bluuebunny/crossref_metadata_2025 huggingface dataset:
Isolated effective Hamiltonians for two nearly degenerate modes coupled by Coriolis and centrifugal terms
Authors: Krishnan Mangala S. , Carrington Tucker
Published: July 2002

A quantum mechanical canonical transformation due to Bogoliubov and Tyablikov (BT), applied previously to eliminate a single Coriolis term coupling two nearly degenerate vibrational degrees of freedom of a polyatomic molecule [J. Chem. Phys. 94, 461 (1991); 95, 1884 (1991)], is generalized. First, we show how to use an angular momentum dependent BT transformation to effectively decouple two vibrational degrees of freedom Coriolis coupled by two components of the angular momentum. This is accomplished by choosing the molecule-fixed axes so that, in the rotated frame, there is only one Coriolis term. Redefining the orientation of the molecule-fixed axes at equilibrium and using the BT transformation enables us to move large off-block matrix elements into vibrational blocks so that we can use second order perturbation theory to treat strong Coriolis coupling. Second, we develop a mixed BT transformation-perturbation theory method to calculate energy levels for molecules for which both Coriolis and centrifugal coupling are present. The method is tested on a two-mode model of formaldehyde using an ab initio Hamiltonian. Third, we combine our BT transformation method and conventional contact transformation theory to derive effective Hamiltonians for nearly degenerate vibrational states coupled by Coriolis and centrifugal terms without resorting to a reduced dimensional model. It is very difficult to use perturbation theory after having applied a BT transformation. We circumvent this problem by first using standard perturbation theory to decouple modes whose zeroth-order energies are well-separated and then applying a BT transformation for the strongly coupled modes. The theory is applied to an experimentally determined Hamiltonian for formic acid.
The effect of nonpolar solvents on Rydberg states: van der Waals complexes of azabicyclooctanes
Authors: Shang Q. Y. , Moreno P. O. , Dion C. , Bernstein E. R.
Published: July 2002

The effect of solvation by nonpolar solvents on the (n,3s) Rydberg states of 1,4-diazabicyclo[2.2.2]octane (DABCO) and azabicyclo[2.2.2]octane (ABCO) is investigated through mass resolved excitation spectroscopy of their van der Waals complexes. The solute/solvent clusters formed in a supersonic expansion include DABCO and ABCO with Ar, n-CmH2m+2 (m=1–7), and CF4 and C2F6. The resulting spectra are analyzed with the help of empirical potential energy calculations of the cluster binding energies, minimum energy structures, van der Waals modes, and potential barriers between the various cluster minimum energy structures. Good agreement is found between the calculated and experimental results for DABCO and ABCO clustered with argon and methane. The spectra of clusters with all other hydrocarbons can be ascribed to only one major geometry for each cluster stoichiometry, despite the fact that calculations yield many stable geometries for each cluster. This apparent lack of agreement between calculations and experiments can be rationalized based on cluster binding energy, zero point energy, and the potential energy barriers between the cluster minima. The observed blue shift of the cluster 000 transition energy as a function of the n-alkane chain length can be qualitatively modeled by a Lennard-Jones potential for the solute–solvent interaction for both the ground and excited states. The model reveals a strong repulsive interaction between the Rydberg state electronic distribution and the solvent molecule. This repulsion depends on the distance between the solvent molecule and the solute molecule nitrogen atom.
Reply to the Comment on: Cation vibrational spectroscopy of trans and gauche n-propylbenzene rotational isomers. Two-color threshold photoelectron study and ab initio calculations
Authors: Takahashi M. , Kimura K.
Published: July 2002

In connection with the Comment by Liu and Zhou [J. Chem. Phys. 98, 0000 (1993)] on our paper [J. Chem. Phys. 97, 2920 (1992)], the following can be pointed out. Although our ab initio calculations with the STO-2G basis set are not sufficient, we have obtained some useful information from symmetry consideration which supports our experimental vibrational assignments of the trans n-propylbenzene cation.
Crystal-field excitations and magnetic properties of TmPO4
Authors: Loong C.-K. , Soderholm L. , Abraham M. M. , Boatner L. A. , Edelstein N. M.
Published: July 2002

The magnetic-excitation spectrum of TmPO4 has been studied using inelastic neutron scattering techniques. Sharp crystal-field transitions were observed in spectra obtained at 15 and 100 K, yielding new information regarding the energy-level structure of the Tm3+ ground-multiplet splitting. The data were analyzed using a Hamiltonian that included the atomic free-ion and crystal-field interactions for an f12 configuration. Using the Tm free-ion parameters derived from optical spectroscopy of Tm3+ diluted in a LuPO4 host, a set of crystal-field parameters were obtained for Tm3+ in pure TmPO4. The calculated magnetic spectra of TmPO4 agree very well with the results of neutron and Raman measurements. The calculated bulk magnetic susceptibility of TmPO4 exhibits a large anisotropy at low temperature and is in good agreement with the experimental data. A significant contribution to the specific heat from the Tm3+ crystal-field states was found at temperatures below 100 K.
Nonlinear swelling of polymer gels
Authors: Durning C. J. , Morman K. N.
Published: July 2002

We present a continuum theory for the nonlinear swelling of polymer gels. The approach is to solve the continuity equations for the network and liquid under the constraint that the instantaneous free energy be minimal. Constitutive relations are needed for the free energy density of the gel, W, and for the liquid diffusion current, J. The James–Guth phantom network model and Bastide’s scaling model are used for W; Fick’s law is used for J which is tantamount to neglecting the gel’s shear modulus relative relative to its osmotic compressive modulus. The theory is applied to the free swelling of networks in good solvents to a semidilute gel for three geometries: sphere, long cylinder, and thin slab. We find a geometry-dependent, nonlocal effect influencing the measurables (gel shape and liquid uptake). This arises from the dependence of the network’s deformation at the gel/liquid interface on the whole deformation field for the sample. The theory gives reasonable agreement with experiment; discrepancies are likely due to nonlocal effects in J not accounted for by Fick’s law.
Molecular gradients and hessians implemented in density functional theory
Authors: Komornicki Andrew , Fitzgerald George
Published: July 2002

We derive expressions for molecular gradients and hessians for the case when the energy is evaluated using density functional theory. Although derivative expressions have been proposed previously, our derivation is based on the unitary exponential parameterization of the wavefunction, and our expressions are valid for local and non–local potentials. Density functional theory, although similar in implementation to standard SCF theory, differs in that it introduces an exchange–correlation term which is density dependent. The presence of such a quantity introduces additional derivative terms which are not present in standard approaches of electronic structure theory. Expressions are derived for both the exact Coulombic repulsion, as well as the case where the density is expressed as a fitted quantity. Given these choices our final equations offer a computationally tractable expression with particular emphasis on conditions which ensure that the computed quantities are numerically correct. We show that although the use of a fitted density allows significant computational savings in the energy and the first derivatives, it introduces additional computational complexity, beyond that normally encountered in traditional electronic structure methods, once second derivatives are evaluated. The evaluation of second derivatives also introduces derivatives of the exchange–correlation potential which have not been previously considered.

The presence of such terms introduces the most serious computational complexity to the evaluation of any second derivative expression based on the density–functional formalism. Our derivation and analysis presents a computationally tractable procedure for the evaluation of all the terms required to compute the first and second derivatives with respect to perturbations such as nuclear coordinates, and external electric fields. Using a general set of response equations for the first order change in the wavefunction, we provide expressions for the evaluation of harmonic frequencies, infrared intensities, and molecular polarizabilities. Our final discussion assesses the computational consequences of using either an exact form for the density, or a fitted form. Although most of our discussion is cast in the form of a closed–shell formalism, extensions to an unrestricted (UHF) formalism are straightforward.
Static and dynamic evanescent wave light scattering studies of diblock copolymers adsorbed at the air/water interface
Authors: Lin Binhua , Rice Stuart A. , Weitz D. A.
Published: July 2002

We report the results of static and dynamic evanescent wave light scattering studies of a monolayer of a diblock copolymer, polystyrene-b-polymethylmethacrylate (PS-b-PMMA) with weight averaged molecular weights (Mw) of 880 000:290 000 supported at the air/water interface. Our studies probe the interfacial structural and dynamic properties of the monolayer on a length scale which is a fraction of the wavelength of light. The static light scattering studies were carried out as a function of polymer surface coverage and temperature; we also report some preliminary data for the dependence of the static structure function on the relative molecular weights of the PS and PMMA blocks. The complementary dynamic light scattering studies were carried out only as a function of surface coverage. Our data suggest that, upon spreading in the air/water interface, PS-b-PMMA (880:290 K) copolymers form thin disklike aggregates containing about 240 molecules. These data are consistent with a model in which each such aggregate is a ‘‘furry disk’’ with a dense core consisting of a layer of collapsed PS blocks atop a thin layer of extended PMMA blocks on the water surface and a brushlike boundary of extended PMMA blocks. The data show that the furry disks diffuse freely when the surface coverage is small, but when the surface coverage is large, they are immobile. Our data also suggest that the furry disks can aggregate to form even larger ‘‘islands’’ of disks with an extension greater than 20 μm. The static structure function of the assembly of furry disks is well described, over a wide range of surface coverage, by the structure factor of a two-dimensional hard disk fluid modulated by a two-dimensional hard disk form factor.
A fluctuation approach to solvation in polar fluids
Authors: Karlström Gunnar , Halle Bertil
Published: July 2002

A phenomenological approach to solvation in polar fluids is proposed that explicitly recognizes the role of thermal fluctuations in the solvent. The probability distributions for the fluctuating electric potential, field, field gradient, etc. in a polar fluid are shown to be Gaussian with a variance determined by the linear (dielectric) response function of the fluid. This Gaussian distribution is used to calculate the solvation free energy of a solute carrying a permanent multipole and/or a polarizability. Unlike the traditional reaction-field approach, the fluctuation approach can describe the solvation of a polarizability by the fluctuating field from the solvent. This contribution to the solvation free energy may be regarded as the zero-frequency part of the electrodynamic dispersion interaction.
Effective Hamiltonians for Coriolis-coupled nearly degenerate modes: Illustrative examples
Authors: Krishnan Mangala S. , Carrington Tucker
Published: July 2002

Using an angular momentum dependent Bogoliubov–Tyablikov (BT) transformation technique effective rotational Hamiltonians are derived for four molecules all of which have a pair of nearly degenerate strongly coupled vibrational modes. The BT transformation is applied to a two-vibrational-mode Hamiltonian obtained by using perturbation theory to eliminate the coupling between all but the nearly degenerate modes. Energy levels computed from the BT effective rotational Hamiltonians and accurate variational calculations agree very well. BT effective Hamiltonians are derived and tested for ozone, formaldehyde, its singly deuterated analog, and chlorodifluoromethane.
Molecular orientational dynamics in solid C70: Investigation by one- and two-dimensional magic angle spinning nuclear magnetic resonance
Authors: Tycko R. , Dabbagh G. , Vaughan G. B. M. , Heiney P. A. , Strongin R. M. , Cichy M. A. , Smith A. B.
Published: July 2002

We present the results of 13C nuclear magnetic resonance (NMR) measurements that probe molecular orientational dynamics in solid C70 in the temperature range 223–343 K. Orientational dynamics affect the NMR line shapes and spin–lattice relaxation rates by modulating the 13C chemical shift anisotropy (CSA). Motionally averaged CSA line shapes, determined from both one-dimensional and two-dimensional magic angle spinning NMR spectra, and relaxation rates are determined for each of the five inequivalent carbon sites in the C70 molecule. Comparisons of the results for the five sites provide evidence for rapid uniaxial molecular reorientation in the monoclinic (T≤280 K) and rhombohedral (280≤T≤330 K) phases and rapid isotropic reorientation in the face-centered cubic (T≥330 K) phase. The orientational correlation time is roughly 2 ns at 250 K and of the order of 5 ps at 340 K.
A uniform Wentzel–Kramers–Brillouin approach to electron transport in molecular gases
Authors: Demeio Lucio , Shizgal Bernie
Published: July 2002

The relaxation of a nonequilibrium distribution of electrons in molecular gases, and the transient and steady electron transport properties are studied with the Boltzmann equation, which accurately accounts for elastic and inelastic electron–moderator collisions. The elastic collision operator is a self-adjoint Fokker–Planck operator, whereas the inelastic collision term is a difference operator. We consider a discretization of the inelastic collision operator which is motivated by the discrete energy losses that occur. For small energy losses, a continuous approximation is introduced such that the inelastic collision term is approximated by a Fokker–Planck operator similar to the elastic collision operator. The transient electron transport properties are studied for an initial electron distribution function in terms of the eigenvalue spectrum of both the elastic and inelastic collision operators. The Fokker–Planck eigenvalue problem is transformed into a Schrödinger equation and the nature of the spectrum is studied in terms of the Wentzel–Kramers–Brillouin and supersymmetric Wentzel–Kramers–Brillouin approximations. The methodology is applied to model systems and the transport of electrons in methane.
Line shapes of strongly radiation-damped nuclear magnetic resonance signals
Authors: Mao Xi-an , Ye Chao-hui
Published: July 2002

Line shapes of strongly radiation-damped nuclear magnetic resonance (NMR) signals are theoretically analyzed. Analytical expressions for the radiation-damped signals in the frequency domain are reported for the first time. It is shown that the line shapes are generally dependent on the flip angle θ0 of the excitation pulse. In the range of 0≤θ0<π, the peak height increases linearly, but the linewidth decreases monotonically with the flip angle. An oscillating feature is predicted for the line shape when θ0≳π/2. The theoretical predictions are confirmed by the experimental results.
Isotropic Raman line widths in binary mixtures of van der Waals liquids: A comparison between isotopically and chemically diluted states of liquid methyl iodide
Authors: Keutel Dietmar , Seifert Frank , Oehme Karl-Ludwig
Published: July 2002

To analyze vibrational depopulation and dephasing mechanisms of polyatomic liquids, the presently most comprehensive study on the dilution behavior of isotropic Raman line widths has been made. By way of example, and following early proposals of Evans and Yarwood [Adv. Mol. Relaxation Int. Processes 21, 1 (1981)], liquid methyl iodide (MeI) was chosen. The low temperature study (T=207 K) covers the detailed concentration dependencies (x) of the isotropic half-widths (FWHH) in total 17 (of 18 possible) polarized fundamentals of CH3I and in the isotopically modified molecules CH2DI, CHD2I, and CD3I (12CD3I) (i) in chemical mixtures with xenon and (partially) CF3I and (ii) in all the six binary isotopic mixtures of the isotopic species above. Furthermore, isotopic and chemical dilutions are combined in ternary mixtures of type (CX3I)0.014−(CY3I)x−0.014−Z1−x with X, Y=H, D and Z=Xe, CF3I, where the trace CX3I has been investigated to study the modulations of the vibrational transition frequency by changes of the intermolecular potential in an isotopically diluted state free of vibrational resonance coupling. The manifold of curves FWHHiso(x) are discussed in terms of a recent Raman line shape theory of Bratos and Tarjus [Phys. Rev. 32A, 2431 (1985)] which explicitly considers the time-dependent cross correlation between the environmentally induced frequency shifts Δωei(t) and those induced by (intermolecular) resonance coupling (RC) ΔωRC(t). Overall, strong indications have been found that intermolecular coupling Δωij=ωi−ωj play a very significant role in line broadening. This holds true not only for the bending modes (as known beforehand), but also for the C–H (C–D) and C–I stretching modes. However, to be effective, these coupling has to be of a strictly resonant nature, i.e., near-resonant transitions with 0.05≤ℏ‖Δωij‖/kBT≪1 are ineffective in line broadening in the same way as strictly off-resonant transitions with ℏ‖Δωij‖/kBT≳1.
Dielectric relaxation derived from collective molecular reorientation of 9-hydroxyphenalenone in the solid state
Authors: Mochida Tomoyuki , Izuoka Akira , Sugawara Tadashi , Moritomo Yutaka , Tokura Yoshinori
Published: July 2002

Temperature dependence of dielectric relaxation of 9-hydroxyphenalenone (1) is measured in the frequency region of 102–107 Hz. We found a characteristic dielectric relaxation in the room temperature phase (phase II: 255–380 K), where the molecular orientation is known to be disordered. We ascribed the dielectric relaxation to the motion of boundaries between domains consisting of left- and right-directed molecules in the molecular chain.
Charged rodlike colloidal suspensions: An ab initio approach
Authors: Löwen Hartmut
Published: July 2002

The interactions and orientational correlations of charged rodlike colloidal particles in a salt-free suspension are calculated using an ‘‘ab initio’’ approach which combines molecular dynamics for the rods and classical density functional theory for the counterions. Both fluid and liquid-crystalline phases are investigated. It is found that the interaction between charged rods can satisfactorily be described by an effective Yukawa-segment model which in general differs, however, from the Derjaguin–Landau–Verwey–Overbeek segment model. For strongly interacting long rods, an exactly soluble Yukawa-segment model, based on a cylindrical Poisson–Boltzmann cell, is proposed which reproduces the ab initio data quite well in the liquid-crystalline phase.
The valence-bond charge-transfer-exciton model for predicting nonlinear optical properties (hyperpolarizabilities and saturation length) of polymeric materials
Authors: Lu Daqi , Chen Guanhua , Goddard William A.
Published: July 2002

A simple theory is developed and applied to the polarizability (α), second hyperpolarizability (γ), and the saturation lengths for nine polymeric materials. The theory is based on a valence bond view of the ground and excited states and considers the excited states as charge transfer excitons. It involves just two parameters which can be extracted from simple molecular orbital calculations and/or from experimental values of band gap and bandwidth. For the one system (oligothiophenes) with experimental data on saturation length, the predicted behavior of α and γ with polymer length is in good agreement with experiment.
9.6 GHz and 34 GHz electron paramagnetic resonance studies of chromium-doped forsterite
Authors: Budil David E. , Park Dong Gon , Burlitch James M. , Geray Roland F. , Dieckmann Rüdiger , Freed Jack H.
Published: July 2002

Chromium-doped forsterite single crystals grown under conditions that produce a high Cr4+/Cr3+ ratio were examined by electron paramagnetic resonance (EPR) at 9.6 and 34 GHz. The crystals were grown in 2–3 atm of oxygen by the floating-zone method starting from polycrystalline chromium-doped forsterite powder synthesized via a sol–gel method. Three crystals with chromium concentrations of 110, 300, and 390 ppm were studied. At 34 GHz, transitions are observed for the laser-active tetrahedral Cr4+ species that are not observable at 9.6 GHz, which improve the resolution and accuracy with which the magnetic parameters can be measured by EPR. In addition, peaks for a non-Kramers species appear at 34 GHz that were not observed at 9.6 GHz. These peaks are not analyzed in detail, but are tentatively ascribed to Cr4+ in the octahedral substitution sites of the crystal. At the highest chromium concentration, the Cr3+ spectra show evidence of direct interaction with Cr4+. A global least-squares fit of the combined 9.6 and 34 GHz data for the 300 ppm crystal gives D=64.26±0.18 GHz, E=−4.619±0.009 GHz, gx=1.955±0.009, gy=2.005±0.040, gz=1.965±0.006, and places the magnetic z axis in the ab plane at an angle of 43.8±0.3° from the b crystallographic axis (in Pbnm).

A method for accurately measuring the Cr4+/Cr4+ ratio using EPR line intensities is given. The EPR linewidth of the Cr4+ center exhibits a strong orientation dependence that is well-modeled by including site variations in the D and E zero-field splittings and in the orientation of the z magnetic axis. The linewidth analysis reveals a high degree of correlation between the distributions in D and E, and a somewhat weaker correlation between E and the z axis orientation. These results are interpreted to suggest that the tetrahedral Cr4+ sites vary mainly in the degree of compression of the tetrahedral cage along the a crystallographic axis. The Cr4+ EPR linewidths increase significantly at higher chromium concentration, but maintain the same qualitative orientation dependence. The EPR data indicate that the major contribution to inhomogeneity in the tetrahedral site, which may be related to the tunable range of the Cr4+ laser center, is distortion induced by chromium substitution into the crystal lattice rather than direct chromium–chromium interactions.
Is the depletion of ozone by HSO an exothermic process?
Authors: Esseffar M. , Mó O. , Yáñez M.
Published: July 2002

The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, as well as other high level ab initio calculations [QCISD(T) and CCSD(T)/6–311++G(3df,2pd, and QCISD(T)/6–311+G(5d2f,2p)] which avoid the additivity scheme of this theory, are used to estimate the heat of formation of HSO. For this purpose, we propose also, as a further improvement of the standard G2 scheme, to evaluate the residual correlation effects by using the QCISD(TQ) method, which is exact in fifth order rather than the usual QCISD(T) procedure. The heat of formation of HSO at 0 K estimated using different reactive processes is −4.2±1.3 kcal/mol. The most significant consequence is that the depletion of ozone by HSO is predicted to be slightly endothermic rather than exothermic, as it has been concluded in other theoretical studies. We have also found that HSO is more stable than SOH, but the energy gap between both isomers predicted by our calculations is smaller than previous reported values.
Domain morphology in a two-dimensional anisotropic mesophase: Cusps and boojum textures in a Langmuir monolayer
Authors: Schwartz Daniel K. , Tsao Mei-Wei , Knobler Charles M.
Published: July 2002

Brewster angle microscopy (BAM) and polarized fluorescence microscopy (PFM) were used to observe domains of a tilted condensed phase in coexistence with an isotropic liquid phase in Langmuir monolayers of pentadecanoic acid. The orientation of the molecular tilt direction inside the domains can be described as a ‘‘virtual boojum’’ texture and a cusp appears in the domain boundary at the vertex of the boojum. The angle subtended by the cusp increases with the domain size for domains greater than 25 μm in diameter. All of these observations are at least qualitatively consistent with the recent predictions of a generalized Wulff construction for deformable materials [J. Rudnick and R. Bruinsma, Phys. Rev. Lett. (submitted)].
Monitoring of slow conformational exchange by doubly selective irradiation in nuclear magnetic resonance spectroscopy
Authors: Boulat Benoit , Rance Mark
Published: July 2002

A new NMR spectroscopic method is described that allows a selective determination to be made of whether two sites are connected by a slow conformational exchange process. An accompanying approximate theory is derived which permits a quantitative estimate to be made of the exchange rate in numerous situations. Numerical simulations are presented to specify the domain of validity of the theory. Experimental verification is demonstrated for an aromatic ring in the bovine pancreatic trypsin inhibitor protein.
Head–tail competition and modulated structures in planar surfactant (Langmuir–Blodgett) films
Authors: Schwartz Daniel K. , Viswanathan Ravi , Zasadzinski Joseph A.
Published: July 2002

We have observed two distinct types of spatially modulated structures (on nanometer length scales) on the surface of Langmuir–Blodgett films of fatty acid salts using atomic force microscopy. Both involve periodic height modulation of the crystalline molecular lattice. The first type, seen in films containing Ba, Ca, and Mg, is commensurate with the molecular lattice and can be described as a regular series of packing defects that are consistent with the close-packing requirements of the alkyl tail groups. The other type, seen in films containing Cd and Mn, is not always commensurate and is less perfectly correlated than the first type. We suggest a correspondence between these two types of modulations and the two predicted by a new toy model which incorporates the competition between the in-plane density preferred by the headgroups in conjunction with the cation and the close-packing requirements of all-trans crystalline alkyl chains.
Controlling spatiotemporal dynamics of flame fronts
Authors: Petrov Valery , Crowley Michael F. , Showalter Kenneth
Published: July 2002

In certain mixtures of fuel and oxidizer, propagating flame fronts may exhibit both stable and unstable cellular structures. Such flames represent spatially extended chemical systems, with coupling from diffusion of heat and reactants. A new algorithm is proposed that allows the stabilization and tracking of a steady, two-cell front through a bifurcation sequence that eventually leads to chaotic behavior. Periodic modes of the front can also be stabilized and tracked. The system is stabilized by monitoring one experimentally accessible variable and perturbing one boundary condition. No knowledge of the detailed dynamics of the system (i.e., the underlying governing equations) is required to implement the tracking method. The algorithm automatically provides information about the locations of the unstable steady states and periodic orbits and the magnitudes of the associated eigenvalues and Floquet multipliers.
Electronic excited state of NO adsorbed on Cu(111): A two-photon photoemission study
Authors: Kinoshita Ikuo , Misu Akira , Munakata Toshiaki
Published: July 2002

Two-photon photoemission was observed from molecularly adsorbed NO on Cu(111) at 110 K. Excitation and photoemission were performed with a laser light of 5 ns duration in the photon energy range from 3.8 to 4.5 eV. The NO-induced photoemission feature was found to be composed of two components: one was attributed to the sequential two-photon photoemission from the ground state of adsorbed NO, the other to the one-photon photoemission from the NO negative-ion state which was formed by hot electrons in the Cu substrate. The two-photon photoemission intensity from the ground state NO was resonantly enhanced at the photon energy of 3.98±0.05 eV. The photon energy dependence was attributed to resonance excitation to the excited state of adsorbed NO. Width of the resonance curve was 0.3 eV, suggesting that the lifetime of the excited state is around 2 fs. The negative-ion state of adsorbed NO was 1.26±0.07 eV above the Fermi level. The energy width of the photoemission feature from the negative-ion state was about 0.4 eV which was fairly narrower than those observed in the inverse-photoemission spectra.
Theoretical investigation of the lowest singlet and triplet states in poly(paraphenylene vinylene)oligomers
Authors: Beljonne D. , Shuai Z. , Friend R. H. , Brédas J. L.
Published: July 2002

Using the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian in combination with configuration interaction techniques, we calculate the optical and photoinduced absorption spectra of poly(paraphenylene vinylene) oligomers containing from two to five phenyl rings; the evolutions with chain length of the singlet–singlet and triplet–triplet excitation energies as well as the values extrapolated for the polymer are in good agreement with experiment. The geometry relaxation phenomena in the first one-photon allowed singlet excited state and in the lowest triplet state are modeled on the basis of either bond-order/bond-length relationships or the formation of (bi)polaron-type defects; the results are compared to those of direct geometry optimizations in the excited state. The different methods consistently lead to more pronounced bond-length modifications in the triplet state than in the singlet state.
Scattering angle dependence of electron impact excitation: Intensity variation within a vibrational progression
Authors: Dillon Michael , Kimura Mineo , Buenker Robert J. , Hirsch Gerhard , Li Yan , Chantranupong Lek
Published: July 2002

Intensity distributions of electronic transitions in O2 and CO within a vibrational progression resulting from electron impact excitation are studied theoretically and experimentally. The multireference single- and double-excitation configuration interaction (MRD-CI) method is used to elucidate details of selected electronic transitions. In particular, the adiabatic MRD-CI approach can account for the variation of the Franck–Condon envelope with scattering angle that has been reported for the B 1Σ+←X 1Σ+ transition in CO and also was recently observed in the B′ 3Σ−u←X 3Σ−g transition of O2. This behavior contrasts with the relative stability of the intensity distribution observed within the CO A 1Π←X 1Σ+ vibrational progression. In the former cases the excited state undergoes changes with internuclear separation because of the presence of an avoided crossing. Since a transition from the zeroth vibrational level in the ground electronic state to an individual vibrational level in the excited electronic state tends to select a particular internuclear distance (R centroid), each vibrational band may behave as a transition to a separate electronic level. This happens because the excited-state wave function undergoes a compositional change with internuclear separation between the adiabatic partners of the avoided crossing.
Orientational disorder in lithium sodium sulfate
Authors: Dharmasena Gamini , Frech Roger
Published: July 2002

Results of Raman, x-ray diffraction, and differential scanning calorimetry studies of lithium sodium sulfate (LiNaSO4) doped with Cd2+ are presented. The x-ray diffraction and differential scanning calorimetry data show that there is no significant change in the room temperature structure over the substitutional range studied. Raman spectra of Cd2+ substituted lithium sodium sulfate are compared with the temperature dependent spectra of pure LiNaSO4 and found to be qualitatively similar, reflecting the increasing breakdown of factor group correlated SO2−4 vibrations with increasing Cd2+ concentration.
Rotational effect on the lifetime of molecular resonances
Authors: Sepulchre J.-A. , Gaspard P.
Published: July 2002

We report a study of the effect of rotation on the activated complex of direct ultrafast dissociation processes. The dissociative potential energy surface is supposed to have a single unstable equilibrium point of saddle type. The vibration–rotation molecular Hamiltonian is expanded around the saddle equilibrium point by the Van Vleck perturbative method of contact transformations. In this way, we obtain the effect of vibration–rotation coupling on the complex energies of the scattering resonances of the activated complex. We show that rotation may induce either a stabilization or a destabilization depending on the values of the imaginary vibration–rotation coupling constants αFν between rotation and the unstable motion along the vibrational normal mode of dissociation. Explicit formulas are given for these constants and their order of magnitude is evaluated for typical molecules.
Ab initio study of styrene and β-methyl styrene in the ground and in the two lowest excited singlet states
Authors: Zilberg Shmuel , Haas Yehuda
Published: July 2002

The structure and vibrational frequencies of styrene and trans-β-methyl styrene in the lowest three singlet states (S0, S1, and S2) have been calculated using ab initio quantum chemical methods. The frequencies are compared with experimental data obtained in the bulk and in a supersonic jet. The calculation shows that in the ground state the molecules have a broad shallow potential as a function of the torsional angle, are essentially planar, but may be slightly bent. In the S1 and S2 states, the molecules are planar; In S1, the main structural change is in the aromatic ring, that is somewhat expanded. In S2, the C=C vinyl double bond elongates, while the C1—Cα single bond becomes shorter, bringing these two bonds to almost equal length. Correlation diagrams connecting ground state vibrational modes with ones belonging to electronically excited states are given; they show that for many out-of-plane modes the vibrational frequencies decrease upon electronic excitation. This is accounted for in terms of the changes in the π electron distribution taking place upon optical excitation that result in decreasing the force constants characterizing these vibrations. The frequencies of most in-plane modes change very little, but mixing between S0 modes is indicated in some cases, and a few vibrations, among them a Kekulé-type mode, undergo considerable change. The relation to the spectroscopy of the corresponding transitions in benzene is briefly discussed.
Time-delayed two-color photoelectron spectra of aniline, 2-aminopyridine, and 3-aminopyridine: Snapshots of the nonadiabatic curve crossings
Authors: Kim Byungjoo , Schick Carolyn P. , Weber Peter M.
Published: July 2002

We present time-delayed two-color photoionization photoelectron spectra of aniline, 2-aminopyridine, and 3-aminopyridine seeded in a cold molecular beam. The molecules are prepared in their S1 electronic states by a picosecond UV laser pulse and ionized by a time-delayed 200 nm probe pulse. The photoelectron spectrum is observed with a time-of-flight spectrometer. All time-delayed spectra reveal only one product of the nonradiative relaxation process. Careful considerations of electronic and vibrational overlap propensity rules for the ionization step lead to the conclusion that the dominant nonradiative decay mechanism in these molecules is the intersystem crossing to a bath of vibrationally excited levels of the T1 electronic state. Our observations reveal no admixtures of T2 or higher triplet levels. The pathway of the nonradiative electronic relaxation in 2-aminopyridine is found to be independent of the initially prepared vibrational states up to 1000 cm−1 of vibrational energy. We find no evidence of intramolecular vibrational relaxation preceding the electronic curve crossing.
State resolved translational energy distributions of Cl and HCl in the ultraviolet photodissociation of chloroethylenes
Authors: Huang Yibo , Yang Yung-An , He Guoxin , Hashimoto Satoshi , Gordon Robert J.
Published: July 2002

Velocity-aligned Doppler spectroscopy was used to measure the speed distribution functions of HCl(v″=0–2,J″) produced in the 193 nm photodissociation of vinyl chloride. A mechanism which is consistent with our present data as well with our earlier work is three center elimination of HCl followed by concerted but nonsynchronous isomerization of the vinylidene fragment, with some of the isomerization energy converted to kinetic energy of the recoiling fragments. In order to explain the vibrational dependence of the translational and rotational energy distributions, the mechanism is assumed to be vibrationally adiabatic. In addition, magic angle Doppler spectroscopy was used to measure the speed distribution functions of Cl atoms produced in the photodissociation of vinyl chloride and the three dichloroethylene isomers. Bimodal energy distributions were observed for both spin–orbit states, with the Cl(2P1/2) fragment having more kinetic energy than Cl(2P3/2). The data are consistent with competitive reaction on two potential energy surfaces. Adiabatic correlation, with some scrambling in the asymptotic region, provide a qualitative explanation of the spin–orbit branching ratios.
Structure in the lowest absorption feature of CdSe quantum dots
Authors: Norris D. J. , Bawendi M. G.
Published: July 2002

We use transient differential absorption experiments to investigate the ‘‘single dot’’ absorption line shape of CdSe quantum dots. We observe both a narrow (full width half maximum ∼5 meV) and a broad (∼50 meV) bleach component within the inhomogeneously broadened first absorption line of our samples. We deduce the single dot absorption line shape which is most consistent with the experimental results. This line shape, which contains structure in the lowest quantum dot absorption feature, explains the large ‘‘Stokes’’ shift seen in the full band edge luminescence of CdSe quantum dots. We discuss the structure within the context of several competing models. The data appear inconsistent with models which use surface states to explain the anomalous emission behavior of II–VI quantum dots. Instead they imply that exciton fine structure is observed in our samples.
Mass-resolved multiphoton ionization spectroscopy of jet-cooled Cl2. II. The (2+1) REMPI spectrum between 76 000 and 90 000 cm−1
Authors: Al-Kahali Mohamed S. N. , Donovan Robert J. , Lawley Kenneth P. , Ridley Trevor
Published: July 2002

The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of Cl2 has been recorded between 76 000 and 90 000 cm−1. The origins of twenty Rydberg states are located, with the s series (4s−8s) generally exhibiting four Ω components and the d series (3d−5d) two components for each value of n. Vibronic coupling between the β(1g) ion-pair state and the 3d and 4d (Ω=1) states is pronounced, necessitating the use of mass-resolved REMPI for the analysis of these vibronically mixed states.
A simple kinetic model of ion production and loss in the laser excitation of nitric oxide
Authors: Kuo C. T. , Hardwick J. L. , Mosely J. T.
Published: July 2002

A kinetic model is proposed to describe the behavior of molecular multiphoton ionization for nitric oxide. The dissociative attachment and several related radiationless processes are included in this model. It helps the understanding of the ion production and loss in the system of resonance enhanced multiphoton ionization of NO.
Brillouin scattering study of pentane at high pressure
Authors: Tkachev Sergey N. , Bass Jay D.
Published: July 2002

Brillouin scattering was used to study the acoustic properties of liquid and crystalline pentane at pressures up to 4.1 GPa and 22.5 °C using a diamond anvil cell. Hypersonic sound velocities of liquid pentane agree with lower frequency ultrasonic results to 2.5 GPa. There is considerable velocity anisotropy for longitudinal (LA) and transverse (TA) acoustic phonons measured on a single crystal grown at 1.81±0.05 GPa. The equilibrium liquid/solid boundary is 1.81±0.05 GPa at 22.5 °C, but liquid pentane can be metastably superpressurized to 3.05±0.05 GPa. Acoustic phonon attenuation decreases rapidly with pressure for liquid pentane.
The thermal diffusivity tensor and lattice dynamics of β-oxygen at high pressure
Authors: Abramson E. H. , Slutsky L. J. , Brown J. M.
Published: July 2002

An experimental approach to the study of thermal transport in the diamond-anvil high pressure cell is described and the elements of the thermal diffusivity tensor of β-oxygen are reported to a pressure of 9 GPa. The anisotropy in the thermal conductivity is found to be opposite in sense to that in the velocity of acoustic waves. Results are interpreted in terms of a lattice dynamical model in which the strong interactions between an atom and its first-, second-, and third-nearest neighbors are treated in tensorial form with parameters evaluated directly from experiment.
Vibronic structure in the low-lying singlet–triplet transitions of benzene and toluene
Authors: Swiderek P. , Michaud M. , Sanche L.
Published: July 2002

Low-energy electron-energy-loss spectra of solid benzene and toluene in the range of the three low-lying triplet states were recorded at a temperature of 15 K. Vibronic structure within the low-lying triplet bands of toluene is observed for the first time. In the case of benzene the high resolution spectra reveal more details in the vibronic structure than known from previous electron- energy-loss spectra. With this information a modified interpretation of the vibronic structure in the first triplet band of benzene is proposed. The difference between the spectra of toluene and benzene is explained by the influence of vibronic coupling on the lowest triplet state. In addition, the systematic broadening of the vibronic levels within the first and second triplet band of toluene is interpreted as an effect of the side group internal rotation.
The bending dynamics of acetylene
Authors: McCoy Anne B. , Sibert Edwin L.
Published: July 2002

The dynamics and spectroscopy of (J=0) acetylene bending degrees of freedom are investigated using a reduced dimensional Hamiltonian. This Hamiltonian is obtained by applying an adiabatic approximation to average the vibrational Hamiltonian over the ground state in the three stretch coordinates. Within this approximation, an effective bend force field is obtained by adjusting force constants in the adiabatic potential to improve agreement between experimental and theoretical eigenvalues. With minor modification, a global bend force field is determined that qualitatively describes the vinylidene vibrations and quantitatively describes the acetylene vibrations. This surface is compared to the results of a recent ab initio calculation. A dispersed fluorescence spectrum out of the excited Ã state, calculated from this model, is found to agree well with results of a recent experimental study.
Quantum, semiclassical and classical dynamics of the bending modes of acetylene
Authors: Sibert Edwin L. , McCoy Anne B.
Published: July 2002

The dynamics and spectroscopy of an adiabatic Hamiltonian, derived by McCoy and Sibert [J. Chem. Phys. 105, 459 (1996)], describing the bending motions of acetylene are presented and discussed. The resulting eigenfunctions of this model are interpreted using classical, semiclassical, and quantum mechanical descriptions of the vibrations. Using perturbation theory, the Hamiltonian describing the bends is reduced to two coupled hindered rotor Hamiltonians. This simple Hamiltonian predicts that local mode dynamics of the bending motion first occurs at about 6000 cm−1 of excitation. This prediction is confirmed by the quantum mechanical studies. The hindered rotor Hamiltonian also predicts that l-type doubling leads classically to stable periodic orbits corresponding to planar motion. The extent of planar type motion is quantified using both semiclassical and quantum mechanical models.
Potential energy surfaces for polyatomic reactions by interpolation with reaction path weight: CH2OH+→CHO++H2 reaction
Authors: Rhee Young Min , Lee Tae Geol , Park Seung C. , Kim Myung Soo
Published: July 2002

An improved algorithm to construct molecular potential energy surfaces for polyatomic reactions is presented. The method uses the energies, gradients, and Hessians, which can be obtained from ab initio quantum chemical calculations. The surface is constructed by interpolating the local quadratic surfaces with reaction path weights. The method is tested with a five-atom reaction system for which an analytic potential energy surface has been reported together with classical trajectory results. An excellent agreement is achieved for energy partitioning in products obtained by trajectory calculation on the original analytic and interpolated surfaces. Reduction of error caused by the use of the reaction path weight is explained.
Light scattering study of viscoelastic properties of a liquid Gallium surface
Authors: Kolevzon V.
Published: July 2002

High frequency capillary waves on a liquid Gallium surface were studied by quasi-elastic light scattering spectroscopy. The observed damping constants of the waves differ greatly from the predictions of classical theoretical treatment of the surface as that of a simple liquid. This is explained in terms of the presence of a surface layer possessing visco-elastic properties which are extracted from the fit of experimental spectra using different theoretical forms. One of the most important results of the data analysis is that the surface dilational viscosity appears to be negative. This result implies that the widely used phenomenological theoretical model should be replaced by one taking into account processes of adsorption and diffusion.
Orientational ordering in the mixed crystal Ar1−x(N2)x: A molecular dynamics study
Authors: Raugei Simone , Cardini Gianni , Schettino Vincenzo , Jodl Hans J.
Published: July 2002

The mixed crystal Ar1−x(N2)x was studied by molecular dynamics simulations in the NPT ensemble for three selected concentrations. The α↔β transition was not observed, instead the low temperature α phase shows a transition to a plastic fcc phase. The mixed crystals with fcc arrangement tend to form orientationally ordered crystals. An orientational glass was obtained and characterized for x≃0.7 and with an hcp structure in agreement with the experimental observations.
Solution of the Percus–Yevick equation for the molecular pair correlation function of a linear solute molecule in a simple liquid
Authors: Franosch T. , Singh A. P.
Published: July 2002

The short-range translational and orientational order around a rigid linear solute molecule immersed in a dense simple liquid is investigated within the Percus–Yevick theory. The orientational correlations induced by the anisotropic solute–solvent interaction are analyzed by a spherical harmonics expansion. The smearing of the local shell structure is studied for increasingly anisotropic model molecules and for different liquid densities.
Proton transfer dynamics in the first excited singlet state of malonaldehyde
Authors: Arias Angela A. , Wasserman Thierry A. W. , Vaccaro Patrick H.
Published: July 2002

The dynamics of proton transfer in the first excited singlet state of malonaldehyde [Ã 1B1(π*n)] have been investigated through the use of high-resolution degenerate four-wave mixing spectroscopy. Observed Ã 1B1–X̃ 1A1 (π*←n) rovibronic features are at odds with previous absorption measurements and cannot be explained readily through the accepted description of tunneling behavior within the Ã 1B1(π*n) potential surface.