagnostic
Authors: Karl R. R. , Lyman J. L.
Published: February 2003

Flowing samples of SF5C1 and SF5C1+H2 were irradiated with 6 J/cm2 of pulsed CO2 radiation in an apparatus with real time chlorine-atom detection capabilities. The experiments were designed to determine if multiple-photon absorption by a sulfur–fluorine stretching oscillation resulted in the cleavage of the weaker sulfur–chlorine bond. It was shown that chlorine atoms are produced in high yield and that the appearance time was shorter than the mean time between collisions. From the known C1+H2 reaction rates the temperature in the photolysis zone was deduced to be 186°C, thus guaranteeing that the reaction is not thermal.
Measurements of some line positions and strengths in the ν4 region of SF6
Authors: Person Willis B. , Kim K. C.
Published: February 2003

Absolute ’’line strength’’ measurements with a diode laser spectrometer are reported for a number of J-manifolds for ν4 of SF6 in the gas phase. These results are analyzed to obtain the ν4 vibrational band strength: S0V≡A= (83±11 km mole−1. A critical discussion is presented of errors in line and band strength measurements in general, and of these results in particular. The experimental procedure was calibrated by measurements on several rotational lines in ν2 of N2O; the vibrational band strength derived for that transition in N2O is S0V=A=7.71±0.76 km mole−1, in excellent agreement with the literature value from total integrated band area measurements. The measured value for S0V for ν4 of SF6 is compared with literature values, and the discrepancy is discussed.
SCF Hartree–Fock calculations of ground state wavefunctions of compressed atoms
Authors: Ludeña Eduardo V.
Published: February 2003

SCF Hartree–Fock wavefunctions have been calculated for the ground state configurations of He, Li, Be, B, C, and Ne assuming that the atoms are enclosed within a spherical box with infinite potential walls. The variation of the total energy as a function of the sphere radius R is reported. The effect of compression on orbital energies is discussed.
Simulation of polymer dynamics. I. General theory
Authors: Fixman Marshall
Published: February 2003

Methods are presented for the simulation of conformational relaxation processes in polymer chains. The chains interact with the solvent through frictional and Langevin forces, have constrained bond lengths and angles, and smooth rotational energy functions. Relationships are obtained between the stochastic difference equations that form the basis for computer simulation, and equivalent Langevin differential equations and Smoluchowski diffusion equations. The presence of constraints and the associated action of potential forces and Langevin forces on the same time scale leads to somewhat intricate algorithms and correction terms. However, the time increment during a simulation step can be increased by a factor of 1000 or more over what is required if vibrational potentials are used to preserve the primary chain structure. Computation time is proportional to chain length in the absence of hydrodynamic interaction and excluded volume forces.
Photo-induced vibrational predissociation of the van der Waals molecule (N2O)2
Authors: Gough T. E. , Miller R. E. , Scoles G.
Published: February 2003

An apparatus is described in which a supersonic beam of nitrous oxide in helium impinges upon a 2 °K silicon bolometer, or alternatively, is detected by a mass spectrometer. Laser excitation of the beam in the vicinity of ν3 of nitrous oxide decreases the flux of the van der Waal’s molecule (N2O)2 reaching the bolometer. Thus, excitation of ν3 is followed by vibrational predissociation of the van der Waal’s bond.
Distant intramolecular interaction between identical chromophores: The n-π* excited states of p-benzoquinone
Authors: Goodman Julie , Brus L. E.
Published: February 2003

The structure and dynamics of the p-benzoquinone n-π* excited states have been investigated in solid Ne host. The two carbonyl excitations are essentially degenerate, with a B1g–Au splitting of +64 cm−1 between the singlets and −11 cm−1 between the triplets. There is no evidence that the lower 3B1g state has a double minimum potential via a pseudo Jahn–Teller distortion. The two triplets have very similar charge distributions and structures, despite their different formal symmetries. Excited singlet vibronic states show evidence for lifetime broadening due to intersystem crossing. Experiments in crystalline hosts are reviewed and reinterpreted. The structure in Ne host is closer to the intrinsic gas phase structure than are the structures in various single crystal environments. Valence bond theory, rather than molecular orbital theory, appears to be the natural description of the electronic structure.
Velocity dependence of the sensitized fluorescence of nitrogen by metastable argon
Authors: Parr T. P. , Martin R. M.
Published: February 2003

The velocity dependence of the excitation of nitrogen to N2(C 3Πu) by metastable argon has been redetermined using a time-of-flight velocity selected argon beam. The cross section velocity dependence is in good agreement with the previous determination using electronic pulsing of the metastable Ar(3P2,0). With mechanical pulsing, which eliminates the dc ground state Ar beam, the cross section function is different, rising more slowly to a maximum at a collision energy of about 0.3 eV and gradually decreasing up to 0.8 eV. All three TOF determinations extrapolate to zero threshold energy. Mechanical pulsing is believed to give the best cross section function, and the differences between this function, the electronically pulsed function, and a function determined using a rotary velocity selector are attributed to exchange collisions in the argon beams. The functions are normalized to the thermal rate constant to give the results in terms of absolute cross section vs collision energy.
Kinetic studies of ArF* and Ar2F* in proton-excited Ar–F2 mixtures
Authors: Chen C. H. , Payne M. G. , Judish J. P.
Published: February 2003

A detailed neutral kinetic scheme for the production and quenching of ArF* and Ar2F* is provided from time-resolved and time-integrated spectra for proton-excited Ar–F2 mixtures at various partial pressures. The mechanism for the production of Ar2F* is concluded mainly from the three-body quenching of ArF* by argon. It is also found that the efficiency to produce ArF* by Ar2* (1u) with F2 is comparable to Ar(3P2) colliding with F2. The two-body quenching rate constants of ArF* and Ar2F* by F2, the three-body quenching rate constant of ArF* by Ar, and the radiative lifetime of Ar2F* are also obtained. The mechanism of ion–ion recombination to produce ArF* is briefly discussed.
Electron attachment to van der Waals polymers of carbon dioxide and nitrous oxide
Authors: Klots Cornelius E. , Compton R. N.
Published: February 2003

Negative ions formed by electron attachment to carbon dioxide and nitrous oxide, expanded through a sonic nozzle, are reported. In each case the dominant ions have the formula O−(M)n, where M is the parent monomer. The appearance potential of O− in each case indicates the absence of ’’hot bands.’’ Ions with the formula (CO2)−n, with n?2, evidently formed by evaporative electron attachment, seem to be stable with respect to autodetachment. The ’’three-body’’ mechanism for attachment of thermal electrons to nitrous oxide and oxygen is discussed.
Electron attachment to van der Waals polymers of water
Authors: Klots Cornelius E. , Compton R. N.
Published: February 2003

Negative ions resulting from electron attachment to van der Waals polymers of water are reported. The most prominent ions observed consititute a homologous series with the stoichiometric formula OH−(H2O)m. They and the positive ions H+(H2O)m are ascribed to ion–molecule half-reactions. The ionization of van der Waals complexes as a method for preparing ’’collision complexes’’ is discussed.
Triplet electronic states of acetylene: c
i
s and t
r
a
n
s structures and energetics
Authors: Wetmore Ross W. , Schaefer Henry F.
Published: February 2003

Molecular electronic structure theory has been used to predict the equilibrium geometries and energies of acetylene in its ground state and lowest triplet electronic states. Both double zeta (DZ) and double zeta plus polarization (DZ+P) basis sets of Gaussian functions were used, in conjunction with self-consistent-field (SCF) and large scale configuration interaction (CI) wavefunctions. The predicted 1Σ+g ground state structure is re(C–C) =1.213 Å, re(C–H) =1.066 Å, in good agreement with experiment, re(C–C) =1.203 Å, re(C–H) =1.060 Å. The first excited state of acetylene is the cis 3B2 state, predicted to lie 3.43 eV above the 1Σ+g ground state. The predicted equilibrium geometry of this cis state is re(C–C) =1.343 Å, ϑe(HCC) =127.8°, re(C–H) =1.090 Å. The three other lowest triplet states are trans 3Bu (Te=3.78 eV), trans 3Au (4.13) eV, and cis 3A2 (4.50 eV). The predicted ordering and CCH bond angles for these states are not entirely consistent with Walsh’s rules.
Effect of centrifugal distortion and anharmonic vibration on the 19F chemical shielding in CH3F
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: February 2003

The temperature and density dependence of the 19F NMR frequency has been measured for CH3F gas. The NMR chemical shielding for 19F nucleus in the isolated CH3F molecule was obtained from 280 to 380 K by extrapolating the frequencies to zero density. This temperature dependence is interpreted in terms of the thermal average of powers of the normal coordinates. The linear term in this expansion is due to centrifugal distortion and anharmonic vibration. Using the cubic force constants and other molecular constants which have been reported for CH3F, an estimate of the dependence of the 19F shielding on the CF bond extension has been obtained.
Internal dynamics of van der Waals complexes. II. Determination of a potential energy surface for ArHCl
Authors: Holmgren Stephen L. , Waldman Marvin , Klemperer William
Published: February 2003

The Born–Oppenheimer angular-radial separation method for calculating ground state properties of atom-diatomic complexes has been used to determine a potential energy surface for ArHCl. Using a nonlinear least squares procedure, the calculated properties from trial surfaces were fit to molecular beam electric resonance data including both radial and angular expectation values. The inclusion of coriolis coupling terms in the Hamiltonian were found to have a small but discernable effect on the calculated properties. Both the number and type of parameters used to describe the surface affected their correlations dramatically. Fitting the angular properties of the complex required the potential to have an anisotropic to isotropic strength ratio of about 1:2. The isotropic portion of the potential could not be uniquely determined from the bound-state data alone, but was fixed by predicted differential elastic scattering cross-sections. In terms of R, the length of the vector connecting the Ar and the center of mass of the HCl, and ϑ, the angle the HCl makes with that vector, the surface has the form: V (R,ϑ) =V0(R)+V1(R) P1(cosϑ) +V2(R) P2(cosϑ), with V0(R) =133 cm−1[0.43 exp{20.[1−(R/3.81 Å)]}−1.43 (3.81 Å/R)6], V1(R) =41.2 cm−1[2.083 exp{10.36[1 −(R/4.259 Å)]}−3.083(4.259 Å/R)7], V2(R) =29.8 cm−1 [1.376 exp{10.36[1.−(R/4.259 Å)]}−2.376 (4.259 Å/R)6].
Random-phase calculation of the circularly dichroic 3000 Å band of (+)−D-camphor
Authors: Texter J. , Stevens E. S.
Published: February 2003

Time-dependent Hartree theory is applied in the random-phase, coupled-oscillator approximation to the calculation of the circular dichroism of the carbonyl π*←n transition in (+)−D-camphor. X-ray evidence indicating a very slight pyramidal distortion of the C(CO)C array in the ground state and the observation of the 000←000 member of a 00vs←000 progression (ν′ s∼1100 cm−1) in both absorption (f∼3×10−5) and circular dichroism lead us to conclude that a principal component of the π*←n transition mechanism is one which is electric-dipole allowed. Each such electric-dipole allowed member is in addition both magnetic-dipole and electric-quadrupole allowed. In coupling the 00vs←000 π*←n transitions to the CC and CH densities of states via a polarizability approximation, it is verified that the circular dichroism arising from the coupling of the electric-dipole transition moment is negligible in comparison with that resulting from the coupling of the magnetic-dipole transition moment with the backbone density of states. Moreover it is shown that the associated electric−quadrupole transition moment dominates the coupling energetics. The observed inhomogeneous dissymmetry, Δε/ε, occurs as a result of additional thermally populated 00vs←0va′0 (ν″ a′∼400 cm−1) and 00vs←va″00 (νa″″∼500 cm−1) progressions which contribute appreciably to the absorption intensity but not to the circular dichroism.
The dynamic Jahn–Teller effect in the electronic ground state of Li3. An a
b i
n
i
t
i
o calculation of the BO hypersurface and the lowest vibronic states of Li3
Authors: Gerber Werner H. , Schumacher Ernst
Published: February 2003

The lowest adiabatic potential energy hypersurface of Li3 is determined using the coupled electron pair approximation (CEPA). The GTO basis used accounts for at least 80% of the valence electron correlation energy. Our calculated binding and ionization energies (34 kcal/mole and 4.0 eV, respectively) are in good agreement with the experimental values of Wu [J. Chem. Phys. 65, 3181 (1976)] (41.5±4 kcal/mole and 4.35±0.2 eV, respectively). We show that vibronic coupling is essential in the electronic ground state of Li3, giving rise to a dynamic Jahn–Teller effect which is treated in a nonharmonic approximation. The lowest vibronic states of Li3 have been calculated by solving the two-dimensional Schrödinger equation for the E′×e′ Jahn–Teller case by a variational method. Vibrational probability densities are presented which allow the discussion of the ground state geometry of Li3. In contrast to previous work, an effective D3h geometry is proposed. This is supported by the calculated rotational energy levels, which can be interpreted as belonging to a symmetric top molecule with spectroscopic constants A=B=0.568 cm−1 and C=0.268 cm−1 (assuming three 7Li isotopes).
Theory of NMR relaxation in macromolecules: Restricted diffusion and jump models for multiple internal rotations in amino acid side chains
Authors: Wittebort R. J. , Szabo Attila
Published: February 2003

The theory required to extract detailed motional information from NMR relaxation times of nuclei in an amino acid side chain containing multiple internal rotation axes attached to a large macromolecule of at least cylindrical symmetry is developed. Emphasis is placed on the analysis of 13C-NMR of protonated carbons where dipolar relaxation is predominant. Extension to other relaxation processes is straightforward. The spectral density from which the relaxation times can be calculated is obtained for various models for the motion of the side chain. The existing theory which assumes that internal rotations are both independent and free is generalized to incorporate excluded volume effects in a heuristic way by restricting the amplitude of the internal rotations. It is found that small amplitude motions are ineffective in causing relaxation. Thus jump models involving a relatively small number of configurations are appropriate to describe the motion of the side chain. The advantage of jump models is twofold: (1) excluded volume effects can be handled simply by including only sterically allowed configurations, and (2) the unrealistic assumption of diffusional models that rotations about different bonds are independent can be removed by considering concerted motions in which the positions of only a small number of atoms are altered. An explicit expression for the spectral density, which is computationally practical when the number of configurations is less than 200, is obtained for a class of jump models in which the dynamics is described by a master equation. A detailed application of the general formalism is made to the case of a lysine side chain whose carbon–carbon backbone is constrained to lie on a tetrahedral lattice. Finally, numerical calculations are presented to illustrate some of the qualitative features of the different models, and the strategy that can be used to obtain motional information from experiment within the framework of these models is discussed.
Scattering kinematics: Transformation of differential cross sections between two moving frames
Authors: Catchen Gary L. , Husain Javed , Zare Richard N.
Published: February 2003

A general method is presented for transforming scattering cross sections between two moving frames. The Jacobian of the transformation is developed for nonrelativistic as well as relativistic particle energies in the case of either elastic or inelastic scattering. Instances of incorrectly used transformations in the literature are discussed.
Light scattering of a concentrated multicomponent system of hard spheres in the Percus–Yevick approximation
Authors: Vrij A.
Published: February 2003

A formalism for light scattering (or small angle x-ray or neutron scattering) from a (concentrated) multicomponent system of spherical (colloidal) particles dispersed in a low-molecular weight solvent is derived in terms of direct correlation functions introduced by Ornstein and Zernike and also in terms of Q functions introduced by Baxter. These functions, which characterize the interparticle interactions, are often theoretically much more accessible than the more familiar pair-distribution functions. The formalism is applied to a multicomponent system of hard spheres treated in the Percus–Yevick approximation. For zero scattering angle, a rather simple, exact expression can be formulated containing mean products of particle diameter and particle scattering amplitude. Some limiting cases are treated in more detail. A series expansion in the particle concentration is given up to the order for which the PY approximation is exact. The influence of polydispersity on light scattering is illustrated for a (generalized) exponential size distribution for cases in which the scattering amplitude is proportional to the second and third power of the particle diameter. The reliability of the PY approximation for hard spheres is discussed.
Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with CO2, CD4, or N2O
Authors: Manzanares I. Carlos , Ewing George E.
Published: February 2003

The vibrational relaxation times of liquid nitrogen doped with CO2, CD4, or N2O at 77 K were measured. Stimulated Raman scattering was used to excite N2 into the ν=1 level and its concentration as a function of time was monitored by spontaneous anti-Stokes Raman scattering. For these solutions rapid V–V energy transfer occurs between N2 (ν=1) and the near resonant vibrational levels of the solute molecules. On a longer time scale is the relaxation of the vibrationally excited solute molecule by V–T, R energy transfer. Comparison of the nitrogen solution data for V–T,R relaxation of CO2 by N2 and N2O self-relaxation with gas phase results is consistent with the suggestion that isolated binary collisions are responsible for vibrational relaxation in the liquid state. Therefore, for these systems as far as vibrational relaxation processes are concerned the liquid may be treated simply as a high density gas. No direct comparisons between liquid state and gas phase measurements were possible for V–T,R relaxation of vibrationally excited N2O or CD4 by N2. However, extrapolations from available data suggest that there may be a change in the vibrational relaxation mechanisms for these and other systems when the phase is changed from gas to liquid.
Theoretical dipole moment derivatives and force constants for HCN
Authors: Liu B. , Sando K. M. , North C. S. , Friedrich H. B. , Chipman D. M.
Published: February 2003

Valence configuration interaction wavefunctions are used to calculate dipole moment derivatives and force constants for linear hydrogen cyanide. The magnitude of the dipole moment is found to decrease when the C–N bond is stretched from its equilibrium value. The implications for hydrogen bonding in crystalline HCN are discussed. Self-consistent field calculations are used to confirm that the SCF dipole derivative with respect to the CN bond length has the wrong sign and to explore the sensitivity of the dipole derivatives to the choice of atomic basis set.
Intermolecular coupling in liquid water
Authors: Kint Saima , Scherer James R.
Published: February 2003

Changes in the Raman spectra of H2O from addition of OD oscillators indicate that there is significant intermolecular coupling of OH stretching vibrations in H2O at temperatures from −10 to 70°C. Measurements on solutions of KI in H2O indicate that the primary effect of salt addition on the spectrum of H2O is to diminish intermolecular coupling of OH oscillators. The disruption of intermolecular coupling by addition of a K+ and I− ion is about nine times more effective than addition of a single OD oscillator.
Ultrasonic investigation of critical phenomena in the binary system triethylamine–water
Authors: Garland Carl W. , Lai Chiu-Nan
Published: February 2003

The acoustic velocity and absorption have been measured in a series of triethylamine–water mixtures, with special emphasis on the region near the consolute point at XH2O=0.921 and 18.23°C. The frequency range from 1 to 27.3 MHz and temperature range from −2 to 20°C have been explored. The ’’effective’’ absorption and dispersion (observed values corrected for noncritical high-frequency contributions) are compared to Kawasaki–Mistura mode-coupling theories. Neither version of this theory is adequate to describe the experimental data, but it is possible to fit the data empirically by introducing two constant-frequency relaxations in addition to a critical mode-coupling contribution. One of these additional relaxations may be related to the structural relaxation of large hydration complexes. The nature of the other is unclear, but it may be an artifact of the fitting procedure indicative of the inadequacy of the critical contribution.
Brownian dynamics with hydrodynamic interactions
Authors: Ermak Donald L. , McCammon J. A.
Published: February 2003

A method for simulating the Brownian dynamics of N particles with the inclusion of hydrodynamic interactions is described. The particles may also be subject to the usual interparticle or external forces (e.g., electrostatic) which have been included in previous methods for simulating Brownian dynamics of particles in the absence of hydrodynamic interactions. The present method is derived from the Langevin equations for the N particle assembly, and the results are shown to be consistent with the corresponding Fokker–Planck results. Sample calculations on small systems illustrate the importance of including hydrodynamic interactions in Brownian dynamics simulations. The method should be useful for simulation studies of diffusion limited reactions, polymer dynamics, protein folding, particle coagulation, and other phenomena in solution.
Model potentials for electron scattering: Converged close coupling calculations for the differential cross section for e−N2 at 30–50 eV
Authors: Onda Kunizo , Truhlar Donald G.
Published: February 2003

We have calculated the elastic scattering and rotational excitation cross sections for e−–N2 scattering at 30 and 50 eV using quantum chemical techniques specially designed to be applicable to elastic and inelastic electron scattering by general polyatomic molecules. The angle dependence of the sum of the elastic and rotational excitation differential cross sections is in good agreement with experiment at all scattering angles at both energies, but at 50 eV the difference from experiment exceeds the experimental uncertainty at small scattering angles and near the minimum of the differential cross section. At large scattering angles the rotational excitation cross sections are predicted to exceed the elastic scattering cross sections. The absolute cross sections agree with experiment at some angles but at other angles are as much as 51% (30 eV or 90% (50 eV) higher; this may be due at least in part to the difficulty of putting the experimental results on an absolute scale.
Partially diffusion-controlled reactions of isolated pairs in condensed media
Authors: Mozumder A.
Published: February 2003

A theory is formulated which, within the framework of the diffusion model, takes into account a reaction probability α (?1) of an isolated pair of reactants at the encounter separation. Re-encounters are envisioned in the liquid phase; however the resultant time-dependent reaction rate can be expressed mainly in terms of the first encounter probability. Other useful information, such as the ultimate escape probability, reaction with a uniformly distributed scavenger, etc., are obtained from the fundamental result, the last by using a Laplace transformation. Model calculations are provided and some actual cases are analyzed with a specific physical interpretation of α.
Analysis of 3ν3 in SF6
Authors: Ackerhalt J. R. , Flicker H. , Galbraith H. W. , King Joe , Person Willis B.
Published: February 2003

Studies of the 3ν3 absorption band in SF6 in the spectral region from 2800 to 2850 cm−1 have been completed with a FT–IR spectrometer at a spectral resolution of 0.04 cm−1. This study confirms the results reported earlier by Kildal on the general shape of this absorption band. The absorption has been studied as a function of temperature, allowing a definite assignment to be made of the ground state features observed in this spectrum at room temperature. The ground state absorption to 3ν3 appears with a double Q branch [estimated band origins at 2830.22 and 2827.62 (±0.06) cm−1]. The intensities and temperature dependence studies confirm the assignment by Fox of these two Q branches to the two F1u components of the 3ν3 state.
Thermal accommodation coefficients by high speed vibration of solid samples. V. Translational and internal gas–solid energy exchange for alkanes and other molecular gases
Authors: Draper Clifton W. , Rosenblatt Gerd M.
Published: February 2003

Translational γtrans and internal γint energy accommodation coefficients of alkanes and other polyatomics on polycrystalline metal foils are determined using a high speed vibration method described previously. For the normal alkanes CnH2n+2(n=1–8) on Fe foil, γtrans increases with n from 0.55 for methane to 0.95 for n-octane. γint also increases from 0.52 to 0.91. For the other gases studied on Al or Fe foils—Cl2, CO2, NH3, C3H6, neo-C5H12, and C6H6—the values of γtrans are 0.94, 0.71, 0.87, 0.75, 0.87, and 0.66; the values of γint are 0.94, 0.54, 0.86, 0.63, 0.75, and 0.54. The Al and Fe foils are covered with layer(s) of oxygen and/or test gas, and presumably appear similar to a given gas. Approximate upper and lower limits for vibrational accommodation coefficients (γvib) can be estimated using the assumption that γint=γrot =γvib, or γtrans=γrot. Comparison is made between vibrational accommodation coefficients estimated in this way and available literature data. Gas–surface energy exchange is examined from the viewpoint of trapping—emphasizing its dominant role in energy accommodation under the conditions of these measurements. It is inferred that γrot is very close to γtrans, but that γvib is less than γtrans, for most of the gases studied. However, γvib approaches γtrans for the heaviest hydrocarbons, C7H14 and C8H16, which have very long surface residence times. For CH4 the value of γint, but not of γtrans, appears exceptional.
Polarization model for water and its ionic dissociation products
Authors: Stillinger Frank H. , David Carl W.
Published: February 2003

In order to achieve a simple description of aggregates of deformable water molecules, a new model has been constructed which treats H+ and O2− particles as the basic dynamical and structural elements. The H+ units are bare protons, while the O2− units possess a form of nonlocal polarizability consistent with their electronic structure. The model yields water molecules which have the correct geometry and dipole moment, and which engage in hydrogen bonding to one another. Minimum-energy structures have been determined for the water dimer and trimer and for small hydrate clusters of H+ and OH−; comparison with relevant experiments and quantum–mechanical calculations is satisfactory.
Elastic scattering of intermediate energy electrons by HCN
Authors: Srivastava S. K. , Tanaka H. , Chutjian A.
Published: February 2003

Utilizing a crossed electron beam–molecular beam geometry and a relative gas flow technique, ratios σ (HCN,ϑ)/σ (He,ϑ) of elastic differential cross sections of HCN to those of He have been measured at electron impact energies of 3,5,11.6, 21.6, and 50 eV and at scattering angles of 20° to 130°. Normalized absolute values of σ (HCN,ϑ) have been obtained by multiplying these ratios by the absolute values of σ (He,ϑ) reported previously. Since the rotational–vibrational structure in HCN was not resolved in the present measurements, the term elastic here includes contributions from elastic scattering, as well as from pure rotational and the 1–0, ν2 vibrational excitations. The elastic differential cross sections have been compared with the predictions of the Born approximation and classical perturbation theory. For angular regions lying between 0° and 20°, and 130° and 180°, σ (HCN,ϑ) values have been obtained by extrapolation. These values have been used to calculate the integral and momentum-transfer cross sections.
Chemical-state effects in Auger electron spectroscopy
Authors: Rye R. R. , Madey T. E. , Houston J. E. , Holloway P. H.
Published: February 2003

We have used Auger spectroscopy as a probe of local chemical environment both in the gas and condensed phases using a systematically chosen series of molecules [H2O, CH3OH, (CH3)2O, CH4, C2H4, and C2H2]. For the series of gas phase molecules, H2O, CH3OH, (CH3)2O, and CH4, where oxygen and carbon are, respectively, in similar bonding arrangements, characteristic fingerprint spectra (methanelike for C and waterlike for O) are shown to result. Additional fine structure, which is dependent on the specific molecular environment, appears on the spectra. In contrast, dramatic differences are observed for the series CH4, C2H2, and C2H4 in which major differences in hybridization exist at the carbon site. H2O, CH3OH, and (CH3)2O were studied both in the gas phase (electron excited) and in the condensed phase (x-ray excited). The O(KVV) (K level–valence–valence transition) and C(KVV) spectra are shown to be similar when comparing the gas–solid results only if the multilayer spectra are properly corrected for electron-loss effects. Some ’’solid-state’’ broadening is observed in the multilayer case. The degree of this broadening is found to be greater in the O(KVV) than for the C(KVV) and a new peak appears in the case of solid H2O which is not present in the gas phase. This is consistent with intermolecular bonding primarily at the oxygen site. These results demonstrate, along with the fingerprint correlations given above, that AES is sensitive to changes in the local environment.