bluuebunny/crossref_metadata_2025 huggingface dataset:
MSA and GMSA for charged hard spheres of different diameters: Comparison with RHNC and MC calculationsa)
Authors: Abramo Maria C. , Caccamo C. , Pizzimenti G.
Published: February 2003

The excess internal energies and the radial distribution functions of systems of charged hard spheres with two species of different diameters are calculated by using the mean spherical approximation (MSA) and the generalized mean spherical approximation (GMSA). The results obtained are compared with reference hypernetted chain (RHNC) and Monte Carlo (MC) calculations performed by other authors. The MSA estimates of the excess internal energies are often very close to the computer simulation values. The MSA radial distribution functions, on the whole, compare qualitatively well with the MC gij(r), being less satisfactory when the difference in the diameters of the particles is large. The GMSA appears to improve in a systematic manner over the MSA and, when the whole range of the physical situations investigated in this paper is considered, the agreement of this theory with the MC simulation can be considered very good. Similar remarks apply to the RHNC, so that it is possible to conclude that the GMSA and the RHNC, in the present case, are more or less equally accurate. Different possibilities both to improve upon some still unsatisfactory aspects of the GMSA, and to apply this approximation in the future in order to describe the physical properties of real ionic liquids, are also discussed.
Molecular beam photoionization study of HgCl2
Authors: Linn S. H. , Brom J. M. , Tzeng W.-B. , Ng C. Y.
Published: February 2003

High resolution (0.14 Å FWHM) photoionization efficiency (PIE) data for HgCl+2 have been obtained in the region 730–1100 Å using an oven-type supersonic beam source. The ionization energies of the spin-orbit states X̃ 2Π3/2g and 2Π1/2g were determined to be 11.380±0.003 and 11.505±0.003 eV, respectively. The major fragment ions observed in the region 600–1300 Å are HgCl+, Hg+, and Cl+ having appearance energies of ≤11.041, 12.594±0.018, and 16.721±0.032 eV, respectively. The latter value allows the calculation of the bond dissociation energy for HgCl to be 21.1±2.2 kcal/mol. This study further revealed members of autoionizing Rydberg series converging to the 2D±5/2 and 2D±3/2,±1/2 states at 16.715±0.003 and 18.65 eV, respectively, which exhibit highly asymmetric Beutler–Fano line profiles. The comparison of the PIE curve for Hg+ from Hg and those for HgCl+2 and its fragments unambiguously shows that autoionizing Rydberg series resolved in this study can be assigned to transitions [(5d)10σ2gσ2uπ4uπ4g] → [(5d)9σ2gσ2uπ4uπ4g 2D±5/2] np and [(5d)10σ2gσ2uπ4uπ4g] → [(5d)9σ2gσ2uπ4uπ4g 2D±3/2,±1/2] np, n≥6, which are mainly associated with the mercury atom in HgCl2.
Triplet correlation functions in the Lennard-Jones fluid: Tests against molecular dynamics simulations
Authors: McNeil William J. , Madden William G. , Haymet A. D. J. , Rice Stuart A.
Published: February 2003

A recent theory of Haymet, Rice, and Madden (HRM) for the pair and triplet correlation functions is tested at liquid state densities against new molecular dynamics results for the Lennard-Jones (12,6) fluid. The HRM integral equation, based on the Born–Green equation and a topological reduction of the diagrammatic expansion of the triplet correlation function, has been solved for a high temperature state (T*=2.74, ρ*=0.80) and is found to give triplet correlation functions in good agreement with the molecular dynamics results. For a lower-temperature state (T*=0.73, ρ*=0.85), where numerical difficulties have thus far frustrated attempts to obtain a self-consistent solution of the HRM integral equation, direct tests of the HRM closure are made using molecular dynamics pair correlation functions to evaluate the diagrams. Although some striking qualitative features of the triplet correlations are correctly described by the HRM closure for this low-temperature state, the HRM approach is not in quantitative agreement with the molecular dynamics results. Test calculations indicate that the principle source of these errors is the neglect of important higher-order diagrams for the triplet correlation function. A reorganization of the diagrammatic series is suggested which may identify the most important of these neglected diagrams. Additional computer simulation results are also reported for the purely repulsive Weeks–Chandler–Andersen (WCA) ‘‘reference’’ fluid and for the underlying hard sphere fluid. The similarity of the pair structures of these fluids, noted by WCA, is also found to hold with high accuracy for the triplet structures. It is suggested that these similarities may be exploited in applying the methods of HRM to the hard sphere fluid.
The mobilities of NO−3, NO−2, NO+, and Cl− in N2: A measure of inelastic energy loss
Authors: Viehland L. A. , Fahey D. W.
Published: February 2003

Ion mobility measurements were made for NO−2, NO−3, NO+, and Cl− ions drifting in N2. The measurements were made in a flow-drift tube apparatus at 300 K as a function of E/N. The results were compared to previous measurements made at low E/N as a function of temperature. The comparison was made at the same ion-neutral collision temperatures as calculated from the Wannier equation. The results agree at low collision temperatures (T∼300 K) but disagree at high collision temperatures (T≥400 K). This disagreement or ‘‘mobility difference’’ is attributed to an inelastic energy loss channel for the ion-neutral collisions in the flow-drift tube measurements. Using recently developed theory for ion-swarm phenomena, the differences in mobilities have been used to calculate the dimensionless ratios of inelastic energy loss to momentum transfer in the ion-neutral collision systems.
Rotational dynamics of the NH+4 tetrahedron in NH+4Cl−
Authors: Gerling Rainer W. , Hüller Alfred
Published: February 2003

The ammonium tetrahedron in NH+4Cl− experiences an orientational potential of considerable strength. Changes of orientation occur through thermally activated jumps across the potential barriers (rotational jumps). Whereas the experimentally determined overall jump rate is in general agreement with deductions from model potentials, the detailed jump geometry is not. The experimental ratio of jump rates ν3/ν4 around the threefold and fourfold symmetry axes of the molecule differs by roughly a factor 40 from its theoretical value. We have performed a detailed numerical study of the rotational Langevin equation in a theoretical model potential. Following the rotational path of an individual ammonium tetrahedron, it turns out that multiple jumps are crucial for the jump dynamics and that their inclusion can explain the experimental results. The probability of multiple jumps depends on the friction which couples a single particle to the many body system. The friction constant has been determined from the comparison with experiment.
Computer experiments on aqueous solution. I. Monte Carlo calculation on the hydration of methanol in an infinitely dilute aqueous solution with a new water–methanol pair potential
Authors: Okazaki Susumu , Nakanishi Koichiro , Touhara Hidekazu
Published: February 2003

Monte Carlo calculations have been carried out both for pure water and an infinitely dilute aqueous solution of methanol at 298.15 K and ordinary density by the Metropolis scheme in NVT ensemble. The total number of molecules is 216, one of which is methanol in the case of aqueous solution. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential is used, whereas a new pair potential is determined for water–methanol interaction from ab initio LCAO SCF MO calculations for more than 450 different dimeric configurations with a STO-3G basis set and subsequent multiparameter fitting of MO data thus obtained to an appropriate potential function with a nonlinear optimization method. For the aqueous methanol solution, 4 500 000 configurations have been generated and the final 1 500 000 are used to obtain average quantities for energy and various distribution functions. It is found that, with the introduction of one methanol molecule, the potential energy and structure of water tend to stabilize as a whole. Such stabilizations in energy and structure are found to be angular dependent, owing to anisotropic nature of methanol–water interactions. Calculations for several regions classified according to the direction from the methanol molecule have been attempted to examine the contribution of hydrophilic and hydrophobic regions to the total hydration structure. The hydrophilic region as a whole does not show a large energetic stabilization, though pair interaction distribution function clearly indicate a larger hydrogen bonding interaction for some water molecules in this region. In contrast to this, the hydrophobic region is characterized by larger overall energy stabilization as a result of the sum of a large number of smaller interactions. Strong hydrogen bondings in the hydrophilic region seem to act cooperatively with the structural stabilization in the hydrophobic region to form a fairly stable hydration structure around methanol. This is clearly shown by density distribution diagrams for water around methanol which visualize an iceberglike structure of water, remarkably in comparison with the structure in pure water.
Behavior of charge-transfer absorption upon passing through the neutral-ionic phase transition
Authors: Jacobsen C. S. , Torrance J. B.
Published: February 2003

The charge-transfer band is determined from reflectance measurements on single crystals of TTF-chloranil from 300 to 45 K, passing through the neutral-ionic phase transition at 84 K. As the temperature is decreased from 300 K toward the transition, hνCT decreases slowly from 0.66 to 0.55 eV, perhaps the lowest energy observed for a charge-transfer band. Below this transition, hνCT increases slightly and a second band appears at higher energy. From the measured oscillator strength, the temperature dependence of the degree of charge transfer has been determined. It is found to increase from ∼0.20 to ∼0.30 as the temperature is lowered from 300 to 100 K. Below TC, the degree of charge transfer rises to a value near 0.70.
Perturbation selection rules for multiphoton electronic spectroscopy of neutral alternant hydrocarbonsa)
Authors: Callis Patrik R. , Scott Thomas W. , Albrecht A. C.
Published: February 2003

A one-electron perturbation which is to significantly enhance a ‘‘forbidden’’ single-photon or multiphoton optical transition between any two states of an alternant hydrocarbon must satisfy pseudoparity selection rules in addition to the usual group theoretical selection rules. Pseudoparity predicts whether vibronic or inductive perturbations, e.g., will be successful, while group theory specifies the proper perturbation symmetries. A principal result is that for a given electronic transition, the roles of vibronic, and inductive perturbations are complementary. Their roles reverse depending on whether the transition requires an even or odd number of photons.
Temperature dependence of hydrodynamic dimensions of polystyrenes in cyclohexane by quasielastic light scattering
Authors: Novotny V. J.
Published: February 2003

The quasielastic light scattering measurements of diffusion coefficients of polystyrene in cyclohexane were performed between the lower critical solution temperatures and 100 °C at several molecular weights. At the theta point, the hydrodynamic radii Rh followed the dependence Rh(Å)=0.24 M0.5z for z-average molecular weights Mz above ∼105. Below the theta point, the hydrodynamic radii contracted less sharply with decreasing temperature than the radii of gyration, reflecting the fact that excluded volume effects are more important for polymer segments separated by large distances. Polymer expansion above the theta point did not reach asymptotic values even at 100 °C for Mz below ∼4×105. Blob theory accounts qualitatively but not quantitatively for polymer contraction and swelling. The hydrodynamic expansion can be related to the excluded volume parameter with satisfactory agreement with the excluded volume theories.
Exothermic reactions in TATB initiated by an electron beam
Authors: Stolovy A. , Jones E. C. , Aviles J. B. , Namenson A. I. , Fraser W. A.
Published: February 2003

Experiments were performed in which confined samples of TATB were initiated by uniform and rapid energy deposition via a penetrating electron beam. The temperature and pressure were obtained in the exothermic region with a resolution of about 2 ms. Information on reaction kinetics was obtained to very high temperatures (≳1000 °C) and pressures (≳ 3000 psi). The absence of thermal gradients greatly simplifies the analysis. Values were obtained for the thermal initiation threshold, the explosion temperature, and kinetic parameters for a consistent reaction below 380 °C. An observed endothermic reaction near 390 °C was shown to be due to an impurity. Non-Arrhenius behavior was observed at higher temperatures. A very fast rise in the reaction rate just above 400 °C is followed by a rapid fall off from extrapolated values at higher temperatures.
Infrared spectroscopic proof of the formation of the structure I hydrate of oxirane from annealed low-temperature condensate
Authors: Bertie John E. , Devlin J. Paul
Published: February 2003

The crystalline clathrate hydrate and deuterate of oxirane were formed by slowly condesning a gaseous mixture of water and oxirane onto a CsI plate at 80 °K. The crystallization was monitored by recording the infrared spectrum periodically. (AIP)
Quantum complex rotation and uniform semiclassical calculations of complex energy eigenvalues
Authors: Connor J. N. L. , Smith A. D.
Published: February 2003

Quantum and semiclassical calculations of complex energy eigenvalues have been carried out for an exponential potential of the form V0r2 exp(−r) and Lennard-Jones (12,6) potential. A straightforward method, based on the complex coordinate rotation technique, is described for the quantum calculation of complex eigenenergies. For singular potentials, the method involves an inward and outward integration of the radial Schrödinger equation, followed by matching of the logarithmic derivatives of the wave functions at an intermediate point. For regular potentials, the method is simpler, as only an inward integration is required. Attention is drawn to the World War II researches of Hartree and co-workers who anticipated later quantum mechanical work on the complex rotation method. Complex eigenenergies are also calculated from a uniform semiclassical three turning point quantization formula, which allows for the proximity of the outer pair of complex turning points. Limiting cases of this formula, which are valid for very narrow or very broad widths, are also used in the calculations. We obtain good agreement between the semiclassical and quantum results. For the Lennard-Jones (12,6) potential, we compare resonance energies and widths from the complex energy definition of a resonance with those obtained from the time delay definition.
The self-consistent cell theory applied to a nonuniform system: The surface of a harmonic crystal
Authors: Bonissent Alain
Published: February 2003

The self-consistent cell theory is applied to a harmonic crystal with a free surface. The integral equation for the effective potential exerted by a given cell on a neighboring molecule is solved analytically. This results in a system of equations for the coefficients of the effective interaction potential. The surface free energy of a xenon crystal is calculated by this method in the quasiharmonic approximation. The results are in fair agreement with the exact values obtained by lattice dynamics calculations.
Pressure broadening of CO infrared lines perturbed by H2 and He
Authors: Picard-Bersellini A. , Charneau R. , Brechignac Ph.
Published: February 2003

We present results of infrared linewidth measurements at low and room temperatures for CO diluted in H2 and He using a tunable diode laser in the 4.7 μm spectral region (CO fundamental band). Information on the intermolecular potential is obtained by comparison with results of dynamical calculations. In the case of CO–H2 pressure broadening (PB), data are compared to direct measurements of inelastic cross sections by infrared–infrared double resonance and to ab initio quantum calculations. In the case of CO–He, experimental PB coefficients are compared to theoretical cross sections derived from two ab initio potential energy surfaces.
ENDOR evidence for crystal imperfections: Case of N-acetylglycine
Authors: Khalaf Fawzi Z. , Miyagawa Ichiro
Published: February 2003

The formation of several very similar radicals in irradiated N-acetylglycine was shown to be associated with the crystal imperfections which were produced during growth, heating, and irradiation processes.
The generalized log-derivative method for inelastic and reactive collisionsa)
Authors: Mrugal/a F. , Secrest Don
Published: February 2003

A generalization of the log-derivative method is presented which is useful for both reactive and nonreactive scattering problems. In the coupled system of radial equations for this problem a first derivative term is included for complete generality. Thus, this method may be used when, as is often the case in reactive or curve crossing problems, the equations contain a first derivative term. When no first derivative term is present and no reactive channels are present, the method reduces to the standard log-derivative method. A reactive scattering problem is solved as an example.
Laser induced fluorescence and absorption measurements of NO in NH3/O2 and CH4/air flames
Authors: Chou Mau-Song , Dean Anthony M. , Stern David
Published: February 2003

Laser diagnostics have been used to probe NO in atmospheric pressure flames. Laser induced fluorescence techniques (LIF) were used to measure relative concentration profiles of NO at fuel equivalence ratios φ=1.28, 1.50, and 1.81 in NH3/O2/N2 flames and φ=1.7 and 1.8 in CH4/air/O2 flames. Laser absorption measurements were made to derive an absolute concentration of NO in a lean NH3/O2/N2 flame. This measured NO concentration agreed well with the calculated equilibrium concentration. The fluorescence signals from rich flames were then calibrated by comparing the fluorescence signals to that of the lean flame where absolute concentrations were derived. In rich NH3/O2/N2 flames NO concentrations decay more rapidly throughout the burnt gases than one would expect from the conventional mechanism of ammonia oxidation. This suggests that new reactions such as NH2+NH2 and NH+NH2 to ultimately yield N2 are important in these rich flames. LIF measurements on the CH4/air/O2 flames were able to resolve the growth and decay of ‘‘prompt NO’’ within the flame front. The LIF technique is estimated to have a sensitivity of better than 1 ppm for NO in these atmospheric pressure flames.
The dynamics of the reaction of 16O(1D)+D2 18O→16OD+18OD
Authors: Guillory William A. , Gericke K. H. , Comes F. J.
Published: February 2003

The detailed energy partitioning in the reaction of a metastable oxygen atom O(1D) with D2O proceeding to two OD molecules has been studied. In order to distinguish the product state distribution between the two chemically identical product molecules OD, the oxygen atom in the heavy water molecule was labeled isotopically. The use of spectroscopic methods allows a complete analysis of the products’ state distribution including such fine details as the distribution of the different Λ components and of the electron spin. The vibrational energy is almost exclusively channeled into the new (16OD) bond, whereas the original (18OD) bond is produced (≳90%) in the ground vibrational state. Both OD radicals show a broad rotational excitation and the rotational energy is equally partitioned among the two bonds. The energy distribution over the rovibrational levels strongly reflects the influence of coincident product molecules emerging from this chemical reaction. The reaction is very direct and must proceed on a time scale which does not allow for efficient energy transfer into all the available phase space.
The HCl–HCl interaction: From quantum mechanical calculations to properties of the liquid
Authors: Votava Chr. , Ahlrichs R. , Geiger A.
Published: February 2003

Analytical HCl–HCl pair potentials are derived from large quantum mechanical calculations at CEPA-(SD) level. The computed well depth for the dimer is 1.9 kcal/mol. Those pair potentials are used in molecular dynamics simulation studies at T=297 K and ρ=0.8354 g/cm3. The results for the static (e.g., pair distribution functions) and dynamic properties are compared with experimental and other molecular dynamics results. Comparison of the results from different simulation runs allows a check on their sensitivity with respect to the pair potential employed. It turns out that the potential yielding the best fit to the computed interaction energies gives a good representation of properties of liquid HCl. However, a readjustment of this potential—in order to account for well-known deficiencies of the quantum mechanical calculations—results in a slight improvement especially in the mean potential energy and the pressure. Finally, we used the computed pair potential to determine the structure and association energy of (HCl)n clusters with cyclic and chain structure.
A study of solid and liquid carbon tetrafluoride using the constant pressure molecular dynamics technique
Authors: Nosé Shuichi , Klein Michael L.
Published: February 2003

The constant pressure molecular dynamics technique originally proposed by Andersen to study fluids and subsequently generalized by Parrinello and Rahman to deal with crystals of arbitrary symmetry has been further extended to treat molecular systems. As a pedagogical example designed to illustrate the utility of this approach, we have investigated the properties of carbon tetrafluoride in its condensed phases using an intermolecular potential based upon atom–atom interactions. In particular, we have explored the effect of changes in temperature and pressure on the orientationally ordered low temperature monoclinic solid. As in the real crystal, isobaric heating to sufficiently high temperature causes the ordered phase to transform spontaneously to a noncubic orientationally disordered phase. The properties of this disordered phase are also examined along with those of the liquid. The atom–atom potential appears to correlate a wide range of experimental data. The possible role of the electrostatic octopole–octopole interactions is also discussed briefly.
Transient kinetics of chemical reactions with bounded diffusion perpendicular to the reaction coordinate: Intramolecular processes with slow conformational changes
Authors: Agmon Noam , Hopfield J. J.
Published: February 2003

Intramolecular reactions inside macromolecules (e.g., binding of ligands to iron inside heme proteins) may often be coupled to slow random fluctuations in the reaction center geometry. This motion is ‘‘perpendicular’’ to the reaction coordinate. It can be described as bounded diffusion in the presence of a binding potential field and an intramolecular rate constant which depends on the perpendicular degree of freedom. The diffusion equation is solved under the appropriate reflective boundary conditions. The transient decay of the total population is multiexponential (power law) for small diffusivity, changing to monoexponential kinetics for large diffusivity. For large times or large diffusivity, direct integration is very tedious, but an eigenvalue expansion converges rapidly. It also allows the calculation of the ‘‘average survival time’’ (an extension of the ‘‘first passage time’’) a natural candidate for replacing the reciprocal rate constant in multiexponential kinetics. An example is given for electron transfer between two loosely bound sites in a macromolecule. The average survival time shows a non-Kramers dependence on diffusivity, of the type found in the binding kinetics in heme proteins.
The adsorption and dissociation of methyl isocyanide on Rh(111)
Authors: Semancik Steve , Haller G. L. , Yates John T.
Published: February 2003

The coordination chemistry of methyl isocyanide (CH3NC), and CO are similar in a number of transition metal complexes, and it is therefore of interest to consider the surface chemistry of these molecules. We have used high resolution electron energy loss spectroscopy (EELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES) to characterize the interaction of CH3NC with Rh(111). At 120 K, EELS measurements indicate that CH3NC adsorbs nondissociatively as a bridged species. As the coverage is increased, vibrational loss features attributable to singly coordinated bonding occur, and multilayers of CH3NC eventually develop. In contrast, CO bonds initially to Rh in atop sites, with a bridged form occurring only at higher coverages. Thermally induced changes in the adsorbed CH3NC have also been monitored; TPD shows that H2, HCN, and N2 are liberated from the surface between 350 and 850 K, and EELS has been used to follow the vibrational mode changes that accompany decomposition. Additional insight was provided by adsorbing CH3NC onto surfaces predosed with O2 or CO, both of which produced some site blocking. Other interactional effects include CO and perhaps carbonate formation when CH3NC reacts with oxygen, and rapid removal of the intensity of the EELS modes associated with CO when CH3NC is adsorbed onto preadsorbed CO. Comparisons are made with previous results for CH3NC adsorption on Ni(111), Ag(311), and alumina-supported Rh clusters.
High energy elastic and inelastic electron scattering by the NH3 molecule-binding effect
Authors: Duguet A. , Lahmam-Bennani A. , Rouault M.
Published: February 2003

From the measurement of high energy elastic and inelastic electron scattering by the NH3 molecule we find that molecular elastic scattering calculation agrees with experiment, but the independent atom model for molecular inelastic scattering does not reproduce the experiment, even when electron correlations are taken into account. This means that the inelastic scattering by molecules is strongly affected by the rearrangement of the charge densities during the bond formation.
Finite concentration fluorescence quenching in the presence of diffusion
Authors: Baird J. K. , McCaskill J. S. , March N. H.
Published: February 2003

A fluorophore is converted from its ground state A to an electronically excited state A* by absorption of a photon hνex. A* decays to A by fluorescence, with a lifetime τ. It is quenched by collision with quencher Q at a diffusion controlled rate k[Q]. We suppose that A and Q are capable of reacting to form a complex AQ, even in the absence of the excitation. AQ forms at a diffusion controlled rate kf[Q] and decays at a rate kb. It is assumed that the quencher concentration is large enough so their depletion in the forward reaction of the final step can be neglected. The Stern–Volmer law is derived for the above four step chemical mechanism; the theory being given to all orders in the concentration of the quenchers. The results are compared with the concentration dependence proposed by Felderhof and Deutch. It is concluded that the predictions of the present work should be amenable to experimental test.
Absolute rate constants for the hydroxyl radical reactions with CH3SH and C2H5SH at room temperaturea)
Authors: Lee J. H. , Tang I. N.
Published: February 2003

Interests in the gas phase oxidation reactions of organic sulfur compounds such as thiols, sulfides, and disulfides have become increasingly important due to their potential role in the formation of tropospheric and stratospheric sulfur containing aerosols. The rate constants for the reaction of OH with CH3SH and C2H5SH have been determined under pseudo-first-order conditions, using the discharge flow-resonance fluorescence technique. The measured room temperature rate constants are (2.56±0.44)×10−11 and (3.67±0.18)×10−11 cm3 molecule−1 s−1 for CH3SH and C2H5SH, respectively. The CH3SH results are compared to those of previous studies, employing different experimental techniques.
Real-time kinetic study of the reaction F+NO2
Authors: Fasano D. M. , Nogar N. S.
Published: February 2003

The title reaction was studied in real time by observing infrared fluorescence resulting from the competitive reaction F+H2. With N2 as a collision partner, the limiting rate constants were found to be k0=9.8±1.6×10−31 cm6 molecule−2 s−1 and k∞=3.2±0.8×10−11 cm3 molecule−1 s−1, as determined by fitting the experimental data to Kassel, and modified Lindemann–Hinshelwood models. Measurments with argon showed it to be slightly less effective as stabilizing the reaction complex. Calculations indicate that both FNO2 and FONO are formed in the reaction, with a product ratio of FONO/FNO2≊7.
Collisional deactivation of vibrationally highly excited polyatomic molecules. III. Direct observations for substituted cycloheptatrienes
Authors: Hippler H. , Troe J. , Wendelken H. J.
Published: February 2003

Vibrationally highly excited ethyl-cycloheptatriene (41 600 cm−1) and isopropyl-cycloheptatriene (41 800 cm−1) molecules in the electronic ground state have been prepared by UV laser excitation inducing one-photon absorption and subsequent internal conversion. Collisional energy loss of these molecules has been followed directly by hot UV absorption spectroscopy. Average energies transferred per collision 〈ΔE〉 have been determined for a series of bath gases. 〈ΔE〉 values were found, for a given bath gas, to increase very slightly with the size of the excited molecule.
An RKR-like inversion procedure for bound–continuum transition intensities
Authors: Child M. S. , Essén Hanno , Le Roy Robert J.
Published: February 2003

A semiclassical RKR-like inversion procedure for determining a repulsive diatomic molecule potential energy curve from structured bound–continuum transition intensity data is derived and tested. The method presumes a knowledge of the (attractive) initial state potential well and of the energy and vibrational assignment of the absorbing or emitting level. Its application to a structured emission continuum of NaK observed by Breford and Engelke [Chem. Phys. Lett. 53, 282 (1979)] yields a potential energy curve which is incompatible with other known properties of this system, a result which confirms the suggestion of Kato and Noda [J. Chem. Phys. 73, 4940 (1980)] that the original assignment of the initial state for this spectrum is in error.
An electron bombardment procedure for generating cation and neutral radicals in solid neon matrices at 4 K: ESR study of 14N2+ and 15N2+
Authors: Knight Lon B. , Bostick J. M. , Woodward R. W. , Steadman J.
Published: February 2003

A new generation technique for matrix isolated cations and neutrals involving the electron bombardment of the matrix sample during deposition is presented. Use of the method for the study of ion-neutral reactions and other types of matrix experiments is discussed. Results for the gas phase electron bombardment of (CH3)2O are compared with electron bombardment of (CH3)2O conducted during neon deposition. The relative amounts of N and N2+ radicals trapped in neon matrices have been measured as a function of electron energies. An ESR analysis of 14N2+ and 15N2+ generated by electron bombardment in neon matrices at 4 K yields: giso=2.0004(2); Aiso(14N)=104.1(6) MHz and Aiso(15N)=146.1(6) MHz. The ESR results are used to estimate the extent of s-p hybridization of N2+ and a comparison with theoretical calculations and several other 13 electron diatomic radicals is made.
The microwave spectrum of the SiN(2Σ+) radical
Authors: Saito Shuji , Endo Yasuki , Hirota Eizi
Published: February 2003

The microwave absorption spectrum of the SiN radical has been detected in a dc glow discharge in a SiCl4 or SiH4/N2 mixture. The three rotational transitions N=2←1, 3←2, and 4←3, have been observed in the frequency region 87 to 175 GHz by using a source-frequency modulation microwave spectrometer. The rotational constant, the centrifugal distortion constant, the spin-rotation coupling constant, and the magnetic hyperfine coupling constants b and c and the electric quadrupole coupling constant of the nitrogen nucleus have been precisely determined. The electronic structure of the SiN molecule has been briefly discussed using the magnetic hyperfine coupling constants and also the quadrupole coupling constant [3.05(23) MHz] of 14N.
Observation of the 1 2Πu ← X 2Σg+ system in Cs2+
Authors: Helm H. , Cosby P. C. , Huestis David L.
Published: February 2003

Predissociation of vibrational levels in the 1 2Πu state of Cs2+ has been observed by monitoring Cs+ photofragments resulting from absorption of laser radiation by the X 2Σg+ ground state. Predissociation of the 1 2Πu state is induced by interaction with the repulsive 1 2Σu+ state and leads to the production of photofragments Cs++Cs in their ground electronic state with center-of-mass separation energies around 1 eV. The dominant features of the 1 2Πu ← X 2Σg+ band system are confined to the wavelength range between 7700 and 8100 Å. A lower limit for the bond energy of the 1 2Πu state, measured relative to its dissociation limit Cs+(1S0)+Cs(2P1/2) is found to be 0.39±0.1 eV. The spin-orbit splitting in the 1 2Πu state is measured to be 280 cm−1.
Laser-induced fluorescence spectrum of the CCN radical with an Ar+ laser
Authors: Hakuta Kohzo , Uehara Hiromichi
Published: February 2003

The laser-induced fluorescence (LIF) spectrum of the CCN radical was observed with an Ar+ laser. The laser line of 465.8 nm coincided with two rovibronic transitions: R1 (23.5) and sR21 (12.5) for A2Δi:(010)Φ−X 2Πr:(010)Δ. The LIF spectrum consisted of the resonant series: A2Δi:(010)Φ−X 2Πr: (v1v2v3)Δ, and the collision-induced bands: A2Δi:(010)Π−X 2Πr:(v11v3)Σ+, Σ−. The analysis yielded the vibrational and the vibronic parameters of the ground electronic state X 2Πr.
A structural interpretation of the vibrational spectra of binary fluorohafnate glasses
Authors: Almeida Rui M. , Mackenzie John D.
Published: February 2003

Binary glass samples have been prepared in the HfF4–BaF2 system, containing between 62–73 mol % HfF4. Their infrared absorption and polarized Raman spectra have been studied. The glass transition and crystallization temperatures have also been measured. The vibrational modes have been assigned and the results were compared with those previously obtained for ZrF4–BaF2 glasses. The cation isomorphic substitution of Zr by Hf allowed us to make a more definite vibrational assignment. The structures of the two glass systems were found to be similar, though not identical. The structural interpretation of the vibrational spectra was consistent with the occurrence of sixfold coordinated Hf atoms, probably forming a network of octahedral chains cross linked by Ba–F ionic bonds, with the degree of bridging increasing with the Hf content of the glasses. Substitution of Ba by Sr did not affect the structure. Due to the large F/Ba ratio of ∼10, there is an overlap of neighboring chains that may lead to sharing of a single nonbridging fluorine atom between two Hf atoms belonging to neighboring chains. The existence, on the average, of two such F atoms for each Hf may raise its effective coordination number from 6 to ∼8, when determined from an x-ray radial distribution function analysis, as recent results show. Vibrational spectroscopy, however, appears to be able to effectively distinguish which is the parent Hf atom to which every fluorine is bonded by a predominantly covalent, directional and shorter bond, as part of a highly symmetric octahedral coordination shell. In this respect, vibrational spectroscopy (particularly Raman) appears to be a more sensitive structural tool than x-ray diffraction.
Molecular g values, magnetic susceptibility anisotropies, and molecular quadrupole moments in 15N2–HF, 15N2–DF, OC–HF, OC–DF, and OC–HCl van der Waals complexes
Authors: Read W. G. , Campbell E. J.
Published: February 2003

The rotational spectrum of 15N2–DF, the rotational Stark effect in 15N2–HF, and the rotational Zeeman effect in OC–HF, OC–DF, 15N2–HF, 15N2–DF, and OC–HCl van der Waals complexes have been studied using pulsed Fourier transform microwave spectroscopy carried out in a Fabry–Perot cavity. The following spectroscopic constants have been obtained in this study: The Zeeman parameters are used to obtain the molecular quadrupole moments. The bulk magnetic susceptibility is estimated and the diamagnetic susceptibilities are calculated. The relationships between the Zeeman parameters measured for the complex and those of the free monomers are discussed. We observe that a simple zero order projection equation for g⊥ and χ∥–χ⊥ is reproducing the experimental values quite well.
Infrared spectrum of the CO2–HCl complex in solid argon at 12 K
Authors: Andrews Lester , Arlinghaus Robert T. , Johnson Gary L.
Published: February 2003

Co-condensation of CO2 and HCl in excess argon at 12 K produced the CO2–HCl hydrogen-bonded complex. Small shifts in the νs and ν3c modes, the low νl fundamental, and a very weak infrared Fermi doublet indicate a weakly bound complex. The more strongly bound CO2–HF and C 18O2–HF complexes gave more intense infrared Fermi doublets. The infrared intensity ratio ν3/ν1 is 200/1 for the CO2–HF complex demonstrating that the HF ligand induces finite electrical asymmetry in the CO2 submolecule.
Operator and algebraic methods for NMR spectroscopy. II. NMR projection operators and spin functions
Authors: Balasubramanian K.
Published: February 2003

We outline double coset and restricted character cycle index methods which generate the equivalence classes of NMR spin functions and the irreducible representations in the representation spanned by spin functions in any equivalence class. Elegant operator methods are developed by which the projection operators of NMR groups can be obtained in terms of the projection operators of composing groups. Thus, the projection operators of NMR groups of many molecules can be obtained without the knowledge of their character tables. Projection operators thus obtained are applied on the set of spin functions in an equivalence class of NMR spin functions (rather than the whole set of spin functions) which are generated by double coset methods to obtain symmetry-adapted NMR spin functions in the composite particle representation. Methods are illustrated with several examples of molecules containing as many as 230 NMR spin functions. The methods developed here are quite general so that they can be applied to NMR of nuclei with multispin states and they do not require the character tables of NMR groups. The techniques outlined here are illustrated with 2, 2, 3, 3-tetramethyl butane for which the symmetry-adapted NMR spin couplings are obtained in the composite particle representation. The NMR Hamiltonian matrix of this molecule which is of order 218×218 is factored into 26×26 matrix by the composite particle treatment. This is further factored into 8 1×1 matrices and 6 4×4, 4 2×2, and 3 8×8 matrices in the symmetry-adapted basis.
Structure of liquid hydrogen fluoride studied by infrared and Raman spectroscopy
Authors: Desbat Bernard , Huong Pham V.
Published: February 2003

Infrared and Raman spectra of hydrogen fluoride, deuterium fluoride, and their isotopic mixtures at various temperatures from 293 to 77 K have been recorded and analyzed in comparison with calculated data obtained from the classical G.F. matrix method applied to hypothetical structures chosen a priori. The best fit lead to the most probable structure of HF at 293 K which is a zig–zag chain with six or seven molecules having an angle of 42° with the average axis. At a lower temperature (190 K), just before solidification, the chains become longer, n=8 with an angle θ≂35 °. These structures fit well with the high dielectric constant of liquid hydrogen fluoride and lead to a calculated Kirkwood factor g1=3.65 at 293 K and g1=5.36 at 190 K which are in perfect agreement with known experimental values.
Surface enhanced Raman scattering (SERS) from pyridine on silver-UHV interfaces: Excitation spectra
Authors: Pockrand I. , Billmann J. , Otto A.
Published: February 2003

Excitation spectra of various surface enhanced Raman lines of pyridine adsorbed to evaporated silver films (1 L exposure) in UHV have been measured. Relative to ordinary scattering from thick pyridine layers on SERS inactive Ag films, every vibration exhibits a resonance profile which peaks at the same Stokes frequency corresponding to 2.0 eV independent of vibration. This value is shifted to the red after additional adsorption of 103 L of pyridine on the SERS active film. The ratio of corresponding Raman line intensities from ’’bulk’’ and ’’surface’’ pyridine varies differently for different vibrations with exciting frequency. The results are discussed with respect to various proposed enhancement mechanisms.
Effects of interlamellar forces on longitudinal acoustic modes of n-alkanes
Authors: Kobayashi Masamichi , Sakagami Kouichi , Tadokoro Hiroyuki
Published: February 2003

Effects of the interlamellar interactions in n-alkane crystals on the longitudinal acoustic modes (LAM), including both the fundamental and the higher overtones, were investigated on the basis of the polarized Raman spectra of n-C28H58, n-C30H62, n-C34H70, and n-C36H74 crystallized in the orthorhombic II form (a double layer polytype of the monoclinic form). The LAM-1 bands of this crystal modification split into a doublet consisting of the (cc) and (bc) polarization components separated by about 6 cm−1. The frequency gaps of the LAM-m bands were found to decrease with m (approximately proportional to m−1). Theoretical considerations on the correlation field splitting and the scattering intensities of the LAM bands led to the conclusion that the observed band splitting was caused by the intermolecular force acting between the successive monomolecular layers. The intrinsic value of the Young modulus of the n-alkane molecules free from the external forces in the crystalline state and the strength of the interlayer force were evaluated from the observed frequencies of the two components of LAM using a simple dynamical model. By comparing the Raman spectra of n-C28H58 and n-C30H62 between the triclinic and orthohombic II modifications, it was concluded that the effect of the lateral force on the LAM was not so significant as that of the longitudinal force.
Effect of the internal rotation of the OH group on the OH stretching vibrational Raman profile of ethanol in the gas phase
Authors: Schiel D. , Richter W.
Published: February 2003

A quantum mechanical calculation was performed to explain the temperature dependence of the OH stretching vibrational Raman profile of ethanol, which mainly consists in a pronounced increase in scattering intensity on the central region between the two band components caused by rotational isomerism. Introducing the interaction between the stretching vibrational and internal rotational motions of the OH group by means of the angular dependence of the OH stretching frequency, which can be directly taken from the gauche–trans splitting of the OH band in the experimental spectrum, leads to the result that the ‘‘fill-up’’ effect observed in the central region can be attributed to vibrational transitions between excited torsional states.
Adsorption and desorption and structural phase transition near T
c on Ni surfaces with atomic-beam scattering
Authors: Wilsch H. , Rieder K. H.
Published: February 2003

The adsorption and desorption of hydrogen, oxygen, carbon monoxide, and ethylene on Ni(110) and of hydrogen on Ni(100) were monitored with He-atomic beam scattering. The attenuation of the specular beam during gas adsorption initially shows a linear dependence on the adsorbate partial pressure and dose, which, when extrapolated, corresponds to a decrease of 100% for about one-half a Langmuir (1 L=1.33×10−4 Pa s). This can be related to total collision cross sections of the adsorbed particles in the range of 30 to 100 Å2, indicating dominance of the attractive interaction of the He atoms with adsorbates in dilute concentrations. Statistical occupation or depletion of sites in the adlayer structures suffice to explain the changes in coherent specular intensity upon adsorption and desorption processes. Near the Curie temperature of Ni (Tc=631 K), diffracted intensities of H2 and He show non-Debye–Waller behavior in their temperature dependence. Connections to changes in carbon surface concentration and to a (structural) phase transition (‘‘surface roughening’’) near Tc are discussed.
Phonon echo in organic solids
Authors: Swiatkiewicz J. , Talapatra G. B. , Kurland R. J. , Prasad Paras N.
Published: February 2003

We report the first observation of a phonon echo in an organic solid. The two-pulse echo and the three-pulse stimulated echo are observed for the d-tartaric acid powder at an acoustic frequency of 20 MHz, the acoustic oscillations being generated by application of the rf pulses. The observed echo behavior is found to be consistent with the previously proposed anharmonic-oscillator model of phonon echo in piezoelectric powders. (AIP)
Velocity field and velocity autocorrelation function in ionic liquids
Authors: Munakata Toyonori
Published: February 2003

A simple theory for the velocity autocorrelation function of two-component ionic liquids is developed with the use of a microscopic velocity field and a mode-coupling method. Our expression for the velocity autocorrelation function consists of contributions from coupling to both mass- and charge-current excitations, each with longitudinal and transverse components. The coupling strength to charge-current excitations is shown to be proportional to the mass ratio of the two ionic species. That is, e.g., cationic motion, the larger the mass ratio m−1/m1 is, the stronger the coupling becomes where m−1 and m1 denote masses of an anion and a cation, respectively. Application to the symmetric molten salt and molten sodium chloride manifests the significance of each contribution.
The contribution of hydrogen bonding to the structure of liquid methanol
Authors: Pettitt B. Montgomery , Rossky Peter J.
Published: February 2003

We present a study of the structure of liquid methanol using the recently proposed extended RISM integral equation and Jorgensen’s transferable intermolecular potentials (TIPS). Results are obtained as a function of molecular ‘‘polarity’’ by scaling the partial Coulombic charges located at molecular interaction sites; at full polarity we compare to computer simulation results for the same potential. At the experimental density it is found that packing forces are predominant in determining many features of the liquid structure, as reflected in site–site correlation functions. The influence of hydrogen bonding is best characterized as a narrowing in the molecular distributions, resulting from an accommodation of the attractive forces within structural alternatives consistent with packing constraints.