bluuebunny/crossref_metadata_2025 huggingface dataset:
T
2 can be greater than 2T1 even at finite temperature
Authors: Laird Brian B. , Skinner James L.
Published: July 2002

The relaxation of a nondegenerate two-level quantum system linearly and off-diagonally coupled to a thermal bath of quantum-mechanical harmonic oscillators is studied. The population and phase relaxation times, T1 and T2, are calculated to fourth order in the system/bath interaction. Focus is on a specific model of the bath spectral density that is both Ohmic (proportional to frequency at low frequency) and Lorentzian, and which has the property that, in the semiclassical or high-temperature limit, it reproduces the stochastic model studied previously by Budimir and Skinner [J. Stat. Phys. 49, 1029 (1987)]. For this fully quantum-mechanical model, it is found that under certain conditions the standard inequality, T2≤2T1, is violated, demonstrating that this unusual result, which was originally derived from the (infinite-temperature) stochastic model, is valid at finite temperature as well.
Optical spectra and structure of oligomeric models of polyparaphenylenevinylene
Authors: Tian B. , Zerbi G. , Schenk R. , Müllen K.
Published: July 2002

This paper treats the vibrational and UV/visible spectra of oligomers of polyparaphe- nylenevinylene (PPV) and lays the background knowledge for the treatment of the spectra and structure of PPV which is treated in the paper that immediately follows. The problem of the molecular conformation of these systems is faced both with MNDO (modified neglect or differential overlap) calculations and with infrared-Raman experiments. Oligomers are certainly twisted in noncrystalline phases while may approach coplanarity in the solid. Fourier transform Raman spectra are recorded and are interpreted in terms of the recently developed theory of the effective conjugation coordinate.
Rotational spectra and structures of van der Waals dimers of Ar with a series of fluorocarbons: Ar⋅⋅⋅CH2CHF, Ar⋅⋅⋅CH2CF2, and Ar⋅⋅⋅CHFCF2
Authors: Kisiel Z. , Fowler P. W. , Legon A. C.
Published: July 2002

Rotational spectra of van der Waals dimers between an argon atom and CH2CHF, CH2CF2, and CHFCF2 have been obtained by pulsed-supersonic nozzle Fourier transform microwave spectroscopy. Analysis of the derived spectroscopic constants shows that the dimers have structures such that for CH2CHF, CH2CF2, and CHFCF2 the Ar atom is positioned over the FCCH, FCF, and FCCF atomic chains with Ar-molecular center-of-mass distances of 3.62 Å, 3.51 Å, and 3.56 Å, and angles between the Ar–cm axis and molecular planes of 48.2°, 72.9°, and 60.5°, respectively. Structures for the three dimers are also predicted with a simple multisite model which describes the anisotropy of the dispersive interaction; both the Ar acceptor site and the atom–atom distances are satisfactorily reproduced.
Picosecond dynamics and molecular aggregation from vibrational dephasing in the fluid phases of some 4-n-alkyl-4′-cyanobiphenyl liquid crystals
Authors: Rothschild Walter G. , Perrot Michel , De Zen Jean-Marc
Published: July 2002

We describe the temperature dependence of the inhomogeneously broadened CN Raman profile I(ω) at ωc ∼2230 cm−1 of the title compounds (n=1, 6, 8, 9, 11, 12) in their isotropic liquid phase and solutions (CHCl3, CCl4) by simulating the oscillator amplitude correlation function by a vibrational equilibrium renewal process in terms of random fluctuations of the oscillator transition frequency ω(t)=ωc +ω1(t) about its central value ωc. To this effect, the autocorrelation function of the frequency shift ω1(t) is expressed as a probability density function (PDF) F̂(t) of recurrence times of the stochastic motional narrowing events in the local environment of the CN oscillators. System-related physical meaning and satisfactory data fit is obtained if F̂(t) is understood as an expansion in terms of parallel, independent exponential relaxation processes with characteristic times τ that are distributed by a PDF ρα(τ)=〈τ〉h(τ)/τ, where α is the dispersion parameter of the extended exponential and 〈τ〉 the expectation of τ. Width and ranges of h(τ) show strong molecule–molecule clustering, possibly indicating a trend with alkyl chain length. At temperatures just above the mesophase–liquid-phase transition, the range of the prevalent relaxation times τ in the local environment of the CN oscillators is of the order of 1–4 ps. Only at temperatures near 570 K or by high dilution in the solvents are the inter- and intracluster forces sufficiently diminished to approximate those of ordinary fluids. We consider our method to give a realistic description of the dynamics of types of macroscopically isotropic fluids where, nevertheless, the shape, size, and polarity of their molecules lead to a degree of aggregation that weakens the identity and the influence of constituent members. The temporary structure of the macroscopically isotropic fluids in the liquid-crystal systems is best understood by admitting a significant presence of randomly distributed local regions of dynamic nematicity, causing temperature-dependent relaxation pathways over 10–50 Å distances.
Resonance enhanced multiphoton ionization photoelectron spectra of CO2. II. Competition between photoionization and dissociation
Authors: Wu Ming , Taylor David P. , Johnson Philip M.
Published: July 2002

Resonance enhanced multiphoton ionization photoelectron spectra (REMPI-PES) of CO2 were obtained via several Rydberg states. Contrary to expected diagonal Franck–Condon factors, long vibrational progressions were seen in each photoelectron spectrum. The dominant vibration varied with the resonant state. A long progression of two quanta of the asymmetric stretch was observed in the PES through nf resonances, while symmetric stretches were more apparent with the 3p resonances. These results may arise from photoionization occurring during molecular dissociation.
C̃→Ã emission in H2O following two-photon excitation: Dissociation dynamics in the Ã state for different initial states
Authors: Zhang J. Z. , Abramson E. H. , Imre D. G.
Published: July 2002

C̃→Ã emission spectra of the water molecule have been observed following two-photon excitation to various vibrational levels of the C̃(1B1) electronic state. Photodissociation dynamics on the Ã(1B1) state potential energy surface were probed through the C̃→Ã emission spectra. The different spectra correspond to photodissociation trajectories in the Ã state starting with a variety of initial conditions. Time-dependent quantum mechanical simulations of the emission spectra show good agreement with the experimental data and provide further insight into the dissociation dynamics on the Ã state surface.
The calculation of highly excited bound-state energy levels for a triatomic molecule by using three-arrangement basis sets and contracted basis functions
Authors: Day Paul N. , Truhlar Donald G.
Published: July 2002

The rovibrational energy levels of the nonrotating H+3 molecular ion have been calculated using a basis set defined in three-arrangement Jacobi coordinates. Energy levels corresponding to the A′1 symmetry are reported up to 22 600 cm−1 above the potential energy minimum and are converged to within 1.8 cm−1, energy levels in the A2 symmetry are reported up to 28 800 cm−1 and are converged to within 0.7 cm−1, and energy levels in the E′ symmetry are reported up to 24 300 cm−1 and are converged to within 1.8 cm−1. The maximum deviation of the reported results from the results of Carter and Meyer [J. Chem. Phys. 93, 8902 (1990)] is about 2 cm−1, and most of the results agree to within 1 cm−1. An advantage to our basis set is that it can represent the full D3h symmetry of this system, and thus the energy levels are assigned to the correct symmetry group unambiguously. In order to improve computational efficiency, contracted basis functions were formed using a successive diagonalization-truncation procedure.
Cage molecules containing elements of groups V and VI. I. Structure determinations using simulated annealing
Authors: Jones R. O. , Seifert G.
Published: July 2002

Density functional calculations with simulated annealing have been performed to determine the structures and relative stabilities of stable isomers of seven-membered molecules containing elements of groups V (phosphorus, arsenic) and VI (sulfur, selenium, and tellurium). Particular attention is focused on structures related to P4S3 and P2S5, including the families PnAs4−nS3−mSem and P4SnTe3−n. We show that the bonding trends in all these molecules can be understood in terms of transferable bond energies.
Permanent dipole moments and two-color multiphoton resonances in the two-level molecule: The rotating wave approximation versus exact results
Authors: Kondo A. E. , Blokker V. M. , Meath William J.
Published: July 2002

The interaction of two continuous wave electric fields (lasers) with a two-level molecule, that has a nonzero difference d, between the permanent dipoles of the two states involved in a multiphoton two-color transition, is studied in the rotating wave approximation and by using exact methods. Analytical rotating wave approximation results for the time-dependent populations of the molecular states and for the resonance profiles are derived and used to help discuss the effects of permanent dipole moments in the two-color absorption problem. Several examples of multiphoton, two-color, two-level resonance profiles calculated in the rotating wave approximation and by using exact techniques, are employed for illustrative purposes. They are used to help quantify the conditions for the validity of the two-color rotating wave approximation, which are considerably more restrictive than the analogous conditions for the one-color rotating wave approximation with d≠0, and to illustrate the interpretive and predictive nature of the rotating wave approximation results. For example, there are many multiphoton, two-color transitions that are available if a molecule possesses permanent dipoles (d≠0). The analytic rotating wave approximation expression for the molecule-two laser coupling can often be used to reliably estimate the field parameters of the two lasers that are required to optimize a given transition relative to others.
Excitation-emission study of continuum Raman and the photofragmentation mapping of IBr
Authors: Levy Izhak , Shapiro Moshe , Yogev Amnon
Published: July 2002

Experiments and theory of the excitation-frequency dependence of the continuum-Raman signal of IBr are described. The theoretical computations, which do not follow the Kramas–Heisenberg expression, are extremely successful in predicting the wavelength dependence of each Stokes line and the ratio between the different overtones. The ‘‘reflection principle’’ or ‘‘photofragmentation mapping’’ of absorption spectroscopy, which postulates the equivalence between the topology of an initial vibrational wave function and the frequency dependence of the cross section, is shown to hold also for continuum-Raman spectroscopy. Our experimental excitation–emission spectra for the low-lying overtones of IBr are a clear reflection of the final nuclear density. In contrast, we show that the excitation-frequency dependence of the high overtones is mainly sensitive to the shape of the intermediate excited electronic states.
Wavelength dependence of electric-field effects on persistent spectral holes
Authors: Hartmannsgruber N. , Maier Max
Published: July 2002

We determined the effective electric dipole moment difference Δμ* of perylene in the amorphous polymer polyvinylbutyral from the changes of persistent spectral holes in an external electric field. The value of Δμ* increases with increasing wavelength of the spectral hole in the inhomogeneous absorption band. The experimental results are compared with calculations of the wavelength dependence of Δμ* using an extension of the theory of pressure broadening and shift of spectral holes of Laird and Skinner [J. Chem. Phys. 90, 3274 (1989)]. The calculations are based on changes of the matrix shift of perylene by the electric field. Best agreement between the calculations and the measurements was obtained when changes of a short-range intermolecular interaction, like the repulsive interaction, by the electric field were assumed to be dominant.
Time-dependent dynamics of electrons and nuclei
Authors: Deumens Erik , Diz Agustin , Taylor Hugh , Öhrn Yngve
Published: July 2002

Using the time-dependent variational principle with a group theoretical coherent state defining the wave functions for electrons and nuclei, a system of coupled, first-order, nonlinear differential equations is obtained for a general molecular system. The equations form a classical Hamiltonian system within a generalized phase space that allows a systematic time-dependent study of molecular processes. The approach is general and provides a computational framework for a variety of properties such as transition and excitation probabilities in atomic and molecular collisions, and molecular spectra such as vibrational spectra with anharmonicities. The basic approximation corresponding to the choice of a single determinantal wave function for the electrons and classical nuclei is analyzed. Illustrative applications to the p+H collision process and to vibrations of the H2O molecule exhibit good agreement with experiment and with other theoretical work.
Coupled internal rotations in substituted ethoxybenzenes: Maximum entropy analysis of nuclear magnetic resonance of nematic solutions data and a
b i
n
i
t
i
o rotameric distributions
Authors: Di Bari Lorenzo , Persico Maurizio , Veracini Carlo Alberto
Published: July 2002

The dipolar couplings observed by means of nuclear magnetic resonance spectra of solutes in nematic solvents (LX-NMR) are used to build the maximum entropy probability distribution for the orientational and the internal variables that describe the conformational state of the solute molecule. This approach is applied to the cases of ethoxy (β,β,β-d3) benzene, 4-fluoro ethoxy (β,β,β-d3) benzene, and 4-chloro ethoxy (β,β,β-d3) benzene that contain two internal degrees of freedom, and yields a bidimensional probability map for the two relevant internal rotations. A detailed map of the ab initio potential for ethoxybenzene is also obtained with full optimization of internal coordinates. The two results are compared, showing that they are in good agreement, which allows one to say that the two rotations are strongly correlated especially in the region of the maximum probability (absolute potential minimum).
CO internal excitation from the reaction: H+CO2→CO+OH
Authors: Nickolaisen Scott L. , Cartland Harry E. , Wittig Curt
Published: July 2002

Time-resolved infrared diode laser absorption spectroscopy has been used to probe CO internal excitation following 193 nm photolysis of 300 K H2S/CO2 samples. Vibrations and rotations are colder than statistical, i.e., Eint(CO) is only ∼1500 cm−1 even though ∼10 000 cm−1 is available for product excitations, assuming modest collisional deactivation of the hot H atoms that undergo reaction. A [v=1]/[v=0] ratio of ∼0.4 is obtained and there is essentially no population at v≥2. Both the v=0 and 1 rotational distributions are cold, peaking at Jmax∼11 and 13, respectively. The vibrational distribution is nascent while the rotational distributions may be partially relaxed, but not enough to alter the main conclusions. Combined with earlier results for OH internal excitations and center-of-mass (CM) kinetic energies, we conclude that at high collision energies there is a propensity toward product CM kinetic energy. In this regime, the reaction cross section rises rapidly with energy and statistical unimolecular rate theory is not applicable, even with a HOCO° intermediate.
Rotational line strengths for the photoionization of diatomic molecules
Authors: Xie Jinchun , Zare Richard N.
Published: July 2002

We derive an expression for the probability that a diatomic molecule AB(n,v,N) in the electronic state n, vibrational state v, and rotational level N yields upon photoionization AB+(n+,v+,N+), where we assume Hund’s case (b) coupling. Our result is formally equivalent to the previous work of Buckingham, Orr, and Sichel [Phil. Trans. Roy. Soc. London, Ser. A 268, 147 (1970)] but differs substantially in that we use spherical tensor methods, which provide insight into the photoionization dynamics in terms of the contribution of different multipole moments. The total interaction term is given by the tensor product of the electric dipole moment operator T(1,μ0) and the multipole moment tensor T(l,m) describing the photoelectron in the lth partial wave. The interaction term is further simplified into a sum of reduced multipole moments T(k,p), where k=l±1 and p=μ0+m. For an isotropic distribution of initial states, the transition probability is given by P(N,N+)=1/3∑k Sk(N,N+)‖μ̄(k,q)‖2, where the factor of 1/3 arises from the use of a beam of polarized light, Sk(N,N+) is a generalized rotational line strength factor, and ‖μ̄(k,q)‖2=‖μl=k+1(k,q) ‖2+‖μl=k−1(k,q)‖2 is the sum of the squares of reduced multipole moment matrix elements.

The summation over k is restricted to even values for a (±)↔(±) transition and to odd values for a (±)↔(∓) transition. Thus, for an unpolarized molecular sample, the integrated photoelectron intensity associated with an N→N+ transition is the incoherent sum of the multipole moments that contribute to this transition, and each such contribution is an incoherent sum over l=k+1 and l=k−1. If the molecular sample is polarized (aligned and/or oriented), then the expression for the N→N+ integrated photoelectron intensity becomes a coherent sum over different k values with the same l value. Moreover, if the photoelectron distribution is angle resolved, then the expression for the N→N+ transition probability is a coherent sum over l values with the same k value when the molecular sample is unpolarized and cannot be separated into incoherent parts when the molecular sample is polarized. The expression for P(N,N+) has been used to fit the results of the photoionization of H2 and NO. In both cases, the fit obtained, which required only one or two adjustable parameters, respectively, agrees well with the experimental data. This treatment may be readily extended to photoionization of polyatomic molecules and to molecules that follow different angular momentum coupling cases.
Lifetime and transition probability measurements of the second positive system of nitrogen by laser-induced fluorescence
Authors: Fukuchi T. , Wuerker R. F. , Wong A. Y.
Published: July 2002

Nitrogen laser-induced fluorescence from a low-pressure (≤10−3 torr) radio frequency produced plasma has been observed for the second positive system of N2. A 45.4±0.4 ns lifetime of the C 3Πu(v′=0) state was experimentally measured from laser-induced fluorescence. In addition, some transition probabilities of the second positive system were deduced from fluorescence signal strengths. The results are compared against values found by electron excitation of nitrogen.
The response of semiflexible liquid crystals to quenched random disorder
Authors: Dadmun M. , Muthukumar M.
Published: July 2002

We have studied the effect of quenched random impurities on the first-order nematic to isotropic transition of trimers using Monte Carlo simulation. We have shown that the introduction of quenched impurities to a liquid crystalline system will lower the transition temperature, round the specific heat peak, and change the order of the transition. This is in agreement with previous calculations that show that the introduction of quenched random impurities can decrease or eliminate the discontinuity of a first-order transition.
Close-coupling wave-packet study of He and Ne sticking on metal surfaces
Authors: Jackson Bret
Published: July 2002

A fully quantum-mechanical study is made of light particles sticking on a model Cu surface. The full molecule-metal wave function is expanded to include states corresponding to elastic scattering and the creation or annihilation of single phonons. The coupled-channel equations for this system are evolved in time numerically using well-known wave-packet techniques. For light particles and low energies, where only single-phonon interactions are important, this is a rigorous treatment of the dynamics and can serve as a benchmark. Probability distributions for energy transfer are computed from the reduced density matrix. These distributions exhibit resonances which correspond to transitions into the gas-metal bound states. The trapping probability is studied as a function of particle mass, energy, and surface temperature for a flat Cu surface. A comparison is made with the forced oscillator model and other approximate methods which are shown to give poor results for the sticking probability of light particles.
Isothermal density tuning of the depolarization ratios from the ν1 mode of methane
Authors: Rose Erik J. , Whitewolf E. , Baglin Frank G.
Published: July 2002

Pure methane and 10 mol % methane in Ar have been studied in the region of methane’s ν1 totally symmetric stretching mode near 2910 cm−1 (ω0, the signal maximum or zeroth spectral moment frequency). Densities from 0.0795 (3 bar) to 15.0 (2100 bar)×1021 mol/c.c. were examined using Ar+ laser illumination at 514.5 nm. Frequency shifts of ν1’s ω0, and the zeroth moment depolarization ratio, ρD, values as a function of pressure were recorded. Previous ω0 shifts vs density were generally confirmed and were extended to higher densities. This allowed a shallow minimum to be observed in this data. It was correlated with the ρD values. These latter values were examined to see whether or not they were as sensitive as the frequency shift results were to the local reorientational friction coefficient tensor fr (potential-energy interactions) in methane and the Ar/CH4 solutions. The initial (3.5–1000 bar data) depolarization results were interpreted with two models of methane in mind. The first, a conventional view that is based upon the concept that higher densities cause distortions in the molecular shape (lower symmetry), and the second, which makes use of the known fact that the symmetry of methane at low pressures is less than Td. Therefore, as the density and fr increase quantum rotation slows and the rotation-vibration interaction effectively ceases and causes the molecules to pack tightly and the well-known Td symmetry is achieved.
A comparison between experimental and theoretical excitation functions for the O++H2 (4A″) system using trajectory calculations over a wide energy range
Authors: González Miguel , Gilibert Miquel , Aguilar Antonio , Sayós R.
Published: July 2002

The O++H2 prototypic ion–molecule reaction was recently studied in our group using the quasiclassical trajectory method on a ground model potential-energy surface of the system inferred from both experimental and theoretical information and reasonable assumptions, fitted using a London–Eyring–Polanyi–Sato surface modified to include an ion–induced-dipole interaction term. The theoretical results previously reported agreed rather well with the available experimental information. In view of the latest experimental findings, we present in this contribution the results of new trajectory calculations on the above-mentioned surface, covering a wide range of relative translational energies of reactants. The accord between the experimental determinations and the present theoretical evaluations is excellent at relative energies below 1.0 eV, while for higher energies the potential-energy surface, even though still yielding reasonably good results, provides a worse agreement. The observed discrepancies should be attributed either to the too attractive character of the surface or to the interfering influence of the competing O++H2→O+H2+ charge-transfer process.
Sensitivity analysis of water thermodynamics
Authors: Zhu Sheng-Bai , Wong Chung F.
Published: July 2002

Sensitivity analysis in various thermodynamic properties of liquid water is performed based on two flexible three-site models. It is found that the determinative factors for the Helmholtz free energy and the system internal energy are the gas-phase O–H bond length and the Lennard-Jones radius. The Lennard-Jones radius also plays a predominating role in evaluating the thermodynamic pressure. On the other hand, the statistical entropy, the heat capacity at constant volume, and the thermal pressure coefficient are mainly controlled by the parameters relating to the O–H bond stretching. In addition, the Kirkwood Gk factor is quite sensitive to the cutoff radius in the reaction field geometry. This study helps understand the thermodynamics in liquid water and provides useful hints for further improving the current water models.
Use of canonical orbital energy derivatives for closed-shell self-consistent-field wave functions
Authors: Yamaguchi Yukio , Remington Richard B. , Gaw Jeffrey F. , Schaefer Henry F. , Frenking Gernot
Published: July 2002

The characteristics of the first and second derivatives of the canonical orbital energies for the closed shell self-consistent-field (SCF) wave function have been studied. The first derivatives of the orbital energies were determined analytically and the second derivatives by the finite difference method. These derivatives were transformed from the Cartesian coordinate system to the normal coordinate system for a variety of stationary points. It is demonstrated that the first and second derivatives of the orbital energies in terms of the normal coordinates provide very useful chemical information concerning the molecular structures and unimolecular reactions for a range of chemical species. Similarities and differences between the present approach and Mulliken–Walsh method are also discussed.
The microphase separation transition in symmetric interpenetrating networks: A Monte Carlo study
Authors: Schulz M. , Binder K.
Published: July 2002

The microphase separation transition of interpenetrating networks was studied in the framework of a Monte Carlo simulation of a simple lattice model of interpenetrating networks. This model is based on introducing suitable cross links in the standard bond fluctuation model of dense systems of linear polymers. For the case of a symmetric composition ratio f=0.5, we found stable lamellar structures for temperatures below the critical temperature Tc and a second-order-like phase transition at Tc from the disordered phase to the ordered structure.
Detection of critical mode convection in the presence of a thermal gradient using chemical waves as a passive indicator
Authors: Su Sunyu , Armstrong Robin L. , Menzinger Michael , Cross Albert , Lemaire Claude
Published: July 2002

The detection of the lowest critical modes, γ1(1) and γ0(1), for thermal convection in a vertical tube is reported. Their visualization was through the distortion they caused to chemical waves propagating in a manganese-catalyzed Belousov–Zhabotinsky medium. A time dependent temperature gradient was allowed to develop in the system and the dynamical behavior of spontaneously occurring trigger waves was recorded as magnetic resonance images. The work also illustrates the potential of magnetic resonance imaging to provide information on the temperature dependence of the propagation velocity of trigger waves and of the frequency of pacemaker activity.
Activation barriers for series of exothermic homologous reactions. III. Reactions of s2 metal atoms with N2O and O2
Authors: Futerko Peter M. , Fontijn Arthur
Published: July 2002

Potential energy curves are derived for the rate-limiting channels of reactions of s2 metal atoms with N2O and O2, for which both reactants are in their ground electronic states. Activation barriers and rate coefficients are derived from these curves and are compared with the available experimental data. The importance of contributions by spin-forbidden channels to the rate coefficients of N2O reactions is evaluated using the Landau–Zener–Stuckelberg probability of potential-curve switching. Calculated rate coefficients thus obtained are in good agreement with the available experimental data. It is established that for the Mg, Ca, and Ba+N2O reactions the interaction energy of the lowest singlet and triplet states increases as the mass of the metal atom increases. Spin-forbidden channels are thus more important for the Ca and Ba+N2O reactions than for the Mg+N2O reaction. Calculated activation barriers for reactions of s2 metal atoms with O2 are found to be in agreement with the available experimental data, the only exception being that for the Zr+O2 reaction. The activation barriers, with the exception of those for the Ni and Zr+O2 reactions, are seen to correlate with the s–p promotion energies plus the ionization potentials of the atoms. Explanations for these disagreements are given.
Electronic states of Yn (n=2–4)
Authors: Dai Dingguo , Balasubramanian K.
Published: July 2002

We compute the geometries and energy separations of several electronic states of Yn (n=2–4). The complete-active-space self-consistent-field (CASSCF) followed by multireference singles+doubles configuration interaction (MRSDCI) calculations which included up to 2.6 million configurations are made in this study. We find two nearly degenerate states, namely, 3T1 and 1A1 of tetrahedral geometry, as candidates for the ground state of Y4. The Y–Y bond lengths are computed as 3.41 and 3.42 Å for 3T1 and 1A1 states, respectively. The electronic states with the rhombus structures are found to be ≥0.34 eV above the tetrahedral ground state for Y4. We found two nearly degenerate electronic states with D3h geometries as candidates for the ground state of Y3 (2A″2 and 2A′1). Two electronic states of Y2, namely, 5Σu− and 1Σg+ (short R) are also studied.
A geometric representation of nuclear modulation effects: The effects of high electron spin multiplicity on the electron spin echo envelope modulation spectra of Mn2+ complexes of N-ras p21
Authors: Larsen Russell G. , Halkides Christopher J. , Singel David J.
Published: July 2002

A theoretical treatment is presented for the analysis of ESEEM spectra of I=1/2 nuclei coupled to an electron spin of high multiplicity, with specific attention to the case of S=5/2. This treatment is shown to account for the observed spectral behavior of 15N and 31P nuclei coupled to Mn2+ in a GDP complex with the protein N-ras p21. The treatment involves the decomposition of the multilevel electron spin system into a set of noninteracting two level systems, an approximation that is valid when the dispersed EPR spectral width is large compared to the microwave excitation bandwidth. The consequent spectral selectivity of the microwave excitation is accounted for, in ESEEM simulations, by attaching a weight to the ESEEM subspectra associated with each EPR transition, and calculating the total ESEEM spectrum as a weighted superposition of the subspectra. The simplest means of estimating the appropriate weight factors—identifying them with the cw EPR intensity of each transition, as deduced by simulation of the EPR spectra—leads to ESEEM simulations that account for the key features of the observed spectra, in particular, features that are peculiar to high multiplicity spin systems. In the studied Mn2+ system, no clear indication of orientation selective effects were found. A simple geometric representation is presented which enables the facile understanding of ESEEM spectra of nuclear spin I=1/2 coupled to an electron spin of high spin multiplicity in orientationally disordered solids. Analytical expressions are derived for the ESEEM frequencies, frequency dispersions and amplitudes. It is shown that in these systems external field variation can lead to an array of spectral line-narrowing and amplitude resonance phenomena analogous to those observed in S=1/2 systems.
Dipolar interactions between NH3 molecules trapped in solid argon. II. Line narrowing during nuclear spin species conversion
Authors: Boissel P. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

A spectacular line narrowing as a function of time is observed in the ν2 and ν4 rovibrational absorption spectra of NH3 trapped in argon matrices, at T=5 K. This effect has been predicted on the basis of a fundamental difference between the two nuclear spin symmetry species; molecules belonging to the spin 1/2 species (J=K=1 rotational states) exhibit dipolar interactions, leading to an inhomogeneous broadening of the lines, whereas spin 3/2 species (J=K=0) are unaffected by this interaction. The system presents some analogy with the quadrupolar orientational glass phase exhibited by N2/Ar dilute solid solutions, and also by solid hydrogen. However, here, after a fast cooling, the concentration of ‘‘active’’ (spin 1/2) species decreases with time as the nuclear spin species conversion proceeds. Concentration effects can then be probed spectroscopically within a few hours, inside the same sample. A quantitative study of the perturbation is performed, using a simple model to calculate the perturbed line profiles. This analysis shows that, as the concentration decreases, NH3 molecules undergo a transition from a dipolar behavior to a nondipolar one.
Line shape of transition to the predissociative level Cs2 D 1Σu+(v, J) and the effects of magnetic field
Authors: Katô Hajime , Kumauchi Takahiro , Nishizawa Kiyoshi , Baba Masaaki , Ishikawa Kiyoshi
Published: July 2002

A collimated cesium beam is crossed at right angles by the laser beam, and the excitation spectrum of the D 1Σu+–X 1Σg+ transition is measured by detecting the emission from the dissociated atom Cs(6p 2P3/2). Line broadening is observed for a number of transitions to the D 1Σu+(v, J) levels, and the line shapes of the isolated lines are observed to be Lorentzian. When an external magnetic field H is applied parallel to the electric vector of the laser light, the linewidths are observed to increase proportionally to H2, and it is attributed to the magnetic predissociation. Some lines are observed to change by the magnetic field from a Lorentzian to an asymmetric line shape accompanied by energy shift of the intensity maximum, and it is identified as originating from the M dependence of both the transition moment <D 1Σu+‖μ‖X 1Σg+≳ and the Zeeman energy of the 3Πu state.
Quantum mechanical calculation for photodissociation of hydrogen peroxide
Authors: Zhang Dong H. , Zhang John Z. H.
Published: July 2002

Quantum dynamics calculations are carried out to study ultraviolet (UV) photodissociation of H2O2 at a photon energy of 248 nm. The photodissociation process of hydrogen peroxide is simulated by the standard two-surface model using an ab initio ground potential energy surface and a simple empirical excited surface. The time-dependent approach is employed in quantum dynamics calculations due to the short-time nature of the dissociation process. In this calculation, two high-frequency OH stretching modes are kept frozen but the remaining four degrees of freedom are treated fully quantum mechanically. The quantum calculation fully utilizes the symmetry properties of the system and each symmetry block is computed separately. The computed rotational state distribution of the OH fragments is in qualitative agreement with the classical calculation of Bersohn and Shapiro, with most of the excess energy being carried away by the relative translational motion of the OH fragments. The effect of torsional mode on the rotational state distribution is investigated by calculating the Franck–Condon factors of photodissociation using torsionally excited bound state wave function. Our calculated rotational state distribution, averaged over contributions of two parity-splitting states, is found to be in good agreement with that observed in experiment.
Spin-density wave, charge density wave, and polarizabilities of C60 in the Pariser–Parr–Pople model
Authors: Willaime F. , Falicov L. M.
Published: July 2002

The Pariser–Parr–Pople Hamiltonian is investigated on a single C60 molecule in the Hartree–Fock approximation. The effect of on-site and two-site electron–electron interactions on the stability of states other than the normal paramagnetic state is studied. In particular (i) a spin-density wave state, with a structure similar to that of the classical ground state of the antiferromagnetic Heisenberg Hamiltonian; and (ii) a charge density wave state which occurs for weakly screened two-site interactions. Linear and nonlinear optical properties are also calculated. With the finite field method and the Mo/ller–Plesset correction in second order, a linear polarizability of 325.5 a.u. and a second hyperpolarizability of 5.87×10−36 esu are found.
A study of the energetics of the Cl2/MgO(001) interface using correlation corrected periodic Hartree–Fock theory
Authors: McCarthy Maureen I. , Hess Anthony C. , Harrison N. M. , Saunders V. R.
Published: July 2002

The energetics of the Cl2/MgO(001) interface were investigated using the ab initio periodic Hartree–Fock (PHF) method and local density functional correlation corrections to PHF theory, as implemented in the program CRYSTAL92. Estimates of the correlation corrected PHF energies are made by post-SCF evaluations of three gradient corrected functionals. The correlation energy is calculated from the fully converged ground state PHF charge density and added to the PHF total energy. This is the first study of interfacial energetics using the correlation corrected PHF theory. PHF and correlation corrected molecule/surface binding energies are reported for seven orientations of the adsorbate with respect to the surface plane. Three of the configurations align the intramolecular axes along the surface normal and the remaining geometries arrange the molecules heat-to-tail, parallel to the surface plane. The most favorable interaction occurs when chlorine approaches a surface oxygen along the normal direction. This site preference is consistent with a classical electrostatic description of the physisorption process. The binding energy increases with decreasing surface coverage. At the most dilute coverage studied (1:8) the PHF binding energy was 4.1 kcal/mol and the correlation corrected binding energies ranged from 9.2 to 10.3 kcal/mol. All three functionals tended to increase the molecule/surface attractions, shorten the molecule/surface equilibrium distance, increase the curvature of the molecule/surface potential energy surface near equilibrium, and reduce the molecule/molecule repulsions.
J-recoupling patterns arising from two chemically equivalent nuclear spins in magic-angle spinning spectra of solids
Authors: Wu Gang , Wasylishen Roderick E.
Published: July 2002

A general description of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra arising from a pair of chemically equivalent nuclear spins is presented in terms of average Hamiltonian theory (AHT). In general, the MAS NMR spectra of such a spin-pair exhibit a spinning frequency dependent four-line pattern from which it is possible to extract the indirect spin–spin coupling constant, J, involving the ‘‘equivalent’’ spin pair. Explicit expressions for the spinning frequency dependence of the four-line pattern have been derived using AHT. In principle, correction terms to any order can be included; however, consideration of correction terms up to and including third order appear to be sufficient to interpret the most important features characteristic of J-recoupled spectra involving chemically equivalent spin pairs. The average Hamiltonian theory predicts three different general types of recoupling patterns. The type of recoupling pattern observed for a particular chemically equivalent spin pair is predicted to depend on the relative magnitudes of the indirect homonuclear coupling constant, J, the direct homonuclear dipolar coupling constant, R, the magnitude of the instantaneous chemical shift difference between the ‘‘equivalent’’ spins in frequency units, and the MAS spinning frequency. All reported examples of spinning frequency dependent MAS NMR spectra arising from a pair of chemically equivalent spins can be understood using the theoretical expressions derived here. As an example, we interpret the unusual J-recoupling pattern observed in 31P MAS NMR spectra of Hg(PPh3)2(NO3)2. The recoupling pattern is unusual in that 2J(P,P) is given by the separation of alternate lines in the four-line pattern. Similar unusual J-recoupling patterns were first reported by Eichele, Wu, and Wasylishen.
Molecular dynamics and vibrational relaxations in liquid nitromethane. II. Raman, coherent anti-Stokes Raman spectroscopy, and transient optical Kerr effects in the totally symmetric ν4 mode in CH3NO2
Authors: Cataliotti Rosario Sergio , Foggi Paolo , Giorgini Maria Grazia , Mariani Leonardo , Morresi Assunta , Paliani Giulio
Published: July 2002

The vibrational relaxation of the ν4 mode of CH3NO2 liquid and its molecular dynamics are investigated. Temperature and solvent effects on vibrational dephasing and correlation times are discussed in terms of the adiabatic impact theory to infer that, in the molecular collision, the attractive part of intermolecular potential is explored. The statistical uncorrelation of rotational and vibrational relaxations is experimentally demonstrated.
A theoretical study of the initial stages of Si(111)–7×7 oxidation. I. The molecular precursor
Authors: Schubert Boris , Avouris Phaedon , Hoffmann Roald
Published: July 2002

We have studied the initial stages of the oxidation of the Si(111) surface using extended Hückel tight-binding calculations. Due to the different dangling bond sites present on the reconstructed Si(111)–7×7 surface, one may expect more than one molecular precursor or dissociated Si–O configuration to be formed. As candidates for the main and kinetically most stable molecular precursor, structures involving O2 associated with a single Si adatom site are proposed. Bridge structures are found to be less stable. However, dissociated species derived from bridge structures play an important role in the oxidation process. In this paper we introduce the computational approach used, and discuss the nature of the molecular precursors. In a second paper the nature of the atomic oxygen containing products and the mechanism of SiO4 formation are discussed.
Bond selectivity in the reaction of NH(a 1Δ) with methanol
Authors: Okada Satoru , Tezaki Atsumu , Yamasaki Katsuyoshi , Matsui Hiroyuki
Published: July 2002

Reaction of NH(a 1Δ) with methanol has been studied by a flash photolysis of HNCO at 193 nm at room temperature (298±2 K). Time dependence and relative concentrations for NH, NH2, NHD, CH3O, H, and D are monitored by a laser induced fluorescence (LIF) technique. From the pseudo-first-order analysis of NH(a 1Δ) decay rate, the rate constants for the reactions NH(a 1Δ)+CH3OH→ products (1), and NH(a 1Δ)+CH3OD→products (2) have been determined to be k1=(1.37±0.10)×10−10 cm3 molecule−1 s−1 in the pressure range of 3–20 Torr Ar or 5 Torr He, and k2=(1.33±0.05)×10−10 cm3 molecule−1 s−1 at the pressure of 5 Torr He, respectively. By comparing the LIF intensity of NH(X 3Σ−) in reaction (1) with that in NH(a 1Δ)+Xe reaction system, the contribution of physical quenching in reaction (1) has been evaluated to be less than 2.4%. Also by comparing the LIF intensities of NHD and NH2 in reaction (2) with those in NH(a 1Δ)+D2/H2 reaction systems, the branching ratio between the pathways producing NHD (2b) and NH2 (2c) has been determined to be k(2b)/k(2c)=23±9. It is elucidated that the insertion of NH(a 1Δ) into O–D bond of CH3OD is completely predominant over C–H bond; i.e., the O–D bond is about 69 times more reactive toward insertion of NH(a 1Δ) than a single C–H bond.
Impurity segregation in Lennard-Jones A/AB heterostructures. II. The effect of impurity size
Authors: Cook Stephen J. , Clancy Paulette
Published: July 2002

Segregational phenomena in Lennard-Jones heterostructures of the type A/A1−xBx were investigated using nonequilibrium molecular dynamics computer simulation techniques. In this paper, the ratio of the size of the ‘‘impurity’’ B atoms relative to that of the substrate A atoms was varied, covering ranges where the B atoms were either smaller than or larger than the A atoms. Ratios of the collision diameter, σR=σB/σA, of 1.20, 1.15, 0.90, 0.85, 0.80, 0.75, and 0.5 were considered. The concentration of the B atoms was also varied. The tendency for segregation of the B atoms into a surface-melted thin film was studied and compared to the segregational behavior exhibited at a moving solid/(bulk) liquid interface, the latter emulating liquid phase epitaxy. When the B atoms are significantly larger than the substrate atoms, as here, either misfit dislocations are nucleated or an amorphous phase is produced depending on the concentration of B atoms and the degree of lattice mismatch. When the B atoms are smaller than the substrate A atoms, there is a more pronounced tendency to maintain crystallinity and to occupy interstitial lattice positions rather than substitutional sites. This asymmetry of segregational behavior depending on whether the impurities are larger than or smaller than the substrate is suggested to be due to the inherent asymmetry of the Lennard-Jones potential. The results of previous vapor phase epitaxial simulations are reviewed in the larger context of the present results for liquid phase epitaxy.
Revised densities of methane to 10 kbar and 200 °C
Authors: Robertson Stanley L.
Published: July 2002

The adoption of new calibration densities at low pressures leads to greatly improved accuracy for densities at high pressures.
Model study of two-dimensional protein aggregates in the nucleation stage of crystallization
Authors: Higo Junichi , Nagayama Kuniaki
Published: July 2002

A two-dimensional (2D) aggregation model was introduced to investigate the nucleation stage of protein crystallization. The constituent protein molecules were positioned at the lattice points of a 2D square. Four specific interaction sites were assigned to the fringe of each molecule, and through these sites the molecules are bound to each other with an effective interaction. Protein molecules were allowed to translate in the vicinity of the lattice points and to rotate within a limited angle. The partition function was calculated for aggregates consisting of 2–16 molecules. Exposed bonds, which connected exposed molecules to the rest part of the aggregates, were the weakest. The peripheral bonds, which connected molecules at the periphery of the aggregates, were weaker than the buried bonds, which were located inside the aggregates. Increasing the size of the aggregate caused both the buried and peripheral bonds to become stronger due to correlation between fluctuating bonds. The more compact an aggregate was, the more stable it was. The free energy was determined by the size, the compactness of the aggregate, and the correlation between bonds. The free-energy gain that results by generating aggregates from individual free molecules was calculated, yielding a critical size, which discriminated growing aggregates from dissociating aggregates.
Configuration interaction calculations with terms linear in the interelectronic coordinate for the ground state of H+3. A benchmark study
Authors: Röhse Robert , Klopper Wim , Kutzelnigg Werner
Published: July 2002

The CISD-R12 method is applied to the ground states of H2 and H+3 at their equilibrium configurations, with both bare nuclear Hamiltonian (BNH) and self-consistent field (SCF) references with eight basis sets that range from 10 s (i.e., ten contracted s-type Gaussians) to 30s20p12d9f. With the largest basis set, the energy of H2 is obtained with an error of 2μEh, while for H+3, the error is probably smaller and the best calculated energy of −1.343 835Eh is supposed to be accurate to all indicated figures. The BNH reference is always superior to the SCF reference. The relativistic corrections are evaluated at the SCF level by means of ‘‘direct perturbation theory’’ both for H2 and H+3. The Hylleraas-SCF methods and related concepts for the construction of an optimum reference function are discussed in the Appendix.