bluuebunny/crossref_metadata_2025 huggingface dataset:
Statistical mechanics of solvent induced forces and vibrational frequency shifts. Low density expansions and Monte Carlo simulations
Authors: de Souza Luís E. S. , Guerin Claudia B. E. , Ben-Amotz Dor , Szleifer Igal
Published: July 2002

Theoretical expressions are presented for the solvent configuration averaged force on a diatomic solute throughout the vapor–liquid density range. Analytical low density expansions and solvent configurational space averages are used to predict solvent induced changes in solute vibrational frequency. Purely classical Monte Carlo simulation results for a system representing bromine (Br2) dissolved in argon agree quantitatively with previous coupled quantum-classical results of Herman and Berne, up to liquid densities. It is found to be impossible to obtain a red gas to liquid shift (such as that typically observed experimentally) in any realistic diatomic system with only binary solvent atom–solute atom interaction potentials. However, redshifts are predicted when a three-atom potential, in which the solute–solvent interaction depends on solute bond length, is introduced.
Spectroscopic probing of aerosol particle interfaces
Authors: Zhang Jian-Xiang , Aker Pamela M.
Published: July 2002

A novel nonlinear Raman spectroscopic technique, which can be used to monitor the chemical composition of an aerosol particle interface, is described. The technique is called morphology-dependent stimulated Raman scattering (MDSRS). Experimental results show that there is a quantifiable relation between MDSRS signal size and the concentration of species being probed. We outline the analytic form of the nonlinear gain equation and prove its reliability by modeling MDSRS signal strengths.
On the characteristics of the orientational motion of liquid chloroform: A mid- and low-frequency Raman study
Authors: Rothschild Walter G. , Cavagnat Raymond M. , Maraval Philippe
Published: July 2002

Our paper presents a fit of an orientational equilibrium renewal process in terms of a sequence of random events of free rotational motion and libratory oscillations [Lindenberg and Cukier, J. Chem. Phys. 62, 3271 (1975)] to new pure-rotational spectra and correlation data of the ν3 mode (367 cm−1) of liquid chloroform–Cl-35 at 300 and 213 K. The model fits are good; they account fully for the frequently observed (but rarely well-simulated) partial regain of the memory of the orientational coherence at intermediate time domains. Further, the physical-statistical picture of the nature of the orientational motion turns out to be that of a librator in a potential-well of a strongly temperature-dependent depth; stages of free rotation are significant but one order of magnitude shorter than the oscillatory regimes, even at room temperature and at ambient pressure. This contrasts notions of rotational motion hitherto held for such systems. Second, we explored the rotational correlation data of the C–H stretching mode ν1, which exhibits much faster vibrational relaxation than mode ν3, to ascertain empirically effects of significant vibration-rotation coupling in ν1. We find that this effect is significant for the longer time domains only. Finally, we report low-frequency depolarized Raman spectra (5–200 cm−1) at 212, 243, 273, and 295 K. They yield a temperature dependence of the peak-frequencies of the reduced Raman intensity (density-of-vibrational-states) which can only be reconciled with a predominance of librational motion within the liquid cage.
The ν1 vibrational predissociation lifetime of (HCN)2 determined from upperstate microwave-infrared double-resonance measurements
Authors: Kerstel E. R. Th. , Lehmann K. K. , Gambogi J. E. , Yang X. , Scoles G.
Published: July 2002

We have successfully applied upper state microwave-infrared double-resonance spectroscopy, with optothermal molecular beam detection, to the determination of the (HCN)2 vibrational predissociation lifetime upon excitation of the outside CH stretching mode (ν1). The measured lifetime is 1.7(5) μs. At this time this is the longest lifetime for a hydrogen bonded complex determined with such precision. We have also been able to accurately measure several upper state transition frequencies, therewith revealing the presence of a perturbation in the spectrum that shifts at least one of the rotational levels by an amount on the order of 1 MHz, and appears to decrease the lifetime of the level measurably. A rate equation approach predicts that this experiment will have a negligible signal to noise ratio for upper state lifetimes much shorter than the infrared (IR) laser–molecular beam interaction time, such as is the case here. In order to explain the presence of a double resonance signal, we have proposed a new mechanism that is based upon a coherent interaction model. The technique of microwave–IR double resonance should be widely applicable to the measurement of dissociation lifetimes in the presently difficult-to-access range from about 100 ns to 100 μs. This is a range in which a significant number of complexes are known to have vibrational predissociation lifetimes.
Ab initio study of cyclobutadiene using the effective valence shell Hamiltonian method
Authors: Martin Charles. H. , Graham R. L. , Freed Karl. F.
Published: July 2002

The effective valence shell Hamiltonian (Hν) method is used to examine the electronic structure of cyclobutadiene. These computations are designed to help guide future experimental studies on this elusive compound, as well as aid in understanding the general features of effective Hamiltonian calculations. Calculations are performed with two qualitatively different valence spaces. The first valence space mimics the valence spaces used in semiempirical methods in that only four π valence-like molecular orbitals are used. The second valence space includes four additional diffuse π molecular orbitals. A [4s5p1d/2s1p] Cartesian Gaussian basis set is used (116 total functions). Our results agree with available spectroscopic vertical ionization potentials to within 0.1 and 0.3 eV for the b2g and b1u orbital ionizations. The calculated vertical excitation energies and ground state automerization barrier height agree well with previous ab initio calculations. For the first time, however, the lowest lying optical transition from the X 1Ag ground state is predicted to occur at 5.99 eV to a Rydberg 1B3u state.
Dynamic multipole polarizabilities and long range interaction coefficients for the systems H, Li, Na, K, He, H−, H2, Li2, Na2, and K2
Authors: Spelsberg Dirk , Lorenz Thomas , Meyer Wilfried
Published: July 2002

Different approaches to the calculation of dynamic polarizabilities are briefly discussed and compared. Using a perturbational approach dynamic multipole polarizabilities are calculated from full valence configuration interaction wave functions. The polarizabilities are expressed in terms of reduced spectra which, in turn, are used to compute the dispersion and induction coefficients for all combinations of the systems listed, including anisotropic contributions.
Nonlinear dynamics of large-amplitude molecular excitation by shaped optical pulse sequences
Authors: Cahn Brian D. , Martens Craig C.
Published: July 2002

The effects of nonlinear dynamical phenomena on strategies for creating large amplitude excitations in molecular systems are studied. A simple area-preserving mapping modeling a Morse oscillator coupled to a second intramolecular degree of freedom is considered. The optical field is approximated by a finite sequence of impulsive interactions with arbitrary amplitudes and time increments. The goal of exciting the system to 75% of its dissociation energy is established. For the uncoupled Morse oscillator, an analytic solution for the optimal spacing between pulses of minimal equal intensity is given. When intramolecular coupling is present, the existence of a nonlinear resonance zone is shown to strongly interfere with the efficacy of this strategy. Dramatic improvements in efficiency can be obtained by employing pulse sequences that explicitly take into account the presence of strong perturbations of the zeroth order phase portrait.
Translational diffusion in a smectic-A phase by electron spin resonance imaging: The free-volume model
Authors: Moscicki Jozef K. , Shin Yeon-K. , Freed Jack H.
Published: July 2002

The method of dynamic imaging of diffusion (DID)-ESR (electron spin resonance) has been utilized to study the anisotropy of translational diffusion of spin probes in the smectic A phase of a eutectic liquid crystal, S2. In particular, the nearly spherical perdeuterated-TEMPONE (PDT) and the rigid and elongated cholestane spin label (CSL) molecules were studied. Whereas D⊥ (the coefficient of diffusion perpendicular to the nematic director) showed simple Arrhenius dependence for both probes, diffusion parallel to the director displayed two different temperature regimes with a changeover of D∥ at t*≊26–27 °C. The regime above (below) t* is characterized by weak (strong) translational ordering. For CSL the ratio D⊥/D∥<1 above t* which indicates nematiclike behavior, but below t* the behavior is more smecticlike, i.e., D⊥/D∥≳1; for PDT D⊥/D∥≳1 over the whole temperature range. A free volume model is developed to interpret the activation energies associated with D⊥ and D∥ (i.e., E⊥ and E∥) in terms of the orientational and translational order parameters for the smectic phase and those for the spin probes. Also included are the variation of the compressibility across the smectic layer and the length of the probe relative to that of the thickness of the smectic layer. The fact that above t* E∥/E⊥ is unity for CSL but a little greater that unity for PDT is interpreted as due to the weaker coupling of the larger CSL molecule to the weak translational ordering and compressibility variation. Below t*, E∥/E⊥ becomes 1.52 and 1.80 for CSL and PDT, respectively, which may be interpreted in terms of enhancement of these smectic features. The free volume model may be used to analyze E∥ and E⊥ for self-diffusion and for a wide range of spin probes, including such very small probes like methane, as a function of the key parameters.
Photodissociation and photoionization and their branching ratio in bisbenzene chromium
Authors: Penner A. , Amirav A. , Tasaki S. , Bersohn R.
Published: July 2002

Bisbenzene chromium (BBC) with an unusually low ionization potential, 5.45 eV is a model molecule to study phenomena near the ionization potential. A beam of BBC molecules cooled by a supersonic expansion was used to measure the absorption spectrum and the excitation spectra for ionization and Cr atom production. The uv spectrum is mainly a Rydberg series terminating at the ionization potential together with progressions in the symmetric benzene–Cr stretch. In the range from 320 to 282 nm, the region of a non-Rydberg absorption band, the excitation spectrum for Cr atom production is quite different from the absorption spectrum. In contrast, in the range 282–227 nm the excitation spectrum for Cr atom production is practically identical with the absorption spectrum. This identity suggests a close to 100% yield of Cr atoms. Near the ionization energy of BBC the yield of Cr atoms decreases while the ionization yield increases until a quite steady branching ratio of three Cr+ ions to one Cr atom is reached. The rates of dissociation of excited BBC into Cr atoms are 0.70±0.02×106 s−1 and 4.2±0.1×106 s−1 at 279.8 and 248 nm, respectively.
Zero-kinetic-energy photoelectron spectroscopy from the Ã 1A
u state of acetylene: Renner–Teller interactions in the trans-bending vibration of C2H+2 X̃ 2Πu
Authors: Pratt S. T. , Dehmer P. M. , Dehmer J. L.
Published: July 2002

Double-resonance excitation via the Ã 1Au state is used to record zero-kinetic-energy photoelectron spectra of acetylene. The analysis of these spectra leads to an improved value of 91 952±2 cm−1 for the adiabatic ionization potential to the C2H2+ X̃ 2Πu ionic ground state. Because the Ã 1Au intermediate state has a trans-bent geometry, transitions from it readily populate the trans-bending vibration of the ground state ion, leading to new information about this mode and its Renner–Teller interactions. The relative intensities of the Renner–Teller components and of the rotational structure within each component also provide information on the dynamics of the photoionization process.
Lifetimes of very high Rydberg states of aromatic molecules
Authors: Chupka William A.
Published: July 2002

Recent experimental results reporting lifetimes of very high Rydberg states of phenanthrene and deuterated phenanthrene together with a theoretical analysis are examined critically. Weaknesses in the theoretical model are pointed out, in particular the proposal that lifetimes of isolated field-free molecules vary as n6 up to values of n≂100 above which very rapid decay occurs by autoionization. Another model, in which the effects of external fields and collisions result in lifetimes which vary in an ideal statistical limit as n5 at lower values of n and rapid destruction at higher values of n, is shown to explain the experimental results equally well. To the extent that such a statistical limit is not attained, nonexponential decay is expected. Decay processes of high Rydberg states of large polyatomic molecules are discussed. Especially in the case of a fused polynuclear aromatic, the isolated molecule with lower values of n is proposed to decay by an internal conversion mechanism in which the rate-determining initial step involves a transfer of energy from the Rydberg electron to the ion core amounting to one or a very few vibrational quanta. The experimentally observed rapid decrease of lifetimes for the highest values of n is attributed to destruction, primarily by ionization, due to such external effects as collisions and fields. A continuity principle requires that such collisional ionization as well as autoionization be associated with excitation of a range of values of n (and therefore of lifetimes for isolated molecules) approaching infinity.
Facile abstraction of chemisorbed D on Si(100) by atomic H
Authors: Koleske D. D. , Gates S. M. , Schultz J. A.
Published: July 2002

The abstraction of chemisorbed deuterium (D) on Si(100) by atomic hydrogen (Hat) is studied in real time. The surface H and D coverages are measured by mass analyzing the recoiled H+ and D+ ion signals during the abstraction reaction. We find that Hat efficiently removes adsorbed D on Si(100) with a low activation energy of 0.8±0.6 kcal/mol and a reaction probability that is 0.36 times the Hat adsorption rate on clean Si(100).
The correction vector approach to linear and nonlinear optical properties of conjugated systems
Authors: Albert I. D. L. , Morley J. O. , Pugh D.
Published: July 2002

The sum-over-states (SOS), time dependent perturbation theory expansion over molecular eigenstates method for the computation of molecular hyperpolarizabilities is briefly reviewed. A correction vector method for the computation of linear and nonlinear optical properties of π-conjugated systems has been devised for use with a singly and doubly excited configuration interaction model (SDCI). The SOS approach and the correction vector method are equivalent when the same basis set of many-electron configurations is used in both. It has been verified that the values obtained from the correction vector method and by direct summation over eigenstates are identical. The correction vector method has been found to be very efficient for larger systems, both in terms of central processing unit (CPU) time and storage requirements. The direct SOS summation has been used to study the approach to the final SDCI values as excited states are added in order of increasing energy. Using the methods described in the previous paragraph, a study of the polarizabilities and hyperpolarizabilities for frequency doubling and tripling in unsubstituted polyenes and in amino-, nitro-, donor/acceptor substituted polyenes, and 4-nitroaniline (pNA) has been made. The basis set has been restricted to singly and doubly excited configurations between π orbitals (πSDCI) which have been extracted from the ground state obtained from a complete neglect of differential overlap (CNDO) calculation. The results are found to be in good agreement with other values reported in literature. They also show that the full πSDCI hyperpolarizabilities of the polyenes are largely accounted for by the contributions of a small number of low-lying excited states, tending to justify the use of simplified models such as the two and three state models in these systems.
Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy of liquid benzene: A Kubo relaxation function analysis
Authors: Joo Taiha , Albrecht A. C.
Published: July 2002

Femtosecond time-resolved coherent anti-Stokes Raman scattering (TRCARS) for the ν1 ring-breathing mode of neat benzene-h6 and benzene-d6 are reported at room temperature (25 °C) and at 6.5 °C. Ultrafast time resolution permits close investigation of the dephasing dynamics at early times. Deviation from the conventional exponential decay is found. The data are analyzed using Kubo line shape theory. The correlation time τc and the rms magnitude Δ of the Bohr frequency modulation are determined for the process responsible for the vibrational dephasing and for the linewidth. Spinning motion is suggested as the stochastic process responsible for the vibrational dephasing in those neat liquids.
I−2 photodissociation and recombination dynamics in size-selected I−2(CO2)n cluster ions
Authors: Papanikolas John M. , Vorsa Vasil , Nadal María E. , Campagnola Paul J. , Buchenau Heinrich K. , Lineberger W. C.
Published: July 2002

Pump–probe techniques are used in conjunction with a tandem time-of-flight mass spectrometer to investigate the I...I− cage recombination dynamics following I−2 photodissociation in size-selected I−2(CO2)n cluster ions. The absorption recovery, which reflects the recombination and vibrational relaxation of the photodissociated I−2, exhibits a strong cluster size dependence in the range of n=13–15. Over this limited cluster size range, the absorption recovery time decreases from ∼40 ps (n≤12) to ∼10 ps (n≥15). In addition, a recurrence is observed at ≊2 ps in the absorption recovery of the larger clusters (n=14–17). This feature results from coherent I...I− motion following photodissociation. Measurement of the absorption recovery with both parallel and perpendicular pump–probe polarizations demonstrates that the pump and probe transition dipoles lie in the same direction. Analysis of the I−2 transition dipole directions shows that the coherent motion takes place on the first two repulsive excited potential surfaces. The two-photon photofragment distribution reflects the solvent cage structure as a function of pump–probe delay time.
Ab initio study of the intermolecular potential of the water–carbon monoxide complex
Authors: Sadlej J. , Buch V.
Published: July 2002

The combination of the supermolecular Mo/ller–Plesset scheme with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface (PES) of the H2O...CO complex. We located three low-energy configurations on the potential energy surface corresponding to two isomeric H-bonded complexes OC...HOH (C structure), CO...HOH (O structure), and a T-shaped structure with CO bonded to the O atom of H2O. The absolute minimum corresponds to the C configuration OC...HOH, involving a nonlinear C...H–O bond. The tilt from the linearity is 11 deg, in agreement with the value derived from the experimental data. The computed binding energies on the fourth-order perturbation theory level are 651 cm−1 for the C configuration, 301 cm−1 for T, and 256 cm−1 for O. The anisotropy of the potential energy surface is analyzed using the perturbation theory. The absolute minimum results from the attractive electrostatic contribution and dispersion energy, which overcome considerable exchange repulsion. A small tilt of 11 deg from the linear H bond is due to the balance of the electrostatic and exchange repulsion terms; the repulsive Heitler–London term is minimal when the angle between the C2V axis of the water molecule and the intermolecular axis is equal to 63.0 deg. The bonding in the T configuration is due largely to the dispersion energy which overcomes strong exchange repulsion. The third O configuration is more stable on the SCF level than on the MP2 level, because of the reversal of the sign of the dipole moment of the CO molecule. The tunneling motion of the water molecule around its c inertial axis was studied and the barrier to exchange of the bound and the free hydrogen atom was determined as 280 cm−1 (1289.470 μhartree).
How large is the effect of 1s correlation on the D
e, ωe, and r
e of N2?
Authors: Bauschlicher Charles W. , Partridge Harry
Published: July 2002

The effect of N 1s correlation on the spectroscopic constants of N2 is studied using the coupled-cluster singles and doubles approach with a perturbational estimate of the connected triples [CCSD(T)] and internally contracted multireference configuration interaction (ICMRCI) techniques. At the ICMRCI level, we obtain a 1s effect of +1.35 kcal/mol on the dissociation energy. However, the effect is found to be smaller when size-extensive methods are used. The 1s effects computed at the CCSD(T) and internally contracted averaged coupled-pair-functional (ICACPF) levels are in excellent agreement. Our best estimate for the effect of 1s correlation is +0.8 kcal/mol on De, +9 cm−1 on ωe, and −0.002 Å on the bond length. Including our estimate for the effect of 1s correlation, we obtain a De of 227.1 using a correlation-consistent polarized-valence sextuple zeta basis set at the ICMRCI+Q level (where the +Q indicates that the Davidson correction has been included). Basis set incompleteness, which is estimated to be 0.7±0.2 kcal/mol, is still the major source of error. The CCSD(T) ten-electron results are found to be in excellent agreement with those obtained at the ICACPF or ICMRCI+Q levels of theory.
Limits of validity for mean field description of compressible binary polymer blends
Authors: Lifschitz Masha , Dudowicz Jacek , Freed Karl F.
Published: July 2002

We examine phase diagrams of model binary polymer blends and estimate the temperature range where mean field theory is applicable. The computations use both compressible and incompressible Flory–Huggins theory and the corresponding random phase approximations. The size of the nonclassical regime is estimated in terms of the Ginzburg number Gi for both the one and two phase regions. Model calculations indicate that compressibility (and thus pressure) can significantly affect widths of the nonclassical region and critical temperatures (Tcrit) for phase separations. The qualitative scaling of Tcrit and Gi (and hence the widths of the nonclassical regime) also differs between compressible and incompressible blends. Incompressible symmetric blends with polymerization index N yield constant values for both the product NGi and the ratio Tcrit/N, while compressible polymer mixtures produce NGi as nonconstant and even possibly as varying nonmonotonically with chain length. Pressure and entropic interaction variations are shown to change miscibility limits and strongly to affect Ginzburg numbers for some blends. Model calculations provide insight into common experimental methods for determining the onset of the nonclassical domain.
Determination of the threefold internal rotation barrier in ArNH3
Authors: Grushow A. , Burns W. A. , Reeve S. W. , Dvorak M. A. , Leopold K. R.
Published: July 2002

The two Σ and four Π states of the weakly bound complex Ar–NH3 correlating to j=2, k=±1 ammonia have been observed by tunable far infrared difference frequency-microwave sideband spectroscopy. The results have been combined with published data to determine a new angular potential energy surface for the system. The barrier to threefold internal rotation of the NH3 about its C3 axis in the complex is estimated to be 25.606(24) cm−1 near the minimum energy (T-shaped) configuration. The potential also exhibits maxima at both symmetric top configurations, with energies approximately 53 and 31 cm−1, respectively above that of the global minimum. The location and splitting between the symmetric and antisymmetric Σ states are indicative of a strong interaction with another pair of unobserved states, most likely the first excited intermolecular stretch built on j=1, k=±1 Ar–NH3.
Three-photon resonance-enhanced ionization spectroscopy of CO(A 1Π−X 1Σ+). I. Influence of phase matching on the appearance and adverse pressure effect of the spectra
Authors: Xu Su , Sha Guohe , He Jinbao , Zhang Cunhao
Published: July 2002

Three-photon resonance-enhanced ionization spectra of pure CO via the A 1Π state have been obtained under a pressure range of 0.1–13 torr. It is found that the spectra can only be observed by using tightly focused laser beams, and its intensity manifests adverse pressure effect, i.e., the signal decreases with the increase in pressure for gas pressures above 1.0 torr, and almost disappears at 13 torr. Convincing experimental evidence is provided to show that the appearance of the spectra is not due to direct three-photon absorption of the fundamental laser beam, but to reabsorption of the laser-induced third harmonic generation (THG) in the CO medium. In other words, under tightly focused conditions, THG reabsorption is much stronger than direct three-photon absorption. Hence, contrary to the assertions given by previous investigators in Xe that the anomalous phenomena in three-photon excitation are caused by destructive interference, we have shown that phase matching, which greatly increases the THG, plays a major role in the three-photon resonance-enhanced ionization spectra of CO(A 1Π−X 1Σ+), and accounts for both the appearance of the spectra and the adverse pressure effect of the spectral intensity. In addition, we have given an analytical treatment of the adverse pressure effect in terms of the phase-matching condition.
Structural effects of high pressure gas on the rotator phases of normal alkanes
Authors: Sirota E. B. , Singer D. M. , King H. E.
Published: July 2002

The effects of high pressure gases (P≤400 bar) on the RII, RI, and RV rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x-ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters: layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases: helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.
Quantum mechanical calculations of collinear reactive collisions at energies above the dissociation threshold: A discrete-variable-representation approach
Authors: Sakimoto Kazuhiro , Onda Kunizo
Published: July 2002

A time-independent method which does not rely on a close-coupling approach has been developed for quantum mechanical reactive/dissociative collisions of collinear atom–diatom systems. The Schrödinger equation represented in hyperspherical coordinates is solved by means of a discrete variable representation. The results for a model H+HD system are compared with those obtained by the time-dependent wave-packet propagation calculation of Leforestier. The present method is powerful for the study of reactive collisions at energies above the dissociation threshold.
Ab initio potential energy surfaces for C2H→C2+H photodissociation
Authors: Duflot D. , Robbe J-M. , Flament J-P.
Published: July 2002

The C2H photodissociation processes have been studied using ab initio multireference perturbative configuration interaction methods. The calculated structural parameters of the two linear lowest lying states X 2Σ+ and A 2Π were found to be in good agreement with previous theoretical and experimental works. Construction of the correlation diagram indicates that C2H cannot photodissociate into C+CH. Therefore, two-dimensional potential energy surfaces of the six lowest lying A′ and A″ doublet states have been calculated as functions of the bond angle and the C–H bond distance, keeping the C–C distance at the equilibrium C2H value. It appears that the 1 2A′, 2 2A′, and 1 2A″ states correlating to the X 2Σ+ and A 2Π states in linear geometry remain bound at all angles, whereas the 3 2A′, 4 2A′, and 4 2A″ are dissociative. However, the structure of the energy surfaces is complicated by the presence of numerous avoided crossings between these states. Transition dipole moments connecting the excited states with the X 2Σ+ and A 2Π states have been also obtained in linear geometry. They were calculated to be very weak, leading to the conclusion that the photodissociation rate of C2H will be negligible below 7 eV. Higher lying states, including Rydberg states, were not considered here, since they cannot dissociate directly into C2+H.
Thermodynamics of a fluid confined to a slit pore with structured walls
Authors: Diestler D. J. , Schoen Martin , Curry Joan E. , Cushman John H.
Published: July 2002

In this article we extend our previous thermodynamic analysis of films confined to slit pores with smooth walls (i.e., plane–parallel solid surfaces without molecular structure) to the situation in which the walls themselves possess structure. Structured-wall models are frequently employed to interpret experiments performed with the surface forces apparatus (SFA), in which thin films (1–10 molecular diameters thick) are subjected to shear stress by moving the walls laterally over one another at constant temperature, chemical potential, and normal stress or load. The periodic structure of the walls is reflected in a periodic variation of the shear stress with the lateral alignment (i.e., shear strain) of the walls. We demonstrate by means of a solvable two-dimensional model that the molecular length scale imposed by the structure of the walls precludes the derivation of a simple mechanical expression for the grand potential analogous to that which holds in the smooth-wall case. This conclusion is borne out by the results of a grand-canonical Monte Carlo simulation of the three-dimensional prototypal model consisting of a Lennard-Jones (12,6) fluid confined between fcc (100) walls. Criteria for the thermodynamic stability of thin films confined by structured walls are derived and applied to the SFA.
Synchronization and spatiotemporal self-organization in the NO+CO reaction on Pt(100). I. Unsynchronized oscillations on the 1×1 substrate
Authors: Veser G. , Imbihl R.
Published: July 2002

The oscillatory NO+CO reaction on Pt(100) has been investigated in the 10−6 mbar range using photoemission electron microscopy (PEEM) as a spatially resolving method. The existence ranges for kinetic oscillations have been mapped out in (pCO,T)-parameter space with fixed pNO=4×10−6 mbar. Kinetic oscillations occur within a partial pressure range of 0.8<pNO/pCO<1.9. In the lower lying of two temperature windows for oscillatory reaction behavior, the oscillations proceed unsynchronized on a 1×1 substrate without exhibiting macroscopic rate variations. Instead, one observes spatiotemporal pattern formation which has been studied in detail. These patterns are dominated by periodic wave trains, which become unstable at lower temperatures, giving rise to spiral waves and irregularly shaped reaction fronts. With decreasing temperature, the front velocity increases, while simultaneously the spatial periodicity of the wave trains becomes larger. In agreement with theoretical predictions by a three-variable model, the local oscillations terminate at the upper T boundary via a Hopf bifurcation and at the lower T boundary via a bifurcation of the saddle-loop type.
Thermochemical properties of gaseous POBr and some H–P–O species
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

Effusion cell beams containing the gaseous species HPO, HPO2, HPO3, and POBr along with other products were generated by the reaction of H2O(g) or Br2(g) with Ca2P2O7(s). The species were identified by mass spectrometry, and were characterized thermochemically from the study of several reaction equilibria. Reaction enthalpies and standard enthalpies of formation were derived from third law analysis of the equilibrium data, yielding the ΔfH298° values HPO, −34.2; HPO2, −110.6; HPO3, −168.8; and POBr, −53.5, all in kcal mol−1. Our new result for POBr is in serious disagreement with a value reported in the literature; there are no previous reports on the other species studied here. A preliminary ΔfH298° value of −60.2 kcal mol−1 was also obtained for POCl(g), this time in reasonable accord with a literature value. The new thermochemical data yield calculated compositions in agreement with measured values for some lamp gas mixtures.
Rotationally resolved nonresonant two-photon ionization of SH
Authors: Hsu C.-W. , Baldwin D. P. , Liao C.-L. , Ng C. Y.
Published: July 2002

The threshold photoelectron (PE) spectrum for nascent SH formed in the ultraviolet photodissociation of H2S has been measured using the nonresonant two-photon pulsed field ionization (N2P-PFI) technique. The rotationally resolved N2P-PFI-PE spectrum of SH indicates that photoionization dynamics favor the rotational angular momentum change ΔN≤0 with the ΔN values up to −3, an observation similar to that found in the PFI-PE spectra of OH (OD) and NO. The ionization energy for SH(X 2Π3/2) is determined to be 84 057.5 ± 3 cm−1 (10.4219 ± 0.0004 eV). The spin–orbit splitting for SH(X 2Π3/2,1/2) is 377 ± 2 cm−1, in agreement with the literature value. This study illustrates that the PFI-PE detection method can be a sensitive probe for the nascent internal energy distribution of photoproducts.
Solvent dynamical effects on bond-breaking electron transfer reactions
Authors: Zhu Jianjun , Spirina O. B. , Cukier R. I.
Published: July 2002

The effects of solvent and intramolecular dynamics on the rates of bond-breaking electron transfer (BBET) reactions is investigated. In the model we adopt, suggested by Saveant [J. Am. Chem. Soc. 109, 6788 (1987)], electron transfer and bond breaking are considered to occur as a concerted process. Thermal equilibrium rate constants kie [i=1(2) denoting the forward (reverse) reaction] are derived and exhibit a characteristic Marcus form, with the reorganization energy equal to the sum of contributions from the solvent, intramolecular vibrational and bond-breaking coordinates. The effect of dynamics on the BBET rate constants is studied by using diffusion-reaction equations. We assume that the intramolecular vibrational coordinate is in equilibrium and the solvent and the bond-breaking coordinates can be out of equilibrium. The survival probabilities are derived analytically with the use of a decoupling approximation. The single exponential decay of the survival probabilities leads to nonthermal-equilibrium rate constants ki that interpolate between the thermal equilibrium kie and diffusion controlled kid rate constants (where motion along the nonequilibrium coordinates control the rate) according to k−1i=k−1ie+k−1id. The diffusion controlled rate constants kid depend on the relaxation times along both the bond-breaking and solvent coordinates. For large activation energies, the fast relaxation will dominate the rate, while for small activation energies, the slow relaxation time will dominate the rate. We also discuss the case of the dynamics along the bond-breaking coordinate being characterized by an energy diffusion process. The rate constant is evaluated for high activation barrier reactions and still has the form given above, with a suitably redefined relaxation time for energy diffusion.
Application of classical liquid state methods to the calculation of optical absorption bands in fluids
Authors: Winn M. D. , Kahl G.
Published: July 2002

Numerical results are presented for a classical model describing optical absorption in a fluid of nonpolar linearly polarizable molecules. The model corresponds to the microscopic Yvon–Kirkwood equations with frequency-dependent molecular polarizability. The dynamic response of the model system to an externally applied electric field is identical to that predicted by the much-studied quantum Drude oscillator model. A fast and reliable numerical method is described, based on that proposed by Gillan for the solution of the Ornstein–Zernike equation of classical liquid state theory, which allows more sophisticated results than those obtained to date. In particular, the evolution of the optical absorption band is studied for hard sphere and Lennard-Jones fluids, in which the molecular center-of-mass positions are described by realistic pair distribution functions. Both neat fluids and impurity systems are considered. A number of spectroscopic properties are calculated, including the renormalized dynamic polarizability and the dynamic dielectric constant.
Homo- and heterocoordination in nonadditive hard-sphere mixtures and a test of the van der Waals one-fluid model
Authors: Jung Jinkyung , Jhon Mushik , Ree Francis H.
Published: July 2002

An equimolar mixture of hard spheres with equal diameter d, having an unequal collision diameter d12 between unlike species, favors a hetercoordinated arrangement if d12 lies between 0.65d and d, and a homocoordinated packing below d12=0.65d. At d12<0.65d, hard spheres of one type move increasingly unaffected by the presence of hard spheres of the other type. This conclusion was drawn from Monte Carlo (MC) simulations over large ranges of density and d12 and originates in the fact that collisions between like-species at d prevent collisions of unlike-species at shorter distance d12<d. The observed shift of hetero- to homocoordination affects equation-of-state data as well. Additionally, a test of the van der Waals one-fluid model of mixtures by the Monte Carlo data shows that this model significantly overestimates the pressure at high density, and hence, is less suitable for nonadditive hard sphere mixtures. As expected, the MC data suggest a fluid phase separation at d12≳d.
C60 vibrates as a hollow sphere
Authors: Ceulemans A. , Fowler P. W. , Vos I.
Published: July 2002

A model for the vibrations of a cage molecule in terms of the motions of an infinitely thin spherical shell is applied to icosahedral C60. When appropriate values are given to the two free parameters, the model accounts for the general pattern, energy ordering, radial, and tangential characteristics of the vibrations, giving results comparable to those from a six-parameter conventional valence force field. In the model, the lowest frequency of any spherical-shell molecule is the ‘‘d’’ squashing deformation that describes tidal waves on a flooded planet.
Eigenstate resolved infrared/infrared double resonance spectroscopy of the 3ν1 overtone band of 1-propyne: Intramolecular vibrational energy redistribution into a Coriolis-coupled bath
Authors: Gambogi Joan E. , Kerstel Erik R. Th. , Lehmann Kevin K. , Scoles Giacinto
Published: July 2002

Sequential infrared/infrared double resonance excitation of an optothermally detected molecular beam has been used to obtain the eigenstate resolved spectrum of the second C–H stretch overtone in propyne near 9700 cm−1. The high resolution and sensitivity of this technique allows for extraction of detailed information about the dynamics of intramolecular vibrational energy redistribution from this highly fractionated spectrum. The analysis suggests a coupling mechanism consisting of anharmonic coupling out of the bright state through a doorway state or first tier, followed by subsequent coupling to a strongly Coriolis mixed bath. The lifetime of the bright state, which is determined by the first step and is not dependent on the total angular momentum, is measured to be about 320 ps for the K=0 clumps and about 210 ps for the K=1 clumps. The root mean square coupling matrix element determined for the J′=0 clump is 0.008 cm−1 and decreases with increasing J′. According to the level spacing and Heller’s F statistics, the spectrum shows evidence indicating that the underlying dynamic behavior is chaotic.
Calculations of magnetic properties. IV. Electron-correlated magnetizabilities and rotational g factors for nine small molecules
Authors: Cybulski Sl/awomir M. , Bishop David M.
Published: July 2002

Calculations of the magnetizabilities and the rotational g factors at the self-consistent-field (SCF) and second-order Mo/ller–Plesset perturbation theory (MP2) levels of theory are reported for H2, N2, F2, HF, CO, HCN, HNC, H2O, and NH3. The sums rules, that verify the reliability of the calculations, are shown to be well satisfied. The second-order correlation corrections to the magnetizabilities are found to be small, thus substantiating the generally observed good agreement between the experimental and SCF results. Vibrational corrections to the properties of the diatomic molecules are given. Very good agreement is found between the experimental and vibrationally corrected MP2 rotational g factors for the diatomic molecules.
Uncovering a minor C–C fission channel in bromoacetyl chloride
Authors: Kash P. W. , Waschewsky G. C. G. , Butler L. J.
Published: July 2002

Previous crossed laser-molecular beam experiments on bromoacetyl chloride investigated the influence of electronic nonadiabaticity on the branching between C–Cl and C–Br fission upon 1[n(O),π*(C=O)] excitation at 248 nm. This note identifies an additional contribution to the data from a minor primary C–C fission channel. Using the distribution of kinetic energies imparted to C–C fission products in bromoacetone following 1[n(O),π*(C=O)] excitation as a characteristic one, we reanalyze the Br+ and Cl+ time-of-flight spectra from bromoacetyl chloride at 248 nm to identify any contribution from C–C bond fission. We find that a small portion of the Br+ signal results from CH2Br fragments from C–C fission. Although this in no way alters the major conclusions from this previous work, it does slightly alter the P(ET) derived for C–Br fission and the C–Cl:C–Br branching ratio from bromoacetyl chloride.
Fine vibrational structure in core-to-bound spectra of polyatomic molecules
Authors: de Miranda M. P. , Beswick J. A. , Parent P. , Laffon C. , Tourillon G. , Cassuto A. , Nicolas G. , Gadea F. X.
Published: July 2002

Near-edge x-ray-absorption fine-structure (NEXAFS) spectra of 1-butene (C4H8), acrylonitrile (C2H3CN) and 1,3-butadiene (C4H6) multilayers were recorded with high resolution (better than 100 meV at the carbon edge) and a theoretical method to interpret the vibrational structure of the observed C1s→π* bands is presented. The method is based on local (quadratic) approximations for the potential energy surfaces of both ground and excited electronic states, with input data obtained by ab initio calculations of normal modes for the ground state and energy gradients for the excited state within the framework of the equivalent core model. The method proved to be useful in qualitative terms, providing information on the nature of the excited modes and on the geometrical changes following the electronic excitation. We have also calculated the C1s→π* spectrum of ethylene (C2H4) and compared it to those obtained for C4H8, C2H3CN, and C4H6. Since the latter can be considered as substituted ethylenes, the validity of the building-block scheme for the vibrational splittings in inner-shell absorption spectra could be assessed by this comparison.
Ab initio computation of semiempirical π-electron methods. II. Transferability of ℋν parameters between ethylene, trans-butadiene, and cyclobutadiene
Authors: Martin Charles H. , Freed Karl F.
Published: July 2002

A series of systematic approximations is applied within the correlated, size-extensive, ab initio effective valence shell Hamiltonian (ℋν) formalism to test such basic assumptions of π-electron theory as the zero-differential-overlap (ZDO) approximation and the transferability of semiempirical matrix elements (called parameters). The systematic approximations (tested in paper I) involve the use of π-electron valence orbitals that are constructed from a set of localized, transferable pπ functions and the neglect of three- and four-body ℋν parameters. Our approximate ab initio ℋν explicitly contains correlation contributions to all its parameters, and some fundamental differences exist between the ab initio ℋν and the standard semiempirical π-electron methods. Correlation contributions are shown to be essential in justifying the ZDO approximation, but some violations of this approximation appear. Transferability of ℋν parameters is often excellent between ethylene, trans-butadiene, and cyclobutadiene provided that the local geometries are made as similar as possible. The ℋν parameters with the largest correlation contributions display the greatest sensitivity to molecular environment, while orbital orthogonalization provides another mechanism inhibiting exact transferability.
Monte Carlo simulations of the structures and optical absorption spectra of Na atoms in Ar clusters, surfaces, and solids
Authors: Boatz Jerry A. , Fajardo Mario E.
Published: July 2002

Optical absorption spectra of Na/Ar systems are calculated by combining the classical Monte Carlo simulation method with a quantum mechanical first-order perturbation scheme [Balling and Wright, J. Chem. Phys. 79, 2941 (1983)] for estimating the energies of the Na* 3p(2P) excited states. The model incorporates many drastic approximations, but contains no adjustable parameters. Our Na/Ar matrix simulations generated relaxed structures for several candidate trapping sites based on various sized vacancies in face-centered-cubic (fcc) solid Ar. Trapping sites for which the equilibrium structures belong to the Oh or Td point groups yielded the experimentally well-known ‘‘triplet’’ absorption line shape; for these cases, the splitting of the degeneracy of the excited Na* 3p(2P) state is due solely to fluctuations away from the equilibrium structures. Simulations of Na/Ar clusters, surfaces, and matrix sites possessing a strong permanent axial asymmetry yielded a widely split ‘‘doublet plus singlet’’ absorption line shape. Despite our success at reproducing several qualitative aspects of the absorption spectroscopy of Na/Ar matrices, our simulations failed to quantitatively reproduce the experimental data. We discuss the major limitations of our model, as well as several possible improvements.
Rotational analyses and angular momentum assignments of [2Π3/2]4d Rydberg states of Br2
Authors: Xu Lei , Wang Ya-fei , Wang Pei-nan , Li Fu-ming
Published: July 2002

Rotational structures and angular momenta of the two observed [2Π3/2]4d gerade Rydberg states of bromine in the region 68 800–72 000 cm−1 have been studied by high resolution resonance enhanced (2+1) multiphoton ionization spectroscopy. The rotational constants have been obtained and the angular momenta of the two [2Π3/2]4d series have been assigned for the first time. These results are further determined and confirmed by computational spectral simulations. The angular momenta of the two observed [2Π3/2]4d Rydberg states are concluded to be Ω=1. Comparison is also made for these two Rydberg states.