bluuebunny/crossref_metadata_2025 huggingface dataset:
A fitting law for rotational transfer rates: An angular momentum model with predictive power
Authors: Osborne Mark A. , McCaffery Anthony J.
Published: July 2002

We have formulated a law for state-to-state rotational transfer (RT) in diatomic molecules based on the angular momentum (AM) theory proposed by McCaffery et al. [J. Chem. Phys. 98, 4586 (1993)]. In this, the probability of angular momentum change in the rotor is calculated by assuming the dominant process to be the conversion of linear to angular momentum at the repulsive wall of the intermolecular potential. The result is a very simple expression containing three variable parameters, each of which has physical significance in the context of the model. Fits to known RT data are very good and suggest strongly that linear to angular momentum change is indeed the controlling process in RT. The parameters of the fit are sufficiently available to give the model predictive power. Using this formulation, RT probabilities may be calculated for an unknown system with little more than the atomic masses, bond length, and velocity distribution. We feel that this represents an important step in the development of a simple physical picture of the RT process.
Feature state assignment and hierarchical and statistical analyses of vibronic and rovibronic level structure of NO2 in the 16 300–25 130 cm−1 region
Authors: Miyawaki Jun , Yamanouchi Kaoru , Tsuchiya Soji
Published: July 2002

The laser induced fluorescence (LIF) spectrum of jet-cooled NO2 in the energy range from 16 300 cm−1 up to the dissociation limit at 25 130.6 cm−1 was measured with an energy resolution of 0.4 cm−1, and vibronic interaction was discussed through a feature state assignment and a hierarchical analysis. By convoluting the spectrum, the feature states representing bending excited levels in the 2B2 state were identified. The hierarchical level structure just below the dissociation limit was interpreted in terms of a stepwise intramolecular vibronic energy redistribution (IVR) caused by the anharmonic couplings within the 2B2 state followed by the vibronic and rovibronic couplings between the 2B2 and 2A1 states. In a higher resolution (∼0.04 cm−1) measurement the transitions to the rovibronic eigenstates just (0–55 cm−1) below the dissociation limit were resolved. The observed vibronic level density having b2 symmetry, ρvib(b2)=1.6/cm−1, in this energy region is derived from the observed peak density, ρpeak=9.6/cm−1, by assuming a strong K mixing. The observed large peak density was ascribed to the large anharmonisity of the ground state potential energy surface near the dissociation limit. The statistical analyses applied to this eigenstate spectrum showed an extremely strong correlation among these eigenstates, indicating the complete IVR. The present results of the statistical analyses near the dissociation limit support the statistical behavior in the dissociation dynamics just above the dissociation limit investigated in our previous paper [J. Chem. Phys. 99, 254 (1993)].
The decay of triplet pyrazine and pyrazine-D4 in supersonic jets: Isotope effects
Authors: Becker Israela , Cheshnovsky Ori
Published: July 2002

We have measured the intersystem crossing (ISC) rates of optically excited triplet pyrazine-d4 in supersonic expansion. ISC rates ranging from 3.3×102 to 1.3×103 were found in the 1154 cm−1 range of excess vibrational energy above the T1 origin. These rates were substantially lower than the ISC rate of pyrazine. Our measurements were accompanied by model calculations of the ISC rates of pyrazine and pyrazine-d4. According to this model, certain vibrational modes, which undergo large frequency reduction in the excited state, regulate the strong vibrational energy dependence of the T1→S0 ISC of pyrazine. The large frequency changes result from the interaction of the near lying 3nπ* and 3ππ* states (the ‘‘proximity effect’’). These calculations account quantitatively for the excess vibrational energy dependence of the ISC rates in pyrazine, as well as for the isotopic substitutional effect.
Study of low-lying electronic states of ozone by anion photoelectron spectroscopy of O−3
Authors: Arnold Don W. , Xu Cangshan , Kim Eun H. , Neumark Daniel M.
Published: July 2002

The low-lying electronic states of ozone are studied using anion photoelectron spectroscopy of O−3. The spectra show photodetachment transitions from O−3 to the X̃ 1A1 ground state and to the five lowest lying electronic states of the ozone molecule, namely the 3A2, 3B2, 1A2, 3B1, and 1B1 states. The geometry of the ozonide anion determined from a Franck–Condon analysis of the O3 X 1A1 ground state spectrum agrees reasonably well with previous work. The excited state spectra are dominated by bending vibrational progressions which, for some states, extend well above the dissociation asymptote without noticeable lifetime broadening effects. Preliminary assignments are based upon photoelectron angular distributions and comparison with ab initio calculations. None of the excited states observed lies below the ground state dissociation limit of O3 as suggested by previous experimental and theoretical results.
State-to-state relaxation of highly vibrationally excited acetylene by argon
Authors: Tobiason J. D. , Fritz M. D. , Crim F. F.
Published: July 2002

Vibrational overtone excitation followed by laser-induced fluorescence (LIF) interrogation determines state-to-state rotational and vibrational relaxation rates of highly energetic acetylene molecules by argon. We initially excite a single rotational state of the vibration 3ν3 (three quanta of C–H stretch, Evib=9640 cm−1) and detect the state (3ν3, Jf=20) in rotational relaxation measurements or the state (ν1+ν2+ν3+2ν4, l=0, Jf=4) in vibrational relaxation measurements. The data, which we acquire under single-collision conditions, show that argon rotationally relaxes acetylene almost half as efficiently as acetylene itself but is relatively inefficient in vibrational relaxation.
Matrix isolation study of the interaction of excited neon atoms with CF4. Infrared spectra of CF+3 and CF−3
Authors: Forney Daniel , Jacox Marilyn E. , Irikura Karl K.
Published: July 2002

When a Ne:CF4 sample is codeposited at approximately 5 K with a beam of neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting sample shows prominent absorptions of CF2 and CF3, as well as a complex absorption pattern between 1650 and 1670 cm−1. Earlier reports of the infrared spectrum of CF+3 produced from various CF3X species and trapped in solid argon are consistent with the assignment of this group of absorptions to ν3 of CF+3. The prediction of those studies that the ν1+ν4 combination band of 12CF+3 should lie near 1625 cm−1 is confirmed. Supplementary observations on Ne:HCF3 and Ne:DCF3 samples support these assignments, as well as that of the ν2(a2″) fundamental of CF+3 at 798.1 cm−1. Ab initio calculations of the structure and vibrational fundamentals of ground-state CF−3 are consistent with the tentative assignment of two infrared absorptions to that species.
A new direct ab initio dynamics method for calculating thermal rate constants from density functional theory
Authors: Truong Thanh N. , Duncan Wendell
Published: July 2002

We present a new direct ab initio dynamics methodology for calculating thermal rate constants from density functional theory (DFT). Dynamical theory is based on a full variational transition state theory plus multidimensional semiclassical tunneling approximations. We have applied this approach to the CH3+H2→CH4+H abstraction reaction using the BH&H-LYP method which is the combination of the hybrid Becke’s half-and-half (BH&H) method for nonlocal exchange and Lee–Yang–Parr (LYP) functional for nonlocal correlation. The 6-311G(d,p) basis set was used in these calculations. To obtain quantitative results, the classical potential energy along the minimum energy path (MEP) was corrected either by scaling to match a more accurate ab initio results for the barrier heights or by carrying out single point calculations at selected points along the MEP at a more accurate level of ab initio molecular orbital (MO) theory. By comparing with our previous QCISD results and experimental rate constants, we found that DFT particular the BH&H-LYP method can provide sufficient accurate potential energy surface information for rate calculations for this system. The present direct DFT dynamics method can be used for reactive dynamics studies of reactions involving large polyatomic molecules from first principles. More work however is still needed to test the accuracy of DFT methods for such calculations.
Treatment of rotational isomeric states. III. The use of biasing potentials
Authors: Straatsma T. P. , McCammon J. A.
Published: July 2002

A technique is described to determine umbrella biasing potentials that can be used to enhance sampling of rotational isomeric states in molecular simulations of polypeptides in water. The analytical biasing potential functions are obtained through fitting of potentials of mean force obtained by thermodynamic integration simulations to a small number of functions used to describe dihedral torsion. The resulting dramatic increase in efficiency of sampling is illustrated by comparison of molecular simulations of the alanine–dipeptide molecule in aqueous solution, with and without the use of the biasing potentials. The same biasing potentials were used in simulations of the alanine–tripeptide and the alanine–heptapeptide in aqueous solution. Similar dramatic increases in sampling efficiency were observed for these simulations, when the biasing potentials were applied, which suggests that these biasing potentials, although determined for the dipeptide, may be transferable to larger peptide chains. It is illustrated how the biasing potentials can be used in biasing potential annealing simulations, leading to rapid folding of peptide chains into aqueous solution structures with low energy.
Structure of the ethylene dimer from rotationally resolved near-infrared spectroscopy: A quadruple hydrogen bond
Authors: Chan M. C. , Block P. A. , Miller R. E.
Published: July 2002

Reported here are the first rotationally assigned near-infrared spectra of the ethylene dimer. The vibrational bands observed in this study correlate with ν9 and ν11 of the ethylene monomer, corresponding to C–H stretching motion. The spectral patterns have been identified as due to the vibrational transitions of a symmetric top molecule with D2d symmetry. This structure of the dimer has been predicted by ab initio calculations as the most stable geometry due to the fact that it enables the formation of four equivalent hydrogen bonds. Both vibrational bands show considerable homogeneous broadening corresponding to vibrational predissociation lifetimes of 0.37 and 0.18 ns for ν9 and ν11 excitation, respectively.
Living poly(α-methylstyrene) near the polymerization line. IV. Extent of polymerization as a function of temperature
Authors: Sarkar Das S. , Ploplis Andrews A. , Greer S. C.
Published: July 2002

We measure the extent of polymerization, Φ, as a function of temperature, T, for living poly(α-methylstyrene) in the solvent tetrahydrofuran, using sodium naphthalide and cesium naphthalide as the initiating species. We compare our measurements of Φ(T) to the theoretical predictions of a mean field model and of a nonmean field model [J. C. Wheeler and P. M. Pfeuty, Phys. Rev. Lett. 71, 1653 (1993)]. We find that both models describe our observations semiquantitatively. The nonmean field model is statistically (but not dramatically) a better description for two of the samples. For the third sample, the mean field description is statistically better, probably because the concentration of monomer/polymer in solvent is the smallest for this sample.
An inverse method for obtaining smooth multidimensional potential energy surfaces: application to Ar+OH A 2∑+(v=0)
Authors: Ho Tak-San , Rabitz Herschel , Choi Seung E. , Lester Marsha I.
Published: July 2002

A regularized inverse method is developed to determine an accurate multidimensional potential energy surface directly from spectral or scattering data. The method properly handles the inherent underdeterminancy, instability and nonlinearity of the problem, and extracts a physically acceptable smooth solution. The method is implemented for Ar+OH(A 2∑+).
Deactivation of two-photon excited Xe(5p56p,6p′,7p) and Kr(4p55p) in xenon and krypton
Authors: Whitehead C. A. , Pournasr H. , Bruce M. R. , Cai Hong , Kohel J. , Layne W. B. , Keto John W.
Published: July 2002

Lifetimes and bimolecular quenching rate constants have been determined for two-photon laser excited states of Xe*(5p56p,5p56p′,5p57p) and Kr*(4p55p) in krypton and xenon buffer gases. Collisional mixing between Kr*5p[5/2]2 and Kr*5p[5/2]3 in krypton is observed and analyzed using a coupled two-state model to obtain the rate of mixing. The measured rate constants for quenching of Xe*(6p′,7p) by krypton are 15%–20% smaller than those measured previously in xenon while bimolecular rates for the Kr*(5p) states are an order of magnitude larger in xenon than those in a krypton buffer. Measurements of state-to-state rate constants for deactivation and excitation transfer are also reported for these states in krypton and xenon buffer gases.
A new photon angular momentum approximation for molecular collisions in intense nonresonant laser fields
Authors: Madsen Dane K. , Eno Larry
Published: July 2002

We here explore a new scheme for dealing with the photon angular momentum effects which arise in the analysis of collisions in intense nonresonant laser fields. The scheme involves approximating the system total angular momentum so as to create a reduced set of dynamical equations depending parametrically upon the relative orientation of laser field axis and system transition dipole moment. The equations are solved as a function of orientation and results are collected using numerical quadrature. We examine the scheme in application to a model collision problem. Comparing with the results of exact calculations we find that the new scheme is very effective in determining the magnitudes of S-matrix elements but not the corresponding phases. The scheme is also compared with an earlier one which involves predynamical orientation averaging of laser–particle coupling elements. We find that the new scheme is at least as accurate as the earlier and is much more tractable computationally.
Intermolecular potentials and rovibrational energy levels of the Ar complexes with HCN and HCCH
Authors: Tao Fu-Ming , Drucker Stephen , Klemperer William
Published: July 2002

The intermolecular potential surfaces for ArHCN and ArHCCH are computed by Mo/ller–Plesset perturbation theory at the fourth-order approximations (MP4) with a large basis set containing bond functions. Rovibrational energies and spectroscopic constants of the two systems are computed from the intermolecular potentials using the collocation method. The intermolecular potential for ArHCN at the MP4 level has a single minimum at the collinear Ar−H−C−N configuration (R=4.56 Å, θ=0°) with a minimum potential energy of Vm=−135.9 cm−1. The bending frequencies, rotational constants, and centrifugal distortion constants of ArHCN and ArDCN calculated using the MP4 potential are in good agreement with experiment. Rovibrational energies with J=0 through 6 arising from j=0 and j=1 levels of HCN are calculated and compared with the experimental transition frequencies. The intermolecular potential surface for ArHCCH has a symmetric double minimum near the T-shaped configuration. The minimum positions at the MP4 level are (R=4.05 Å, θ=60° and 120°) and the minimum potential energy is Vm=−110.9 cm−1. The rotational constants and bending frequency of ArHCCH arising from the MP4 potential are calculated and compared with experiment. The anisotropy of the MP4 potential is slightly underestimated. The effects of monomer bending vibration on the ArHCN and ArHCCH potentials are studied by additional calculations. The potential anisotropy of ArHCN decreases, whereas that of ArHCCH increases as the monomer vibration is taken into account. This might be partially responsible for the discrepancies between the theoretical predictions and experiment.
Optimized atomic Lennard-Jones 6–12 parameters for simulating pVT properties of a realistic polymethylene melt
Authors: Widmann Albert H. , Laso Manuel , Suter Ulrich W.
Published: July 2002

From the comparison of experimental low-pressure pVT data for a short alkane with the results of Monte Carlo simulations in the NpT ensemble of an atomistically detailed model of polymethylene (PM) with explicit hydrogens, we have obtained Lennard-Jones parameters that allow accurate prediction of pVT behavior for liquid long-chain alkanes at high pressure. The parameters were obtained from the Slater–Kirkwood formula and fitted to the experimental density of n-pentane at 0.1 MPa; they faithfully reproduce experimental data for chains up to C23H48 (n-tricosane) and pressures up to 100 MPa over a wide temperature interval.
Induced dipole moments in the ν2/ν4 dyad of SnH4
Authors: Loëte M. , Coquard M.-P. , Ohshima Y. , Takami M.
Published: July 2002

The linear Stark effect measured in the ν2/ν4 dyad of SnH4 by Doppler-free infrared–infrared double resonance spectroscopy has been analyzed. Fifteen double resonance signals in the vibrationally excited state and 12 previously reported signals in the ground state have been fitted simultaneously to an effective Stark Hamiltonian. The centrifugal distortion moment μ0 and the two vibration-induced moments μ44 and μ24 are determined to be μ0=1.499(11)×10−5 D, μ44=−1.317(42)×10−2 D, and μ24=3.94(23)×10−2 D.
Structure function of linear and star polymers in the small wave vector regime
Authors: Bishop Marvin , Clarke Julian H. R. , Freire Juan J.
Published: July 2002

Off-lattice Monte Carlo simulations employing the pivot algorithm are used to generate ideal and excluded-volume linear and star polymers in two and three dimensions. The second, fourth, sixth, and eighth moments of the average monomer–monomer separation are calculated from the resulting configurations. The coefficients in the expansion of the structure factor are computed from universal ratios of these moments. The values found for excluded-volume chains are smaller than the ideal chain values and the differences are greater in two dimensions than in three dimensions. The structure function is also computed in the simulations for three-dimensions. There is excellent agreement for ideal linear chains and five-arm stars with the Debye and Benoit functions, respectively. The first order renormalization group ε expansion prediction for excluded volume polymers is in reasonable agreement with the simulation data.
Molecular dynamics simulations of athermal polymer blends: Comparison with integral equation theory
Authors: Stevenson Craig S. , Curro John G. , McCoy John D. , Plimpton Steven J.
Published: July 2002

Molecular dynamics simulations of a binary, athermal blend of chains consisting of 50 sites were carried out over a range of compositions at an overall packing fraction of 0.465. The species in the blend interact with Lennard-Jones repulsive interactions chosen to give effective hard core diameters of 1.015 and 1.215 for the A and B sites, respectively. The intramolecular structure of the chains was seen to be weakly dependent on composition, with the mean-square end-to-end distance of the minority component at low volume fractions tending towards that of the host component. The three intermolecular radial distribution functions gAA(r), gBB(r), and gAB(r) were observed to increase with increasing concentrations of the A chains. Polymer reference interaction site model (PRISM) calculations were performed on this same blend and very good agreement was found between the theory and the simulation over a range of compositions. Significant nonrandom mixing was observed from the molecular dynamics simulations on local length scales; PRISM theory was able to accurately predict these nonrandom local features in the packing.
The dissociation energies of CH4 and C2H2 revisited
Authors: Partridge Harry , Bauschlicher Charles W.
Published: July 2002

The bond dissociation energies of CH4, C2H2, and their fragments are investigated using basis set extrapolations and high levels of correlation. The computed bond dissociation energies (De) are accurate to within 0.2 kcal/mol. The agreement with the experimental (D0) values is excellent if we assume that the zero-point energy of C2H is 9.18 kcal/mol. The effect of core (carbon 1s) correlation on the bond dissociation energies of C–H bonds is shown to vary from 0.2 to 0.7 kcal/mol and that for C–C bonds it varies from 0.4 to 2.2 kcal/mol.
Correlations in the liquid interfaces of simple liquids
Authors: Stecki J. , Toxvaerd S.
Published: July 2002

From molecular-dynamics simulation of liquid–liquid interface at low temperatures between two simple liquids, we determine two point correlation functions, also for the liquid–vacuum interface. True Fourier vector in-plane dependence, correlations parallel and perpendicular to the interface, susceptibilities, and surface scattering factor (in the Born approximation) are discussed. The Bhatia–Thornton transformation is generalized to inhomogeneous systems and applied. Effects of capillary waves, surface reconstruction, surface layering, and prominence of bulk structures are found and discussed.
Dynamics of aggregation of tungstic acid
Authors: Miyakawa K. , Iwata Y. , Takase A.
Published: July 2002

The dynamics of aggregation of tungstic acid was investigated as a function of time, using static and dynamic light scattering. Two types of kinetic behaviors were observed depending on the magnitude of the system parameter: One is the power-law growth and the other the exponential growth. At an intermediate value of the system parameter, the crossover from the power-law behavior to the exponential one was also observed. However, none of the aggregation kinetics observed had the characteristic rates: Those were directly controlled by an amount of hydrogen chloride added. Thus the universality in aggregation may not hold for the present colloidal system.
Overtone spectrum of the CH chromophore in CHI3
Authors: Marquardt Roberto , Gonçalves Norberto Sanches , Sala Oswaldo
Published: July 2002

The infrared spectrum of iodoform vapor was measured between 900 and 10 000 cm−1 in a heatable multipass cell with an interferometric Fourier transform spectrometer and temperatures up to 433 K. Twenty-five bands (including hot bands) could be assigned as fundamentals, overtone, or combination bands of the CH stretching and bending vibrations. The observed data are interpreted in terms of variational calculations using internal and normal coordinates and some evidence is given for the existence of a strong coupling constant ‖ksbb′‖=50 cm−1 between the two vibrational modes with subpicosecond redistribution of energy. Off-diagonal anharmonic constants between the CH stretching and bending manifold and the molecular frame vibrations have been determined.
Five-dimensional local mode-Fermi resonance model for overtone spectra of ammonia
Authors: Kauppi Esa , Halonen Lauri
Published: July 2002

A five-dimensional local mode-Fermi resonance model for overtone spectra of the ammonia molecule has been constructed. The model Hamiltonian is expressed in terms of curvilinear internal valence coordinates and it includes the three stretching vibrations and the doubly degenerate bending vibration. The symmetric bending vibration associated with the inversion motion has been excluded. Thus the model is useful for energy levels with the totally symmetric bending vibration on its ground state. Vibrational energy levels have been calculated using van Vleck perturbation theory. Nonlinear least-squares method has been used to optimize potential energy parameters. Observed vibrational band origins for 14NH3, 14ND3, and 14NT3 have been employed as data. A fit with the standard deviation of 5.5 cm−1 has been obtained using one set of isotope invariant potential energy parameters. The optimised potential energy surface compare well with results of ab initio electronic structure calculations and with results of customary anharmonic force field calculations.
High pressure studies of the acridine/fluorene photoreaction: Vibration assisted tunneling
Authors: Chan I. Y. , Dernis Mitchell S. , Wong Chung M. , Prass B. , Stehlik D.
Published: July 2002

We report a multifaceted investigation of the hydrogen transfer photoreaction in acridine-doped fluorene crystals at higher temperature. The purpose is to elucidate the role of vibrationally assisted tunneling in this reaction system. Raman experiments were conducted at various pressures and 77 K to document the change of vibrational frequency for the promoting mode(s). Upon compression, a line with a large pressure coefficient emerges from under the strong phonon mode at 96.5 cm−1. Through polarization studies under pressure, we have identified it as a molecular butterfly mode of B1 symmetry. We have measured the reaction rate at 150 K in order to examine the effect of a suggested promoting mode at ∼440 cm−1. The reaction rate again increases exponentially with pressure, but with a significantly higher pressure coefficient than that at 1.4 and 77 K. Mode patterns based on a recently published [J. Phys. Chem. 98, 12 223 (1994)] normal coordinate analysis of fluorene are used to help establish the promoting modes for this reaction. This consideration suggests that the 95 and 238 cm−1 modes are likely promoting modes in addition to the 125 cm−1 libration. A computation of the Franck–Condon factor for the H-transfer process indicates that a small population of a high overtone of a promoting mode may make a disproportionally large contribution to the reaction rate. This calculation fails to account for the greater pressure coefficient of the reaction rate at higher temperature. Instead, such an increase may come partly from a greater compressibility at higher temperature.
A semiclassical self-consistent-field approach to dissipative dynamics. II. Internal conversion processes
Authors: Stock Gerhard
Published: July 2002

A semiclassical time-dependent self-consistent-field (TDSCF) formulation is developed for the description of internal conversion (IC) processes in polyatomic molecules. The total density operator is approximated by a semiclassical ansatz, which couples the electronic degrees of freedom to the nuclear degrees of freedom in a self-consistent manner, whereby the vibrational density operator is described in terms of Gaussian wave packets. The resulting TDSCF formulation represents a generalization of familiar classical-path theories, and is particularly useful to make contact to quantum-mechanical formulations. To avoid problems associated with spurious phase factors, we assume rapid randomization of the nuclear phases and a single vibrational density operator for all electronic states. Classically, the latter approximation corresponds to a single trajectory propagating along a ‘‘mean path’’ instead of several state-specific trajectories, which may become a critical assumption for the description of IC processes. The validity and the limitations of the mean-path approximation are discussed in detail, including both theoretical as well as numerical studies. It is shown that for constant diabatic coupling elements Vkk′ the mean-path approximation should be appropriate in many cases, whereas in the case of coordinate-dependent coupling Vkk′(x) the approximation is found to lead to an underestimation of the overall relaxation rate.

As a remedy for this inadequacy of the mean-path approximation, we employ dynamical corrections to the off-diagonal elements of the electronic density operator, as has been suggested by Meyer and Miller [J. Chem. Phys. 70, 3214 (1979)]. We present detailed numerical studies, adopting (i) a two-state three-mode model of the S1−S2 conical intersection in pyrazine, and (ii) a three-state five-mode and a five-state sixteen-mode model of the C̃→B̃→X̃ IC process in the benzene cation. The comparison with exact basis-set calculations for the two smaller model systems and the possible predictions for larger systems demonstrate the capability of the semiclassical model for the description of ultrafast IC processes.
The unimolecular dissociation of HCO: A spectroscopic study of resonance energies and widths
Authors: Tobiason Joseph D. , Dunlop James R. , Rohlfing Eric A.
Published: July 2002

We use dispersed fluorescence (DF) and stimulated emission pumping (SEP) spectroscopies on the B̃ 2A′–X̃ 2A′ system of jet-cooled HCO to measure the vibrational energies, resonance widths, and relative fluorescence intensities of 73 bound and resonance states supported by the ground-state potential energy surface. The SEP experiments use both two-color resonant four-wave mixing (RFWM-SEP) and the more conventional technique in which SEP signals are obtained from fluorescence depletion (FD-SEP). Where applicable, RFWM-SEP provides superior spectra to those obtained with FD-SEP, which is susceptible to saturation broadening that can prevent accurate determinations of resonance widths. The observed bound and resonance states span an energy range of 2000–21 000 cm−1 and comprise a wide range of vibrational excitation among the three vibrational modes, including states with 1–12 quanta of excitation in the C–O stretch, 0–5 quanta of bending excitation, and 0–1 quanta of excitation in the C–H stretch. The widths are markedly mode-specific and often display striking, nonmonotonic variations with increasing C–O stretch excitation. We compare our results to those of previous DF and SEP studies and to recent dynamical calculations of resonance energies and widths that use realistic potential surfaces derived from ab initio calculations. The resonance widths are particularly sensitive gauges of the unimolecular dissociation dynamics and provide stringent tests of theoretical potential surfaces.
The dynamics of symmetric polystyrene–polybutadiene diblock copolymer melts studied above and below the order–disorder transition using dynamic light scattering
Authors: Papadakis Christine M. , Brown Wyn , Johnsen Robert M. , Posselt Dorthe , Almdal Kristoffer
Published: July 2002

This communication describes dynamic light scattering measurements on three low molar mass symmetric polystyrene–polybutadiene diblock copolymer melts. Above but close to the order–disorder transition, four modes contribute to the time-dependent correlation function. In the depolarized geometry there is a very fast, broad relaxation typifying the segmental reorientational dynamics and a slower, single-exponential mode related to chain orientation and stretching. In the polarized experiments two additional processes are seen: the heterogeneity mode related to the self-diffusion of individual copolymer molecules and a slow mode representing the translational diffusion of large (≂100 nm) clusters. Just below the order–disorder transition (ODT), the slowest mode is shifted to very long times. The heterogeneity mode gives rise to depolarized scattering below the ODT because of the anisotropy induced by the lamellar structure. With the lowest molar mass copolymer above the ODT, it could be seen that the amplitude of the mode related to chain orientation and stretching decreases the deeper the sample is in the disordered state. A new subtraction technique was used for data analysis.
Vector correlation studies of HO2 photodissociation at 220 nm
Authors: Lock Michael , Barnes Rhett , Sinha Amitabha
Published: July 2002

The 220 nm photolysis of the hydroperoxyl radical, HO2, is investigated by probing the ejected OH fragments using Doppler and polarization spectroscopy. Analysis of the measured line profiles reveals that the OH fragments are predominately (84%) formed with the partner oxygen atom in its electronically excited 1D state with a smaller component (16%) being associated with oxygen atoms in the 3P ground electronic state. Measurement of OH fragment internal state distribution indicates that the 23 200 cm−1 of available energy is primarily released as electronic excitation of the oxygen atom (fel=0.57) and to a lesser extent as relative translation of the products (ftr=0.41). The internal degrees-of-freedom of the OH fragment receive very little of the available energy and are found to be fairly cold (fvib<0.004 and frot=0.014). For the primary O(1D) dissociation channel the measured 〈μ⋅v〉 correlation is strongly positive (βμv=0.61) indicating a preference for parallel alignment of the electronic transition moment and the recoil velocity vector in HO2, consistent with the excited state being of A″ symmetry. All other bipolar moments are close to zero for this pathway (βμJ=−0.10, βvJ=−0.04, βμvJ=−0.06) independent of the probed rotational quantum state of the OH fragment. For the minor O(3P) pathway a comparable set of bipolar moments is obtained. An investigation into the source of OH fragment rotation reveals that the combined contributions from out-of-plane rotation, generated by initial parent thermal motion about A-inertial axis, and in-plane rotation, generated by the combination of bending mode zero-point energy and final state interaction on the excited potential energy surface, result in negligible 〈v⋅J〉 correlation in the photodissociation of a thermally distributed sample of HO2 at 300 K.
Dynamics of chains in excluded volume conditions: Frequency-dependent viscosity of regular star chains
Authors: Rey Antonio , Freire Juan J.
Published: July 2002

A theoretical treatment based on the Rouse–Zimm scheme is formulated to compute the relaxation times of an isolated elastic polymer chain with intramolecular interactions mimicking the excluded volume conditions. From these relaxation times, the intrinsic viscosity and the frequency-dependent viscosity of linear and regular star-branched chains have been computed. The conformational averages needed in the calculations are evaluated through a Monte Carlo sampling, to avoid any assumption on the distribution function of intramolecular distances. We observe an interesting curvature in the plots of the viscoelastic moduli G′ and G″ against the frequency, which seems to be due to excluded volume interactions. Finally, the calculation and shape of the stress correlation function is discussed in terms of the constraining effects imposed by excluded volume in the vicinity of the branching point.
Comprehensive analysis of the A–X spectrum of I2: An application of near-dissociation theory
Authors: Appadoo D. R. T. , Le Roy R. J. , Bernath P. F. , Gerstenkorn S. , Luc P. , Vergès J. , Sinzelle J. , Chevillard J. , D’Aignaux Y.
Published: July 2002

High resolution absorption spectra of the A 3Π1u–X 1Σ+g system of I2, consisting of some 9552 lines of some 79 bands spanning the vibrational range v′=0–35 and v″=3–17, have been recorded and analyzed. A fit to them which uses the previously determined accurate molecular constants for the X 1Σ+g state yields an accurate new set of molecular constants for the A state, including the Λ doubling constants. The A-state vibrational and inertial rotational constants, as well as mechanically consistent centrifugal distortion constants, are represented by near-dissociation expansions, yielding an accurate representation of the experimental data which also provides a reliable global representation of all observed and unobserved vibration–rotation levels of this state.
Semiclassical calculation of cumulative reaction probabilities
Authors: Spath Bruce W. , Miller William H.
Published: July 2002

It is shown how the rigorous quantum mechanical expression for the cumulative reaction probability (CRP) obtained by Seideman and Miller [J. Chem. Phys. 96, 4412; 97, 2499 (1992)], N(E)=4 tr[ε̂r⋅Ĝ*(E)⋅ε̂p⋅Ĝ(E)], which has been the basis for quantum calculations of the CRP for simple chemical reactions, can also be utilized with a semiclassical approximation for the Green’s function, Ĝ(E)≡(E+iε̂−Ĥ)−1=(iℏ)−1∫∞0 exp(iEt/ℏ)exp(−i(Ĥ−iε̂)t/ℏ). Specifically, a modified Filinov transformation of an initial value representation of the semiclassical propagator has been used to approximate the Green’s function. Numerical application of this trajectory-based semiclassical approximation to a simple one-dimensional (barrier transmission) test problem shows the approach to be an accurate description of the reaction probability, even some ways into the tunneling regime.
The shape dependence of the solute–solvent interactions in a liquid crystalline phase: A computer simulation study
Authors: La Penna Giovanni , Foord E. K. , Emsley J. W. , Tildesley D. J.
Published: July 2002

Molecular dynamics simulations have been performed on a single hexane molecule fixed in a single shape, defined by a rotation angle φ about the center C–C bond, in a collection of 256 rigid particles interacting with each other through the Gay–Berne potential. The system is in a nematic liquid crystalline phase. Seven separate simulations were done, in each case with the hexane in a different shape, and the results are used to test theoretical models for conformationally dependent potentials of mean torque, which have been used to calculate averaged values of internuclear dipolar couplings. The results suggest that the truncation of the potentials of mean torque at second-rank terms is a good approximation when calculating dipolar couplings, but that there are inaccuracies in the shape dependence of the potentials.
Microscopic simulation of phase transition in interacting ionic gels
Authors: Aalberts Daniel P.
Published: July 2002

Ionic polymer networks are studied using the bond fluctuation method. Gels are known to exist in expanded or collapsed phases as demonstrated by Tanaka et al.’s experiments on ionic gels. Two interactions—the quality of the solvent and the work done by a gas of counterions—suffice to characterize the first-order phase transition indicated in these two-dimensional simulations. A technique is introduced which prevents local attractive interactions from hindering global relaxation.
Calculations of the low temperature pressure broadening of HCO+ rotational spectral lines by H2
Authors: Liao Qing , Herbst Eric
Published: July 2002

Calculations on the pressure broadening by H2 of HCO+ rotational spectral lines have been performed in the temperature range 11–300 K. Recent low temperature measurements of the pressure broadening of the J=3–2 line at 11–30 K are reproduced to within 10–20% by the results of a capture theory. The results of Anderson theory in this temperature range, on the other hand, are low by a factor of up to ≊3 compared with experiment. A hybrid method is proposed, which converges to the capture theory at low temperatures and the Anderson theory at high temperatures.
Low-energy electron scattering by halomethanes: Elastic and differential cross sections for CF4
Authors: Gianturco F. A. , Lucchese R. R. , Sanna N.
Published: July 2002

Low-energy quantum calculations are carried out for electrons scattering by CF4 molecules in their ground electronic states. The corresponding elastic cross sections (rotationally summed) are obtained as integral quantities and as angular distributions, i.e., differential cross sections (DCS), over a range of collision energies from ≊3 eV up to 35 eV. The exact static exchange (ESE) results compare well with experiments and with previous calculations. The inclusion of a model polarization potential is shown to generally improve results, especially at low collision energies and in the small-angle scattering region.
Photodissociation dynamics of Ã state ammonia molecules. II. The isotopic dependence for partially and fully deuterated isotopomers
Authors: Mordaunt David H. , Dixon Richard N. , Ashfold Michael N. R.
Published: July 2002

The technique of H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the photodissociation dynamics of the mixed isotopomers NH2D and NHD2 following the excitation to the v2′=0 and 1 levels of their lowest lying Ã 1B1 (C2v) excited electronic states. Peaks in the resulting total kinetic energy release (TKER) spectra are assigned to levels of the NH2, NHD, or ND2 fragments with a wide range of quantum numbers Ka for rotation about their a inertial axes, and with N=Ka, N=Ka+1, or N=Ka+2 as appropriate. These data provide the first measurements of high rotational levels for the ground electronic state of the NHD radical. The least squares fitting of all these spectra, and those resulting from NH3 and ND3, to the best calculated NH2, NHD, and/or ND2 rotational term values provides accurate estimations of the respective N–H and N–D bond dissociation energies D00 across the whole series. These values are D00(H–NH2)=37 115±20 cm−1 (4.602±0.002 eV); D00(H–NHD)=37 240±50 cm−1; D00(H–ND2)=37 300±30 cm−1; D00(D–NHD)=37 880±60 cm−1; and D00(D–ND2)=38 010±20 cm−1. The differences between these values are fully consistent with differences in zero-point energies and lead to a mean value of De=40 510±25 cm−1. Dissociation of NH2D or NHD2 through their (Ã−X̃) 210 bands to give an NHD product leads to TKER spectra with a much higher statistical character than those leading to an NH2 or ND2 product, and to those obtained following excitation through the 000 bands. This is rationalized in a semiquantitative manner in terms of a varying contribution to the dissociation rate of the parent molecules from internal conversion (IC) to high levels of their respective ground states. Nuclear permutation symmetry appears to play an important role both for the IC rates and for the subsequent branching between product channels.
Models for statistical decomposition of metal clusters: Vibrational frequency distributions
Authors: Shvartsburg Alexandre A. , Ervin Kent M. , Frederick John H.
Published: July 2002

The application of statistical theories to the decomposition kinetics of metal clusters requires the estimation of the vibrational frequency distributions. We adapt elastic theories developed for bulk metals and fine particles to generate a physically reasonable frequency distribution model for small metal clusters. Results obtained from this elastic cluster model compare favorably with previously reported experimental heat capacity data for fine particles. In addition, predictions of the present model are shown to correlate very well with experimentally determined trends in metal cluster cohesive energies. The elastic cluster model is then applied to the statistical unimolecular decay kinetics of metal clusters and compared with results found using earlier theoretical models. The present model predicts slower rates of decomposition in comparison with the other models. These results suggest that the binding energies extracted from experimental photodissociation and collision-induced dissociation measurements may have been slightly overestimated in the past.
Isotopic substitution of a hydrogen bond: A near infrared study of the intramolecular states in (DF)2
Authors: Davis Scott , Anderson David T. , Farrell, John T. , Nesbitt David J.
Published: July 2002

High resolution near infrared spectra of the two high frequency intramolecular modes in (DF)2 have been characterized using a slit-jet infrared spectrometer. In total, four pairs of vibration–rotation–tunneling (VRT) bands are observed, corresponding to K=0 and K=1 excitation of both the ν2 (‘‘bound’’) and ν1 (‘‘free’’) intramolecular DF stretching modes. Analysis of the rotationally resolved spectra provides vibrational origins, rotational constants, tunneling splittings and upper state predissociation lifetimes for all four states. The rotational constants indicate that the deuterated hydrogen bond contracts and bends upon intramolecular excitation, analogous to what has been observed for (HF)2. The isotope and K dependence of tunneling splittings for (HF)2 and (DF)2 in both intramolecular modes is interpreted in terms of a semiclassical 1-D tunneling model. High resolution line shape measurements reveal vibrational predissociation broadening in (DF)2: 56(2) and 3(2) MHz for the ν2 (bound) and ν1 (free) intramolecular stretching modes, respectively. This 20-fold mode specific enhancement parallels the ≥30-fold enhancement observed between analogous intramolecular modes of (HF)2, further elucidating the role of nonstatistical predissociation dynamics in such hydrogen bonded clusters.
A high resolution photoionization study of Ne and Ar: Observation of mass analyzed threshold ions using synchrotron radiation and direct current electric fields
Authors: Hsu C.-W. , Lu K. T. , Evans M. , Chen Y. J. , Ng C. Y. , Heimann P.
Published: July 2002

Using the high resolution vacuum ultraviolet (vuv) photon source provided by the monochromatized undulator synchrotron radiation of the Chemical Dynamics Beamline at the Advanced Light Source, we have measured the photoionization efficiency (PIE) spectrum for Ne in the energy range of 21.56–21.67 eV at a wavelength resolution of 0.3 meV [full width at half-maximum (FWHM)]. The PIE spectra for Ne obtained using 0.76 and 2.4 V/cm electric fields reveal autoionizing features attributable to the Rydberg states Ne[2p5ns′(1/2)1; n=14–29] and Ne[2p5nd′(3/2)1; n=12–35] converging to the spin–orbit excited Ne+(2P1/2) state. The positions of these Rydberg states are compared to previous experimental results and those calculated using the quantum defects and IE for Ne+(2P1/2) given in Moore [Natl. Stand Ref. Data Ser. Natl. Bur. Stand. 35 (1971)]. We have also observed mass analyzed threshold ions (MATI) for Ne formed in the Ne+(2P3/2,1/2) states. For Ar, only the MATI peak for Ar+(2P3/2) is observed. The failure to observe the MATI peak for Ar+(2P1/2) is attributed to shorter lifetimes of high-n Ar[3p5ns′(1/2)1] and Ar[3p5nd′(3/2)1] Rydberg states compared to the minimum time required for separating the prompt Ar+ ions from the field ionized Ar+ ions in this experiment. The MATI peaks for Ne+(2P3/2,1/2) achieves a resolution of 0.7 meV (FWHM). These MATI spectra for Ne and Ar reported here represent the first such studies made using a cw vuv light source and dc electric fields.
Electronic spectroscopy of jet-cooled benzylidenecyclobutane, a sterically hindered styrene
Authors: Manea V. P. , Cable J. R.
Published: July 2002

The electronic spectrum of the styrene derivative, benzylidenecyclobutane, seeded in a supersonic jet expansion has been recorded using resonantly enhanced two-photon ionization spectroscopy. The main vibronic features in the spectrum are associated with a low frequency progression assigned to the torsional motion of the phenyl ring. Analysis of the observed torsional levels reveals an excited state potential energy surface characteristic of a planar equilibrium geometry which undergoes large amplitude motion and a ground state surface having a minimum at a torsional angle of 25° between the phenyl and vinyl groups. Ab initio calculations of the ground state torsional potential surface predict a minimum in the range of 28°–26°, depending on the size of the basis set. In these structures the cyclobutane ring adopts a puckering angle between 17° and 19°. Deuterated isotopomers have also been synthesized and their corresponding photoionization spectra analyzed to reveal the mixing between the torsion and other low frequency modes such as cyclobutane ring puckering. The extent of this mixing is found to be sensitive to the sites of deuteration on the molecule.