bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoionization mass spectrometric studies of AsHn (n=1–3)
Authors: Berkowitz J.
Published: July 2002

The photoion yield curves of the free radicals AsH and AsH2, prepared by the reaction of H atoms with AsH3, have been measured. The adiabatic ionization potential of AsH (forming AsH+, X 2Π1/2) is 9.641±0.008 eV. Autoionizing Rydberg states are observed and analyzed to converge to an a 4Σ− state lying 1.94 eV higher in energy. The adiabatic ionization potential of AsH2 (forming AsH+2, X̃ 1A1) is 9.443±0.007 eV. The 3B1 state of AsH+2 is conservatively estimated from the spectrum to lie 0.60–1.46 eV higher in energy, with the lower figure expected to be close to the true value. In addition, the ion yield curves of AsH+3, AsH+2, and AsH+ from photoionization of AsH3 have been measured. From these measurements, the adiabatic ionization potential of AsH3 is 9.82±0.01 eV, the appearance potential of AsH+2 (+H) is 12.69±0.01 eV, and that of AsH+(+H2) is 11.295±0.05 eV. The latter two measurements, when combined with the corresponding ionization potentials, yield D0(H2As–H)=74.9±0.2 kcal/mol and D0(HAs–H)=66.5±0.2 kcal/mol. The value of D0(As–H), as deduced from these measurements, depends upon an accurate heat of atomization of AsH3, which in turn requires an accurate value for ΔH○f0 (As,g). An analysis of alternative values is presented, from which D0(As–H)=64.6±0.7 kcal/mol (2.80±0.03 eV) is obtained. When these stepwise bond energies, and earlier results on PHn and NHn, are compared with the semiempirical model of Goddard and Harding, the largest discrepancy occurs for NHn. An analysis of successive ionization potentials Pn, PnH, PnH2 (Pn=N, P, As), and also Ch, ChH, ChH2 (Ch=O, S, Se) based on the same philosophy again shows a large departure from prediction for the first row elements, but fair agreement for the second and third row hydrides. The deviation of the first row hydrides from the Goddard–Harding model is attributed to the substantial ionic character of these systems.
Pulsed laser photolysis kinetics study of the O(3P)+ClO reaction
Authors: Nicovich J. M. , Wine P. H. , Ravishankara A. R.
Published: July 2002

A pulsed laser photolysis technique has been employed to study the kinetics of the important stratospheric reaction O+ClO→k1Cl+O2 in N2 buffer gas over the temperature and pressure ranges 231–367 K and 25–500 Torr. 351 nm pulsed laser photolysis of Cl2/O3/N2 mixtures produced Cl atoms in excess over O3. After a delay sufficient for the reaction Cl+O3→ClO+O2 to go to completion, a small fraction of the ClO was photolyzed at 266 nm to produce O(3P). The decay of O(3P) in the presence of an excess, known concentration of ClO was then followed by time-resolved resonance fluorescence spectroscopy. We find that k1 is independent of pressure, but that k1(T) increases with decreasing temperature. Our results suggest that the Arrhenius expression k1(T)=(1.55±0.33)×10−11 exp{(263±60)/T} cm3 molecule−1 s−1 is appropriate for modeling stratospheric chemistry. Errors in the Arrhenius expression are 2σ and represent precison only. The absolute accuracy of k1 at any temperature within the range studied is estimated to be ±20%. Our results agree with other recent measurements of k1 at 298 K but give significantly faster rate coefficients at stratospheric temperatures. A few measurements of the rate coefficient for the reaction ClO+ClO→k7products were also carried out. These measurements were necessary to assess the time dependence of [ClO].
Vibrational quenching of NO+(v) in collisions with CH4 from 0.04 to 1.2 eV
Authors: Richter R. , Lindinger W. , Ferguson E. E.
Published: July 2002

The vibrational relaxation of NO+(v=1) ions in collisions with CH4 molecules has been measured in a flow drift tube experiment from thermal energy to 1.2 eV relative kinetic energy. The quenching rate constant has a broad minimum as a function of collision energy in the ∼0.5–0.6 eV range. These results are interpreted in terms of complex formation at low energies (KEcm ≪U) followed by vibrational predissociation in the complex and by a Landau–Teller nonadiabatic energy transfer at large relative kinetic energies (KEcm ≫U), where U is the interaction well depth.
Overtone-induced isomerization of allyl isocyanide
Authors: Segall Jeffrey , Zare Richard N.
Published: July 2002

The rate of isomerization of allyl isocyanide to allyl cyanide is monitored using Stern–Volmer kinetics when various features of the 5–0 and 6–0 C–H overtone stretch region are excited. Previously, Reddy and Berry found that the isomerization rates varied from band to band and that the variations were not monotonic with excitation energy. They attributed this behavior to nonstatistical effects. It is found in this study that the isomerization rates vary within an overtone band by up to a factor of 1.8, in addition to the variations from band to band seen previously. This observation rules out the possibility that the overtone bands are purely lifetime broadened. In addition, the photoisomerization rate increases with increasing temperature. An increase of 60 K increases the photolysis yield in the 5–0 C–H region by a factor of 3–4, while increases in the 6–0 C–H region are smaller, 25%–60%. These effects can be qualitatively explained if the overtone spectrum of allyl isocyanide is inhomogeneously broadened because of the presence of vibrational ‘‘hot bands.’’ In this case, molecules excited by the photolysis laser will have varying amounts of initial thermal energy depending on where the laser is tuned within a band. A simple model for the effects of hot bands on the isomerization rate is in good agreement with experimental results. It is not necessary to invoke nonstatistical effects in order to explain the observed overtone-pumped isomerization rates of allyl isocyanide.
Polymer chain dynamics derived from the Kramers potential: A treatment of the Rouse model with and without excluded volume interaction
Authors: Frisch H. L. , Pistoor N. , Sariban A. , Binder K. , Fesjian S.
Published: July 2002

We suggest that at least for single polymer chains relaxation times of typical dynamical processes can be obtained from biased sampling Monte Carlo computations on equilibrium data. We show this explicitly for the viscosity difference relaxation time τ in the Rouse or Zimm model (with any suitable, iterated or not, preaveraging of the Oseen–Burgers tensor) chain. We briefly discuss the corrections of such theory due to the excluded volume effect. We illustrate by performing the biased average of the equilibrium radius of gyration of Rouse chains of up to 150 monomer beads on a cubic lattice which yields directly τ.
Determination of chemisorption geometries for complex molecules by using near-edge x-ray absorption fine structure: Ethylene on Ni(100)
Authors: Zaera F. , Fischer D. A. , Carr R. G. , Gland J. L.
Published: July 2002

We have derived theoretical expressions for the incident beam angular dependence of the absorption cross section for ethylene chemisorbed on single crystal surfaces with linearly polarized light. The changes in the π and σ resonance intensities for C 1s near-edge x-ray absorption fine structure (NEXAFS) spectra can be related to the geometry of chemisorption relative to the surface. The derived equations have been used to analyze experimental data in order to determine the structure of chemisorbed ethylene on Ni(100). At low temperatures ethylene bonds with the molecular axis parallel to the surface but with the molecular plane tilted 50°. The geometry of vinyl fragments formed by annealing the crystal to 180 K was also determined. The C–C bond is bent 35° from the surface normal in this case, with the plane of the molecule tilted by 65°.
Improved potential energy surface for He–CO2
Authors: Beneventi L. , Casavecchia P. , Vecchiocattivi F. , Volpi G. G. , Buck U. , Lauenstein Ch. , Schinke R.
Published: July 2002

The anisotropic potential energy surface of He–CO2 is determined by the simultaneous analysis of newly measured high resolution total differential cross sections, differential energy loss spectra, new low temperature second virial coefficients, new diffusion, and viscosity data. The calculations are carried out in the infinite-order-sudden approximation. The repulsive anisotropy of the potential is determined from the rotationally inelastic cross sections and the quenching of the diffraction oscillations, while the absolute scale is fixed by the position of these oscillations. The second virial coefficient data are essentially sensitive to the general features of the spherical effective potential well. The transport data are then correctly predicted by this potential surface which differs both in the anisotropy and the spherical part from the recently derived multiproperty fit potential for this system.
A crossed laser-molecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm
Authors: Ray Urmi , Hou Hui Qi , Zhang Zhvangjian , Schwarz Wolfgang , Vernon Matt
Published: July 2002

A crossed laser-molecular beam study of the one and two photon dissociation mechanism of bis (cyclopentadienyl) iron (ferrocene, FeCp2) has been performed at 193 and 248 nm. By combining electron bombardment mass spectroscopy with time-of-flight (TOF) measurements, the photodissociation mechanism at 193 nm is shown to have two distinct mechanisms. (1) FeCp2+hν→FeCp*+Cp; (2) FeCp+2hν→FeCp+Cp, FeCp→Fe+Cp. For the first mechanism, which accounts for less than 5% of the photodissociation events, the FeCp* velocity distribution is quantitatively consistent with a statistical dissociation producing FeCp in an excited, ligand field electronic state. The velocity distributions of the Cp and Fe fragments produced by the second mechanism (FeCp is an unstable intermediate) are also in excellent agreement with microcanonical calculations for both Cp elimination steps using the known metal–ligand bond energies of ferrocene. For the second mechanism, dissociation occurs on the lowest potential energy surface for each Cp elimination. Although one photon is energetically sufficient to remove one Cp ligand from ferrocene, RRKM calculations of the lifetime indicate that Cp elimination is extremely slow for dissociation along the ground electronic state potential energy surface. Hence, after internal conversion to the ground electronic state, the large photon absorption cross section (∼4 Å2) for the experimental irradiation conditions allows additional photons to be absorbed until the dissociation rate exceeds the up pumping rate. The large photon energy causes the dissociation rate to increase by many orders of magnitude for each additional photon absorbed. Consequently, there is strong selectivity for the total number of photons absorbed. Both mechanisms, occurring on two different electronic potential energy surfaces, suggest that dissociation induced by excitation of the ligand-to-metal charge transfer states accessed at 193 nm can be quantitatively described as a statistical, unimolecular decomposition. At 248 nm, the measured product velocity distributions are qualitatively consistent with the mechanism deduced from the 193 nm results, but the energy available for translation at this wavelength is too small to extract quantitative product

translational energy distributions which are required to independently test the applicability of the statistical dissociation model.
Surprising similarities in the spectator decay of oxygen core-excited CO, CO2, and OCS
Authors: Carroll T. X. , Thomas T. D.
Published: July 2002

De-excitation electron spectra of oxygen core-excited and core-ionized CO, CO2, and OCS are compared. The spectra for de-excitation of the core-ionized species are quite distinctive and show only broad similarities. Those for de-excitation of the neutral core-excited species to two-hole, one-particle states are, in contrast, strikingly similar to each other both in peak shapes and positions. Differences that are apparent in the normal Auger spectra largely disappear when an extra electron is added to the lowest unoccupied (antibonding) molecular orbital. It is suggested that the antibonding effect of the spectator electron is large enough to make the core-excited oxygen weakly bound to the rest of the molecule in both the core-excited and final states. As a result, the spectra are all similar to that of a core-excited oxygen atom.
Determination of the structure of mixed argon–xenon clusters using a finite-temperature, lattice-based Monte Carlo method
Authors: Robertson Daniel H. , Brown Franklin B. , Navon I. Michael
Published: July 2002

The energy-optimized structures of all mixed Ar–Xe clusters containing 7, 13, and 19 atoms have been determined using a finite-temperature, lattice-based Monte Carlo procedure, which incorporates a highly efficient, memoryless, quasi-Newton-like conjugate gradient algorithm. This involves locating the global minima on the corresponding potential energy surfaces constructed from pairwise-additive Lennard-Jones potentials. For these systems, this optimization procedure has been found to be much more efficient than the more generally applicable simulated annealing method. Based on these energy-optimized structures, substitution sequences have been presented and discussed.
Measurements and calculations of CO2 room-temperature high-pressure spectra in the 4.3 μm region
Authors: Hartmann J. M.
Published: July 2002

Measurements of the absorption by pure CO2 in the 4.3 μm region are presented. They refer to the temperature of 291 K and densities in the 1–80 amagat range. Calculations based on the addition of Lorentzian contributions strongly overestimate the absorption since they do not account for line overlapping. The energy corrected sudden approximation (ECSA) is used to model line mixing within the impact approximation. The scaling parameters of this model are deduced from line-broadening data. This calculation strongly underestimates the absorption in the spectral region near the edges of the vibrational band, whereas it overestimates the absorption in the far wings of the lines. This is attributed to the impact approximation which does not account for the frequency dependence of the relaxation operator. This dependence is roughly determined from our results; it is in good qualitative agreement with the few corresponding available results.
Optical–optical double resonance studies of rotational autoionization of NO
Authors: Pratt S. T. , Dehmer J. L. , Dehmer P. M.
Published: July 2002

Optical–optical double resonance spectroscopy is used to probe Rydberg series converging to the first ten rotational levels of NO+ X 1Σ+, v+=0. Above the lowest ionization threshold, rotational autoionization of Rydberg series converging to higher thresholds is observed. Predissociation of these Rydberg states is found to compete with rotational autoionization in much the same manner as predissociation competes with vibrational autoionization in the region of the first few vibrational limits of NO+. The presence of this competing decay process, which has a decay rate similar to that of rotational autoionization, permits the comparison of rotational autoionization rates for different changes in rotational quantum number (ΔN+). Rotational autoionization by ΔN+=2 is found to be faster than by ΔN+=1 or 3. This results from the requirement that ΔN+=even processes require interactions between levels that both have even or both have odd values of orbital angular momentum l, while ΔN+=odd processes require interactions between levels of which one has even l and the other has odd l. In NO, the latter interactions are known to be quite weak. The electric field dependence and pressure dependence of the ionization threshold are also discussed.
Raman spectroscopy of fast pre-reactive, reactive, and post-reactive molecular motions in liquids
Authors: Viot P. , Tarjus G. , Borgis D. , Bratos S.
Published: July 2002

A theory is presented to study the effect on the isotropic Raman spectra of pre-reactive, reactive, and post-reactive molecular motions in the case of fast conformational changes. The reaction dynamics is pictured as a diffusion along the reaction coordinate and is supposed to obey a one-dimensional Smoluchowski equation. The resulting correlation functions exhibit several modes, some of which are generated by pre-reactive and post-reactive processes. Their spectral manifestation is studied in detail.
Ground and low-lying states of FeH+ as derived from a
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o self-consistent field and configuration interaction calculations
Authors: Sodupe M. , Lluch J. M. , Oliva A. , Illas F. , Rubio J.
Published: July 2002

The electronic states of FeH+ dissociating into Fe+ (6D) and H (2S) have been studied at the restricted open Hartree–Fock (ROHF) and configuration interaction (CI) level of theory, using a pseudopotential approach and considering or not relativistic effects. The ground state appears to be a 5Δ at the ROHF level but inclusion of correlation effects leads to a 5Π ground state. Nevertheless both states exhibit in all cases a near degeneracy. The calculated D0 is about 90% of the experimental value at the CI level whereas ROHF calculations account only for about 25% of the experimental dissociation energy.
Compact expressions for the radial electronic density functions for the 2S states of three-electron systems
Authors: King Frederick W. , Dressel Paul R.
Published: July 2002

The radial electronic density function D0(r) has been evaluated in closed form for the 2S states of three-electron systems that are described by Hylleraas-type wave functions. The density function D0(r) can be reduced to the form: D0(r)=𝒥I=17 𝒥K=0gI 𝒜IKrKe−αIr. Numerical values of the expansion coefficients 𝒜IK, summation limits gI and exponents αI are determined for the ground states of the following members of the Li I isoelectronic series: Li, Be+, B2+, C3+, N4+, O5+, F6+, and Ne7+. A discussion is given on the constraints that must be imposed on the choice of the basis set for the Hylleraas wave function, in order that D0(r) be reducible to the aforementioned compact analytic form. Expectation values for several moments 〈rn 〉 are calculated using D0(r). The electron–nuclear cusp condition is evaluated for the wave functions used to determine D0(r) for each member of the Li I sequence examined in this investigation.
Experimental estimation of orientational correlation in liquid bromine
Authors: Misawa M.
Published: July 2002

An approximate method to deduce orientational correlations in liquids directly from experimental structure factors has been proposed and applied satisfactorily to liquid bromine. The result suggests that the most probable orientation realized in liquid bromine is ‘‘staggered parallel (∥)’’ orientation rather than ‘‘T’’ orientation proposed so far. The molecules in the ∥ orientation are located at about 4.15 Å which is the shortest separation from a central one, while the other molecules located beyond this distance are effectively uncorrelated in orientation. The orientation is found to be very similar to that observed in the crystalline state. Simple consideration of intermolecular potentials calculated for various orientations by use of a model potential including two-center Lennard-Jones potential with idealized point quadrupole supports the present results qualitatively.
Rate constants, branching fractions, and energy disposal for the H+ClO and H+SF reactions
Authors: Wategaonkar S. J. , Setser D. W.
Published: July 2002

The H+ClO and SF reactions have been isolated and studied by infrared chemiluminescence in a fast flow reactor. The OH product channel is favored over the HCl channel by a factor of 4.5 and the total rate constant is (7.7±1.9)×10−11 cm3 s−1 for the H+ClO reaction. Both sets of products are accessed from a bound singlet intermediate with HCl+O(3 P) formed by a singlet–triplet surface crossing in the exit channel; the energy disposal is 〈fV (OH)〉=0.45 and 〈fV (HCl)〉=0.31. The H+SF reaction gives only HF+S(3P), but the energy disposal differs dramatically from the HCl channel of the ClO reaction. This difference arises from changes in the thermochemistry, which result in an earlier crossing to the HSF triplet surface followed by release of repulsive energy as the HF separates from the S(3P) atom.
Ion–ion recombination studies in ambient helium and argon at atmospheric densities
Authors: Lee H. S. , Johnsen R.
Published: July 2002

We have measured the dependence on gas density of the recombination rate for the reaction H3O+(H2O)n+NO−3→neutrals for n=2,3 in helium at densities from 0.3 to 1.2 NL and in argon from 0.2 to 0.9 NL (NL =Loschmidt’s number). We find that three-body recombination accounts for only part of the total observed recombination rate. The enhancement of mutual neutralization by the ambient gas appears to dominate the recombination process at low densities. A simple model is given which reproduces the experimental data fairly well.
Rotational excitations of methane molecules in a nonequilibrium krypton matrix
Authors: Prager M. , Langel W.
Published: July 2002

The rotational transitions of 1.5% CH4 isolated in a Kr matrix by condensing the mixed gas onto a cold (T=7 K) surface have been measured in the energy range <1.2 meV using a high resolution neutron scattering technique. The spectra are similar to those observed with CH4/ Ar. Most methane molecules are found at weakly disturbed substitutional fcc and hcp sites with weak rotational potentials. About 30% of the molecules are on disordered sites and display a broad distribution of stronger potentials.
Linearized velocity disturbances and dynamic Oseen tensors in simple shear and elongational flows
Authors: Rabin Y. , Wang S. Q. , Creamer D. B.
Published: July 2002

We investigate the velocity disturbance produced by point-like sources of force in viscous fluids subjected to unbounded, simple shear, and elongational flows of arbitrary strength. The response to the body force is given by the dynamic Oseen tensor which is constructed using the propagator for the linearized Navier–Stokes equation. Application of the method of characteristics leads to ‘‘Kelvin mode’’-type solutions for the velocity disturbances and explicit analytical expressions are obtained for the propagators and Oseen tensors in the above mentioned flows.
Electrostatic spectrum and dielectric constant of nonpolar hard sphere fluids
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study the dielectric constant of a system of hard spheres with polarizable point dipoles at their center. The deviations from the Clausius–Mossotti formula are found at six volume fractions by computer simulation on a system with periodic boundary conditions. We also find the spectral density in the Stieltjes representation of the dielectric constant. This is directly related to the corresponding spectral density of a two-phase composite. The spectral density also characterizes the response of the system when the dipoles are modeled as Lorentz harmonic oscillators. The detailed structure of the spectrum, which we find by computer simulation, provides a severe test for theory.
Extracting laws of decay in the femto–picosecond range from autocorrelation functions
Authors: Remacle F. , Desouter-Lecomte M. , Lorquet J. C.
Published: July 2002

The formalism of the resonance states is used to derive approximate expressions of the unimolecular law of decay resulting from a specific excitation. These expressions contain no cross terms and wash out the quantum interferences. We propose a method to relate them to an experimentally observable quantity, viz., the autocorrelation function C(t) obtained as the Fourier transform of a spectral profile, which is available even when the spectrum is poorly resolved. For a specific excitation, the exact initial rate of decay (valid up to the dephasing time T1) is equal to the initial slope of ‖C(t)‖2. The subsequent time evolution can be obtained by averaging ‖C(t)‖2 over its oscillations. This generates a function ‖C(t)‖2av whose area (from time T1 onwards) is directly related to an average decay lifetime. At times t>T1, a good approximation to the average decay curve Pav(t) can be derived by multiplying ‖C(t)‖2av by an appropriate constant. The method is exemplified on various diatomic and triatomic models. As an application to a real system, we study the B̃ 2B2 state of H2O+ which is coupled to the Ã 2A1 state via a conical intersection. State B̃ is found to undergo an ultrafast intramolecular relaxation with a lifetime of (1.6±0.2) 10−14 s.
Hole and antihole profiles in nonphotochemical hole-burned spectra
Authors: Lee In-Ja , Hayes John M. , Small Gerald J.
Published: July 2002

The shapes of the real-phonon and pseudophonon sideband holes (PSBH) which occur in hole-burned spectra in amorphous solids are both wavelength and burn time dependent. The theory previously proposed to simulate hole shapes in the short burn time limit is extended to examine the PSBH shapes at arbitrary times and at different burn wavelengths. The simulated spectra are compared with experimental data for tetraphenyl porphin in polystyrene. It is also shown how the simulated spectra may be used to deconvolve the antihole spectrum from the hole spectrum.
Use of semigrand ensembles in chemical equilibrium calculations on complex organic systems
Authors: Alberty Robert A. , Oppenheim Irwin
Published: July 2002

When chemical potentials of some reactants are externally fixed, the equilibrium pressures of other reactants can be calculated thermodynamically using Legendre transforms of the usual thermodynamic potentials and statistical mechanically using semigrand ensembles. The use of the semigrand isobaric partition function for complex organic systems involving chemical reactions is discussed. This partition function provides a simplified, but comprehensive, view of chemical equilibrium within homologous and related series, which involve infinite numbers of species. The transformed thermodynamic properties for equilibrium systems of this type are expressed in terms of the molecular partition functions of the various species.
The vibrational spectra of molecular ions isolated in solid neon. I. CO+2 and CO−2
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:CO2 sample was codeposited at approximately 5 K with a beam of neon atoms that had been excited in a microwave discharge, an absorption appeared at 1421.7 cm−1, very near the gas-phase band center for the antisymmetric stretching fundamental (ν3) of CO+2. Detailed isotopic substitution studies support the assignment of this absorption to that fundamental of CO+2, as well as of an absorption at 1658.3 cm−1 to ν3 of CO−2. In earlier studies of the charge transfer interaction of an alkali metal with CO2, this vibration of CO−2 had been strongly perturbed by coordination with the alkali metal cation. In the present experiments, the threshold for electron photodetachment from CO−2 was observed in the visible spectral region. Evidence was also obtained for the stabilization of the O2C⋅⋅⋅OCO− cluster anion.
Vibrational predissociation in the H–F stretching mode of HF–DF
Authors: Fraser G. T. , Pine A. S.
Published: July 2002

The high-resolution infrared spectrum of the K=1–0 subband of the H–F stretching vibrational band of the hydrogen-bonded HF–DF complex has been recorded using a molecular-beam electric resonance optothermal color-center-laser spectrometer. The spectrum exhibits minor perturbations and vibrational predissociation linewidths of 23±2 MHz full width at half-maximum for comparison to the 11±1 MHz widths found for the corresponding mode of the homonuclear HF–HF dimer.
Coherent anti-Stokes Raman spectroscopy of shock-compressed liquid oxygen
Authors: Schmidt S. C. , Moore D. S. , Shaw M. S. , Johnson J. D.
Published: July 2002

Vibrational spectra of liquid oxygen, shock compressed to several high-pressure/high-temperature states, were obtained using single-pulse multiplex coherent anti-stokes Raman scattering (CARS). The experimental spectra were compared to synthetic spectra calculated using a semiclassical model for the CARS intensities and best fit vibrational frequencies, peak Raman susceptibilities, and Raman linewidths. Up to the maximum shock pressure of 9.6 GPa, the vibrational frequencies were found to increase monotonically with pressure. An empirical fit, which could be used as a pressure/temperature/frequency calibration standard, showed that the Raman frequency shifts could be accurately described by linear pressure and temperature dependences. Above ≊9 GPa, the liquid oxygen opacity at 632.8 nm increased rapidly, presumably because of proximity (collision)-induced absorption. Calculations showed that the induced absorption did not resonantly enhance the CARS spectra, but did attenuate the laser beams and the CARS signals. The measured linewidths suggest that the vibrational dephasing time decreased to approximately 1 ps at the highest pressures.
Quantum effects on ion–dipole capture rate coefficients
Authors: Marković Nikola , Nordholm Sture
Published: July 2002

A recent theoretical study of the ion–dipole capture process using classical variational and effective potential methods is extended to the quantum regime. Capture rate coefficients are calculated for the ion–dipole potential energy surface utilizing a model where the vibrations are frozen, the rotations are quantized and the translational motion is classical. Results from a simple adiabatic capture theory, variational transition state theory, and an effective potential method are presented and compared with the corresponding classical rate coefficients and with results from classical trajectory calculations for H+3 ions colliding with HCl, HCN, and CS. Comparison is also made with other theoretical and experimental results.
An electron spin-echo envelope modulation study of the lowest triplet state of pyridine-d5: Spin-density distribution and structure
Authors: Buma W. J. , Groenen E. J. J. , Schmidt J. , de Beer R.
Published: July 2002

Recently we have shown that the lowest triplet state (T0) of pyridine, incorporated in a single crystal of benzene, may be studied by electron spin-echo (ESE) spectroscopy. From the nitrogen hyperfine structure in the ESE detected electron paramagnetic resonance (EPR) spectra, we were able to conclude that pyridine, a planar molecule in the ground state, becomes nonplanar upon excitation into T0. Here we report the results of a detailed investigation of this distortion and of the electronic nature of the lowest triplet state of pyridine-d5. We have performed electron spin–echo envelope modulation (ESEEM) spectroscopy. From the modulation spectra, the electron-nuclear double resonance (ENDOR) frequencies corresponding to the various deuterium nuclei are obtained. Analysis of the dependence of these frequencies on the orientation of the magnetic field with respect to the triplet fine-structure axes system allows for a determination of the deuterium hyperfine and quadrupole tensors. From these tensors and the known nitrogen hyperfine tensor, the structure and spin-density distribution of pyridine in its lowest triplet state are deduced. Pyridine adopts upon excitation into T0 a boatlike structure, in which the nitrogen atom is tilted by about 40° with respect to the plane through the ortho- and meta-carbon atoms and the para-carbon/para-deuterium fragment by about 10° with respect to this plane. Thereby the hybridization of the ortho-carbon atomic orbitals strongly deviates from that for aromatic hydrocarbons and becomes almost sp3; the hybridization of the atomic orbitals on the other carbon atoms changes much less, while the nitrogen atomic orbitals remain sp2 hybridized. Approximately half of the spin density is found to be localized on the nitrogen atom with a remarkable distribution over the atomic orbitals: the π orbital carries 40%, the n orbital 10%. The rest of the spin density is distributed over the para- (30%) and ortho-carbon atoms (10% each). The lowest triplet state of pyridine is neither an nπ* state nor a ππ* state, but a state of mixed character as the result of a strong vibronic coupling between the 3B1 (nπ*) and 3A1 (ππ*) states.
Photostimulated desorption of NO chemisorbed on Pt(100) at 193 nm
Authors: Mase Kazuhiko , Mizuno Seigi , Yamada Masamichi , Doi Ichiro , Katsumi Tohru , Watanabe Shuntaro , Achiba Yohji , Murata Yoshitada
Published: July 2002

Photostimulated desorption of NO chemisorbed on a Pt(100) surface at 80 and 300 K has been studied with an ArF excimer laser (λ=193 nm) and a positive-ion measurement system. NO+ is the only ion species observed when a NO-saturated Pt(100) surface is irradiated with laser light. The NO+ yield is proportional to the third power of laser fluence. The translational-energy distribution of the NO+ ions is independent of laser fluence. We propose a two-step model as the most probable NO+ formation mechanism. The first step is desorption of neutral NO induced by valence-electron excitation in chemisorbed NO with one-photon absorption. Then, the desorbed NO is ionized in the vicinity of the surface via the two-photon nonresonant ionization process. Relatively large desorption cross sections estimated from the decay of the NO+ yield support the above NO+ formation mechanism.
Incorporating advantages of time-dependent dynamics in time-independent collision methods: Early asymptotic analysis
Authors: Singer Sherwin J. , Lee Sungyul , Freed Karl F.
Published: July 2002

Transition amplitudes that may be written as matrix elements of a Green’s operator are quite usefully cast as temporal correlation function of localized wave packets. In many situations, the correlation function is nonzero only for a short time while the wave packet is close to its initial position. The transition amplitude is only sensitive to the potential near the initial location of the wave packet. We derive time-independent analogs of the above described features of time-dependent collision theory for matrix elments of the time-independent Green’s function. Transition amplitudes are shown to be completely independent of the potential outside the Franck–Condon region when the true wave function can be approximated by a (primitive or uniform) semiclassical form in the outer region. The reaction coordinate can be separated into a strongly interacting Franck–Condon region, to be treated by standard close coupling methods, and an asymptotic region for which no dynamical calculations are required in the semiclassical limit. Because this is a spatial, rather than temporal, separation, our semiclassical method is successful in situations, such as predissociation, where time-dependent methods fail. Also, we can evaluate total cross sections by using basis sets that are optimal for the Franck–Condon region without ever considering the transition to asymptotic target states. Numerical illustrations are provided for predissociations in Na2 and Br2.
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o study of CN− impurity centers in alkali halides: Lattice stabilization of excited electronic states
Authors: Tellinghuisen Joel , Ewig Carl S.
Published: July 2002

The lowest electronic states of 1Σ+, 3Σ+, 1Π, and 3Π symmetry in the cyanide anion are calculated at the multiconfiguration self-consistent field (MCSCF) level using simple point-charge models to simulate the ionic environment in the cubic alkali halide crystals. The electrostatic potential of the lattice is essential to stabilize the excited states against autodetachment, yet the resulting spectroscopic properties are remarkably insensitive to gross changes in the lattice, including deletion of all but the six nearest-neighbor cations. The lowest excited state—the state responsible for an observed UV emission spectrum of CN− in some alkali halides—is shown to be 3Σ+, as in the isoelectronic N2 and NO+ molecules, rather than 3Π, as in CO. The properties of the ground electronic state are further examined at the SCF level in clusters of six alkali ions. The cations produce a ‘‘ compression’’ of the anion, decreasing the internuclear distance and increasing the vibrational frequency from the point-charge results and thus yielding better agreement with experiment. Attempts to determine the orientational potential of the ground state of CN− in the lattices remain inconclusive, due to basis set limitations; however there are strong indications that in the 〈100〉 orientation favored by CN− in the sodium halides, the anion prefers an off-center location with the Na–N distance appreciably shorter than the Na–C distance. An examination of methods used to extract spectroscopic constants from pointwise tabulated potentials indicates that fits to closed-form potentials are better than fits to polynomials in (R-Re ) and to methods which entail numerical solution of the vibrational wave equation for the tabulated potential.
Elastic constants of the disordered phase of CBr4 by Brillouin spectroscopy
Authors: Zuk J. , Brake D. M. , Kiefte H. , Clouter M. J.
Published: July 2002

The technique of Brillouin spectroscopy has been used to determine the adiabatic elastic constants of single crystals of the orientationally disordered phase (phase I) of CBr4 .The values of the elastic constants at 333 K are C11 =42.2, C12 =33.6, and C44 =16.1 (in units of kbar) and suggest that there is strong rotation–translation coupling in the k=0 acoustic wave vector regime. Because x-ray Laue diffraction was not possible in solid CBr4 , the necessary orientation of the single crystals was determined by also using the Brillouin scattering spectra.
Mercury 6 3P
J relaxation and intramultiplet mixing under collisions with H2, HD, D2, and He
Authors: Bras N. , El Afghani A. , Butaux J. , Jeannet J. C. , Perrin D.
Published: July 2002

Absolute rate constants for quenching and intramultiplet mixing of Hg(63P0,1,2) by collisions with H2, HD, and D2 have been determined. A smooth variation of these coefficients with the isotopic form of hydrogen has been observed. For He only an upper limit of the rate constants has been obtained. Attempts to explain the 3P1←3P2 transfer by simple mechanisms are described. The surface crossing mechanism seems the most adapted. But a theory more elaborate than the one presented here should be developed, including both the rotational and the vibrational degrees of freedom of the molecule.
Effect of optical resonances on the second harmonic response from Ag(111) and Ag(110) in solution
Authors: Georgiadis R. , Neff G. A. , Richmond G. L.
Published: July 2002

The wavelength dependence of the second harmonic response from native single crystal silver electrodes is examined for Ag(111) and Ag(110). Both the rotational anisotropy and potential dependence of the second harmonic response are significantly different at the two wavelengths examined, 1064 nm and 532 nm excitation. These differences are attributed to optical resonances between the incident or outgoing radiation and surface electronic or bulk electronic transitions of the native metal. The anisotropy at 532 nm reflects the increased contribution of surface interband transitions as this higher excitation energy. It is proposed that the potential dependence at 532 nm excitation involves resonances with surface electronic states on the metal.
High-pressure nuclear magnetic resonance study of the dynamical solvent effects on internal rotation of N,N-dimethyltrichloroacetamide
Authors: Xie Chuan-Liang , Campbell Douglas , Jonas Jiri
Published: July 2002

The pressure dependence of the hindered rotation about the amide C–N bond of N,N-dimethyltrichloroacetamide (DMTCA) has been studied at 282.3 K in n-pentane and methylcyclohexane solvents using the high resolution, high pressure nuclear magnetic resonance (NMR) technique. The experimental rotation rate k decreases with increasing pressure in both solvents and the correlation of the rates with solvent viscosity η shows that the rotation falls into the strongly coupled diffusive regime. Interpretation of the experimental k vs η dependence in terms of the Kramers’ model fails to account for the leveling off of the rate constant at high viscosities. The Grote–Hynes theoretical model, which assumes frequency-dependent friction, reproduces well the observed rate behavior with viscosity of the solvent.
Molecular dynamics simulations of the structure and dynamics of confined polymer melts
Authors: Bitsanis I. , Hadziioannou G.
Published: July 2002

We performed a series of molecular dynamics simulations investigating the static and dynamic properties of polymer melts confined between planar solid surfaces. The solid–melt interface was found to be very narrow (approximately two segment diameters) and independent of chain length. Inside the interface the segment density profile was oscillatory, the bond orientation altered between directions parallel and normal to the solid surface, and the chain ends accumulated very close to the wall (in the absence of strong wall–segment attraction). The oscillations of the segment density profile were weaker and were dampened faster than those of a simple fluid density profile next to the same solid surface. This reflected the reduced ability of sequences of connected segments (chains) to layer themselves against a solid surface because of restrictions on their configurations imposed by the chain connectivity requirement. This effect made the solid–melt interface even narrower than that of a simple fluid. Only the chain portions lying inside the interface had their shape affected by the wall.

Chain statistical segments inside the interface assumed orientations parallel to the wall. In the absence of wall–segment attraction, the size of the statistical segments inside the interface was unaffected. This situation resulted in an apparent decrease of the radius of gyration normal to the wall an apparent increase of the radius of gyration parallel to the wall and spatial independence of the total radius of gyration. The wall effect was gradually diminished and chains assumed their bulk dimensions when their center-of-mass was so far from the solid surface that no portions of the chain could reach the interface (i.e., at a distance comparable to the bulk radius of gyration). The microscopic dynamics of chain portions inside the interface were strongly anisotropic. The mobility increased in the direction parallel to the wall and decreased normal to the wall. This fact was caused by the angular asymmetry of the segment–segment collisions inside the interface, i.e., by the same mechanism that induces the segment layering. The total mobility inside the neutral wall–melt interface was identical with that in the bulk reflecting the fact that the average segment density inside the interface had essentially the bulk value. The presence of strong wall–segment attraction increased the average interfacial density above the bulk value and lowered the mobility of the interfacial chain portions in all directions. The mean-square displacement of the chain center-of-mass during a certain time interval was affected by the solid only if the chain had a portion of itself inside the interface for a fraction of this time interval. The longest relaxation time of the chains, a property that cannnot be localized properly on a length scale smaller than the interfacial width, exhibited a weak and strongly diminishing with chain length spatial dependence.
Chaos in the Showalter–Noyes–Bar-Eli model of the Belousov–Zhabotinskii reaction
Authors: Lindberg David , Turner Jack S. , Barkley Dwight
Published: July 2002

The observation of robust, large-scale chaos in the Showalter–Noyes–Bar-Eli model of the Belousov–Zhabotinskii reaction is reported. The chaos observed is comparable to that found in CSTR experiments at low flow rates.
Excited states of ZnS:Mn and ZnSe:Mn stabilized by the Jahn–Teller active E mode: Calculations by semiempirical self-consistent field methods
Authors: Stavrev K. K. , Kynev K. D. , St. Nikolov G. , Breza M.
Published: July 2002

The splitting of the quartet excited states of Mn2+ doped in ZnS and ZnSe due to coupling with the normal E vibration of tetrahedral MnS4 and MnSe4 clusters is studied by semiempirical methods which neglect the differential overlap between atomic orbitals. Stable excited state geometries of the impurity centers are found only if (i) the second neighbor Zn ions are included in the treated clusters and (ii) the collective E vibration of all the atoms neighboring the central Mn ion is taken into account.
Decomposition of silane on Si(111)-(7×7) and Si(100)-(2×1) surfaces below 500 °C
Authors: Gates S. M. , Greenlief C. M. , Beach D. B. , Holbert P. A.
Published: July 2002

Using static secondary ion mass spectrometry (SIMS) to observe the silicon hydride species formed by silane adsorption on atomically clean single crystal silicon surfaces, two distinct adsorption mechanisms are identified. Dissociation to SiH3 plus H occurs on the Si(100)-(2×1) surface, which contains pairs of dangling bonds located on Si dimers (with Si–Si distance ≊2.4 Å). In contrast, SiH2 formation in the adsorption step is indicated on the Si(111)-(7×7) surface, where adjacent dangling bonds are separated by more than 7 Å. Lower limits on the silane reactive sticking coefficient (SR) are evaluated using hydrogen coverage (ΘH) measurements after calibrated SiH4 exposures, and this limit is ≊10−5 for 25 °C gas and 100–500 °C surface temperatures. Within experimental error, SR is the same for both mechanisms on the two clean surfaces (ΘH near zero). Dependence of SR on ΘH is reported at 400 °C for both surfaces, and differences appear as ΘH exceeds 0.1 H/Si. Silane adsorption is weakly activated on Si(111)-(7×7), as evidenced by enhanced adsorption as TS is increased between 50 and 400 °C.
Monte Carlo simulation of dense polymer systems on a lattice
Authors: Geyler Stefan , Pakula Tadeusz , Reiter Johannes
Published: July 2002

At present the collective motion algorithm is the only Monte Carlo technique which allows the simulation of monodisperse polymer melts on a lattice at a volume fraction of unity. Here we describe the algorithm in detail and show that it fulfils the condition of detailed balance. It yields chain dimensions which are slightly smaller than chain dimensions obtained for polydisperse systems or for systems at volume fractions smaller than unity, as expected. The computational expense to obtain an independent sample containing a constant number of chains increases as about the 3.6th power of chain length.
Photochemistry of adsorbed phenyl iodide: Desorption and photofragmentation dynamics
Authors: Villa E. , Dagata J. A. , Lin M. C.
Published: July 2002

The photodissociation and photodesorption dynamics of iodobenzene adsorbed at 150 K on LiF substrates has been investigated at 222 nm using the time-of-flight/resonance-enhanced multiphoton ionization/mass spectrometry technique. The Maxwell–Boltzmann (MB) translational temperatures (TB) calculated for the phenyl radical, atomic iodine in the I (2P3/2) and (2P1/2) states, and biphenyl were 1200, 1500, and 1050 K, respectively. The desorbed parent molecule (detected by electron ionization) was found to be translationally colder, with TB≊900 K. Analysis of translational energy distributions indicate that the adsorption–dissociation mechanism of iodobenzene differs significantly from that of the methyl halides.
Production and depletion of long-lived charged non-bond-alternating defects in polyacetylene with mid-band-gap photolysis
Authors: Hoener Carolyn F.
Published: July 2002

Photoinduced IR absorptions at 1368, 1289, and 640 cm−1 with much longer lifetimes and lower excitation energies than those previously reported have been observed in polyacetylene. These ‘‘persistent’’ photoinduced IR absorptions were observed after the photolysis of polyacetylene at temperatures below 125 K with ν>5500 cm−1. The IR absorption frequencies are similar to the previously reported transient (τ≊1 ms) photoinduced IR absorptions which are attributed to charged solitons. The persistent photoinduced IR absorptions reported here decay slowly in the dark over the course of several hours. The disappearance of these absorptions can be accelerated by photoexcitation of an electronic transition with an onset near 2800 cm−1 and an absorption maximum near 4000 cm−1. Two mechanisms are proposed to explain the production of IR-active defects at such low photolysis frequencies.