bluuebunny/crossref_metadata_2025 huggingface dataset:
Proton affinities of diacetylene, cyanoacetylene, and cyanogen
Authors: Deakyne Carol A. , Meot-Ner (Mautner) Michael , Buckley Thomas J. , Metz Ricardo
Published: July 2002

The proton affinity of cyanoacetylene HCCCN was determined by pulsed high pressure mass spectrometric equilibrium measurements as 180.1±1 kcal/mol. Ion cyclotron resonance (ICR) bracketing experiments yielded the proton affinity of diacetylene HCCCCH as 180±1 kcal/mol and of cyanogen NCCN as 161±2 kcal/mol. Ab initio calculations at the 6-31+G** level are used to evaluate the heats of formation of HCCCCH as 105 and of HCCCN as 87 kcal/mol. The ab initio results show also that protonation of diacetylene on a terminal carbon converts a destabilizing antibonding interaction between the triple bonds in the neutral molecule into an attractive interaction, leading to a significant shortening of the C2–C3 bond.
Basis set methods for describing the quantum mechanics of a ‘‘system’’ interacting with a harmonic bath
Authors: Makri Nancy , Miller William H.
Published: July 2002

The case of a system (e.g., a one-dimensional reaction coordinate) coupled to a ‘‘bath’’ of many harmonic oscillators is treated by quantum mechanical basis set methods. By choosing the basis set for the bath to incorporate the coupling explicitly, it is shown how the bath can then be eliminated to obtain an effective Hamiltonian matrix for only the system. Numerical calculations are carried out which show that, even in the zeroth version of the approach, the effect on the system (e.g., the tunneling splitting in a double-well potential) of coupling to the bath is described well, even when the effect is extremely large.
Rotational spectra and structures of the Ar2–H/DF trimers
Authors: Gutowsky H. S. , Klots T. D. , Chuang Carl , Schmuttenmaer C. A. , Emilsson Tryggvi
Published: July 2002

The feasibility of studying weakly bound, neutral van der Waals clusters through their microwave rotational spectra has been shown by identifying and characterizing Ar2–H/DF. Detection of the trimeric complexes was accomplished with the Flygare Mark II Fourier transform spectrometer, modified to better utilize its inherent sensitivity. Twenty-six b-dipole transitions (all ee↔oo) were observed in the 3–18 GHz region for Ar2–HF, and 19 for Ar2–DF, and their hyperfine structure analyzed. Fitting of the line centers with Kirchhoff’s NBS program gave values (all in MHz) for Watson’s determinable parameters of 3576.508(1), 1739.139(1), and 1161.054(1) for the rotational constants A″, B″, and C″ in Ar2–HF, and of 3506.791(1), 1744.056(1), and 1155.636(1) in Ar2–DF. The centrifugal distortion constants τ1, τ2, τaaaa, τbbbb, and τcccc were found to be −0.2039(3), −0.0522(1), −0.5353(5), −0.1159(1), and −0.02021(1) in Ar2–HF, and −0.2116(7), −0.0528(2), −0.4941(7), −0.1123(1), and −0.0191(1) in Ar2–DF. Three small sixth order terms were necessary to fit the data within experimental error. The trimer is found to be a planar T-shaped, highly asymmetric top (κ=−0.521) with the equilibrium position of the H/DF on the C2v figure axis (the b axis), the H/D end pointing toward the two argon atoms. The structure can be viewed as a composite of three dimers, the Ar2 dimer and two Ar–H/DF’s. In the trimer the Ar–Ar and the two Ar–H/DF distances are a bit longer (0.04 and 0.01 Å) than in the dimers, suggesting the presence of second-order effects. This picture is supported by an approximate, pseudotriatomic analysis of the force field, based on the four independent planar centrifugal distortion constants. The cluster is quite ‘‘floppy.’’ The hyperfine structure shows that torsional oscillations of the H/DF are comparable with those in the Ar–H/DF dimers, but anisotropic, being 3° larger in-plane than out-of-plane. The in-plane torsional oscillation of the Ar2 is considered.
Electronic structure of polyimide
Authors: Brédas J. L. , Clarke T. C.
Published: July 2002

We present a valence effective Hamiltonian (VEH) electronic structure calculation on the polyimide poly[N,N′-bis(phenoxyphenyl)-pyromellitimide], also called PMDA (pyromellitic dianhydride)–ODA (oxydianiline). The most important result is that the valence electronic bands are calculated to be very narrow, even though the calculation is performed on a perfectly stereoregular chain. For instance, the highest 35 occupied valence bands (i.e., the electronic bands with binding energies lower than 18 eV) have widths smaller than 0.25 eV. That result implies that even small disorder effects will tend to strongly localize the electronic processes in polyimide. From the band structure, we calculate the density of valence states and produce a theoretical simulation of the UPS spectrum. Good agreement with the recently published UPS experimental spectrum is obtained.
The nonequilibrium electromotive force. I. Measurements in a continuously stirred tank reactor
Authors: Keizer Joel , Chang On-Kok
Published: July 2002

Based on a statistical thermodynamic theory, it has been predicted [J. Keizer, J. Chem. Phys. 82, 2751 (1985)] that at nonequilibrium steady states the electromotive force (EMF) of an electrochemical cell will differ from the local equilibrium value given by the Nernst equation. We describe here experiments designed to test this prediction for aqueous solutions of Fe2+ and Fe3+ in sulfate buffer. Using a continuously stirred tank reactor driven by a peristaltic pump, a feed solution containing Fe2+ and Fe3+ was mixed with a second feed solution containing the oxidant sodium peroxydisulfate Na2S2O8. The reaction leads to a steady nonequilibrium mixture, which at acidic pH in sulfate buffer is composed of Fe2+ and the ferric sulfate complexes FeSO+4 and Fe(SO4)−2. The EMF of this half-cell was measured vs a saturated calomel reference electrode as a function of residence time in the reactor. These potentials were compared to the Nernst potential calculated on the basis of the concentration ratio of Fe2+ to total Fe3+ at the steady states. The Nernst potential was reproducibly larger than the measured EMF by values that depended on the concentration ratio of Fe2+/Fe3+ in the feed solution and the residence time. The largest deviations were −1.8 mV, which occurred when the Fe2+/Fe3+ ratio was small and the residence time was about 40 s. We have ruled out streaming potentials, junction potentials, and incomplete mixing as the origin of this effect. We show that the dependence of the nonequilibrium portion of the EMF on feed concentrations and residence time is in good agreement with calculations based on methods that are described in the second paper in this series.
The nonequilibrium electromotive force. II. Theory for a continuously stirred tank reactor
Authors: Keizer Joel
Published: July 2002

In previous work [J. Keizer, J. Chem. Phys. 82, 2751 (1985)] we used statistical nonequilibrium thermodynamics to predict a non-Nernstian component to the electromotive force (EMF) for half-reactions involving reactants at nonequilibrium steady states. In this paper we present a simple theory for calculating the nonequilibrium component of the EMF based on the elementary transport processes occurring in a continuously stirred tank reactor (CSTR). The calculations utilize the density–density correlation function, which is obtained from the statistical theory of nonequilibrium thermodynamics. This gives rise to an expression for the second partial derivatives of the generalized entropy, or sigma function, which is used to calculate generalized chemical potentials. The generalized chemical potentials are related to the EMF through a generalization of the Nernst equation. The calculations presented here depend on the residence time in the CSTR, reaction rate constants, feed line concentrations in the CSTR, and the diffusion constants of reactants and products. A characteristic diffusion length is used to represent the length scale below which turbulent mixing effects are not important. Calculations with the theory are carried out for several different reaction mechanisms, including A+B⇄C; A+B⇄C, D+E⇄B; A+B⇄2B; and A+B→C+D, A+D→C+E. Values of the nonequilibrium EMF depend on the mechanism as well as all of the transport parameters cited above. For a plausible choice of the diffusion length, corrections to the Nernst formula can be as large as 10–15 mV. Specific calculations for the reaction of Fe2+ with S2O2−8 are shown in the preceding paper to agree with experimental measurements on this system in a CSTR.
B̃ 4∑−u–X̃ 4∑−g laser excitation spectrum of C+2
Authors: Rösslein Matthias , Wyttenbach Martin , Maier John P.
Published: July 2002

The B̃ 4∑−u–X̃ 4∑−g electronic transition of C+2 has been detected in the gas phase by recording its laser excitation spectrum. The rotational structure is resolved and the 0–0, 1–0, and 0–1 bands are discussed. The origin lies at 19 730.6(2) cm−1. The analysis shows that only odd rotational lines are present whereas in the also recorded spectrum of 13C+2, a 3:1 intensity alternation is seen, the even lines being the strongest. The assignment follows on the basis of the rotational structure, isotopic effects and comparison with the data obtained recently in a Ne matrix and with theoretically calculated values. The spectroscopic constants Te,ωe,ωexe,B0,B1 are given for the B̃4 ∑−u and X̃ 4∑−g and C+2.
Connected-diagram expansions of effective Hamiltonians in incomplete model spaces. I. Quasicomplete and isolated incomplete model spaces
Authors: Kutzelnigg Werner , Mukherjee Debashis , Koch Sigurd
Published: July 2002

In this and the following paper, we formulate a Fock space theory for incomplete model spaces (IMS) that applies both to coupled-cluster expansions and to perturbation theory. We stress in this paper that the concept of the ‘‘connected’’ nature of extensive quantities like an effective Hamiltonian Heff is more fundamental than the ‘‘linkedness’’ that is conventionally used in many-body perturbation theory. The ‘‘connectedness’’ of Heff follows when the wave operator W is multiplicatively separable into noninteracting subsystems. This is ensured by writing W as an exponential Fock space operator with the exponent connected. It is demonstrated in particular that the connectedness of the exponent in W requires that the normalization condition of W be separable as well. Unlike the situation in a complete model space, the definition of ‘‘diagonal’’ or ‘‘nondiagonal’’ operators depends generally on the particular m-valence IMS. There are, however, special categories of IMS, the ‘‘quasicomplete’’ and the ‘‘isolated’’ model spaces, for which these definitions are possible without reference to the particular IMS. The formal properties of these IMS are discussed. It is shown that for the quasicomplete model space, the intermediate normalization is not separable, while it is so for the isolated model space.
An effective Hamiltonian study of molecular clusters
Authors: Suck Salk S. H. , Lutrus C. K.
Published: July 2002

Earlier we presented only a qualitative description of a semiempirical effective Hamiltonian method which was successfully applied to the molecular clusters of hydrogen-bonded (H2O)n. Here a comprehensive analysis of this method is introduced by means of its generalization. Unlike the earlier study, emphasis is placed upon the physical properties of small clusters concerning the energy of formation and the entropy of formation, in order to find propensity rules associated with prenucleation process. It is pointed out that the predicted energy of formation of the molecular clusters at 0 K increases with size, showing slow convergence to the bulk value. Qualitative estimations of entropy changes for small clusters are made, in order to examine entropy effects on the stability of the clusters at finite temperatures. In addition, it is found that the first ionization potential rapidly converges to the work function of ice and the dipole moments of the clusters of open structures tend to linearly increase with increasing cluster size.
ArF laser flash photolysis of phenol and anisole
Authors: Kajii Yoshizumi , Obi Kinichi , Nakashima Nobuaki , Yoshihara Keitaro
Published: July 2002

Absorption spectra of phenoxy radical in the gas phase are observed in the range of 210–300 nm in the ArF excimer laser (193 nm) flash photolysis of phenol and anisole. An intense absorption band is obtained around 230 nm and a broad band around 270–300 nm. In the photolysis of anisole, simultaneous formation of methyl and phenoxy radicals makes it possible to determine the molar extinction coefficient of phenoxy radical which is determined to be 7800 M−1 cm−1 at 230 nm. The phenoxy radicals are produced in the time range of shorter than 1 ns from phenol, anisole, pentafluorophenol, and pentafluoroanisole on irradiation with the ArF laser light. The photodissociation is not explained in terms of the hot molecule mechanism but of predissociation.
Dipoles induced by long-range interactions between centrosymmetric linear molecules: Theory and numerical results for H2⋅⋅⋅H2, H2⋅⋅⋅N2, and N2⋅⋅⋅N2
Authors: Bohr J. E. , Hunt K. L. C.
Published: July 2002

We have derived symmetry-adapted expressions for the dipole moments of pairs of D∞h molecules interacting at long range, in a form useful for line shape analyses of collision-induced rototranslational spectra. Our results are complete to order R−7 in the intermolecular separation R. In addition to quadrupolar and hexadecapolar induction effects, results to this order include induction due to nonuniformities in the local field acting on a molecule (E-tensor induction), back induction, and polarization due to dispersion forces. The dispersion terms are computed within our recently developed reaction field model, from which we have obtained accurate integral expressions for the dipole coefficients, and approximations in terms of static susceptibilities and dispersion energy coefficients. For H2⋅⋅⋅H2, H2⋅⋅⋅N2, and N2⋅⋅⋅N2, numerical results for the dipole coefficients are tabulated. While quadrupolar induction dominates the long-range dipole, other induction effects are evident in the far-infrared collision-induced spectra. Over the range of validity of the model, E-tensor induction, back induction, and dispersion effects are generally smaller than hexadecapolar induction, but appreciable. The magnitudes of the dipole coefficients and the orientation dependence of each polarization mechanism determine its contributions to the observed collision-induced absorption spectra.
Energy dissipation in matrix-isolated silver atoms: A time-resolved fluorescence study
Authors: Wiggenhauser H. , Schroeder W. , Kolb D. M.
Published: July 2002

The fluorescence from optically excited Ag atoms in Ar, Kr, and Xe matrices has been investigated in a time-resolved synchrotron-radiation study. A detailed energy dissipation model could be established from a systematic analysis of rise and decay times of all the observed fluorescence bands after pulsed excitation into the Ag (4d105p)2P1/2,3/2 levels, and by setting time windows between the excitation pulses in emission and emission-yield spectroscopy. Although the overall wavelength dependence of the decay time follows the λ3 law, the decay time is independent of λ within a given emission band. Finally, the role of energy transfer between Ag atoms and dimers for the evaluation of decay times is briefly addressed.
Vibrational phase relaxation at surfaces: The role of lateral interaction
Authors: Persson B. N. J. , Hoffmann F. M.
Published: July 2002

We have studied the temperature and coverage dependence of the vibrational line shape of the C–O stretch vibration for CO on Ru(001). We find that the narrow IR line profile which is observed for the ordered structure of CO on Ru(001) is due to the strong dipole–dipole coupling and that for low CO coverage (or in the isotopic dilution limit), where the lateral interaction is effectively zero, the linewidth of the C–O stretch mode increases in accordance with theory. The implication of this result is briefly discussed.
On the bond distance in methane
Authors: Bowen-Jenkins Philippa , Pettersson Lars G. M. , Siegbahn Per , Almlöf Jan , Taylor Peter R.
Published: July 2002

The equilibrium bond distance (re) in methane has been optimized using coupled-pair functional and contracted CI wave functions, and a Gaussian basis that includes g-type functions on carbon and d-type functions on hydrogen. The resulting bond distance, when corrected for core–valence correlation effects, agrees with the experimental value of 2.052 a0 to within the experimental uncertainty of 0.002 a0. The main source of error in the best previous studies, which showed discrepancies with experiment of 0.007 a0, is shown to be basis set incompleteness. In particular, it is important that the basis set be close to saturation, at least for the lower angular quantum numbers.
Radiative lifetime and quenching of the Ã 2A1 state of the CH3O radical
Authors: Lin Shiaw-Ruey , Lee Yuan-Pern , Nee J. B.
Published: July 2002

The laser-induced fluorescence technique has been employed to study the first excited electronic state Ã 2A1 of CH3O. Vibrational levels v′3 =0–7 of the Ã state were excited and fluorescence decay rates were measured. Compared to a value 2.2±0.2 μs at v3=0, a significant decrease of decay rate was found at 277.6 nm (0.89 μs, v′3=7). Excitation to higher levels further reduced the lifetime. The predissociation threshold was estimated to be slightly above 276 nm (36 220 cm−1). The electronic quenching rates of the Ã state (v′=0–2) by CH3OH, O2, NO2, and N2 have also been determined and are compared with those of OH in the Ã state.
Broad-band structured fluorescence from NaI
Authors: Bower R. D. , Chevrier P. , Das P. , Foth H. J. , Polanyi J. C. , Prisant M. G. , Visticot J. P.
Published: July 2002

The emission spectrum of a molecular beam of NaI has been measured following excitation by the 248 nm KrF line of an excimer laser. The fluorescence structure, and the lifetime (17±2 ns), indicate the presence of a bound upper state not previously characterized. We identify this state as correlating with Na(3 2P)+I(5 2P3/2) atoms. Through numerical simulation using a Morse function representation for the upper state, we have determined approximate values for the well depth of 530±260 cm−1, well breadth of ∼1.8 Å−1, and equilibrium separation of ∼3.4 Å for this state. As a consequence of the breadth of this weakly bound excited state the bound→bound line spectrum spans a broad spectral range (87% of the ground state well depth) entirely comparable with spectra attributed to free→bound emission.
Energetics and spin- and Λ-doublet selectivity in the infrared multiphoton dissociation DN3→DN(X 3Σ−, a 1Δ)+N2(X 1Σ+g): Experiment
Authors: Stephenson John C. , Casassa Michael P. , King David S.
Published: July 2002

Multiphoton vibrational excitation of deuterated hydrazoic acid, DN3, by a CO2 laser (I=10 GW/cm2) leads to dissociation forming DN in both X 3Σ− (spin forbidden) and a 1Δ (spin allowed) electronic states. Under collisionless conditions, the nascent DN fragments were probed via laser induced fluorescence, to determine initial product state distributions. The DN(X 3Σ−) molecules are formed predominantly in the symmetric F1 and F3 spin–rotation states with little population (≤6%) in the antisymmetric F2 levels. There is no significant population (<3%) in excited DN(3Σ−) vibrational levels. The distribution of rotational states is Boltzmann-like, characterized by a rotational ‘‘temperature’’ of about 920 K for the F1, F3 states and 500 K for F2 levels. Doppler profiles showed a large kinetic energy release of about 10 100 cm−1 total in the triplet channel. The DN(1Δ) products are formed preferentially in the symmetric Δ(A′), e-labeled lambda doublet levels: Δ(A′)/Δ(A″)=1.44. The DN(1Δ) is formed with no vibrational excitation (<2%); the rotational states are populated Boltzmann-like with a rotational ‘‘temperature’’ of 425 K. Doppler profiles give a total kinetic energy of about 1500 cm−1 in this channel. These observations give information about the distribution of energy in the reactant, the location of the barriers to dissociation, and the geometry of the transition states. Alexander, Werner, and Dagdigian (accompanying article) show that the observed DN(3Σ−) spin- and DN(1Δ) Λ-doublet selectivities reflect the symmetry properties of a planar transition state and that the low degree of DN(3Σ−) rotational and vibrational excitation is also expected from the transition state geometry.
Nonadiabatic theory of triatomics: General formalism and application to Renner–Teller and conical-intersection effects
Authors: Petrongolo Carlo
Published: July 2002

A general theory of nonadiabatic effects in molecular spectra of bound triatomic molecules is presented. Within the nonrelativistic approximation and the expansion method, the formalism is exact and allows a unified study of the various nonadiabatic interactions between two or more electronic states. A full, spinless rovibronic Hamiltonian is used and the rotation, inversion, and nuclear permutation symmetries are considered for defining a rovibronic expansion basis which is symmetry adapted for ABC and AB2 molecules. The Hamiltonian matrix elements on the rotoelectronic-angular basis are obtained for singlet electronic states, by fully considering both the nuclear and the electronic structure. The equations are then specialized to three-state Renner–Teller and conical intersection effects in the diabatic representation, and are approximated and compared with previous works. Finally, the expressions of the dipole line strengths are derived and the singularities of the adiabatic representation of conical intersections are discussed.
Proton transfer in benzoic acid crystals: A chemical spin–boson problem. Theoretical analysis of nuclear magnetic resonance, neutron scattering, and optical experiments
Authors: Skinner J. L. , Trommsdorff H. P.
Published: July 2002

The double proton transfer in benzoic acid crystals can be described by a double-minimum potential. At low temperatures one need consider only the two lowest energy eigenstates, which must be coupled to the crystalline phonons in order to obtain relaxation. Thus the benzoic acid system provides a well-defined chemical example of the spin–boson Hamiltonian. Within this model the tunneling relaxation between localized states occurs by one-phonon emission or absorption. Alternatively, at high temperatures the proton transfer is thermally activated. With this simple picture in mind we analyze NMR T1 relaxation experiments. The temperature-dependent proton transfer rate that emerges from the NMR analysis is in good agreement with inelastic neutron scattering experiments. Optical transitions of a dye probe have also been used to determine proton transfer rates in crystalline benzoic acid. Our model allows us to discuss both doped and pure crystal experiments within a unified framework. Thus, we find that all three different experimental probes yield results that are consistent with our simple theoretical picture. From our results we can determine the proton–phonon coupling constant.
Spin-forbidden radiative decay involving quasidegenerate states. Application to the B 1Σ+→a 3Π transition in MgO
Authors: Yarkony David R.
Published: July 2002

The spin–orbit induced interactions among the low-lying X, B 1Σ+, a 3Π, A 1Π, and c 3Σ− states of MgO are studied with emphasis on the experimentally observed B 1Σ+→a 3Π spin–forbidden dipole-allowed radiative transition. A dressed diabatic states approach is used in which the spin–orbit induced perturbation of the (zeroth order) a 3Π0+ and a 3Π1 fine structure states is partitioned into a contribution from the zeroth order X 1Σ+0+ and A 1Π1 states and contributions from all other remaining states of 1Σ+0+ and 1Π1 symmetry. This perturbation–partitioning approach extends a recently developed technique for determining spin–orbit perturbed wave functions directly in the CSF basis based on first order perturbation theory [J. Chem. Phys. 83, 1168 (1985)] to situations involving quasidegenerate electronic states. The wave functions in question are expanded in second order configuration state function spaces of between 60 000–100 000 terms. The parallel [ μ∥(B 1Σ+0+, v=0; a 3Π0+ v=n)] and perpendicular [ μ⊥(B 1Σ+0+, v=0; a 3Π1, v=n)] components of the spin–forbidden dipole-allowed transition moment for the B 1Σ+, v=0→a 3Π, v=n transition were obtained. It was found that μ⊥(B 1Σ+0+, v=0; a 3Π1, v=n) peaks at n=1 and that for 0≤n≤3 we have μ⊥(B 1Σ+0+,v=0; a 3Π1, v=n) >μ∥ (B 1Σ+0+,v=0; a 3Π0+,v=n)]. The spin–orbit induced mixings of the zeroth order X 1Σ+0+ and A 1Π1 states with the a 3Π0+ and a 3Π1 states are responsible for the preponderance of the observed B→a (0,0) and (0,1) transition moments.
A theoretical study of methyl chemisorption on Ni(111)
Authors: Schüle Josef , Siegbahn Per , Wahlgren Ulf
Published: July 2002

Calculations including electron correlation have been performed for methyl adsorbed on nickel clusters mimicking the Ni(111) surface. The chemisorption energies are evaluated following a recently developed scheme, in which the cluster is prepared for bonding. In this way cluster dependent oscillations of the chemisorption energies are largely eliminated. By also using calculated vibrational frequencies of the adsorbed methyl an almost certain assignment of the preferred chemisorption site is obtained. Methyl is found to adsorb in the threefold hollow site with a chemisorption energy in the range 50–55 kcal/mol. The origin of the soft C–H frequencies observed experimentally is a charge transfer from the metal into the C–H antibonding orbitals. The only weak sign of a direct metal–carbon–hydrogen interaction in the calculations is that the C–H frequency is slightly lower for an eclipsed compared to a staggered orientation of methyl in the threefold hollow. The present results are compared to previous experimental and theoretical results for methyl adsorbed on metal surfaces.
Relaxation and flow mechanisms in ‘‘fragile’’ glass-forming liquids
Authors: Stillinger Frank H.
Published: July 2002

Fragile glass-forming liquids are characterized by strongly non-Arrhenius temperature dependence of shear viscosity, and by unusually high heat capacity in the supercooled liquid above the glass transition. Using the inherent structure formalism, a theoretical model is developed to explain these characteristics. The vitrifying liquid is viewed as a dynamic patchwork of relatively strongly bonded (but amorphous) molecular domains that are separated by irregular walls of weakened bonds. Mean domain diameter ξ rises as temperature declines to minimize wall free energy, but the process is self-limiting due to intradomain frustration energy. Shear flow occurs by a ‘‘tear and repair’’ mechanism involving elemental wall areas of mean size ξ2. The relation of the present model to the tiling models for glass formation is explored, and helps to establish the presence of anomalous heat capacity in the supercooled liquid. The analysis suggests in strongly supercooled fragile liquids that the Stokes–Einstein formula may underestimate self-diffusion constants, and that the Adam–Gibbs relation for mean relaxation time in terms of calorimetric entropy may display systematic errors.
Site percolation and Vogel–Fulcher behavior on picosecond time scales in concentrated electrolytes: Raman spectra of aqueous solutions of LiSCN and KSCN
Authors: Rothschild Walter G. , Perrot Michel
Published: July 2002

In this paper we further explore the applicability of a vibrational T2 process based on the extended-exponential modulation model [Rothschild, Perrot, and Guillaume, J. Chem. Phys. 87, 7293 (1987)] to Raman correlation data of concentrated aqueous solutions of LiSCN and KSCN [Katō, Mol. Phys. 48, 1119 (1983); Katō and Takenaka, Mol. Phys. 46, 257 (1982)]. In general, the values of dispersion parameter α in the modulation function exp[−(t/τ)α], obtained from the fit of the theory to the isotropic correlation data of the CN oscillator, predict the prevalence of interrelated, collective dynamic processes in the medium that are the cause of the instantaneous oscillator transition frequency shifts (motional narrowing). In particular we predict, from the observed concentration dependence of α, strong short-time (fraction to several ps) cation–water–anion interactions that, in the more concentrated LiSCN–H2O systems at 303 K, are above a site percolation threshold with a value of α∼0.3 (close to that found in glasses). The expectation value of t, 〈t〉=τΓ(1+1/α), becomes critical near a concentration of 5 mol/ℓ  and shows a pronounced Vogel–Fulcher-type temperature dependence (T0=250 K) in the 10 mol/ℓ  LiSCN–H2O system over a range 0.45–76 ps. However, since α approaches its limiting value=1 at the highest temperature reported (353 K), the large-cluster cation–water–anion distributions in LiSCN–H2O must be rather tenuous. In contrast, the characteristics of α and of 〈t〉 for the KSCN–H2O systems agree with the relatively weak cation–water forces; the (inverse) concentration dependence of α is linear, its temperature dependence is flat, and the Vogel–Fulcher-type temperature behavior of 〈t〉 for the 10 mol/ℓ  solution stretches merely from 0.7 to 1.4 ps.
An electron diffraction study of alkali iodide vapors
Authors: Hartley J. G. , Fink M.
Published: July 2002

The structures of NaI, KI, RbI, and CsI in the vapor phase were determined via standard electron diffraction counting techniques with nozzle temperatures ranging from 770–866 K. The data analysis yielded the structure parameters and the ratio of the monomeric to dimeric compounds. The mean amplitudes of vibration for the monomeric molecules were held at values based on microwave frequencies and the harmonic potential approximations. The corrected monomer distances re agree well with microwave-spectroscopy results. Rhombic geometries with bonded dimer distances ranging from 3.00 to 3.57 Å and I–M–I angles that ranged from 95° to 102° were observed. The dimer mole fraction in the vapor varied from 3% to 18%. There was no significant amount of any trimeric species observed in the vapor. The bonded distance of KI was found to be equal to the interionic separation of the crystal. The observed dimer mole fractions were compared with various theoretical predictions.
Spectroscopic characterization of the X(1+) and A(3+) states of CdNe, CdAr, CdKr, and CdXe
Authors: Kvaran Agust , Funk David J. , Kowalski Andrzej , Breckenridge W. H.
Published: July 2002

We report the spectroscopic characterization of the X(10+) and A(30+) states of CdNe, CdAr, CdKr, and CdXe. The van der Waals molecules were created in a free jet supersonic expansion and studied by low and high resolution laser-induced fluorescence. CdAr was also studied by dispersed fluorescence. A method of analyzing rotationally structured vibrational bands of overlapping isotopic spectral contributions is discussed. Spectroscopic parameters are obtained from computer simulations of CdNe and CdAr spectra and from analysis of vibrational isotope splittings for CdKr and CdXe. CdNe: r″e (X state)=4.26±0.05 Å, re(A state) =3.62±0.05 Å, D′e(A state) =77 cm−1; CdAr: r″e(X) =4.33±0.04 Å, r′e(A) =3.45±0.03 Å, De(A) =325 cm−1; CdKr: D′e(A) =513 cm−1 and CdXe: De(A) =1086 cm−1.
Intersymmetry and Coriolis-assisted intermode vibrational energy transfers in CD3Cl from infrared double-resonance measurements
Authors: Doyennette L. , Ménard-Bourcin F.
Published: July 2002

The time-resolved infrared (IR) double-resonance method has been applied to the study of collision-induced vibrational energy transfers in CD3Cl. The fast energy transfer between the Coriolis-coupled levels of the v2=1 and v5=1 vibrational states of CD3Cl was investigated both in the pure gas and in mixtures with argon yielding rate constants, respectively, equal to (1.18±0.08)107 and (2.12±0.78)106 s−1 Torr−1. This fast intermode transfer is found to occur only between rovibrational levels of the same symmetry type. The V–V transfer between the A and E symmetry levels has also been observed, as well as the V–V transfer between the two isotopic species CD335Cl and CD337Cl. The measured rate constants for these processes are very close and equal to (6.1±0.4)105 s−1 Torr−1. The rate constant for the vibrational thermalization was also determined and found to be (2.5±0.5)103 s−1 Torr−1, in agreement with previous studies.
Non-Boltzmann rotational and inverted spin–orbit state distributions for laser-induced desorption of NO from Pt(111)
Authors: Richter Lee J. , Buntin Steven A. , Cavanagh Richard R. , King David S.
Published: July 2002

The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.
The electronic spectrum of NOCl: Photofragment spectroscopy, vector correlations, and a
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o calculations
Authors: Bai Y. Y. , Ogai A. , Qian C. X. W. , Iwata L. , Segal G. A. , Reisler H.
Published: July 2002

The electronic absorption spectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of ab initio calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ-doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion-cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ-doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low-lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized rNO and ClNO angle as a function of rClN, as well as the angular dependences of the potentials and oscillator strengths of the singlet–singlet transitions. The following assignments are proposed for the main absorption bands: (1) E band—T1(1 3A″)←S0(1 1A′); the transition borrows intensity by mixing with remote singlet states, predominantly the 4 1A′ state; (2) D and C bands—S1(1 1A″)←S0(1 1A′); the C band corresponds to excitation of ν′1 in S1; (3) B band—S3(2 1A′)←S0(1 1A′); (4) A band—S5(4 1A′)→S0(1 1A′). The assignments proposed here are in full agreement with all the experimental observations and the results of the calculations. Despite the shallow minima calculated for the T1 and S1 surfaces, dissociation on all the surfaces is fast, and the implications of the results to the dissociation dynamics are discussed.
Persistent infrared laser induced spectral holes in hydrogen bonded polymers
Authors: Shirk James S. , Pong Richard G. S. , Snow Arthur W.
Published: July 2002

Persistent spectral holes were burned into the profile of the hydrogen bonded OH stretching absorption, near 3500 cm−1, of three polymeric alcohols. The mechanism is an IR induced conformational change of a H-bonded OH group to weaken the hydrogen bond. In one polymer, an empirical quantum efficiency of about 5×10−3 was measured and a hole depth of 20% of the absorption was achieved with a low power cw laser. The conformational changes occur near the bond which is excited and before extensive intramolecular vibrational randomization.
Half-collision dynamics: Exclusive production of Cd(5s5p 3P2) from the predissociation of Cd(5s5p 1P1)⋅Xe electronic states
Authors: Funk David J. , Breckenridge W. H.
Published: July 2002

The CdXe(C 1Π1) state, correlating with Cd(5s5p 1P1)+Xe, undergoes efficient spin–orbit predissociation via the repulsive c̃ 3∑1 state, producing Cd(5s5p 3P2)+Xe exclusively. This is consistent with the fact that the Cd(5s5p 1P1) atomic state is deactivated to Cd(5s5p 3P2 on nearly every collision with a Xe atom. The CdXe(D 1∑+0) state, excited above its dissociation limit, produces not only Cd(5s5p 1P1)+Xe but also a small yield of Cd(5s5p 3P2)+Xe. A mechanism is proposed in which the D 1∑+0 and C 1Π1 states are mixed via rotationally induced L uncoupling, leading to Cd(5s5p 3P2)+Xe by C 1Π1–c̃ 3∑1 predissociation. This has implications related to the dynamical concept of ‘‘orbital following.’’ The dissociation energy D0 of the CdXe(X 1∑+0) ground state is also determined to be 176±5 cm−1.
A theoretical study of the energy of hypofluorous acid, HOF
Authors: Pople John A. , Curtiss Larry A.
Published: July 2002

An ab initio molecular orbital study of the dissociation energy of HOF based on the newly developed G1 method is presented. The results indicate that the value of ΔH○f0 of gaseous HOF derived by Berkowitz, Appelman, and Chupka should be reexamined and that the best experimentally derived value of ΔH○f0 of HOF is −19.9 kcal/mol, corresponding to a dissociation energy D0(HO–F) of 47.5 kcal/mol.
Evidence for valence hole localization in the Auger decay and fragmentation of carbon and silicon tetrafluorides
Authors: Lapiano-Smith D. A. , Ma C. I. , Wu K. T. , Hanson D. M.
Published: July 2002

Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time-of-flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F2+ ions following fluorine 1s excitation in SiF4 provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two-hole final state that persists on the time scale of fragmentation. In contrast, no F2+ was observed for CF4, which indicates that fragmentation for this molecule is more characteristic of a delocalized two-hole state.
Application of Badger’s rule to third row metal diatomics
Authors: Weisshaar James C.
Published: July 2002

We extend the range of applicability of Badger’s rule, an empirical correlation between ωe and re , to diatomic molecules that include one or two metal atoms from the third row of the periodic table. For such M2 and MO species, the accuracy of the correlation competes with that of ab initio calculations in certain cases. We use the correlation along with experimental values of ωe to estimate re for two electronic states each of Cu+2 and of TiO+. For Cu+2 , we obtain 2.35±0.10 Å for X 2Σ+g and 2.22±0.15 Å for the excited 2Π state at T0=1.143 eV. For TiO+, we obtain 1.54±0.05 Å for X 2Δ and 1.57±0.05 Å for the B 2Σ+ state at T0=1.39 eV.
Spectra in the chaotic region: Methods for extracting dynamic information
Authors: Gomez Llorente Jose M. , Zakrzewski Jakub , Taylor Howard S. , Kulander Kenneth C.
Published: July 2002

Nonlinear dynamics is applied to chaotic unassignable atomic and molecular spectra with the aim of extracting detailed information about regular dynamic motions that exist over short intervals of time. It is shown how this motion can be extracted from high resolution spectra by doing low resolution studies or by Fourier transforming limited regions of the spectrum. These motions mimic those of periodic orbits (PO) and are inserts into the dominant chaotic motion. Considering these inserts and the PO as a dynamically decoupled region of space, resonant scattering theory and stabilization methods enable us to compute ladders of resonant states which interact with the chaotic quasicontinuum computed in principle from basis sets placed off the PO. The interaction of the resonances with the quasicontinuum explains the low resolution spectra seen in such experiments. It also allows one to associate low resolution features with a particular PO. The motion on the PO thereby supplies the molecular movements whose quantization causes the low resolution spectra. Characteristic properties of the periodic orbit based resonances are discussed. The method is illustrated on the photoabsorption spectrum of the hydrogen atom in a strong magnetic field and on the photodissociation spectrum of H+3 . Other molecular systems which are currently under investigation using this formalism are also mentioned.
Relativistic effects in molecules: Pseudopotential calculations for PbH+, PbH, PbH2, and PbH4
Authors: Schwerdtfeger P. , Silberbach H. , Miehlich B.
Published: July 2002

Nonrelativistic, relativistic, and semiempirical pseudopotentials for the Pb atom have been generated to replace the chemically inert core electrons for investigating the effects of relativity and correlation on molecular properties of PbH+, PbH, PbH2, and PbH4. Spin–orbit effects are taken into account by using a quasirelativistic two-spinor pseudopotential. The relativistic bond contraction is found to be dependent on the Pb(6s) orbital participation in the Pb–H bond (Δrelre : 0.04 Å for PbH+, PbH, and PbH2 and 0.07 Å for PbH4). The calculated and measured values agree excellently [e.g., re (PbH)=1.839 Å; expt. 1.839 Å]. The inert pair effect for the lead hydrides will be discussed.
Excited state intermolecular proton transfer in matrix isolated β-naphthol/ammonia complexes
Authors: Brucker G. A. , Kelley D. F.
Published: July 2002

The spectroscopy and proton transfer dynamics of matrix isolated β-naphthol⋅(NH3)n, n=1,2...complexes have been studied. The complexes are formed by annealing of β-naphthol/NH3/argon matrices. The annealing studies indicate that the n=3, and probably n=4, complexes undergo excited state intermolecular proton transfer (ESPT), and this assignment is confirmed by comparison of experimental and simulated spectra. Time resolved emission studies indicate that the ESPT time is about 20 ps. These results are discussed in terms of simple tunneling theories.
Q dependence of rotational excitations of CH4 molecules in argon
Authors: Asmussen B. , Gerlach P. , Press W. , Prager M. , Blank H.
Published: July 2002

Rotational excitations of CH4 molecules, matrix isolated in solid argon, are observed with inelastic neutron scattering (INS). In the argon matrix the orientational crystalline field Vc(Ω) leads to weakly hindered rotation. Experimental data are compared with the predictions of the ‘‘Extended James–Keenan model’’ for solid CH4 II. Deviations from complete orientational disorder are studied via the Q dependence of the scattered intensities. The transition energies for CH4 in Ar differ considerably from the corresponding transition energies in solid CH4 II due to the stronger crystalline field, while the transition matrix elements seem to be rather similar. The parameters β4=7.5 and β6=−25 of the expansion of the crystalline field into octahedral rotor functions are estimated from the experimental data and compared with predictions, based on different empirical potentials between nonbonding atoms.
Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and CO2
Authors: Ruoff R. S. , Emilsson T. , Chuang C. , Klots T. D. , Gutowsky H. S.
Published: July 2002

Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2–HF, –HCl, –HCF3, and –CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN–Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T-shaped with C2v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B0 is 699.204 and 467.408 MHz, respectively, and DJ is 162 and 87 Hz; for Y=HCF3, B0 is 305.742 MHz and DJ and DJK are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B0+C0)/2 is 452.426 MHz and DJ is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B0’s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers. The B0’s are relatively insensitive to the position of the central HCN but give r1+r2 accurately. With this limitation, r1 and r2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r1 are 0.069, 0.054, 0.030, and 0.004 Å and in r2, 0.043, 0.062, 0.042, and 0.052 Å. The shrinkage in r1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN–Y dimer.
Electronic ground state assignment for O+4: Neon matrix electron-spin resonance investigation
Authors: Knight Lon B. , Cobranchi S. T. , Petty Jeff
Published: July 2002

The O+4 radical cation has been generated by two different methods and trapped as an isolated ion in neon matrices at 4 K for electron-spin resonance (ESR) investigation. The O+4 species is a potentially important atmospheric ion for which little experimental or theoretical information is available. These ESR results indicate that the electronic ground state of O+4 is a nonlinear quartet (S=3/2) with the three unpaired electrons occupying primarily 2p(π) orbitals. The fine structure-parameters obtained from an analysis of the ESR spectrum are D=1.875(5) and E=0.0350(5) cm−1 . The isotopic ions16,17 O+4 were also generated but the 17 O hfs could not be fully analyzed because of the large number of overlapping hyperfine lines.
Resonant two-photon ionization-photoelectron spectroscopy of Cu2: Autoionization dynamics and Cu+2 vibronic states
Authors: Sappey Andrew D. , Harrington Joel E. , Weisshaar James C.
Published: July 2002

Resonant two-photon ionization of gas phase Cu2 in a cold molecular beam in conjunction with time-of-flight photoelectron spectroscopy provides new vibronic state spectroscopic information for the dimer cation Cu+2. One color ionization via the 0–0, 1–0, and 2a–0 bands of Smalley’s System V neutral Cu2 resonant states (J←X transition) accesses Cu+2 states in the range 0–1.4 eV. The electron kinetic energy measurements slightly refine the first adiabatic ionization energy of Cu2 to I1(Cu2)=7.899±0.007 eV. We observe two electronic states of Cu+2 which we assign as X 2Σ+g and an excited 2Π spin–orbit pair of sublevels with origins at T0(2Π3/2)=1.143±0.002 eV and T0(2Π1/2)=1.256±0.002 eV. The absence of spin–orbit splitting identifies the ground state 2Σ symmetry; the spin–orbit splitting of 898±8 cm−1 identifies the excited states as 2Π. Within X 2Σ+g we observe a remarkably long vibrational progression, perhaps extending from v=0–80. The vibrational intervals determine the constants ωe=188±4 cm−1 and ωexe=0.75±0.09 cm−1. The 2Π vibrational intervals determine ωe=244±6 cm−1. The adiabatic bond dissociation energy of ground state Cu+2 is D0(Cu+–Cu)=1.84±0.08 eV. The intensity pattern of the X 2Σ+g vibrational bands exhibits multiple peaks whose positions and amplitudes are sensitive to the resonant J state vibrational level. For 0–0 excitation, we observe reproducible band intensity alternation. We present preliminary mass spectral and photoelectron data indicating that the cause of the highly non-Franck–Condon band intensities is excitation of long lived, dissociative autoionization states which undergo extensive nuclear motion on the time scale of electron ejection. We propose an autoionization mechanism that includes a description of the Cu2 J state and explains the observed phenomena invoking only one electron transition.