bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of the magnetic properties of Nd2Fe17 and Nd2Fe17Hx(x=3, 4.9) in high magnetic field
Authors: Isnard O. , Guillot M.
Published: July 2002

Through high magnetic field magnetization measurements performed in field up to 200 kOe, the magnetic properties of ferromagnetic compounds Nd2Fe17, Nd2Fe17H3 and Nd2Fe17H4.9 are investigated in the temperature range 4.2–300 K. The sample preparation and the interstitial element location in the unit cell are reminded. Powder samples embedded in a resin and then aligned at room temperature in a 10 kOe field are prepared in order to obtain oriented samples. Although the influence of the hydrogen insertion on the saturation magnetization is weak, the anisotropy constants are very sensitive to the hydrogen content and decrease strongly in the two hydrides. In Nd2Fe17H3 an abrupt field-induced transition is observed in a large temperature range. It shows that the Nd anisotropy is deeply modified by the hydrogen insertion.
Molecular beam epitaxial growth and structural properties of HgCdTe layers on CdTe(211)B/Si(211) substrates
Authors: Yang B. , Xin Y. , Rujirawat S. , Browning N. D. , Sivananthan S.
Published: July 2002

The nucleation and growth of Hg1−xCdxTe on CdTe(211)B/Si(211) substrates by molecular beam epitaxy are comprehensively studied by in situ reflection high energy electron diffraction and transmission electron microscopy. Microtwins are observed to be formed at the interface, but are overgrown later as the growth proceeds. It is shown that the three-dimensional growth at the nucleation stage of HgCdTe on CdTe(211)B/Si and CdZnTe(211)B is more likely due to the higher surface energy of Hg1−xCdxTe than that of CdTe(211)B and CdZnTe(211)B.
The distribution of conformational disorder in the high-temperature phases of the crystalline n-alkanes
Authors: Maroncelli M. , Strauss H. L. , Snyder R. G.
Published: July 2002

The distributions of conformational defects that exist in the high-temperature phase II (also referred to as the hexagonal or rotator phase) of the crystalline n-alkanes C21 and C29 have been measured by an infrared CD2-substitution technique and have been accounted for in terms of a lattice model that provides freedom for longitudinal displacement of the chains. The defects consist almost entirely of gtg′ kinks distributed nonuniformly along the chain. The uneven distribution is indicated in the variation in the concentration of gauche bonds measured at various sites along the chain. The highest concentration is at the chain ends, and the concentrations at interior sites decrease exponentially in going toward the middle. To explain the distribution we used a modification of a lattice model that had been successfully applied to the lipid bilayer. Comparison of observed distributions with those computed from the model indicates that the factors that determine the shape of the distribution are quite different in the n-alkane and bilayer cases. For the bilayer, the dominant factor is the variation in the lateral density of chains; for the n-alkane, the dominant factor is associated with longitudinal displacement of the chains.
Application of a diabatic distorted wave approximation to the study of X⋅ ⋅ ⋅H2 (X=He, Ne, Ar) van der Waals molecules
Authors: Roncero O. , Miret-Artés S. , Delgado-Barrio G. , Villarreal P.
Published: July 2002

A diabatic rotational decoupling scheme is applied to X⋅ ⋅ ⋅H2 van der Waals molecules using the anisotropic potentials of Tang and Toennies [J. Chem. Phys. 68, 5501 (1978); 74, 1148 (1981)]. We have developed the method proposed by Beswick and Requena [J. Chem. Phys. 72, 3018 (1980)] in an entirely numerical way. Attention is focused on the rotational predissociating levels and on the shape resonances of these systems. Among these resonances, a narrow orbiting and a narrow overbarrier resonances are found for the complexes Ne–H2 and Ar–H2, respectively. They should be amenable to experimental observation in molecular beam scattering as well as infrared spectroscopic studies.
Cooperative single-quantum excitations of a closed-ring polymer chain
Authors: Freedhoff Helen S.
Published: July 2002

In this paper, we study the collective single-quantum excitations of a ring of N identical molecular units, of arbitrary radius R relative to the radiation wavelength λ. We first calculate the single-quantum eigenstates of the system for N=2,...,6 and N large, their energies and their lifetimes due to interaction with the radiation field. For large N, we study the collective emission by the ring as a function of direction, comparing the pattern with that created by a collectively excited linear chain of molecular units. Finally, we calculate the emission spectrum of the ring for arbitrary initial excitation, and the spectrum of the radiation absorbed by the ring from an external electromagnetic field.
Energy relaxations in CS2 and in NO2–N2O4 vapors following pulsed laser excitation at 337.1 nm by probe beam deflection measurements
Authors: Sontag H. , Tam A. C. , Hess P.
Published: July 2002

The pulsed photothermal generation and probe-beam deflection technique was applied to monitor nonradiative relaxation and transport processes in CS2 and in NO2–N2O4 vapors following nitrogen laser excitation at 337.1 nm. A theoretical model is developed to describe the signal shape, and to derive the thermal diffusivity and energy relaxation time. The experiments were performed over a vapor pressure range of typically 1–100 Torr with a time resolution of 1 μs. In CS2 vapor of pressure above 10 Torr at room temperature, a slow heat release component is observed attributed to the photochemical formation of (CS)n particulates (‘‘laser-snow’’). In NO2–N2O4 vapor, the experimental data provide the thermal diffusivity values in the pressure range of 1–100 Torr; these measurements support a ‘‘frozen equilibrium’’ model and indicate that any contribution due to the diffusional transport of chemical enthalpy for this system is small in this pressure range at room temperature. These experiments demonstrate that the photothermal probe-beam deflection technique can provide useful information on heat-release rates, for example, due to particulate formation, as well as measurement of thermal diffusivity, for example, in hostile and dissociative gas systems.
A double many-body expansion of the two lowest-energy potential surfaces and nonadiabatic coupling for H3
Authors: Varandas Antonio J. C. , Brown Franklin B. , Mead C. Alden , Truhlar Donald G. , Blais Normand C.
Published: July 2002

We present a consistent analytic representation of the two lowest potential energy surfaces for H3 and their nonadiabatic coupling. The surfaces are fits to ab initio calculations published previously by Liu and Siegbahn and also to new ab initio calculations reported here. The analytic representations are especially designed to be valid in the vicinity of the conical intersection of the two lowest surfaces, at geometries important for the H+H2 reaction, and in the van der Waals regions.
Laser magnetic resonance in supersonic plasmas: The rotational spectrum of SH+
Authors: Hovde David C. , Saykally Richard J.
Published: July 2002

The rotational spectrum of v=0 and v=1 X 3Σ− SH+ was measured by laser magnetic resonance. Rotationally cold (Tr=30 K), vibrationally excited (Tv=3000 K) ions were generated in a corona excited supersonic expansion. The use of this source to identify ion signals is described. Improved molecular parameters were obtained; term values are presented from which astrophysically important transitions may be calculated. Accurate hyperfine parameters for both vibrational levels were determined and the vibrational dependence of the Fermi contact interaction was resolved. The hyperfine parameters agree well with recent many-body perturbation theory calculations.
Theory of electronic branching in the photodissociation of HI and DI
Authors: Levy I. , Shapiro M.
Published: July 2002

Potential curves, nonadiabatic couplings, and transition dipoles of the first four (3Π1, 1Π1,3Π0, and 3∑1) excited electronic states of HI are extracted from various photodissociation experiments of HI in the near UV. Excellent agreement with these experiments regarding the overall absorption spectrum, the I*(2P1/2)/I(2P3/2) branching ratio, and the β anisotropies of the two channels at various wavelengths, has been achieved. On the basis of the electronic parameters thus extracted, the full dynamics of the HI photodissociation at all excitation energies in the near UV have been unravelled. The data obtained for HI is used to predict the photodissociation dynamics of DI. These predictions are in good agreement with available experimental results. Subsequent work will deal with prediction of the resonance Raman scattering of HI/DI and the control of the I*(2P1/2)/I(2P3/2) branching ratio by elliptical control using the Asaro–Brumer–Shapiro scheme.
CO(v=1) population lifetimes of metal–carbonyl cluster compounds in dilute CHCl3 solution
Authors: Heilweil E. J. , Cavanagh R. R. , Stephenson J. C.
Published: July 2002

Tunable infrared picosecond pulses in the 5 μ region have been used for time-resolved pump–probe measurements of the population relaxation lifetime (T1) of CO(v=1) stretching vibrations in a series of metal–carbonyl cluster compounds in room temperature chloroform solution. T1 was the same for symmetric (ν=2084 cm−1, 90±10 ps) and antisymmetric (2014 cm−1, 87±10 ps) modes of the dicarbonyl Rh(CO)2(C5H7O2); T1 was the same for the B1 (2092 cm−1, 710±130 ps) and B2 (2036 cm−1, 750±90 ps) modes of Rh2(CO)4Cl2. Similarly long T1 times were found for Rh4(CO)12 (2075 cm−1, 610±65 ps) and Rh6(CO)16 (2077 cm−1, 700±100 ps). The molecule Co4(CO)12 has also been compared to the corresponding rhodium analog and it exhibits an initially fast relaxation of 47±5 ps followed by a slower 396±70 ps decay. The transient response of the more complex systems to the single frequency experiment is found to be sensitive to frequency and can exhibit bleaching, absorption, and a combination of these effects. Such behavior is attributed to overlap of the IR pulse with v=1 to v=2 and higher transitions in the M4(CO)12 (M=Rh or Co) and Rh6(CO)16 molecules. The long CO(v=1) T1 values for the metal cluster molecules suggest relaxation via multiquantum transfer of vibrational energy to adjacent M–C stretch and M–C–O bend vibrations; energy transfer to vibrational or electronic states of the central metal core seems unimportant in determining T1 for these systems.
Velocity-aligned Doppler spectroscopy
Authors: Xu Z. , Koplitz B. , Wittig C.
Published: July 2002

The technique of velocity-aligned Doppler spectrosocopy (VADS) is presented and discussed. For photolysis/probe experiments with pulsed initiation, VADS can yield Doppler profiles for nascent photofragments that allow detailed center-of-mass (c.m.) kinetic energy distributions to be extracted. When compared with traditional forms of Doppler spectroscopy, the improvement in kinetic energy resolution is dramatic. Changes in the measured profiles are a consequence of spatial discrimination (i.e., focused and overlapping photolysis and probe beams) and delayed observation. These factors result in the selective detection of species whose velocities are aligned with the wave vector of the probe radiation kpr, thus revealing the speed distribution along kpr rather than the distribution of nascent velocity components projected upon this direction. Mathematical details of the procedure used to model VADS are given, and experimental illustrations for HI, H2S, and NH3 photodissociation are presented. In these examples, pulsed photodissociation produces H atoms that are detected by sequential two-photon, two-frequency ionization via Lyman-α with a pulsed laser (121.6+364.7 nm), and measuring the Lyman-α Doppler profile as a function of probe delay reveals both internal and c.m. kinetic energy distributions for the photofragments. Strengths and weaknesses of VADS as a tool for investigating photofragmentation phenomena are also discussed.
Thermal kinetics in small systems
Authors: Klots Cornelius E.
Published: July 2002

Standard microcanonical rate-constant theory can be cast in a form which closely resembles and, in the case of a sufficiently large medium, reduces to that of the canonical ensemble. This relation permits a new understanding of the role of an activation energy in a chemical reaction. A facile methodology is also proposed for the extraction of thermodynamic data from kinetic studies of small media.
State-selective ionization of nitrogen in the X 2Σ+g
v+=0 and v+=1 states by two-color (1+ 1) photon excitation near threshold
Authors: Trickl T. , Cromwell E. F. , Lee Y. T. , Kung A. H.
Published: July 2002

Efficient generation of N+2X 2Σ+g(v+=0,1) is demonstrated by an extreme-ultraviolet +visible (1+1) photon excitation scheme. The c′4 1Σ+u (v′=0,1) Rydberg states are used as intermediates. The N2 molecules are ionized near the v+=0 and v+=1 ionization thresholds. The autoionizing ndδg 1Δg Rydberg series with the rotational quantum numbers N=4, 5, and 6 (in the limit of l uncoupling) are observed for the first time. Extrapolation of the N=4 series yields an ionization energy of N2 of 125 666.959(67) cm−1.
A density functional theory of polymer phase transitions and interfaces
Authors: McMullen William E. , Freed Karl F.
Published: July 2002

We employ density functional methods to derive the free energy and grand potential functionals appropriate to homopolymers and blends. The grand potential functionals are minimized by the single-monomer densities of the blends or homopolymers, and the nonideal portions of the free energies possess functional Taylor expansions whose coefficients are related to monomer–monomer direct correlation functions. In the limit that the polymerization indices become unity, the formalism reduces to that of atomic systems. By absorbing parts of the ideal free energy functionals into the nonideal contribution to the free energies, we demonstrate the formal equivalence of the theory of polymers to that of nonuniform atomic systems. The polymer formalism also reproduces the stability analyses of polymer phase transitions deduced by regarding dense polymeric liquids as atomic fluids. Nevertheless, for homogeneous polymeric fluids, the ideal free energy functionals become the well-known Flory–Huggins expressions for the entropy of mixing different homopolymers and the entropy of dissolving homopolymers in solvent. This suggests that numerical calculations, based on the correct ideal free energy functionals, will prove superior to calculations for polymers that derive from theories of atomic systems. We discuss extensions to block copolymers and mention numerous applications to polymer phase transitions and interfacial phenomena.
Molecular motion of n-alkane chains in urea inclusion adducts studied by analysis of Raman band profiles
Authors: Kobayashi Masamichi , Koizumi Hiroshi , Cho Yonhae
Published: July 2002

A quantitative analysis of the temperature dependence of the motional broadening of the Raman bands associated with the guest alkane molecules accommodated in the hexagonal phase of urea inclusion adducts of even-numbered n-alkanes of n–C14H30 through n–C22H46 was performed according to the site-hopping theory. The profiles of a certain polarization component of the following five bands; the CH2 antisymmetric stretch νa(CH2) (XY), the CH2 scissoring δ(CH2)(ZZ) and the CH2 twist t(CH2)(XZ), and the symmetric and antisymmetric CC stretches νs(CC)(ZZ) and νa(CC)(XZ)[Z∥c,X,Y⊥c], were found to be reproducible by a single Lorentzian function in the whole temperature range covering both the orthorhombic and hexagonal phases. Difference in the broadening behavior among the five Raman bands was interpreted quantitatively in terms of the equation derived previously by the authors. For each adduct, the value of the potential barrier height E* to the rotational motion of the alkane molecules was obtained in a good constancy from the temperature dependence of the half-width of the bands which belonged to different symmetry species and exhibited different broadening behavior. The value of E* for the C-16 adduct agreed well with the barrier height calculated on the basis of van der Waals intermolecular potential functions and the crystal structure. The E* value was found to increase monotonically with an increase in the chain length of the guest n-alkane molecule, except for an anomalous increase at C-20, as has been observed in the chain-length dependence of the transition temperature between the orthorhomic and hexagonal phases. The end-gauche content of the guest molecules in the hexagonal phase was evaluated as about 5 mol% irrespective of the chain length. For the alkane vibrations, bandshifts with variation in temperature were measured and analyzed according to the libration–torsion theory presented by Wood et al.
A
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o potentials and pressure second virial coefficients for CH4–H2O and CH4–H2S
Authors: Woon David E. , Zeng Peitao , Beck Donald R.
Published: July 2002

A b initio Hartree–Fock (HF) and second-order many-body perturbation theory (E2) were used to generate data for pair potentials of the mixtures CH4–H2S and CH4–H2O. The points were fit with site–site functions consisting of terms for interactions of the following types: exponential repulsion and R−6 and electrostatic attraction. The basis sets of Gaussian functions were developed by systematically adding polarization functions to maximize the zeroth-order dipole polarizability; the counterpoise method was used to correct basis set superposition error. The best well depths calculated for CH4–H2S and CH4–H2O were −0.0227 and −0.0215 eV, respectively (with no experimental results available for comparison). The potentials were evaluated by examining errors in the dipole polarizabilities and pressure second virial coefficients.
Third-order nonlinearity and two-photon-induced molecular dynamics: Femtosecond time-resolved transient absorption, Kerr gate, and degenerate four-wave mixing studies in poly (p-phenylene vinylene)/sol-gel silica film
Authors: Pang Yang , Samoc Marek , Prasad Paras N.
Published: July 2002

Femtosecond response and relaxation of the third-order optical nonlinearity in a newly developed poly (p-phenylene vinylene)/sol-gel silica composite are investigated by time-resolved forward wave degenerate four-wave mixing, Kerr gate, and transient absorption techniques using 60 fs pulses at 620 nm. Using a theoretical description of two- and four-wave mixing in optically nonlinear media, it is shown that the results obtained from simultaneous use of these techniques yield valuable information on the real and imaginary components of the third-order susceptibility. In the composite material investigated here, the imaginary component is derived from the presence of a two-photon resonance at the wavelength of 620 nm used for the present study. This two-photon resonance is observed as transient absorption of the probe beam induced by the presence of a strong pump beam. It also provides fifth-order nonlinear response both in transient absorption and in degenerate four-wave mixing. The fifth-order contributions are derived from the two-photon generated excited species which can absorb at the measurement wavelength and therefore modify both the absorption coefficient and the refractive index of the medium.
Properties of urea–water solvation calculated from a new a
b i
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o polarizable intermolecular potential
Authors: Åstrand P.-O. , Wallqvist A. , Karlström G. , Linse P.
Published: July 2002

In this work, we present a quantum chemical-based urea–water potential which includes many-body effects by using explicit polarizabilities, and mimics the dipole and quadrupole moments of the individual molecules. We compare this potential in detail with explicit ab initio calculations and with other potentials used in molecular simulations of urea–water systems. Several deficiencies in earlier work are pointed out, such as the quality of the basis set, the accuracy of the electrostatics, and polarization effects. In the minimum energy configuration of the urea–water dimer, the water molecule is engaged in two hydrogen bonds with urea, forming a cyclic structure, with an energy of −10.9 kcal/mol. In order to test the reliability of using polarizable molecular models in simulations of dipolar solutes in liquid water, a molecular dynamics simulation with one urea molecule and 210 water molecules was carried out. The hydration of urea is characterized by the ability of urea to fit into the water structure as a ‘‘waterlike’’ molecule. The dynamics of the system was investigated by studying diffusion and relaxation processes. The relative values of the diffusion coefficients for urea and bulk water are preserved, but as in a previous polarizable water simulation, their magnitudes are too small. The shell waters are more tightly bound to urea than bulk waters are to each other. Supported by previous simulations, we conclude that urea does not act as a water structure breaker and that this effect is unimportant in biological systems.
Predominant decay channel for superexcited organic molecules
Authors: Koizumi Hitoshi
Published: July 2002

Photoabsorption and photoionization cross sections of organic molecules are systematically compared for elucidating relation between superexcitation and ionization. The cross sections examined are of alkenes, alkanes, alcohols, and ethers in the energy range of about 2 eV above the first ionization potential. Although the photoabsorption cross sections are much different from one another, the photoionization cross sections are similar in each molecular group. This result indicates that ions are formed only through direct photoionization and most of superexcited molecules dissociate to neutral fragments. Ionization efficiency curves are calculated under the assumption of no autoionization, and they well reproduce the observed ionization curves, which mainly depend on energy difference between the first and second ionization potentials.
Influence of disorder on viscosity of undercooled melts
Authors: Avramov I.
Published: July 2002

The influence of disorder on shear viscosity of undercooled melts is described by the formula η=η0⋅exp(θ/T )a. It is shown that this expression reproduces the existing experimental data with a reasonable accuracy for a broad range of substances of various nature (silicates, borates, germanates, phosphates, Se, glucose, glycerin, As2Se3, etc.). Angell and Sichina [Ann. NY Acad. Sci. 279, 53 (1976)] distinguish between ‘‘strong’’ and ‘‘fragile’’ liquids on the basis of their heat capacity. The present model provides a physical ground for such classification. Also the isostructural viscosity of glasses is easily described. The present approach gives a natural explanation to the empirical rule of Bimen and Kauzmann.
π-bonded dimers, preferential pairing, and first-order desorption kinetics of hydrogen on Si(100)–(2×1)
Authors: D’Evelyn Mark P. , Yang Yuemei L. , Sutcu Leyla F.
Published: July 2002

Evidence is presented that π-bonding is present in the surface dimers on clean Si(100)–(2×1) and, as a consequence, that hydrogen atoms preferentially pair on surface dimer atoms even at low coverage. We propose a simple lattice gas model in order to calculate the equilibrium distribution of doubly- and singly-occupied dimers as a function of coverage and temperature, and show that even a very conservative estimate of the enthalpy difference between hydrogen on doubly- and singly-occupied dimers can explain the observed first-order desorption kinetics.
p-fluorotoluene. I. Methyl (CH3 and CD3) internal rotation in the S1 and S states
Authors: Zhao Zhong-Quan , Parmenter Charles S. , Moss David B. , Bradley Andrew J. , Knight Alan E. W. , Owens Kevin G.
Published: July 2002

Supersonic jet S1-S0 spectroscopy (resonance-enhanced multiphoton ionization, fluorescence excitation, and dispersed single vibronic level fluorescence) has been used to determine the S1 and S0 internal rotation energy level structure of p-fluorotoluene with a CD3 methyl rotor as well as to extend observations of the CH3 rotor structure. The observed rotor energy levels 2≤m≤8 for both species in both states are fit by a simple sixfold hindered rotor Hamiltonian for which the rotor inertial constants B and the internal rotation potential energy barriers V6 are evaluated. V6 may be obtained independently from B by observations of ΔE3, the observed splitting of the 3a″1 and 3a″2 rotor levels. Numerical solution of the wave equation shows that the perturbation theory relationship V6=−2ΔE3 holds well for any reasonable B value. Correspondingly, the B constant may be obtained from other level energies without appreciable sensitivity to (reasonably) assumed barrier heights. Earlier microwave and S1-S0 fluorescence results are combined with the present work to produce a set of preferred values for these constants. The values in cm−1 for the S0 state are B=5.46 (2.82) and V6=−4.77 (−4.77) for CH3 (CD3) rotors. The S1 values are B=4.90 (2.54) and V6=−33.0 (−25.2). The 20% barrier height reduction occurring on transformation from a CH3 to a CD3 rotor is similar to that observed in other systems. Calculation implies that the staggered conformer is the minimum energy configuration for both electronic states. Many of the S1-S0 rotor transitions are forbidden, and a discussion is given of induced intensity mechanisms that involve coupling of internal rotation to overall rotation or coupling of internal rotation to electronic motion. Substantial energy-level perturbations often occur for states with m≥5. A survey of B values and hindered rotation constants for 30 species with methyl rotors attached to aromatic rings reveals some general correlations.
Scattering delay times and transition rates for continuum resonance Raman scattering: Detailed derivations via the time-dependent approach and applications to 79Br2
Authors: Hartke B. , Kiefer W. , Kolba E. , Manz J. , Strempel J.
Published: July 2002

Scattering delay times between photon absorption and emission, and transition rates for continuum resonance Raman scattering are derived in a coherent and detailed fashion, within the time-dependent approach pioneered by Heller, Imre, and others. The resulting expressions for the Raman scattering delay times are related to similar expressions for particle scattering delay times derived by Eisenbud, Wigner, and Smith in terms of S matrices. These expressions are valid both for the ideal cases of sharp photon frequencies, and for realistic cases of finite frequency distributions or laser profiles. The Raman transition rates for this type of resonance scattering have a golden-rule-type expression implying the familiar selection rules and symmetry of two-photon transitions, similar to the Fermi–Pauli golden rule for single photon transitions. Applications to 79Br2 yield ultrashort Raman scattering delay times in the 10 fs domain.
Thermochemistry of CHn, SiHn (n=0–4), and the cations SiH+, SiH2+, and SiH3+: A converged quantum mechanical approach
Authors: Grev Roger S. , Schaefer Henry F.
Published: July 2002

We have determined 0 K heats of formation of CHn and SiHn (n=0–4) as well as the cations SiH+, SiH2+, and SiH3+ using large atomic natural orbital basis sets and coupled cluster methods including all single, double, and (perturbatively) triple excitations [CCSD(T)]. Core-correlation effects on the bond dissociation energies have been explicitly evaluated. For the intermediate hydrides CHn and SiHn (n=1–3), heats of formation are determined from theoretical bond dissociation energies in two ways: using experimental heats of formation of the H and C (or Si) atoms; and using experimental heats of formation of the H atom and the parent hydrides CH4 (or SiH4). In principle, this procedure allows us to place rigorous upper and lower bounds on the heats of formation of the intermediate hydrides. Because our theoretically predicted atomization energies are already of high quality, estimation of remaining deficiencies in the one-particle basis sets can be obtained from extrapolation of observed trends in atomization energies upon basis set expansion. These extrapolated results are in outstanding agreement with experimental values where they are known to high accuracy. For the SiHn compounds, a serious problem occurs: our predicted atomization energy of SiH4 is larger than that obtained from experimental heats of formation for the silicon atom and silane. Thus either relativistic effects on the atomization energy of SiH4 are large, or the experimental heats of formation of Si and SiH4 are incompatible. Excepting the atomization energy of SiH4, and thus the heats of formation of Si and SiH4, none of our other SiHn thermochemical predictions (properly interpreted) are clearly incompatible with experiment. Furthermore, our theoretical predictions are again in outstanding agreement with experimental determinations that are most certain.
Photoelectron spectroscopy of CN−, NCO−, and NCS−
Authors: Bradforth Stephen E. , Kim Eun Ha , Arnold Don W. , Neumark Daniel M.
Published: July 2002

The 266 nm photoelectron spectra of CN−, NCO−, and NCS− have been recorded with a pulsed time-of-flight photoelectron spectrometer. The photoelectron spectrum of CN− has also been recorded at 213 nm revealing transitions to the A 2Π state as well as the ground X 2Σ+ state of the CN radical. The following adiabatic electron affinities (EAs) are determined: EA(CN)=3.862±0.004 eV, EA(NCO)=3.609±0.005 eV, and EA(NCS)=3.537±0.005 eV. The adiabatic electron affinity of cyanide is in disagreement with the currently accepted literature value. Our measurement of the electron affinity of NCS confirms recent theoretical estimates that dispute the literature experimental value. By Franck–Condon analysis of the vibrational progressions observed in each spectrum, the change in bond lengths between anion and neutral are also determined. For NCO− this yields R0(C–N)=1.17±0.01 Å and R0(C–O)=1.26±0.01 Å, and for CN− the equilibrium bond length is found to be Re(C–N)=1.177±0.004 Å. The gas phase fundamental for CN− is determined for the first time: ν=2035±40 cm−1.
Hydrogen bonding between the water molecule and the hydroxyl radical (H2O⋅HO): The global minimum
Authors: Xie Yaoming , Schaefer Henry F.
Published: July 2002

Ab initio quantum mechanical self-consistent-field (SCF) and single and double excitation configuration interaction (CISD) methods have been used in a new study of hydrogen bonding between the water molecule and the hydroxyl radical. The basis sets used are double-zeta plus polarization (DZP) and triple-zeta plus double polarization (TZ2P). The two energetically low-lying minima are 2A′ and 2A″ states, with hydrogen bonding occurring between the oxygen atom in the water molecule and the hydrogen atom in the hydroxyl radical; the 2A′ state has a slightly (∼0.3 kcal/mol) lower energy. The comparable planar C2v symmetry structure lies <0.1 kcal/mol higher in energy. The hydrogen bond distance H...O of the 2A′ is ∼1.94 Å, which is close to that of the water dimer. The ground state dissociation energy is 5.6 kcal/mol, which is larger than that of the water dimer. The predicted infrared spectra for these structures are also reported.
Effect of intermolecular interactions on vibrational-energy transfer in the liquid phase
Authors: Bakker H. J.
Published: July 2002

We present an experimental and theoretical study of the relaxation after excitation of the C–H stretch vibration for dilute solutions of CHCl3, CHBr3, and CHI3 dissolved in different solvents and for the pure liquids CHCl3 and CHBr3. Experimentally, we study the relaxation with pump–probe experiments using picosecond infrared pulses and we found that the vibrational-energy transfer takes place via an intramolecular vibrational-relaxation process followed by an intermolecular energy transfer. Theoretically, we develop a new model for the description of intramolecular vibrational energy transfer. In this model the important effects of the intermolecular interactions on the rate of intramolecular energy transfer are described. From the comparison between experiment and theory the relevant parameters for intramolecular vibrational-energy transfer can be identified.
Coherence effects between intramolecular vibrational relaxation and dissociation in triatomic van der Waals systems
Authors: Roncero Octavio , Villarreal Pablo , Delgado-Barrio Gerardo , Halberstadt Nadine , Janda Kenneth C.
Published: July 2002

Time-independent [Halberstadt and co-workers, J. Chem. Phys. 96, 2404 (1992); 97, 341 (1992)] and time-dependent quantum mechanical calculations that describe the intramolecular vibrational relaxation (IVR) of Ar...Cl2 are used to develop analytical models for this process. It is shown that time-resolved experiments should reveal an oscillatory dissociation rate. It is found that the oscillations will be different for different rotational levels, and may tend to wash out if insufficient state selection is achieved in the initial excitation step. This may explain why no such oscillations were observed for Ar...I2. It is also predicted that the observed product state rotational distribution will change with the initially excited rotational state.
Slow motion electron-spin resonance line shapes of spin-labeled molecules undergoing restricted rotational diffusion in a cone: Applications to lysolecithin/water systems
Authors: Liang Zhichun , Westlund Per-Olof
Published: July 2002

The restricted diffusion-in-a-cone (DC) model is formulated within the context of a slow-motional electron-spin resonance (ESR) line shape to describe the local reorientation of amphiphile molecules in lipid bilayers. A special program is designed to reduce the computation amount, and the computational time for the DC model is comparable to that for the familiar Maier–Saupe (MS) potential model. Special emphasis is placed on comparing the two potential models in ESR line shape calculations. Illustrative analyses of ESR spectra are presented for some selected bilayers. It is found that the DC model is particularly suitable for describing the local dynamic states of single chain systems. The simulation results show that while the two models give similar values for the reorientational rates, the DC model predicts lower local molecular orders.
Infrared external reflection spectroscopic studies of phase transitions in Langmuir monolayers of stearyl alcohol
Authors: Buontempo Joseph T. , Rice Stuart A.
Published: July 2002

There is a steadily growing body of evidence that much of the information available concerning the properties of Langmuir monolayers concerns systems which are not in a state of equilibrium. What are now known to be unrelaxed monolayers of stearyl alcohol exhibit a high surface pressure phase transition whose signature in the isotherm changes from a ‘‘kink’’ (0–8 °C), to a small nearly flat region (8–15 °C), and finally to another kink (at higher temperatures). We have carried out external reflection infrared spectroscopic studies of relaxed monolayers of stearyl alcohol along a representative isotherm from each of the mentioned temperature regions. The results of our studies indicate that in each region the surfactant hydrocarbon chain becomes more ordered (i.e., has fewer gauche conformations) as the surface pressure is increased. We find that (i) at 5 °C, throughout the surface pressure range where the change in intramolecular chain order occurs, the collective tilt of the hydrocarbon chains remains nearly constant. For this isotherm the kink signals the point at which the hydrocarbon chains have achieved a very high degree of intramolecular order, perhaps the all-trans limit. (ii) In the 11 °C monolayer, in the phase often referred to as ‘‘superfluid’’, the intramolecular disorder in the hydrocarbon chains is measurably greater than in the equivalent phases in the higher and lower temperature regions. We also show that in the relaxed monolayer the nearly flat portion of the isotherm observed in the unrelaxed monolayer is almost totally absent, leaving only a kink very similar to those observed at higher and lower temperatures. (iii) At 25 °C, although the hydrocarbon chains in the relaxed monolayer attain a relatively high degree of intramolecular order, the high surface pressure phase transition is not observed. Instead, the surface pressure levels off at a value below that at which the transition is expected, and below the equilibrium spreading pressure. We suggest that the leveling off at the surface pressure signals that molecules from the monolayer are forced into solution. (iv) At low surface pressure the spectroscopic data imply that the monolayer consists of a heterogeneous array of ordered islands.
An assessment of the role of dipoles on the density-of-states function of disordered molecular solids
Authors: Dieckmann A. , Bässler H. , Borsenberger P. M.
Published: July 2002

The effect of permanent dipoles on the density-of-states (DOS) function of an array of hopping sites is examined by summing the contributions of individual dipoles to the total potential at a given site. For relative dipolar site concentrations ≳0.1, the DOS function is of Gaussian shape. At lower concentrations, it approaches a Lorentzian. Good agreement with literature data on the dipolar contribution to the DOS function for hole transport in molecularly doped polymers and molecular glasses differing with regard to dipole moment and concentration of dipolar moieties is found.
Equations of state for hard-sphere chain fluids
Authors: Mitlin Vladimir S. , Sanchez Isaac C.
Published: July 2002

A method of modeling thermodynamic properties of hard-sphere chain fluids is considered. The method is based on the Baxter’s results for the Percus–Yevick theory. We discuss the recent reformulation of Baxter’s results given by Chiew for athermal linear chain molecules and show that it fails as soon as one considers more complicated molecules, in particular, the proper transition to the thermodynamic limit is not always possible. An improved formulation is given which enables us to obtain previous results and not yield divergencies. Examples of the equations of state are given. It is shown that the equation of state for branched chains is the same as for linear chains. For cyclic chain molecules, the equation of state is found to be independent of chain length and polydispersity. Experimental data obtained by S. J. Clarson in 1985 confirms this surprising result. Limitations of the method are discussed; it is shown that it fails in the low-density limit as soon as the molecules with cyclic fragments are considered.
Photoassociation of Kr–F collision pairs in the ultraviolet
Authors: Schloss J. H. , Jones R. B. , Eden J. G.
Published: July 2002

The bound←free absorption (photoassociation) of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs has been studied in the ultraviolet in Kr/F2 gas mixtures by laser pump–probe techniques. Experiments carried out at 248 nm with two time-delayed excimer laser pulses demonstrate that KrF(B2 Σ1/2+) molecules are produced by the photodissociation of F2, followed by the photoassociation of Kr–F collision pairs by a single ∼5 eV photon. The saturation behavior of this photoassociation process indicates that the currently accepted value for the KrF(B→X) stimulated emission cross section (2.6×10−16 cm2) is too large by at least a factor of 2. A revised value for σSE of (1.2±0.6)×10−16 cm2 is suggested. By indirectly monitoring the temporal decay of the atomic fluorine number density owing to reformation of F2, the rate constant for the recombination of F atoms in a Kr background by three-body (2F+Kr) collisions has been determined to be kr=2×10−32 cm6 s−1.
Terminal rovibrational distribution of Na2 in a sodium supersonic beam
Authors: Zalicki P. , Billy N. , Gouédard G. , Vigué J.
Published: July 2002

We have measured the terminal rovibrational distribution of Na2 in a molecular beam produced by a supersonic expansion of pure sodium at source pressure × nozzle diameter products p0d=0.02 and 0.32 Torr cm. The rovibrational distribution was determined by the laser-induced fluorescence technique and covers the 550–1200 and 550–2700 cm−1 internal energy ranges, respectively. The highest vibrational state for which molecules were detected is v″=15. For the lower stagnation pressure, the internal energy distribution is well relaxed and can be described by a temperature of about 350–400 K. For the p0d=0.32 Torr cm experiment, the distribution is nonthermal with a large population excess for the levels with a high internal energy. The local temperature exceeds 460 K at 2000 cm−1. A striking result is that the rovibrational distribution depends almost exclusively on the internal energy (i.e., it does not depend much on the partitioning of this energy between vibration and rotation). This new result does not seem to be in contradiction with previous experiments which tested only a small set of levels. We discuss the mechanisms of the rovibrational relaxation during the sodium beam expansion. Recent experiments have shown that this relaxation is a very efficient process and we believe that it can explain our observations. We attribute the population excess of the rovibrational levels with a high internal energy to the process of dimerization in the expansion. Finally, a nonstatistical behavior of the Na2 ortho/para population was observed for the vibrational levels v″=10–15. Such an effect for hydrogen molecules formed by three-body recombination has been predicted by Roberts within the resonance complex theory of this process [R. E. Roberts, J. Chem. Phys. 54, 1423 (1971)].
Short range structure of B2O3–Cs2O glasses analyzed by x-ray diffraction and Raman spectroscopy
Authors: Iwadate Yasuhiko , Igarashi Kazuo , Hattori Takeo , Nishiyama Shin , Fukushima Kazuko , Mochinaga Junichi , Igawa Naoki , Ohno Hideo
Published: July 2002

Structural analyses of pure B2O3, B2O3(90 mol%)–Cs2O (10 mol%) and B2O3 (80 mol%)–Cs2O (20 mol%) glasses were made by an x-ray diffraction method and the effects of Cs2O addition were investigated on the short range structure of the glasses. Furthermore, Raman spectroscopic studies were applied to analyze the interatomic interactions between boron and oxygen atoms. Due to x-ray diffraction, pure B2O3 was confirmed to have BO3 triangles which formed the so-called boroxol ring structure. When Cs2O was added to B2O3, the boroxol ring structure still remained but some parts of BO3 triangles was definitely converted to BO4 tetrahedra. This result was also supported by Raman spectroscopic observations. The other important consequences were that a Cs atom was surrounded by six O atoms, and four Cs–O interatomic distances were about 3 Å but two pairs 3.5 Å, indicating that the distorted Cs–O octahedra might exist in the B2O3–Cs2O glasses. This spatial arrangement of Cs and O atoms was quite different from the corresponding crystal structure in the standpoint of the enormous decrease in coordination number (the number of O atoms around a Cs atom).
Secondary ions produced from gaseous and frozen H2O under energetic (MeV/amu) Ar ion impact
Authors: Matsuo T. , Tonuma T. , Kumagai H. , Shibata H. , Tawara H.
Published: July 2002

Mass/charge spectra of secondary ions produced under energetic (∼MeV/amu) Ar ion impact on gaseous and frozen H2O targets are measured using a magnet-type mass spectrometer. The ion spectra are found to be quite different between these target phases. Multiply charged Oi+ ions with charge states i=2–6, as well as fragmented H+, O+, and OH+ ions, are clearly observed in the gas target, whereas the production of multiply charged ions is strongly suppressed in the frozen target. The most intense ion species is the parent H2O+ ion in the gas target; the H3O+ ion in the frozen target. The most outstanding feature in the frozen target is that the production of cluster ions of the type (H2O)nH+ (n=1–31), and their intensities, decreasing as the cluster size n increases, show anomalies between n=4 and n=5 and also around n=20, 21, and 22. Negatively charged cluster ions with formula (H2O)nO− and (H2O)nOH− are also efficiently produced in the frozen target.
Inclusion of the geometric phase in quantum reactive scattering calculations: A variational formulation
Authors: Wu Xudong , Wyatt Robert E. , D’Mello Michael
Published: July 2002

We present a method for including the geometric phase in quantum reactive scattering computations based on the log derivative version of the Kohn variational principle. A new variational functional is developed which includes the influence of the geometric phase through modifications in the momentum operators. The system investigated is a two-dimensional reactive scattering model which includes the vector potential induced by the magnetic field of an infinitely long solenoid. The coordinates used in this model are analogous to Jacobi coordinates used in atom–diatom systems. Some interesting features of this study include the gauge invariance of the scattering probabilities, symmetry adaptation of the wave function, and the behavior of the probability density in the presence of the geometric phase.
Numerical study of the phase diagram of a mixture of spherical and rodlike colloids
Authors: Bolhuis Peter , Frenkel Daan
Published: July 2002

Fluid–fluid phase separation in a simple model for a colloid dispersed in a solution of stiff rodlike polymers is studied by means of Gibbs ensemble Monte Carlo simulations. The fluid–solid transition for this model is computed using first-order perturbation theory. The phase diagrams clearly show that a stable critical point is not possible for polymer lengths smaller than 30% of the colloidal particle diameter. At very large rod length, the fluid coexistence curve can be derived directly from the hard-sphere equation of state.
Site-dependent vibrational coupling of CO adsorbates on well-defined step and terrace sites of monocrystalline platinum: Mixed-isotope studies at Pt(335) and Pt(111) in the aqueous electrochemical environment
Authors: Kim Chung S. , Tornquist Wade J. , Korzeniewski Carol
Published: July 2002

Infrared spectroscopy is applied to probe qualitative structural features of the adlayers formed by CO at step sites and on terrace planes of Pt(335){Pt(S)-[4(111)×(100)]} in the aqueous electrochemical environment. The C–O stretching vibrational features are reported for adlayers formed from 12CO/13CO isotopic mixtures over a wide range of CO surface coverages. At saturation, the predominant spectral features are associated with the vibrational modes of terrace-CO in terminal (atop) coordination environments. The position of the 12CO and 13CO spectral features and their relative intensity are examined for several 12CO/13CO fractions, and they are shown to display the characteristics of a strongly coupled system.

In comparison with corresponding mixed isotope spectra for CO at Pt(111) electrodes, intermolecular coupling for terrace-CO on the (111) surface planes of Pt(335) is observed to be significantly stronger, reflecting the higher CO surface coverages on the edge sites and the terrace sites of the Pt(335) surface plane. At low coverages, spectral features associated with edge-CO are discerned, and the intermolecular coupling for atop CO is weaker than for corresponding coverages of CO at Pt(111). The weak coupling at low coverages is attributed to the exclusive CO occupation at the step edges, which confines the intermolecular coupling to one dimension, in the direction along the step edges. For all coverages, values are determined for the dynamic dipole–dipole coupling parameter (Δνd) and the chemical (static–dipole) shift parameter (Δνs). Values for Δνs are generally small at all coverages. Values for Δνd are small (<8 cm−1) at low coverages, where CO forms one-dimensional structures along the step edges, and they increase to large values (∼42 cm−1) at coverages that coincide with the growth of two-dimensional structures on the terrace planes. The majority of measurements were made for the Pt(335) electrode at potentials in the classical double-layer region, although dipole coupling parameters are also reported for Pt(335)/CO at potentials in the hydrogen adsorption region, where Δνd approaches zero at low coverages.