bluuebunny/crossref_metadata_2025 huggingface dataset:
Large local energy fluctuations in water. II. Cooperative motions and fluctuations
Authors: Ohmine Iwao , Tanaka Hideki , Wolynes Peter G.
Published: July 2002

Large local energy fluctuations in liquid water and their physical origin are investigated by using classical molecular dynamics (MD) calculation and quenching techniques. Performing a trajectory calculation of 100 ps, it is found that large rotational motions of individual water molecules, which are always associated with potential energy destabilization of 10–20 kcal/mol, occur once in about 10 ps. The stabilization and destabilization of the individual water molecules are induced by cooperative motions. In order to analyze these cooperative motions in the liquid water, the water structures are quenched to their local minima (called the inherent structures). Comparing the inherent structures successively visited by the system, it is found that collective motions of about 10–40 molecules localized in space occur in unstable regions. The potential energy fluctuation of an individual molecule can reach up to 15 kcal/mol even in the inherent structures. The strong potential energy correlation among neighboring molecules indicates these cooperative motions cause the ‘‘flip–flop’’-type energy exchanges; as a molecule is stabilized, another is to be unstabilized and vice versa. A flip-flop motion does not involve a (large) energy barrier but causes large energy fluctuations of the individual molecules. A large portion of potential energy fluctuations of the individual water molecules is accounted for as the superposition of fluctuations in the inherent structures and those in the normal modes build upon these structures.
On the quenching of helium 2 3S: Potential energy curves for, and nonadiabatic, relativistic, and radiative couplings between, the a 3Σ+u, A 1Σ+u, b 3Πg, B 1Πg, c 3Σ+g, and C 1Σ+g states of He2
Authors: Yarkony David R.
Published: July 2002

This work considers the possible role of nonadiabatic effects in the collisional quenching of He(2 3S). The electronic structure aspects of a nonadiabatic-radiative decay mechanism are analyzed. In this mechanism the a 3Σ+u state is coupled by relativistic, rotational, and radiative interactions to the A 1Σ+u state which serves as a gateway to the X 1Σ+g (electronically quenched) state of He2 through the spin-allowed dipole-allowed bound–free transition A 1Σ+u →X 1Σ+g. State averaged MCSCF/second-order CI wave functions for the ground X 1Σ+g state, and the excited, a 3Σ+u, A 1Σ+u, b 3Πg, B 1Πg, c 3Σ+g, and C 1Σ+g states (referred to here as the primary space) of He2 were determined. Using these

wave functions all interstate matrix elements of the form 〈Ψ0(J)‖Ô‖Ψ0(I)〉 were determined for (i)Ô=ĤBP≡Ĥso+Ĥss where Ĥso and Ĥss are, respectively, the spin–orbit and dipolar spin–spin interactions in the Breit–Pauli approximation, (ii) Ô=L̂e, where L̂e is the total electronic orbital angular momentum operator, and (iii) Ô=μ̂ where μ̂ is the dipole moment operator. In the nonrotating molecule these interactions give rise to the spin-forbidden dipole-allowed radiative transitions (b 3Πg, c 3Σ+g) →A 1Σ+u. However a complete description of these radiative decay processes requires consideration of interactions originating outside the primary space. Thus in this work the spin-forbidden, dipole-allowed perpendicular, μ⊥ (J, A 1Σ+u0+),  J=c 3Σ+g1, b 3Πg1 and parallel, μ∥(b 3Πg0+, A 1Σ+u0+), transition moments were determined using quasidegenerate perturbation theory. The computed potential energy curves, coupling matrix elements, and dipole moments permit a fully quantum mechanical analysis of the nonadiabatic-radiative quenching mechanism. A preliminary phenomenological analysis of aspects of this process is provided.
Rate constant calculations on the C++HCl reaction
Authors: Dateo C. E. , Clary D. C.
Published: July 2002

Calculations of the rate constant at several temperatures for the reaction C+(2P)+HCl(X 1∑+) are presented. A quantum mechanical treatment of ion–molecule reactions which combines a rotationally adiabatic capture with a centrifugal sudden approximation is used to obtain rotational state-selective cross sections and rate constants. Ab initio SCF (TZ2P) methods are employed to obtain the long- and short-range electronic potential energy surfaces. This study indicates the necessity of incorporating the multisurface nature of open-shell systems for some ion–molecule reactions. The spin–orbit interactions are treated within a semiquantitative model. Results fare better than previous calculations which used only classical electrostatic forces, and are in good agreement with CRESU and SIFT measurements at 27, 68, and 300 K.
Adsorption kinetics and isotopic equilibration of oxygen adsorbed on the Pd(111) surface
Authors: Guo Xingcai , Hoffman Alon , Yates John T.
Published: July 2002

Oxygen adsorption on the Pd(111) surface has been studied at 100 and 300 K by temperature programmed desorption (TPD) and isotopic mixing experiments. At 100 K, the sticking coefficient is determined to be 1 up to the coverage of 0.3 O/Pd. The saturation coverage is 0.62 O/Pd, 27% of which dissociates during thermal desorption. Three molecular desorption processes are observed with the activation energy of 7.6, 9.1, and 12.3 kcal/mol, respectively. At 300 K, the sticking coefficient increases with coverage from ∼0.14 at zero coverage to 0.87 at θ≊0.05 O/Pd, then decreases to zero at a saturation coverage of 0.25 O/Pd. The desorption activation energy of 53 kcal/mol is calculated for the associative desorption process with a lateral repulsive interaction of 0.7 kcal/mol. Based on the isotopic mixing results and previous high resolution electron energy loss spectroscopy (HREELS) data, a more complete picture concerning adsorption, conversion, equilibration, desorption, and dissociation processes is suggested.
Rate constants for the reaction O+D2→OD+D by the flash photolysis–shock tube technique over the temperature range 825–2487 K: The H2 to D2 isotope effect
Authors: Michael J. V.
Published: July 2002

Rate constants for the reaction of O(3P) atoms with deuterium, O+D2→OD+D, have been measured over the temperature range 825–2487 K. The experimental method that has been used is the flash photolysis–shock tube (FPST) technique. This technique utilizes atomic resonance absorption spectroscopy (ARAS) to monitor O-atom depletion in the presence of a large excess of reactant, D2. The measurement is made in the stagnant reflected shock wave region. Thus, shock heating simply serves to prepare the gas density and temperature for a flash photolytically induced absorption photometric experiment. The results that have been obtained between 825 and 2487 K can be represented by the Arrhenius expression: k=(3.22±0.25)×10−10 exp(−7293±98 K/T) cm3 molecule−1 s−1. The average deviation of the present data from this equation is ±17%. An alternative three parameter expression that represents the data to within ±16% is k=1.95×10−15 T1.45 exp(−5250 K/T) cm3 molecule−1 s−1. When the recent results of Zhu, Arepalli, and Gordon (the preceding paper) are considered, a three parameter expression can be determined for the temperature range, 343–2487 K. This combined result is k=2.43×10−16 T1.70 exp (−4911 K/T) cm3 molecule−1 s−1. The average deviation of the data from this equation is ±16%, whereas the data of Gordon and co-workers agree to within ±5%. The combined result is compared to earlier experimental results and, also, to theoretical calculations by Bowman, Wagner, Walch, and Dunning; Garrett and Truhlar; and Joseph, Truhlar, and Garrett. The present result is used along with recent data for O+H2 to specify the experimental isotope effect, kH2/kD2, over the experimental temperature range. Lastly, the experimental rate constant ratio is compared to the theoretical predictions.
High-resolution angle- and energy-resolved photoelectron spectroscopy of NO: Partial wave decomposition of the ionization continuum
Authors: Allendorf Sarah W. , Leahy David J. , Jacobs Dennis C. , Zare Richard N.
Published: July 2002

We report a two-color high-resolution energy- and angle-resolved study of the photoelectrons produced in the (1+1′) REMPI of NO via rotational levels of the A 2Σ+ v=0 state. We find markedly different photoelectron angular distributions arising from production of ions in different rotational states (ΔN=0,±1,±2 transitions in the ionization step). We also observe that the ΔN=±2 angular distributions are very sensitive to the intermediate state alignment. A model is put forward in which experimental observables (angle- and energy-resolved photoelectron spectra) are used to determine the attributes (relative amplitudes and phase shifts) of a small number of interfering continuum channels that contribute to the ionization step as well as the fraction of parallel character of the ionization step. Nearly 70% of the ejected photoelectrons are associated with the ΔN=0 ionization transition; the partial wave composition of these electrons is dominated by p character. The less important ΔN=±1 peaks have both s- and d-wave character. The ΔN=±2 photoelectron peaks exhibit far more f-wave than p-wave character because destructive interference nearly removes the p-wave contribution to the angular distribution. The partial wave decomposition is used to predict angular distributions resulting from excitation of the intermediate state by different rotational branch transitions; these predictions are in excellent agreement with the measured distributions.
Evaporation of covalent clusters: Unimolecular decay of energized size-selected carbon cluster ions (C+n, 5≤n≤100)
Authors: Radi P. P. , Hsu M. T. , Brodbelt-Lustig J. , Rincon M. , Bowers M. T.
Published: July 2002

The unimolecular decay of energized size-selected carbon clusters (C+n, 5≤n≤100) is investigated. The clusters are produced in a laser-generated plasma on the surface of a graphite rod. Directly extracted cations that decay on a μs time scale are probed in a double-focusing, reverse-geometry mass spectrometer. The unimolecular decomposition rates are extracted from metastable fraction measurements. We observe a dramatic discontinuous increase in the decay rate constant as a function of cluster size around mass C+c0 (factor of 5 to 10). Additionally, low rate constants, relative to the neighbors, are found for C+50, C+60, and C+70. The results are rationalized by postulating a phase transition from small ‘‘rigid’’ clusters for n<30 to larger ‘‘molten’’ entities for n>30. In this model local deviations in rate constant reflect the thermodynamic stabilities of the clusters. A further consequence of this model is that ‘‘magic’’ numbers in the mass spectrum originate primarily from the intrinsic stability of the clusters with respect to evaporation and not from a kinetic growth mechanism.
Radiative lifetimes and collisional deactivation of two-photon excited xenon in argon and xenon
Authors: Bruce M. R. , Layne W. B. , Whitehead C. A. , Keto J. W.
Published: July 2002

Radiative lifetimes and bimolecular rates have been determined for two-photon laser excited states of Xe* (5p56p, 5p56p′, 5p57p) in argon and xenon buffer gases. The collisional deactivation rates are found to be very large for the Xe 6p′ and 7p states [∼(2–5)×10−10 cm3/s] while the rates for the Xe 6p states are comparatively smaller [∼(0.06–1.2)×10−10 cm3/s]. In general, the quenching rates in argon are about a factor of 2 smaller than the xenon quenching rates for the same excited state with the notable exception of Xe 6p[1/2]0. For Xe 6p[1/2]0, a multicomponent decay has been observed in argon buffer gases. The second component is attributed to collisional coupling to Xe 5d[1/2]1 which lies 132.3 cm−1 below Xe 6p[1/2]0. Quench rates determined from the collisionally induced VUV fluorescence from Xe 5d[1/2]1 at 125 nm are in excellent agreement with this assignment. Furthermore, these experiments have unambiguously identified the product channel involved in the curve crossings observed in studies of optical collisions [N. Böwering, T. D. Raymond, and J. W. Keto, Phys. Rev. Lett. 52, 1880 (1984)].
Dynamical structure of water: Low-frequency Raman scattering from a disordered network and aggregates
Authors: Rousset J. L. , Duval E. , Boukenter A.
Published: July 2002

Low-frequency inelastic light scattering of water is observed from a temperature equal to 80 down to −20 °C in the supercooled regime. For energies higher than 3 cm−1 it is shown that the major part of the light scattering is Raman scattering. A broadband with a maximum at 50 cm−1 is interpreted as scattering from transverse acoustic modes of a disordered network. A weaker scattering which shifts towards the Rayleigh line when the temperature decreases is well explained by Raman scattering from the oscillations of water molecule aggregates which grow when the temperature decreases.
Synchrony and the emergence of chaos in oscillations on supported catalysts
Authors: Schüth F. , Song X. , Schmidt L. D. , Wicke E.
Published: July 2002

Complex behavior of an oscillating reaction on an array of catalytic regions was modeled using a distributed model consisting of a 10×10 matrix with ten randomly placed oscillators coupled via heat transfer. Each single oscillator was described by a very simple system of two differential equations chosen to model the CO/NO reaction on supported Pd. However, the coupled system exhibited very complex behavior with periodic oscillations, period multiplications, beat structures and chaos. All of these complex features predicted by the model have also been found in experiments. In addition, the model was able to predict the experimentally observed tendencies in the development of complex behavior with changing reaction conditions. Changing distributions of the cells or introducing small differences in the description of the single oscillators did not change the principal features of the system. However, the transition to chaos could be changed in that for one distribution of active cells a Feigenbaum sequence to chaos was found but not in another. With one oscillator twice as active as the rest of the cells, the high activity cell was dominant and entrained the whole system, leading to completely synchronized behavior. Also in this situation, however, chaotic responses were found, when the high activity oscillator was in a stable ignited state. Possible generalizations of this treatment to other oscillating systems are discussed.
Near-infrared absorption spectra of 16O3 and 18O3: Adiabatic energy of the 1A2 state?
Authors: Anderson S. M. , Morton J. , Mauersberger K.
Published: July 2002

New absorption spectra of the 1A2(0,v2,1)←1A1(0,0,0) bands of 16O3 and 18O3 near 1 μ are reported. The behavior of vibronic band isotope shifts for low v2 suggests that the lowest point on the 1A2 surface lies 9990±70 cm−1 above the 1A1 minimum. This result is relatively insensitive to the vibrational assignment. Accounting for zero-point and binding energies of the ground state places the 1A2 minimum very close to the O+O2(v=0) dissociation limit, not low enough to support the zero-point energy of a bound state. Implications regarding recent speculation on the role of this and other electronically excited states in ozone photochemistry are discussed.
Excitation energies from the coupled cluster singles and doubles linear response function (CCSDLR). Applications to Be, CH+, CO, and H2O
Authors: Koch Henrik , Jensen Hans Jo/rgen Aa. , Jo/rgensen Poul , Helgaker Trygve
Published: July 2002

The linear response function for a coupled cluster singles and doubles wave function is used to calculate vertical electronic energies for the closed shell system Be, CH+, CO, and H2O. It is shown that excitations of single electron replacement character can be described accurately in such an approach. Improved convergence is obtained using a preconditioned form of the coupled cluster linear response matrix.
Rotationally resolved dicopper (Cu2) laser-induced fluorescence spectra
Authors: Page Ralph H. , Gudeman Christopher S.
Published: July 2002

We have used an RF sputter source to produce gas-phase copper dimer molecules for study with laser-induced fluorescence spectroscopy. Rotationally resolved spectra of transitions from the electronic ground (X) state to the A, B, C, G, and J states have been obtained. Previous rotational analyses of the X, A, and B states are confirmed. Analysis of the X–C spectrum shows the C state to have Π(Ω=1) symmetry, and a bond length of 2.26 Å. Its vibrational constants are refined slightly. The J state has a bond length of 2.15 Å, and shows evidence of extensive perturbation. Emission lifetimes at the bandheads of its lowest two vibrational states are 80 ns. Franck–Condon factors for the X–C and X–J transitions have also been obtained. The new J state bond length supports conclusions drawn from a recently obtained photoelectron spectrum of Cu2: the cation ground state indeed corresponds to loss of a 4sσ electron, and the excited state is due to removal of a 3d electron. Bond strengths and distances in the various states of the neutral are rationalized in terms of 4sσ bonding orbitals and nonbonding (slightly repelling) 3d cores; creation of a 3d vacancy allows 3d shell bonding to occur, increasing bond strengths and vibrational frequencies slightly.
Saturation studies of H2O and HDO near 3400 cm−1 using intense picosecond laser pulses
Authors: Vodopyanov K. L.
Published: July 2002

The effect of significant decrease of water absorptivity for the intense picosecond laser radiation at λ=2.79 and 2.94 μm being near the center of the OH stretching mode absorption band was discovered. In case of pure water a thermal mechanism dominated: A very fast temperature rise led to weakening of H-bonds and consequently to the absorption band shift towards higher frequencies. As a result, a considerable (up to 10 times) decrease in the optical density at the laser frequency was obtained. In the second case of HDO diluted in D2O the temperature effects were eliminated and a pure spectroscopic saturation of the v=0 to v=1 vibrational transition was displayed. The saturation intensity as high as Is=2.5.1011 W/cm2 in this case gives the value of energy relaxation time of the OH excited state to be in the range 0.3 ps <T1 <0.6 ps. The width of the homogeneously broadened component of the fundamental OH band in HDO spectrum is evaluated to be ≥ 40 cm−1.
The determination of accurate dipole polarizabilities α and γ for the noble gases
Authors: Rice Julia E. , Taylor Peter R. , Lee Timothy J. , Almlöf Jan
Published: July 2002

The static dipole polarizabilities α and γ for the noble gases helium through xenon have been determined using large flexible one-particle basis sets in conjunction with high-level treatments of electron correlation. The electron correlation methods include single and double excitation coupled-cluster theory (CCSD), an extension of CCSD that includes a perturbational estimate of connected triple excitations, CCSD(T), and second-order perturbation theory (MP2). The computed α and γ values are estimated to be accurate to within a few percent. Agreement with experimental data for the static hyperpolarizability γ is good for neon and xenon, but for argon and krypton the differences are larger than the combined theoretical and experimental uncertainties. Based on our calculations, we suggest that the experimental value of γ for argon is too low. Adjusting this value would also bring the experimental value of γ for krypton into better agreement with our computed result. The MP2 values for the polarizabilities of neon, argon, krypton, and xenon are in reasonable agreement with the CCSD and CCSD(T) values, suggesting that this less expensive method may be useful in studies of polarizabilities for larger systems.
The time dependent solution of the Smoluchowski equation: Kinetics of escaping from the well for different dimensionalities
Authors: Shushin A. I.
Published: July 2002

A recently developed method of analytical solution of the Smoluchowski equation is applied to the investigation of the kinetics of diffusional escape from a potential well for different space dimensionalities n. The kinetics is described by the time dependence of the well occupation probability Nn(t). The formulas derived are valid for times longer than the time t0=a 2/D of relaxation within the well U(r) [a is the Onsager’s radius determined by U(a)=kT ]. In the absence of absorption (or reaction) within the well the kinetics for low n=1,2 is found to be nonexponential at any time from the very beginning and for any depth of the well. The characteristic escaping time, however, significantly depends on the well depth. At longer times we obtain long time tails Nn(t)∼t−n/2 (n=1,2), typical for corresponding free diffusion processes. A simple general analytical expression for N1(t) is derived. In the limit of high reactivity within the well N1(t) appears to be exponential at moderately short t, but at long times it changes to the asymptotic function ∼t−3/2 (unlike the weak reactivity tail ∼t−1/2 ). In the two-dimensional (2D) case in the high reactivity limit the kinetics is also exponential at short t. The long time asymptotics somewhat differs from the weak reactivity one (∼t−1):N2(t)∼1/t ln2 t. It is shown that the three-dimensional (3D) escaping process is described by the same expression as the one-dimensional (1D) one in the high reactivity limit, i.e., at moderately short t, N3(t)∼exp(−Wt), while at long times N3(t)∼t−3/2. Some possible extensions of the method are discussed.
Mechanisms of methanol decomposition on Pd {111}
Authors: Rebholz M. , Kruse N.
Published: July 2002

The decomposition of methanol (CH3OD) on a Pd{111} single crystal surface has been investigated by combined static secondary ion mass spectrometry (SSIMS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). The SSIM spectra contain a number of ions, including CH+x(x=0–3), PdCH+3, PdO+ (or PdOD+ ), PdOCH+3 and PdnCO+(n=1–3). These ions indicate the presence of methyl, oxygen (hydroxyl), methoxy, and carbon monoxide in the adsorbed layer. The absolute and relative ion intensities have been found to depend on the methanol gas pressure and surface temperature. The latter has been varied in a linear time program (TPSSIMS) in order to monitor changes in the composition of the adsorbed layer. The reaction route leading to CO and hydrogen involves methoxy formation and decomposition. Thus O–H and C–H bond breaking occur in successive steps. Steady methoxy surface concentrations, maintained by adsorption and thermal desorption (after recombination with hydrogen), build up with a delay time after accumulation of adsorbed CO. The detection of methyl and hydroxyl/oxygen suggests that O–C bond activation takes place as well. Adsorbed methyl is thermally and chemically stable up to temperatures of 500 K. TPD reveals the formation of H2O, HDO, and D2O as a result of the recombination reaction between OHad/Oad and hydrogen/deuterium atoms. The origin of the CH+3 ions detected in (TP)SSIMS is twofold. They are formed in some amounts by fragmentation of a methoxy species; large amounts, however, result from the direct sputter desorption of a methyl adsorbate. At temperatures above 300 K the surface concentration of methoxy is small and, consequently, the contribution from fragmentation can be disregarded. C 1s XP spectra have been taken at temperatures between 90 and 453 K. The data give corroborating evidence for the presence of surface species containing carbon atoms in different chemical environments. A clear distinction between adsorbed CO and CHx has become possible.
Potential energy surfaces, quasiadiabatic channels, rovibrational spectra, and intramolecular dynamics of (HF)2 and its isotopomers from quantum Monte Carlo calculations
Authors: Quack Martin , Suhm Martin A.
Published: July 2002

We report analytical representations of the six-dimensional potential energy hypersurface for (HF)2, the parameters of which are closely adjusted to low energy experimental properties such as hydrogen bond dissociation energy (D0=1062 cm−1 ) and vibrational–rotational spectra in the far and mid infrared. We present a detailed analysis of properties of the hypersurface in terms of its stationary points, harmonic normal mode amplitudes, and frequencies for the Cs minimum and C2h saddle point and effective Morse parameters and anharmonic overtone vibrational structure for the hydrogen bond and the HF stretching vibrations. The comparison between experimental data and the potential energy surface is carried out by means of accurate solutions of the rotational–vibrational Schrödinger equation with quantum Monte Carlo techniques, which include anharmonic interactions between all modes for the highly flexible dimer. Two extensions of the quantum Monte Carlo technique are presented, which are based on the clamped coordinate quasiadiabatic channel method and allow for the approximate calculation of excited rotational and vibrational levels.

Predictions include dissociation energies D0 for isotopomers (XF)2 with X=μ, D, T (D0=477; 1169; 1217 cm−1 ). Unusual anharmonic isotope effects predicted for the out-of-plane bending fundamental ν6 [378; 276; 295; and 358 cm−1 for (HF)2, (DF)2, (HFDF), (DFHF)] can be understood in simple terms. Centrifugal effects both for the high frequency a-axis rotation and low frequency c-axis rotation are accurately calculated for the vibrational ground state and some excited states, with a best equilibrium center of mass distance Req.ab=5.14a0 between the HF monomers. A very large anharmonic interaction constant x46≊−16 cm−1 is predicted for the hydrogen bond vibration ν4 and for out-of-plane bending ν6. This leads to assignment of our earlier experimental observation of a band at 383 cm−1 as ν6+ν4−ν4(K=1←0) at almost exactly the predicted position. The fundamental ν4 is predicted at 130±10 cm−1. A new, indirect assignment of our experimental data gives ν4≊125 cm−1. Monte Carlo calculations are presented for quasiadiabatic channels and transition states for hydrogen bond dissociation. We present a discussion of symmetry correlations for these channels and symmetry effects in predissociation by rotation, nuclear spin symmetry, and parity violation. Large effects from zero point energy on the three-dimensional quantum centrifugal barriers for rotational predissociation are found. On the basis of the new data, a much improved statistical mechanical estimate for the equilibrium 2HF=(HF)2 is obtained.
On the resonance structure of the adiabatic hypersurfaces of small doubly charged helium clusters He++n, n=3–6
Authors: Hogreve H.
Published: July 2002

A b initio calculations on doubly positively charged helium systems were performed in the adiabatic approximation by employing the MRDCI procedure and Gaussian base set of various sizes. The resulting potential energy surfaces are studied under the particular aspect of identifying structures that could lead to bound or resonance states. Although for none of the considered systems true bound states were found, there exist metastable resonance states for various configurations. For He++3, the metastable ground state occurs in a collinear symmetric configuration, and for He++4 in a planar geometry with D4h symmetry; in both cases, the binding has a covalent character. Metastable configurations of the larger systems He++5 and He++6 arise as the result of such He++3 or He++4 subsystems to which neutral He are attached by polarization forces. Our discussion of the dynamical implications of the calculated adiabatic potential surfaces includes in particular possibilities of eventual formation and dissociation reactions for the trimer ion He++3.
Pressure and pH dependence of the structure of a fatty acid monolayer with calcium ions in the subphase
Authors: Shih M. C. , Bohanon T. M. , Mikrut J. M. , Zschack P. , Dutta P.
Published: July 2002

We have studied the effect of pressure and pH on Langmuir monolayers of heneicosanoic acid in the presence of calcium ions in the subphase. At very low pH the monolayer is unaffected by calcium ions; as the pH is increased, the head groups are pulled closer together and the chain tilt at zero pressure decreases. As a result, the phase transitions seen along the isotherms occur at lower pressures. When the isotherms become featureless, the chains are untilted at all pressures. At very high pH, a new untilted structure is observed.
Initial state effects in the vibrational predissociation of hydrogen fluoride dimer
Authors: Bohac E. J. , Marshall Mark D. , Miller R. E.
Published: July 2002

The state-to-state vibrational predissociation dynamics of the hydrogen fluoride dimer has been investigated in detail using a newly developed instrument which gives both initial state selection and photofragment state determination. Results are reported for a wide variety of initial states associated with the ν1 and ν2 vibrations. The final state distributions universally indicate that the preferred dissociation channels correspond to the production of one HF fragment that is highly rotationally excited and another that is not. This is explained in terms of an impulsive dissociation mechanism which proceeds from a geometry close to that of the equilibrium structure of the dimer. We find that nearly degenerate initial states can have rather different final state distributions. In particular, there is a distinct difference between the upper and lower members of the tunneling doublet, which is most likely attributable to their related symmetries. The dissociation energy (D0) is determined to be 1062±1 cm−1.
Structural phenomena related to associative and dissociative adsorption of water on Ni(110)
Authors: Callen B. W. , Griffiths K. , Kasza R. V. , Jensen M. B. , Thiel P. A. , Norton P. R.
Published: July 2002

The adsorption of water on Ni(110) has been studied by thermal desorption spectroscopy (TDS), work function (ΔΦ), Fourier transform infrared reflection–absorption spectroscopy (FTIR-RAS), low energy electron diffraction (LEED), and electron-stimulated desorption ion angular distribution (ESDIAD). The major findings of this study are the following: (1) Water molecules in the chemisorbed c(2×2) half-monolayer do not cluster and the plane of the molecules is highly inclined to the surface normal; (2) no ESDIAD evidence of oriented water dimers is observed at 130 K and no FTIR activity is observed following adsorption at 80 K until multilayers are populated; (3) water has been measured to partially dissociate at a minimum temperature of 205±2 K; (4)the binding energy of water to the Ni(110) surface is increased by H-bonding to adsorbed hydroxyls (produced by the partial dissociation) in a linear OH(ad)/H2O complex with planar symmetry in the [001] direction. This bonding results in a higher temperature desorption state (A1).
Probing picosecond processes with nanosecond lasers: Electronic and vibrational relaxation dynamics of heme proteins
Authors: Li P. , Sage J. T. , Champion P. M.
Published: July 2002

We discuss the technique of resonance Raman saturation spectroscopy and present experimental results that probe relaxation processes in heme proteins following electronic excitation in the Soret band. The observable relaxation time scales are limited by the laser excitation rate kL rather than by the laser pulse width (∼10 ns). Analysis of the data using a theory that separates the electronic and vibrational relaxation leads to electronic ground state recovery times τ=6.4±2.0 ps for ferrocytochrome c, τ=4.8±1.5 ps for deoxymyoglobin, and τ=2.0±0.7 ps for deoxyhemoglobin. The Raman depolarization ratio is predicted to increase at high laser flux, due to the preparation of a partially oriented sample by photoselective excitation. Such effects are observed in heme systems and the relaxation times extracted from the depolarization analysis are in good agreement with those derived from measurements of Raman intensity saturation. Studies of the asymmetric broadening of the ν4 mode of cytochrome c at high laser flux reveal that the line shapes in the Stokes and anti-Stokes region are inequivalent.

Time-reversal symmetry dictates that this broadening is due to an underlying Raman band associated with an excited electronic state that is populated at high laser flux. Similar line broadening effects, observed in hemoglobin and myoglobin samples, are also shown to arise from Raman scattering of excited electronic states rather than Rabi broadening [Alden et al., J. Phys. Chem. 94, 85 (1990)] or anharmonic coupling to vibrationally hot low frequency modes [Petrich et al., Biochemistry 26, 7914 (1987)]. Quantitative analysis of Stokes and anti-Stokes Raman scattering determines the heme vibrational temperature as a function of laser flux and leads to a description of the Raman intensities that differs significantly from that of Lingle et al. [J. Phys. Chem. 95, 9320 (1991)], which ignores electronic saturation effects and is based on the scattering properties of a two-level system. For cytochrome c, we use a simple thermal transport model to extract a value for the product of the heme area and the coefficient of surface heat transfer between the heme and the surrounding protein. This leads to a ∼4 ps time constant for the short-time exponential phase of heme cooling.
High-resolution and symmetry-resolved N and O K-edge absorption spectra of NO
Authors: Kosugi Nobuhiro , Adachi Jun-ichi , Shigemasa Eiji , Yagishita Akira
Published: July 2002

High-resolution electron-yield and ‘‘symmetry-resolved’’ ion-yield spectra have been measured at the N and O K edges of a 2Π open-shell molecule, NO. Several Rydberg transitions converging to the 3Π and 1Π ionization thresholds are found. The core-to-valence and core-to-Rydberg transitions are interpreted with the help of ab initio self-consistent-field configuration-interaction and frozen-core calculations for the core-excited states with three open shells. It is found that the equivalent-core model (N*O=O2) breaks down in discussing the state ordering of the three N 1s–2pπ* excited states, 2Δ, 2Σ−, and 2Σ+. It is important to consider explicitly the core hole and the exchange repulsion between the core and π* electrons.
Isotopic labeling as a tool to establish intramolecular vibrational coupling: The reaction of 2-propanol on Mo(110)
Authors: Uvdal P. , Wiegand Benjamin C. , Serafin J. G. , Friend C. M.
Published: July 2002

The reactions of 2-propanol on Mo(110) were investigated using temperature programmed reaction, high resolution electron energy loss, and x-ray photoelectron spectroscopies. 2-Propanol forms 2-propoxide upon adsorption at 120 K on Mo(110). The 2-propoxide intermediate deoxygenates via selective γ C–H bond scission to eliminate propene as well as C–O bond hydrogenolysis to form trace amounts of propane. The C–O bond of 2-propoxide is estimated to be nearly perpendicular to the surface. Selective isotopic labeling was used to establish the coupling between the C–O stretch and modes associated with the hydrocarbon framework. The degree of coupling was strongly affected by bonding to the surface, primarily due to weakening of the C–O bond when 2-propoxide is bound to Mo(110). Selective isotopic labeling was, therefore, essential in making vibrational assignments and in identifying key reaction steps. Only a small kinetic isotope effect was observed during reaction of (CD3)(CH3)CHOH, consistent with a substantial component of C–O bond breaking in the transition state for propene elimination. Coupling of the C–O stretch to motion of the methyl group is also suggested to be important in the transition state for propene elimination.
Rotational spectrum, molecular structure, and 201Hg nuclear quadrupole coupling constants of HCNHg
Authors: Shea J. A. , Campbell E. J.
Published: July 2002

High resolution rotational spectra for 21 isotopic species of the hydrogen cyanide–mercury complex have been measured in the 7–15 GHz region using a Balle/Flygare pulsed Fourier transform microwave spectrometer. Rotational constants and 14N nuclear quadrupole coupling constants are (in MHz), for HC14N202Hg: B0=1265.6627(7), DJ=0.020 669(14), χN=−0.695(12), and for DC14N202Hg: B0=1201.8338(16), DJ=0.021 213(45), and χN=−0.748(19). The 201Hg nuclear quadrupole coupling constants are, for HC15N201Hg: −5.97(13) MHz, and for DC15N201Hg: −6.03(24) MHz. The atomic arrangement is HCN...Hg with a zero-point vibrationally averaged center-of-mass separation of 4.05 Å for the most abundant HC14N202Hg species. The equilibrium structure was not determined. HCNHg does not exhibit the extreme centrifugal distortion and isotopic sensitivity effects seen in ArHCN [K. R. Leopold, G. T. Fraser, F. J. Lin, D. D. Nelson, Jr., and W. Klemperer, J. Chem. Phys. 81, 4922 (1984)].
Theory of resonant two-photon dissociation of Na2
Authors: Chen Zhidang , Shapiro Moshe , Brumer Paul
Published: July 2002

The quantum scattering theory of resonant two-photon (ω1+ω2) dissociation is developed and applied to Na2 photodissociation. In the energy range considered, photodissociation primarily occurs via excitation to the A 1Σu state, intersystem crossing to the b 3Πu state, and subsequent excitation to the triplet continuum. Photodissociation rates to produce Na(3s)+Na(3d), Na(3s)+Na(4s), and Na(3s)+Na(3p) are reported as a function of both ω1 and ω2. Characteristic features due to spin–orbit coupling and to multiple product production are observed and discussed.
Luminescence from pure and doped solid deuterium irradiated by keV electrons
Authors: Stenum B. , Schou J. , So/rensen H. , Gürtler P.
Published: July 2002

Solid samples of pure and doped deuterium were irradiated by keV electrons and the luminescence of these samples was measured between 200 and 700 nm. In order to separate the intrinsic emissions from impurity-induced luminescence, deuterium was doped with standard air impurities like N2, H2O, O2, and CO. In purified solid deuterium, an emission at 275 nm was observed for the first time and attributed to a transition in D*3 molecules produced via ionization of D2, formation of D3+, and subsequent recombination with electrons. Film-thickness dependence as well as timing experiments corroborate this interpretation.
Spectroscopic constants and potential energy curves for 15 electronic states of Ag2
Authors: Zhang Hongxing , Balasubramanian K.
Published: July 2002

We employ the complete active space multiconfiguration self-consistent method followed by large-scale configuration interaction (CI) calculations which included d-electron correlations and 5p orbitals of Ag for 15 electronic states of the silver dimer (Ag2). The spectroscopic constants (re,Te,ωe) and potential energy curves for these states are computed. Our computed constants are in good agreement with the known experimental values for the X 1Σg+, A 1Σu+, and B 1Πu states. We also predict the spectroscopic constants of several other states [3Σu+,3Πu,1Σg+(II), 3Σg+,1Πg,3Πg,3Σu+(II)] which are yet to be observed.
Total and partial decay widths in vibrational predissociation of HF dimer
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

Coupled channel golden-rule quantum mechanical calculations of total and partial decay widths are reported for vibrational predissociation of HF dimer. In this calculation, an ab initio potential energy surface is employed in both bound and scattering calculations but the vibrational coupling is given by an electrostatic interaction. The computed lifetime of HF dimer is 4.0 and 4.8 ns for ν2+ and ν2− (‘‘bound-H’’) excitation, and 11 ns for both ν1+ and ν1− (‘‘free-H’’) excitations. These theoretical lifetimes are compared to the experimental values of about 1 ns for the ν2 excitation and about 20 ns for the ν1 excitation. We also present rotational state distributions of HF monomers which are in qualitative agreement with the experimental measurement of Bohac et al. [J. Chem. Phys. 96, 6681 (1992)].
Direct evaluation of phase coexistence by molecular simulation via integration along the saturation line
Authors: Kofke David A.
Published: July 2002

Thermodynamic integration along a path that coincides with the saturation line is proposed as an efficient means for evaluation of phase equilibria by molecular simulation. The technique allows coexistence to be determined by just one simulation, without ever attempting or performing particle insertions. Prior knowledge of one coexistence point is required to start the procedure. Integration then advances from this state according to the Clapeyron formula—a first-order ordinary differential equation that prescribes how the pressure must change with temperature to maintain coexistence. The method is unusual in the context of thermodynamic integration in that the path is not known at the outset of the process; results from each simulation determine the course that the integration subsequently takes. Predictor–corrector methods among standard numerical techniques are shown to be particularly well suited for this type of integration. A typical integration step along the saturation line proceeds as follows: An increment in the temperature is chosen, and the saturation pressure at the new temperature is ‘‘predicted’’ from previous data (the initial coexistence datum and/or previous simulations). Simultaneous but independent NPT simulations of the coexisting phases are initiated at the said conditions. Averages taken throughout the simulations are repeatedly used to ‘‘correct’’ the estimate of the pressure to convergence. Thus strictly the pressure is not fixed during the simulation. Vapor–liquid coexistence of the van der Waals model is first used to study the numerical integration method without the complications of molecular simulation. In a second application the phase envelope of the Lennard-Jones model fluid is computed, and many variations of the technique are examined. Overall, the results are remarkably consistent and in agreement with previous simulation studies. Difficulty is encountered upon approach of the critical point, but, by artificially coupling the simulation volumes, the method remains effective in this regime so long as a suitably small integration step is employed. Many extensions and improvements of the technique are discussed.
Binary icosahedral clusters: Atom and electron counting rules
Authors: Teo Boon K. , Zhang Hong , Kean Yin , Dang Hy , Shi Xiaobo
Published: July 2002

The first part of this paper deals with the enumeration of all possible stereoisomers (or polytypes) of a binary (two-colored) icosahedron (either noncentered or centered) via Polya’s theorem. The numbers of achiral and chiral structures for each combination of the two types of atoms (or colors) A and B are determined. In cluster chemistry, A and B may represent two different kinds of atoms (e.g., main-group elements vs transition metals) in a heteronuclear cluster, two different ligand environments in a homonuclear cluster, or atoms and holes (vacant sites) in an icosahedral cluster with missing vertices. The second part deals with the electronic requirements of binary icosahedral clusters where A and B may represent main-group elements and transition metals, or, atoms and holes. Significantly different electron counts are observed for centered vs noncentered icosahedral clusters, depending upon the nature of the interstitial atom. Thus, icosahedral clusters can be classified into four broad categories according to their electron counts: Type I: the majority are those with 13 electron pairs as represented by noncentered icosahedral clusters, or those centered with main-group elements or early transition metals as the interstitial atom; Type II: those with more than 13 electron pairs as exemplified by centered icosahedral clusters with late transition metals as the interstitial atom; and Types III and IV: those with less than 13 electron pairs as observed for centered Au icosahedral clusters. The atom and electron counting rules developed are rationalized in terms of molecular orbital theory. The utilities of the atom and electron counting rules in systematizing structural and spectroscopic data of icosahedral clusters (in both solid-state and gas-phase) as well as in rationalizing the variations of the electron counts in different icosahedral cluster systems are discussed.
The low lying electronic states of O−3
Authors: Koch Wolfram , Frenking Gernot , Steffen G. , Reinen D. , Jansen M. , Assenmacher W.
Published: July 2002

The energies of the three lowest lying excited states of the ozonide anion (1 2B2, 1 2A1, 1 2A2) at the optimized geometry of the X 2B1 ground state are theoretically predicted at the MRCI-SD level of theory using large atomic natural basis sets. The calculated vertical excitation energy Tv=2.85 eV for the 1 2A2←X 2B1 transition, which has a large transition moment, is in good agreement with the experimental results for the isolated O3− anion in host matrices and solution between 2.69–2.81 eV. The state symmetry forbidden transition 1 2B2←X 2B1 is calculated with Tv=2.26 eV. The excitation energy for the 1 2A1←X 2B1 transition is theoretically predicted with Tv=2.28 eV and a very low transition moment. The theoretical results are also discussed in comparison with recently observed low lying absorption bands of O3−.
Apparent and real values of photochemical hole-burning parameters. Sulfonated tetraphenylporphin doped in polyvinyl alcohol
Authors: Murase Norio , Horie Kazuyuki
Published: July 2002

This paper presents real values of photochemical hole-burning (PHB) parameters for dye-doped polymer systems. The cross section for purely electronic zero-phonon absorption σ*0, quantum efficiency of hole formation η and full-width at half-maximum of inhomogeneous broadening Δωi are determined by the least-squares fitting method. Our sample is sulfonated tetraphenylporphin doped in polyvinyl alcohol at 20 K. The determined values are σ*0 = (2.3 ± 0.2)× 10−15 cm2, η=(1.6±0.6)×10−2, and Δωi=(223±7) cm−1. The errors are of the order of a standard deviation. The η is much greater than previously reported values. The apparent quantum efficiencies of hole formation are derived under the assumption that the lowest-energy absorption band consists of transitions of one type. They exhibit a marked wavelength dependence, but their values are well explained quantitatively by the real values. There is no need to consider any wavelength dependence of σ*0 and η, at least at the initial stage of burning. The purely electronic transition component is about half of the lowest-energy absorption band. The other component is ascribed to at least two kinds of vibronic transitions of the dye. The Debye–Waller factors for these two vibronic transitions are roughly estimated to be in the order of 10−2. This result means the site-selectivity in PHB is not perfect in a wide range of the lowest-energy absorption band.
Rotational diffusion of uniaxial probes in biaxial liquid crystal phases
Authors: Berggren E. , Tarroni R. , Zannoni C.
Published: July 2002

We discuss the reorientation of a cylindrically symmetric probe in a biaxial orthorombic medium, such as a biaxial nematic. We write down and solve for the first time the rotational diffusion equation for a rod-like or disc-like uniaxial probe reorienting in a medium of biaxial symmetry. We calculate first and second rank correlation functions both in the uniaxial and biaxial phase, giving the possibility of following the evolution of dynamic observables of interest (e.g. spectral densities) through the uniaxial–biaxial phase transition. Nonperturbative and approximate analytical solutions are provided.