bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized Gaussian wave packet dynamics
Authors: Huber Daniel , Heller Eric J.
Published: July 2002

We propose an extension of the semiclassical Gaussian wave packet dynamics to eliminate the three main restrictions of this method. The first restriction is that the wave packet is forced to remain Gaussian. This is correct only for quadratic, linear, or constant potentials. The second restriction is that the method is, in general, not able to treat most classically forbidden processes. The third restriction is that the norm is conserved only for Gaussian wave packets. For a superposition of Gaussians this is no longer true. We can eliminate these restrictions by an extension of the method into complex phase space, keeping time real.
Collisional broadening of CO2 IR lines. II. Calculations
Authors: Rosenmann L. , Hartmann J. M. , Perrin M. Y. , Taine J.
Published: July 2002

The ability of available theoretical models in describing broadening mechanisms is tested for the CO2–O2, CO2–CO2, and CO2–N2 systems. It is shown that the Anderson–Tsao–Curnutte theory is inaccurate since short-range forces can contribute significantly to broadening. We use the approach of Robert and Bonamy, but the usual expansion of the atom–atom potential to the fourth order around the intermolecular distance appears insufficient at short distances for these particular systems. We propose a better representation of the radial dependence of the atom–atom potential, while keeping the previous analytical expression of the cross section. Satisfactory results are obtained for both the rotational quantum number dependence of room-temperature CO2–O2, CO2–CO2, and CO2–N2 half-widths and the evolution of CO2–N2 broadening with temperature. It is shown that the isotropic part of the potential involved in the trajectory calculation must be coherently deduced from the atom–atom interaction potential.
Sequential clustering reactions of Si+ with silane: A theoretical study of the reaction mechanisms
Authors: Raghavachari Krishnan
Published: July 2002

The mechanisms of the sequential clustering reactions of Si+ with SiH4 have been studied by means of accurate quantum chemical techniques using polarized basis sets and including the effects of electron correlation and zero-point corrections. In accordance with the experimental results of Mandich, Reents, and Jarrold, the reactions lead to the formation of Si2H+2, SiH+4, Si4H+6, and finally Si5H+10. The study of the detailed reaction pathways including the necessary transition structures leads to the assignment of specific isomeric products in each reaction step. The specific isomers assigned are H2Si–Si+, H3Si–SiH–Si+, (H3Si)2Si–Si+, and (H3Si)3Si–SiH+. The bottleneck in the reaction sequence is due to the formation of the branched product Si5H+10 where elimination of H2 is not easily possible due to the saturation of the bonding involving the central silicon atom. Isotopic exchange reactions which have been seen experimentally are also rationalized by this mechanism. Quantitative comparisons are made between our calculated energy barriers and those derived by Mandich et al. from phase space calculations using the experimental reaction rates. The mean absolute deviation between the two results for seven different energetic quantities is only ≂3 kcal/mol.
Infrared multiphoton dissociation of RDX in a molecular beam
Authors: Zhao Xinsheng , Hintsa Eric J. , Lee Yuan T.
Published: July 2002

Infrared multiphoton dissociation (IRMPD) of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in a molecular beam has been performed in order to investigate the mechanism of RDX thermal decomposition. A beam of molecules was crossed by a pulsed TEA CO2 laser and velocity distributions of the various products were measured by the time-of-flight (TOF) technique as a function of the laboratory angle using a mass spectrometric detector. The dissociation channels, their branching ratios, and the translational energy distributions of the products were determined. In contrast to the conventional view of simple bond rupture through loss of NO2 as the dominant primary channel in RDX decomposition, it was found that the dominant primary channel is concerted symmetric triple fission to produce three CH2N2O2 fragments which subsequently undergo secondary concerted dissociation to produce HCN, H2CO, HONO (or HNO2), and N2O. A total of two primary and four secondary dissociation channels were observed. Concerted reactions predominate over simple bond rupture not only in the number of channels (four vs two) but also in the amount of products. A fair amount of translational energy release through concerted reaction channels was observed, which is significant for an explanation of the energies of RDX decomposition.
Resonant Raman scattering of controlled molecular weight polyacetylene
Authors: Schen M. A. , Chien J. C. W. , Perrin E. , Lefrant S. , Mulazzi E.
Published: July 2002

Polyacetylene, (CH)x, films of 500, 5300, 10 500, and 100 000 Daltons number average molecular weights (Mn ) were synthesized using the titanium tetra-n-butoxide/triethyl aluminum-catalyst/cocatalyst system and examined using resonant Raman scattering techniques. Before isomerization, trans segments are found to exist mainly as short, isolated sequences independent of Mn. After thermal isomerization, theoretical analysis of the RRS spectra using the Brivio, Mulazzi model indicate the ratio of long trans conjugated segments (N≥30) to short trans conjugated segments (N≤30) is significantly larger for 100 000 Dalton polymer in comparison to polymer of 10 500 Mn and below. For samples below 10 500 Daltons, no clear relationship between actual polymer molecular weight and G is observed. Optimization of the isomerization conditions for 100 000 Dalton polymer results in trans-(CH)x with a G=0.80. These results suggest that not until very long molecular chains are obtained can samples composed principally of long conjugated segments be obtained. It is proposed that defects which arise during and after the polymerization limit the content of long segments. Ambient, short term oxidation of 100 000 Mn polymer shows a decrease in G from 0.80 to 0.70. Low level chain oxidation or doping is shown to preferentially occur within long conjugated segments.
The effect of higher than double excitations on the F+H2→FH+H barrier
Authors: Bauschlicher Charles W. , Langhoff Stephen R. , Lee Timothy J. , Taylor Peter R.
Published: July 2002

The classical barrier height and saddle point geometry are computed using the averaged coupled-pair functional (ACPF) method, correlating both seven and nine electrons. The size-consistent ACPF method indicates that 2s correlation substantially reduces the barrier. The excellent agreement between the ACPF and MRCI+Q results at both the seven and nine electron level provides additional support for the +Q correction. Thus the ACPF treatment supports a low (1.65 kcal/mol) barrier. The seven-electron CISDTQ results are shown to be consistent with the MRCI+Q and ACPF results.
Production of anion–atom complexes by electron stimulated desorption
Authors: Sanche L. , Parenteau L.
Published: July 2002

We report the formation of the anion–atom complexes ArO− and ArCl− by electron stimulated desorption from multilayer argon films containing O2, N2O, and Cl2 molecules.
Analysis and deconvolution of some J′≠0 rovibronic transitions in the high resolution S1←S fluorescence excitation spectrum of pyrazine
Authors: Siebrand Willem , Meerts W. Leo , Pratt David W.
Published: July 2002

Fluorescence excitation spectra are reported for several J′≠0 rotational members of the 000 band of the S1(1B3u)←S0(1A1g) electronic transition of pyrazine at a resolution of about 10 MHz. The transitions studied, namely R(0)–R(2) and P(2)–P(4), are each split into a large number of sharp lines ascribed, as in the case of the previously studied P(1) (J′=0) transition, to coupling with the lowest triplet state T1(3B3u). Despite this complexity, we show in this paper that it is possible to separate the lines into clusters of transitions that terminate in the same K′ component of the electronically excited, mixed S1–T1 state. This demonstrates that K′ is a good quantum number, at least at low J′ in the zero-order S1 state. From this analysis, we determine the rotational constants of the S0 and S1 states. We also determine: (i) the relative cluster intensities; (ii) the coupled T1 level densities; and (iii) by using standard deconvolution techniques, the S1–T1 coupling matrix elements, each as a function of J′,K′. Cluster intensities decrease with increasing J′, but K′=0 clusters are significantly less intense than K′≠0 clusters in the fluorescence excitation spectra. Observed triplet level densities in each cluster exceed by an order of magnitude the calculated density of rovibronic states if selection rules appropriate to the D2h point group are taken into account. Neither the observed level densities nor the coupling matrix elements (which vary from less than 5 MHz to more than 500 MHz) show a clear-cut systematic dependence on J′ or K′, although K′=0 levels appear to be more strongly coupled than K′≠0 levels. Possible explanations for these results and their implications for intersystem crossing dynamics in the isolated molecule are discussed.
Excited rovibronic level dependence of the magnetic quenching of slow fluorescence of pyrazine vapor
Authors: Ohta Nobuhiro , Takemura Takeshi
Published: July 2002

Excited rotational level dependence of the external magnetic field effects both on intensity and on decay of fluorescence of pyrazine vapor has been carefully examined for the zero-point vibrational level in S1 with a field strength of 0–170 G. The magnetic quenching of the slow fluorescence becomes more effective with increasing rotational quantum number J′ of the excited level, and the field strength at which the amount of fluorescence quenching becomes one-half of the total amount of quenching at the saturated fields is roughly proportional to (2J′+1)−1. The magnetic quenching is also found to depend on K′ of the excited level. The rotational level dependence of the magnetic quenching of the slow fluorescence is related to a difference in the number of the triplet levels coupled to the optically excited singlet rovibronic level, based on the spin decoupling mechanism of the singlet–triplet mixed level.
A new contribution to spin-forbidden rare earth optical transition intensities: Analysis of all trivalent lanthanides
Authors: Burdick G. W. , Downer M. C. , Sardar D. K.
Published: July 2002

We expand upon recent quantitative calculations which show that numerous parity- and spin-forbidden linear optical transitions observed in trivalent rare earth ions acquire a major fraction of their intensity from hitherto neglected contributions involving spin–orbit linkages within excited configurations. Extending our revised analysis of observed linear absorption intensities for Gd3+ and Eu3+ to the other nine trivalent lanthanides with visible wavelength transitions, we find that the new contributions are at least as important in these other lanthanides, and are often comparable to standard contributions. We also show that spin-forbidden ΔS=1 transitions are selectively modified over the spin-allowed ΔS=0 transitions. In addition, we examine contributions from linkages with g orbital excited states in the light of recent electronic Raman scattering data for lanthanide phosphates, and show that these long neglected terms may actualy dominate the transition intensity. With substantial modification of previously fitted phenomenological parameters, however, the good fit with observed intensities is retained.
Temperature dependence of the lowest excited singlet-state lifetime of all-t
r
a
n
s-β-carotene and fully deuterated all-t
r
a
n
s-β-carotene
Authors: Wasielewski Michael R. , Johnson Douglas G. , Bradford Eric G. , Kispert Lowell D.
Published: July 2002

A 4 ps, 450 nm laser pulse was used to electronically excite all-trans-β-carotene and all-trans-β-carotene-d56 in 3-methylpentane. The transient absorption spectra of these molecules were measured as a function of temperature down to 20 K. In all cases the 400–500 nm electronic absorption band of each carotene bleaches and a new absorption band near 560 nm appears immediately upon excitation. These bands recover with single exponential kinetics: τ=8.1±0.5 ps for all-trans-β-carotene, and τ=10.5±0.6 ps for all-trans-β-carotene-d56 at 294 K. These recovery times increase by about a factor of 2 in glassy 3-methylpentane, and are nearly independent of temperature from 100 to 20 K. The weak dependencies of the lowest excited single-state lifetime of all-trans-β-carotene on deuteration and temperature are discussed in terms of nonradiative decay mechanisms within carotenoids.
Molecular beam studies of the photodissociation of benzene at 193 and 248 nm
Authors: Yokoyama A. , Zhao X. , Hintsa E. J. , Continetti R. E. , Lee Y. T.
Published: July 2002

The photodissociation processes of benzene following excitation at 193 and 248 nm have been studied by molecular beam photofragmentation translational spectroscopy. When benzene was excited to the 1 B1u state by absorption at 193 nm, dissociation occurred through three primary channels, C6H5+H (80%), C6H4+ H2 (16%), and C5H3+CH3 (4%), following internal conversion to the vibrationally excited ground state. When benzene was excited to the 1 B2u state at 248 nm, two primary dissociation channels, C6H4+H2 (96%), and C5H3+CH3 (4%), were observed. Photodissociation to produce two C3H3 was induced by two photon absorption of benzene at both 193 and 248 nm. Numerous secondary photodissociation processes of the primary photoproducts were also observed at both 193 and 248 nm.
Generalized Brownian dynamics. II. Vibrational relaxation of diatomic molecules in solution
Authors: Smith D. E. , Harris C. B.
Published: July 2002

A simple classical stochastic model for diatomic vibrational relaxation, based on the generalized Langevin equation, is presented. The memory function in the generalized Langevin equation is determined directly from equilibrium force autocorrelation functions for the individual atoms of the diatomic dissolved in the solvent of interest. A simple autoregressive (AR) procedure, developed in a preceding paper [D. E. Smith and C. B. Harris, J. Chem. Phys. 92, xxx (1990)], is used for modeling the memory functions to arbitrary order. This model is tested on the system of iodine in Lennard–Jones xenon using fourth order AR approximations for the memory functions, and is found to be very effective in reproducing data from molecular dynamics simulations at two very different densities. Results are discussed in terms of the simplifying assumption that the solvent interaction with the diatomic can be characterized by equilibrium dynamics of single atoms in solution.
Angular momentum relaxation in binary collisions. Comparison of cross sections
Authors: Jameson Cynthia J. , Jameson A. Keith
Published: July 2002

The values of the collisional efficiency for rotational angular momentum transfer bij are compared for various probe molecules (i=NNO, CO2, CO, N2, CF4, CH4, and SF6 ) with different collision partners ( j=same set as i plus Ar, Kr, Xe, and HCl). The trends in bij are consistent with an underlying kinematic factor which is the same as the collisional efficiency for angular momentum transfer in the perfectly rough hard sphere model of Chandler, modified by electronic factors which depend primarily on the anisotropy of the molecule and secondly on the polarizability and anisotropy of the collision partner.
Thermophysical properties of gaseous refrigerants from speed of sound measurements. I. Apparatus, model, and results for 1,1,1,2-tetrafluoroethane R134a
Authors: Goodwin A. R. H. , Moldover M. R.
Published: July 2002

The speed of sound in gaseous 1,1,1,2-tetrafluoroethane (R134a) has been obtained between 233.16 and 340 K from measurements of the frequency of the radial acoustic resonances of a gas-filled spherical cavity. Perfect-gas heat capacities and second and third acoustic virial coefficients have been calculated from the results. The acoustic virial coefficients are used to estimate the density virial coefficients B(T) and C(T) and an effective square-well potential. The estimates of B(T) are consistent with B(T) deduced from high-quality equation-of-state measurements; those for C(T) are slightly inconsistent. The apparatus and its calibration with argon are described.
A perturbation method for calculating vibrational dynamic dipole polarizabilities and hyperpolarizabilities
Authors: Bishop David M. , Kirtman Bernard
Published: July 2002

Perturbation formulas are derived for calculating the vibrational dynamic polarizability (αv) and hyperpolarizabilities (βv and γv ) of polyatomic molecules. These formulas, based on an initial harmonic oscillator approximation, include corrections for mechanical anharmonicity (cubic) terms in the vibrational potential and electrical anharmonicity (quadratic) terms in the dependence of the electrical field polarization potential on nuclear coordinates. Results are presented for FH and CO2. In the former case, comparison is made to ‘‘exact’’ numerical values. Fully documented computer programs for the perturbation treatment are available.
Photoionization mass spectrometric studies of the transient species Si2Hn (n=2–5)
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The transient species Si2H5, Si2H4, Si2H3, and Si2H2 have been prepared for the first time, by reactions of F atoms with Si2H6. The species are generated in situ and studied by photoionization mass spectrometry. The adiabatic ionization potentials are (in eV) Si2H5, 7.60±0.05; Si2H4, 8.09±0.03; Si2H3, <7.59; and Si2H2, 8.20+0.01−0.02. In addition, two fragment appearance potential curves were measured, Si2H+3 (Si2H5) and Si2H+2 (Si2H4). By combining these ionization potentials with appearance potentials obtained here and from Si2H6, the following heats of formation (kcal/mol at 0 K) have been inferred: Si2H5, <63.3 (59.2); Si2H4, 67.9±0.9; Si2H3 (∼96); Si2H2, <100.7 (90.2–95.6). The quantities in parentheses are probable values, but less well defined. Structures and structural changes resulting from photoionization are discussed. In particular, Si2H2 and its cation are very likely cyclic (doubly bridged). Heats of formation of cations are also obtained, from which the proton affinities of Si2H2 (199±3 kcal/mol) and Si2H4 (199.9 kcal/mol) are deduced. The structural and energetic differences between corresponding silicon and carbon hydrides are discussed in detail.
Effect of breaking cylindrical symmetry on photoelectron angular distributions resulting from resonance-enhanced two-photon ionization
Authors: Reid Katharine L. , Leahy David J. , Zare Richard N.
Published: July 2002

An expression is derived for the photoelectron angular distribution (PAD) following (1+1′) resonance-enhanced multiphoton ionization (REMPI) of a molecule with linearly polarized light beams. When the two polarization vectors are parallel, cylindrical symmetry exists, and the PAD depends only on θ, the angle between the linear polarization vector of the ionizing radiation and the electron ejection direction. When the polarization vectors are perpendicular, cylindrical symmetry is broken, and the PAD shows φ and θ dependence. For an arbitrary angle between the two polarization vectors, the angular distribution ceases to have reflection symmetry. This breaking of cylindrical symmetry causes interference effects in the REMPI process that are readily described using a density matrix formalism. As an example, the (1+1′) REMPI of NO via its A 2Σ+ state is considered.
Potential energy surfaces near intersections
Authors: Atchity Gregory J. , Xantheas Sotirios S. , Ruedenberg Klaus
Published: July 2002

The topographies of two potential energy surfaces are examined in the vicinity of their intersection. A brief account of the basic theory is given and the possible surface types are discussed explicitly. Two main patterns are found. One of these (‘‘peaked’’) has the character of a tilted double cone in that the lower (upper) surface decreases (increases) in all directions from the intersection which is a point where an infinite number, in fact, all orthogonal trajectories emanate. The other pattern (‘‘sloped’’) results when both surfaces are monotonically sloped and touch each other along the slope, with most orthogonal trajectories bypassing the intersection. When the latter pattern prevails, the intersection can lie on a steepest descent line which originates at a transition state and hence may qualify as a reaction path model. An intermediate pattern, involving a horizontal slope on both surfaces, is also possible. The topographical patterns also differ markedly with respect to the bunching of the steepest descent lines. In general, the latter tend to veer away from the intersection on the lower surface favoring bifurcations, but are funneled towards the intersection on the upper surface, making the vicinity of the intersection a region favoring radiationless transitions. The various cases are classified and illustrated through quantitative graphs of contours and orthogonal trajectories.
Fluorescence-detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses
Authors: Scherer Norbert F. , Carlson Roger J. , Matro Alexander , Du Mei , Ruggiero Anthony J. , Romero-Rochin Victor , Cina Jeffrey A. , Fleming Graham R. , Rice Stuart A.
Published: July 2002

We introduce a novel spectroscopic technique which utilizes a two-pulse sequence of femtosecond duration phase-locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in-phase, in-quadrature, and out-of-phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule.

These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct determination of wave packet phase evolution is possible because phase locking effectively transforms the interferogram to a frame which is referenced to the optical carrier frequency, thereby eliminating the high (optical) frequency modulations. This has the effect of isolating the rovibrational molecular dynamics. The phase locking scheme is demonstrated for molecular iodine. The excited state population following the passage of both pulses is detected as the resultant two-beam dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from the vibrational dynamics of the molecule on the electronically excited potential energy surface. In addition, coherent interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted herein by use of a simple analytic model, by first order perturbation theory and by quantum mechanical wave packet calculations. We find the form of the interferogram to be determined by the ground state level from which the amplitude originates, the deviation from impulsive preparation of the wave packet due to nonzero pulse duration, the frequency and anharmonicity of the target vibrational levels in the B state, and the detuning of the phase-locked frequency from resonance. The dependence of the interferogram on the phase-locked frequency and phase angle is investigated in detail.
On the dynamics of moderately crosslinked networks
Authors: Sommer Jens-Uwe
Published: July 2002

We show, that the dynamics of moderately crosslinked polymer networks may exhibit stretched exponential behavior only due to the broad distribution of chain lengths, with an exponent of 1/3. The obtained response function obeys the crosslink density time scaling.
Statistical mechanical theory for the plectonemic DNA supercoil
Authors: Hearst John E. , Hunt Nathaniel G.
Published: July 2002

The eigenfunctions for circular boundary conditions of the differential equation first used by Harris and Hearst in 1966 to represent the dynamic properties of the wormlike coil have now been applied to the closed circular coils of high writhe. In order to avoid problems of knotting and excluded volume, the discussion here has been restricted to three-dimensional eigenfunctions with near plectonemic symmetry, i.e., eigenfunctions which cross each constant z plane only twice. It is concluded that at the natural levels of superhelical density that are found for DNA in vivo, the DNA free of protein must be in a highly reduced configurational entropy state. The impact of this conclusion on issues of entanglement, chromosome replication and segregation, and chromosome organization are discussed.
Analysis of the network topology in liquid water and hydrogen sulphide by computer simulation
Authors: Ruocco G. , Sampoli M. , Vallauri R.
Published: July 2002

The analysis of Voronoi polyhedra for liquid water and hydrogen sulphide, at different temperatures, has been performed by using the molecular configurations generated by computer simulation of the liquids with realistic potential models. Some topological and metric properties of the Voronoi polyhedra have been calculated and their distributions are studied. In addition, the cross correlation between pairs of metric quantities are also investigated. The latter correlations are found to be more relevant for a clear distinction between the two systems examined here. In particular, the cross correlation between the potential energy of a molecule and the volume of the corresponding Voronoi polyhedron makes it clear that the interpretation of the anomalous physical properties of water in terms of local volume has to be revised.
Vibrational predissociation of ArH2O
Authors: Bissonnette C. , Clary D. C.
Published: July 2002

Accurate close-coupling calculations are used to investigate the vibrational predissociation of ArH2O as a function of the overall rotation J of the van der Waals complex. A full vibrational and rotational basis of H2O states is used in the calculation. The potential energy surface is of a form due to Cohen and Saykally and derived from far-infrared spectra, with an additional term to introduce the dependence on the vibrations of H2O. The linewidths calculated in this work show a maximum at J=6 and it is found that Fermi resonances affect dramatically the magnitude of the calculated linewidths. Good agreement with experimentally measured linewidths of Nesbitt and Lascola is achieved and the calculations provide a simple picture for the J dependence of the linewidths.
Complexes of oxygen with benzene and hexafluorobenzene
Authors: Grover J. Robb , Hagenow Gerd , Walters Edward A.
Published: July 2002

The complexes of C6H6⋅O2, (C6H6)2O2, and C6F6⋅O2 were studied by photoionization using synchrotron radiation. Dissociation energies were measured to be D0(C6H6⋅O2)=1.6±0.3 kcal mol−1, D0[(C6H6⋅O2)+]=3.4±0.3 kcal mol−1, D0(C6F6⋅O2)=2.1±0.4 kcal mol−1, and D0[(C6F6⋅O2)+]=3.2±0.4 kcal mol−1. We calculate from the above that D298(C6H6⋅O2)=0.4±0.4 kcal mol−1, verifying that the benzene–oxygen interaction is only a ‘‘contact’’ at room temperature. The dissociation energies of the heterodimer ions are much smaller than those of the homodimer ions of their constituents [viz. 15, 11, and 7 kcal mol−1 for (C6H6)+2, (O2)+2, and (C6F6)+2. ] Sharp onsets were observed for C6H6⋅O2→(C6H6⋅O2)+ and C6F6⋅O2→(C6F6⋅O2)+, at 9.172±0.004 and 9.856±0.003 eV, respectively, measurements made possible by autoionization in the threshold region. Surprisingly, the heterodimer ion (C6H6⋅O2)+ is formed essentially entirely from neutral C6H6⋅O2, with no contribution from fragmentation of larger clusters. Production of C6H6O+ and C6F6O+ from C6H6⋅O2 and C6F6⋅O2 could not be detected, but is easily observed from mixed trimers. The first onsets occur at 14.10±0.05 and 14.10±0.09 eV, respectively, and are markedly higher than the thresholds. A second onset for C6F6O+ occurs at 14.7–15.0 eV.

In addition, dips are observed near 590 Å in the yield spectra for both C6H6O+ and C6F6O+, where the well-known window resonances of the oxygen (c 4Σ−u)3sσg Rydberg states occur. Thus there is evidence for the participation of two different mechanisms. The former data are consistent with a mechanism in which the organic moiety is first photoionized to produce an excited ion that then dissociates the oxygen, where one of the oxygen atoms is captured by the ion. The latter data support a mechanism in which the O2 moiety is the chromophore, where O+, formed by predissociation of [O+2]* from autoionization of a Rydberg state, is captured by a benzene molecule. In either case, the product is born excited, and to be observed must be stabilized by excitation and ejection of the third component of the original trimer.
Hilbert space multireference coupled-cluster methods. II. A model study on H8
Authors: Kucharski S. A. , Balková A. , Szalay P. G. , Bartlett Rodney J.
Published: July 2002

The performance of various coupled-cluster (CC) approaches using both single and multideterminantal references is investigated for the (quasi-)degenerate states of molecular systems, where inclusion of higher excitations (or equivalently nondynamic correlation) proves to be needed. The prototype system H8 represents an adequate model for our study, where we can vary the degree of degeneracy from a completely degenerate situation to a nondegenerate one in a continuous way. To obtain a reliable benchmark for our CC results, the full configuration interaction (FCI) and large-scale complete active space configuration interaction (CAS CI) calculations, respectively, are performed for a variety of geometries and states. The convergence of the approximate single reference CC approaches is found to be extremely sensitive to the level of degeneracies involved. In the nondegenerate case the standard CC method with single and double excitations is found to be quite satisfactory; in the (quasi-)degenerate situations, however, the inclusion of triple excitations and noniterative quadruple excitations is needed to furnish semiquantitative values of correlation energies. The alternative treatment of nondynamic correlation using a multideterminantal Hilbert space coupled-cluster (MRCC) method demonstrates the power of this approach, which provides a balanced description of both dynamic and nondynamic correlation in the degenerate region for all the investigated states of H8. Its convergence for nondegenerate situations, however, is less satisfactory, being affected by an intruder state problem.
The reactions of positive and negative halogen ions with Cl2 and Br2
Authors: Španěl P. , Tichý M. , Smith D.
Published: July 2002

A selected ion flow tube study has been carried out at 300 K of the reactions of some atomic and molecular positive and negative halogen ions with Cl2 and Br2 from which the rate coefficients k and ion product distributions have been determined. For the energetic F+ ion reactions, dissociative charge transfer is the dominant process, while for the Cl+ ions, only nondissociative charge transfer occurs. For the less energetic Br+ and I+ reactions, dihalogen molecular ions are important products. All these positive ion reactions proceed quite efficiently, i.e., the k are appreciable fractions of kc, their respective collisional rate coefficients, except for the reactions of Cl2 with the lower energy ions of the spin–orbit triplet of I+, i.e., I+(3P1,0), for which k∼0.07kc, this being due to the endothermicities of the reactions. The molecular ion Cl2+ undergoes rapid nondissociative charge transfer with Br2, a process which is, of course, endothermic for the reaction of Br2+ with Cl2 and so no reaction is observed. The less-energetic atomic negative ion reactions proceed—via atom exchange—in which the atomic negative ion of the reactant molecular species and a dihalogen molecule are produced. For those reactions that are exothermic, the k are, within error, equal to (2/3)kc, implying that they proceed via complexes which separate statistically back to reactants (1/3) and forward to products (2/3). Both the Br−+Cl2 and Cl−+Br2 reactions are somewhat less efficient (i.e., k<2/3kc), a result of the slight endothermicities of the reactions. Of the molecular negative ion reactions, electron transfer is the major process in the Cl2− reaction with Br2, whereas the reaction of Br2− with Cl2 proceeds relatively slowly producing the triatomic ion BrCl2−.
Reversal of the isotopic effect in the surface behavior of binary polymer blends
Authors: Hariharan Arvind , Kumar Sanat K. , Russell Thomas P.
Published: July 2002

We report neutron reflectivity measurements on the surface behavior of isotopic polystyrene blends of symmetric and disparate molecular weights near an air surface. For the symmetric blends we find, in agreement with past findings, that the segments of the deuterated polymer always partition to the air surface. These results, which are driven purely by energetic effects, can then be modeled in the framework of a mean-field lattice theory with a constant surface energy difference parameter. In contrast, for the asymmetric blends we find that the segments of either polymer can partition to the surface, and the controlling variable is the disparity in the molecular weights of the two components. These new results, which cannot be predicted with the constant density lattice models utilized for symmetric blends, can be modeled if one balances the energetic preference of placing the deuterated segments near the surface with entropic effects, which are caused by the presence of a density gradient at the air surface, preferring the surface segregation of the short chains. These findings emphasize the need for the inclusion of free-volume effects when modeling the segregation to a free surface, and we show that a recent mean-field compressible lattice model does capture these effects adequately.
Renormalization group theory for fluids
Authors: White John A. , Zhang Sheng
Published: July 2002

Some extensions of renormalization group methods to fluids are discussed that may facilitate the development of a general renormalization group theory for real fluids that is capable of predicting their thermodynamic properties globally, including both at the critical point and away from the critical point, from a specification solely of the microscopic interactions among the constituent molecules. The extensions include application to virial series and free energies for freely moving molecules (as contrasted with Hamiltonian methods used for fixed lattices of molecules); inclusion of contributions from fluctuations of very short wavelengths, comparable to the range of the attractive forces; and evaluation of the scale factor for fluctuation amplitudes. An approximate theory incorporating these new features is formulated and illustrated in a simple application to the thermal behavior of n-pentane in a large extended neighborhood of its critical point.
Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the Ã–X̃ emission spectrum
Authors: Bowman Joel M. , Gazdy Bela , Bentley Joseph A. , Lee Timothy J. , Dateo Christopher E.
Published: July 2002

We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled-cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero-point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH-overtone states up to 23 063 cm−1. We also report a simulation of the Ã–X̃ stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even-bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited Ã state. The potential for the Ã state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the Ã state. In addition, the Ã-state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis-switching arguments of a number of previously unassigned states in the SEP spectrum.
Potential energy surface and large amplitude motions of the water–carbon dioxide complex
Authors: Makarewicz J. , Ha Tae-Kyu , Bauder A.
Published: July 2002

The results of ab initio investigations of the equilibrium structure and the potential energy surface of water–carbon dioxide are presented. It has been found that the complex has two equivalent, slightly nonplanar equilibrium configurations. A hydrogen-bonded structure has not been found. Large amplitude van der Waals motions of the complex have been studied and the vibrational and rotational energy levels have been calculated. The results show that the complex behaves like a quasiplanar molecule, in agreement with the interpretation of experimental data.
Molecular photoionization cross sections by the Lobatto technique. I. Valence photoionization
Authors: Wilhelmy Immanuel , Ackermann Lutz , Görling Andreas , Rösch Notker
Published: July 2002

A method for the calculation of electronic continuum wave functions is presented which is based on the logarithmic derivative version of the Kohn (LDK) variational principle. The variational principle is cast into algebraic form by introducing a finite basis set that consists of spherical harmonic Gaussian-type functions (GTOs) and of Lobatto shape functions with the latter representing the translational part of the basis. A local effective potential which is obtained from density functional theory results in fairly accurate photoionization cross sections. Also studied are asymptotic corrections to the effective potential for the photoelectron which, in many cases, lead to improved results. The Lobatto procedure is applied to the diatomics N2 and CO and to benzene which may be regarded as a prototype for larger non spherical symmetric systems for which the method is targeted. For the two diatomics, results in excellent agreement with experiment have been found. For benzene the results are compared to those obtained by the Stieltjes–Tchebychev (ST) imaging technique and by the continuum multiple scattering (CMS) method which both have been applied to similar effective local potentials. Comparison with the ST imaging technique shows that the LDK Lobatto (LDKL) method provides qualitatively similar results, but the LDKL cross sections are of higher resolution and allow a more detailed analysis because of the explicit determination of the continuum wave function. For most of the valence orbitals of benzene the CMS method does not lead to satisfactory agreement with experiment due to the well-known deficiencies of this technique. The LDKL method implemented with a combined basis set does not suffer from the limitations of the ST and the CMS methods, but remains applicable to larger-size molecules.
Temperature-dependent vibrational relaxation in polyatomic liquids: Picosecond infrared pump–probe experiments
Authors: Tokmakoff A. , Sauter B. , Fayer M. D.
Published: July 2002

Vibrational lifetimes of metal carbonyl solutes in two liquids were studied as a function of temperature from the melting points to the boiling points. Picosecond infrared pump–probe experiments were performed at the absorption maxima of the T1u CO stretching vibration (∼1980 cm−1) of Cr(CO)6 and W(CO)6 dissolved in carbon tetrachloride (CCl4) and chloroform (CHCl3). The temperature dependencies of the lifetimes in CCl4 are dramatically different from those in CHCl3. A decrease in the vibrational lifetime with temperature is seen for both metal carbonyls in CCl4. However, the vibrational lifetime of W(CO)6 in CHCl3 becomes longer as the temperature is increased, while the lifetime of Cr(CO)6 decreases only slightly. To understand the vibrational dynamics in these systems it is necessary to consider the temperature dependencies of the thermal populations of low frequency phonons (instantaneous normal modes) of the liquids, the phonon density of states, and the anharmonic coupling matrix elements.
Thermochemistry of gaseous manganese oxides and hydroxides
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

The reactive vaporization of MnO(c) in the presence of O2 and H2O/D2O has been studied by effusion-beam mass spectrometry, and the new species MnO2, MnOH, Mn(OH)2, and MnO(OH) have been identified. The identities of the hydroxides were confirmed from deuterium isotopic substitution. Reaction enthalpies and standard enthalpies of formation have been derived from equilibrium measurements, yielding the ΔfH°298 values for the gaseous species MnO2, −17.6; MnOH, 0.1; Mn(OH)2, −95.9; and MnO(OH), −41.1, all in kcal mol−1. For use in the analysis, thermal functions were calculated from known or estimated molecular constants. Bond energies derived from the results fall in a logical and consistent sequence.
Thermodynamics of fluids in quenched disordered matrices
Authors: Rosinberg M. L. , Tarjus G. , Stell G.
Published: July 2002

Using the replica method, we derive the thermodynamic relations for a fluid in equilibrium with a quenched porous matrix. In particular, the appropriate Gibbs–Duhem equation is obtained as well as the equivalence between grand canonical and canonical ensembles. The exact compressiblity and virial equations are derived. Whereas the compressibility equation remains a direct and practical way to obtain the adsorption isotherm, the virial equation involves terms which do not relate easily to the properties of the fluid/matrix system. This explains the inconsistency between previous theoretical predictions and computer simulation results.
Understanding colloidal charge renormalization from surface chemistry: Experiment and theory
Authors: Gisler T. , Schulz S. F. , Borkovec M. , Sticher H. , Schurtenberger P. , D’Aguanno B. , Klein R.
Published: July 2002

In this paper we report on the charging behavior of latex particles in aqueous suspensions. We use static light scattering and acid–base titrations as complementary techniques to observe both effective and bare particle charges. Acid–base titrations at various ionic strengths provide the pH dependent charging curves. The surface chemical parameters (dissociation constant of the acidic carboxylic groups, total density of ionizable sites and Stern capacitance) are determined from fits of a Stern layer model to the titration data. We find strong evidence that the dissociation of protons is the only specific adsorption process. Effective particle charges are determined by fits of integral equation calculations of the polydisperse static structure factor to the static light scattering data. A generalization of the Poisson–Boltzmann cell model including the dissociation of the acidic surface groups and the autodissociation of water is used to predict effective particle charges from the surface chemical parameters determined by the titration experiments. We find that the light scattering data are best described by a model where a small fraction of the ionizable surface sites are sulfate groups which are completely dissociated at moderate pH. These effective charges are comparable to the predictions by a basic cell model where charge regulation is absent.
Translational diffusion of transient radicals created by the photoinduced hydrogen abstraction reaction in solution: Anomalous size dependence in the radical diffusion
Authors: Terazima Masahide , Okamoto Koichi , Hirota Noboru
Published: July 2002

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2-propanol) are investigated by using the transient grating (TG) method. In all the reaction systems, D’s of the transient radicals, as well as those of the parent molecules, can be measured simultaneously. The results clearly show slower diffusive motions of the radicals, at least of the π radicals in the hydrogen abstraction reaction systems, compared with those of the parent molecules. D’s of the parent molecules usually agree well with the calculated values based on the Spernol and Wirtz modification of the Stokes–Einstein (SE) relation. Although the measured D’s of the radicals are closer to the values predicted by the simple SE equation, the agreements and the ratio of D between the radicals and its parent molecules depend on the molecular size. The ratio becomes closer to unity as the molecular size becomes large. Possible origins of this dependence are discussed.
Exact full-dimensional bound state calculations for (HF)2, (DF)2, and HFDF
Authors: Zhang Dong H. , Wu Qian , Zhang John Z. H. , von Dirke Michael , Bačić Zlatko
Published: July 2002

Detailed results of the converged full-dimensional 6D quantum calculations of the vibrational levels of (HF)2, (DF)2, and HFDF, for total angular momentum J=0, are presented. The ab initio 6D potential energy surface by Quack and Suhm was employed. This study provides a comprehensive description of the bound state properties of the HF dimer and its isotopomers, including their dissociation energies, frequencies of the intermolecular vibrations, tunneling splittings, and extent of wave function delocalization. Quantum number assignment of the calculated eigenstates by plotting different cuts through the wave functions worked rather well for (HF)2, but proved to be much harder for (DF)2 and HFDF, indicating stronger vibrational mode mixing in these species. The ground-state tunneling splitting for the HF dimer from our exact 6D calculations, 0.44 cm−1, is very close to that from a previous 4D rigid-rotor calculation, 0.48 cm−1 [J. Chem. Phys. 99, 6624 (1993)]. This is in disagreement with the result of a recent 6D bound state calculation for (HF)2 by Necoechea and Truhlar, which gave a ground-state tunneling splitting a factor of 3.7 times larger than the 4D result.