bluuebunny/crossref_metadata_2025 huggingface dataset:
The a
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o model potential method. Main group elements
Authors: Huzinaga Sigeru , Seijo Luis , Barandiarán Zoila , Klobukowski Mariusz
Published: July 2002

In this paper we present the ab initio core model potential method, in which the Coulomb and exchange core operators Jc and Kc are represented as accurately as possible using adequate local and nonlocal potentials, and the valence basis set is optimized in atomic MP SCF calculations following the variational principle. Nonrelativistic model potential parameters and valence basis sets are presented for the main group elements from Li to Xe. The pilot SCF molecular calculations on the ground states of N2, P2, As2, Sb2, and CaO show a good agreement between all-electron and model potential results, in particular the changes observed in molecular results due to improvements of the valence part of the all-electron basis set are reproduced by the model potential calculations.
Parallel algorithms for quantum chemistry. I. Integral transformations on a hypercube multiprocessor
Authors: Whiteside Robert A. , Binkley J. Stephen , Colvin Michael E. , Schaefer Henry F.
Published: July 2002

For many years it has been recognized that fundamental physical constraints such as the speed of light will limit the ultimate speed of single processor computers to less than about three billion floating point operations per second (3 GFLOPS). This limitation is becoming increasingly restrictive as commercially available machines are now within an order of magnitude of this asymptotic limit. A natural way to avoid this limit is to harness together many processors to work on a single computational problem. In principle, these parallel processing computers have speeds limited only by the number of processors one chooses to acquire. The usefulness of potentially unlimited processing speed to a computationally intensive field such as quantum chemistry is obvious. If these methods are to be applied to significantly larger chemical systems, parallel schemes will have to be employed. For this reason we have developed distributed-memory algorithms for a number of standard quantum chemical methods. We are currently implementing these on a 32 processor Intel hypercube. In this paper we present our algorithm and benchmark results for one of the bottleneck steps in quantum chemical calculations: the four index integral transformation.
Atom scattering from disordered surfaces: Randomly corrugated hard walls and the sudden approximation
Authors: Dacol D. K. , Rabitz H. , Gerber R. B.
Published: July 2002

We used a randomly corrugated hard wall model and the sudden approximation to analyze two experiments on atom scattering from disordered surfaces. In one, the structural surface disorder was caused by ion bombardment. In the other, the disorder was due to an incomplete overlayer of adsorbed atoms. We also present a study of the scattering of a rigid rotor by a randomly corrugated hard wall using the sudden approximation.
Vibrational relaxation of highly excited molecules: Mode specific vibrational energy transfer from SF6 to N2O
Authors: Koshi Mitsuo , Vlahoyannis Yannis P. , Gordon Robert J.
Published: July 2002

Vibrational energy transfer from SF6 to N2O was studied by using a pulsed CO2 laser for multiphoton excitation of SF6 and monitoring the 4.5 μm emission from N2O(v3=1). The initial average energy of SF6 was selected by varying the fluence of the laser. The vibrational temperature of the ν2 mode of N2O was determined by measuring the attenuation of the fluorescence with a cold gas filter. The vibrational temperature of the ν3 mode was estimated by equating the SF6 and N2O(ν3) vibrational temperatures at the fluorescence maximum. The principal finding of this study is that N2O(ν2) is preferentially excited. While the mode specificity of the VV transfer declines with energy, it is greater than expected statistically. We also observed that the relaxation of ν2 is an order of magnitude slower than the relaxation of ν3. A kinetic model which accounts for most of the observations shows that VV transfer from SF6 to N2O occurs from high lying states of SF6 while the decay of N2O occurs by back transfer to the discrete levels of SF6. The slow relaxation of the bending mode of N2O indicates the existence of a bottleneck in VV transfer between N2O(ν2) and the discrete levels of SF6, which may be caused by a mechanism involving long-range attractive forces.
Collisional relaxation of DF(v=1) and HF(v=1) by the DF dimer
Authors: Rensberger K. J. , Robinson J. M. , Crim F. F.
Published: July 2002

Overtone vibration-laser double resonance measurements determine the vibrational relaxation rates of DF(v=1) and HF(v=1) by the DF dimer. Both monomers are efficiently relaxed by the dimer at a rate that is 20% of the gas kinetic rate. The similarity of the rate constants for the two systems, which have very different energy defects, indicates that the relaxation occurs by collision complex formation and energy redistribution, rather than direct vibration-to-vibration energy transfer from the monomer to the dimer.
Quantitative study on the photoexcitation process of SiF4 at 49–120 nm
Authors: Suto Masako , Wang Xiuyan , Lee L. C. , Chuang T. J.
Published: July 2002

The photoabsorption and fluorescence cross sections of SiF4 were measured in the 49–120 nm region using synchrotron radiation as a light source. Absorption bands were tentatively assigned to Rydberg states and their oscillator strengths were determined. The fluorescence was dispersed to identify the emitting species. In the 49–58 nm region, the emitter is attributed to the excited SiF+4 (D̃) ion. In the 92–98 nm region, the fluorescence is likely produced by the excited SiF*3 radical. Photodissociation process of SiF4 in the extreme ultraviolet region is discussed.
Close-coupling calculations of transport and relaxation cross sections for H2 in Ar
Authors: Hutson Jeremy M.
Published: July 2002

Close-coupling calculations of effective cross sections for transport and relaxation processes in Ar+H2 mixtures are performed, using both the recent TT3(6,8) potential of Le Roy and Hutson and the older BC3(6,8) potential of Carley and Le Roy. The cross sections calculated are those involved in sound absorption, thermal diffusion, shear viscosity, pressure broadening and shifting of rotational Raman lines, depolarized Rayleigh light scattering, nuclear magnetic relaxation, and the Senftleben–Beenakker effects. Comparisons with experimental results are made wherever possible.
Sideband intensities in two-dimensional NMR spectra of rotating solids
Authors: Herzfeld Judith , Roberts James E. , Griffin Robert G.
Published: July 2002

In the presence of magic angle sample spinning (MASS), two-dimensional solid-state NMR spectra of magnetically dilute I=1/2 nuclei split into rotational sidebands spaced at the spinning frequency in both dimensions. The intensities of these sidebands contain information on the sizes and relative orientations of the interactions present in the two experimental time domains. Here we extend an earlier analysis of sideband intensities in one-dimensional MASS spectra to two-dimensional MASS spectra. The resulting equations are applicable to heteronuclear and homonuclear correlation spectra, dipolar/chemical shift spectra, and slow chemical exchange spectra. We use the equations to deduce the orientation of the 15N chemical shift tensor in the amide group of L-asparagine⋅H2O from the sideband intensities in the 15N–1H dipolar/chemical shift spectrum. This application to an AX2 system enlarges on previous work with AX systems and may be generalized to AXn systems.
Application of the complex rotation method to the study of resonance states of atoms at a corrugated surface
Authors: Maniv Tsofar , Engdahl Erik , Moiseyev Nimrod
Published: July 2002

The complex rotation method is applied to the calculation of complex poles of the scattering matrix for atoms selectively trapped at a corrugated, static surface. The method is found to be extremely efficient and accurate even for highly corrugated surfaces, for which the use of more conventional methods is known to be in trouble. The method also provides insight into the trapping processes, revealing that for a simple harmonic corrugation there is a critical value of the corrugation amplitude, below which the trapping process is dominated by the coupling between adjacent diffraction channels, while above this critical value the coupling between more distant channels dominates.
A theoretical study of the infrared vibrational intensities of CH3F
Authors: Sosa Carlos , Schlegel H. Bernhard
Published: July 2002

The atomic polar tensors, vibrational frequencies, and infrared intensities were calculated for methyl fluoride using ab initio molecular orbital methods. Calculations were carried out using several split-valence basis sets (3-21G, D95, 6-31G, 6-311G) augmented with various numbers of diffuse and polarization functions. The CID/6-31G(d) optimized geometry was found to agree well with the experimental geometry and was used for all of the calculations. Atomic polar tensors were computed by analytical differentiation of the dipole moment at the Hartree–Fock level, and by numerical differentiation at the CISD level. The Hartree–Fock limit of the dipole moment is estimated to be 2.06±0.02 D at the CID/6-31G(d) optimized geometry. Compared with the observed, anharmonic vibrational frequencies, the calculated frequencies are an average of 8±4% too high at the HF/6-311++G(3d,3p) level, with larger errors for bending modes than for stretches. When correlation corrections are included the errors are significantly smaller and more uniform, 4.4±1.0%. Infrared intensities are much more sensitive to basis set effects than frequencies, requiring multiple sets of polarization functions, diffuse functions, and correlation corrections. A useful relation has been found between the calculated molecular dipole moment and the intensity of the CF stretching mode.
Self-consistent interaction potential for a molecule adsorbed on a dielectric surface: A symmetric top molecule on an ionic crystal
Authors: Girard Christian , Girardet Claude
Published: July 2002

An iterative self-consistent determination of the long range interaction energy between an admolecule and a ionic crystal is performed within the scheme of local and response potentials and the definition of the generalized electric susceptibilities of the two partners. The multipolar (electrostatic+induction) contributions and the quantum (dispersion+empirical short range) terms are determined as a sum of interactions between the molecule and the atomic planes parallel to the surface, constituting the crystal. The discrete structure of each plane is described with an increasing accuracy by increasing the order of the Fourier expansion in the reciprocal planar lattice. A semianalytical expression of each contribution is given for a symmetric top molecule adsorbed on a NaCl surface as a function of the location of the center of mass and of the orientation of the molecule.
Hydrogen atom transfer reactions of He+ and Ne+ with H2, D2, and HD
Authors: Ervin Kent M. , Armentrout P. B.
Published: July 2002

The hydrogen atom transfer reactions of helium(1+) and neon(1+) ions with isotopic molecular hydrogen (H2, D2, and HD) are investigated using guided ion beam techniques. These reactions are exothermic, but are known to be extremely slow at thermal energies. The cross sections for formation of HeH+ (HeD+) and NeH+ (NeD+) exhibit thresholds at high relative translational energies, 8 to 12 eV c.m. Unusual isotope effects are observed in the reaction with HD. The deuteride product is formed exclusively near threshold, while the hydride product predominates at higher energies. Reaction mechanisms involving Rydberg excited states of HeH+2 and NeH+2 are discussed.
The noncoincidence effect in the ν3 band of liquid methyl fluoride
Authors: Zyung Taehyoung , Wilde Richard E.
Published: July 2002

The 11 cm−1 splitting observed in the ν3 region of the Raman spectrum of liquid methyl fluoride is explained as arising from the noncoincidence effect. An interpretation of the solid-state infrared and Raman spectra is based upon correlation-field splitting effects. An analysis of these two effects suggests that the transition-dipole moment increases from 1.31×10−6 C kg−1/2 in the gaseous state to 2.28×10−6 C kg−1/2 in the solid state.
Direct IR laser absorption spectroscopy of jet-cooled CO2HF complexes: Analysis of the ν1 HF stretch and a surprisingly low frequency ν6 intermolecular CO2 bend
Authors: Lovejoy Christopher M. , Schuder Michael D. , Nesbitt David J.
Published: July 2002

High sensitivity, tunable laser direct absorption methods are exploited to obtain high resolution IR spectra (Δν≲0.001 cm−1) of weakly bound CO2HF complexes in a pulsed supersonic slit jet expansion. Transitions from the ground vibrational state corresponding to a single quantum excitation of the ν1 HF stretch are observed and analyzed with a semirigid linear molecule Hamiltonian. The observed increase in both B (+1.75%) and D (+55%) upon ν1 excitation is inconsistent with the commonly used diatomic approximation, and is not possible to rationalize for a nearly linear upper state geometry with small amplitude zero point motion of the intermolecular CO2 bend coordinate. We consider an alternative centrifugal straightening mechanism which predicts large centrifugal distortion effects due to end over end rotation of a complex with a nonlinear vibrationally averaged geometry in a weak bending potential. In support of this interpretation, hot band spectra are observed arising from bend excited complexes significantly populated in the 16 K expansion; intensity based estimates of the internal excitation indicate a surprisingly low bend frequency of 10±5 cm−1. A preliminary analysis of the spectra as l doubling in a Π←Π vibrational hot band for a linear equilibrium geometry is presented. An alternative interpretation of the spectrum as asymmetry doubling of a K=1←1 rotational hot band for a bent geometry is also considered. This latter interpretation is more consistent with the data and predicts a CO2 bend angle in the complex for K=1 between 25° and 30°. Linewidths for the upper vibrational states in CO2HF exhibit homogeneous broadening 3–4 times in excess of the apparatus resolution. Voigt analysis of the absorption line shapes indicates linewidths (FWHM) of 136±16 MHz and independent of J state; this corresponds to a relaxation lifetime of 1.1±0.2 ns for HF in the complex.
Spectroscopy of reactive intermediates: Electron photodetachment from solvated anions
Authors: Moylan Christopher R. , Dodd James A. , Han Chau-Chung , Brauman John I.
Published: July 2002

Electron photodetachment spectra of two complex anions, C6H5CH2OHF− and CH3OHOCH−3, have been recorded, using an ion cyclotron resonance spectrometer to generate, store, and detect the ions. The threshold energies determine the potential well depths of the ion–molecule reactions for which the anions are presumable intermediates. The detachment energies for C6H5CH2OHF− and CH3OHOCH−3 are ≤3.05±0.06 and 2.26±0.08 eV, respectively. The ability to measure absolute photodetachment cross sections is demonstrated in the latter case. The electron affinity of C6H5CH2O is 2.14±0.02 eV (49.3±0.5 kcal/mol). One would predict that complex ions of the form ROHF− where ROH is less acidic than HF would have poor Franck–Condon factors for adiabatic threshold photodetachment. This prediction is confirmed; five such ions display cross sections smaller than about 10−20 cm2 at all wavelengths above 365 nm. A useful correlation between ion structure and observation or nonobservation of visible/near-UV photodetachment is therefore demonstrated.
Catastrophes and stable caustics in bound states of Hamiltonian systems
Authors: Delos J. B.
Published: July 2002

Caustics—envelopes of families of classical trajectories, or boundaries between classically allowed and forbidden regions—correspond to singular points of a phase-space surface called a Lagrangian manifold. According to catastrophe theory, only a limited number of types of caustics are stable under general perturbations of the manifold. Most of the caustics that are found in calculations correspond to members of the canonical list of elementary catastrophes. However, there are some exceptions—examination of trajectories of typical Hamiltonian systems shows that stable structures exist which are not in accord with the stability theorem of catastrophe theory. These exceptional cases are discussed in this paper. They arise because of the special form of the typical Hamiltonian of physical systems.
Radiative lifetimes of selected rovibronic triplet levels of the CO molecule
Authors: Strobl K. H. , Vidal C. R.
Published: July 2002

Radiative lifetimes of individual rovibronic levels of the perturbed a′ 3Σ+(v=14) and the e 3Σ−(v=5) states of the CO molecule have been measured under collision-free conditions using state selective excitation and measuring the temporal decay of the subsequent fluorescence. The accuracy of the measurements was typically 3% to 5%. The unperturbed triplet lifetimes and the corresponding spin-orbit interaction constants have been determined. Excellent agreement was found between the measured and the calculated lifetimes based on data of Field and co-workers. It is also shown that under favorable conditions lifetime measurements are significantly more sensitive to the mixing coefficients ‖β‖2 than wavelength measurements.
Computed potential surfaces for six low-lying states of Ni3
Authors: Walch Stephen P.
Published: July 2002

SCF/CCI calculations are presented for selected portions of the potential surfaces for six low-lying states of Ni3. The calculations use the effective core potentials developed by Hay and Wadt. For near equilateral triangle geometries, four states were studied, all of which arise from three Ni atoms in the 4s13d9 state. The 4s electron configuration here is 4sa′214se1, which leads to Jahn–Teller distortion analogous to Cu3. All of these states are within 0.04 eV of each other. For linear geometries two states were studied. The first linear state, which arises from three Ni atoms in the 4s13d9 state, is 0.16 eV higher than the corresponding near equilateral triangle state. The second linear state, which has one 4s23d8 center atom and the other two atoms in the 4s13d9 state, is estimated to be nearly degenerate (within 0.01 eV) with the near equilateral triangle structures.
Radiative lifetime and quenching of the 3p 4D state of atomic nitrogen
Authors: Copeland Richard A. , Jeffries Jay B. , Hickman Albert P. , Crosley David R.
Published: July 2002

The radiative lifetime of nitrogen atoms in the 3p 4D0 state is determined to be 43±3 ns, and the total removal rate constants from the excited 3p 4D0 state of nitrogen atoms are measured for collisions with He, Ne, Ar, Kr, Xe, and N2. In a low pressure discharge flow reactor, the 3p 4D0 state is prepared by two-photon excitation from the 4S0 ground state of atomic nitrogen. Time-resolved fluorescence from the 3p 4D0→3s 4P transition monitors the temporal evolution of the population in the 3p 4D0 state. As the rare gases become heavier with a more complex electron cloud, the quenching rate constants increase from less than 0.6×10−11 cm3 s−1 for He to a value of 66±12×10−11 cm3 s−1 for Xe. Collision mechanisms which might account for such a dramatic increase are discussed.
Mechanisms of laser interaction with metal carbonyls adsorbed on Si(111)7×7: Thermal vs photoelectronic effects
Authors: Gluck N. S. , Ying Z. , Bartosch C. E. , Ho W.
Published: July 2002

Ultrahigh vacuum studies of the interaction of 514 nm radiation from a cw Ar ion laser and its second harmonic at 257 nm with mono- and multilayer coverages of Mo(CO)6, W(CO)6, and Fe(CO)5 adsorbed on Si(111)7×7 at 90 K using thermal desorption spectroscopy (TDS), laser induced desorption spectroscopy, high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy were performed. A model for the temperature rise of the sample due to cw laser heating is developed. By directly measuring the substrate temperature, these experiments were able to distinguish between photoelectronic and thermal effects active in the decomposition and desorption mechanisms of the adsorbed carbonyls. Results from TDS and HREELS show that Mo(CO)6 and W(CO)6 are molecularly adsorbed, while Fe(CO)5 partially dissociates upon adsorption. The decomposition of adsorbed Mo(CO)6 is caused by electronic excitation due to direct absorption of the 257 nm radiation. Irradiation with 514 nm radiation results in no photochemistry. The same mechanism is dominant for adsorbed W(CO)6 and Fe(CO)5; however, new excitation mechanisms are available to these molecules that lead to bonding changes in W(CO)6 and Fe(CO)5 with 514 nm irradiation. The photodecomposition products of the adsorbed carbonyls are found to be different from the gas-phase decomposition products. The surface stabilizes the adsorbed carbonyls, preventing complete removal of all the CO ligands. Desorption of CO due to photoelectronic excitation is found to occur via sequential single photon absorption and extraction of CO ligands. Evidence of clustering of carbonyl fragments was observed after 257 nm irradiation.
AgH, Ag2, and AgO revisited: Basis set extensions
Authors: Martin Richard L.
Published: July 2002

An extended basis set has been developed for Ag which significantly improves the agreement between theoretical and experimental spectroscopic parameters for AgH, AgO, and Ag2. The major improvement comes about as a result of the improved treatment of electron correlation in the Ag d shell upon the introduction of f functions. Their inclusion produces very slight differences at the SCF level, but significant reductions in re and increases in ωe and De in the Mo/ller–Plesset perturbation theory expansion. At the MP4(SDTQ) level, typical results are 0.02 Å too long for re, 4% too low for ωe, and 10 kcal too small for De. From a pragmatic standpoint, MP2 give results very similar to this at a much reduced level of effort.
Surface reaction pathways of methylamine on the Ni(111) surface
Authors: Chorkendorff I. , Russell J. N. , Yates J. T.
Published: July 2002

The interaction and bond scission sequence of methylamine, CH3NH2, on Ni(111) have been investigated by means of Auger electron spectroscopy and temperature programmed desorption under UHV conditions in the temperature range 87–800 K. Comparisons have been made to the NH3/Ni(111) and N/Ni(111) systems. Methylamine is found to absorb molecularly through its lone pair up to ∼330 K after which a dehydrogenation channel opens which competes with the desorption channel. The sequence of the initial bond breaking was investigated by measuring the initial reactive sticking coefficient of deuterium labeled molecules at 363 K. Methylamine decomposition was found to take place through both ends of the molecule, initially with a slight rate preference at the C end. The CN residue left on the surface by the dehydrogenation process resulted in self-poisoning of the active sites. This residual species was found to decompose at ∼530 K, leading to N2 desorption and the diffusion of carbon into the crystal at 700–800 K.
A quasiharmonic model calculation for two non-Markovian far-infrared spectra of diatomic polar molecules in a rare-gas liquid. II. Correlation functions of the interaction and spectral application to a HCl–Ar solution
Authors: Hernández A. Calvo , Velasco S. , Mauricio F.
Published: July 2002

In paper I, the theoretical TTOC and PTOC far-infrared absorption coefficients of diatomic polar molecules in rare-gas liquid have been expressed in terms of correlation functions of the Hamiltonian which describes the diatomic–medium interaction. In this paper these correlation functions are derived using a harmonic model for the liquid structure which takes into account some peculiarities such as dynamical behavior and geometrical arrangement of atoms in the liquid. The correlation functions and the different spectral contributions which appear in the spectral theory of paper I are numerically analyzed for a HCl–Ar solution. Comparison between theoretical and experimental spectra shows that the best agreement is obtained with the PTOC approach.
Accurate width and position of lowest 1S resonance in H− calculated from real-valued stabilization graphs
Authors: Bentley John , Chipman Daniel M.
Published: July 2002

The use of real-valued stabilization graphs for calculation of resonance energies and widths is considered in connection with the lowest Feshbach resonance state of H−, nominally (Φ2s)2. One problem that arises is the difficulty of generating stabilization graphs with well defined avoided crossings between the resonance state of interest and nearby interacting continuum states. For this purpose, criteria are developed for selection of basis sets and CI lists and for determination of suitable stabilization parameters. Another problem is the extraction of resonance parameters from the stabilization graph. We study one particular analytic continuation procedure recently proposed by Isaacson and Truhlar. Criteria for separation of physical from nonphysical solutions of the complex energy stationary point, for determination of the necessary numerical precision for the input real eigenvalues, and for other details of the method have all been examined. The results for H−, even with a modest Gaussian basis set and partial CI list, are in excellent agreement both with experiment and with more elaborate calculations by other methods. It is concluded that the method is capable of describing electronic resonances to high accuracy and shows promise for application to systems more complicated than H−.
Adsorption of polar molecules at a wall: Monte Carlo simulations and integral equations
Authors: Russier V. , Rosinberg M. L. , Badiali J. P. , Levesque D. , Weis J. J.
Published: July 2002

In this paper we investigate the influence of an angular-dependent adsorption potential on the structure of a dipolar hard sphere fluid in the vicinity of a hard wall. We report Monte Carlo (MC) simulation results for the density profile, the orientational structure, the polarization and the electrical potential profiles and make detailed comparisons with the theoretical results obtained from integral equations theories. We propose a new version of the reference linearized hypernetted chain approximation (RLHNC) which clears up its relation with the mean spherical approximation (MSA). Good agreement with MC results is reached when one takes as a reference system the dipolar fluid in the absence of adsorption potential.
Nonexponential picosecond trapping in poly-(N-vinylcarbazole). A theoretical study
Authors: Sienicki K. , Winnik M. A.
Published: July 2002

A theoretical model for excitation trapping in poly-(N-vinylcarbazole) in the presence of excimer dissociation is presented. We assume a time-dependent trapping coefficient in the form k(t)=b+ct−1/2, which reflects both the diagonal disorder and quasi-one-dimensionality of carbazole hopping sites [Kauffmann et al., J. Chem. Phys. 85, 3566 (1986)]. The equations obtained for monomer and excimer fluorescence decays are given in terms of Laplace transforms. Respective equations for fluorescence decays have been obtained also in the case when excimer formation is time dependent. Equations for monomer and excimer quantum yields are also given.
Vibrational relaxation in the A 3Σ+u state of N2 in rare gas matrices
Authors: Pan S. L. , Zumofen G. , Dressler K.
Published: July 2002

The electron–beam excited A 3Σ+u–X 1Σ+g luminescence spectrum of N2 diluted in solid Ne, Ar, and Kr is investigated. The observed bands show a doublet structure, indicating that there are at least two distinct emitting sites of the N2 molecules in Ar and Kr matrices. For these matrices the population evolutions in the vibrationally excited A 3Σ+u states exhibit independent cascading decays within each band component for levels v′=3–6 while the populations of the lower levels v′=0–2 show temporal decays which can be attributed to the matrix-induced A 3Σ+u–X 1Σ+g radiation. For the Ne matrices long-living emission from the v′=0 and short-living and weak emission from the v′=1 and v′=2 levels are detected, respectively. The observed vibrational relaxation and its dependence on temperature and on impurities added to the solid is discussed on the basis of multiphonon-relaxation and energy–transfer models.
Krypton dimers: Resonance-enhanced MPI via g
e
r
a
d
e states in the region of atomic 4p55p configurations
Authors: Echt O. , Cook M. C. , Castleman A. W.
Published: July 2002

The two-photon resonant absorption of Kr2(1Σ+g) has been measured via (2+1) multiphoton ionization studies in the energy range 91 750–92 420 cm−1. Two vibrational progressions are observed. One of them is attributed to the atomic 4p5(2P3/2)5p[5/2]2 state, implying D′0 =220 cm−1. A long progression is assigned as correlating with the 4p5(2P3/2)5p[3/2]2 state, implying D0 ≥1480 cm−1.
Inelastic neutron scattering study of the torsional and CCC bend frequencies in the solid n-alkanes, ethane–hexane
Authors: Nelligan W. B. , LePoire D. J. , Brun T. O. , Kleb R.
Published: July 2002

High resolution inelastic neutron scattering (INS) spectra of the n-alkanes (n=2 to 6) have been measured, using the crystal analyzer spectrometer at the intense pulsed neutron source, Argonne National Laboratory, for energy transfer less than 1000 cm−1. In this paper we present these data and extend the valence force field for n-alkanes of Schachtschneider and Snyder by modifying the torsion potential and the C–C–C bending force constants. The modified force constant set provides a much improved match of the frequencies obtained from normal modes calculations to our experimental data in the energy region below 1000 cm−1 and a minimal perturbation of the frequencies at higher energies. The amplitude of the scattering peaks measured by the crystal analyzer spectrometer is shown to be closely related to the hydrogen density of states. The data also show peaks produced by lattice mode excitations. These data are not analyzed but are compared with data from the literature.
Spectroscopic studies of cryogenic fluids: Benzene in argon and helium
Authors: Nowak R. , Bernstein E. R.
Published: July 2002

Energy shifts and bandwidths of the 610 vibronic feature of the 1B2u←1A1g optical absorption spectrum of benzene dissolved in supercritical argon and helium, and in liquid argon are reported as a function of pressure, temperature, and density. Benzene/Ar solutions display red shifts of the 610 transition with increasing density but the dependence is found to be nonlinear at high densities. Benzene/He solutions evidence blue shifts of the 610 transition as a function of increasing density which also becomes nonlinear at high densities. Only small spectral shifts are recorded if the density is kept constant and pressure and temperature are varied simultaneously. In addition, a small density independent temperature effect on the transition energy shift is identified. Experimental results are compared to dielectric (Onsager–Böttcher and Wertheim) and quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute absorption energy. Reasonably good agreement between experiment and theory is found only for the benzene/Ar system at relatively low densities. The theory fails to predict energy shifts for both the benzene/He and high density benzene/Ar systems. This result is different from the findings for the benzene/N2 and benzene/C3H8 solutions and can be interpreted qualitatively in terms of competition between dispersive attractive and repulsive interactions as a function of density. The failure of the theory to describe these transition energy shifts is attributed to the omission of explicit repulsive interactions terms in the theoretical models employed.
Coupling model interpretation of dielectric relaxation of poly(vinyl acetate) near T
g
Authors: Rendell R. W. , Ngai K. L. , Mashimo S.
Published: July 2002

Recent dielectric relaxation measurements on poly(vinyl acetate), PVAc, by Nozaki and Mashimo (NM) have demonstrated temperature and frequency dependences for well-annealed samples near the glass transition temperature Tg which cannot be explained by conventional theories or concepts. NM concluded that a new concept has to be introduced to interpret the dielectric behavior of PVAc near Tg. We demonstrate that the coupling model of relaxation is able to provide this new concept and can quantitatively describe the dielectric behavior at all temperatures. NM have carried out dielectric relaxation measurements between T=84.77 and 26.85 °C on a PVAc sample with Tg=31 °C. The samples were carefully annealed at each temperature to ensure that it is in equilibrium. NM found that the asymmetric dielectric loss curves become broader as temperature is lowered toward Tg with the most significant changes occurring near Tg. Furthermore the frequency of maximum loss fm was found to be described by the WLF equation (expected to be valid on the basis of free volume models) only at high temperatures. Extrapolation of the WLF curve to the glass transition region deviates significantly from the measurements. All of these phenomena are expected by the coupling model. The model predicts a relation between the shape of the dielectric loss curves and properties of the relaxation time. We demonstrate that the temperature dependence of fm is quantitatively predicted at all temperatures, including the deviations from WLF behavior near Tg. The same coupling model was previously shown to resolve several other problems in polymer physics near Tg.
Recurrence relations for the Dunham coefficients and analytic expressions of the diagonal radial matrix elements for an anharmonic oscillator
Authors: Hadinger G. , Tergiman Y. S. , Hadinger G.
Published: July 2002

Analytic recurrence relations for the calculation of the Yun Dunham coefficients are established, thereby allowing to generate all the set of the Yun from the Yu0 coefficients. Then, expressions for diagonal radial matrix elements of any power of the internuclear displacement from equilibrium in an arbitrary vibration–rotation level of the 1Σ-state diatomic molecule follow in terms of Yu0 only. As an illustrative application, all the set of Yun coefficients previously published are found again by using the computer algebraic manipulation language reduce. A number of diagonal matrix elements of CO, HBr, and HCl have been symbolically computed and compared with previous available results.
Trajectory studies of hyperthermal Xe scattering from GaAs(110)
Authors: Lim Carmay , Tully John C. , Amirav Aviv , Trevor Paula , Cardillo Mark J.
Published: July 2002

We present the results of stochastic classical trajectory simulations of the scattering of a nonreactive gas, xenon, from a semiconductor surface of known structure and electronic and vibrational properties, GaAs(110). The range of incident energies considered is 1 to 8 eV, in order to make direct comparison with results of molecular beam experiments reported in the accompanying paper. We employed a 48-atom three-layer slab of GaAs, with periodic boundary conditions in two dimensions and frictional and stochastic forces in the third (surface normal) dimension. Pairwise additive Lennard-Jones potentials describe the gas–surface interaction. The calculations reproduce the large energy exchange and surprising structural sensitivity observed experimentally. Energy transfer is dominated by an initial binary interaction of the Xe with a single Ga or As atom. The repetitive collision nature of this binary encounter produces angular scattering patterns very similar to those obtained for scattering of light atoms, including ‘‘rainbow maxima.’’ The mechanism of energy deposition may have implications to ‘‘gentle’’ sputtering by neutral atom bombardment and to collision induced excitation of electron–hole pairs.
A collinear quantal study of vibrational predissociation and prereaction of van der Waals molecules
Authors: Engel Volker , Schinke Reinhard , Pollak Eli
Published: July 2002

It was recently suggested that vibrational excitation of van der Waals molecules could lead to prereaction instead of predissociation. A mechanism for vibrational prereaction based on tunneling of a light atom is proposed and tested. The effect of van der Waals wells on the reactivity of collinear MuD2, ClHBr, and ClHCl systems is studied. We find that vibrational prereaction is sensitive to the ratio of tunneling and vibrationally nonadiabatic interaction. If tunneling dominates, prereaction will take place. This is the case for the ClHBr and ClHCl systems. The interplay between reaction probability and photodissociation cross sections is studied. We conclude that vibrational prereaction will lead to an increase in reaction probability and an increase in the formation of products in photodissociation. This study suggests that vibrational prereaction could be observed at least in principle in light atom transfer systems.
Nascent vibrational and rotational distributions of CH+ (A 1Π) produced from the charge transfer reaction of He+ with C2H2 at thermal energy
Authors: Yamaguchi Sumio , Tsuji Masaharu , Nishimura Yukio
Published: July 2002

The CH+(A 1Π−X 1Σ+) emission has been observed from the He++C2H2 reaction at thermal energy under low pressure conditions. The nascent vibrational distribution for v′=0−3 was nearly exponential with a Boltzmann temperature of 2300±300 K. The nascent rotational temperature varied from 3000±500 K for v′=0 to 2000±1000 K for v′=3. The fractions of the available energy deposited into rotation 〈fr〉 and vibration 〈 fv〉 were evaluated to be 21% and 9%, respectively.
Experimental evidence for the time dependence of the metastable decay rate of Ne cluster ions: A further key to the magic number problem
Authors: Märk T. D. , Scheier P.
Published: July 2002

Clusters of Ne atoms formed in a supersonic nozzle expansion have been studied by electron ionization mass spectrometry. In particular, we have found that the metastable decay of Ne cluster ions is very nonexponential, thus confirming a recent theoretical prediction of Klots applying quasiequilibrium theory to the problem of unimolecular fragmentation of clusters.
The carbon chemical shift tensor in polyoxymethylene
Authors: Maas W. E. J. R. , Kentgens A. P. M. , Veeman W. S.
Published: July 2002

The complete chemical shift tensor of the carbons in crystalline polyoxymethylene is determined via the two-dimensional dipolar correlated 13C chemical shift NMR technique. The orientation of the tensor is determined by the local tetrahedral symmetry of the C2H2O2 unit and not by the overall symmetry of the helical chains. In an accompanying paper this shift tensor is used to study ultraslow motions in polyoxymethylene.
The ultraviolet spectra of size-resolved (benzene)N, N=2–60
Authors: Hahn Mee Y. , Paguia Angelo J. , Whetten Robert L.
Published: July 2002

Photoabsorption spectra of benzene clusters BN containing up to N=60 molecules have been obtained on an N-resolved basis over the B2u region of the spectrum (λ=265–245 nm). The resonant enhanced two-photon ionization of a supercooled cluster beam yields distinctive spectra for each value of N. The energy of the 610 resonance is found to decrease monotonically with increasing N, but at N=50 is still far from condensed-phase values.
Resonant two-photon dissociation of HD+ via Fano formalism
Authors: Chakrabarti M. K. , Bhattacharyya S. S. , Saha Samir
Published: July 2002

We have studied the resonant two-photon dissociation (TPD) of vibrationally excited HD+ by both 1sσg→1sσg vibrational and 1sσg→2pσu electronic transitions. Following Armstrong et al., a quantum mechanical description of the laser field enabled us to define the intermediate resonant state as a ‘‘pseudodissociating state’’ of the molecule–radiation system. This makes our theory formally similar to that of single-transition (photon) autoionization problem as formulated by Fano. This similarity has been exploited to obtain the intensity dependent line shapes of the resonant TPD of HD+. Resonant TPD cross sections from the vi=6, Ji=0 level of the ground 1sσg state of HD+ are calculated with a linearly polarized infrared laser radiation in the wavelength range 16 530–16 565 Å and at different intensities ranging from 1.53×108–3.83×1013 W/cm2. Effects of laser intensity on the linewidth, line shift, and cross sections are found to be very different for the two types of transitions. Our previous results of nonresonant TPD cross sections are reproduced via this model.
Neutron scattering from a series of compatible polymer blends: Significance of the Flory χF parameter
Authors: Brereton M. G. , Fischer E. W. , Herkt-Maetzky Ch. , Mortensen K.
Published: July 2002

In polymer blends the Flory χF parameter, describing the short range interactions between chain segments, is frequently represented by a structureless point-like interaction. However, in the mean field approximation, the influence of this parameter is directly enhanced by the degree of polymerization of the chains. This large enhancement is also effective with regard to the short range spatial structure of χF(r) and makes it possible to detect these structural details from a small angle neutron scattering experiment. We present a new way of representing the neutron data so as to explicitly determine these details and we demonstrate the method for the system polytetramethyl carbonate/deuterated polystyrene. The temperature dependence displayed by this system appears to be anomalous within the mean field approximation. However, we show that it can be successfully interpreted in terms of a structural model for χF(r) which consists of spatially separated positive and negative contributions within a 1–10 Å range. This form is supported by examining the relation of χF to the pseudopotential interactions between the segments. In the experimental part of this work we also compare our model with SANS data from two other systems (a) polyvinylmethyl ether/deuterated polystyrene, and (b) polyparamethyl styrene/deuterated polystyrene. The last two show a ‘‘normal’’ behavior and are consistent with a relatively structureless χF parameter.
Consistently averaged hydrodynamic interaction for Rouse dumbbells: Translational diffusivity
Authors: Öttinger Hans Christian
Published: July 2002

The recently developed Hookean dumbbell model with consistently averaged hydrodynamic interaction is used to discuss the translational diffusivity of polymers in dilute solutions undergoing homogeneous flows. An anisotropic diffusivity, different diffusion tensors for different definitions of translational diffusivity, and important effects caused by the distribution of the mass of a single polymer molecule in space are found. The results of this paper imply that the well-known Nernst–Einstein equation can not be generalized to the diffusion of polymers in flowing solutions.
Adiabatic decomposition of mass-selected alkali clusters
Authors: Bréchignac C. , Cahuzac Ph. , Roux J.-Ph. , Pavolini D. , Spiegelmann F.
Published: July 2002

The decomposition of metastable photoionized mass-selected alkali clusters is investigated using a tandem time-of-flight spectrometer. Na+n and K+n are found to decompose mainly by the evaporation of either a single neutral atom or a neutral dimer in a time scale of about 10 μs. The predominant fragmentation channels are found to follow the adiabatic dissociation channels associated with the lowest energies accordingly to our CI calculations. In the light of our experimental and theoretical results a comparison between the different calculations of the absolute atomization energies available in the literature is presented.
Dynamical temporal evolution of molecular IR absorption spectra observed with picosecond CO2 laser pulses
Authors: Mukherjee P. , Kwok H. S.
Published: July 2002

The variation of the collisionless absorption spectral shapes for vibrationally excited SF6 and C2F5Cl molecules as a function of the picosecond probe pulse duration was determined using a two-IR frequency pump–probe scheme. The experiments revealed a spectral narrowing with decreasing pulse duration, similar to a recent observation in C3F7I with a single laser pulse. For C2F5Cl, the vibrational temperature dependence of the spectral narrowing was also studied. The observed experimental results are shown to be consistent with a dynamic multitier classification of energy levels in the QC of polyatomic molecules. An empirical calculation was used to evaluate the relevant parameters associated with the intramolecular coupling. Specifically, the time constants for such a coupling are evaluated to be >20 ps for all the molecules studied. The effect of variable density of states on the time constant and spectral width is also discussed on the basis of the C2F5Cl data. It was found that both the redshift and the degree of spectral broadening increased with the internal energy of the molecule. The redshift was also observed to saturate at 〈n〉∼13. This dynamical spectral measurement is distinguished from the usual static spectral measurements in that the laser–molecule interaction time is used as a parameter, thus circumventing the usual pitfalls of inferring dynamical information from spectral measurements.
Effect of pressure and temperature on the Raman spectra of solid CO2
Authors: Olijnyk H. , Däufer H. , Jodl H.-J. , Hochheimer H. D.
Published: July 2002

High pressure Raman studies up to 18 GPa have been made on solid CO2 at room temperature and low temperatures. Changes in the spectral features of external modes are used to draw conclusions on the structural behavior under compression. Anharmonic contributions to the intermolecular potential and their pressure dependence are determined from the pressure and temperature shift of librational frequencies. Anharmonic contributions to the intramolecular potential are determined by pressure tuning of the Fermi resonance.
Viscoelastic relaxation of segment orientation in dilute polymer solutions. III. Results in the optimized Rouse–Zimm approximation to the rotational isomeric state hierarchy of models
Authors: Perico Angelo
Published: July 2002

The viscoelastic theory of Perico and Guenza for the TCFs probed in different experiments on segment relaxation is compared with the master equation approach of Helfand and applied to detailed rotational isomeric state polymer models in the optimized Rouse–Zimm dynamic approximation. In the free draining limit, for the vector autocorrelation function of a segment in central position on an ideally flexible chain, a zero order modified Bessel function form is obtained as previously found for the Helfand’s conformational time correlation function. The correlation time for the alignment memory function is explicitly calculated for the polyethylene, isotactic polypropylene, isotactic polystyrene, and polydimethylsiloxane.
The effect of small cluster environment on molecular oscillator strengths and spectra
Authors: Liver Naomi , Nitzan Abraham , Amirav Aviv , Jortner Joshua
Published: July 2002

In this paper, we present an electrostatic theory of the effect of small atomic cluster environment on molecular oscillator strengths and spectra. The molecular dipole is represented by a classical oscillating charge distribution and the cluster atoms by spherical polarizable particles. From our general theoretical results, we calculate the effect of cluster size and geometry on the molecular radiative lifetime and the spectral shift. These properties exhibit a considerable sensitivity to the cluster and molecular geometry, i.e., the cluster size and structure, the molecule–atom(s) distance(s), the spatial size of the molecular transition dipole, and molecular orientation within the cluster. The reduced plot of the relative change in the oscillator strength vs the relative frequency shift seems to have universal character and is useful in analyzing and predicting experimental trends. Available experimental results are consistent with the theoretical predictions.