bluuebunny/crossref_metadata_2025 huggingface dataset:
Assignment of Franck–Condon enabled vibrations in the NV1←X̃ transition of cyclopentadiene
Authors: McDiarmid Ruth , Gedanken Aharon
Published: July 2002

The absorption spectra of the NV1←X̃ transition of high pressure samples of cyclopentadiene and cyclopentadiene-d6 were measured and analyzed. One hot band was observed in each isotopomer. This band was identified as the ring deformation mode. It was shown to correlate with the first vibrational subband of the electronic transition. A second subband of this transition, observed only in cyclopentadiene, was identified as the ■CH wag. The implications of these results on the previous interpretation of this transition as a progression in solely the C■C stretching vibration [J. Chem. Phys. 93, 3850 (1990)] are discussed.
Radiative lifetimes and reactivity of metastable NO+(a 3Σ+,v) and O+2(a 4Πu,v)
Authors: Marx R. , Yang Y. M. , Mauclaire G. , Heninger M. , Fenistein S.
Published: July 2002

The radiative lifetimes and reactivity of metastable NO+(a 3Σ+) and O+2(a 4Πu) have been investigated using a triple cell ion cyclotron resonance (ICR) spectrometer. Radiative lifetimes of (100±20) ms and (135±25) ms have been found for NO+(a 3Σ+) v≥1 and v≥0 respectively, with Ar and CO2 as monitor gases. The overall reaction rate of NO+(a 3Σ+,v≥1) with Ar (6.5×10−10 cm3 s−1 ) is 1 order of magnitude larger than the rate of v=0 determined in flowing afterglows. CO2 reacts at collision rate. For O+2 the overall lifetime of v≥0, determined with Ar as monitor, is (30±6) ms in between the two values (7 and 130 ms) found by Bustamente et al. using the photodissociation technique. The possible reasons for the longer lifetimes determined in previous ICR experiments is discussed. In the absence of any ab initio calculation, the experimental results on NO+(a 3Σ+) may only be compared to values calculated using an approximated perturbation model. Unfortunately, the reliability of this model, also used in the previous studies, is very limited because of the large uncertainty on the coupling between the metastable a 3Σ+ and the perturbing radiative A 1Π states.
Rate processes on moderately smooth surfaces
Authors: Zhdanov V. P.
Published: July 2002

To describe the effect of lateral interactions between adsorbed particles on the kinetics of rate processes on uniform surfaces, the lattice–gas models are usually employed. Traditionally, these models are based on the idea that the equilibrium positions of adsorbed particles are fixed rigidly on the surface. This assumption may be incorrect in many real systems (including such traditional systems as CO or hydrogen on different metals), where the activation energy for surface diffusion is not very high (or is low) compared to lateral interactions between nearest-neighbor adsorbed particles (i.e., where the surface is smooth). In the present paper, a general theory is developed to take into account the influence of shifts of equilibrium position of adparticles on the kinetics of various rate processes on solid surfaces. The theory is applicable to the case when the activation energy for surface diffusion exceeds lateral interactions between adparticles. The shifts of equilibrium positions are shown to result in significant changes in the rate constants of various processes. The effect of shifts on the coverage dependence of the apparent Arrhenius parameters for desorption, thermal desorption spectra, and the coverage dependence of the chemical diffusion coefficient is also demonstrated in detail.
The photophysics and photochemistry of complexes of s-tetrazine and acetylene
Authors: Morter Cheryl L. , Wu Yenchune R. , Levy Donald H.
Published: July 2002

The photochemistry and photophysics of complexes of s-tetrazine (T) with acetylene have been studied in a supersonic free jet using laser induced fluorescence spectroscopy. Two complexes of the form T–C2H2 and a complex of the form T–(C2H2)2 were observed. One of the monoacetylene complexes has a stacked structure and one has a planar structure. In all cases low frequency vibrational structure was observed in the emission spectrum, and this allowed measurement of the frequencies of intermolecular vibrations. At some levels of excitation, all three complexes had broad features in their emission spectra. It was shown that in these cases, the density of states was sufficiently high that the molecular complexes were in the statistical limit, and therefore the broad structure was attributed to intramolecular vibrational relaxation (IVR). At higher levels of excitation, dissociation of the complexes was observed, and in some instances both IVR and dissociation were observed. Rate constants were measured by comparing the relative intensity of emission produced by these two decay channels with the intensity of resonance fluorescence. These rate constants were found to vary depending on the complex and upon the initially excited state of a given complex. When dissociation was observed, not all energetically allowed vibrational states were observed. The diacetylene complex was observed to dissociate to produce both monoacetylene complexes, but never the monomer.
Laser induced fluorescence measurements of the X 2Σ+ state of CN produced by electron impact on CH3CN
Authors: Darrach M. , McConkey J. W.
Published: July 2002

Laser induced fluorescence (LIF) measurements of CN production following electron impact on CH3CN are carried out over an energy range from 30 to 300 eV. Cross sections for production of CN (X, v=0) over this range are obtained using the cross section for production of N+2 (X, v=0) as standard. The CN fragments are determined to be rotationally ‘‘cold’’ suggesting that dissociation occurs by stretching of the C–C bond. Rotational ‘‘temperatures’’ increased by a factor of 2 (to 80 K) as the incident electron energy was increased from 30 to 300 eV. LIF measurements of the translational energies of the CN fragments revealed a group with a surprisingly low translational temperature of 130 K.
Viscoelastic behavior of water/AOT/n-heptane microemulsions by ultrasonic absorption investigation
Authors: D’Aprano A. , D’Arrigo G. , Goffredi M. , Paparelli A , Liveri V. Turco
Published: July 2002

The ultrasonic absorption in water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane system has been measured at 25 °C, as a function of the water/AOT molar ratio (X) and of the volume fraction (φ) of the micellar phase. The ultrasonic spectra have been found strongly dependent on φ and nearly independent on X. In particular, as φ increases, ultrasonic dynamics shifts from a single to a distribution of relaxation times with occurrence of viscoelastic effects. Such findings have been correlated to the increased connectivity of the micellar network.
Investigation of orientational correlations and structural transitions in neat liquids
Authors: Rozhdestvenskaya N. B. , Smirnova L. V.
Published: July 2002

The temperature dependence of depolarized light scattering intensity (DLSI) and nuclear magnetic resonance (NMR) chemical shift were measured for hexafluorobenzene in the temperature range from 10 to 63 °C. The DLSI curve is not monotone, consists of regions of different slope, and has a dip near 47 °C. The chemical shift curves also consists of different slope straight regions whose separation temperatures are coincident with anomalies on the DLSI curve. As DLSI is directly dependent on effective anisotropy of molecules, the authors suggest that in liquid C6F6, the regions of different slope correspond to molecular structures with different local order and symmetry. The authors derive the conclusion that in liquid C6F6 there are transitions between local structures generated by dimers with small differences between their minimum potential energy.
Computer simulation of a model 2–2 electrolyte: Multiple time-step molecular dynamics
Authors: Smith David E. , Kalyuzhnyi Yu. V. , Haymet A. D. J.
Published: July 2002

A popular model of a 2–2 electrolyte is simulated using a multiple time-step molecular dynamics algorithm. Many authors have used this particular model to test approximate statistical mechanical theories. The ions are assumed to be the same size, and to interact via a continuous potential which behaves as a Coulomb potential at large distances and as an inverse ninth power repulsion at short distances. The ions are embedded in a dielectric continuum of fixed dielectric constant, here taken to be 78.358 to model water at 25 °C. The multiple time-step algorithm outlined here is essential to the success of the simulation. The structure, thermodynamics, and clustering of ions in the electrolyte are examined over the full range of concentrations. Significant corrections are found to earlier Monte Carlo simulations at the concentration 0.005 M, and new results are presented at the concentration 0.001 M.
Fine-structure selective collisional energy transfer in spherical top molecules: Evidence for a symmetry-based mechanism from rovibrational eigenfunctions
Authors: Parson Robert
Published: July 2002

Recent state-resolved experiments have shown that rotational energy transfer in collisions of vibrationally excited spherical top molecules is remarkably selective with respect to the fine structure components of the rovibrational states. In a recent paper [J. Chem. Phys. 93, 8731 (1990)], these results were rationalized on the basis of symmetry arguments and the Harter–Patterson theory of spectral clustering. The present paper provides numerical evidence for those assertions. Matrix elements of an atom–spherical top interaction potential are calculated using numerically accurate wave functions from spectroscopic Hamiltonians and using the approximate wave functions given by the Harter–Patterson theory. Agreement between the two calculations is satisfactory and both confirm the propensity rules derived previously, suggesting that the proposed mechanism does in fact operate in these systems.
Calculations on vibrational predissociation of Ar–OH (A 2Σ+)
Authors: Chakravarty C. , Clary D. C. , Esposti A. Degli , Werner H.-J.
Published: July 2002

An ℛ-matrix algorithm is developed for executing vibrational predissociation calculations within the Golden Rule approximation. The algorithm is used to calculate vibrational predissociation linewidths and OH product rotational distributions for the quasibound states of Ar–OH (A2 Σ+, v=1). An ab initio potential energy surface obtained with the coupled electron pair approximation is used in the computations. The theoretical results are compared with experiment. The high anisotropy of the system is found to strongly favor vibrational to rotational energy transfer and the product OH (A2 Σ+, v=0) molecules are produced in highly excited rotational states. Quasibound states associated with excited bending levels are predicted to dissociate more rapidly than those assigned to the ground bend. For metastable states with the same bending quantum number, linewidths are predicted to decrease with increase in the intermolecular stretching quantum number.
Measurement of static polarizabilities on s-tetrazine
Authors: Heitz S. , Weidauer D. , Hese A.
Published: July 2002

Laser Stark spectroscopy has been performed on s-tetrazine molecules in a collimated, seeded supersonic beam. The Stark splittings and shifts of the rR(0)00 and the pP(1)10 lines in the vibrationless A 1B3u←X 1Ag transition could be measured in a static homogeneous electric field. The determined polarizability anisotropy of the A 1B3u state is 1/2(P′aa+Pbb)−P′cc =6.7(2) Å3, and of the X 1Ag ground state 1/2(P■aa+P″bb) −P■cc=0.8(2) Å3. The difference in the average polarizability P=1/3(Paa+Pbb+Pcc) between these two states was found to be ΔP=P′−P″=1.4(2) Å3.
Calculation of the ‘‘absolute’’ free energy and the entropy of classical solids from the motion of particles
Authors: Rickman J. M. , Phillpot S. R.
Published: July 2002

It is shown that an analysis of the dynamics of a classical solid’s constituent particles can be used to accurately calculate its ‘‘absolute’’ free energy and entropy. We validate the method by calculating the free energy and entropy of a defect-free, Lennard-Jones crystal and then comparing these results with those determined by applying an independent integration method. The relation between this ‘‘mechanical’’ (as opposed to ‘‘ensemble’’) method and other methods is discussed.
Heterogeneous and homogeneous critical points of polymer distributions
Authors: Brannock G. R.
Published: July 2002

General equations for the convergence of more than two phases (heterogeneous multiple critical points) and the merging of critical points associated with phase regions (homogeneous multiple critical points) are developed in the first half of this paper and are applied to three polymer distributions in the second half. At a heterogeneous point the spinodal surface is locally translation invariant along a local zero eigenvector field of the stability matrix and at a homogeneous point the gradient of the spinodal surface is null. Local invariance is extended to (1) field parameters (as temperature or pressure) along the streamlines of the zero eigenvector field on the spinodal surface and to (2) the classical approach with Legendre transform free energy of one composition variable. The heterogeneous critical conditions of Solc for a mixture of two homopolymers with molecular weight distribution are confirmed. For a copolymer with mer distribution, or equivalently a mixture of regularly interacting components, these equations are just successive moment expressions of a Gaussian distribution. The last example concerns the local destabilization of a homopolymer/copolymer mixture by copolymer mer distribution. Solc’s formalism can be extended to match these results also. Homogeneous multiple critical points are forbidden by molecular weight distribution in the first example and by a mer distribution that represents more than two components in the last two examples. A simple Landau expansion describing the local convergence of phases and critical points about a ternary tricritical point emphasizes the invariance of the critical point hierarchy with eigenvector scaling that was employed in developing all examples.
Intramolecular energy transfer and mode-specific effects in unimolecular reactions of disilane
Authors: Schranz Harold W. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

Intramolecular energy transfer rates and pathways in disilane Si2H6 have been investigated in detail by analysis of the envelope functions of the time variation of the uncoupled normal-mode kinetic energies [J. Chem. Phys. 89, 5680 (1988)] and by a new method that involves the Fourier transform of the local-mode ‘‘bond energies.’’ The results show that the total intramolecular vibrational relaxation (IVR) rate out of a given mode is generally much faster than the total dissociation rate. However, many of the individual mode-to-mode rate coefficients are significantly smaller than this rate. Consequently, IVR is not globally rapid on the time scale of the reactions. The Si–Si and local modes relax over a much longer time scale than the Si–H modes. This observed decoupling of sets of internal modes is interpreted to mean that phase space is not explored ergodically on the time scale of the reactions, even at internal energies significantly greater than the dissociation thresholds. The present results are consistent with and complementary to our earlier observation of trajectory rate coefficients that are considerably larger than corresponding statistical phase space predictions computed on the same potential-energy surface [J. Chem. Phys. 94, 0000 (1991)]. As a consequence, we find numerous mode-specific effects present in the system. Trajectory rates are found to be very sensitive to the nature of the initial energy partitioning. The computed kinetic isotope effects also show evidence of mode-specific chemistry. These data are consistent with the principle that a total intramolecular energy transfer rate from a given vibrational mode that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure statistical behavior and an absence of mode-specific chemistry.
The dynamics of CO oxidation on Pd, Rh, and Pt studied by high-resolution infrared chemiluminescence spectroscopy
Authors: Coulston George W. , Haller Gary L.
Published: July 2002

The dynamics of carbon monoxide oxidation on Pd, Rh, and Pt foils were probed under nearly collision-free conditions using high resolution infrared chemiluminescence. Auger electron spectroscopy was used to verify the absence of impurities on the surfaces. The reactants were supplied to the surface through a free jet nozzle source, while a Fourier transform infrared spectrometer operating at 0.012 cm−1 resolution was used to fully resolve the rotational structure of several vibrational transitions in the product CO2 (22 in the case of Pd). In all cases, the product CO2 is vibrationally excited and the apparent vibrational temperatures are in the same order as the peak reaction rates, i.e., Pd>Pt>Rh. The surface coverage of oxygen on Pd was varied by changing the CO:O2 ratio and the surface temperature and, in both cases, increasing oxygen coverage causes an increase in vibrational excitation of product CO2. On Pt and Rh, the apparent temperatures of different vibrational modes are similar, while on Pd, those levels involved in Fermi resonances that are traditionally called the symmetric stretch levels are selectively populated. From these results, evidence that the activated complex is bent on Pd relative to Pt and Rh and, in all cases, is aligned more or less along the surface normal is presented.
Ionic solids at high pressures and elevated temperatures: MgO (periclase)
Authors: Allan N. L. , Braithwaite M. , Cooper D. L. , Mackrodt W. C. , Wright S. C.
Published: July 2002

An equation of state for MgO is calculated for conditions of high pressure and temperature using atomistic simulations within the quasiharmonic approximation with two-body potentials and a simple shell model. It is shown that experimental Hugoniot data can be reproduced accurately; other important properties are also examined including various quantities of geophysical interest. Results for the Helmholtz free energy and internal energy surfaces A(V,T) and U(V,T) are presented in parametrized form.
The Doppler spectra of O(1D) from the photodissociation of O2, NO2, and N2O
Authors: Shafer Neil , Tonokura Kenichi , Matsumi Yutaka , Tasaki Shintaro , Kawasaki Masahiro
Published: July 2002

The Doppler profiles of the O(1D) products from the photodissociation of O2 at 157.6 nm and NO2 and N2O at 205.47 nm are detected by a resonance enhanced multiphoton ionization technique. The translation energy and angular distributions are deduced for the O(1D) atoms. Present results indicate that (a) O2 (B 3Σ−u) has a short dissociative lifetime when irradiated at 157.6 nm, (b) NO(2Π) photofragment from NO2 at 205.47 nm is mostly vibrationally excited, and (c) most of the available energy is released as kinetic energy when N2O is photodissociated at 205.47 nm.
Segmental distribution functions for a micelle comprised of small symmetric diblock copolymers (short chain amphiphiles)
Authors: Rodrigues Klein , Mattice Wayne L.
Published: July 2002

The micelle formed by short chain amphiphiles (small diblock copolymers) in a selective solvent has been simulated on a cubic lattice. The system contains twenty chains, each with ten beads of A and ten beads of B, at a volume fraction of 0.0376. These chains form a single micelle when the pairwise interaction of nonbonded beads in the insoluble block contribute an energy of −0.5 kT, all other interaction energies being zero. The micelle consists of an internal core that is a close packed region occupied almost exclusively by the insoluble blocks, and a corona, the outer portion of which consists primarily of solvent and the soluble blocks. However, the interface between these two regions is not sharp, as judged by distribution functions for the two types of beads, and for the junctions between the blocks. Letting 〈r 2j〉 denote the mean square radius of gyration of the junctions, the close packed core of insoluble beads extends from the center of mass no farther than ∼0.5 〈r 2j〉. The free ends of the insoluble block have a mean square radius of gyration of 0.88 〈r 2j〉, which places them, on average, farther from the center of mass than a randomly chosen bead in the insoluble block. The results are not compatible with models for a diblock copolymer micelle that assume a sharp interface, nor with models that assume the free ends of the insoluble block seek out the center of mass, or are randomly distributed throughout the core.
Vacuum ultraviolet photodissociation and photoionization studies of CH3SCH3 and CH3S
Authors: Nourbakhsh S. , Norwood K. , Yin H.-M. , Liao C.-L. , Ng C. Y.
Published: July 2002

We have measured the translational energy releases of the laser photodissociation processes CH3SCH3+hν (193 nm)→CH3+CH3S [process (1)] and CH3SCH2+H [process (2)]; and CH3S+hν (193 nm)→S+CH3 [process (3)]. The onsets of the translational energy distributions for photofragments of processes (1) and (2) allow the direct determination of 74.9±1.5 and 91±2.5 kcal/mol for the dissociation energies of the CH3–SCH3 and H–CH2SCH3 bonds at 0 K, respectively. The threshold observed for S formed by process (3) is consistent with the conclusion that the production of S(3P) is small compared to S(1D). The photoelectron–photoion coincidence (PEPICO) spectra for CH3SCH+3, CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ resulting from photoionization of CH3SCH3 have been measured in the wavelength region of 900–1475 Å. The PEPICO study allows the construction of a detailed breakdown diagram for the formation of CH3SCH+2, CH3S+ (or CH2SH+ ), and CH2S+ from energy-selected CH3SCH+3 ions.
Adsorbed polymers: Neutron reflectivity and concentration profiles
Authors: Guiselin O. , Lee L. T. , Farnoux B. , Lapp A.
Published: July 2002

We have determined by neutron reflectivity the volume fraction profile, φ(z) of poly(dimethylsiloxane) chains in toluene adsorbed at the free surface of the solution. φ(z) is found to vary as (a/z)4/3 conforming to scaling laws proposed by de Gennes. The extension of the profile is of the order of the radius of gyration. To arrive at these results, the use of two different isotopic compositions and the simultaneous analysis of results obtained with various polymer chain lengths and concentrations are necessary. The validity of several models has been tested. With a few realistic hypotheses compatible with already existing experimental data, it appears that only the self-similar profile is acceptable for the system studied.
Local-mode effects on the rotational structure of the first stretching overtone band system of stibine, SbH3
Authors: Halonen Marjo , Halonen Lauri , Bürger Hans , Moritz Peter
Published: July 2002

A high-resolution Fourier-transform infrared spectrum, resolution about 0.006 cm−1, of the first stretching overtone band system of stibine has been measured and rotationally analyzed in detail up to J′=22 for both 121SbH3 and 123SbH3 isotopic species. The rotational levels of the A1 and E vibrational states have been analyzed simultaneously by including vibration–rotation coupling terms in the Hamiltonian for the two isotopic species. The A1/E vibrational energy-level separation has been found to be 0.0727(8) cm−1 for 121SbH3 and 0.0763(8) cm−1 for 123SbH3 with the A1 state being higher in energy in both cases. Fits including 1445 transitions with the standard deviation of 0.0024 cm−1 for 121SbH3 and 1322 transitions with the standard deviation of 0.0022 cm−1 for 123SbH3 have been obtained by optimizing altogether 33 upper-state parameters in both cases. A simple local-mode model is shown to account very well for the vibrational dependence of the rotational constants and the coefficients of vibrationally off-diagonal H22 operators.
Micelle size and order in lyotropic nematic phases from nuclear spin relaxation
Authors: Quist Per-Ola , Halle Bertil , Furó István
Published: July 2002

Nuclear magnetic resonance (NMR) relaxation of quadrupolar nuclei is introduced as a new method for determining micelle size and nematic order in lyotropic nematic mesophases from the dependence of the spin relaxation rates on molecular diffusion over the curved micelle surface. The approach is illustrated by an experimental study of two uniaxial nematic phases (the calamitic NC and discotic ND phases of the sodium dodecyl sulphate/decanol/water system) using two nuclei: 2H in the α-deuterated surfactant and 23Na in the counterions. The two nuclei yield similar results: an apparently temperature independent axial ratio of 3–4 in both phases and a nematic order parameter which decreases from ca. 0.9 (0.75) at the lowest temperature to ca. 0.6 (0.5) at the highest temperature in the NC (ND) phase. As compared to the predictions of the Maier–Saupe theory, the nematic order parameter in the NC phase is considerably larger and decreases more strongly as the nematic–isotropic transition is approached.
High-lying rovibrational states of floppy X3 triatomics by a new D3h symmetry adapted method: Application to the H+3 molecule
Authors: Bačić Z. , Zhang J. Z. H.
Published: July 2002

A new D3h symmetry adapted approach to accurate calculation of high-lying rovibrational (J>0) states of floppy X3 triatomic molecules is presented, extending our recent work on J=0 bound states of D3h triatomics [Z. Bačić and J. Z. H. Zhang, Chem. Phys. Lett. 184, 513 (1991)]. Three sets of Jacobi coordinates are employed, allowing construction of basis sets with full S3 permutation symmetry of the three identical nuclei. Inclusion of S3 symmetry reduces substantially the size of the final matrix eigenvalue problem and the computational effort involved. It also assures unambiguous, correct symmetry assignment of the calculated rovibrational levels. Our method is especially suitable for rotating D3h molecules with strongly coupled large amplitude motions of the three atoms. An efficient quasiadiabatic diagonalization and truncation scheme is incorporated into our methodology. Application to H+3 for J=1 produced the total of 426 well converged, symmetry assigned states up to ∼24 000 cm−1 above the ground state, far more than in any previous calculation for rotating H+3.
Fluorescence excitation spectra of the S1 states of isolated trienes
Authors: Petek Hrvoje , Bell Andrew J. , Christensen Ronald L. , Yoshihara Keitaro
Published: July 2002

First observation of fluorescence for simple, linear trienes is reported. S1←S0 fluorescence excitation spectra of hexatriene and octatriene indicate large differences between the S0 and S1 potential energy surfaces. Activation energy of <200 cm−1 for the S1 state nonradiative decay is tentatively ascribed to isomerization.
Phase diagram and structure of the bicontinuous phase in a three-dimensional lattice model for oil–water–surfactant mixtures
Authors: Ciach A.
Published: July 2002

The regions of stability of the bicontinuous, lamellar, oil-rich, water-rich, and disordered phases and the phase equilibria are determined for a three-dimensional lattice model of surfactant mixtures. Analytical expressions for the density distributions in the bicontinuous phase are obtained in the vicinity of the critical temperature Tc. A tricritical point is calculated analytically, and is found to be located either on the transition between the disordered phase and the bicontinuous phase or on the transition between the disordered phase and coexisting oil- and water-rich phases, depending on the interaction strengths. A structure factor in the bicontinuous phase is calculated. Connection with microemulsions is discussed.
A theoretical study on the ionic states with analysis of vibrational levels of the photoelectron spectrum of ketene (C2H2O and C2D2O )
Authors: Takeshita Kouichi
Published: July 2002

A b initio calculations are performed to study the molecular equilibrium structure and the vibrational levels of the low-lying five ionic states, 1 2B1, 1 2 2B2, 2 2B1, 2 2B2, and 2A1 of ketene (C2H2O and C2D2O. ) The theoretical intensity curve obtained by the Franck–Condon factors for the ionization transitions are also reported and compared with the photoelectron spectrum of C2H2O and C2D2O. A number of new assignments of the vibrational levels of the photoelectron spectrum are proposed.
Influence of compressibility and monomer structure on small angle neutron scattering from binary polymer blends
Authors: Dudowicz Jacek , Freed Karl F.
Published: July 2002

The compressible random phase approximation (RPA) is combined with the lattice cluster theory to study how compressibility and monomer molecular structure affect small angle neutron coherent scattering intensities and quantities like the apparent radius of gyration and correlation length in binary polymer blends. Blend compressibility implies that the partial structure factors Sαβ(k) depend on three macroscopic interaction parameters χαβ, which, in turn, are functions of (at least) three microscopic van der Waals energies, temperature, pressure, polymerization indices, and monomer molecular structures. The {χαβ} are determined from the lattice cluster theory for the extended lattice model of compressible, structured monomer binary blends. The traditional incompressible RPA analysis is applied to our computed Sαβ(k) and uncovers deficiencies in extracting the apparent radius of gyration and correlation length. The calculated structure factor, apparent radius of gyration, and correlation length for polystyrene/polyvinylmethylether blends are compared with the experimental data of Han et al. without using new adjustable parameters.
Ion rotational distributions for near-threshold photoionization of H2O
Authors: Lee M.-T. , Wang Kwanghsi , McKoy V. , Tonkyn R. G. , Wiedmann R. T. , Grant E. R. , White Michael G.
Published: July 2002

Ion rotational distributions for single-photon VUV photoionization of the 1b1 orbital of the X̃ 1A1 ground state of the jet-cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic-like.
Photoinitiated charge transfer in N2O+–Ar
Authors: Bieske E. J. , Soliva A. M. , Friedmann A. , Maier J. P.
Published: July 2002

Vibrationally structured electronic transitions of N2O+–Ar have been observed by measuring the wavelength-dependent yields of the photodissociation reactions to yield N2O+ or Ar+. There appear to be four structured overlapping electronic band systems which are distinguished by vibrational spacings and by their propensity towards production of either N2O+ or Ar+. Variations in the Ar+/N2O+ photoproduct ratio with wavelength are explained as due to vibrational predissociation on different potential-energy surfaces correlating with either Ar+ or N2O+ products. The first band system, observed exclusively at the N2O mass, has its origin close to 445 nm, corresponding approximately to the difference in the energies of N2O+[X 2Π3/2]+Ar[1S0] and N2O[1Σ+]+Ar+[2P3/2] and is assigned as an intracluster charge-transfer transition. Two strong band systems situated to higher energy are assigned as transitions to the two additional electronic states which are expected to correlate with 2P3/2 and 2P1/2 Ar+ and N2O[1Σ+] products. While excitation of these two bands results almost exclusively in Ar+ production, a fourth weaker band near 342 nm leads to N2O+ and appears likely to be a transition to a state correlating with an excited vibronic state of N2O+[A 2Σ+(1,0,0)]+Ar[1S0]. The different band systems exhibit extensive vibrational progressions involving the deformation of the bond between the N2O and the Ar. The shift in the onset of the first charge transfer from the difference in the Ar and N2O ionization potentials combined with the appearance energy for Ar+ production allow tentative estimates of 690 and 1340 cm−1 to be made for the dissociation energies of the lowest and first excited states of N2O+–Ar.
193 nm photodissociation dynamics of nitromethane
Authors: Moss D. B. , Trentelman K. A. , Houston P. L.
Published: July 2002

Multiphoton ionization spectroscopy and time-of-flight mass spectrometry have been used to determine nascent photofragment energy distributions for several of the products of the 193 nm photolysis of nitromethane (CH3NO2). Internal energy distributions have been obtained for CH3 and NO(X 2Π), and translational energy distributions for CH3, NO(A 2Σ+), and O(3P). The production of two NO electronic states (X and A) and the appearance of two peaks in the translational energy distributions of the CH3 and O fragments are consistent with earlier proposals of a two-channel dissociation. The major channel produces CH3 and NO2(1 2B2), some of the latter having sufficient internal excitation to further dissociate to NO(X) and O. The minor channel is believed to produce NO2 in a different electronic state which subsequently absorbs a second 193 nm photon and dissociates to yield NO(A) and O. The major channel NO2 dissociation dynamics are fit well by an impulsive model, while the minor channel apparently partitions much of the available energy into NO(A) vibration and/or rotation.
Rigid bender analysis of van der Waals complexes: The intermolecular bending potential of a hydrogen bond
Authors: Nesbitt David J. , Lovejoy Christopher M.
Published: July 2002

High resolution ir data on weakly bound OCOHF complexes formed in a slit supersonic expansion reveal a progression of extremely low frequency vibrational levels associated with the bending of the OCO–HF hydrogen bond. In a previous paper [J. Chem. Phys. 93, 7716 (1990)], we presented a spectroscopic analysis of the fundamental, combination and hot bands observed, corresponding to transitions between vlbend=00, 11, 20, 22, and 31, where vlbend denotes quanta of OCOHF skeletal bend excitation with l units of vibrational angular momentum. In this paper, we analyze the rotationally resolved data in terms of the rigid bender formalism of Hougen, Bunker and Johns to determine an explicit angular potential, V(θ), for the OCOHF complex in both the HF ground (vHF=0) and vibrationally excited (vHF=1) state.

The OCOHF ground state (vHF=0) potential is dominated by quartic and sextic angular terms, and thus is surprisingly shallow with respect to the bending angle. This quasilinear vibrational behavior is characterized by wide amplitude bending wave functions with zero point motion extending from −38° to +38°. In contrast, the OCOHF excited state (vHF=1) exhibits a significantly bent equilibrium geometry with a hydrogen bond bend angle of 31°±5°, corresponding to a cylindrically symmetric, noncolinear minimum in the potential. This shift in equilibrium geometry upon vHF excitation is quantitatively responsible for promoting Δvbend=0,2,... combination band vibrational modes, in analogy with Franck–Condon progressions in a bent←linear electronic transition.

The predissociation lifetimes for vHF=1 excited OCOHF vary systematically with vlbend, and can be analyzed in terms of a geometry dependent predissociation rate which increases with bending of the hydrogen bond angle. These empirical bending potentials are in qualitative agreement with, but quantitatively much shallower than predicted by previous electrostatic and ab initio calculations, and differ fundamentally from the traditional notions of a relatively stiff, linear hydrogen bond. The present results on the hydrogen bond potential surface for O=C=O---HF are consistent with statistical analyses of–C=O---H–N hydrogen bond angles obtained from x-ray crystallographic studies of proteins.
A laser study of the blue electronic transitions of CaS
Authors: Jarman C. N. , Hailey R. A. , Bernath P. F.
Published: July 2002

Three new electronic transitions of CaS have been observed in the blue region of the spectrum. The vibrational bands in these transitions span thousands of cm−1 owing to a large change in geometry. Approximately 1000 cm−1 portion of the spectrum containing these bands has been recorded at high resolution by dye laser excitation spectroscopy and analyzed. The new states all have Ω=1, and are tentatively labeled G 1Π, F 1Π, and f 3Π1. Spectroscopic constants have been generated for all three upper states and for the X 1Σ+ state from a fit to 4340 individual lines. These constants have been used to generate potential curves and Franck–Condon factors.
A dynamic Monte Carlo method suitable for molecular simulations
Authors: Kotelyanskii Michael J. , Suter Ulrich W.
Published: July 2002

A dynamic Monte Carlo (MC) method for simulations has been presented which is similar to the well-known Brownian dynamics method, but is suitable for many systems and can readily be adopted to isobaric simulations. This dynamic MC has been applied to several systems and shows similar accuracy and improved efficiency compared with routine methods. It samples configuration space approximately according to the equilibrium probability density and is applicable to NPT simulations of rather complex systems, also with rigid constraints.
Selective bond fission in methyl mercaptan at 193 nm via radial derivative coupling between the 2 1A″ and 1 1A″ adiabatic electronic states
Authors: Keller J. S. , Kash P. W. , Jensen E. , Butler L. J.
Published: July 2002

We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.
Activation barriers for series of exothermic homologous reactions. II. Reactions of s1 metal atoms with N2O
Authors: Futerko Peter M. , Fontijn Arthur
Published: July 2002

We recently showed [J. Chem. Phys. 95, 8065 (1991)] that the activation barriers of reactions between ground state metal atoms and N2O correlate with the s-p promotion energies plus the ionization potentials of the metal atoms. A semiempirical calculation method was developed there to interpret these observations. However, the treatment of the alkali metals required an ad hoc assumption. It is shown here that, by considering interaction potentials as functions of the distances between the metal and oxygen atoms, agreement between theory and experiment is obtained without any such hypothesis.
Quasielastic light scattering study of the formation of inhomogeneities in gels
Authors: Suzuki Yoshiharu , Nozaki Kae , Yamamoto Tetsuya , Itoh Kiichi , Nishio Izumi
Published: July 2002

The inhomogeneities in gels, in particular the mechanism of the formation of the inhomogeneities, were studied. The gels used in this report are water based poly(acrylamide) gels (AA gels) and poly(N-isopropylacrylamide) gels (N-IPA gels). It is known that the cross linkers in AA gels are water insoluble at low temperature and that the linear components in N-IPA gels are water insoluble at high temperature. By changing the gelation temperature at which gels are made, we can control the formation of the inhomogeneous structure in the samples. In order to investigate the inhomogeneities in the gels, two experiments were performed by using the standard laser light scattering method. For the characterization of the degree of inhomogeneities in gels, the position dependence of scattered light intensity from homogeneous and inhomogeneous gels were measured. It was found that the position dependence was a very good indicator of gel inhomogeneities. We also observed the change of the autocorrelation function of scattered light intensity from a sample undergoing the gelation process. It was found that the profile of the correlation functions measured from pregel solution, which is going to be inhomogeneous gel, were different from the correlation functions measured from pregel solution which is going to be homogeneous gel. From the correlation function measured from the sample which will be inhomogeneous gels, two relaxation modes were observed−one is the fast decaying mode which is expected to be observed from ideal gels or ideal semidilute solutions (the cooperative diffusion) and another mode is the slow decaying mode which is not expected to be observed from ideal gels or ideal semidilute solutions. From the experimental results, we discussed the mechanism of the formation of inhomogeneities in gels.
Stochastic linear model for nonadiabatic condensed phase reactions. Weak coupling limit
Authors: Shushin A. I.
Published: July 2002

Condensed phase nonadiabatic transitions are studied in a semiclassical two state linear model. The effect of condensed phase is modeled by stochastic motion along the classical coordinate and by stochastic fluctuations of parameters of semiclassical Hamiltonian: slopes of diabatic terms, coupling of terms, and coordinate of avoided crossing. In the weak coupling limit simple formulas for nonadiabatic transition rate are derived without any assumptions about the mechanisms of stochastic classical motion and fluctuations of parameters. The rate appears to be independent of the mechanism of motion. The dependence on fluctuating parameters is expressed only through simple averages over parameter distributions, independent of correlation times of fluctuations.
Nonadiabatic effects in the statistical adiabatic channel model: The atom+diatom case
Authors: Dashevskaya E. I. , Nikitin E. E. , Troe J.
Published: July 2002

Nonadiabatic coupling between exit adiabatic channel states is considered for an atom+diatom collision complex. Nonadiabatic transition probabilities are estimated on the basis of a sequential two-state correlation diagram which qualitatively reproduces adiabatic potential barriers arising from an anisotropic interaction in a system ‘‘point charge–linear dipole rotor.’’ The implication of nonadiabatic interaction in changing statistical adiabatic channel predictions for product distribution functions are briefly discussed.
Anisotropic velocity distribution of desorbing product in carbon monoxide oxidation on palladium (110)
Authors: Matsushima Tatsuo , Shobatake Kosuke , Ohno Yuichi , Tabayashi Kiyohiko
Published: July 2002

Significant anisotropy was found in the velocity distributions of desorbing product CO2 from a Pd(110) surface. The velocity distributions were determined by a cross-correlation time-of-flight technique combined with angle-resolved thermal desorption. Heating the coadlayer of CO and oxygen produces five peaks in the CO2 formation spectrum; P1– (around 420 K), P2– (∼370 K), P3– (∼300 K), P4– (∼230 K), and P5–CO2 (∼170 K). The translational temperature of each CO2 is much higher than the corresponding surface temperature, and increases in the sequence of P1– <P2– <P3– <P4– <P5–CO2. It decreases rapidly with an increase in the desorption angle perpendicular to the surface trough and more slowly parallel to it. This anisotropy is correlated to the reaction site symmetry.
Observation of torsional motion in the ground-state cation of jet-cooled tolane by two-color threshold photoelectron spectroscopy
Authors: Okuyama Katsuhiko , Cockett Martin C. R. , Kimura Katsumi
Published: July 2002

High-resolution threshold photoelectron spectra of jet-cooled tolane (C6H5–C≡C–C6H5) have been obtained by two-color (1+1′) resonantly enhanced multiphoton ionization via the S1 state {1B1u(ππ*)}. From the threshold photoelectron spectra, the adiabatic ionization energy has been determined to be Ia = 63 917 ± 3 cm−1 (7.9243±0.0004 eV). A vibrational progression due to torsional motion in the cation ground state was observed up to its fifth quantum level with a fundamental frequency of 54 cm−1, showing small anharmonicity. From these results, we have evaluated the potential barrier height to the torsional motion to be 1980±60 cm−1. The threshold photoelectron spectra obtained via several S1 vibronic levels exhibit dominant peaks due to the Δv=0 ionization transitions. This fact indicates that no significant change occurs in molecular structure upon photoionization with removal of the antibonding π electron from the S1 state. Threshold photoelectron bands arising from formally forbidden odd quantum transitions in the torsional mode were also clearly observed with small intensities. The mechanism of the appearance of such a nontotally symmetric mode is discussed.
Reaction dynamics on barrierless reaction surfaces. II. Microcanonical variational transition states
Authors: Lim Kieran F. , Keir Roland I.
Published: July 2002

A previous quasiclassical trajectory investigation of a generic proton-transfer reaction [Lim and Brauman, J. Chem. Phys. 94, 7164 (1991)] suggested the existence of both the usual centrifugal barrier transition state and a ‘‘dynamic’’ nonenergetic transition state for association on a barrierless potential surface. This paper reports a microcanonical variational transition state theory investigation of the same potential surface. The dynamic transition state is found to fulfill the variational criterion of a minimum in the sum of states. Implications for ion/molecule reactions are discussed.
Laser spectroscopy of the CaOH A 2Π–X 2Σ+ (020)–(000) band: Deperturbation of the Fermi resonance, Renner–Teller, and spin–orbit interactions
Authors: Li Mingguang , Coxon John A.
Published: July 2002

The A 2Π–X 2Σ+ (020)–(000) band of CaOH has been observed using cw dye laser excitation, and rotationally analyzed. The measured line positions, in combination with data obtained in our laboratory for the A 2Π–X 2Σ+ (100)–(000) band, have been employed in a global deperturbation of the A(100) 2Π and A(020)κ 2Π and μ 2Π vibronic states that simultaneously takes account of Fermi resonance, the Renner–Teller effect, and spin–orbit coupling. All the measured line positions are reproduced to within the measurement accuracy of 0.005 cm−1. The Renner–Teller and Fermi resonance parameters in the CaOH A 2Π state have been determined: εω2=−36.569 19(97) cm−1 and W1=10.300 58(54) cm−1.
Excimer formation dynamics in pure and mixed naphthalene clusters
Authors: Saigusa Hiroyuki , Sun Sheng , Lim Edward C.
Published: July 2002

The excited-state dynamics of the pure naphthalene clusters and of the mixed clusters involving two naphthalenes solvated with durene have been investigated as a function of the excess vibrational energy of the initially excited clusters. All of the clusters studied exhibit excimer fluorescence, depending strongly on the excess vibrational energy and on the solvation number. The isomerization rate has been measured by detecting a buildup in the excimer fluorescence and used as a probe of the cluster geometry. The trimer requires an excess energy of 870 cm−1 to promote the excimer formation with a rise time of 32 ns, suggesting a structurally less favorable process. The dimer and tetramer undergo rapid isomerization at much lower energies. The solvent addition is found to impede excimer formation, due to geometry restriction. The singly solvated cluster exhibits an excimer evolution time as slow as 60 ns.