bluuebunny/crossref_metadata_2025 huggingface dataset:
Living poly(α-methylstyrene) near the polymerization line. VII. Molecular weight distribution in a good solvent
Authors: Das S. Sarkar , Zhuang Jeuwen , Andrews A. Ploplis , Greer S. C. , Guttman C. M. , Blair W.
Published: July 2002

We have measured the molecular weight distribution (MWD) in a case of equilibrium polymerization. We have studied the time development of the MWD of “living” bifunctional poly(α-methylstyrene) in tetrahydrofuran after a quench to 21 K below the polymerization temperature, Tp. We see an intermediate Gaussian distribution evolving toward a final exponential distribution, as expected from theoretical considerations. We see a longer equilibration time for the number average molecular weight (Mn) as well as for the weight average molecular weight (Mw) than for the monomer concentration ([M)], whereas theories predict that Mn and [M] will relax together and that Mw will take much longer. We attribute the delayed equilibration and a second peak at about Mn/4 to the effects of ionic aggregation of the living polymers. We have also studied the equilibrium MWD of this system as a function of the temperature below Tp, and thus as a function of the number average degree of polymerization (L). These measurements and the time study discussed above are the first experimental evidence that the equilibrium MWD for an organic polymer in a state of equilibrium polymerization is an exponential/Flory–Schulz distribution, and is consistent with scaling predictions. Near Tp and at low L, we observe a deviation from the exponential distribution, which may be evidence of the effect of a chain-length dependence of the equilibrium constant for polymerization, or of the effects of polydispersity on correlations due to excluded volume. In addition, the measured L is about two times less than that expected from the initiator concentration; this could result from ionic aggregation or from chain transfer reactions.
Competition of different ionization pathways in K2 studied by ultrafast pump–probe spectroscopy: A comparison between theory and experiment
Authors: Nicole C. , Bouchène M. A. , Meier C. , Magnier S. , Schreiber E. , Girard B.
Published: July 2002

We present a comparison of experiment and theory of ultrafast one-color pump–probe multiphoton ionization spectrocopy of K2. The wave packet propagation in the A 1Σu+ state and in the (2) 1Πg Rydberg state is monitored in detail by changing systematically the pump and probe wavelength from 779 nm to 837 nm. The measured total ionization rates as a function of the delay time between pump and probe are shown to depend sensitively on the pump and probe wavelengths used and exhibit drastic changes and a variety of fascinating structures as the direct observation of inward and outward wave packet detection and frequency doubling of the detected wave packet oscillation. The time dependent quantum mechanical wave packet calculations are in excellent agreement with the experimental results and allow a clear interpretation of different ionization pathways and mechanisms observed in the femtosecond ion signal.
Rationalizing the effects of modified electrostatic interactions in computer simulations: The dielectric self-consistent field method
Authors: Boresch Stefan , Steinhauser Othmar
Published: July 2002

The dielectric self-consistent field method, a novel tool to study solvated systems based on continuum electrostatics, is introduced. It permits the qualitative and even semiquantitative calculation of orientational correlation functions, i.e., it gives insights into the orientational structure of a solute–solvent system. Further, modified Coulomb potentials and periodic boundary conditions can easily be integrated. One possible application is rapid, yet detailed methodological studies of the effects resulting from the various modified electrostatic interactions that are used regularly in computer simulations with explicit solvent molecules. As an example, we report the distance dependent Kirkwood g-factor and ion–dipole correlation functions of a solvated glycine zwitterion obtained with a simple cutoff, a shifted potential, two reaction field techniques, and Ewald summation. For the reaction fields and Ewald summation, conducting and adjusted dielectric boundary conditions are compared.
Influence of steps on the interaction between adsorbed hydrogen atoms and a nickel surface
Authors: Hanbicki Aubrey T. , Darling S. B. , Gaspar D. J. , Sibener S. J.
Published: July 2002

We have studied the effect of an extended array of defects on the two-dimensional phase behavior of adsorbed hydrogen on a Ni surface using helium atom scattering. Specifically, the interaction of hydrogen with the stepped Ni(977) surface was examined and compared with similar interactions with the flat Ni(111) surface. The phase behavior of hydrogen on Ni(977) is qualitatively the same as that of hydrogen on Ni(111); however, the temperature at which the order–disorder transition occurs is elevated. On the stepped surface, the ordered (2×2)-2H phase exists at a temperature 40 K higher than on the flat surface. This reversible phase transition is second order and is best fit with Tc=310 K and β=0.12, indicative of two-dimensional Ising behavior. Stabilization of the ordered phase is attributed to pinning from the step edges. The cross section for diffuse elastic He scattering by adsorbed hydrogen and the temperature-dependent domain size of ordered hydrogen along the step edges are also discussed.
Orbital momentum profiles and binding energy spectra for the complete valence shell of propane
Authors: Zheng Y. , Pang W. N. , Shang R. C. , Chen X. J. , Brion C. E. , Ghanty T. K. , Davidson E. R.
Published: July 2002

The orbital momentum profiles and binding energy spectra for the complete valence shell of propane are reported. The experiment has been performed using a high energy resolution (ΔE=0.95 eV FWHM) multichannel (e,2e) electron momentum spectrosocopy spectrometer at an impact energy of 1200 eV plus the binding energy. The measured binding energy spectra are compared and consistent with PES data available in the literature and also with the predictions of Hartree–Fock, Green’s function and MRSD-CI methods. A strong splitting observed in the inner valence energy spectra due to electron correlation and ion relaxation effects is confirmed by MRSD-CI calculations. The experimental momentum profiles have been compared with calculations obtained using the target Hartree–Fock method with a minimum basis set and also a very large basis set. Density functional theory calculations using B3LYP functionals as well as large basis set MRSD-CI calculations are also reported. The agreement between theory and experiment for the shape of the orbital electron momentum distributions is generally good.
The effect of intermolecular interactions on the electric properties of helium and argon. II. The dielectric, refractivity, Kerr, and hyperpolarizability second virial coefficients
Authors: Koch Henrik , Hättig Christof , Larsen Helena , Olsen Jeppe , Jo/rgensen Poul , Fernández Berta , Rizzo Antonio
Published: July 2002

The dielectric, refractivity, Kerr, and hyperpolarizability second virial coefficients for the helium and argon gases are evaluated for a wide range of temperatures using a semiclassical approach and the high quality frequency-dependent interaction induced electric polarizabilities and second hyperpolarizabilities of the previous paper. For helium and argon we obtain satisfactory agreement with most of the experimental data for the dielectric and the refractivity second virial coefficients. Our results confirm that the helium gas second Kerr virial coefficient is very small at temperatures beyond 70 K. For argon we obtain a very good agreement with a recent experimental determination at 632.8 nm, whereas we suggest that previous experimental results for 458 nm might be inaccurate. The ESHG results indicate a possible disagreement between a recent experimental determination and the semiclassical ansatz for the second hyperpolarizability virial coefficients.
Semiempirical method for calculating structure and band gap of semiconducting polymers
Authors: Yoder G. , Dickerson B. K. , Chen A.-B.
Published: July 2002

A semiempirical method that yields accurate band gaps and atomic positions in sp2-hybridized, organic, semiconducting polymers has been obtained. This method is a tight-binding calculation where most of the parameters are determined via an ab initio local density approximation method leaving only π–π interactions to be adjusted empirically. Once fitted for one or two systems, the method can be applied to other similar systems without further adjustment. Accurate results have been demonstrated for a number of semiconducting polymers that contain C, S, N, and H.
Choosing a good representation of the quantum state wave functions for semiclassical surface hopping calculations
Authors: Herman Michael F.
Published: July 2002

Simple curve crossing problems have shown that the probability of transition between quantum states for the fast degrees of freedom becomes small at low energies, if the adiabatic representation is employed for the quantum state wave functions. Conversely, the transition probability approaches unity at high energies in the adiabatic representation, but it is small if the diabatic representation is employed. Methods are developed and numerically tested for choosing a representation, which is intermediate between the adiabatic and diabatic representations, so as to approximately minimize the interstate coupling in semiclassical surface hopping methods. The reduction in the coupling should diminish the importance of multihop trajectories in surface hopping procedures, resulting in a corresponding reduction in the computational effort required for accurate calculations.
Calculations on the Jahn–Teller configurations of the benzene cation
Authors: Muller-Dethlefs Klaus , Peel J. Barrie
Published: July 2002

The analysis of the zero-electron-kinetic-energy photoelectron spectra of benzene has led to a reinvestigation by ab initio methods of the electronic states of the C6H6+ cation resulting from Jahn–Teller distortions on ionization. The calculations involving a range of currently used methodologies all verify that the two cation configurations, B2g2 and B3g2 of D2h symmetry, resulting from removal of an electron from the e1g(π) degenerate MOs of C6H6, comprise a true minimum and a transition state, differing only slightly in energy. These are linked through the in-plane b1g vibration, confirming that b1g is actually a pseudorotational coordinate. Hence C6H6+ exhibits similar structural floppiness to the cations of methane and cyclopropane although with a much smaller barrier to pseudorotation than for these smaller species. These results support the general proposition that such Jahn–Teller distortions associated with molecular ionization (of stable closed-shell hydrocarbon molecules of high symmetry) generally involve a quadratic contribution which leads to a single global minimum cation structure, with all other derived stationary states being transition states each characterized by a single imaginary vibration frequency.
Rotational state-to-state energy transfer of NH2(Ã 2A1) in beam-gas condition
Authors: Dong Feng , Li Xuechu , Zhang Min , Wang Xiuyan , Lou Nanquan
Published: July 2002

The detailed study on the rotational state-to-state energy transfer (RSET) of NH2(Ã 2A1) at a relative translational energy of about 360 cm−1 has been reported herein, by combining self-breakdown pulsed dc discharge and a photon counting technique. The transferred populations from the parent levels to the daughter levels have been obtained through the spectral simulations. It is clear that the rotational state distributions depend not only on the rotational quantum numbers of the initial and collisionally populated states, but also on the quantum number Ka. It is interesting to find that the probability of RSET behaves differently depending on the rotational quantum numbers of the parent levels, i.e., the exponential angular momentum transfer law (AMT) is preferred with the relatively low rotational levels, while the exponential energy gap law is preferred with the relatively high levels. According to the sudden approximation and the theory of Osborne et al., the experimental results are explained qualitatively.
A two-dimensional deuterium nuclear magnetic resonance study of molecular reorientation in sugar/water glasses
Authors: Wachner Andrew M. , Jeffrey Kenneth R.
Published: July 2002

Two-dimensional nuclear magnetic resonance time domain analysis of the stimulated echo experiment is used to study the molecular reorientation of deuterated glucose molecules in a glucose/water glass at about 15 °C above the glass transition. By measuring both the cos–cos and sin–sin two-time single particle correlation functions it is shown that the reorientation is isotropic, which enables the determination of the relaxation time T1Q and the rotational correlation time. By fitting the cos–cos and sin–sin correlation functions with a modified Kohlrausch–Williams–Watts (KWW) function and comparing the resulting parameters with simulations, it is shown that the reorientation is dominated by small angle jumps (≈3°), but there is also a small fraction of large angle (≈34°) jumps present.
Singlet excited states of Be2
Authors: Pecul Magdalena , Jaszuński Michal , Larsen Helena , Jørgensen Poul
Published: July 2002

The potential curves of low-lying singlet states of Be2 are computed in the hierarchy of coupled-cluster (CC) models: CCS, CC2, CCSD, and CC3 from the ground-state energy and linear response excitation energies. The results are compared with full configuration interaction (FCI) values. For single-excitation dominated transitions the convergence of the excitation energies toward FCI is smooth with a reduction in the error of about a factor of 3 at each level of the CC hierarchy. This reduction is only seen from CCSD to CC3 for the double-excitation dominated transitions. CC3 gives accurate potential curves and spectroscopic constants of the excited states dominated by single excitations. For the double-excitation dominated states the CC3 equilibrium distances and vibrational frequencies agree reasonably well with FCI, while the bonding energies and the 0-0 excitation energies have larger errors. We analyze the influence of the frozen-core approximation, basis-set effects and the counterpoise correction on the ground-state energy, the excitation energies, and molecular constants. Considering all these effects, we provide an improved estimate of the calculated properties.
The semirigid vibrating rotor target model for atom-polyatom reaction: Application to H+H2O→H2+OH
Authors: Zhang Dong H. , Zhang J. Z. H.
Published: July 2002

The semirigid vibrating rotor target (SVRT) model for the polyatomic reaction has been applied to the reaction of H+H2O→H2+OH using the time-dependent wave packet approach. Since the SVRT model for a general atom–polyatom reaction involves only four-mathematical dimensions (4D), the SVRT dynamics calculation for H+H2O requires much less computational effort than the exact full-dimensional treatment. Numerical calculation shows that by properly choosing the values for the excluded degrees of freedom, excellent results are obtained for the computed reaction probability, cross section, and rate constant. The present numerical calculation for H+H2O reaction from the initial ground state clearly demonstrates that the SVRT model for the polyatomic reaction provides an accurate and practical approach for computational study of chemical reactions involving polyatomic molecules.
Structure and interaction between ionic layers
Authors: Hernández-Contreras M. , Pincus P.
Published: July 2002

The effect of asymmetric charge distribution on the structural properties of a lamellar system of ionic layers was determined with the use of the hypernetted chain theory. Due to electrostatic interaction and positional correlations between ions in all different layers, there appear layer pairs attractive forces which enhance when the ionic charge and concentration per layer is increased or when the distance of separation in reduced. The innermost layer pairs show the strongest attractive interaction due to an increased enhancement of ionic correlations. Asymmetry in the ionic concentration in the normal direction to the planes and moderate layers separation, h, produce a nonmonotonous pressure curve as a function of h for the more exterior layer pairs whereas at longer separations this pressure shows the universal asymptotic power law of correlated charge fluctuations of a simple two layer system, P∼−1/h3.
Quasi-elastic Raman scattering and free volume in isotropic pressure-densified atactic poly(methyl methacrylate) glasses
Authors: Schmidt M. , Brodin A. , Jacobsson P. , Maurer F. H. J.
Published: July 2002

We have made a series of isotropic pressure-densified atactic poly(methyl methacrylate) (PMMA) glasses with densities ranging from 1.1823 g/cm3 to 1.1963 g/cm3, in order to investigate how the glassy dynamics, measured by Raman scattering in the frequency range of 0.1–10 THz, depends on the thermomechanical history of the glasses. In particular, we investigate whether there is a correlation between the strength of the fast relaxation dynamics and the free-volume characteristics, such as the average free-volume size and the free-volume fraction. The fast relaxation strength is deduced from the quasielastic light scattering (QES) intensity; the free-volume size is measured with positron annihilation lifetime spectroscopy (PALS); and the free-volume fraction is determined from pressure–volume–temperature (PVT) data and the Simha–Somcynsky equation-of-state (EOS) theory. Our temperature-dependent Raman and PALS measurements show that both the QES intensity and the ortho-positronium (o-Ps) lifetime increase smoothly with the temperature. On the other hand, the relaxation strength of the densified glasses at room temperature is, within experimental error, independent of the density, whereas both the o-Ps lifetime from PALS and the free volume fraction from the PVT data decrease significantly with pressure densification. We therefore conclude that the fast relaxation intensity cannot be explained on the basis of free-volume quantities, contrary to a recently suggested correlation between the QES intensity and the free volume [V. N. Novikov et al., J. Chem. Phys. 107, 1057 (1997)]. On the other hand, the observed behavior is consistent with another recently suggested model [V. N. Novikov, Phys. Rev. B 58, 8367 (1998)] which attributes the fast relaxations to vibration anharmonicity. Moreover, we show that the measured increase in the refractive index of the pressure-densified glasses fits the Lorentz–Lorenz equation and compares well to predictions of the relative change in refractive index with formation pressure from our own as well as published PVT data.
Rescaling of diffusion coefficients in two-time scale chemical systems
Authors: Strier Damián E. , Ponce Dawson Silvina
Published: July 2002

We study reaction–diffusion systems which involve processes that occur on different time scales. In particular, we apply a multiscale analysis to obtain a reduced description of the slow dynamics. Under certain assumptions this reduction yields a new set of reaction–diffusion equations with rescaled diffusion coefficients. We analyze the Selkov model [E. E. Selkov, Eur. J. Biochem. 4, 79 (1968)] and the ferrocyanide–iodide–sulfite reaction [E. C. Edblom et al., J. Am. Chem. Soc. 108, 2826 (1986)] to determine whether the rescaling in this case may account for the difference of diffusivities that the formation of certain types of patterns requires.
Polyenes vs polyynes: Efficient π-frame for nonlinear optical pathways
Authors: Lee Jin Yong , Suh Seung Bum , Kim Kwang S.
Published: July 2002

The electronic and vibrational hyperpolarizabilities of polyenes NH2–(HC=CH)n–NO2 and polyynes NH2–(C≡C)n–NO2 have been investigated. As the chain length increases, the first hyperpolarizabilities increase drastically for polyenes, while they increase moderately for polyynes. Furthermore, in polyenes the vibrational first hyperpolarizabilities are almost the same as the electronic first hyperpolarizabilities, while in polyynes the former is much smaller than the latter. An analysis of the electron densities of HOMOs of polyenes and polyynes confirms that the polyene π-frame is more efficient for long pathways of nonlinear optical molecules, while the polyyne π-frame is slightly better for very short pathways. In both polyenes and polyynes, the IR intensities arise mainly from the terminal carbon sites, while the Raman intensities, from the central carbon sites.
Photo- versus pulsed field-ionization of individual high n (n≲57) Rydberg states: Resolved d,g series of benzene
Authors: Siglow Klaus , Neusser Hans Jürgen
Published: July 2002

Low l Rydberg states of the polyatomic molecule benzene in the range 37≲n≲56 are resolved for the first time by double resonance excitation using narrow-band ultraviolet (UV)-laser pulses. The short lived Rydberg states are detected by photoexcitation of the ionic core with the same laser pulses followed by fast autoionization of the Rydberg states. The Rydberg spectrum obtained shows clear substructure of individual high n Rydberg peaks with components of quantum defects δl=0.00, 0.015, and 0.04. It is compared with a delayed pulsed field ionization spectrum with a delay of 8 μs. The bright low l core-penetrating zero order states cannot be observed in the delayed pulsed field ionization experiment for n<50 since there is no mixing with the long-lived Stark manifold of noncore-penetrating high l Rydberg states in the residual electric stray fields present in our apparatus which is necessary to lengthen the lifetime to survive the delay of 8 μs. Our results for the observed low l Rydberg states agree with previous results from Rydberg spectroscopy of low n Rydberg states in benzene. This is an important step towards observation of continuous Rydberg series from n=3 up to n=56. Above n=56 the different n,l Rydberg states are mixed in the electric stray fields into a single n Stark manifold.
Calculation of the vibrational linewidth and line shape of Raman spectra using the relaxation function. I. Method and application to nitrogen
Authors: Kooi M. Eline , Smit Floris , Michels Jan P. J. , Schouten Jan A.
Published: July 2002

The spectral line shape of the fundamental vibration of nitrogen is calculated from molecular dynamics simulations by determining the Fourier transform of the relaxation function. It has been applied to the fluid phase at various pressures and temperatures, and to solid δ-N2. The validity of the assumption that the spectrum at relatively high temperatures and pressures can be calculated by assuming that these systems are in the fast modulation regime (Δτc≪1), has been verified. A deviation of the vibrational line shape from the motional narrowing limit has been found for fluid nitrogen at low pressure, with a Kubo parameter, Δτc, equal to 0.23, and for the vibrational line of the molecules on the a sites in δ-N2, with Δτc equal to 0.075. It is concluded that the value of the Kubo parameter is not an unambiguous criterion for the fast modulation regime. Moreover, a detailed comparison reveals a difference in the dynamical behavior of the molecules on the a and c sites. It is shown that this procedure can also be used if one does not know whether or not closely spaced lines are to be expected. The present procedure is suited to calculate line shapes in the intermediate Kubo regime, e.g., in concentrated mixtures, where no simple relations are available. Finally it is shown that in nitrogen at low density and 126 K the ratio of the correlation time of the frequency autocorrelation function and the dephasing time is smaller than in CH3I, where the n dependence of the vibrational overtone is subquadratic.
Correlation effects in molecular diffusion in zeolites at infinite dilution
Authors: Jousse Fabien , Auerbach Scott M. , Vercauteren Daniel P.
Published: July 2002

Molecular diffusion in zeolites is often resumed to a random walk between specific adsorption sites within the channels and cavities of the materials. Several types of correlations between the steps of the walk come to precise this assumption: kinetic correlations due to the incomplete relaxation of the molecule in its final site, vacancy correlations arising at high loading because molecules are blocking each other, and geometrical correlations because zeolite channels and cages can boast nonsymmetric sites. The first and last correlation effects can be observed at infinite dilution. In this article we present a way of calculating an exact diffusion coefficient at infinite dilution as a function of the microscopic rate constants, taking into account both geometric and kinetic correlation effects. This is achieved by cutting the molecular motion into uncorrelated sequences of jumps, where all jumps inside one sequence are correlated to each other. This method is applied to study geometrical correlations of benzene in NaY, comparing with kinetic Monte Carlo data of Saravanan et al. [C. Saravanan, F. Jousse, and S. M. Auerbach, J. Chem. Phys. 108, 2162 (1998)], and both kinetic and geometrical correlations of ethane in silicalite, comparing with molecular dynamic simulations of Kärger et al. [J. Kärger, P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 110, 1163 (1999)].
Ab initio determination of americium ionization potentials
Authors: Tilson Jeffrey L. , Shepard Ron , Naleway Conrad , Wagner Albert F. , Ermler Walter C.
Published: July 2002

The first three ionization potentials of americium are calculated using ab initio spin–orbit configuration interaction techniques. These results are favorably compared to available experimental and previous theoretical work. The lowest two ionization potentials are accurately determined using wave functions constructed as simple single and double substitutions from a self-consistent field reference configuration with scalar relativistic effects included through an averaged relativistic pseudopotential. A determination of the third ionization potential to comparable accuracy requires inclusion of the spin–orbit operator and significant intermediate coupling with a resulting configuration expansion length in excess of 1.9 million double-group adapted functions. The solution to this problem was achieved by application of a new parallel spin–orbit configuration interaction component to the COLUMBUS Program System. A decomposition of the ionization potential calculation into parts either sensitive or largely insensitive to the spin–orbit operator is favorably tested, leading to hybrid calculations of improved accuracy.
Self-consistent fluid variational theory for pressure dissociation in dense hydrogen
Authors: Juranek Hauke , Redmer Ronald
Published: July 2002

Recent developments in shock-wave experimental techniques have allowed the 100 GPa pressure range in hydrogen to be probed. In recently reported single-shock-wave laser-driven experiments, the principal Hugoniot was determined up to 300 GPa. It has been shown that dissociation of hydrogen molecules becomes operative under such ultrahigh pressures. Various models have been developed which treat pressure dissociation approximately. In this paper we generalize standard fluid variational theory to a two-component system with a reaction (dissociation). From the free energy, other thermodynamic functions such as the internal energy and entropy are derived. Comparison with other approaches and the shock-wave data is performed.
Ab initio potential energy surface for the Ar(1S)+OH(X2Π) interaction and bound rovibrational states
Authors: Klos Jacek , Chalasinski Grzegorz , Berry Mary T. , Kendall Rick A. , Burcl Rudolf , Szczesniak M. M. , Cybulski Slawomir M.
Published: July 2002

Adiabatic potential energy surfaces for the A′2 and A″2 states of the Ar(1S)–OH(X2Π) complex were calculated using supermolecular unrestricted fourth-order Møller–Plesset perturbation theory and a large correlation consistent basis set supplemented with bond functions. The potential energy surface (PES) of the A′ state has two minima. The global minimum from the unrestricted coupled-cluster calculations with single, double, and noniterative triple excitations occurs for the collinear geometry Ar–H–O at R=7.08a0 with a well depth of De=141.2 cm−1. There is also a local minimum for the skewed T-shaped form, whereas the Ar–O–H arrangement corresponds to a saddle point. The PES of the A″ state also has two minima, which occur for the two collinear isomers. A variational calculation of the bound rovibrational states was performed. The calculated binding energy, D0=93.8 cm−1, and the energies of the bound vibrational states are in good agreement with experiment [see Berry et al., Chem. Phys. Lett. 178, 301 (1991) and Bonn et al., J. Chem. Phys. 112, 4942 (2000), preceding paper].
Ab initio based study of the ArO− photoelectron spectra: Selectivity of spin–orbit transitions
Authors: Buchachenko A. A. , Jakowski Jacek , Chałasiński Grzegorz , Szczȩśniak M. M. , Cybulski S. M.
Published: July 2002

A combined ab initio atoms-in-molecule approach was implemented to model the photoelectron spectra of the ArO− anion. The lowest adiabatic states of Σ and Π symmetry of ArO and ArO− were investigated using the fourth-order Møller–Plessett perturbation theory including bond functions. The total energies were dissected into electrostatic, exchange, induction, and dispersion components. The complex of Ar with atomic oxygen is only weakly bound, primarily by dispersion interaction. The Π state possesses a deeper minimum (Re=3.4 Å,De=380 μEh) than the Σ state (Re=3.8 Å,De=220 μEh). In contrast, the anion complex is fairly strongly bound, primarily by ion-induced dipole induction forces, and the Σ state possesses a deeper minimum at shorter interatomic distances (Re=3.02 Å,De=3600 μEh) than the Π state (Re=3.35 Å,De=2400 μEh). The Σ–Π splittings in both systems are mainly due to differences in the exchange repulsion terms. Atoms-in-molecule models were used to account for the spin–orbit interaction, and to generate adiabatic relativistic potentials and wave functions. Collisional properties, diffusion, and mobility coefficients of O and O− in Ar, and absolute total Ar+O scattering cross sections, were calculated and found to agree well with the available experimental data. The photoelectron spectra were simulated within vibronic model, and were found in excellent agreement with the experimental measurements. The bimodal electron kinetic energy distribution was shown to stem from the strong selectivity of spin–orbit transitions, which split into two dense groups, depending on the initial electronic state of the anion. The latter feature cannot be described without explicit consideration of electronic intensity factor.
Structure and rovibrational analysis of the [O2(1Δg)v=0]2←[O2(3Σg−)v=0]2 transition of the O2 dimer
Authors: Biennier Ludovic , Romanini Daniele , Kachanov Alexander , Campargue Alain , Bussery-Honvault Béatrice , Bacis Roger
Published: July 2002

The rotationally resolved absorption spectrum of the O2 dimer involving the [O2(1Δg)v=0]2←[O2(3Σg−)v=0]2 transition has been recorded near 632.6 nm by continuous wave Cavity Ring Down Spectroscopy in a supersonic slit jet expansion of pure O2. A quadratic dependence of the absorption in the jet versus the stagnation pressure is observed. A rotational temperature of 12 K is derived from the (O2)2 rotational analysis. The high spectral resolution of the CW-CRDS measurements limited by the residual Doppler broadening in the jet and the low rotational temperature allow the first rotational analysis in this open-shell complex. The same spectrum was also recorded by Intracavity Laser Absorption Spectroscopy and the comparison of the performances of the two methods is discussed. Among more than 600 lines measured between 15 800 and 15 860 cm−1 from the CW-CRDS spectrum, 40 were assigned to the RP0, RQ0, and RR0 branches of two subbands associated with B1−←A1+ and A1+←B1− transitions between the ground and excited rovibrational levels, labeled following the G16 permutation inversion representation. Forty five lines were assigned to PP2, PQ2, and PR2 branches of two subbands associated with B1−←A1+ and A1+←B1− transitions. The subbands centered at 15 808.401(49) [A1+←B1−] and 15 813.134(37) cm−1 [B1−←A1+] for those arising from K=0, and at 15 812.656(20) [A1+←B1−] and 15 818.277(35) [B1−←A1+] when arising from K=2, are analyzed considering (O2)2 as a slightly asymmetric prolate top. The rotational analysis of the two K=0 subbands leads to very close values of the effective rotational constant, Bp=(B+C)/2, for both A1+ and B1− levels: 0.095 cm−1 for the [O2(3Σg−)v=0]2 lower states and 0.063 cm−1 for the [O2(1Δg)v=0]2 excited states, in close agreement with theoretical values. The H geometry is confirmed as the most stable for the ground electronic singlet state. A distance between the two monomers of 6.1 a0 and 7.5 a0 is derived for the ground and excited singlet states. Similar results are obtained from the two K=2 subbands. A vibrational assignment is given for the two rotationally analyzed subbands (K=0) and proposed for the main features of the whole band.
The third-order polarizability γ of C60: The role of low-lying two-electron excited singlet states Ag and Hg
Authors: Nomura Yasushi , Miyamoto Takashi , Hara Toshiki , Narita Susumu , Shibuya Tai-ichi
Published: July 2002

The third-order polarizability γ of the C60 molecule has been calculated for the third harmonic generation at several incident frequencies using two different schemes of the sum-over-state (SOS) method, and the contribution of the low-lying singlet excited states as the second intermediate states has been analyzed. The group-theoretical analysis of the SOS expression clarifies that the 1Ag and 1Hg states are the only states that contribute to the γ as the second intermediate states. For the numerical analysis, the electronic states previously obtained in the semiempirical CNDO/S approximation with the singly and doubly excited configuration interaction method are used. It is found that the inclusion of the doubly excited configurations is essential in evaluating the γ. Those excited 1Ag and 1Hg states which make significant positive contributions to the γ are practically the doubly excited states. In order to secure error-free SOS calculations, equivalent but independent calculations have been also carried out with the frequency-dependent moment method.
Microscopic calculation of surface linear optical response in crystals of para-nitroaniline
Authors: Malagoli M. , Munn R. W.
Published: July 2002

Previous theoretical results for surface linear optical response in molecular crystals are extended to treat response to the external applied field, the bulk macroscopic field, and the surface macroscopic field. They are used to calculate planewise dipole sums, surface local electric fields, surface linear susceptibilities, and surface refractive indices for surfaces normal to the [001] and [101] directions of the centrosymmetric para-nitroaniline crystals. For each direction, two different terminations give different results. Surface effects are stronger normal to the surface for the [001] direction, and in the surface for the [101] direction. Surface effects decrease sharply into the bulk, being negligible beyond neighboring layers of molecules. The present results provide input for calculations of surface-induced quadratic nonlinear optical response, reported in the following article.
Electron-pair radii and relative sizes of atoms
Authors: Koga Toshikatsu
Published: July 2002

The electron-pair intracule (relative motion) h(u) and extracule (center-of-mass motion) d(R) densities represent probability densities for the interelectronic distance and the center-of-mass radius of any pairs of electrons, respectively. For 102 atoms from He (atomic number Z=2) to Lr (Z=103), we report that electron-pair radii R2i and R2e, defined by h(R2i)=c2i and d(R2e)=c2e, have good linear correlations with the relative sizes R1 of atoms introduced based on the single-electron density ρ(r) such that ρ(R1)=c1, where c1, c2i, and c2e are constants common to the 102 atoms. It is also shown that an interesting relation R2e≅R2i/2 holds, if c2e is set equal to 8c2i.
X-ray and molecular dynamics study of liquid structure in pure methylchloromethane compounds ((CH3)4−nCCln)
Authors: Rey R. , Pardo L. C. , Llanta E. , Ando K. , López D. O. , Tamarit J. Ll. , Barrio M.
Published: July 2002

We have studied liquid structure for a whole family of methylchloromethane compounds ((CH3)4−nCCln), exploiting the interplay of x-ray diffraction measurements and molecular dynamics (MD) computations. To this end we report for the first time x-ray spectra for 1,1,1-trichloroethane (n=3), and 2,2-dichloropropane (n=2), together with a new determination for carbon tetrachloride (n=4). A consistent set of molecular models for MD simulation has also been developed for the full family, providing excellent accord with thermodynamic properties (vaporization enthalpy and density over the full liquid phase), and with diffraction data alike. The theoretical results have allowed the interpretation of the salient features in the experimental spectra and of the trends peculiar to this family of compounds, basically characterized by the suppression of one of the two main peaks in the spectrum as the number of chlorines is diminished. A numerical method that constructs radial correlation functions for ideal dimer geometries has served to explore the most probable structures between nearest neighbors. We have concluded that structure at short intermolecular distances cannot be assigned to any clear-cut geometry. Instead, it can be explained by a combination of corner-to-face and interlocked configurations, with a contribution (dependent on the compound) of face-to-face configurations.
A Monte Carlo study of the pattern formation during transitions in n-alkane crystals
Authors: Yamamoto T. , Matsumoto S. , Hirose M.
Published: July 2002

The rotator phase transition in n-alkane was recently found to show a characteristic pattern of preferred growth along the b-axis (shorter axis) of the unit cell. In order to investigate the origin of this anisotropic growth, a pattern formation in n-alkane crystal is studied during the transition between the low temperature orthorhombic (LO) phase and the rotator (R) phase by use of the Monte Carlo method. Of possible factors that will influence the growth pattern, we here concentrate on the mode of chain packing by assuming that the chains have rigid planar zigzag conformation and are placed in a regular orthohexagonal lattice. The herringbone order in the LO phase is found to develop rather quickly resulting characteristic domains with the (100) and (110) boundaries. The domain boundaries run preferentially along the b-axis at lower temperatures and are considered as a stacking fault or antiphase boundary. The transition between the LO phase and the R phase is found to exhibit a characteristic pattern, where the R phase domains grow preferentially along the b-axis. All these behaviors are shown to originate from different energies of the (100) and (110) boundaries.
Identification of trihalide anions in bis(ethylenedithio)tetrathiafulvalene salts by Raman spectroscopy
Authors: Wojciechowski R. , Ulanski J. , Lefrant S. , Faulques E. , Laukhina E. , Tkacheva V.
Published: July 2002

In order to distinguish individual contributions of trihalide anions in β-like structures of the (BEDT-TTF)2X organic conducting single crystals [where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X stands for I3, I–I–Br, Br–I–Br, Br–I–Cl, Cl–I–Cl or mixed trihalide anions], low frequency Raman spectra have been recorded at 25 K. This technique permitted us to establish a presence of a ternary mixture of I3, I–I–Br, and Br–I–Br anions in one of the nonstoichiometric salts and an existence of Br–I–Br, Br–I–Cl, and Cl–I–Cl anions in another one. It is not possible to detect these anions independently using the x-ray diffraction method. The ab initio calculated interatomic bond lengths and vibrational frequencies of trihalide anions are in good agreement with experimental data, which confirmed the assignment of observed low-frequency Raman bands. At 80 K the Raman spectra were taken for a fixed linear polarization of the incident laser light, and the examined single crystals were rotated in π/8 increments. In this way, an anisotropy of trihalide anion’s polarizability has been investigated to confirm the collinear arrangement of anions and the proposed assignment of Raman bands.
A quantum molecular dynamics study of exciton self-trapping in conjugated polymers: Temperature dependence and spectroscopy
Authors: Kobrak Mark N. , Bittner Eric R.
Published: July 2002

We examine the dynamics of exciton self-trapping in conjugated polymer systems using mixed quantum-classical molecular dynamics. The model treats the exciton as a two-dimensional quantum mechanical wave function representing a particle/hole quasiparticle interacting with a classical vibrational lattice [M. N. Kobrak and E. R. Bittner, J. Chem. Phys. 112, 5399 (2000)]. We show that the dynamics are influenced strongly by thermal disorder in the lattice, and that there is a dramatic change in the self-trapping mechanism as temperature increases. At low temperatures, the rate of localization is limited by the time required for the vibrational lattice to respond to the creation of the particle–hole pair, while at higher temperatures thermal disorder permits localization on time scales limited primarily by electronic response. We simulate the time-resolved fluorescence spectrum for the model system, and compare the temperature dependence of the spectrum to recent time-resolved fluorescence upconversion studies on polydiacetylene derivatives.
Comment on “The near infrared, visible, and near ultraviolet overtone spectrum of water” [J. Chem. Phys. 111, 2444 (1999)]
Authors: Campargue A. , Bertseva E. , Naumenko O.
Published: July 2002

On the basis of new results obtained by intracavity laser absorption spectroscopy, a number of HDO lines have been identified in the E-PAPS electronic archive of H2O absorption lines provided in the paper entitled “The near infrared, visible and near ultraviolet overtone spectrum of water” [J. Chem. Phys. 111, 2444 (1999)].
Photophysical properties of hexa-functionalized C60 derivatives: Spectroscopic and quantum-chemical investigations
Authors: Coheur P.-F. , Cornil J. , dos Santos D. A. , Birkett P. R. , Liévin J. , Brédas J. L. , Walton D. R. M. , Taylor R. , Kroto H. W. , Colin R.
Published: July 2002

The photophysical properties of hexa-functionalized C60 derivatives (C60Cl6, C60Ph5Cl, C60Ph5H, and two C60Ph5OH isomers) have been investigated by means of UV–visible absorption and fluorescence spectroscopy in cyclohexane solution and quantum-chemical calculations derived from semiempirical Hartree–Fock approaches. A very good agreement is obtained between the measured absorption and fluorescence spectra and the calculated excitation energies, thus allowing for a detailed assignment of the optical features. The effects of symmetry and chemical nature of the functional groups on the ground-state and spectroscopic properties of the C60 derivatives are discussed.
Optimal packing of polydisperse hard-sphere fluids. II
Authors: Blaak Ronald
Published: July 2002

We consider the consequences of keeping the total surface fixed for a polydisperse system of N hard spheres. In contrast with a similar model [J. Zhang et al., J. Chem. Phys. 110, 5318 (1999)], the Percus–Yevick and Mansoori equations of state work very well and do not show a breakdown. For high pressure Monte Carlo simulation we show three mechanically stable polydisperse crystals with either a unimodal or bimodal particle-size distribution.
Ultraviolet photodissociation of HCl in selected rovibrational states: Experiment and theory
Authors: Regan Paul M. , Ascenzi Daniela , Brown Alex , Balint-Kurti Gabriel G. , Orr-Ewing Andrew J.
Published: July 2002

Experimental and theoretical methods have been applied to investigate the effect of internal parent excitation on the ultraviolet photodissociation dynamics of HCl (X 1Σ+) molecules. Jet-cooled H35Cl molecules within a time-of-flight mass spectrometer were prepared by infra-red absorption in the following quantum states: v=1, J=0 and J=5; v=2, J=0 and J=11; v=3, J=0 and J=7. The excited molecules were then photodissociated at λ∼235 nm and the Cl(2Pj) photofragments detected using (2+1) resonance enhanced multiphoton ionization. The results are presented as the fraction of total chlorine yield formed in the spin–orbit excited state, Cl(2P1/2). The experimental measurements are compared with the theoretical predictions from a time-dependent, quantum dynamical treatment of the photodissociation dynamics of HCl (v=1−3, J=0). These calculations involved wavepacket propagation using the ab initio potential energy curves and coupling elements previously reported by Alexander, Pouilly, and Duhoo [J. Chem. Phys. 99, 1752 (1993)]. The experimental results and theoretical predictions share a common qualitative trend, although quantitative agreement occurs only for HCl (v=2).
Evidence of triplet ethylene produced from photodissociation of ethylene sulfide
Authors: Qi Fei , Sorkhabi Osman , Suits Arthur G.
Published: July 2002

Tunable synchrotron radiation has been used to probe the dissociation dynamics of ethylene sulfide, providing selective determination of the translational energy distributions of both excited (1D) and ground-state (3P) sulfur atoms, with momentum-matching to the ethylene cofragments. The results suggest the presence of a channel giving S(3P) in conjunction with triplet ethylene C2H4 (3B1u), and allow for the first experimental measure of the energy of the latter species near the equilibrium geometry, in which the two methylene groups occupy perpendicular planes.
Collision-induced dissociation dynamics of Ar2+ at high levels of vibrational excitation
Authors: Chiu Yu-hui , Pullins Steve , Levandier Dale J. , Dressler Rainer A.
Published: July 2002

The collision-induced dissociation (CID) dynamics of the Ar2++Ar collision system are investigated at different Ar2+ internal energy distributions in a guided-ion beam (GIB) apparatus. The internal energy of reactant ions, assumed vibrational in a first approximation, is controlled by varying the position of ionization in a supersonic jet, electron impact ion source. Three conditions are investigated: cold, in which the ions are produced as vibrationally relaxed as possible; intermediate, in which a substantial shift in the CID onset is observed; hot, in which the apparent CID threshold is at near thermal collision energies. The vibrational distribution of the Ar2+ ions is probed at the same conditions by measuring the kinetic energy release of photofragment Ar+ following Σg+2←Σu+2 photodissociation. The derived internal energy distributions are then used to model the observed CID cross sections with a modified line-of-centers approach to assess vibrational effects in the single-collision cross sections. The intermediate CID cross sections are consistent with a negligible vibrational enhancement beyond the statistical predictions. A substantial increase in cross section is observed when going from intermediate to hot conditions, despite a weak increase in internal energy, as apparent from the photodissociation measurements. Contributions from metastable states, not registered in the photodissociation experiment, can explain this disparity.