bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical study on the mechanism of charge transfer state formation of 4-(N,N-dimethylamino)benzonitrile in an aqueous solution
Authors: Kato Shigeki , Amatatsu Yoshiaki
Published: July 2002

The mechanism of charge transfer (CT) state formation of excited state 4-(N,N-dimethylamino)benzonitrile (DMABN) in an aqueous solution has been studied theoretically. Ab initio configuration interaction (CI) calculations were carried out for the potential energy surfaces of ground and excited state DMABN. The potential surface of second excited S2 state was represented by a superposition of three diabatic states, one is of the ion pair type and the other two of the neutral ones, to facilitate the calculations in a polar solution. The intermolecular pair potentials between DMABN and H2O were developed with the aid of electron distributions in DMABN obtained from ab initio calculations. These potential functions were applied to determine the geometries of DMABN–H2O complex and the results were compared with the available experimental data. Monte Carlo simulation calculations were further performed for the aqueous solution of DMABN. The potentials of mean force for the torsional angle of dimethylamino group revealed that the S2 state potential profile is remarkably altered due to the solvation and the twisted intramolecular CT state becomes a stable point on the surface while this point corresponds to the top of potential barrier in the gas phase. The origin of broad emission band at a longer wavelength region observed in the experiments were discussed on the basis of present calculations. In order to elucidate the mechanism of CT state formation, the reaction free energy surfaces were constructed as the function of solvation coordinate and amino torsional angle. The results obtained here were that: (a) the shape of free energy curve of S2 state is far from a parabolic form along the solvation coordinate while the S1 state curve is nearly parabolic; and (b) the torsional coordinate is required to undergo a deformation to reach the transition state region of CT state formation reaction.
Time frame viewpoint of resonance Raman enhancement of a non-totally symmetric vibration
Authors: Lee Soo-Y.
Published: July 2002

The time frame approach is presented to describe the resonance Raman enhancement of a non-totally symmetric vibration due to vibronic coupling between two nearby electronic states. It is shown that in contrast to the resonance Raman enhancement of a totally symmetric vibration, the resonant contribution to the Raman scattering tensor for a non-totally symmetric vibration arises from the electronic state that is vibronically coupled to the electronic state first reached by the incident light. The second-order differencing method of Kosloff is used to numerically solve the coupled time-dependent Schrödinger equation in an application to pyrazine. In the case where the vibronically coupled states have the same oscillator frequency, it is shown that a unitary transformation of the wave packets leads to an immediate picture of the adiabatic states, as well as provide a qualitative explanation of various factors on the Raman intensity.
van der Waals vibrational dependence in the vibrational predissociation dynamics of OH–Ar
Authors: Berry Mary T. , Brustein Mitchell R. , Lester Marsha I.
Published: July 2002

The OH–Ar vibrational predissociation lifetimes and OH product rotational state distributions are shown to change with van der Waals (vdW) state selection within the manifold of OH–Ar vibrational states correlating with OH A2Σ+(v′=1)+Ar(1S0). Excitations to pure vdW stretching levels result in similar product state distributions, but predissociation lifetimes that vary from 30±8 ps at v′vdW=0 to greater than 150 ps at vvdW=6. Excitations to assigned vdW bend-stretch combination bands result in product state distributions which differ from those observed after excitation of the pure vdW stretch and those differences are attributed to the form of the bending wave function. Rotational constants and band positions for OH–Ar features in the OH A2Σ+–X2Π3/2 0–0, 1–0, 1–1, 2–1, 1–2, and 2–2 regions are also presented. The spectroscopic analysis reveals details about the radial portion of the intermolecular potential between Ar (1S0) and hydroxyl radicals in the ground X2Π3/2 and excited A2Σ+ states. Vibrational excitation of the OH moiety induces measurable perturbations in the interaction potentials along the OH–Ar vdW stretching coordinate for both electronic states. These changes are reflected in the vibrational predissociation rates.
A coupled cluster calculation of the quadrupole polarizability of CO
Authors: Maroulis George , Thakkar Ajit J.
Published: July 2002

The quadrupole moment (θzz) and quadrupole polarizability C tensor are calculated for the ground state of carbon monoxide. The results are based on finite-field energies computed using the coupled-cluster double excitation model (commonly known as CCD), corrected by fourth-order contributions from single and triple excitations computed with CCD amplitudes. The final values are θzz=−1.50 ea20, Cxx,xx =25.98 e2a40E−1h, Czz,zz =47.01 e2a40E−1h and C̄=56.03 e2a40E−1h.
Effective temperature and relaxation in complex systems
Authors: Rajagopal A. K. , Ngai K. L. , Teitler S.
Published: July 2002

A homogeneous complex system, i.e., one with interacting modes of the same type, that is in thermodynamic equilibrium is described formally in terms of effective independent modes. This formal description causes an apparent increase in system entropy. It is argued that this apparent increase in entropy is accompanied by the introduction of an effective temperature T* that is lower than the actual temperature T of the complex system; T*/T=f, 0<f≤1 with equality when complexity (interaction among modes) is negligible. The concept of effective temperature is considered in the context of the phenomenology of the coupling models for relaxation in complex systems with Arrhenius behavior. This allows f to be identified with 1-n where n is a key parameter in coupling model descriptions of relaxation dynamics for such systems. One thereby obtains a relation between n and the interactions among modes of a homogeneous complex system in equilibrium.
Combined infrared and ultraviolet-visible spectroscopy matrix-isolated carbon vapor
Authors: Kurtz Joe , Huffman Donald R.
Published: July 2002

Infrared and UV-visible absorption spectra have been measured on the same sample of matrix-isolated carbon vapor in order to establish correlations between absorption intensities of vibrational and electronic transitions as a function of sample annealing. A high degree of correlation has been found between the IR feature at 1998 cm−1 recently assigned to C8 and a UV absorption feature at about 3100 Å. Thus, for the first time, direct evidence is given for the assignment of one of the unknown UV-visible features of the long-studied matrix-isolated carbon vapor spectrum.
The morphology of symmetric diblock copolymers as revealed by neutron reflectivity
Authors: Anastasiadis Spiros H. , Russell Thomas P. , Satija Sushil K. , Majkrzak Charles F.
Published: July 2002

The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene (PS), and polymethylmethacrylate (PMMA), as a function of the total molecular weight of the copolymer where either block is perdeuterated. It is shown that the hyperbolic tangent function, as opposed to a linear or cosine-squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains. The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50±3 Å. This interface is also found to be identical to that between PS and PMMA, homopolymers. However, using measured values of the Flory–Huggins interaction parameter for PS and PMMA, current theoretical treatments cannot describe the observed widths of the interface.
Extended adiabatic formalism for computing thermodynamic properties of a quantum system coupled to a nonadiabatic bosonic bath
Authors: Coalson Rob D.
Published: July 2002

The solution procedure for computing properties of a quantum system coupled to an environment of harmonic oscillators in the adiabatic (low oscillator frequency) limit is extended into a general formalism capable of treating nonadiabatic effects. Starting from a path integral representation of the quantum propagator, the standard sum over intermediate configurations of the system (which is represented via a discrete set of base states) is replaced by integrations over continuous Gaussian auxiliary variables. In the adiabatic limit only one auxiliary variable is needed; more variables are required as the nonadiabaticity of the oscillator bath increases. We demonstrate numerically that large nonadiabatic effects can be computed with relatively few auxiliary variables. In particular solvation energies and localization probabilities calculated via our Extended Adiabatic prescription for a strongly nonadiabatic (multimode) ‘‘ohmic’’ bath are compared to results obtained via the effective adiabatic approximation method of Carmeli and Chandler [J. Chem. Phys. 82, 3400 (1985)]. Complete agreement is found. Advantages of the extended adiabatic method for more complicated applications are discussed.
Sensitivity of elastic gas–surface scattering to the potential: A functional sensitivity approach based on wave packet dynamics
Authors: Viswanathan R. , Shi S. , Vilallonga E. , Rabitz H.
Published: July 2002

Functional sensitivity analysis is used to study the effect of potential structure upon the elastic scattering of He atoms from a one-dimensional surface. The calculations are implemented by computing the total scattering wave functions from a wave packet calculation by a Mo/ller wave operator method. The functional sensitivities of the various diffraction probabilities for several angles of incidence and surface corrugation are studied. The method is extended to examine the role of potential structure for a surface with adsorbed impurities. It was observed that the various diffraction processes draw from local regions of the potential in very different ways. At high angles of incidence for back scattering and particularly for strong surface corrugation, the large protruding portions of the surface cast a ‘‘shadow’’ of lower dynamical sensitivity. Results of this type should ultimately be insightful for the inversion of experimental data to obtain the interaction potential.
Intramolecular dynamics of collisionally excited metal⋅⋅⋅ligand complexes in the energy localization range
Authors: Shin H. K.
Published: July 2002

We report trajectory calculations on intramolecular energy flow and bond dissociation of a model system OC⋅⋅⋅Pt⋅⋅⋅CO in a collision energy range, where the energy transfer to the complex localizes in one of the two Pt⋅⋅⋅CO bonds. The collision model is an Ar atom incident on the left-hand-side terminal atom of the complex containing a vibrationally excited ligand (OC*⋅⋅⋅Pt⋅⋅⋅CO or OC⋅⋅⋅Pt⋅⋅⋅CO*) in a collinear configuration. In this energy range, essentially all of the energy transferred to the complex localizes in the right-hand-side Pt⋅⋅⋅CO bond for a time corresponding to many vibrational periods before dissociation or intramolecular energy flow occurs. The existence of this energy localization time zone and its effect on intramolecular dynamics are discussed in detail at various collision energies. Energy localization leads to long delay for bond dissociation and strong dependence of bond dissociation on the initial excitation site. Time delay for the dissociation of the left-hand-side Pt⋅⋅⋅CO bond is particularly long, and this inefficient dissociation process is discussed in terms of intramolecular recrossing of the vibrational energy across the heavy mass barrier. At the threshold, dissociation always occurs at the side where the excited ligand is present: Ar+OC*⋅⋅⋅Pt⋅⋅⋅CO→Ar+OC*+Pt⋅⋅⋅CO or Ar+OC⋅⋅⋅Pt⋅⋅⋅CO* →Ar+OC⋅⋅⋅Pt+CO*.
Theoretical study of the nitrogen-atom hyperfine coupling constant. II
Authors: Bauschlicher Charles W.
Published: July 2002

The N hyperfine coupling constant has been computed using multireference configuration interaction (MRCI) and averaged coupled pair functional (ACPF) treatments in very large one-particle basis sets. Unlike previous calculations, no selection of configurations has been performed. The calculations again illustrate the difficulty of computing the N hyperfine coupling constant accurately. The best MRCI result of 10.7 MHz is larger than the accurate experimental value of 10.4509 MHz, while the best ACPF result (10.3 MHz) is smaller than experiment. The difference between this work and previous calculations is discussed.
An interpretation of molecular fragmentation dynamics in terms of the resonance spectrum
Authors: Jolicard G. , Atabek O.
Published: July 2002

Quasidegenerate perturbation theory using a Bloch wave operator formalism is applied to the calculation of the rich resonance spectrum of a model polyatomic molecule undergoing photodissociation. The sharp structures (peaks and dips patterns) of the total dissociation rate as well as the partial probabilities to obtain the fragments at given internal states for various incident photon energies, are interpreted in terms of tunneling, shape or Feshbach resonances.
Chemical reaction dynamics: Combination of two models
Authors: Evans Glenn T. , van Kleef Eddy , Stolte Steven
Published: July 2002

The methods of classical kinetic theory for rigid nonspherical bodies are used to calculate bimolecular reaction cross sections for gas phase atom–diatom (A+BC→AB+C) reactions. The pre-collisional conditions for reaction are determined by the angle-dependent line of normals (ADLN) model whereas the post-collisional properties, such as the angular momentum and recoil energy, of the reaction products are determined by the direct interaction with product repulsion (DIPR) model. Derived cross sections are functions of the pre-collisional orientation of the diatom, the initial and final relative velocity, and the rotational angular momentum of the AB product. The hybrid, ADLN–DIPR model is applied to the analysis of the cross sections for the Ba+N2O reaction. Interpretation of the cross-section data requires both the ADLN and DIPR models and no collection of reasonable parameters in either the ADLN or the DIPR models individually can simply account for the measured cross sections.
Some results for hard spheres at a hard wall using the inhomogeneous Percus–Yevick equation and a modified Lovett–Mou–Buff–Wertheim equation
Authors: Plischke Michael , Henderson Douglas
Published: July 2002

Some results are reported for the contact value of the density profile of hard spheres near a hard wall using the inhomogeneous Percus–Yevick equation for pair correlation functions and the Lovett–Mou–Buff–Wertheim (LMBW) equation to relate the pair functions to the singlet density profile. It is found that improved results are obtained if the LMBW is modified as suggested by Quintana et al.
Modeling complex intramolecular processes using constrained molecular dynamics
Authors: Lambrakos S. G. , Peyrard M.
Published: July 2002

A phenomenological approach based on constrained molecular dynamics for modeling complex intramolecular processes on the characteristic time scale of intermolecular interactions is presented. One feature of this approach is that it allows modeling of energy transfer between intermolecular degrees of freedom and internal molecular modes on a longer time scale than is possible using more detailed methods. In addition, by using time-dependent or conditional constraints, this approach can be extended for the modeling of molecular dissociation and subsequent energy release in energetic materials. In this paper this approach is used to construct a model for energy transfer between lattice modes and the internal modes of a large molecular group. The mathematical properties of constrained molecular dynamics that are important for accurately modeling internal processes on an intermolecular time scale are examined. Results of simulations of shocks propagating through a reactive crystal using the present model demonstrate the utility of constrained molecular dynamics for modeling energy transfer in complex systems are presented.
Electronic structure of vinylthio radical (CH2CHS): A theoretical study
Authors: Yamaguchi Makoto , Momose Takamasa , Shida Tadamasa
Published: July 2002

A predictive ab initio MO calculation was performed for the thus far undetected sulfur analog of the vinoxy radical. Multiconfigurational self-consistent field (MCSCF) and symmetry adapted cluster expansion–configuration interaction (SAC–CI) wave functions were used to describe the electronic structure of the radical. The symmetry species of the ground state was found to be 2A″ as in the case of the vinoxy radical. The crucial difference between the two radicals is that the thio-radical is more or less characterized as a sulfur-centered radical, CH2=CH–S⋅ whereas the oxy-radical is regarded as a carbon centered ⋅CH2–CH=O in their ground state. Excitation energies, normal modes, Franck–Condon factors, hyperfine coupling parameters, and electron affinity were predicted.
Analysis of the absorption spectrum of the N
V1 transition of cyclopentadiene
Authors: Sabljić A. , McDiarmid R.
Published: July 2002

The ultraviolet absorption spectra of cyclopentadiene and cyclopentadiene-d6 were measured in the NV1 transition region. The spectra were analyzed to determine the excited state properties. The prominent vibrational progression was identified as a progression in the (predominantly) a1 C=C stretching vibration. A second, unidentified, vibrational interval was observed in the cyclopentadiene spectrum and inferred to exist in the spectrum of cyclopentadiene-d6. The excited state potential surface was deduced to be displaced 0.2 Å from the lower, predominantly along the C=C normal coordinate. The excited state lifetime was determined to be 37 fs at the origin and decrease at a rate inversely related to the excited state vibrational quantum number.
Intramolecular and solvent dynamics in femtosecond pump–probe spectroscopy
Authors: Bosma Wayne B. , Yan Yi Jing , Mukamel Shaul
Published: July 2002

A density-matrix theory is applied to the calculation of femtosecond pump–probe experiments on solvated polyatomic molecules. Specific calculations are performed for a two-mode system, in which an optically active solute mode is represented by a pair of harmonic oscillators in the state representation, and the solute is modeled as an overdamped harmonic mode, using the Wigner phase space representation. No restrictions are placed on the relative time scales of the solute vibrational period, the solvent relaxation time scale and the pump pulse duration. The calculations demonstrate the physical phenomena observed in recent experiments on solvated dyes, both in the hole-burning limit, where the pump pulse is long compared to the relevant solute and solvent time scales, and the impulsive limit, for which the pump is short compared to the solute vibrational period.
Solvent relaxation and electron back transfer following photoinduced electron transfer in an ensemble of randomly distributed donors and acceptors
Authors: Lin Y. , Dorfman R. C. , Fayer M. D.
Published: July 2002

The role of solvent relaxation in electron back transfer following electron transfer from an optically excited donor to randomly distributed acceptors is treated theoretically. The solvent dynamics are included by using a time dependent electron back transfer rate function, Keff(R,t). The solvent relaxation is parameterized by τr, the relaxation time, D, the solvent energy diffusion constant, and Δq, the potential barrier height difference between the nonequilibrium solvent state formed upon ion creation and the relaxed solvent state. The expression for the ensemble averaged donor cation state population probability, 〈Pct(t)〉, as a function of these solvent relaxation parameters is derived. Numerical calculations are presented. Relationships among 〈Pct(t)〉, the intermolecular interaction parameters, and solvent relaxation parameters provide detailed insights into the distance and time dependence of the flow of electron probability in an ensemble of donors and acceptors. The theoretical expressions can be used to calculate experimental observables such as the transient grating signal.
A new method for determining water–cation short-range potentials for modeling water in aluminosilicate lattices
Authors: Aloisi G. , Barnes P. , Catlow C. R. A. , Jackson R. A. , Richards A. J.
Published: July 2002

The problem of deriving realistic interaction potentials for modeling hydrated sites within aluminosilicate structures has been considered. It is argued that since a variety of cationic sites is involved, the effect of the lattice must be taken into account explicitly, both in the derivation and in the use of H2O–cation potentials. A new method for doing this is described with examples for H2O–Si4+ and H2O–Al3+ in quartz and/or zeolite-A. A more detailed analysis for H2O–Ca2+ in zeolite-A and cement structures is presented.
Identification of a defect chain motion in n-alkanes by means of nuclear magnetic resonance spin–lattice relaxation time measurements
Authors: Basson Ilsa , Reynhardt Eduard C.
Published: July 2002

Proton spin–lattice relaxation time measurements in the rotating frame as a function of inverse temperature have revealed shallow minima not previously observed for a series of n-alkanes ranging from n-C22H46 to n-C41H84. These minima cannot be interpreted in terms of reorientations of methyl groups or entire chains. The most likely reorientational model is one in which a thermally activated trans–gauche defect motion modulates the internuclear dipolar interactions. The results suggest that the gauche defect equilibrium orientation has a higher energy than that of the ordered all-trans equilibrium orientation. Activation energies for the motion as well as energy differences between the two orientations have been extracted from the results for these n-alkanes.
Infrared spectroscopy and Mie scattering of acetylene aerosols formed in a low temperature diffusion cell
Authors: Dunder T. , Miller R. E.
Published: July 2002

A method is described for forming and spectroscopically characterizing cryogenic aerosols formed in a low temperature gas cell. By adjusting the cell pressure, gas composition and flow rate, the size distribution of aerosol particles can be varied over a wide range. The combination of pressure and flow rate determine the residence time of the aerosols in the cell and hence the time available for the particles to grow. FTIR spectroscopy, over the range from 600 to 6000 cm−1, is used to characterize the aerosols. The particle size distribution can be varied so that, at one extreme, the spectra show only absorption features associated with the infrared active vibrational bands and, at the other, they display both absorption and Mie scattering. In the latter case, Mie scattering theory is used to obtain semiquantitative aerosol size distributions, which can be understood in terms of the interplay between nucleation and condensation. In the case of acetylene aerosols, the infrared spectra suggest that the particles exist in the high temperature cubic phase of the solid.
Calculations of vibrational state mixing leading to intramolecular vibrational energy redistribution in S1 anthracene: Comparison with quantum beat experiments
Authors: Bullock Warren J. , Adams Dieter K. , Lawrance Warren D.
Published: July 2002

A simple model for the anharmonic coupling constants has been used to calculate vibrational state mixing in S1 anthracene. The aim of the calculations is to provide insight into the vibrational state mixing responsible for intramolecular vibrational energy redistribution (IVR). The calculations include all vibrations of the appropriate symmetry within a 100 cm−1 interval centered about the state of interest. The calculations are compared with experimental measurements of quantum beats in S1 anthracene [P. M. Felker and A. H. Zewail, J. Chem. Phys. 82, 2975 (1985)]. These experiments involved an investigation of rotational effects that established the coupling to be anharmonic in origin. We show that in order for the experimental data to be explained by anharmonic coupling alone, the high-order anharmonic terms must be reasonably large. This implies that the anharmonic expansion converges quite slowly for EVIB≲2000 cm−1 in anthracene, in contrast with spectroscopic data for small molecules. Anthracene does not appear to be unique with regard to its IVR behavior, and consequently we suggest that slow convergence of the anharmonic expansion will prove to be the norm for large molecules. As a consequence of the slow convergence, direct coupling through high-order anharmonic terms is an important coupling mechanism. The model used to determine the anharmonic coupling constants is not specific to anthracene, and it is anticipated that it will be possible to predict vibrational state mixing in other molecules using the parameters deduced for anthracene.
Light scattering with evanescent waves: Intermolecular interference and the structure factor for an ideal flexible chain at an interacting interface
Authors: Gao Jun , Freed Karl F. , Rice Stuart A.
Published: July 2002

We report the evaluation of the structure factor for polymer chains near an interface when illuminated with evanescent light. The evanescent light is generated when there is total internal reflection of light at an interface between two dielectric media. We begin with a general virial expansion for the scattering intensity in powers of the polymer surface concentration to define the nature of the extrapolation to the infinite dilution limit, and to thereby extract the structure factor in that limit. The scattering per unit surface area is a function of the chain surface concentration when the polymers are attached to the surface, and a function of the bulk polymer concentration when the chains are distributed throughout the solution. The single chain scattering function is evaluated for Gaussian chains interacting with an impenetrable interface and either terminally anchored to the surface or free to move throughout the solution. The structure factors are obtained for the independent in-plane and out-of-plane scattering geometries, which provide information, respectively, about the chain dimensions parallel and perpendicular to the interface. The dependence of the evanescent wave scattering on such parameters as the scattering angle and the polymer-surface interaction parameter is explained on the basis of the exponential decay of the evanescent wave intensity away from the interface.
Diode laser probing of I*(2P1/2) Doppler profiles: Time evolution of a fast, anisotropic velocity distribution in a thermal bath
Authors: Cline Joseph I. , Taatjes Craig A. , Leone Stephen R.
Published: July 2002

The relaxation of a nonthermal translational population distribution of fast I*(2P1/2) atoms dilutely dispersed in a gaseous bath at thermal equilibrium is studied by time-resolved Doppler spectroscopy. The fast, anisotropic velocity distribution of I* atoms is produced by pulsed laser photolysis of n-perfluoropropyl iodide (n-C3F7I) at 266 nm. A frequency-narrowed, GaAsInP diode laser is tuned across the iodine (2P1/2,F=3←2P3/2,F=4) transition at 1315 nm to measure the Doppler gain profile of the I* photofragments. The velocity distribution is expressed as a separable product of a radial speed function and an angular function describing the anisotropy. The collision-induced time evolution of both the speed and anisotropy components of the nascent velocity population distribution relaxing to form a 300 K Maxwellian equilibrium distribution is determined. The thermalization dynamics of I* are studied for a heavy bath gas (n-C3F7I) and a light (He) bath gas. In the case of the heavy bath gas the anisotropy is removed by collisions 2.5 times faster than the speed is thermalized, while for the light bath gas the anisotropy and speed relaxation occur on the same time scale. The velocity and angular distributions of the I* photofragment from the 266 nm photolysis of n-C3F7I are also reported.
Statistical intramolecular vibrational relaxation and its hindrance: The fundamentals of (CH3)3C–C≡CH and (CH3)3Si–C≡CH
Authors: Lehmann K. K. , Pate B. H. , Scoles G.
Published: July 2002

We have measured the fundamental acetylenic C–H stretch near 3.0 μm of (CH3)3C–C≡CH and (CH3)3Si–C≡CH using an optothermal, molecular beam spectrometer. We find that the individual R(J) lines of the hydrocarbon are Lorentzian with a FWHM of 800 MHz indicating statistical intramolecular vibrational relaxation (IVR) with a 400 ps lifetime. The R(J) lines of the silicon compound are clearly asymmetric and, in addition, show a FWHM of about 150 MHz indicating a much longer (>2 ns) lifetime. The increase in IVR lifetime in the larger density of states molecule may be due to reduced kinetic coupling resulting from the heavier Si atom.
Analysis of vibronic-mode coupling in azulene by fully resonant coherent four wave mixing
Authors: Carlson Roger J. , Wright John C.
Published: July 2002

Fully resonant, three laser, coherent Stokes Raman scattering (CSRS) is used to probe the vibrational correlations and couplings associated with the S0−S1 transition of azulene in a low temperature mixed molecular crystal. Unlike incoherent resonant emission, CSRS is not complicated by extra resonances due to relaxed emission. The spectra obtained confirm most of the assignments of the S1 vibronic fundamentals made by incoherent spectroscopies. In addition, however, the vibronic counterpart of the vibrational fundamental at 900 cm−1 is correlated to a previously unobserved vibronic resonance at 842.3 cm−1. Evidence of matrix moderated coupling, including possible site splitting and Fermi resonances involving lattice phonons, is given for the 677, 825, 917, 941, and 900 cm−1 vibrational levels.
Nanosecond and microsecond study of probe reorientation in orthoterphenyl
Authors: Hyde Patrick D. , Evert Todd E. , Ediger M. D.
Published: July 2002

Time-resolved optical measurements of probe reorientation in orthoterphenyl have been performed. Rotational correlation functions have been measured from 100 ps to 1 ms for anthracene and anthanthrene, while correlation functions were observed over a narrower time window for 9-phenylanthracene and 9,10-diphenylanthracene. The rotational correlation time of anthracene follows the temperature dependence of η/T, indicating that anthracene rotation is well coupled to the dynamics responsible for the main glass transition. At temperatures above 30 °C, rotational correlation times depend upon probe size. In this region, orthoterphenyl behaves as a simple liquid. At temperatures near −5 °C, observed rotational correlation times are roughly independent of probe size. These results are consistent with the existence of domains of associated orthoterphenyl molecules. At lower temperatures, near and below the glass transition temperature, a partial reorientation of anthracene associated with the β process is observed. These results imply that the β process in orthoterphenyl involves the partial reorientation of most molecules rather than the total reorientation of a small fraction of the molecules.
Statics and dynamics of organic charge-transfer crystals: The orientational phase transition in A-TCNB
Authors: Brose Karl-Heinz , Luty Tadeusz , Eckhardt Craig J.
Published: July 2002

Crystal potential energy calculations and lattice dynamical calculations are presented for the high-temperature and low-temperature phases of the charge-transfer complex crystal of anthracene with 1,2,4,5-tetracyanobenzene using the method of atom–atom potentials. For several different sets of the ‘‘exp-6’’ potential parameters the crystal energy was minimized with respect to the lattice constants and orientations and positions of the molecules in the unit cell. These calculations show good agreement with the experimental values. The equilibrium crystal structures were used to calculate the lattice dynamics in the harmonic approximation for both crystal modifications. In the phonon dispersion curves lattice modes were discovered that have imaginary frequencies which may be attributed to instabilities of the crystal. Application is made to the orientational phase transition occurring in this crystal. The results suggest that the high-temperature phase is dynamically disordered and that the phase transition is predominantly two-dimensional in the (ab) plane of the crystal and is driven by a librational phonon mode with soft mode behavior. In addition, instabilities are indicated in the stacking direction of molecules. The variance of the results with the parametrization of the potential energy function is studied.
Rotational state dependence of external magnetic field effects on yield and decay of fluorescence of s-triazine vapor
Authors: Ohta Nobuhiro , Takemura Takeshi
Published: July 2002

Effects of external magnetic field (H) on intensity and decay of fluorescence of s-triazine vapor have been examined under collision-free conditions following excitation at the individual rotational lines belonging to the 610 or 620 absorption band of the S0→S1 transition. The fluorescence intensity is reduced by H and the value of the fluorescence lifetime is increased by H, as far as the slowly decaying portion is concerned. The efficiency of the magnetic quenching of fluorescence depends not only on the vibrational level, but also on the rotational level excited and a good correlation in rotational state dependence between the fluorescence lifetime at zero field and the efficiency of the magnetic quenching of fluorescence is found at 620. As J′ of the excited level increases, the lifetime of the slow fluorescence increases, while the magnetic quenching becomes more effective. It is shown that both external magnetic field and molecular rotation play a role to increase the level density of the triplet state coupled to S1.
Hard-sphere fluids inside spherical, hard pores. Grand canonical ensemble Monte Carlo calculations and integral equation approximations
Authors: Sloth Peter
Published: July 2002

Density profiles and partition coefficients are obtained for hard-sphere fluids inside hard, spherical pores of different sizes by grand canonical ensemble Monte Carlo calculations. The Monte Carlo results are compared to the results obtained by application of different kinds of integral equation approximations. Also, some exact, analytical results for the partition coefficients are given, which are valid in the case of (very) small pores or at low density, respectively.
Structures, barriers for rotation and inversion, vibrational frequencies, and thermodynamic functions of ethyl, α-fluoroethyl, and α,α-difluoroethyl radicals: An a
b i
n
i
t
o study
Authors: Chen Yonghua , Rauk Arvi , Tschuikow-Roux E.
Published: July 2002

The potential energy surfaces of CH3CHF and CH3CF2 radicals were studied by ab initio molecular orbital methods using the gaussian 86 system of programs at the UHF/6-31G* level of theory. CH3CHF has C1 symmetry and exists as a single pair of enantiomeric conformations. The difluoro species has only a single stable structure, of Cs symmetry. The barriers to rotation about the C–C bond, and the barriers hindering pyramidal inversion at the nonplanar radical center were located for each compound by analytical methods. The monofluoro and difluoro species had similar rotation barriers at the UMP2/6-31G*//6-31G* level, 1.68 and 2.26 kcal/mol, respectively, after inclusion of zero-point vibrational energy differences. The inversion barriers were substantially different, 0.54 and 10.45 kcal/mol, respectively. Vibrational frequencies, as well as moments of inertia for overall and internal rotations, are reported for each species and for the ethyl radical. Calculated heat capacities, entropies, enthalpies, and free energy functions are tabulated as a function of temperature. Use of several isodesmic or homodesmic reactions to obtain a value for the heat of formation of CH3CHF at 298 K is discussed. Good experimental values are available for ethyl and CH3CF2. Adopting reference values for ΔH0f,298 of 28.36, −17.3, and −72.3 kcal/mol for CH3CH2, CH3CHF, and CH3CF2, respectively, values for ΔH0f,T, ΔG0f,T and log10Kf for all three radicals are reported as a function of temperature in the range 0–1500 K. Comparison of the theoretical and experimental data for CH3CH2 suggests that the error introduced by use of harmonic HF/6-31G*//6-31G* frequencies is ±1 cal mol−1 K−1 for both heat capacity and entropy.
An a
b i
n
i
t
i
o study of the structure and vibrational spectra of allyl and 1,4-pentadienyl radicals
Authors: Szalay P. G. , Császár A. G. , Fogarasi G. , Karpfen A. , Lischka H.
Published: July 2002

MCSCF and MR-ACPF calculations on the structure and vibrational frequencies of allyl (2-propenyl) and 1,4-pentadienyl radicals are presented. Using different levels of theory the instability problem in allyl has been investigated in great detail. Based on these conclusions fully optimized structures and force constants were calculated at the three-orbital CAS-MCSCF and at the MR-ACPF level for allyl. To correct for systematic errors, an empirical scaling was also applied on the MCSCF force field. The scaled MCSCF and the unadjusted MR-ACPF force constants were used to calculate the vibrational frequencies. A reassignation of the experimental infrared spectrum is also given. Using the scale factors optimized for allyl the scaled MCSCF force field and vibrational frequencies of pentadienyl were also calculated.
Dynamical correlations in liquid hydrogen–sulphide
Authors: Ricci M. A. , Ruocco G. , Rocca D. , Vallauri R.
Published: July 2002

The analysis of the density fluctuations in liquid H2S has been performed by applying a recently proposed theoretical method. Computer simulation results for the correlation functions and the corresponding spectra are reported and compared with the theoretical predictions. A discussion of the differences between the results obtained for H2S and H2O, at comparable thermodynamic states, is carried out. The nature of the interaction potential is invoked to explain the different behavior of the two systems, and the strong anisotropy of the H2O potential which mimics the effect of hydrogen bond is suggested to be the origin of the collective behavior of the density fluctuations propagating through the hydrogen atoms.
Simultaneous measurements of refractive index and density in a microemulsion near a critical end point
Authors: Rebbouh N. , Beysens D. , Lalanne J. R.
Published: July 2002

The behavior of the density and the refractive index of a microemulsion of sodium dodecyl sulfate, water, n-pentanol, and n-dodecane is investigated near its critical end point. Measurements are made simultaneously and both are to an accuracy of a few ppm. In contrast to the density data which can be fitted to a regular, linear behavior in the whole temperature range (28.3–30.5 °C), the refractive index exhibits a pronounced anomaly near the critical point. The temperature dependence of such an anomaly cannot be described by a power law behavior. However, it can be understood as a chemical change connected to morphological modifications in the dispersed phase, a phenomenon connected to the close vicinity of sponge-like phases. This approach accounts well for the data.
Photodissociation of iodine complexes in relation to the cage effect
Authors: Philippoz Jean-Michel , Monot René , van den Bergh Hubert
Published: July 2002

The photodissociation I2M+hν→I2+M was studied for M=C2H6 and M=(C2H5)2O with excitation above the dissociation limit of I2 B state. Vibrational population distributions of the I2(B) product molecules are reported. The results are compared to previous studies of the dissociation of similar iodine complexes and to studies of the cage effect in high density gases and liquids.
Ultraviolet-light-induced oxide formation on GaAs surfaces
Authors: Lu Z. , Schmidt M. T. , Podlesnik D. V. , Yu C. F. , Osgood R. M.
Published: July 2002

We present a study of deep-ultraviolet-light-enhanced (4.1<hν<5.1 eV) oxygen reactions on GaAs from submonolayer to several monolayers coverage. The reaction is nonthermal and does not involve gas-phase excitation or dissociation of O2. Our experiments show a distinct wavelength and coverage dependence for the photoenhancement. X-ray photoelectron spectroscopy has been used to examine the chemical nature of the oxygen adsorbate and the GaAs oxides in order to find intermediate reaction species and evidence of the reaction pathways. The roles of photons and photogenerated carriers in the reaction enhancement mechanism are discussed. The results indicate that a mechanism based on photoemission of electrons into the growing oxide film is most in accord with the experimental observations. Such electron emission would increase the field-driven transport of oxygen to the GaAs interface.
The Newton variational functional for the log-derivative matrix: Use of the reference energy Green’s function in an exchange problem
Authors: Ramachandran B. , D’Mello Michael , Wyatt Robert E.
Published: July 2002

The Newton Variational Principle for the log-derivative matrix (the Y-NVP) is studied in the context of a collinear exchange problem. In contrast to the integral equation methods that calculate the K or the T matrices directly, the matrix elements of the log-derivative Newton functional can be made independent of the scattering energy. This promises considerable savings in computational effort when state to state transition probabilities are calculated at several energies, since the matrix elements of the functional need be calculated only once. Green’s functions defined with respect to a reference energy, called the reference energy Green’s functions (or the REGFs), play a central role in the Y-NVP functional. The REGFs may be defined with or without reference to asymptotic channel energies. If channel dependent REGFs are used, the Y-NVP converges at the same rate as the GNVP for the K or T matrices, when the scattering energy is the same as the reference energy. On the other hand, channel independent REGFs permit even further reductions in computational effort.

We use both types of REGFs in the present paper, and compare the rates of convergence. These comparisons show that the convergence rate of the method is not significantly altered by the type of REGF used. Further, we show that the Y-NVP is able to achieve rapid convergence of reactive transition probabilities over a large range of scattering energies, even when scattering resonances are present. An analysis of the computational effort required for each part of the calculation leads to the conclusion that a Y-NVP calculation using a channel independent REGF requires essentially only the same amount of computer time as a log-derivative Kohn (Y-KVP) calculation, while, presumably, offering faster convergence.
Resonant vibrational excitation of methylamine by low energy electron impact
Authors: Motte-Tollet F. , Hubin-Franskin M.-J. , Collin J. E.
Published: July 2002

The study of the excitation functions of the NH2 and CH3 stretching modes of methylamine in the 4–12 eV electron impact energy range shows evidence of a broad shape resonance built on the electronic ground state and centered at 7.5 eV. This resonance is formed by the trapping of the incident electron in the second unoccupied molecular orbital and is of A″ symmetry. The elastic and inelastic angular differential cross sections measured in the 10°–90° range suggest a dominant p character. A second shape resonance located at higher energy contributes mainly to the enhancement of the CH3 deformation modes and is very likely of A′ symmetry. A comparison is made between the resonances observed in NH3 and CH3NH2. The effect of the methyl substitution is briefly discussed.
Sensitivity of the chlorite–thiosulfate system: A stochastic approach
Authors: Peeters P. , Baras F. , Nicolis G.
Published: July 2002

The chlorite–thiosulfate reaction provides us with an example of isothermal chemical reaction giving rise to explosive-like behavior in batch condition. Experimental results show that this phenomenon is characterized by high sensitivity, reflected in particular by the fact that reaction times occur randomly despite the care taken to prepare initially the system in identical states. In the present paper a reaction scheme reproducing explosive dynamics analogous to the experimental data is proposed and the effects of fluctuations of different origins on the reaction time are examined. As it turns out, both internal and external fluctuations induce a significant broadening of the probability distribution of explosion times as well as an asymmetry in its shape. The effect of random initial conditions is also analyzed and shown to affect deeply the explosive behavior.
Infrared–microwave double resonance spectroscopy of molecular ions: HN+2
Authors: Ho W. C. , Pursell C. J. , Weliky D. P. , Takagi K. , Oka T.
Published: July 2002

Infrared–microwave double resonance spectroscopy has been used to study the simple molecular ion HN+2. This is the first application of this powerful method to a molecular ion. The HN+2 ion was produced in a hollow cathode discharge cell, which served two purposes. First, the hollow cathode discharge is capable of producing relatively large concentrations of ions at the low pressures required for saturation. Also, the hollow cathode serves as a waveguide for the propagation of the microwave radiation. A color center laser was used as the infrared source. Its high power was essential in this work for saturation of the infrared transitions. The HN+2 ion was chosen as a test case for double resonance because of the availability of detailed spectroscopic information and its ease of production. We have observed the pure rotational transition J=1←0 in both the ground and ν1 states using the ν1 infrared fundamental. The determined molecular constants are eqQ=−5.88(10) MHz and B=46 586.895(18) MHz for the ground state, and eqQ=−5.71(17) MHz and B=46 208.986(27) MHz for the ν1 state. The high sensitivity of the double resonance technique has also allowed us to study the direct l-type doubling transitions in the ν2 and ν1+ν2 states using the ν1+ν2←ν2 infrared hot band. To the best of our knowledge, this is the first centimeter wave spectrum of a molecular ion. The determined molecular constants are q=254.966(11) MHz and qJ =−2.83(17) kHz for the ν2 state, and q=258.912(32) MHz and qJ =−3.15(65) kHz for the ν1+ν2 state.
The emission spectrum of helium hydride. IV. Bands near 4100 and 4600 Å
Authors: Ketterle Wolfgang
Published: July 2002

The emission bands of helium hydride near 4100 and 4600 Å were analyzed for 4HeH and 3HeD. They are assigned to the emission of the three coupled states 4p 2Π, 4d 2Π, and 4d 2Δ to the A 2Σ+ (4100 Å) and B 2Π (4600 Å) states. These bands are analogous to the emission of n=3 states near 5300 and 6000 Å analyzed previously. Only the A″ parity components of n=4 states were observed, the states of A′ parity decay predominantly by predissociation. Several weak lines around 4100 Å were assigned to the transition 5d→B 2Π.
The emission spectrum of helium hydride. V. Characterization of low-lying Rydberg states (n=2–5)
Authors: Ketterle Wolfgang
Published: July 2002

The low-lying Rydberg states of helium hydride (n=2–5) are characterized. Absolute binding energies and quantum defects are derived from spectroscopic data. Isotope shifts and scaling of molecular constants with principal quantum number n are discussed. Comparison with available ab initio calculations shows good agreement. For nd states the transition from Hund’s case (b) to (d) is treated. Their splitting is mainly caused by the quadrupole moment of HeH+. Some so far unassigned groups of spectral lines were shown to involve n=2–4 Rydberg states with vibrational excitation.