bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution laser spectroscopy of NO2 just above the X̃  2A1–Ã 2B2 conical intersection: Transitions of K−=0 stacks
Authors: Biesheuvel C. A. , Bulthuis J. , Janssen M. H. M. , Stolte S. , Snijders J. G.
Published: July 2002

The visible absorption spectrum of NO2 is very dense and irregular, and shows signs of a chaotic frequency and intensity distribution in the higher energy region. The complexity of the spectrum is related to a conical intersection of the potential energy surfaces of the two lowest electronic states. Above the conical intersection strong vibronic interactions lead to hybrid eigenstates, which can be viewed as mixtures of low vibrational levels of the electronically excited state and high vibrational levels of the electronic ground state. As a contribution to the elucidation of the nature of the vibronic bands of NO2 we have measured high-resolution spectra of a number of vibronic bands in the region between 10 000 and 14 000 cm−1 by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by a bolometer, and in some cases, laser-induced fluorescence was detected. The hyperfine structure is dominated by the Fermi-contact interaction and the magnitude of this interaction is a direct measure of the (electronic) composition of the hybrid eigenstates. In the present paper we have restricted our analysis to transitions of K−=0 stacks. The fine- and hyperfine structure of each rotational transition can be analyzed by using an effective Hamiltonian approach. The very good agreement that is found between the calculated transition strengths and the measured line intensities is evidence that in the spectral region studied, rovibronic interactions play a minor role. The composition of the hybrid eigenstates is compared with ab initio calculations reported in the literature, leading to the conclusion that measurements of the hyperfine structure are a helpful tool in characterizing vibronic bands.
Accurate ab initio rovibronic spectrum of the X 1Σg+ and B 1Σu+ states in Ar2
Authors: Fernández Berta , Koch Henrik
Published: July 2002

Using recently developed coupled-cluster response methods, accurate potential curves for the X 1Σg+ and B 1Σu+ electronic states of Ar2 are evaluated and the lowest rovibronic energy levels are calculated. The results are in very good agreement with the experimentally available spectroscopic data, and suggest a reassignment of the experimental vibrational numbering in the B 1Σu+ state.
Rotational state dependence of collision induced line broadening and shift at low temperature
Authors: Flatin Daniel C. , Goyette Thomas M. , Beaky Matthew M. , Ball Christopher D. , De Lucia Frank C.
Published: July 2002

The dependence on rotational state of the pressure broadening and shift cross sections of the H2S–H2 and H2S–He systems is reported. The 110–101 and 211–202 transitions of H2S were studied over the temperature range 1–600 K, which includes both the low temperature quantal and the ambient semiclassical collision regimes. At low temperature collisions are mediated by the resonant formation of quasibound complexes rather than the more familiar “action-at-a-distance” mechanisms characteristic of ambient collisions. Additionally, because internal excitation of rotational states can play an important role in the formation of weakly bound complexes, the relation between the rotational energy level spacing hν, and the relative kinetic energy kT plays an important role. The low temperature regime is of interest both because of the interaction mechanism and because it is the regime characteristic of the interstellar medium. Qualitatively, the most interesting features were observed at low temperature, where significantly larger (∼×2) pressure broadening cross sections were measured for the 110–101 than for 211–202.
Hierarchies of nonclassical reaction kinetics due to anisotropic confinements
Authors: Ahn Jaewook , Kopelman Raoul , Argyrakis Panos
Published: July 2002

In contrast to classical chemical reaction kinetics, for diffusion limited chemical reactions the anisotropy of the geometry has far reaching effects. We use tubular two and three-dimensional spaces to illustrate and discuss the dimensional crossover in A+B→0 reactions due to dimensional compactification. We find that the crossover time tc=Wα scales as α=β/(a−b), where a, b, and β are given by the earlier and the late time inverse density scaling of ρ−1∼ta and ρ−1∼tbWβ, respectively. We also obtain a critical width Wc below (above) which the chemical reaction progresses without (with) traversing a two or three-dimensional Ovchinnikov–Zeldovich (OZ) reaction regime. As a result we find that there exist different hierarchies of dimensionally forced crossovers, depending on the initial conditions and geometric restrictions. Kinetic phase diagrams are employed, and exponents are given for various Euclidean and fractal compactified geometries, for the A+B and A+A elementary reactions. Monte Carlo simulations illustrate some of the kinetic hierarchies.
Modeling self-assembling of proteins: Assembled structures, relaxation dynamics, and phase coexistence
Authors: Vekhter Benjamin , Berry R. Stephen
Published: July 2002

A string-of-beads model used previously to describe folding of a polypeptide into a β-barrel is transformed into four-strand model of a self-assembling system, which also produces a β-barrel. In molecular dynamics (MD) simulations, both isothermal and variable-temperature (annealing), the system behaves much like a typical small cluster, insofar as it exhibits the dynamic coexistence of several phase-like forms over ranges of temperature. A three-state analytic model, then a four-state model, supplemented by degeneracies inferred from the MD simulations, yield partition functions and phase diagrams that reproduce the simulations rather well.
Thermal desorption from ordered chemisorbed phases studied by helium scattering: Oxygen on Ag(110)
Authors: Canepa M. , Terreni S. , Narducci E. , Mattera L.
Published: July 2002

We relate the helium specular beam intensity during adsorbate desorption (He desorption curve) to the instantaneous surface coverage. In this way the He desorption curve is shown to provide a picture of the desorption process which can be fruitfully compared to the one coming from thermal desorption mass spectra (TDS), obtained under strictly comparable experimental conditions. We tested the combination of thermal energy atom scattering (TEAS) and TDS in the case of the associative desorption from long range ordered O(2×1)–Ag(110) phase. Using the so-called overlap approach and assuming intense adsorbate–adsorbate (and vacancy–vacancy) attractions along Ag–O–Ag rows we obtain an instantaneous coverage which is in good agreement with TDS results. He desorption curves confirm the extreme sharpness of the desorption transition further indicating that the tails of the TDS peak bear small contributions from oxygen which did not belong to the O(2×1) phase. Opportunities and limitations inherent to the use of the He desorption curve (and its first derivative) as a marker of the temperature position and sharpness of the desorption transition are also addressed.
Conformation of interacting lysozyme by polarized and depolarized light scattering
Authors: Chirico Giuseppe , Beretta Sabrina , Baldini Giancarlo
Published: July 2002

The fluctuations of the polarized and depolarized light scattered by lysozyme solutions in acetate buffer and in 60% w/w glycerol–acetate mixtures have been studied by measuring the correlation function with a resolution of 12.5 ns. This result has been achieved by processing two replicas of the same scattered signal with two separate detectors and computing their cross correlation. The correlograms have been investigated at various temperatures and protein concentration at pH≃4.6 and buffer ionic strength ≃45 mM. The rotational relaxation times obtained from depolarized scattering have been found to lie in the range 150–400 μs, depending on the solution temperature, and no appreciable concentration dependence has been observed. On the other hand, the mutual translational diffusion coefficients derived from polarized scattering have been found to be strongly dependent on protein concentration. The main result is that the protein hydrodynamic radius, obtained by polarized photon correlation measurements is fully consistent with that estimated from depolarized photon correlation measurements, once the data are rescaled for temperature, viscosity, and the effects of protein–protein and salt–protein interactions.
Chemical potential of model benzene fluids using expanded ensemble Monte Carlo simulations
Authors: Khare A. A. , Rutledge G. C.
Published: July 2002

Chemical potential for model benzene fluid at liquid densities is calculated using the expanded ensemble method, modified to permit continuous sampling in λ-space. The density is varied from 0.2 g/cm3 to 0.9 g/cm3. Three different temperatures (T=300 K, 400 K, and 600 K) are considered to provide data below and above the critical temperature (Tc=562.16 K). Two different potential models, the Evans-Watts model and the Claessens’ model, are considered, both of which are rigid hexagonal models with six sites interacting by a Lennard-Jones potential. Despite its simplicity, we found that the results obtained using Claessens’ model are in very good agreement with experimental data.
Determining the molecular Aharonov–Bohm phase angle: A rigorous approach employing a molecular properties based adiabatic to diabatic states transformation
Authors: Yarkony David R.
Published: July 2002

Recently there has been considerable interest, not to mention controversy, concerning a key aspect of the molecular Aharonov–Bohm (MAB) effect: the construction of the phase angle, induced by geometric phase effect, whose gradient is the vector potential characteristic of MAB theory. In the past this angle was constructed from explicit knowledge of the locus of the seam of conical intersection. Here it is shown how a phase angle that satisfies the requirements of MAB theory can be determined without a priori knowledge of the locus of points of conical intersection. This approach has important implications for direct dynamics. It is a corollary of a recent analysis that showed that diagonalizing the matrix of virtually any symmetric (real-valued Hermitian) electronic property operator in the subspace of states that intersect conically generates a transformation that removes all of the singularity of the derivative coupling at a conical intersection. Key aspects of this method are illustrated by considering the dipole moment operator near a point on the 1 3A″–2 3A″ seam of conical intersection in CH2.
Langmuir monolayers as disordered solids: Strain-tilt-backbone coupling and natural order parameters for the swiveling transitions
Authors: Luty Tadeusz , Swanson David R. , Eckhardt Craig J.
Published: July 2002

Multistability of Langmuir monolayers, in particular those composed of fatty acids, is reflected by a very rich and complicated phase diagram. We argue that strain-tilt-backbone coupling determines the behavior described by that diagram. Following the solid state approach, we show that a natural order parameter set is defined by thermal averages of spherical harmonics and strain tensor components. In addition, we show that the backbone order parameter can be conveniently represented by an elastic dipole tensor. Treating Langmuir monolayers as disordered solids, we have derived an orientational entropy contribution to the free energy. The swiveling transition between L2(L2h) and L2′(L2*) phases is discussed in detail and is conveniently described in terms of the proposed order parameters. We discuss why the change in the distortion direction of the 2D unit cell tracks the change in tilt direction of the molecules, and also why the cell dimensions involved in the swiveling transition are the same in both phases.
Coupling of phase transitions in Langmuir monolayers
Authors: Jiang M. , Zhong Fan , Xing D. Y. , Dong Jinming
Published: July 2002

A Ginzburg–Landau theory is presented to study coupled first-order gas–liquid phase transition and second-order orientational phase transition in Langmuir monolayers. We show that the coupling between the two phase transitions has a strong influence on the orientational phase transition, making the polar order phase separated from the isotropic one. Such a phase separation will substantially suppress the onset of the transition and the growth of the polar order parameter, which accords with the existing experiments.
An ab initio study of He–F2, Ne–F2, and Ar–F2 van der Waals complexes
Authors: Chan Kenneth W. , Power Trevor D. , Jai-nhuknan Jaran , Cybulski Sławomir M.
Published: July 2002

Single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] has been used to calculate the ground state potential energy surfaces for He–F2, Ne–F2, and Ar–F2 van der Waals complexes. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions (aug-cc-pVTZ+bf). Single point calculations for approximate minima have also been performed with a larger quadruple zeta basis set (aug-cc-pVQZ+bf). For He–F2 and Ar–F2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–F2 the CCSD(T) interaction energies of the two configurations are virtually the same. The linear configuration of each complex has been found to be much more sensitive than the T-shaped one to the changes of the F–F bond length with the interaction becoming weaker when the F–F bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. High-order correlation corrections have been found to play an important role in determining the relative stability of the linear and T-shaped configurations. The harmonic approximation zero-point vibrational energy for He–F2 exceeds the depth of both wells. For Ne–F2 the zero-point vibrational energy is greater for the linear configuration and, because of that, the complex has a T-shaped ground vibrational state. When the zero-point vibrational energy is taken into account for the Ar–F2 complex the linear and the T-shaped configurations are found to have nearly identical energies.
Correlation energies for some two- and four-electron systems along the adiabatic connection in density functional theory
Authors: Colonna François , Savin Andreas
Published: July 2002

Switching on the electron–electron interaction connects the Kohn–Sham to the physical system. The correlation energy, the only unknown energy component in this process, is determined at fixed density, using a technique based on the Lieb Legendre transform definition of the universal density functional. Results are shown for this adiabatic coupling process for He,Ne8+,Be,Ne6+ as well as for the exponential densities nN,ζ(r)=N(ζ3/π)e−2ζr (N=2 or 4; ζ⩾1; for N=4 degeneracy is present and 1D and 3P are analyzed). The data are fitted to a rational approximant and appear to be in good agreement with those given by the less computationally demanding Harris–Jones adiabatic connection.
Photoionization spectrum and ionization energy of CH3SCl
Authors: Cheng Bing-Ming , Chew Eh Piew , Liu Chin-Ping , Yu Jen-Shiang K. , Yu Chin-hui
Published: July 2002

Photoionization-efficiency (PIE) spectra in the wavelength range 108–144 nm were measured for CH3SCl generated from the reaction system Cl/Cl2/CH3SH(CH3SSCH3) in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as the source of radiation. According to the PIE spectrum of CH3SCl thus obtained, the ionization energy (IE) is (9.147±0.007) eV. Based on GAUSSIAN-2 calculations, the observed ionization of CH3SCl near the threshold region is likely to form from singlet CH3SCl to doublet CH3SCl+; the calculated IE 9.140 eV agrees well with the experimental value. Two vibrational frequencies of singlet CH3SCl and one of doublet CH3SCl+ were found to be (244±60) cm−1, (513±60) cm−1, and (631±60) cm−1, respectively.
Towards the ab initio determination of strictly diabatic states, study for (NaRb)+
Authors: Romero T. , Aguilar A. , Gadea F. X.
Published: July 2002

A generalization of the effective metric approach is proposed and applied both for the calculation of radial couplings and for the determination of diabatic states along a single coordinate using the formalism of variational effective Hamiltonian theory. The application to the ionic molecule NaRb+ shows that strictly diabatic states are obtained even for very short distances where a huge number of crossings are observed. Polarization and electronic delocalization effects are estimated. A comparison with various diabatization methods is performed. The proposed method brings a significant improvement with respect to the existing ones.
Femtosecond laser interactions with methyl iodide clusters. I. Coulomb explosion at 795 nm
Authors: Ford J. V. , Zhong Q. , Poth L. , Castleman A. W.
Published: July 2002

The study of the interaction of femtosecond laser radiation with matter, especially clusters, has blossomed in recent years due to advances in laser technology. One aspect of this interaction is Coulomb explosion. This effect occurs when the repulsive energy of like charges, known as Coulomb repulsion, overcomes the cluster’s total cohesive energy, causing the cluster to disintegrate into charged fragments. In this study, the interactions of methyl iodide clusters, formed in a supersonic expansion using argon and helium as carrier gases, were investigated at 795 nm using a Ti:Sapphire femtosecond laser. The resulting atomic and cluster ions were analyzed in a reflection time-of-flight mass spectrometer. The focus of these studies was the elucidation of the effects of carrier gas and laser wavelength on the laser-cluster interactions leading to Coulomb explosion. To achieve these goals, the effects of different carrier gases, laser power, cluster distribution, and the resulting Coulomb explosion energies were examined. A secondary consideration was to examine the experimental results with regard to the Coherent Electron Motion and Ionization Ignition models.
Atomic classes: Rearrangement processes
Authors: Sim Eunji , Patashinski Alexander Z. , Ratner Mark A.
Published: July 2002

Glass formation and glassy dynamics are studied using a mobile cluster, static boundary model. For the two-dimensional Lennard-Jones glass, we examine the nature of the rearrangement processes that characterizes glassy dynamics, the correlation functions of the hexadic order parameter, the temperature dependence of the distribution of relaxation times, and the dependence of these properties both on the disorder within the static boundary and on the temperature. The mobile cluster passes from a low temperature disordered material to an intermediate temperature amorphized material to a high temperature liquid; during this thermal history, the distribution of relaxation times exhibits nonexponential behavior in the glassy region, and becomes Debye-like in the liquid phase. We see a chain of local structural rearrangements in a small cluster of eight particles representing characteristic elementary kinetic events in this very simple model of glass behavior. The possibility to interpret these events in terms of defect motion is discussed.
On the problem of fitting many-body potentials. I. The minimal maximum error scheme and the paradigm of metal systems
Authors: López M. J. , Jellinek J.
Published: July 2002

Issues involved in fitting a semiempirical potential, such as the choice of a goodness criterion, selection of fitting quantities (properties), ascription of the degree of importance to the individual members of a fitting set, etc., all of which affect the outcome of a fitting procedure, are addressed and analyzed. The minimal maximum error fitting scheme is implemented to obtain the values of the parameters of the Gupta-type potential for nickel. The subtleties of this potential, one of which is that only four of its five parameters are independent, and their implications are pointed out and discussed. The resulting values of the parameters for nickel are analyzed and compared with those obtained in earlier studies.
Photophysics of collision-free thiophosgene: Confirmation of the role of the promoting mode in radiationless transitions
Authors: Moule David C. , Lim Edward C.
Published: July 2002

We show here that the inefficiency of S1(A2)→S0(A1) and S2(A1)→S1(A2) internal conversion in gaseous thiophosgene (Cl2CS) has its most likely origin in the absence of an a2 vibration that can vibronically couple S1 with S0 and S2. The finding confirms the critical role the promoting mode plays in electronic radiationless transitions.
X-ray and electron scattering intensities of molecules calculated using density functional theory
Authors: Smith Garry T. , Tripathi Awadh N. , Smith Vedene H.
Published: July 2002

The elastic and total intensities for x-ray and high-energy electron scattering from the ten-electron hydride series has been calculated from Kohn–Sham orbitals using the BLYP, B3LYP and LSDA functionals, and compared to the previous Hartree–Fock and singles and doubles configuration interaction (SDCI) results of Wang [J. Wang, A. N. Tripathi, and V. H. Smith, Jr., J. Chem. Phys. 101, 4842 (1994)] in the same basis. In those cases where density functional theory (DFT) provides a significantly better electron density than Hartree–Fock, the pair density and hence total scattering intensity for x-rays is also better reproduced, especially in the low s region. The asymptotic behavior of the scattering curves from the DFT methods is poorer than Hartree–Fock due to the inability of DFT to reliably predict the density at the nucleus, the electron–electron distribution at zero-electron separation, and the second moment of the electron–electron distribution.
Structure of bridging polymers
Authors: Swenson J. , Smalley M. V. , Hatharasinghe H. L. M.
Published: July 2002

A four-component clay–polymer–salt–water system, consisting of n-butylammonium vermiculite, poly-(ethylene oxide) (PEO), n-butylammonium chloride, and heavy water was studied by wide-angle neutron diffraction. The one-dimensional nature of the system makes it ideal for model studies of colloids. Previous results had shown that if the size of the polymer, given as the end-to-end distance, is larger than the spacing between the clay platelets the polymer chains bridge between the platelets and draw them together. In order to further investigate the bridging mechanism we have studied the structure of the polymer chains using H/D isotope substitution of the polymer. The volume fractions of clay r and PEO v were r=0.05 and v=0.04, respectively, and the salt concentration c was 0.1 M. The results indicate that a significant part of the polymer chains are directly bound to the clay surfaces, and that the remaining (at least 50%) polymer segments have a Gaussian-like distribution in the middle between the clay layers. Previous studies of the corresponding three-component system without added polymer had shown the surface to be covered with two layers of water molecules. The new results show that the ethylene oxide segments displace water molecules immediately adjacent to the clay surfaces, bonding directly to them by physical adsorption. Due to the polymer induced reduction of the interlayer spacing it is likely that each polymer chain is adsorbed on both clay surfaces. The findings are consistent with our newly proposed model for polymer bridging flocculation.
Optical origin of efficient and selective trans↔cis isomerization in photoexcited polyenes: An ab initio calculation
Authors: Zhang G. P. , Zong X. F. , George Thomas F.
Published: July 2002

An ab initio calculation is carried out to investigate the optical origin of highly-efficient and selective trans↔cis isomerizations in a short polyene. It is found that the highly-efficient isomerization is closely related to the highly-selective optical transitions. These dipole-allowed transitions occur selectively at two configurations with the dihedral angles θ11,12=60° and 120°, where the system can be excited or de-excited and is also able to undergo cis or trans isomerization effectively. For many configurations which are not favorable to the isomerization, the transition matrix elements are basically zero. This rules out many unfavorable transitions at these configurations, which is why photoinduced isomerization is so efficient.
Enhancing systematic motion in molecular dynamics simulation
Authors: Wu Xiongwu , Wang Shaomeng
Published: July 2002

Molecular dynamics (MD) simulation is an important approach for conformational search. The conformational searching efficiency in MD simulation is greatly limited by the systematic conformational change or motion. We developed a new MD simulation method to enhance the conformational search efficiency through accelerating the systematic motion. In this work, we describe the theoretical basis and the simulation algorithm of this method for atomic systems. In this method, systematic motion is accelerated by a guiding force derived from a local free-energy surface defined for a system. Under certain approximations, the guiding forces can be estimated from the force information the system experienced in the past through a memory function, and these forces are used to guide the current motion in the same simulation. Therefore, this guiding force is called the self-guiding force and this kind of simulation is called the self-guided molecular dynamics (SGMD) simulation. We have performed detailed analysis of the characteristics of the SGMD in terms of energetic, structural, and dynamic properties with a Lennard-Jones argon system. In addition, we demonstrated the enhanced conformational search efficiency of the SGMD method through the ergodic measure and the crystallization of liquid argon.
Comparison of methods for calculating the properties of intramolecular hydrogen bonds. Excited state proton transfer
Authors: Kar Tapas , Scheiner Steve , C̆uma Martin
Published: July 2002

A series of molecules related to malonaldehyde, containing an intramolecular H-bond, are used as the testbed for a variety of levels of ab initio calculation. Of particular interest are the excitation energies of the first set of valence excited states, nπ* and ππ*, both singlet and triplet, as well as the energetics of proton transfer in each state. Taking coupled cluster results as a point of reference, configuration interaction-singles–second-order Møller–Plesset (CIS–MP2) excitation energies are too large, as are CIS to a lesser extent, although these approaches successfully reproduce the order of the various states. The same may be said of complete active space self-consistent-field (CASSCF), which is surprisingly sensitive to the particular choice of orbitals included in the active space. Complete active space–second-order perturbation theory (CASPT2) excitation energies are rather close to coupled cluster singles and doubles (CCSD), as are density functional theory (DFT) values. CASSCF proton transfer barriers are large overestimates; the same is true of CIS to a lesser extent. MP2, CASPT2, and DFT barriers are closer to coupled cluster results, although yielding slight underestimates.
Crossed beam reaction of the cyano radical, CN(X  2Σ+), with methylacetylene, CH3CCH (X  1A1): Observation of cyanopropyne, CH3CCCN (X  1A1), and cyanoallene, H2CCCHCN (X  1A′)
Authors: Huang L. C. L. , Balucani N. , Lee Y. T. , Kaiser R. I. , Osamura Y.
Published: July 2002

The chemical dynamics to form cyanopropyne, CH3CCCN (X 1A1), and cyanoallene, H2CCCHCN (X 1A′), via the neutral–neutral reaction of the cyano radical, CN (X 2Σ+), with methylacetylene, CH3CCH (X 1A1), is investigated under single collision conditions in a crossed molecular beam experiment at a collision energy of 24.7 kJ mol−1. The laboratory angular distribution and time-of-flight spectra of the C4H3N products are recorded at m/e=65, 64, 63, and 62. The reaction of d3-methylacetylene, CD3CCH (X 1A1), with CN radicals yields reactive scattering signal at m/e=68 and m/e=67 demonstrating that two distinct H(D) atom loss channels are open. Forward-convolution fitting of the laboratory data reveal that the reaction dynamics are indirect and governed by an initial attack of the CN radical to the π electron density of the β carbon atom of the methylacetylene molecule to form a long lived CH3CCHCN collision complex. The latter decomposes via two channels, i.e., H atom loss from the CH3 group to yield cyanoallene, and H atom loss from the acetylenic carbon atom to form cyanopropyne. The explicit identification of the CN vs H exchange channel and two distinct product isomers cyanoallene and cyanopropyne strongly suggests the title reaction as a potential route to form these isomers in dark molecular clouds, the outflow of dying carbon stars, hot molecular cores, as well as the atmosphere of hydrocarbon rich planets and satellites such as the Saturnian moon Titan.
On the conformational structure of a stiff homopolymer
Authors: Kuznetsov Yu. A. , Timoshenko E. G.
Published: July 2002

In this paper we complete the study of the phase diagram and conformational states of a stiff homopolymer. It is known that folding of a sufficiently stiff chain results in formation of a torus. We find that the phase diagram obtained from the Gaussian variational treatment actually contains not one, but several distinct toroidal states distinguished by the winding number. Such states are separated by first order transition curves terminating in critical points at low values of the stiffness. These findings are further supported by the off-lattice Monte Carlo simulation. Moreover, the simulation shows that the kinetics of folding of a stiff chain passes through various metastable states corresponding to hairpin conformations with abrupt U-turns.
Population and coherence control by three-pulse four-wave mixing
Authors: Brown Emily J. , Pastirk Igor , Grimberg Bruna I. , Lozovoy Vadim V. , Dantus Marcos
Published: July 2002

Control of coherence and population transfer between the ground and excited states is reported using three-pulse four-wave mixing. The inherent vibrational dynamics of the system are utilized in timing the pulse sequence that controls the excitation process. A slight alteration in the pulse sequence timing causes a change in the observed signal from coherent vibration in the ground state to coherent vibration in the excited state. This control is demonstrated experimentally for molecular iodine. The theoretical basis for these experiments is discussed in terms of the density matrix for a multilevel system.
High-level ab initio computations of structures and relative energies of two isomers of the CO2 trimer
Authors: Tsuzuki Seiji , Klopper Wim , Lüthi Hans P.
Published: July 2002

The two-body interaction energies of the cyclic (C3h) and noncyclic (C2) trimers at the basis set limit (15.55 and 16.55 kJ/mol) were estimated by means of second-order Møller–Plesset perturbation theory including terms linear in the interelectronic distance (MP2-R12 approach). Coupled cluster calculations with single and double substitutions and with noniterative triple excitations [CCSD(T)] were carried out to correct the MP2-R12 two-body interaction energies for the effect of higher excitations, and also for the evaluation of the three-body interaction energies. The calculated bonding energies of the trimers using this model calculation (16.82 and 17.20 kJ/mol) suggest that the two isomers are nearly isoenergetic. The three-body terms are small (only of the order of 1 kJ/mol), stabilizing the cyclic trimer, but destabilizing the noncyclic trimer.
Photoinduced changes in the structure of a van der Waals complex. Three-dimensional inertial axis reorientation in the S1←S electronic spectrum of indole-Ar
Authors: Korter Timothy M. , Küpper Jochen , Pratt David W.
Published: July 2002

Rotationally resolved S1←S0 electronic spectra of the van der Waals complexes indole-Ar and N-deuterated-indole-Ar in the gas phase are described. Both spectra exhibit inertial axis reorientation. A comparison of the parameters derived from fits of the two spectra with those obtained from a previous study of the bare molecule [Berden et al., J. Chem. Phys. 103, 9596 (1995)] shows that the Ar atom in indole-Ar is attached above the indole plane, displaced toward the N atom in the five-membered ring, in both electronic states. However, the measured center-of-mass coordinates of the Ar atom in the principal axis frames of indole in the two states are different, leading to the observed axis reorientation in the high resolution spectra. The S1 state in indole itself is shown to be the 1Lb state, which has a different electronic distribution from that of the ground state. Thus, the Ar atom “moves” when the photon is absorbed because there are significant differences in the intermolecular potential energy surfaces of the two states.
Potential energy function and vibrational states of N2CO+
Authors: Hochlaf M. , Léonard C. , Ferguson E. E. , Rosmus P. , Reinsch E.-A. , Carter S. , Handy N. C.
Published: July 2002

A six-dimensional potential energy function (PEF) for the electronic ground state of N2CO+ (X 2A′) has been generated by electronic structure calculations using the restricted open shell coupled cluster RCCSD(T) approach. The ion has a planar trans equilibrium structure with: RNN=1.106 Å, RNC=1.905 Å, RCO=1.127Å, θNNC=175.3°, and θNCO=129.1°. Variational calculations of the vibrational states (J=0) have yielded the following anharmonic wavenumbers for the fundamentals: ν1 (NN stretch) 2287.2, ν2 (CO stretch) 2071.0, ν3 (CN stretch+NCO bend) 546.9, ν4 (CN stretch+NCO bend) 215.2, ν5 (in-plane NNC bend) 123.4, ν6 (out-of-plane NNC bend) 133.8 (all values are in cm−1). For fixed equilibrium coordinates except one, the barriers to linearity have been calculated to be 8 cm−1 for the NNC and 2260 cm−1 for the NCO moieties, the torsional barrier to be 35 cm−1. It has been found that the ν3 and ν4 modes are strongly coupled, the in-plane ν5 and out-of-plane ν6 bending modes possess an inverse anharmonicity and fall into clusters. Using complete active space self-consitent-field CASSCF approach on the collinear cuts of the PEF for low-lying excited states several conical intersections between the Π2 and the Σ+2 states have been located.
Nascent rotational distribution and energy disposal of the CaH product in the reaction of Ca(4s4p 1P1)+H2→CaH(X 2∑+)+H
Authors: Liu Dean-Kuo , Chen Jye-Jong , Nien Chia-Fu , Lin King-Chuen
Published: July 2002

We have obtained for the first time the nascent rotational population distributions of CaH(v=0 and 1) in the reaction of Ca(4s4p 1P1) with H2. The rotational, vibrational, and translational fractions of product energy disposal may accordingly be evaluated to be 0.19±0.02, 0.33±0.02, and 0.48±0.02, respectively. Compared to the prior prediction, the low rotational and high vibrational fractions obtained suggest that the reaction mechanism should favor the collinear approach.
A reexamination of exchange energy functionals
Authors: Tsuneda Takao , Suzumura Toshihisa , Hirao Kimihiko
Published: July 2002

Conventional exchange functionals are examined through the one-parameter progressive (OP) correlation functional from physical and numerical points of view. With an exchange functional that obeys the fundamental conditions of the exact exchange functional, the OP functional has been proved to satisfy all fundamental conditions of the exact correlation functional. In this paper, we discuss whether conventional exchange functionals satisfy these conditions or not, and propose some strict conditions for exchange functionals that are required to obey the fundamental conditions. By combining the exchange functionals with the OP correlation functional, we also evaluate chemical properties for the G2 set of molecules, and confirm that the correction for exchange functionals obviously contributes to the improvement of calculated results.
Cluster-surface collisions: Characteristics of Xe55- and C20–Si[111] surface bombardment
Authors: Cheng Hai-Ping
Published: July 2002

Molecular dynamics (MD) simulations are performed to study the cluster-surface collision processes. Two types of clusters, Xe55 and C20 are used as case studies of materials with very different properties. In studies of Xe55–Si[111] surface bombardment, two initial velocities, 5.0 and 10.0 km/s (normal to the surface) are chosen to investigate the dynamical consequences of the initial energy or velocity in the cluster-surface impact. A transition in the speed of kinetic energy propagation, from subsonic velocities to supersonic velocities, is observed. Energy transfer, from cluster translational motion to the substrate, occurs at an extremely fast rate that increases as the incident velocity increases. Local melting and amorphous layer formation in the surfaces are found via energetic analysis of individual silicon atoms. For C20, the initial velocity ranges from 10 to 100 km/s. The clusters are damaged immediately upon impact. Similar to Xe55, increase in the potential energy is larger than the increase in internal kinetic energy. However, the patterns of energy distribution are different for the two types of clusters. The energy transfer from the carbon clusters to Si(111) surface is found to be slower than that found in the Xe clusters. Fragmentation of the carbon cluster occurs when the initial velocity is greater than 30 km/s. At 10 km/s, the clusters show recrystallization at later times. The average penetration depth displays a nonlinear dependence on the initial velocity. Disturbance in the surface caused by C20 is discussed and compared to the damage caused by Xe55. Energetics, structures, and dynamics of these systems are fully analyzed and characterized.
Single state NH(X 3Σ−,v=0,J,N) preparation for state-to-state studies
Authors: Rinnenthal Jan Leo , Gericke Karl-Heinz
Published: July 2002

A new method is presented to examine state-to-state rotational energy transfer in ground state NH(X 3Σ−,v=0,J,N). NH(X 3Σ−) is generated via state selective stimulated emission pumping using the strongly forbidden NH(a 1Δ→X 3Σ−) intercombination transition around 794 nm after foregoing photodissociation of HN3 at a wavelength of 266 nm. Products are detected by laser induced fluorescence (LIF). Chemically relevant collision dynamics including spatial processes can be studied for the first time in v=0 of the electronic ground state. State-to-state rate constants for inelastic collisions of NH(X 3Σ−,v=0,J=3,N=3) with Ne are presented.
A nonequilibrium ensemble formalism: Criterion for truncation of description
Authors: Ramos J. Galvão , Vasconcellos Áurea R. , Luzzi Roberto
Published: July 2002

In the framework of a nonequilibrium statistical ensemble formalism, consisting of the so-called Nonequilibrium Statistical Operator Method, we discuss the question of the choice of the space of thermohydrodynamic states. We consider in particular the relevant question of the truncation of description (reduction of the dimension of the state space). A criterion for justifying the different levels of truncation is derived. It depends on the range of wavelengths and frequencies which are the relevant ones for the characterization, in terms of normal modes, of the thermohydrodynamic motion in a nonequilibrium open system. Applications to the cases of thermal-sensitive resins and of n-doped polar semiconductors are done, numerical results are presented, and experimental observation is discussed.
Multilevel geometry optimization
Authors: Rodgers Jocelyn M. , Fast Patton L. , Truhlar Donald G.
Published: July 2002

Geometry optimization has been carried out for three test molecules using six multilevel electronic structure methods, in particular Gaussian-2, Gaussian-3, multicoefficient G2, multicoefficient G3, and two multicoefficient correlation methods based on correlation-consistent basis sets. In the Gaussian-2 and Gaussian-3 methods, various levels are added and subtracted with unit coefficients, whereas the multicoefficient Gaussian-x methods involve noninteger parameters as coefficients. The multilevel optimizations drop the average error in the geometry (averaged over the 18 cases) by a factor of about two when compared to the single most expensive component of a given multilevel calculation, and in all 18 cases the accuracy of the atomization energy for the three test molecules improves; with an average improvement of 16.7 kcal/mol.
Characterization of dynamical product-state distributions by spectral extended cross-correlation: Vibrational dynamics in the photofragmentation of NH2D and ND2H
Authors: Reid Jonathan P. , Loomis Richard A. , Leone Stephen R.
Published: July 2002

The spectral cross-correlation method [Jacobson et al., J. Chem. Phys. 107, 8349 (1997)], developed for the identification and extraction of spectroscopic patterns, is extended to the analysis of product-state dynamical data from photofragmentation. Fragment product state vibrational distributions for the photodissociation of ammonia and deuterated ammonia species are extracted. Since chemical isolation of the mixed isotopic parent molecules is prohibited, the photodissociation dynamics of all four parent species (NH3, NH2D, ND2H and ND3) are studied simultaneously at 193.3 nm. The electronic emission spectra from the NH2(Ã 2A1), ND2(Ã 2A1), and NHD(Ã 2A1) fragments are recorded by time-resolved Fourier transform infrared spectroscopy. Spectral signatures for the photodissociation products from each parent species are extracted by the cross-correlation method. The formalism is derived to extend the spectral cross-correlation method to dynamical reactive product state information. The application of the cross-correlation method to such a system, in which there is significant overlap of the quantum states of the product species from different parent molecules, represents a powerful new tool to extract the dynamical information of reactive chemical processes from a series of complex spectra. As an example, the relative quantum yields for breaking the N–H or N–D bonds of the parent molecules to form the electronically excited state products, as a function of NH2, ND2, and NHD bending vibrational state, are determined with only limited knowledge of the spectroscopic assignments. The application of the spectral cross-correlation method to probe dynamical chemical processes in other types of reactive systems is discussed.
High-resolution laser spectroscopy of NO2 just above the X̃2A1–Ã2B2 conical intersection: Transitions of K−=1 stacks
Authors: Biesheuvel C. A. , Bulthuis J. , Janssen M. H. M. , Stolte S. , Snijders J. G.
Published: July 2002

The complexity of the absorption spectrum of NO2 can be attributed to a conical intersection of the potential energy surfaces of the two lowest electronic states, the electronic ground state of 2A1 symmetry and the first electronically excited state of 2B2 symmetry. In a previous paper we reported on the feasibility of using the hyperfine splittings, specifically the Fermi-contact interaction, to determine the electronic ground state character of the excited vibronic states in the region just above the conical intersection; 10 000 to 14 000 cm−1 above the electronic ground state. High-resolution spectra of a number of vibronic bands in this region were measured by exciting a supersonically cooled beam of NO2 molecules with a narrow-band Ti:Sapphire ring laser. The energy absorbed by the molecules was detected by the use of a bolometer. In the region of interest, rovibronic interactions play no significant role, with the possible exception of the vibronic band at 12 658 cm−1, so that the fine- and hyperfine structure of each rotational transition could be analyzed by using an effective Hamiltonian. In the previous paper we restricted ourselves to an analysis of transitions of the K−=0 stack. In the present paper we extend the analysis to transitions of the K−=1 stack, from which, in addition to hyperfine coupling constants, values of the A rotational constants of the excited NO2 molecules can be determined. Those rotational constants also contain information about the electronic composition of the vibronic states, and, moreover, about the geometry of the NO2 molecule in the excited state of interest. The results of our analyses are compared with those obtained by other authors. The conclusion arrived at in our previous paper that determining Fermi-constants is useful to help characterize the vibronic bands, is corroborated. In addition, the A rotational constants correspond to geometries that are consistent with the electronic composition of the relevant excited states as expected from the Fermi-constants.
Fourier transform infrared isotopic study of the C12 chain trapped in solid Ar
Authors: Ding X. D. , Wang S. L. , Rittby C. M. L. , Graham W. R. M.
Published: July 2002

A Fourier transform infrared investigation of the C13 isotopic spectrum of linear C12 in solid Ar coupled with density functional theory calculations at the B3LYP/cc-pVDZ level has resulted in the identification of the ν8=1997.2 and ν9=1818.0 cm−1 stretching modes. Agreement between experimental measurements and theory is very good. The identification of a third mode ν7=2140.6 cm−1 is considered tentative.
Rotational spectra of four of the five conformers of 1-pentene
Authors: Fraser G. T. , Xu L.-H. , Suenram R. D. , Lugez C. L.
Published: July 2002

The rotational spectra of four of the five expected conformers of 1-pentene, together with their monosubstituted 13C isotopic forms, have been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. One of the conformers has Cs point-group symmetry while the other three conformers have C1 point-group symmetry. The measurements are compared to results from molecular modeling calculations using the MM3 molecular-mechanics force field of Allinger et al. and to ab initio electronic structure calculations (MP2/6-31G*, MP2/6-311G*, MP4/6-31G*, MP4/6-311G*). Both types of calculations suggest the existence of five distinct conformers of 1-pentene, four of C1 symmetry and one of Cs symmetry. Both the MM3 and ab initio rotational constants deviate from the measured values by ⩽5%. The relatively high barriers between the four conformers limit the conformational cooling in the expansion, allowing all four conformers to be observed at the <2 K rotational temperature of the molecular beam. Efforts to identify the fifth conformer were unsuccessful, presumably due to its reduced intensity, which makes it difficult to identify its spectral pattern from among the plethora of weak unassigned lines due to impurities, complexes, and possible vibrationally excited conformers. The fifth conformer is predicted to have the highest energy of the five conformers of 1-pentene, as well as a low-energy barrier (109 cm−1 at MP2/6-311G* level) for conformational isomerization.