bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized transition state theory calculations for the reactions D+H2 and H+D2 using an accurate potential energy surface: Explanation of the kinetic isotope effect
Authors: Garrett Bruce C. , Truhlar Donald G.
Published: February 2003

Rate constants are calculated for the reactions D+H2→DH+H and H+D2→HD+D and compared to measured values. An accurate potential energy surface, based on the ab initio calculations of Liu and Siegbahn, was used. Rates were calculated using both conventional transition state theory and canonical variational theory. In the former, the generalized transition state dividing surface is located at the saddle point; in the latter it is located to maximize the generalized free energy of activation. We show that, in the absence of tunneling corrections, locating the generalized-transition-state dividing surface variationally has an important quantitative effect on the predicted rate constants for these systems and that, when tunneling is included, most of the effect of using a better dividing surface can be included in conventional transition state theory for these systems by using a consistent transmission coefficient for quantal scattering by the vibrationally adiabatic potential energy curve. Tunneling effects are important for these reactions even for temperatures larger than 400 K. We show how to separate classical recrossing effects from quantal corrections on reaction-coordinate motion in both the transmission coefficients and the kinetic isotope effects. Our most complete calculations are in excellent agreement with most of the measured rate constants and kinetic isotope effects.
Nonequilibrium decay effects in diffusion-controlled processes
Authors: Weaver D. L.
Published: February 2003

It is shown how to include decay or desorption at a boundary in systems with diffusive kinetics. A quantity, the mean equilibrium time, is defined and calculated for one-dimensional diffusion. Both finite and infinite systems are considered.
Infrared chemiluminescence investigation of the reactions of methyl radicals with oxygen and fluorine
Authors: Moss Mary G. , Hudgens J. W. , McDonald J. D.
Published: February 2003

The technique of arrested relaxation infrared chemiluminescence has been used to study two methyl radical reactions. The reaction of methyl radicals with fluorine produces methyl fluoride with a statistical distribution of vibrational energy. In this reaction approximately half the energy available to the methyl fluoride product goes into vibration. The reaction of methyl radicals with oxygen atoms produces formaldehyde with so much vibrational energy that we can only put a lower limit of 30 kcal/mole on its vibrational energy.
Segregation of co-adsorbed species: Hydrogen and carbon monoxide on the (111) surface of rhodium
Authors: Williams Ellen D. , Thiel Patricia A. , Weinberg W. H. , Yates John T.
Published: February 2003

The co-adsorption of CO and H2 on Rh(lll) at low temperature (∼100 K) has been studied using thermal desorption mass spectrometry (TDS) and low energy electron diffraction (LEED). The probability of adsorption of CO on rhodium pretreated with hydrogen has been found to vary nonlinearly with the amount of hydrogen on the surface. In addition, the effect to surface hydrogen on the CO LEED patterns indicates near complete segregation of hydrogen and CO. These results can be explained qualitatively in terms of a strong repulsive CO–hydrogen interaction and a mobile precursor model of CO adsorption.
Dynamics of cross relaxation in modulated systems: Application to nuclear magnetic double resonance of glassy polymers
Authors: Cheung T. T. P. , Yaris Robert
Published: February 2003

A statistical mechanical analysis, based on Mori’s generalized Langevin equation, is presented for the dynamics of cross relaxation in modulated systems. Relaxation rate constants are given by Onsager’s kinetic coefficients where the effects of lattice (heat bath) modulations are examined in terms of the spectral density of lattice motions as well as the transitions among the energy levels involved. Applications of the theory to nuclear magnetic double resonance relaxations of natural abundant 13C in glassy polymers are discussed. For a moderate rf field of 30–40 G (32–43 kHz), it is found that whenever lattice motions have sufficient power at these frequencies, a torsional amplitude of a few degrees suffices to yield a rotating frame longitudinal spin relaxation time T1ρ(C) of the order of milliseconds. This is much shorter than the rigid lattice value of T1ρ(C) which is equal to T12 (ADRF), the cross relaxation time under adiabatic demagnetization in rotating frame condition. The validity of the Markoffian approximation which leads to the usual time independent relaxation rate constant is examined. It seems that for proton deficient systems this approximation breaks down for the cross relaxation in matched spin–lock experiments. The origin of this breaking down is discussed and the interpretations of the cross relaxation T12(SL) in spin–lock experiments are also given.
31P chemical shift and relaxation study of the pseudo-one-dimensional ferroelectric transition in CsD2PO4
Authors: Blinc R. , Zupančič I. , Lahajnar G. , Slak J. , Rutar V. , Verbec M. , Žumer S.
Published: February 2003

The 31P chemical shift tensors σ have been determined above and below Tc. The spin–lattice relaxation time T1 (31P) was measured as a function of temperature at 109.3 and 36.4 MHz. The paraelectric 31P chemical shift tensor was found to be the average of the two tensors observed in the ferroelectric phase. This shows that the phase transition is triggered by the ordering of the O–H(2) ⋅⋅⋅O deuterons. The intrinsic O–H(2) ⋅⋅⋅O deuteron intrabond jump time is, in the absence of the interactions, of the same order as in KD2PO4. The anomalously short T1 (31P) is the result of an ’’anisotropy slowing down’’ of the order parameter fluctuations due to the pseudo-one dimensional nature of correlations in CsD2PO4.
Trajectory study of atomic recombination reactions. VIII. The energy transfer mechanism
Authors: Burns George , Young Aaron W.
Published: February 2003

The energy transfer (ET) mechanism of atomic recombination reactions, X+X=X2*, X2*+M→X2+M, where X2* is a classically unstable orbiting quasidimer consisting of two X atoms and M is a third body, has been commonly used for the past forty years. Presently it is studied in some detail using the 3-D trajectory calculation technique and the Monte Carlo method of sampling. Since this mechanism approximates a pure three body problem by considering two independent and consecutive two body collisions, it introduces an error in trajectory calculations. In order to improve upon this approximation, a more reasonable modified method of trajectory sampling is made. It is found that even in one of the most favorable cases where the ET mechanism is predominant, i.e., for the 2I+He=I2+He reaction, the error introduced by the ET mechanism is of the order of two in the rate constant. It is concluded, in light of the present findings, that earlier as well as present trajectory studies for halogen atom recombination in helium are consistent with experimental data. The existing discrepancy between computed and experimental results can be accounted for by the approximations used in trajectory calculations. On the other hand, the combined Monte Carlo sampling and experimental errors cannot account for the existing discrepancies.
Melting of a monolayer in an external field
Authors: Toxvaerd So/ren
Published: February 2003

The melting in a two dimensional system of Lennard-Jones particles in an external field proportional to the field at a (111) surface of a face-centered cubic crystal of the same particles is analyzed. Molecular dynamics calculations show a reversible first order phase transition. The freezing of the system into a perfect hexagonal lattice is performed over several thousand timesteps. The freezing temperature is found to be in good agreement with the temperature obtained from Ross’s melting law.
Electron and heavy particle temperature dependent quenching rate constants of XeF*.
Authors: Trainor Daniel W. , Jacob J. H. , Rokni M.
Published: February 2003

Two and three body quenching rate constants were determined at 300 and 500 °K for a number of quenching gases common to XeF* laser mixtures (Ne, Xe, F2). Among these is the three body quenching of XeF4 by F2 and Xe. In addition, the quenching rate for the reaction XeF*+e− was measured and this process was found to be one of the major quenching channels under typical laser operating conditions.
Trajectory study of dissociation reactions. The single-ensemble method
Authors: Kutz H. Douglas , Burns George
Published: February 2003

A new method for the calculation of nonequilibrium effects in diatom dissociation reactions is presented. It is based on the use of a single statistical ensemble, which is studied by the multiple collision 3-D trajectory calculation technique. Statistical fluctuations, which in the past prevented accurate determination of such observables as steady state rate constants and reaction cross sections, have now been reduced by a factor of better than ten, as compared with earlier work. Such an improvement in statistical error makes possible the detailed study of various features of the nonequilibrium steady state. Several approximations of earlier multiple-collision trajectory calculations are examined, and results on the dissociation of Br2 in Ar at 3500 °K are presented. For this particular system in the steady state, the internal energy bias parameter was found to be equal to three.
An effective Hamiltonian for the valence states of ethylene generated by canonical transformations
Authors: Westhaus Paul , Moghtaderi M.
Published: February 2003

We calculate the vertical N→T, N→V, and T→V transition energies in the ethylene molecule by diagonalizing the effective valence shell Hamiltonian obtained by our first principles canonical transformation–cluster expansion formalism. Calculations are performed for five different partitions of the one-electron space into core, valence, and excited subspaces. Excellent results of 4.61, 7.67, and 3.06 eV are obtained for the three respective transitions, provided that the many-electron states arising from a given partition meet two criteria. In particular, we find it is necessary to include both a localized and a diffuse π* orbital explicitly in the valence space. In addition, care must be taken in partitioning the one-electron space so as to avoid the quasidegeneracy between a state in the model space and one of the same symmetry in the orthogonal complement.
Proton–H2 scattering on an a
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o CI potential energy surface. II. Combined vibrational–rotational excitation at 4.67 and 6 eV
Authors: Schinke Reinhard
Published: February 2003

Infinite-order-sudden calculations have been performed at 4.67 and 6 eV on the ab initio CI potential energy surface determined recently by Schinke, Dupuis, and Lester. The vibrational degree of freedom has been treated exactly by solving vibrationally coupled radial equations. The rotationally summed differential cross sections for vibrational excitation are in good agreement with the measurements of Schmidt, Hermann, and Linder. It is shown that the rotational excitation cross sections in the vibrational ground state near the rainbow angle are almost exclusively determined by the potential between 2.5a0 and 5a0 proton–H2 separations. In this region only the V2 term of an expansion into Legendre polynomials is nonvanishing and is a factor of ∼3 smaller for the new surface than for the Giese and Gentry analytic potential. These differences result in a dramatic decrease of the rotational excitation cross sections in the rainbow region so that the present theoretical transition probabilities are in much better agreement with the experiments than our previous sudden vib–rotor calculations utilizing Giese and Gentry’s surface.
Polarized spectroscopy of partially oriented molecules. A general formulation for two-photon processes in uniaxial samples
Authors: Michl Josef , Thulstrup Erik W.
Published: February 2003

General expressions are given for linearly polarized intensity in a two-photon experiment on a uniaxial assembly of partially aligned molecules of arbitrary shape and symmetry, without any assumptions concerning the detailed nature of the uniaxial orientation distribution. The formulas are valid for electric dipole interactions and apply to measurements such as ordinary and resonant Raman scattering, two-photon absorption, photoluminescence, and transient or permanent photoinduced dichroism measured in stretched polymers, nematic liquid crystals, or lipid bilayers. Two examples are worked out in some detail: nonresonant Raman scattering and photoluminescence of symmetrical molecules. A discussion of the physical significance of the orientation factors and of their interrelation is given.
Spontaneous cavitation in a n-octanelike fluid
Authors: Weber Thomas A. , Helfand Eugene
Published: February 2003

Simulations are reported for a system of n-octanelike molecules at fixed volume, with various strengths of attractive potential. Spontaneous cavitation is observed to occur with the production of both cylindrical and lamellar cavities. The molecules are shown in all cases preferentially to align perpendicular to the cavity/liquid interface. The cavitated samples have a higher percentage of trans bonds in comparison with the homogeneous fluid samples.
Chemical rectifiers: One-way catalysis
Authors: Ross Robert T.
Published: February 2003

Chemical reactions can have net rates which have an asymmetric dependence on the sign of the free energy of reaction, and are thus analogous in function to rectifiers. Reactions whose forward rate varies as a large positive power of reactant concentrations, and as a large negative power of product concentrations, have an asymmetry which far exceeds that of an ideal electrical rectifier. A catalyst which switches cooperatively between active and inactive forms, in the manner of regulatory enzymes, can be such an extraordinarily good rectifier. For example, an enzyme of the Monod–Wyman–Changeux type, having six binding sites, and catalyzing a simple isomerization, is capable of catalyzing a forward reaction 107 times faster than the maximum rate of the reverse reaction. This rectification ratio is achievable for an enzyme which catalyzes the forward reaction at 20% of the rate permitted by the turnover number of the catalytic site, and with a free-energy drop of only 3 kcal/mol. Such chemical rectifiers may improve the efficiency of photochemical solar energy conversion.
Vibrational resonance coupling and the noncoincidence effect of the isotropic and anisotropic Raman spectral components in orientationally anisometric molecular liquids
Authors: Wang C. H. , McHale Jeanne
Published: February 2003

An expression for the splitting parameter associated with the noncoincidence of the peak positions of the polarized and depolarized Raman spectral components of a totally symmetric band in a molecular liquid has been derived. The derivation is based upon an angular dependent vibrational resonance coupling Hamiltonian, and has induced a more general interaction Hamiltonian than the induced dipole–induced dipole interaction. It has been shown that short-range orientational order is not essential for the noncoincidence effect, although the presence of short-range order can affect the magnitude of the splitting parameter. An example is given to show that the noncoincidence effect is not necessarily due to the induced dipole interaction mechanism. The present result shows that any angular dependent resonance coupling mechanism can give rise to the noncoincidence effect.
Energy distribution of the fragments produced by photodissociation of CS2 at 193 nm
Authors: Yang S. C. , Freedman A. , Kawasaki M. , Bersohn R.
Published: February 2003

Gaseous CS2 was dissociated at 193 nm into CS and S. The translational and internal energy distributions of the CS fragments were measured. There is now overwhelming evidence that the upper electronic state S3 is predissociative. In fact there are three upper states of importance, the initially excited S3, a state which dissociates to CS(X 1Σ) and S(1D) and a triplet state which dissociates to CS(X 1Σ) and S(3P). The CS fragments were rotationally excited with an average rotational energy ∼3.5 kcal/mole. The vibrational populations were also strongly inverted for both the 1D and the 3P dissociations and their surprisal plots were linear. CS fragments were found with v?7. Of the dissociations resulting in CS fragments with v?6, 80±10% of the S atoms are produced in the 1D state and 20±10% in the 3P state.
A
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o potential energy curves for the low-lying electronic states of N22+
Authors: Cobb M. , Moran T. F. , Borkman R. F. , Childs R.
Published: February 2003

Potential energy curves have been calculated for N2(X 1Σg+), N2+(X 2Σg+), and for the low-lying 3Σg−, 1Σg+, 3Πu, 1Πu and 3Πg states of N22+ using SCF techniques. Two different Gaussian basis sets were employed and N22+ curves were determined over a range of internuclear distance from 1.80 to 4.00 bohr. These curves were then used to calculate equilibrium internuclear distances, harmonic and anharmonic vibrational constants, and ionization potentials for the various N22+ states.
Effects of solvent on the excitation profile of the ν2 line of β-carotene
Authors: Hoskins L. C.
Published: February 2003

The excitation profile of the ν2 line of β-carotene has been examined as to its solvent dependency. Excitation profiles in acetone, cyclohexane, ethyl alcohol, and toluene show that the origin of the ν2 line in the resonance Raman spectrum is a vibronic coupling with the 0–0 and 2–0 virtual transitions of a single electronic state, presumably the excited π* state. The solvent shifts in the excitation profile correlate with the observed solvent shifts in the visible spectrum. It is shown that solvents can cause large perturbations to the intensity of the ν2 line.
Comments on a semigroup formalism for the description of phase and population relaxation processes
Authors: Kosloff Ronnie , Rice Stuart A.
Published: February 2003

Population and phase relaxation in two molecular models is studied. The analysis is based on recent work by Lindblad, in which the time evolution of an N-level quantum system is described using the coresponding generator of a completely positive semigroup. One of the models studied corresponds to a diatomic molecule in a dense medium. New results are obtained for the influence of two quantum transitions on population and phase relaxation. The other model studied corresponds to the case of exchange dephasing in a polyatomic molecule embedded in a dense medium. The formalism permits inclusion of coupling terms neglected by Harris, Shelby, and Cornelius; it is shown that inclusion of these terms leads to a rich range of line shapes, which information is not available from the approximation used by Harris et al.
Visible and UV multiphoton ionization and fragmentation of polyatomic molecules
Authors: Boesl U. , Neusser H. J. , Schlag E. W.
Published: February 2003

It is now known that tunable dye laser light leads to resonance enhanced photoionization of molecules in a mass spectrometer via multiphoton absorption. In some cases this process also leads to ion fragmentation. Time delayed tandem laser pulses of different wavelengths are employed to study the mechanism of this multiphoton ionization and fragmentation process in benzene. The experimental results strongly indicate that excitation does not proceed up an autoionization ladder of the neutral molecule, rather the multiphoton process is shown to be due to the climbing of two independent ladders, one up to the ionization potential in the neutral molecule, the second in the new species of molecular parent ions produced. Hence multiphoton ionization could also form a new basis of ion spectroscopy.
Experimental and theoretical shakeup studies. III. The 1s shakeup in CH4, NH3, and H2O
Authors: Creber D. K. , Tse J. S. , Bancroft G. M.
Published: February 2003

The core 1s level x-ray photoelectron spectra of CH4, NH3, and H2O have been recorded to high statistics. Several satellite peaks are resolved in each spectrum. The shakeup peaks are assigned with the aid of Xα–SW calculations. A comparison of the ten electron hydride MO diagrams with the neon atomic energy levels reveal the Rydberg character of the empty excited state orbitals. The shakeup transitions in the respective molecules can also be interpreted from atomic models constructed from the experimental shakeup spectrum of neon. The 2p contribution to the total relaxation energy is calculated from a spectral weight analysis of the shakeup spectra.
Crystal binding energies of rutile type compounds
Authors: Shanker Jai , Jain V. K. , Agrawal S. C.
Published: February 2003

An analysis of the crystal binding of fourteen rutile type compounds is performed in the present paper. The radii and electronic polarizabilities of ions in the crystalline state are calculated using Ruffa’s energy level analysis and an empirical relation between polarizability and radius. Using the electronic polarizabilities so obtained we have estimated the van der Waals dipole–dipole and dipole–quadrupole potentials following the Slater–Kirkwood variational formulation. These potentials are then used to calculate the cohesive energies of the rutile structure crystals under study by adopting an inverse power form as well as an exponential form for the short range repulsion. The results obtained in the present paper are discussed and compared with available experimental data and recent theoretical investigations.
Crossed molecular beams kinetics: BaO recoil velocity spectra from Ba+CO2 and O2
Authors: Parr Timothy P. , Freedman Andrew , Behrens Richard , Herm Ronald R.
Published: February 2003

Laboratory (LAB) angular and time-of-flight (TOF) recoil velocity spectra of BaO product produced by crossing a thermal Ba beam with a CO2 (for several collision energies) or O2 nozzle beam have been measured in a molecular beam apparatus equipped with an electron bombardment ionizer–quadrupole mass filter detector. Product center-of-mass recoil angle and energy distributions have been fit to the data by convoluting the c.m.→LAB transformation over measured speed distributions of both beams and the bandpass of the TOF analyzer. Both reactions produce symmetric product c.m. angular distributions which are indicative of the formation of long-lived collision intermediates.
ENDOR study of x-irradiated single crystals of N-acetylglycine—A reinvestigation
Authors: Chacko V. P. , McDowell C. A. , Singh B. C.
Published: February 2003

Electronic and molecular structure of the stable radical produced by room temperature x-irradiation of single crystals of N-acetylglycine has been established from detailed Electron Spin Resonance (ESR) and Electron Nuclear Double Resonance (ENDOR) studies. Our findings indicate that the radical structure and the nature of the wave function of the unpaired electron are significantly different from those reported in a previous ’’negative’’ ENDOR study. In agreement with earlier ESR studies, we find that the stable radical at room temperature is CH3CONHĊHCOOH. ENDOR lines due to the α-proton, three equivalent methyl protons, and six exchangeable protons have been identified and the hyperfine tensors determined. Spin density distribution in the radical has been compared with results of INDO–MO calculations. According to our findings the radical exists in four slightly different conformations. Isotopic substitution with 15N and deuteration of the exchangeable protons were helpful in the analysis of the complex ESR and ENDOR spectra. It is demonstrated that the radical concentration in the crystal has to be very carefully controlled for the success of an ENDOR investigation.
Configurational properties of aromatic polyesters with sulfur atoms in their structure
Authors: Riande E. , Guzmán J. , Román J. San
Published: February 2003

The dipole moments of poly (thiodiethylene glycol terephthalate) chains were determined as a function of temperature by means of dielectric constant measurements in dioxane. The experimental results were found to be in fair agreement with theoretical results based on a rotational isomeric state model in which the required conformational energies were obtained from previous configurational analysis on poly(ethylene terephthalate), poly(diethylene glycol terephthalate) and poly(thiodiethylene glycol). Since poly(thiodiethylene glycol terephthalate) can be considered an alternating copolymer of ethylene terephthalate and thioethylene units, its configuration-dependent properties were compared with those of poly(ethylene terephthalate) and poly(ethylene sulfide). It was found the flexibility of the copolymer, as expressed by the partition function, intermediate to that of its parent homopolymers. The theoretical results also indicate that the dimensions of poly(thiodiethylene glycol) are similar to those of poly(ethylene terephthalate) while its dipole moment ratio resembles that of poly(ethylene sulfide).
On the barrier for rotation about the C–C bond of the ethyl radical
Authors: Pacansky J. , Coufal H.
Published: February 2003

The observation of only one CH stretching for the almost completely deuterated ethyl radical HCD2CD2, leads to the conclusion that the CH bonds in methyl group of the ethyl radical are equivalent. This is consistent with free rotation about the C-C bond of the ethyl radical. (AIP)
Avoided-crossing molecular beam spectroscopy of SiH4 and GeH4
Authors: Itano Wayne M. , Ramsey Norman F.
Published: February 2003

SiH4 and GeH4 have been investigated by an avoided-crossing, molecular beam, rf resonance technique previously applied to CH4. In this method, the magnetic field is adjusted so as to create avoided crossings between (I=1, mI=1) levels and (I=0) levels in the (J=2) rotational state. rf transitions between these levels were observed. Values were obtained for the scalar and tensor spin–rotation constants ca and cd, for the tensor spin–spin coupling constant d, and for the rotational g factor gJ. The values for SiH4 are ca=4.13(10) kHz, cd=11.077(25) kHz, d=8.41(43) kHz, and gJ=−0.27131(34) and for GeH4 are ca=3.73(53) kHz [3.83(30) kHz], cd=9.493(58) kHz, d=7.2(25) kHz, and gJ=−0.1082(13) [−0.10851(47)]. The numbers in parentheses are estimates of 90% confidence limits. The values in brackets are based on a theoretical calculation of d in GeH4. The present results are compared with previous determinations of ca, cd, and gJ made by other molecular beam techniques and with theoretical calculations of d. and gJ made by other molecular beam techniques and with theoretical calculations of d.
Multiphoton dissociation of SF6 by a molecular beam method
Authors: Schulz P. A. , Sudbo/ Aa. S. , Grant E. R. , Shen Y. R. , Lee Y. T.
Published: February 2003

The dynamics of infrared multiphoton excitation and dissociation of SF6 has been investigated under collision-free conditions by a crossed laser–molecular beam method. In order to understand the excitation mechanism and to elucidate the requirements of laser intensity and energy fluence, a series of experiments have been carried out to measure the dissociation yield dependences on energy fluence, vibrational temperature of SF6, the pulse duration of the CO2 laser, and the frequency in both one and two laser experiments. Translational energy distributions of the primary dissociation product SF5, measured by time-of-flight and angular distributions and the dissociation lifetime of excited SF6 as inferred from the observation of secondary dissociation of SF5 into SF4 and F during the laser pulse suggest that the dynamics of dissociation of excited molecules is dominated by complete energy randomization and rapid intramolecular energy transfer and can be adequately described by RRKM theory. An improved phenomenological model including the initial intensity dependent excitation, a rate equation describing the absorption and stimulated emission of single photons, and the unimolecular dissociation of excited molecules is constructed based on available experimental results. Our studies show that although the energy fluence of the laser determines the dissociation yield of molecules in the quasicontinuum, the role played by the intensity of the laser in multiphoton dissociation is more significant than just that of overcoming the intensity dependent absorption in the lowest levels. Once molecules are excited beyond the dissociation energy, the average level of excitation of the dissociating molecules will be significantly influenced by the laser intensity for a given energy fluence when the rate of decomposition starts to compete with the rate of up-excitation.
Vibrational relaxation in jet-cooled alkyl benzenes. I.Absorption spectra
Authors: Hopkins J. B. , Powers D. E. , Smalley R. E.
Published: February 2003

Fluorescence excitation spectra of a series of alkylbenzenes cooled in a supersonic free jet have been obtained for the first 1000 cm−1 region of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. The series includes all n-alkylbenzenes up to n-hexyl together with isopropyl- and tert-butylbenzene. As with toluene, the spectra in this region for all alkylbenzenes is found to be dominated by vibrations of five ring modes: 6a, 6b, 1, 12, and 18a. Three of these—the ’’system modes’’ (6b, 12, 18a)—are found to be largely invariant to changes in the alkyl chain length and type— the ’’bath.’’ For n-alkylbenzenes with chain length of three or higher, spectra of two distinct conformations are observed with roughly equal intensity. These conformations are distinguished by configuration about the 1–2 carbon–carbon bond of the alkyl chain. When this configuration is trans [n-(t)-alkylbenzene], the alkyl chain extends away from the phenyl ring leaving the ring free to van der Waals complex binding on both sides of the ring. The He2–n-(t)-alkylbenzene van der Waals complex is thus observed. When the conformation about the 1–2 bond is gauche [n-(g)-alkylbenzene] the alkyl chain partially covers one side of the phenyl ring. This produces a small red-shift of the spectrum (a self-induced solvent shift) and prohibits the formation of the ring-centered dihelium van der Waals complex.
Energy level analysis of Np3+:LaCl3 and Np3+:LaBr3
Authors: Carnall W. T. , Crosswhite H. , Crosswhite H. M. , Hessler Jan P. , Edelstein N. , Conway J. G. , Shalimoff G. V. , Sarup R.
Published: February 2003

The polarized absorption and fluorescence spectra of 0.01–5% Np3+ doped into single-crystal LaCl3 were measured at moderate and high resolution in the range to 50 000 cm−1 at 298, 77, and 4 K. The 150 crystal field components identified were fit by a parametrized model which has previously been shown to provide an excellent account of the energy level structures observed for lanthanides doped into LaCl3. This constitutes the first successful evaluation of the crystal field interaction for an actinide ion in LaCl3 in which J mixing effects were explicitly included. The crystal field parameters for Np3+:LaCl3, B20=163, B40=−632, B60=−1625, and B66=1028, all in cm−1, were determined to be approximately twice as large as those for the analogous lanthanide, Pm3+:LaCl3. From analysis of Zeeman patterns in the fluorescence spectra, the ground state was found to be doubly degenerate, having crystal quantum numbers ±2 and a parallel splitting factor 0.17 LU (Lorentz unit).
MCSCF/CI ground state potential energy surface, dipole moment function, and gas phase vibrational frequencies for the nitrogen dioxide positive ion
Authors: Hopper Darrel G.
Published: February 2003

The ground state potential energy surface for the nitrogen dioxide positive ion, NO+2X 1Σ+g(Σ+,A1,A′), has been scanned with a correlated wave function to obtain directly, for the first time, the gas phase equilibrium geometry, force constants, vibrational frequencies, and dipole moment function. The wave function for this scan was constructed from a double-zeta plus polarization one-electron basis with a 12 configuration MCSCF determination of the orbital basis for a full valence 1Σ+g configuration interaction expansion. The calculated equilibrium bond length is 1.12 Å. The vibrational frequencies are computed to be ν1=1514, ν2=679, and ν3=2614 cm−1 The present ab initio results differ significantly from crystalline spectroscopic studies and are, thus, the best values available for the gas phase vibrational frequencies. The dipole moment function is nonzero at the Σ+, A1, and A′ geometries included in the potential surface scan, and is obtained here to provide for the future a priori calculation of the infrared band intensities.
The collision of two linear rotors: A scaling theoretical analysis of the H2–H2 and HF–HF systems
Authors: DePristo Andrew E. , Rabitz Herschel
Published: February 2003

The energy corrected sudden (ECS) scaling theory is applied to rotation–translation (R–T) and rotation–rotation (R–R) energy transfer in the H2–H2 and HF–HF systems. The ECS scaling predictions are in very good agreement with the exact quantal values. Two physically important results are particularly transparent in the scaling analysis: (1) The well-known effectiveness of ortho H2 in rotational relaxation can be traced directly to the higher tensor order interactions which are also operative in para H2 for j?2; and (2) transitions in which rotational quanta are conserved differ appreciably from those in which rotational quanta are not conserved. Scaling predictions, using different values for the average collision range lc, are compared to the exact quantal cross sections. The results indicate that essentially the same value for lc wil be determined by inspection of the dynamical solutions and by the scaling analysis. This point is of importance for future applications of the ECS scaling theory.
Spectroscopic B constants for Rydberg states of H3 and D3
Authors: Carney Grady D.
Published: February 2003

A new technique was used for deriving spectroscopic constants by reference to the known constants of a related electronic state. Applications of this method to several diatomic molecules revealed its usefulness and encouraged its use with the triatomic hydrogen molecules. Spectroscopic B0 constants for Rydberg states of H3 and D3 are obtained. Predicted and observed B0 values for the upper state of the 3d(A1′)→A2″(2p) band are 21.87 and 21.96 cm−1, respectively.
Molecular pair effects and the breakdown of Onsager’s dielectric theory
Authors: Berkowitz M. , Adelman S. A.
Published: February 2003

The source of breakdown of Onsager’s dielectric constant formula for simple polar fluids of high dielectric constant is identified as a purely electrostatic two cavity effect. An estimate of this effect yields a Kirkwood g factor of the order of the dipolar strength parameter 4π/3 ρu2β.
X-ray photoemission studies of atom implanted matrices: Ni in carbon
Authors: Gibbs R. A. , Winograd N. , Young V. Y.
Published: February 2003

Nickel, in low concentration, has been implanted in carbon foil. Both the Ni 2p3/2 and the L3M45M45 Auger levels are shifted 0.9 eV relative to nickel foil, while the valence band level is shifted by 1.2 eV. The characteristic energy loss peak associated with the core and valence levels of bulk nickel is observed on the core and valence levels of implanted nickel. An analysis is presented to show that the characteristic loss peak is consistent with 3d→4s or 4s→4p shake up processes. Model-based spectra are consistent with an initial state configuration... 3d94s1 for implanted nickel atoms.
Nonlinear laser spectroscopy in nitric oxide studied through VUV harmonic generation
Authors: Wallace Stephen C. , Innes K. K.
Published: February 2003

Two and three-photon spectra have been obtained by single and double resonance enhancements of the elements of the third-order nonlinear susceptibility (χ(3)), which describe four-wave sum mixing into the vacuum ultraviolet (VUV). Results are reported for nitric oxide, in which the A 2Σ+−X 2Π transition is utilized for two-photon resonant enhancement in χ3ω(3). Relative intensities of the recorded rotational structure agreed well with the predictions of theory based on multiphoton transition probabilities, with the exception of cases where there was accidental double resonant enhancement of χ(3), occurring through both two and three-photon transition probabilities. Finally, in studies designed to explore double resonance effects in χ(3)(2ω1+ω2), two lasers were used to probe simultaneous contributions from the A 2Σ+ and M 2Σ+ states of NO. Signals were further enhanced by a factor of 103 to values as large as 10−4 of the input dye laser intensity.
Low energy vibrational modes of carbon monoxide on iron
Authors: Kroeker R. M. , Hansma P. K. , Kaska W. C.
Published: February 2003

The energy range 300 to 600 cm−1 contains bending and metal–carbon stretching vibrations for carbon monoxide chemisorbed on transition metals. Tunneling spectroscopy reveals vibrations at 436, 519, and 569 cm−1 for carbon monoxide chemisorbed on alumina-supported iron particles. Measured vibrational mode shifts with the isotopes 13C16O and 12C18O suggest that the lower two are bending vibrations and that the highest one is an iron–carbon stretching vibration for the structure Fe=C=O. Comparisons with infrared data on supported iron particles, iron particles in oil, and iron carbonyls suggest the complementary nature of infrared and tunneling spectroscopy in deducing the nature of adsorbed species.
Structure and dynamics of associated molecular systems. IV. The orientationally disordered phase I of solid DCl
Authors: McDonald Ian R. , Bounds David G. , Klein Michael L.
Published: February 2003

Two recently proposed models for the intermolecular interaction between hydrogen chloride molecules have been used in a molecular dynamics simulation of the plastic crystal phase I of DCl. The simulated static structure factors are compared with neutron scattering data. Of the two models considered, only that which incorporates an explicit hydrogen bond interaction is able to account for the observed structure. For this model, examination of the deuteron motion reveals that rotational diffusion proceeds predominantly via 60 ° flips of the molecules. The dynamical structure factor has been calculated for selected values of the momentum transfer together with the power spectra associated with the autocorrelation functions of the center of mass and angular velocities. The results are discussed in the light of inelastic neutron scattering data.
Phase transition effects: A crystallographic characterization of the temperature dependency of the crystal structure of the 1:1 charge transfer complex between anthracene and tetracyanobenzene in the temperature range 297 to 119 K
Authors: Stezowski John J.
Published: February 2003

Crystal structures of the charge transfer complex between anthracene and tetracyanobenzene, A:TCNB, C14H10: C10H2N4, at three temperatures (297, 234 and 226 K) above the order–disorder phase transition at 206 K and at four temperatures (202, 170, 138, and 119 K) below it have been determined from x-ray diffraction data. The space group of the room temperature phase was assigned as Cm with a=9.528(2), b=12.779(3), c=7.441(2) Å and β=92.39(2) degrees at 297 K and for the low temperature phase is P21/a with a=9.457(1), b=12.689(2), c=7.325(1) Å and β=92.98(1)° at 119 K; Z=2 in both phases. The electron densities, plotted for both the donor and acceptor molecules at each temperature, indicate that there is dynamic librational disorder in the A molecules that gives rise to a single average orientation in the room temperature phase. Below the phase transition, two temperature dependent, symmetry related, orientations are observed; the libration diminishes slowly as a function of temperature in this phase. There are also modest reorientations, relative to the room temperature phase, in the TCNB molecules as the crystal is cooled. Crystal packing indicates that a network of intermolecular dipole–dipole interactions between the heteroatomic acceptor molecules accounts for their lack of libration comparable to that of the donors.
Enthalpies of sublimation and vapor pressures of 2,2′-bis-1,3-dithiole (TTF), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and TTF–TCNQ (1:1)
Authors: de Kruif C. G. , Govers H. A. J.
Published: February 2003

Torsion and mass loss effusion techniques are applied simultaneously to measure the saturation vapor pressures as a function of temperature. The enthalpies of sublimation are derived from the temperature dependence of vapor pressure. Overall mean values for both techniques are: TTF, ΔH0(344.91 K) = (95.3±1) kJ mol−1; TCNQ, ΔH0(431.74 K) = (126.1±1) kJ mol−1; TTF–TCNQ(1:1), ΔH0(410.65 K) = (130.0±2) kJ mol−1. The temperatures given are midrange values at which the saturation vapor pressure of each sample is 0.4 Pa. It is shown that the TTF–TCNQ sample evaporates stoichiometrically but not as a complex.
Time resolved molecular electronic energy transfer into a silver surface
Authors: Rossetti R. , Brus L. E.
Published: February 2003

Experimental techniques are described for preparation of three layer sandwich matrices on evaporated metal surfaces at 4 °K. These structures enable molecule–surface interactions to be studied as a function of distance to the surface. Classical electromagnetic energy transfer theory approximately agrees with experiment for the system pyrazine–silver at distances greater than about 125 Å. Below 125 Å, there is some evidence that either classical theory fails, or that surface structure causes slower energy transfer rates. Classical electromagnetic energy transfer theory is shown to imply qualitatively different analytical behavior at long distance (≳400 Å) than at short distances.
Reply to Bordewijk’s comment ‘‘On the relationship between the Kirkwood correlation factor and the dielectric permittivity’’
Authors: Stillinger Frank H.
Published: February 2003

The comments of Bordewijk’s1 on the relationship between the Kirkwood correlation factor and the dielectric permittivity are disputed.(AIP)
Volume and temperature dependence of the Raman spectrum in phase II of solid CH4
Authors: Medina F. D.
Published: February 2003

Two samples of solid CH4 of different densities were grown. Grüneisen parameters of 2.6 and 3.1 were obtained for the librational mode and one translational mode in phase II. The former indicates an anisotropic intermolecular potential with a strong volume dependence. We conclude that the II–III phase transition involves a sudden, simultaneous tetragonal distortion of the lattice and ordering of the orientationally disordered molecules of phase II, with a slow reorientation of the ordered molecules. The librational mode of phase II does not go soft near the transition to the disordered phase I. The temperature dependence of the linewidth for this mode can be explained by a thermally activated reorientation of the ordered molecules in phase II with an activation energy of 59 cm−1.
193 nm photodissociation of H2S: The SH internal energy distribution
Authors: Hawkins W. G. , Houston P. L.
Published: February 2003

The dynamics of H2S and D2S photodissociation at 193 nm have been investigated using an ArF excimer laser to photolyze the parent compound and a tunable, doubled dye laser to probe the energy content of the SH or DH radicals via the laser induced fluorescence technique. The SH radicals are produced with nearly Boltzmann rotational distributions characterized by temperatures of 375±15 and 220±15 °K for the SH(2Π3/2) and SH(2Π1/2) states, respectively. No vibrationally excited SH or SD fragments were observed. An upper limit on the SH(v=1)/SH(v=0) ratio of 0.005 was determined. The ratio of SH(2Π3/2) to SH(2Π1/2) was 3.75±0.20. Although roughly 20 000 cm−1 of energy is available to the fragments, the average internal energy of the SH fragments is only 320±20 cm−1. Thus, most of the available energy must appear in the relative recoil coordinate between the H and SH fragments. This observation is in good agreement with the conclusions of previous studies and with a quasidiatomic kinematic picture of the dissociation.
Single and double ionization of nitrogen dioxide by electron impact from threshold up to 180 eV
Authors: Stephan K. , Helm H. , Kim Y. B. , Seykora G. , Ramler J. , Grössl M. , Märk E. , Märk T. D.
Published: February 2003

Electron impact ionization of nitrogen dioxide has been studied as a function of electron energy up to 180 eV with a double focussing mass spectrometer Varian MAT CH5 and an improved Nier type electron impact ion source. We report the first observation of NO2++ produced via electron impact. Relative partial ionization cross section functions have been obtained for the processes NO2+e→NO2++2e and NO2+e→NO2+++3e. An accurate measurement of the cross section ratio q (NO2++/NO2) /q(NO2+/NO2) has been performed. In addition an estimate of the absolute partial cross sections has been made normalizing the measured relative cross section against the Ar+ cross section with the method of molecular effusive flow. Using the nth root extrapolation the following ionization potentials have been derived from the cross section functions near threshold: NO2+ = (10,4±0,3) eV and NO2++ = (35,0±0,5) eV.
Intramolecular nonradiative transitions of gaseous small molecules
Authors: Takahashi K.
Published: February 2003

A theory of intramolecular nonradiative transition is proposed for gaseous small molecules. Kubo’s cumulant expansion theorem, the decay theory of Goldberger and Watson, and the Mori theory are all included. In the latter, a line shape of rovibronic level is given as Lorenzian at the center and Gaussian-like at the extremities. The theory explains two experimental evidences, which are, first, the linearity of the Stern–Volmer equation, with its saturation at high pressure, second, the fact that the nonradiative rate is faster in a gas state than in condensed media.
A genealogical electronic coupling procedure incorporating the Hartree–Fock interacting space and suitable for degenerate point groups. Application to excited states of BH3
Authors: Swope W. C. , Schaefer H. F. , Yarkony D. R.
Published: February 2003

The use of Clebsch–Gordan-type coupling coefficients for finite point groups is applied to the problem of constructing symmetrized N-electron wave functions (configurations) for use by the Hartree–Fock SCF and CI methods of determining electronic wave functions for molecular systems. The configurations are eigenfunctions of electronic spin operators, and transform according to a particular irreducible representation of the relevant group of spatial operations which leave the Born–Oppenheimer Hamiltonian invariant. The method proposed for constructing the configurations involves a genealogical coupling procedure. It is particularly useful for studies of molecules which belong to a group which has multiply degenerate irreducible representations. The advantage of the method is that it results in configurations which are real linear combinations of determinants of real symmetry orbitals. This procedure for constructing configurations also allows for the identification of configurations which have no matrix element of the Hamiltonian with a reference configuration. It is therefore possible to construct a Hartree–Fock interacting space of configurations which can speed the convergence of a CI wave function. The coupling method is applied to a study of the ground and two excited electronic states of BH3 in its D3h geometry. The theoretical approach involved Hartree–Fock SCF calculations followed by single and double substitution CI calculations, both of which employed double-zeta plus polarization quality basis sets. The B–H bond length was found to be 1.194 Å for the ground state, 1.26 Å for a 1E″ state lying 6.08 eV above the ground state, and 1.24 Å for a 3E″ state lying 5.48 eV above the ground state. Energies were calculated only for D3h geometries of the molecule, so the excited states may not be stable with respect to C2v distortions. In fact, since both excited states are spatially degenerate, they are expected to exhibit Jahn–Teller distortion. Finally, the effect of including only Hartree–Fock interacting configurations rather than all single and double substitution configurations in the CI calculation is demonstrated to be energetically quite small.