bluuebunny/crossref_metadata_2025 huggingface dataset:
The interaction of CO and Pt(100). II. Energetic and kinetic parameters
Authors: Thiel P. A. , Behm R. J. , Norton P. R. , Ertl G.
Published: February 2003

We have investigated the mechanism and driving force of the CO-induced phase transition of the Pt(100) surface. In a preceding paper, we have concentrated on the mechanism by which CO removes the surface reconstruction. As discussed, the clean reconstructed (hex) surface of Pt(100) is more stable than the unreconstructed (1×1) phase, but if the CO coverage on hex exceeds a small critical value (θ≳0.05) nucleation of (1×1) patches occurs and proceeds until, at θ=0.5, the entire surface has been converted to the (1×1) phase. In this paper we present data which quantitatively describe the energetics of this system. Values for the heats of adsorption of CO on both the reconstructed and unreconstructed phases, as well as values for the preexponential factors for the desorption rates, are determined from quasiequilibrium LEED measurements at different coverages. The difference in the low coverage heats of adsorption of CO on the hex and (1×1) phases (27.5 vs 37.5 kcal/mol) is the driving force in the Pt phase transformation during adsorption. On the other hand, during desorption, the hex phase does not return until the coverage has decreased below 0.3. This measurement allows an estimation of the difference in stability between the clean (1×1) and hex phases: about 9–13 kcal per mole of Pt atoms. The transition between the two phases which are in equilibrium [COgas+hex-Pt(100) vs COads+(1×1)-Pt(100)] shows a hysteresis due to kinetic limitations. These kinetic effects are characteristic of a nucleation process involving a critical coverage.
Wigner method studies of ozone photodissociation
Authors: Sheppard Maurice G. , Walker Robert B.
Published: February 2003

Ultraviolet absorption spectra (180-300 nm) out of vibrationally excited ozone are calculated using the classical Wigner method in conjunction with both ab initio and empirically adapted potential energy surfaces. Final state rovibrational distributions following photodissociation at 266 nm are calculated using a Wigner density in combination with classical trajectories.
Dissociative attachment in HCl, DCl, and F2
Authors: Bardsley J. Norman , Wadehra J. M.
Published: February 2003

Resonant scattering models, using nonlocal decay widths, are developed for dissociative attachment of slow electrons to diatomic molecules. Cross sections are obtained for HCl and DCl in several initial rotational and vibrational states, and the dependence of the average attachment cross section upon the rotational and vibrational temperature is examined. For F2 the cross section for ground state molecules agrees well with experiment above 0.2 eV but shows no zero energy peak. The attachment cross section is higher for vibrationally excited molecules, but the enhancement is much less than that found in H2 and HCl.
Anisotropic orientation in molecular monolayers by infrared spectroscopy
Authors: Rabolt J. F. , Burns F. C. , Schlotter N. E. , Swalen J. D.
Published: February 2003

Fourier transform IR measurements of Langmuir–Blodgett monolayers have been made with both grazing incidence reflection and transmission techniques. The reflection arrangement at incident angles from 85° to 88° has the electric field component perpendicular to the substrate surface while the standard transmission arrangement has the electric field component in the plane of the film. These techniques were used to explore the anisotropic arrangement of cadmium arachidate molecules perpendicular and parallel to the monolayer plane. The implication of these results for characterizing the orientational anisotropy of chemical groups in thin polymer films is also discussed. In addition, the question of uniaxial and biaxial orientation in the monolayer plane was addressed because of the importance of intermolecular packing on both the extent and rate of polymerization in unsaturated fatty acid monolayer components which form synthetic membranes.
Self-broadening and self-shifting of ammonia lines in the 2ν2 band
Authors: Baldacchini G. , Marchetti S. , Montelatici V. , Sorge V. , Buffa G. , Tarrini O.
Published: February 2003

We have measured pressure broadening and shifting of some transition lines of ammonia 14NH3 in the 2ν2 band. The agreement between the experimental results and the Anderson theory is satisfactory. We also discuss the possibility of obtaining the value of the dipole moment in the ν2=2 vibrational state from pressure broadening data.
Advances in calculating Raman excitation profiles by means of the transform theory
Authors: Stallard B. R. , Champion P. M. , Callis Patrik R. , Albrecht A. C.
Published: February 2003

Powerful transform methods for calculating Raman excitation profiles from the resonant absorption spectrum have recently been introduced. In the present work, previous results are extended to include both Condon and non-Condon sources of scattering and individual transform laws are presented for a number of commonly encountered symmetry conditions. In contrast to time correlator based methods, here the Kramers–Heisenberg polarizability expression, itself, serves as the starting point. Furthermore, the adiabatic approximation is limited to only one dimension, that of the scattered vibration. The remaining space of the resonant molecular eigenstates is formally free of many of the encumbering effects which find expression within the usual application of the adiabatic approximation in 3N-6 dimensions. This leads to a set of approximations which formally differ from those previously identified with the transform method. Their nature is less restrictive and encourages the view that the transform method is quite fundamental and deserves wide, though not uncritical, application. After deriving the basic set of transform laws, and discussing the underlying approximations, an illustration of the interference between Condon and non-Condon sources of scattering is presented. Then the transform law, at the Condon level, is shown to predict a strong discrimination against the scattering of low frequency modes. Finally, the problems involved in applying the transform theory to an ensemble of molecules are examined. Theoretical models as well as results from cytochrome-c are used as examples.
Inverse temperature dependencies in the quenching of singlet oxygen O2(1Σ+g) by CO2 and N2O studied with a discharge flow/shock tube
Authors: Borrell Patricia M. , Borrell Peter , Grant Kevin R.
Published: February 2003

The rates of quenching of O2(1Σ+g) by CO2 and N2O have been measured at 293 K and between 600 and 1300 K by studying the time-resolved emission in a discharge flow/shock tube experiment. For CO2, the quenching constant rises slightly from 2.09±0.23×108 dm3 mol−1 s−1 at 293 K to a maximum and then falls to 1.44×107 dm3 mol−1 s−1 at 1300 K. The behavior for N2O is similar, rising slightly from 5.89±0.25×107 dm3 mol−1 s−1 at 293 K and falling to 1.65×107 dm3 mol−1 s−1 at 1250 K. The detailed results are given in Figs. 5 and 8. The low temperature results agree well with previous work. A comparison is made of the decline in rate with the model of Parmenter, which unfortunately does not fit, although his suggested correlation of cross section with well depth is reasonable at room temperature. It is suggested that various temperature dependences found for these and other quenchers indicate that curve crossing occurs in those systems close to the potential energy minimum and so makes the rates very sensitive to the nature of the quenching molecule. An additional emission is found at longer times in the N2O system, which is attributed to chemiluminescence from NO2 formed by decomposition of N2O.
Resonant multiphoton optogalvanic detection of atomic oxygen in flames
Authors: Goldsmith J. E. M.
Published: February 2003

Atomic oxygen has been detected in flames using resonant multiphoton optogalvanic spectroscopy, with an estimated sensitivity of a few parts per million.
Mutual neutralization in rare gas halides
Authors: Whitten Barbara L. , Morgan William Lowell , Bardsley J. Norman
Published: February 2003

The cross sections for mutual neutralization in Ne++F−, Ar++F−, and K++Cl are obtained from two-state close coupling calculations. The Landau–Zener approximation gives adequate results in each case. The rate of neutralization is studied as a function of ambient gas density. Near atmospheric pressure, mutual neutralization is more likely than excimer formation of Ne++F− collisions, but less likely in Ar++F− collisions.
Energetics, wave functions, and spectroscopy of coupled anharmonic oscillators
Authors: Sage Martin L. , Williams J. A.
Published: February 2003

The structure of the energy levels, wave functions, and spectroscopy of two identical coupled Morse oscillators is explored as a function of anharmonicity and of the strength of kinetic and potential coupling constants. Changes in anharmonicity are found to make relatively small changes in properties of the coupled oscillators. Natural local modes are used to discuss the nature of exact wave functions.
Wetting transitions. II. First order or second order?
Authors: Teletzke G. F. , Scriven L. E. , Davis H. T.
Published: February 2003

We resolve a controversial difference between two recently proposed theories of the equilibrium contact angle, the angle at which a fluid interface meets a solid surface, by investigating integral and gradient theories of fluid at a solid. van der Waals integral theory, of which Sullivan’s model is a special case, in fact admits either a first- or second-order transition from a nonzero contact angle to perfect wetting as a critical point is approached. So also does gradient theory, of which the constant coefficient version we examined previously is a special case.
Emission spectra of CF3 radicals. I. UV and visible emission spectra of CF3 observed in the VUV photolysis and the metastable argon atom reaction of CF3H
Authors: Suto Masako , Washida Nobuaki
Published: February 2003

Two new emission bands of the CF3 radical were observed in the UV and visible regions. These emission bands were produced in the VUV photolysis of CF3H by argon resonance lines and also in the reaction of metastable argon atoms with CF3H. The UV emission bands are in the region from 180 to 300 nm. Vibrational and rotational relaxation occurred by the addition of three atmospheric pressures of helium as evidenced by the appearance of a discrete band structure. The visible emission bands are in the region 450–750 nm. The relaxation is observed by the addition of helium; however, no structure is observed in the visible emission bands (probably the band is a continuum). The sum of ΔH for the process CF3H→CF3+H and the energy of the short wavelength limit of the UV emission spectrum agree well with the energy of the incident light. It was suggested that the transition of the UV emission is from an electronically excited bound state to the ground state of CF3 and that of the visible emission is from an electronically excited bound state to an electronically excited lower repulsive state. The spectral region of the UV emission band agreed well with the absorption spectrum of CF3 observed by Glänzer et al.
Emission spectra of CF3 radicals. IV. Excitation spectra, quantum yields, and potential energy surfaces of the CF3 fluorescences
Authors: Washida Nobuaki , Suto Masako , Nagase Shigeru , Nagashima Umpei , Morokuma Keiji
Published: February 2003

Cross sections, quantum yields, and threshold energies for the production of UV and visible emission bands of CF3 from photodissociation of CF3Br and CF3Cl were measured in the 115–130 nm region. The quantum yields for the production of UV and visible emission bands for 0.2–1.0 and 1%–5%, respectively, both for the photolysis of CF3Br and CF3Cl. Appearance energies of the UV and visible emission are (9.41±0.05) and (9.39±0.02) eV, respectively, in the photolysis of CF3Br and (10.10±0.08) and (10.08±0.07) eV, respectively, in the case of CF3Cl. These results show that the upper state energies of the UV and visible emissions are very close to 6.4 eV above the CF3 ground state. Potential energy surfaces of CF3 radicals were calculated by the ab initio CI method. Possible transitions suggested for the UV and visible emission bands are 2A′1 (Rydberg state)→1A″2 (ground state) and 1E′ and/or 2A″2 (both are Rydberg states)→1 A′1 (Rydberg state), respectively. These upper states 2A′1, 1E′, and 2A″2, are most stable in the D3h structure.
HgBr(B) formation by electron impact in HgBr2/noble gas discharges
Authors: McGeoch M. W. , Hsia J. C. , Klimek D. E.
Published: February 2003

The HgBr(B–X) transition forms the basis of an efficient laser at 502 nm. The HgBr(B) state is produced by electric discharge in gas mixtures containing HgBr2. We report measurements of the HgBr(B–X) fluorescence efficiency as a function of electric field in electron beam sustained HgBr2/noble gas discharges. These measurements indicate that the dominant HgBr(B) formation channel is via direct electron impact on HgBr2 by the process HgBr2+e−  → HgBr(B)+Br+e−.A Boltzmann analysis is used to derive an empirical cross section for this process.
Electron scattering by methane: Elastic scattering and rotational excitation cross sections calculated with ab initio interaction potentials
Authors: Abusalbi Najib , Eades Robert A. , Nam Tonny , Thirumalai Devarajan , Dixon David A. , Truhlar Donald G. , Dupuis Michel
Published: February 2003

We calculate ab initio interaction potentials for electron-methane scattering and use them to perform converged scattering calculations for the electronically and vibrationally elastic rotational-state-to-rotational- state cross sections at 10 eV impact energy. The effective potential has static, local exchange, and polarization terms calculated from extended-basis-set Hartree–Fock wave functions for both unperturbed and polarized methane molecules. The polarization potential includes nonadiabatic effects in the semiclassical local kinetic energy approximation, and for comparison we also perform calculations based on the adiabatic polarization potentials. Five to 12 terms are retained in the angular expansion of the various parts of the interaction potential and the coupled channels calculations involved 41 total angular momenta, with 1–33 coupled channels for each. The resulting rotationally summed integral cross sections are in excellent agreement with recent experiments for scattering angles 40° and larger, but are larger than the experiment at small scattering angles. The rotationally inelastic cross sections for the full potential are smaller than those for the adiabatic potential by about a factor of 2.
Effects of a strong external magnetic field on the reduction of cobalt and iron oxides: Confirmation
Authors: Aylmer D. , Rowe M. W.
Published: February 2003

We have confirmed reports by Rowe et al. indicating that a strong (4.6 kOe) magnetic field influences the rate of reduction of some cobalt and iron oxides. In addition to our earlier gravimetric methods, we simultaneously carried out an evolved gas analysis such as that described by Gallagher et al. The discrepant results may arise from differences in experimental conditions used by the two groups. We used isothermal reduction, whereas Gallagher et al. used constantly increasing temperatures. Perhaps the latter are not sensitive to small changes in rate. (AIP)
On inversion of the Williams–Watts function for large relaxation times
Authors: Helfand Eugene
Published: February 2003

For glassy materials relaxation as a function of time is generally well fitted with the Williams–Watts function exp[−(t/τ0)β] with β≊1/3. One frequently interprets such nonexponential behavior in terms of a superposition of exponential relaxation processes with density ρβ(τ). We have determined ρβ(τ), corresponding to the Williams–Watts function, as an asymptotic (divergent) series. The leading behavior is ρβ(τ)∼τ−1+1/2β/(1−β) exp[−(1−β)(βτ/τ0)β/(1−β)]. This compliments the discussion of ρβ(τ) given by Lindsey and Patterson in terms of a convergent series, but one numerically difficult to evaluate for large τ. The density ρβ(τ) for β=1/3, 1/2, and 2/3 is expressed in closed form in terms of known functions. Suggestions for the extraction of the spectral density for long relaxation times for experimental data are given, applicable to relaxation processes slower than exponential.
Total and double differential cross sections of ion-pair formation in collisions of K atoms with SnCl4 and CCl4
Authors: Lacmann K. , Maneira M. J. P. , Moutinho A. M. C. , Weigmann U.
Published: February 2003

The reaction K+ACl4→K++(A−Cl4)−* with A=Sn and C was examined as a function of the collision energy from threshold up to about 40 eV in the c.m. system. Total cross sections of the mass-selected negative ions and doubly differential cross sections (energy and angle) of the K+ ions have been determined. Electron affinities, bond energies, and electronic excitation were calculated from the appearance potentials. In addition, the total cross sections for SnCl4 were measured and are contrasted with the earlier results of CCl4 from Dispert and Lacmann. Although both parent molecules have the same electron affinity within their error limits (2.2 eV for SnCl4 and 2.0 eV for CCl4) and the same dissociation energy for the negative ions of 1.4±0.2 eV, the product ion yields differ drastically. The main negative ion yield in K+SnCl4 results from SnCl−4 formation (over 80%). Its lowest dissociation channel leads to SnCl−3 formation, while Cl− ions are the main ions produced (90%) from CCl4, with only 7% leading to CCl−3+Cl formation at higher energies. These results support orbital energy considerations of electron addition to SnCl4 and CCl4 as applied to the results of reactive collisions of these molecules. The electron affinity and an electronically excited state of SnCl3 have been also determined. Morse potentials of CCl−4 and SnCl−4 were fitted to the experimental results of energy loss measurements from this work. The vertical electron affinities thus derived are 1.15 eV for SnCl4 and −1.0 eV for CCl4.
On the relation between diffusion, sedimentation, and friction
Authors: Kops-Werkhoven M. M. , Vrij A. , Lekkerkerker H. N. W.
Published: February 2003

Since there exists in the literature some discrepancies in the formulation of the relation between diffusion, sedimentation, and friction of colloidal particles in solution, we rederive the relation between these transport phenomena in this paper. From the application of irreversible thermodynamics it is shown, that the relation between the experimentally accessible quantities like diffusion coefficient, sedimentation velocity, and osmotic compressibility can be obtained unambiguously. The relation between sedimentation velocity and frictional coefficient is discussed in detail, where we give an argumentation for the right expression between these two quantities.
Constant temperature molecular dynamics calculation on Lennard-Jones fluid and its application to watera)
Authors: Tanaka H. , Nakanishi Koichiro , Watanabe Nobuatsu
Published: February 2003

Constant temperature molecular dynamics calculation has been carried out on Lennard-Jones liquid simulating argon. This method, proposed recently by Andersen, intends to transform the system from microcanonical to canonical ensemble and to keep the temperature of the system a constant value by the generation of random velocities when molecules exchange their energies with heat reservoir with a certain probability. A simple scheme is given for the estimation of collision probability and the effect of introducing this probability on dynamic behavior is examined in detail for 108 argon atoms as a test simulation. It is established that a collision probability of 0.01 is sufficient to realize the constancy of temperature reasonably well with no appreciable disturbance in the dynamic behavior. A model of pure water with Matsuoka–Clementi–Yoshimine (MCY) potential has also been simulated in the same manner. In the case of 216 MCY water at 298.15 K, the temperature difference is only 0.73 K with a collision probability of 0.005. Various static properties of MCY water have been calculated with reasonable agreement with those by previous Monte Carlo calculation, and moreover, the dynamic behavior calculated for MCY water gives a satisfactory picture on both translational and rotational motions in water, including reasonable agreement of diffusion coefficient with experimental datum.
On the calculation of transfer rates between impurity states in solids
Authors: Jackson Bret , Silbey Robert
Published: February 2003

We comment on the calculation of energy transfer in solids, showing that the calculation can be done to infinite order in the phonon-impurity interaction using the small-polaron transformation. When the final result is expanded, the results of low order perturbation theory are recovered, and, in addition, new terms are found which are due to cross products.
Theory of laser-stimulated surface processes. I. General formulation for the multiphonon relaxation of a vibrationally excited adatom
Authors: Beri A. C. , George Thomas F.
Published: February 2003

A general expression is obtained for the rate of energy transfer from an adatom-surface bond in an excited vibrational state due to IR laser irradiation to vibrational modes of the solid. The rate is seen to involve time Fourier transforms of powers of a correlation function for the displacement of different lattice particles at different times, reciprocal-space Fourier components of a suitably chosen pair potential between the adatom and each lattice atom and a weight factor associated with the laser-excited state of the adatom-surface bond. This ‘‘factorization’’ of the energy transfer rate into components depending on the properties of the laser, those of the adatom-surface bond and those of the solid provides a simple physical picture of the relaxation process within a rigorous framework and strongly suggests many points of departure towards more phenomenological approaches to laser-stimulated surface processes.
Multiple-quantum NMR in solids
Authors: Yen Yu-Sze , Pines A.
Published: February 2003

Multiple-quantum NMR has typically been observed in small groups of spins in isolated molecules. Due to the profusion of spin transitions in a solid, individual lines are unresolved. Excitation of high quantum transitions by normal schemes is thus difficult. To ensure that overlapping lines add constructively and to enhance sensitivity, time-reversal pulse sequences are used to generate all lines in phase. Up to 22-quantum 1H absorption in solid adamantane is observed.
First observation of bound–continuum transitions in the laser-induced A 1Σ+u–X 1Σ+g fluorescence of Na2
Authors: Verma K. K. , Bahns J. T. , Rajaei-Rizi A. R. , Stwalley William C. , Zemke W. T.
Published: February 2003

We report an interesting spectrum of Na2 excited by a Kr+ (5682 Å) laser which shows a long series of R–P doublets in the region 5600–8000 Å and a continuum with three very broad maxima beyond 8000 Å. Our spectral analysis reveals that the laser populates the v′=34, J′=50 level in the A1Σ+u state from where Na2 molecules fluoresce not only to the bound vibrational levels of the entire ground state potential well (3≤v″≤56) but also to the continuum levels above the well. We have made an independent theoretical quantitative prediction of the continuous emission and the agreement between experiment and theory is found to be excellent. Almost the entire (99.6%) ground state RKR potential is constructed using the bound state experimental data which leads to a more accurate value of the dissociation energy (D″e=6024±6 cm−1). The feasibility of a continuously tunable near infrared Na2 laser based upon this radiative dissociation process is discussed. Finally, we present a comprehensive bibliography for the Na2 molecule similar to that given by Hessel and Vidal for Li2 [J. Chem. Phys. 70, 4439 (1979)].
Spectroscopy of large molecular complexes in supersonic jets
Authors: Even Uzi , Jortner Joshua
Published: February 2003

In this paper we report the results of an experimental study of laser-induced fluorescence spectroscopy of M⋅Xn van der Waals complexes, consisting of medium-sized molecules (X=methane, carbon tetrachloride, ammonia, water, methanol, acetonitrile, benzene, and toluene) bound to a large aromatic molecule (M=tetracene and fluorene). These large complexes were synthesized in pulsed supersonic jets of He seeded with M and X. The spectral features assigned to S0→S1 electronic-vibrational excitations of these large M⋅X1 complexes, which are characterized by narrow linewidths (FWHM) of 2–4 cm−1, were attributed to three distinct categories: (i) A vibrationless excitation of M⋅X1, (ii) vibrational excitations of M in M⋅X1, and (iii) excitations of the vibrations involving relative motion of M and X in the M⋅X1 complex.
Statistical mechanics of electrolytes and polyelectrolytes. III. The cylindrical Poisson–Boltzmann equation
Authors: Ramanathan G. V.
Published: February 2003

The Poisson–Boltzmann equation for the potential due to an infinitely long, cylindrical polyion in a dilute ionic solution is studied and certain properties of its solution are proved. It is shown that when the charge density on the polyion exceeds a critical value, the surrounding ionic solution separates into two essentially independent phases. The inner phase which is close to the polyion consists predominantly of counterions. An explicit solution to the distribution function in this phase is obtained to the leading order and its validity is proved. This function depends only on the polyion charge density, and not on the concentration of the ionic solution. The distribution function in the outer phase depends upon the ionic strength but, to the leading order, not on the polyion charge density as long as it exceeds the critical value. It is shown that as the charge density on the polyion is increased, a proportionate number of counterions appears in the inner phase so as to neutralize the effect of this increase on the outer phase. It is proved that the potential, to its leading order, in the outer phase for any polyion charge density higher than the critical value, is the same as that due a polyion carrying a charge density equal to the critical value. The nonlinear dependence of the surface value of the potential on the polyion charge density is proved.
On the validity of the energy sudden approximation
Authors: Chang B. , Eno L. , Rabitz H.
Published: February 2003

This paper contains an examination of the conditions under which the energy sudden (ES) approximation may be expected to be valid. Our approach involves using dimensional analysis to identify (dimensionless) quantities which control energy suddenness and in this fashion three sets of ES criteria emerge. One involves the relative kinetic energy between collision partners and the energy spacing of the internal states of interest; another the strength of the coupling interaction and the same spacing; and a third involves the masses of the colliding molecules and component atoms. We discuss the relationship between these conditions and the justifications given by earlier workers for adopting the ES approximation and then the mass conditions in particular are used as the basis for certain broad statements concerning the applicability of the ES method within nonreactive diatom–diatom and reactive atom–diatom collisions. Finally, a number of avenues for further development of this work are discussed.
Theoretical studies of surface diffusion: A strategy for enhanced sampling
Authors: Doll J. D. , McDowell H. K. , Valone S. M.
Published: February 2003

Recent studies have demonstrated the feasibility and utility of numerical molecular dynamics simulation methods in the study of surface diffusion. Such methods have yielded surface diffusion constants which are in quantitative agreement with those measured by field ion microscope techniques. This letter describes an attempt to reduce the rather large computational effort involved in these studies by devising improved numerical sampling techniques.
Shear viscosities away from the melting line: A comparison of equilibrium and nonequilibrium molecular dynamics
Authors: Holian Brad Lee , Evans D. J.
Published: February 2003

Doubts about the validity of the nonequilibrium molecular dynamics (NEMD) methods of computing shear viscosity have persisted, partly because of the apparent disagreement (∼25%) between NEMD and equilibrium Green–Kubo (GK) results for the Lennard-Jones system near its triple point. This region of the phase diagram near the melting line is the so-called ‘‘molasses’’ regime where the tail of the shear-stress autocorrelation function is quite large, deviating from ‘‘exponential’’ decay at a level of about 10%. In order to see whether the effects of the ‘‘molasses tail’’ might be obscuring a more profound difference between NEMD and GK results, we have carried out independent NEMD and GK calculations for a state in the LJ fluid far away from this troublesome molasses region, namely at a temperature twice critical and a density between the triple and critical points. We find the NEMD and GK results for the linear shear viscosity to be in good agreement.
Dynamics of hydrogen atom and proton transfer reactions. Nearly degenerate asymmetric case
Authors: Babamov V. K. , Lopez V. , Marcus R. A.
Published: February 2003

A method for approximate treatment of collinear H atom or proton transfer reactions AH+B→A+HB which involve two nearly degenerate vibrational states is developed. The method is based on constructing a diabatic representation of the problem, i.e., one in which the reactant and the product vibrational states are distinguished as such throughout the collision. The diabatic representation is constructed with the aid of the adiabatic one discussed in an earlier paper. The treatment can also be applied to symmetric exchange reactions, in which case it yields good agreement with other results.
The He–O2 anisotropic van der Waals potential from the interplay of theory and scattering experiments
Authors: Faubel M. , Kohl K. H. , Toennies J. Peter , Gianturco F. A.
Published: February 2003

The interaction of helium atoms with ground state O2 molecules is examined over a wide range of internuclear distances that fully sample the extension of the shallow well region. A previous multiproperty study of its spherical part is extended to obtain the shape of its anisotropic behavior accurately by a best-fit procedure which involves very precise state-to-state measurements of rotationally inelastic cross sections and IOSA calculations of total and partial differential cross sections. The final form of the interaction is discussed and analyzed with respect to previous findings for similar systems.
Chloro-complexes formation in a ZnCl2–CdCl2 aqueous solution: An x-ray diffraction study
Authors: Paschina G. , Piccaluga G. , Pinna G. , Magini M.
Published: February 2003

A solution containing 1 mol/l of ZnCl2 and CdCl2 was examined by x-ray diffraction in order to study the mean environments of the two cations. The results were compared with those obtained from two solutions in which the same salts were separately dissolved. Zn2+ ions turned out to be hexa-coordinated both in the single and the mixed solution, the coordination shell being composed of about five water molecules and one Cl− ion. On the other hand, Cd2+ ions, which in the CdCl2 solution are nearly hexa-coordinated, become tetra-coordinated, in the mixed solution. This transition is accompanied by a significant increase of the Cd2+–Cl− complexation degree.
Computer simulation of liquid anisotropy. IV. Terms to second order in the external field of force
Authors: Evans M. W.
Published: February 2003

The method of molecular dynamics computer simulation is extended to deal with liquid anisotropy induced by the interaction of an external electric (or magnetic) field with anisotropy of polarizability. The resulting Langevin functions follow accurately the analytical results of Kielich. The new method is applied to extract the details of rotation/translation interaction in a moving frame of reference. The appropriate first order correlation functions are much more oscillatory in the presence of a strong external field.
Crystal-split electronic states of an atom in a rare gas crystal. Calculation of the absorption and fluorescence spectra of trapped oxygen (3P, 1D, 1S) atoms
Authors: Maillard Daniel , Fournier Janine , Mohammed Hassan H. , Girardet Claude
Published: February 2003

A method is proposed to determine the (crystal perturbed) energy levels of a guest species at any point inside the trapping site of an inclusion distorted host crystal. It relies on available pair potential energy curves, usually deduced from ab initio data supplemented by long-range dispersion energy when necessary. It is applied here to oxygen in its atomlike 3P, 1D or 1S state imbedded in Ne, Ar, Kr, and Xe matrix and allows reproduction of transition energies within 0.03 eV, and other experimental features. Its broad field of application could find many other exploitations, some of which are proposed.
Multiphoton NMR spectroscopy on a spin system with I=1/2
Authors: Zur Y. , Levitt M. H. , Vega S.
Published: February 2003

Multiple quantum effects in double frequency (df) pulsed NMR experiments on multilevel spin systems are studied. In these experiments, the spin systems are irradiated by two rf fields, applied simultaneously. A general theoretical description of these experiments is presented using the theory of Shirley for time dependent Hamiltonians. Multiphoton resonance conditions are given and time independent fictitious spin-1/2 Hamiltonians are derived using his perturbation theory treatment. With these Hamiltonians, the evolution of the spin systems during df irradiation is approximated. The example of df NMR experiments on an I=1/2 spin system is discussed. High order perturbation theory is developed to describe the time evolution of this spin system at a three photon resonance. Accurate computer calculations are performed to examine this time evolution. Df NMR experiments are performed on the single 31P(I=1/2) transition of phosphoric acid to check the theoretical results. The three photon resonance condition for these measurements is studied as a function of the rf irradiation intensities and of the resonance offsets of the two frequency components of df irradiation fields. Special NMR pulse cycles are used to study the dependence of the phase of the coherence created by df pulses on the initial rf phases of the two frequency components. Spin-echo type of experiments are developed to detect the spiral motion of the magnetization vector of the I=1/2 spin system at the three-photon resonance.
On the nonradiative and radiative decay rates and a modified exponential energy gap law for 4f–4f transitions in rare-earth ions
Authors: van Dijk J. M. F. , Schuurmans M. F. H.
Published: February 2003

A modified exponential energy gap law for nonradiative decay has been derived for 4f–4f transitions where only a few phonons participate in the transition. Its preexponential factor varies by only a factor of 10 for different host materials and is purely electronic in nature. The modified law is the usual law (giving a factor of 105 variation) modified by using an energy gap which is effectively two (maximum) phonon energies smaller than the energy difference between the initial and the final electronic state. A general relation between the radiative and nonradiative decay rates has been constructed. For 4f–4f transitions this relation enables one to predict the nonradiative decay rate from a knowledge of the radiative decay rate to within one order of magnitude accuracy.
The microwave spectrum and molecular structure of N2–HCl
Authors: Altman R. S. , Marshall M. D. , Klemperer W.
Published: February 2003

The structure of the van der Waals molecule N2–HCl has been determined by molecular beam electric resonance spectroscopy. The microwave spectrum is well fitted by a linear hydrogen-bonded structure with the following spectroscopic constants: Constant              14N2–H 35Cl   15,14N2–H35Cl   14,15N2–H35Cl                   In addition, spectra for the two chlorine isotopes of the complex formed using 15N2 are presented and analyzed. Electric quadrupole coupling hyperfine structure of 14N and 35,37Cl, is used to determine the vibrationally averaged orientation of the submolecules. The HCl submolecule makes an average angle of 26° with the a axis of the complex in all isotopes studied, while the average angle for the N2 submolecule is estimated to be 19°. The hydrogen–bond length is 2.42 Å in the linear configuration which is virtually identical to the hydrogen-bond length in OC–HCl. It is shown that the electric dipole moments of a series of N2 and CO complexes are not simply related to the polarizabilities of N2 and CO. Analysis of the induced difference in the quadrupole coupling constants of the two nitrogen nuclei indicates a transfer of 0.036 of an electron from the outer to the inner nitrogen which will affect the observed dipole moment.
Kinetic studies of ArCl* and Ar2Cl*
Authors: Liegel J. , Spiegel H. , Sauerbrey R. , Langhoff H.
Published: February 2003

Electron beam excited mixtures of argon and helium with different chlorine donors have been investigated in order to study the optical emission characteristics as well as the formation and quenching processes of ArCl* and Ar2Cl*. Emission from the ArCl* (B–X) and ArCl* (D–X) transitions is observed. Due to the competition between formation of ArCl* and Cl2*, Cl* is identified to be their common precursor. Three body quenching of ArCl* yields Ar2Cl* which radiates around (245±5) nm with a bandwidth (FWHM) of 35 nm. Quenching rate constants for Ar2Cl* and a radiative lifetime of (240±40) ns have been determined.
Laser-induced fluorescence of the RbCs molecule
Authors: Katô Hajime , Kobayashi Hiromi
Published: February 2003

Laser-induced fluorescence of the RbCs molecule has been observed when a mixture of rubidium and cesium was irradiated by the 5682 Å line of Kr+ laser. A series of fluorescence lines of transitions to the vibrational levels up to v″=122 is observed and a continuous spectrum with banded diffraction structure is contiguous to it. The spectroscopic constants of the X 1Σ+ ground state are determined from the analysis of the fluorescence, and the potential curve is constructed by the RKR procedure. The dissociation energy De is determined to be 3833±5 cm−1 from the Birge–Sponer extrapolation. The potential curve of the excited state is estimated from the analysis of the fluorescence intensity.
Proton chemical shift tensors in hydrogen-bonded dimers of RCOOH and ROH
Authors: Rohlfing Celeste McMichael , Allen Leland C. , Ditchfield Robert
Published: February 2003

In an effort to explain empirically observed correlations between certain structural features of hydrogen bonds and corresponding proton chemical shift parameters determined by single crystal multiple pulse NMR, a theoretical investigation is made of 14 O–H⋅⋅⋅O dimers involving substituted carboxylic acids and alcohols. Nuclear magnetic shielding tensors are calculated for the protons in each hydrogen bond system. Certain linear dependences are observed, in agreement with experimental data, that indicate that the chemical shift tensor is a sensitive measure of hydrogen bonding that can yield more information than the isotropic shift alone. The apparent correlation between the O⋅⋅⋅O separation and isotropic chemical shift is seen to result from the strong dependence of the perpendicular shielding component on the former quantity. The lack of a similar correlation for the chemical shift anisotropy is explained in terms of the parallel component of the shielding tensor. An interpretation of such trends in these proton chemical shift tensors is proposed in terms of existing models of hydrogen bonds.
On the calculation of time correlation functions in quantum systems: Path integral techniquesa)
Authors: Thirumalai Devarajan , Berne Bruce J.
Published: February 2003

A Monte Carlo method for calculating quantum mechanical time correlation functions is presented. In this method the time correlation function is calculated at several values along the pure imaginary axis of the complex time plane such that 0<it<β, where (β)−1 is the temperature of the system. Using the periodicity of the imaginary time correlation function, the real time behavior is inferred by analytic continuation. The method is used to calculate the absorption spectra of two model systems.
Fluorescence cross sections and electronic transition moments for the A 2Σ+→X 2Π transition in HCl+ by photoionization. Comparison with the ab initio calculations
Authors: Ibuki Toshio , Sato Nobuyuki , Iwata Suehiro
Published: February 2003

The electronically excited HCl+ (A 2Σ+) ions have been produced by the direct photoionization with helium and neon resonance lines. The emission bands A 2Σ+(v′=0–6)→X 2Π (v″=0−2) were detected. The absolute fluorescent cross sections of the transition were determined at 58.4 and 73.6 nm. The ab initio configuration interaction calculations were carried out for the A 2Σ+ and X 2Π states. The excellent agreement between the experimental and ab initio results was obtained for the Franck–Condon factors, electronic transition moments, and potential energy curves.
A reinterpretation of the CH−2 photoelectron spectrum
Authors: Sears Trevor J. , Bunker P. R.
Published: February 2003

We simulate the photoelectron spectrum of CH−2 using the known bending potential functions of CH2 in the X̃ 3B1 and ã 1A1 states, together with a bending potential function and temperature for CH−2 in the X̃ 2B1 state that are adjusted in order to fit the experimental spectrum. We find that in order to reproduce the spectrum we must have a CH−2 temperature of about 2500 K and a singlet–triplet separation in CH2 of 3250±200 cm−1 (0.40±0.02 eV or 9.3±0.6 kcal/mol). This leads to a value for the electron affinity of X̃ state CH2 of 0.65±0.02 eV. In the simulation we include rotational levels with J (or N) up to 20, which means that over 500 000 rovibronic transitions are involved.
Theoretical study of the freezing of polystyrene sphere suspensions
Authors: Shih Wei-Heng , Stroud D.
Published: February 2003

We present a theoretical study of the freezing curve of suspensions of charged polystyrene spheres (polyballs) in water. The spheres are assumed to have the same size and charge, and to interact via a modified Debye–Hückel potential. The free energy of the liquid phase is calculated as a function of sphere density and pH of the solution, using a variational procedure in which an effective hard-sphere diameter is the variational parameter. The freezing curve is obtained from a ‘‘Lindemann criterion’’ that the volume fraction occupied by the effective hard spheres should be a constant. The resulting curve is confirmed by analogous Lindemann calculations for the solid phase, and by a calculation of the curve along which liquid and solid phase free energies are equal. For a ‘‘point-like’’ Debye-Hückel interaction for which corrections due to finite polyball radius are neglected, the melting curve is reentrant: For fixed electrolyte concentration, the fluid first crystallizes, then remelts, as the polyball density increases. Reentrance disappears when realistic size corrections are incorporated. The modified melting curve then agrees fairly well with experiment. It is concluded that the freezing of monodisperse polyball suspensions can be understood within the framework of the classical theory of dense fluids with short range repulsive interactions.
Emission, optical–optical double resonance, and excited state absorption spectroscopy of matrix isolated chromium and molybdenum atoms
Authors: Pellin M. J. , Gruen D. M. , Fisher T. , Foosnaes T.
Published: February 2003

Making use of a combination of time-resolved emission, optical–optical double resonance, and excited state absorption spectroscopy, it has been possible to assign virtually all spectral features with energies below the z7P0 state of matrix isolated Cr atoms. The a5S state located at 7593 cm−1 in the free gaseous Cr atom has lifetimes of 6.32 and 5.1 s in Ar and Kr matrices, respectively. Matrix perturbations on Cr emission lines are small (<±100 cm−1). The dependence of nonradiative decay rates on the local density of states is elucidated. The magnitude of matrix shifts for a particular transition is correlated with the electronic configurations of ground and excited states and it is pointed out that states having only ‘‘s’’ electrons in addition to ‘‘d’’ electrons maintain their gas phase energy relationships in the matrix environment. Direct fluorescence is observed from the z7P0 level of Mo to the 7s ground state. The spin-orbit splitting of the ‘‘relaxed’’ z7P0 state is 690 cm−1, slightly lower than the 707 cm−1 splitting of the free gaseous Mo atom.
A symmetrized generalized log-derivative method for inelastic and reactive scattering
Authors: Mrugala F. , Secrest Don
Published: February 2003

A new invariant imbedding approximate solution algorithm related to the log-derivative method is derived. The method makes use of the symmetry of the scattering equations and is particularly well suited to problems with a first derivative term. Comparisons of efficiency are made in the use of this method on both a reactive scattering problem and a nondiabatic inelastic scattering problem.
Dynamics of liquid state chemical reactions: Vibrational energy relaxation of molecular iodine in liquid solution
Authors: Brooks C. L. , Balk M. W. , Adelman S. A.
Published: February 2003

The dynamics of vibrational energy relaxation of highly excited molecular iodine in three monatomic solvents is studied via stochastic classical trajectory simulations based on the molecular timescale generalized Langevin equation (MTGLE) of motion for liquid state chemical reactions [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)]. Also presented for comparison purposes are parallel studies based on a matrix Langevin equation of motion characterized by friction coefficients which depend on the instantaneous I2 internuclear separation R. The qualitative features of the energy relaxation may be interpreted as effects arising from modifications of the solute dynamics due to molecular timescale correlations between its motion and that of its solvation shells. Such dynamical solvent effects are realistically described by the MTGLE equation of motion but not by the Langevin equation. Thus, for example, the marked slowdown of the rate of I2 energy relaxtion in simple solvents when the I2 vibrational quantum number drops below a solvent-dependent critical value, earlier predicted by Nesbitt and Hynes, is predicted by MTGLE dynamics but not by Langevin dynamics. Finally, practical algorithms for numerically constructing the MTGLE and Langevin equations for specific solute–solvent systems are presented.