bluuebunny/crossref_metadata_2025 huggingface dataset:
The nuclear hexadecapole interaction of iodine-127 in cadmium iodide measured using zero-field two dimensional nuclear magnetic resonance
Authors: Liao Ming-Yuan , Harbison Gerard S.
Published: July 2002

Two dimensional nuclear quadrupole correlation spectroscopy has been used to measure simultaneously the two nuclear spin transition frequencies for the iodine resonance of cadmium iodide at zero magnetic field. Because of the layer structure and resultant polymorphism in this substance, conventional zero-field NMR spectra are inhomogeneously broadened by several hundred kHz. However, the 2D spectra obtained by our method are subject only to homogeneous linewidth, which for this compound is of the order of 5 kHz. This greatly increased precision allows more accurate evaluation of the nuclear hexadecapole coupling in cadmium iodide; which we measure as 0.216±0.514 kHz (i.e., indistinguishable from zero). This gives a maximum value of the nuclear hexadecapole moment several times lower than that recently reported for the same nucleus in potassium iodide.
On the vibronic structure of the absorption spectra of radical cations of some polycyclic aromatic hydrocarbons
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

The vibronic structure of electronic absorption spectra of four polyacenic radical cations is investigated by means of quantum chemical calculations. We use the semiempirical quantum consistent force field/π electron method (QCFF/PI) augmented by a configuration interaction (CI) scheme that includes all excited configurations having at most one electron in the originally empty molecular orbitals and the ab initio restricted open shell Hartree–Fock (ROHF) method. Displacement parameters for the totally symmetric modes are calculated for different electronic transitions and the resulting Franck–Condon structure is compared with the experimental spectra. The analysis reaffirms the initial interpretation of the absorption spectrum of the cation of naphthalene and reinterprets the recently observed spectrum of the cation of anthracene. First ever analysis of the absorption spectrum of the radical cation of pyrene and perylene is given. For the former, we demonstrate that the unusually large frequency value of the highest ag mode active in the strong absorption band of astronomical interest (443.8 nm) is due to vibronic coupling. For the latter, a resonance vibronic coupling mechanism between manifolds of the 1 2B3g and 1 2B2g states is suggested to explain the doublet structure of the origin of the transition to the latter state.
Anionic states of LiFLi
Authors: Gutowski Maciej , Simons Jack
Published: July 2002

Our ab initio theoretical calculations indicate that the LiFLi− anion possesses two bound states which are of 1Σ+g and 3Σ+u symmetry and which have vertical detachment energies of 0.94 and 0.89 eV, respectively. The LiFLi+ cation and the first four electronic states of the neutral LiFLi were also considered. The Renner–Teller splitting for the excited 2Πu state of the neutral is reported. The neutral and both ions are thermodynamically stable.
Fluorescence spectra and torsional potential functions for trans-stilbene in its S and S1(π,π*) electronic states
Authors: Chiang Whe-Yi , Laane Jaan
Published: July 2002

The fluorescence excitation spectra and dispersed fluorescence spectra of trans-stilbene have been recorded and analyzed. Vibrational assignments for the eight low-frequency modes have been made for both the S0 and S1(π,π*) electronic states, and these differ substantially from those of previous workers. Two-dimensional kinetic and potential energy calculations were carried out in order to determine the potential energy surfaces for the two phenyl internal rotations ν37 and ν48. The function V(φ1,φ2)= 1/2V2(2+cos 2φ1+cos 2φ2)+V12 cos 2φ1 cos 2φ2 +V12′ sin 2φ1 sin 2φ2, with V2=1550 cm−1, V12=337.5 cm−1, and V12′ = 402.5 cm−1 for the S0 state and with V2=1500 cm−1, V12=−85 cm−1, and V12′ = −55 cm−1 for the S1(π,π*) state fits the observed data (nine frequencies for S0 and six for S1) extremely well. The barriers to simultaneous internal rotation of both phenyl groups are given by twice the V2 values. The fundamental frequencies for these torsions are ν37=9 cm−1 and ν48=118 cm−1 for the S0 state and ν37=35 cm−1 and ν48=110 cm−1 for the S1 excited state. The third torsion ν35, which is the internal rotation about the C=C bond, was assigned at 101 cm−1 for the S0 state based on a series of overtone frequencies (202, 404 cm−1, etc.). For S1, ν35=99 cm−1 based on observed frequencies at 198, 396 cm−1, etc.

Kinetic energy calculations were also carried out for this mode, and a one-dimensional potential energy function of the form V(θ)=1/2V1(1−cos θ)+1/2V2(1−cos 2θ)+1/2V4(1−cos 4θ) was utilized to reproduce the frequencies for the ground state. For the excited state, an additional V8 term was added in order to fit the data for the trans potential energy well. The data indicate that the trans→twist barrier for the S1 state is higher than 1400 cm−1. However, a somewhat revised frequency assignment would be compatible with a barrier of 1250 cm−1, which is close to the value of 1200 cm−1 determined from dynamics studies.
Selective population of spin–orbit levels in the autoionization of a polyatomic molecule: Branching ratios and asymmetry parameters for the Tanaka–Ogawa Rydberg series in CO2
Authors: Parr A. C. , Dehmer P. M. , Dehmer J. L. , Ueda K. , West J. B. , Siggel M. R. F. , Hayes M. A.
Published: July 2002

The spin–orbit selectivity of angle-resolved photoelectron spectra was used to provide new information on the electronic structure, symmetry, and decay dynamics of members of the autoionizing Tanaka–Ogawa Rydberg series in CO2. This represents the first time that spin–orbit selectivity has been used to obtain such information for a polyatomic molecule. The spin–orbit photoelectron branching ratios were used to show that the angular momentum quantum number λ of the excited Rydberg electron does not change upon autoionization. Furthermore, a consideration of the present results together with previous calculations of the relative intensities of the discrete and continuum ionization channels shows that the most probable electron configuration for the Tanaka–Ogawa Rydberg series is ...(πu)3(πg)4ndδg and that autoionization proceeds primarily via a dδg→εfδu process for the totally symmetric vibronic components of the ion. The asymmetry parameter β was determined for individual spin–orbit components of the various vibronic bands of the X̃ 2Πg state and is discussed in terms of recent theoretical calculations. The Rydberg series appears to be well described by Ωcω coupling, even for relatively low principal quantum numbers. The general utility of this technique for autoionizing Rydberg states and its extension to multiphoton ionization of Rydberg states that lie below the first ionization threshold are discussed.
N–O versus N–N bond activation in reaction of N2O with carbon cluster ions: Experimental and ab initio studies of the effects of geometric and electronic structure
Authors: Resat Marianne Sowa , Smolanoff Jason N. , Goldman Ilyse B. , Anderson Scott L.
Published: July 2002

We report a combined experimental and theoretical study of the reaction of small carbon cluster cations with N2O aimed at understanding the reaction mechanism and how it is affected by the electronic and geometric structure of the C+n reactants. Cross sections for reaction of C+n (n=3–12) with N2O were measured over a collision energy range from 0.1–10 eV, using a guided ion beam tandem mass spectrometer. Ab initio calculations were used to examine the structure and energetics of reactant and product species. Small clusters, which are linear, react with no activation barrier, resulting in either oxide or nitride formation. The branching between oxide and nitride channels shows a strong even–odd alternation, with even clusters preferentially forming nitrides. This appears to be correlated with an even/odd alternation in the ionization potential of the CnN. The larger, monocyclic C+n have activation barriers for reaction, and a completely different product distribution. Secondary reactions of the primary oxide and nitride products were studied at high N2O pressures. Products containing two O or two N atoms are not observed, but it is possible to add one of each. Possible reaction mechanisms are discussed and supported by thermochemistry derived from spin restricted ab initio calculations.
High resolution infrared spectroscopy of cyclobutane: A study of vibrational mode coupling involving large amplitude, low frequency modes
Authors: Li H. , Miller C. Cameron , Philips Laura A.
Published: July 2002

The high resolution IR spectrum of cyclobutane in a supersonic molecular beam was obtained for the region of 2981 to 2991 cm−1. The spectrum reveals four overlapping bands suggestive of vibrational mode coupling in the C–H stretching region. Ground state combination differences demonstrate that these bands originate from two different ground states, the symmetric and asymmetric ring puckering states. Evidence of vibrational mode coupling is present in all four bands. The coupling depends on both J and the symmetry of the puckering state. A model coupling scheme involving two qualitatively different types of couplings is developed to explain the observed spectrum. Symmetry restrictions and the interaction between molecular rotation and ring puckering qualitatively accounts for the dramatically different coupling behavior between the two ring puckering states.
Adsorption of polyelectrolytes onto curved surfaces
Authors: von Goeler F. , Muthukumar M.
Published: July 2002

The conditions for adsorption of a polyelectrolyte chain in a salt solution onto uniformly charged spherical and cylindrical surfaces are derived using a combination of variational procedures and the ground state dominance approximation. Explicit formulas are obtained for the dependence of adsorption characteristics on temperature, the Debye screening length, polyelectrolyte charge density, molecular weight, and curvature. The density profile of adsorbed polyelectrolytes is determined as a function of these parameters.
Vibron dynamics in naphthalene crystal
Authors: Panero Cristina , Bini Roberto , Schettino Vincenzo
Published: July 2002

The polarized infrared spectrum of naphthalene single crystal has been studied between 800 and 1600 cm−1. The dependence on temperature of the linewidth of eight Davydov components of internal modes has been studied in the 10–240 K temperature range. The deconvolution of the experimental line shapes allows us to extract a constant inhomogeneous contribution to the line broadening, due probably to crystal defects, over all the temperature range. The bandwidth data are interpreted on the basis of elementary relaxation mechanisms. In particular, at low temperature, the vibrational decay is dominated by three-phonon depopulation processes. Above 80 K, higher order dephasing mechanisms generally contribute to the vibron dynamics. The calculated decay compares well with experimental results. The infrared results are compared with the relaxation data presented for the Raman modes in previous papers.
The dielectric constant of polar fluids and the distribution of the total dipole moment
Authors: Kusalik P. G. , Mandy M. E. , Svishchev I. M.
Published: July 2002

This article examines the distribution of fluctuations of the total dipole moment M in polar fluids. Detailed results, including average energies, pressures, and dielectric constants, are reported from molecular dynamics simulations of a dipolar soft-sphere system in reaction field (RF) and periodic boundary conditions (PBC). Both cubic and truncated octahedral simulation cells are employed and we demonstrate that the properties of interest are insensitive to our choice of cell geometry. Some dependence upon RF cutoff is observed, particularly in results for the static dielectric constant. The distribution of instantaneous values of M shows no anisotropy, and we argue that the isotropic probability distribution function P(g), where g is the Kirkwood correlation factor and depends only upon the magnitude of M, is sufficient to fully characterize the fluctuations in the total moment for a large sample of a polar liquid. We demonstrate that the functional form for P(g) previously tested for a dipolar fluid in PBC [P. G. Kusalik, Mol. Phys. 80, 225 (1993)] holds for RF boundary conditions; we also find that this form is obeyed by other polar liquids such as water and methanol. The distribution function P(g) is found to be particularly effective at detecting finite size effects in simulations of polar liquids. We also show how knowledge of the functional form for P(g) can be efficaciously used to achieve significant reductions in the computational resources required to determine a static dielectric constant for a polar liquid.
Absolute transition probability measurement of valence-shell electronic structure of CF4 and CCl4: Bethe surfaces, and dipole-dominated preionization-edge valence and Rydberg states
Authors: Ying J. F. , Leung K. T.
Published: July 2002

Absolute generalized transition probabilities or generalized oscillator strengths (GOSs) of valence-shell electronic transitions of CF4 and CCl4 have been determined as functions of energy loss and momentum transfer (i.e., the Bethe surfaces) using angle-resolved electron energy loss spectroscopy (EELS) at an impact energy of 2.5 keV. A low-lying feature at 7.2 eV was observed for CCl4 and was attributed predominantly to electronic transitions from the Cl 3p nonbonding orbitals (2t1, 7t2, and 2e) to a C–Cl σ* antibonding orbital (7a1), based on the result of a single-excitation configuration interaction excited-state calculation. The experimental GOS profile of this low-lying feature was found to have a shape characteristic of a mixture of dipole-allowed and nondipole transitions with relative maxima at momentum transfers of 0 and ∼0.9 a.u., respectively. GOS profiles of other low-lying discrete transitions below the first ionization edge at 12.6 and 13.7 eV in CF4 and at 8.7 and 9.7 eV in CCl4 were also determined and found to have secondary minima and maxima, in addition to the strong maximum at zero momentum transfer. These excitation features were assigned mainly to Rydberg transitions originated from the nonbonding highest occupied molecular orbitals (HOMOs) with t1 symmetry and the second HOMOs with t2 symmetry. The extrema in the GOS profiles have been discussed by considering the spatial overlap of the initial-state and final-state orbital wave functions. Tentative assignments for the rest of the valence-shell energy-loss features of CF4 (5–200 eV) and CCl4 (5–150 eV) were also inferred from the term values reported previously.
Theoretical analysis of the charge-storage states in diphenylpolyenes with one to seven double bonds
Authors: Lögdlund M. , Brédas J. L.
Published: July 2002

The semiempirical Austin Model 1 (AM1) and the nonempirical pseudopotential valence effective Hamiltonian (VEH) methods have been applied for the investigation of the doping-induced electronic and geometrical changes in a series of conjugated molecules. The series consists of diphenylpolyenes with an even number of carbons and extends from stilbene to diphenyltetradecaheptaene, i.e., one to seven double bonds in the polyene part of the molecule. The densities of valence states, as calculated using VEH, are directly compared with the experimental valence band spectra, as recorded by ultraviolet photoelectron spectroscopy, as a function of increasing sodium exposure of the molecular solids. The charge-storage states in the series are discussed in terms of soliton–antisoliton- and polaronlike states, induced upon doping (reduction). In the cases of diphenyltetradecaheptaene and diphenyldodecahexaene (i.e., seven and six double bonds in the polyene part of the molecules), charge-storage states in the form of soliton–antisoliton pairs are induced even at the lowest doping levels, up to saturation at about two sodiums per molecule. In contrast, polaronlike charge storage states are formed at low to intermediate doping levels in diphenyldecapentaene down to stilbene (i.e., molecules with five to one double bonds in the polyene part of the molecules). However, at saturation, all the molecules in the series are able to accommodate two sodium atoms, resulting in soliton–antisoliton-like charge storage states.
Observation and identification of the g 6Φ–a 6Δ transition in the blue system of FeH at 493 nm
Authors: Carter Robert T. , Brown John M.
Published: July 2002

The blue system of FeH, at 493 nm, has been recorded with a sample at near-ambient temperatures by laser excitation spectroscopy. With the help of dispersed fluorescence studies, all the strong lines in this region of the spectrum have been assigned to the (0,0) band of the g 6Φ–a 6Δ transition, where the lower state lies about 1890 cm−1 above the ground 4Δ state. Assignments have been made to ΔΩ=+1 and ΔΩ=0 subbands involving the lowest three spin components in each state. Term values have been determined for the levels of the three lowest spin components of the a 6Δ state and thereby for the g 6Φ11/2 spin component, which were hitherto unknown. It is established that the a 6Δ state of FeH is inverted and that the vibrational interval ΔG1/2 varies markedly between the spin components of this state:   6ΔΩ    9/2  7/2  5/2 ΔG1/2/cm−1  1622  1569  1582.   There is evidence that the a 6Δ state is perturbed by levels of other, nearby electronic states and it is not possible to model its levels with a simple effective Hamiltonian. The radiative lifetime of FeH in the two lowest spin components of the g 6Φ state has been measured as 39±2 ns.
Dissociation dynamics of CH+4 core ion in the 2A1 state
Authors: Furuya Kenji , Kimura Katsumi , Sakai Yasuhiro , Takayanagi Toshinobu , Yonekura Nobuaki
Published: July 2002

Threshold-photoelectron photoion coincidence (TPEPICO) spectra of CH4 have been observed with synchrotron radiation at the excitation to the 2A1 (ν1=0−3) ionic states as well as to the 4pt2 Rydberg (ν1=0−4) states. In all the TPEPICO spectra observed, the CH+3 band shape was almost rectangular, which suggests that the translational and internal energy distributions of CH+3 are very narrow. The total kinetic energy releases (KERs) have been estimated from the CH+3 band shape. As a result, it was found that the CH+3 species were in an electronically excited state. There was a narrow distribution of the total KERs and similarity in the TPEPICO CH+3 band shapes between the spectra at the 2A1 ionic state and the 4pt2 Rydberg state excitations, which led to the conclusion that the Rydberg electron is just a spectator and the dissociation of the core ion plays an important role in dissociation through the 4pt2 Rydberg state. Similar results have also been obtained for CH+2 and CH+ productions. However, on the other hand, an H+ fragment has been observed only at the 2A1 state excitation. It showed a band with a long tail in the slower flight time region. The total average KERs and the decay rates have been estimated from band shape simulation. From these results, it has been found that a dissociation limit of the H+ ion exists just below the 2A1 ionic state. The dissociation mechanisms through the 4pt2 Rydberg state have been discussed in detail in comparison with those of the 2A1 ionic state.
On the two reaction pathways of planar HCN isomerization. II
Authors: Ling Song
Published: July 2002

We show, by applying the theory of a periodic orbit moving on the inverted potential surface, that the two isomerization pathways of planar HCN are not equivalent at energies below the potential barrier (tunneling). Moreover, rotation alleviates tunneling while it hinders, as demonstrated in the earlier study [Ling, J. Chem. Phys. 100, 8554 (1994)] barrier cross over. This concludes our investigations of the equivalence (or not) of the two reaction pathways of planar HCN isomerization, at energies both above and below the reaction barrier.
Crossed beam rovibrational energy transfer from S1 glyoxal. IV. Reduced mass effects and an overview of the inelastic scattering characteristics from four initial levels
Authors: Gilbert Brian D. , Parmenter Charles S. , Krajnovich Douglas J.
Published: July 2002

Crossed molecular beam studies of rotationally and rovibrationally inelastic scattering of S1 glyoxal from H2 and He have been extended to one additional light gas, D2, and to two heavy gases of identical masses, Kr and cyclohexane, C6H12 (84 amu). Laser excitation was used to prepare glyoxal in its 00 level with K′=0 and 0≤J′≤10. Dispersed fluorescence detection was used to observe the final K′ and vibrational states of the inelastic scattering. The relative scattering cross sections for D2 and He collisions are identical to within experimental error and differ substantially from those of H2. The Kr and C6H12 cross sections are also a matched set. These results show that the competition among the approximately 25 observable scattering channels is far more sensitive to the reduced mass of the collision than to variation in the intermolecular potential or even the internal structure of the target gas. An overview of rotational and rovibrational scattering in glyoxal from four vibrational levels (00, 72, 51, and 81) extending to εvib=735 cm−1 is used to uncover generalities and insights about the energy transfer. For all four initial levels the vibrational state changes are highly selective. The detectable channels are always limited to ±1 quantum change in only one of the 12 modes, specifically ν7′ = 233 cm1, the lowest frequency mode. The cross sections for vibrational state change are surprisingly large relative to those for pure rotationally inelastic scattering.

Many cases occur with the light target gases where the ΔK resolved cross sections for rovibrational interactions are nearly equal to those for pure rotationally inelastic scattering with equivalent energy transfer ΔE. Scattering from 72, K′=0 glyoxal contains examples with both H2 and He where the rovibrational cross sections actually exceed those for rotational scattering. Plots of the entire set of cross sections [rotational (ΔK) plus rovibrational (Δυ7=+1)] against ΔE are essentially superimposible for He scattering from 00, 51, and 81 glyoxal. In contrast, scattering from 72 glyoxal with the active mode initially excited is distinctive. For all initial levels, the distribution of cross sections for different ΔK within rotational channels differs from that within rovibrational channels. It is further seen in these comparisons that the change in angular momentum ΔK rather than ΔE controls the relative sizes of cross sections within these channels. The theoretical predictions of Clary, Kroes, and Rettschnick are in accord with these trends and distinctions, agreeing even on some rather subtle points.
Multidimensional dynamics in the electron stimulated desorption of ammonia from Pt(111)
Authors: Burns A. R. , Stechel E. B. , Jennison D. R. , Li Y. S.
Published: July 2002

We characterize the electron stimulated desorption of neutral ammonia (NH3 and ND3) from Pt(111) with vibrational and rotational quantum resolution by using (2+1) resonance enhanced multiphoton ionization detection. Two significant isotope effects appear: (1) the desorption yield of NH3 is three times that of ND3 and (2) NH3 desorbs with considerably more ‘‘spinning’’ rotational energy than does ND3. We find virtually identical translational energy distributions for each desorbate and roughly equal vibrational energy distributions. Vibrational excitation is found exclusively in the ν2 symmetric deformation or ‘‘umbrella’’ mode, independent of isotope. These effects cannot be explained by desorption induced by vibrational energy transfer. Instead, desorption is the result of excitation of a 3a1 electron principally on the N atom, which causes the pyramidal NH3 adsorbate to rapidly invert. Ab initio calculations of two-dimensional potential energy surfaces (intramolecular bond angle and surface bond length) reveal that near-inverted molecules deexcite to a repulsive hard wall of the adsorbate–substrate interaction and desorb. Spinning excitation derives from the rotational barrier of the inverted molecule. Both isotope effects are direct consequences of desorption via inversion. In general, multidimensional dynamics must be considered in the study of stimulated surface processes. Our calculations also indicate that excited-state forces at equilibrium molecule–surface distances are an order of magnitude less than those derived from a currently accepted image-potential model.
Resonance enhanced multiphoton ionization spectroscopy of the NF molecule: 1,3Φ 3d and 4d Rydberg states
Authors: Boggis S. A. , Dyke J. M. , Hughes A. N. , Keszthelyi T. , Richter R. , Tabrizchi M.
Published: July 2002

The (3dδ)1,3Φ and (4dδ)1,3Φ Rydberg states of NF have been investigated by multiphoton ionization (MPI) spectroscopy. These states were observed as two-photon resonances in an overall (2+1) MPI process from NF a1Δ produced from the F+N3 reaction. Ab initio calculations performed at the multireference double excitation configuration interaction level showed that the excited Φ states were of Rydberg character with configurations of ...1π45σ22π1(3dδ)1 and ...1π45σ22π1(4dδ)1. The 3Φ←a1Δ two-photon transitions were found to derive their intensities from spin–orbit and spin-uncoupling interactions in the 1Φ3, 3Φ4, 3Φ3, and 3Φ2 upper states. Analysis of the rotationally resolved bands, using a model which includes these factors, allowed the spin–orbit constant, aπ, for the 2π valence orbital to be derived as (159.0±1.0) cm−1. Rotationally resolved envelopes recorded for the (3dδ)1Φ, v′=0, 1←a1Δ, v″=0 and (3pπ)1Σ+, v′=0←a1Δ, v″=0 (2+1) NF MPI bands, could be simulated reasonably well assuming a Boltzmann rotational distribution for the a 1Δ state at ≊180 K. Experiments showed, however, that this was not a true measure of the initial state distribution because of predissociation or perturbation effects in the resonant intermediate state. Assignments for other two-photon resonant NF MPI bands observed in this work are also suggested.
An ab initio study on the ground state HBO–BOH system
Authors: Richards Claude A. , Vacek George , DeLeeuw Bradley J. , Yamaguchi Yukio , Schaefer Henry F.
Published: July 2002

Ab initio electronic structure theory has been employed in order to investigate the ground state potential energy hypersurfaces of the HBO–BOH system. Geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities of two equilibrium and two transition state [inversion (bending through linear geometry) and isomerization] structures were determined at the self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory using three basis sets. The theoretically predicted geometries and physical properties agree very well with available experimental values. At the highest level of theory employed in this study, CCSD(T) using triple zeta plus double polarization with higher angular momentum function [TZ2P(f,d)] basis set, the linear HBO molecule is predicted to be 45.0 kcal/mol more stable relative to the bent BOH species; with the zero-point vibrational energy (ZPVE) correction this energy separation becomes 44.4 kcal/mol; the classical barrier height for the inversion motion of the bent BOH molecule is predicted to be 3.5 kcal/mol and the barrier height with the ZPVE correction is 3.0 kcal/mol; the classical activation energy for the isomerization (1,2 hydrogen shift) reaction BOH→HBO is determined to be 29.4 kcal/mol and the activation energy with the ZPVE correction is 26.6 kcal/mol.
Dynamic nuclear magnetic resonance frequency shifts for spin 1/2 nuclei coupled to efficiently relaxed spin≳1/2 nuclei
Authors: Werbelow Lawrence G. , London Robert E.
Published: July 2002

Recently, it has been demonstrated that outside of the extreme narrowing regime, interference (cross-correlation) of dipole–dipole and quadrupole interactions can result in visually dramatic second order nuclear magnetic resonance (NMR) frequency shifts for spin one-half nuclei which are scalar coupled to spin-one nuclei. In the work presented here, the consequences of this dynamic frequency shift are examined in greater detail. Previous work has also shown that interferences between quadrupole and anisotropic chemical shieldings can provide significant differential line broadening across the spin one-half multiplet. These complementary calculations are extended to embody the general two spin system, I=1/2, S≳1/2, in the high field limit where extreme narrowing does not obtain.
Time-dependent discrete variable representations for quantum wave packet propagation
Authors: Sim Eunji , Makri Nancy
Published: July 2002

We present an efficient method for exact wave function propagation with several degrees of freedom based on time-dependent discrete variable representations (TD-DVR) of the evolution operator. The key idea is to use basis sets that evolve in time according to appropriate reference Hamiltonians to construct TD-DVR grids. The initial finite basis representation is chosen to include the initial wavefunction and thus the evolution under the bare zeroth order Hamiltonian is described at each time by a single DVR point. For this reason TD-DVR grids offer optimal representations in time-dependent calculations, allowing significant reduction of grid size and large time steps while requiring numerical effort that (for systems with several degrees of freedom) scales almost linearly with the total grid size. The method is readily applicable to systems described by time-dependent Hamiltonians. TD-DVR grids based on the time-dependent self-consistent field approximation are shown to be very useful in the study of intramolecular or collision dynamics.
Comment on ‘‘Infrared external reflection spectroscopy of adsorbates on dielectric substrates: Determining adsorbate orientation in Langmuir monolayers’’ [J. Chem. Phys. 98, 5825 (1993)]
Authors: Hoffmann Helmuth , Mayer Ulrich
Published: July 2002

In a reply to a recent paper by J. T. Buontempo and S. A. Rice [J. Chem. Phys. 98, 5825 (1993)], it is shown that incorrect expressions were used for the parallel and perpendicular components of the absorption indices k, which lead to erroneous intensities in the calculated infrared reflection spectra. Recalculation of the results with the correct equations shows that a molecular orientation analysis based on the dichroic intensity ratios, as proposed in that paper, generally does require a knowledge of the isotropic absorption coefficients.
Computer simulation study of the surface polarization of pure polar liquids
Authors: Brodskaya E. N. , Zakharov V. V.
Published: July 2002

Clusters containing 64 molecules of water or bromine were investigated by the molecular dynamics method. Several water models and a bromine model were taken into consideration in order to test the modern theory of the surface potential of polar liquid and to find out the dipole and quadrupole contribution to this potential. The results of the simulations confirm strongly the theoretical conclusion that equally with the dipole contribution there is a purely quadrupolar contribution to the surface potential.
Interadsorbate interactions in the c(4×2) NO/Ni(111) system
Authors: Stirniman M. J. , Li Wei , Sibener S. J.
Published: July 2002

Electron energy loss spectroscopy has been used to map the dispersion of the dipole active internal NO stretch and of the NO frustrated translation, which has not been previously observed, in the c(4×2) NO/Ni(111) system. The dispersion of the dipole active mode was fit with a model that assumed electrostatic dipole–dipole coupling (including image dipoles) between the adsorbates. The frustrated translation, on the other hand, showed no dispersion to within the resolution of the experiment across the entire surface Brillouin zone of the Ni(111) substrate. These measurements reveal new information on interadsorbate interactions in an important model system.
Rotational spectra and structures of the C6H6–HCN dimer and Ar3–HCN tetramer
Authors: Gutowsky H. S. , Arunan E. , Emilsson T. , Tschopp S. L. , Dykstra C. E.
Published: July 2002

A comparative study has been made of the rotational properties of C6H6–HCN and Ar3–HCN, observed with the Balle/Flygare pulsed beam, Fourier transform microwave spectrometer. C6H6–HCN is found to be a prolate symmetric top and Ar3–HCN an oblate one, both with the H in the middle. The rotational constants B0, DJ, and DJK of the parent species are 1219.9108(4) MHz, 1.12(3) kHz, and 18.32(8) kHz for C6H6–HCN, and 886.4878(1) MHz, 10.374(2) kHz, and 173.16(1) kHz for Ar3–HCN. Rotational constants are reported for the isotopic species C6H6–H13CN, -HC15N, and 13CC5H6–HC15N, and for Ar3–HC15N and -DCN. Analysis of the 14N hyperfine interaction χ finds its projection on the figure axis to be −4.223(4) MHz in C6H6–HCN and −1.143(2) in Ar3–HCN. They correspond to average projection angles θ between the HCN and figure axes of 15.2° and 45.3°, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 Å in C6H6–HCN and 3.47 Å in Ar3–HCN. While the rotational properties of C6H6–HCN are ‘‘normal,’’ those of Ar3–HCN display a long list of ‘‘abnormalities.’’

They include a J-dependent χ(14N) similar to that of Ar–HCN; a very large projection angle θ; large centrifugal distortion including higher-order terms in HJ and HJK; splitting of the K=3 transitions into J-dependent doublets; and the ready observation of an excited vibrational state. These behavioral differences are related qualitatively to the interaction surfaces for the two clusters, calculated with the molecular mechanics for clusters (MMC) model, and discussed. The potential minimum for C6H6–HCN is smooth, circular, steep except for a flat bottom, and deep (1762 cm−1). That for Ar3–HCN is tricuspid, with large gullies, and shallow (507 cm−1). In addition to the dispersion forces, the dominant interaction forming C6H6–HCN is between the benzene quadrupole moment and the HCN dipole moment, a strong 4-2 potential. That in Ar3–HCN is polarization of the spherical Ar by the HCN dipole and quadrupole moments, a weak 0–2,4 potential.
Electronic mechanism of the surface enhanced Raman scattering
Authors: Nakai Hiromi , Nakatsuji Hiroshi
Published: July 2002

Electronic mechanism of the surface enhanced Raman scattering (SERS) is investigated by an ab initio molecular orbital theory. The time-dependent Hartree–Fock method is used to calculate the polarizability of the surface–molecule interacting system. For representing the surface effect, we use small solid clusters Ag2, Ag10, K2, Pd2, and MgO. The present method succeeds in describing the enhancement of the Raman intensity for the adsorbed CO molecule. The maximum intensity of the Ag10CO system is calculated to be seven orders of magnitude larger than that of the free CO molecule. Furthermore, the wavelength dependence of the Raman intensity calculated by the Ag10CO system agrees with the experimental spectrum. The electronic mechanism of the SERS is due to the resonance transitions, in which the surface polarization and the surface–molecule interaction are very important. This mechanism explains the orientation- and distance-dependencies in the surface–adsorbate interacting systems.
The infrared spectrum of He–HCO+
Authors: Nizkorodov S. A. , Maier J. P. , Bieske E. J.
Published: July 2002

The vibrational predissociation spectrum of the He–HCO+ proton bound complex has been recorded in the 3 μm (C–H stretch) region by monitoring the HCO+ photofragment current. A rotationally resolved, parallel band is observed, red shifted 12.4 cm−1 from the ν1 transition of free HCO+. Analysis in terms of a diatomiclike Hamiltonian yields B″=0.2900±0.0002 cm−1, D″=(1.00±0.06)×10−5 cm−1, B′=0.2898±0.0010 cm−1, and ν1=3076.313±0.010 cm−1. Localized perturbations to ν1 rotational levels are observed and are tentatively ascribed to interactions with combination vibrational states made up of quanta of the CO stretch and HCO+ bend, and those of the low frequency intermolecular stretches and bends. Rotational linewidths are laser bandwidth limited suggesting a lower limit of approximately 250 ps for the lifetime of the ν1 level.
Spectroscopy of the fluoromethylene radicals HCF and DCF by 2+1 resonance enhanced multiphoton ionization spectroscopy and by ab initio calculation
Authors: Irikura Karl K. , Hudgens Jeffrey W. , Johnson Russell D.
Published: July 2002

Spectra of fluoromethylene radicals, HCF and DCF, have been observed between 304 and 325 nm using mass-resolved, resonance enhanced multiphoton ionization (REMPI). Singlet HCF is generated by sequential reactions of atomic fluorine in either methane or methyl fluoride. The spectra arise from two-photon resonances between the ground state X̃ 1A′ and the previously unobserved Ẽ 1A′ (3p) Rydberg state. The band origins lie at T0=62154±2 cm−1 (321.69±0.01 nm) and T0=62175±2 cm−1 (321.58±0.01 nm) (2σ) above the ground states for HCF and DCF, respectively. Vibrational resolution and partial rotational resolution are obtained, with ν2′=1128±4 cm−1 and ν3′=1614±4 cm−1 for HCF and ν1′=2095±5 cm−1, ν2′=872±3 cm−1, and ν3′=1582±4 cm−1 for DCF (2σ). Under typical experimental conditions, the strongest peak for HCF is due to the 210310 resonance at 308.23±0.04 nm and the strongest peak for DCF is due to the 220 resonance at 312.85±0.01 nm. The results of vibrational and rotational analyses are consistent with predictions from ab initio calculations, which also yield the adiabatic ionization potential IPa=10.06±0.05 eV (2σ).
Pseudospectral calculation of near-dissociative local mode states for the bifluoride anion HF−2
Authors: Bramley M. J. , Corey G. C. , Hamilton I. P.
Published: July 2002

Using a recently reported global potential energy surface, we calculate all vibrational levels of the HF−2 anion up to the dissociation threshold. The equilibrium geometry of the bifluoride anion is linear with the H atom between the F atoms. The vibrational wave functions are symmetric or antisymmetric with respect to reflection in a plane bisecting the F–F axis. We focus on nearly degenerate pairs of symmetric and antisymmetric levels lying close to the dissociation energy. Sums and differences of these levels are local mode states for which the H atom is localized on one of the F atoms. These near-dissociative local mode states, which can exist above the threshold for dissociation into F− and HF or FH and F− fragments, have been proposed as candidates for spectroscopic experiments which probe the dynamics and structure of the transition state in the unimolecular dissociation of polyatomic molecules. Energies of the low-lying vibrational levels, as well as those around the dissociation energy, are presented. Wave functions of highly vibrationally excited states, lying slightly below and slightly above the dissociation threshold, are analyzed graphically.
Theory of the influence of end conditions on self-contact in DNA loops
Authors: Coleman Bernard D. , Tobias Irwin , Swigon David
Published: July 2002

Explicit solutions of the equations of Kirchhoff’s theory of elastic rods are employed to derive properties of the tertiary structure of a looped segment of DNA that is subject to geometric constraints imposed at its end points by bound proteins. In appropriate circumstances small changes in such boundary data cause a nearly planar loop to undergo a continuous and reversible transition that can be described as a 180° rotation taking the loop from an uncrossed to a singly crossed structure in which sequentially separated base pairs are brought into proximity. Expressions are derived relating points and angles of crossing to end conditions, and results are presented that facilitate the calculation of changes in elastic energy during such transitions.
Density dependence of rotational relaxation of supercritical CF3H
Authors: Okazaki Susumu , Matsumoto Masayuki , Okada Isao , Maeda Katsutoshi , Kataoka Yosuke
Published: July 2002

Polarized and depolarized Raman scattering measurement and molecular dynamics (MD) calculations have been performed for supercritical CF3H at various densities along an isotherm higher than Tc by about 6 K in order to investigate the density dependence of rotational relaxation. The rotational autocorrelation functions obtained from both methods, which are in satisfactory agreement with each other, showed liquid-like diffusional decay for the fluid at densities higher than ρc. The function changed in shape continuously to a nearly free-rotor-like one at the lowest density going through the oscillatory ones at intermediate densities. The detailed analysis based upon the MD trajectories has been done in order to clarify the relaxation mechanism at each density. Applicability of the J-extended diffusion model was also examined. They showed that the density dependence of the rotational relaxation may be explained in terms of the states of molecular aggregation in the fluid.
Calculation of NEXAFS spectra for surface–adsorbates: Hydroxyl on Cu(111)
Authors: Pettersson Lars G. M. , Ågren Hans , Vahtras Olav , Carravetta Vincenzo
Published: July 2002

Using OH adsorbed on the copper(111) surface as model system we demonstrate the feasibility of calculations of NEXAFS spectra of chemisorbed species. We employ a static exchange method that is implemented for large systems through direct atomic orbital techniques. The general cluster convergence of the spectra ranging up to 49 cluster atoms is investigated and found to be satisfactory. The special character of the strong metal to adsorbate charge transfer involving occupied OH orbitals and of the OH–metal ionic bond are shown to have particular consequences for both the analysis and the computation of the NEXAFS spectra. For the larger clusters the discrete spectra are dominated by adsorbate to cluster transitions, which can be understood from the similarity of electronic structure of the adsorbed OH moiety with that of free OH− for which all transitions end up in the continuum.
Rotationally inelastic scattering of NCO(X̃ 2Π)+Ar: Mass effects
Authors: Lai Lih-Huey , Chiu Yu-Hui , Liu Kopin
Published: July 2002

State-resolved integral cross sections for the inelastic scattering of the Hund’s case (a) radical NCO(X̃ 2Π) by Ar have been measured in a pulsed, crossed-beam apparatus. An extensive comparison has been made between its results and those for NCO+He. Substantial differences were found in nearly every aspect. It was conjectured that the observed ‘‘mass effects’’ arose mainly from the difference in interaction time which changed the dynamics from predominantly an impulsive type for NCO+He to a regime governed by multiple encounter collision for NCO+Ar.
Control of the dimensionality of the Fermi surface of metallic monolayers by chemisorption
Authors: Burdett Jeremy K. , Canadell Enric
Published: July 2002

A theoretical study of the Fermi surface for a square lattice of group 6 transition metal atoms shows that although most of this surface is made up of closed loops, it results from the weak hybridization of both one-dimensional (1D) and two-dimensional Fermi surfaces. Our study shows that the number of 1D portions of the Fermi surface can be changed by chemisorption. These 1D features of the Fermi surface may well be the origin of some structural instabilities observed in clean and hydrogen covered surfaces. The influence of the coverage, nature of the chemisorbed species, and interlayer interactions on the surface is reported.
Correlation effects and entropy-driven phase separation in athermal polymer blends
Authors: Singh Chandralekha , Schweizer Kenneth S.
Published: July 2002

Polymer reference interaction site model (PRISM) theory with the Percus–Yevick closure approximation has been applied to investigate the intermolecular correlations, effective chi-parameters, and spinodal phase separation of athermal binary polymer blends. These model mixtures are composed of structurally asymmetric semiflexible chains interacting via purely hard core potentials. In strong contrast to PRISM predictions for the idealized Gaussian thread model, nonlocal entropy-driven phase separation is predicted under certain conditions. By examining the intermolecular pair correlation functions we identify the physical driving force as local packing frustration associated with the different backbone stiffnesses of the blend components, which is propagated to macromolecular scales by chain connectivity and persistence. These entropic packing effects display many nonuniversal features including a sensitive dependence on chain length, blend composition, monomer volume difference, and both the mean and relative aspect ratios of the polymers. The sensitivity of the athermal blend fluctuation phenomena to local chain rigidity and nonzero liquid compressibility is emphasized. For model parameters characteristic of most flexible polymers of experimental interest the athermal packing frustration effect is found to generate only a small amount of thermodynamic incompatibility. Perturbative estimates of the enthalpic chi-parameters associated with (local) structural asymmetries suggest they are much more important than the purely entropic contribution for hydrocarbon alloys such as the polyolefins. Recent incompressible field theories for athermal conformationally asymmetric blends are derived within the liquid state integral equation framework by identifying an alternative, mean-field-like closure approximation coupled with the imposition of a zero compressibility constraint.
Photodissociation of HNO3 at 193 nm: Near-infrared emission of NO detected by time-resolved Fourier transform spectroscopy
Authors: Yeh Pey-Shiun , Leu Gen-Hou , Lee Yuan-Pern , Chen I-Chia
Published: July 2002

Rotationally resolved emission of NO, produced from photolysis of HNO3 at 193 nm, in the near infrared region (8900–9300 cm−1) was recorded with a step-scan Fourier-transform interferometer at a resolution of 0.1 cm−1. The emission is assigned as NO D 2Σ+−A 2Σ+ (v′,v″)=(0,0) band with rotational states N′=17–42. Emission from selective rotational states of NO D 2Σ+ was observed when HNO3 was photolyzed with an ArF excimer laser having a narrow bandwidth ≊0.01 nm. The experimental results indicate that the D 2Σ+ state of NO is formed via absorption of another 193 nm photon by NO (v″=1) in the ground electronic state. The measured distribution of intensity implies that NO is produced highly rotationally excited; the most likely mechanism for formation of NO is from the unstable NO2 fragment undergoing secondary dissociation.
Threshold and cage effect for photodissociation of H2O in solid Ne and Ar
Authors: Cheng Bing-Ming , Lo Wen-Jui , Lai Lih-Huey , Hung Wen-Ching , Lee Yuan-Pern
Published: July 2002

We determined the threshold for photodissociation of H2O to form OH in matrices by dissociating a matrix sample in situ on scanning the energy of synchrotron radiation and by detecting the increase in intensity of laser-induced fluorescence of OH. The derived threshold, 6.89±0.04 eV for H2O in solid Ar, is almost identical to the previously reported value of 6.9±0.04 eV. The photodissociation threshold in solid Ne, 6.87±0.02 eV, is within experimental uncertainty of that determined in solid Ar. They are identical to recently determined threshold of absorption of H2O in solid Ne and Ar. Hence, photodissociation of H2O in matrices near the threshold is determined by absorption from the ground X̃ state to the repulsive Ã state. The thresholds of absorption of H2O correspond to ∼1.7 eV excess energies of dissociation; at these energies sudden exit should dominate, consistent with the results from H2S photolysis in matrices.