bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental studies of the vapor phase nucleation of refractory compounds. I. The condensation of SiO
Authors: Nuth Joseph A. , Donn Bertram
Published: February 2003

An experimental apparatus has been built in which a quantitative study of the vapor phase nucleation of refractory particles can be performed. With this equipment, measurements have been made of the critical partial pressure (Pc) of SiO necessary to initiate avalanche nucleation in the SiO–H2 system as a function of ambient temperature in the range 750 K⩽T⩽1000 K. The condensate produced at low temperatures is Si2O3 rather than the more stable oxides SiO or SiO2. At high temperatures a mixture of Si2O3 and amorphous SiO2 is found. The SiO2 may be the result of annealing or oxidation on the collector after the particles had been formed as less stable clusters in the vapor. Analysis of Pc vs T using classical nucleation theory yields a surface energy of ∼500  ergs/cm2 for the particles. Despite the fact that this is in reasonable agreement with values for Si2O3 obtained from the literature, several inconsistencies in the analysis are noted. It is argued that classical nucleation theory is not applicable to this system.
Effect of atomic reagent approach geometry on reactivity: Reactions of aligned Ca(1P1) with HCl, Cl2, and CCl4
Authors: Rettner Charles T. , Zare Richard N.
Published: February 2003

The reactivity of Ca(1P1) with HCl, Cl2, and CCl4 has been studied as a function of Ca(1P1) alignment with respect to the initial average relative velocity vector of the reagents in a beam-gas scattering geometry. While the total chemiluminescence cross section for the Ca(1P1)+HCl reaction is insensitive to Ca(1P1) alignment, the branching into the CaCl(A 2Π) and CaCl(B2Σ+) states depends markedly on the approach direction of the atomic p orbital. Parallel approach of the Ca p orbital favors CaCl(B2Σ+) formation while perpendicular approach favors CaCl(A2Π). A weak dependence of these effects on CaCl(B2Σ+) vibrational state is observed. The analogous reaction with Cl2 shows a strong preference for perpendicular p-orbital approach in both chemiluminescent product channels, which is most prominant for the CaCl(A2Π) state. In contrast, the reaction with CCl4 displays no significant dependence on approach geometry. For Ca(1P1)+Cl2, a chemi-ionization channel is observed, showing a preference for perpendicular alignment intermediate between that for the CaCl(A2Π) and CaCl(B2Σ+) channels. Chemiluminescence spectra, absolute chemiluminescence cross sections, branching ratios, and emission polarizations are also presented. Results are interpreted in terms of an electron-jump model in which the symmetry of the reagents is preserved during a reactive encounter.
Oscillator strength anomalies as described by the AGP model
Authors: Goscinski O. , Weiner B. , Elander N.
Published: February 2003

The antisymmetrized geminal power wave function (AGP) used together with excited states generated by operators satisfying the vacuum condition leads to simple forms for the transition moments. The variation and possible vanishing of the transition moments can be expressed exclusively in terms of the geminal expansion coefficients.
MSXα calculation of adiabatic potential energy surfaces of Cu+ in sodium chloride lattice in the a1g subspace. Incidence of the copper–chlorine distance on the electronic structure
Authors: Chermette H. , Pedrini C.
Published: February 2003

The multiple scattering Xα method applied to an embedded CuCl5−6 cluster is used to describe the electronic structure of Cu+ in NaCl. Several calculations have been performed with different Cu–Cl distances in order to reach the equilibrium distance. The harmonic force constant of the vibrational breathing deformation of the cluster, deduced from the total energy curve vs the bond distance is in large agreement with experimental results, while the equilibrium distance is predicted slightly larger than the NaCl bulk value.
General occurrence of polar twisted minima in the ionic singlet excited surfaces of polyenes
Authors: Nebot-Gil Ignacio , Malrieu Jean-Paul
Published: February 2003

Except for a few cases, the lowest closed shell SCF solutions for the twisted geometries of polyenes involves delocalized nonpolarized MO’s, and represents a meaningless mixture of radicalar gound state and ionic excited states. Higher (quasi) solutions allow a realistic approach of the polarized ionic states. Orthogonality to the radicalar ground state may be imposed without important loss of energy. Further extensive CI does not modify significantly the situation. These statements are illustrated through ab initio calculations of butadiene and hexatriene. This approach allows us to show that slight distortions stablize polar minima even in ’’homosymmetric’’ molecules such as all-trans hexatriene twisted around the central bond. The existence of polar twisted minima in the ionic singlet excited surfaces of polyenes seems therefore a general phenomenon.
Terminal distributions of rotational energy in free jets of CO2 by infrared emission spectrometry
Authors: Venkateshan S. P. , Ryali S. B. , Fenn J. B.
Published: February 2003

By means of Fourier transform infrared spectrometry we have analyzed the radiation from supersonic free jets of CO2 issuing from heated nozzle sources. The terminal rotational energy contents deduced from the spectra are consistent with those obtained by other methods. The distributions of this energy are clearly non-Boltzmann. A two-temperature model provides remarkably faithful descriptions of the observed distributions and leads to a simple characterization of the departure from rotational equilibrium in terms of a rotational entrophy. The results show that the extent of thls departure increases with increasing source density. Competition between rotation-rotation and rotation-translation exchanges seems to provide a plausible explanation for the observed behavior.
Nonphotochemical hole burning and antihole production in the mixed molecular crystal pentacene in benzoic acid
Authors: Olson R. W. , Lee H. W. H. , Patterson F. G. , Fayer M. D. , Shelby R. M. , Burum D. P. , Macfarlane R. M.
Published: February 2003

Results of narrowband hole burning experiments and picosecond photon echo experiments on pentacene in benzoic acid are presented. On burning, several new discrete pentacene absorptions (antiholes) are created. These spontaneously revert to the unburned form at low temperatures, but may also be induced to revert by optical excitation. Deuteration of the host acidic protons markedly changes the pentacene fluorescence lifetime and hole burning and recovery behavior, indicating the strong interaction of pentacene and the benzoic acid dimer hydrogen bonds. Hole burning and antihole formation are attributed to hydrogen bond tautomerism of benzoic acid dimers near excited pentacenes. Two mechanisms for this tautomerization are suggested. The homogeneous linewidth of the pentacene S0→S1 transition is measured by both photon echo and hole burning experiments. A preliminary study of the homogeneous linewidth measured by hole burning indicates the transition broadens with an 11 cm−1 thermal activation energy, which differs from measurements made by other methods.
Homofusion and heterofusion of triplet excitons in a ‘‘real’’ organic mixed crystal: Biphenyl host-naphthalene and pyrene guests system
Authors: Talapatra Gautam Buddha , Misra T. N.
Published: February 2003

The prompt and delayed emission spectra of a biphenyl host with naphthalene and pyrene both as guests mixed crystal have been studied. The fluorescence of naphthalene and pyrene and the excimer fluorescence of pyrene have been observed. The delayed emissions are observed from pyrene and naphthalene as delayed fluorescence of pyrene, phosphorescence of naphthalene, and pyrene. The investigations of excitation spectra of these emissions have shown to be very useful to understand the mechanism of delayed fluorescence generation. The heterogeneous defect-naphthalene and homogeneous naphthalene–naphthalene triplet–triplet annihilation are responsible for the production of delayed fluorescence of pyrene. The kinetic analyses from the decays and buildups have been made. The delayed luminescence decays at various temperatures have found to be nonexponential. However, in the long time region they are nearly exponential. Even in the long time region, the pyrene delayed fluorescence decays and buildups are nonexponential in the temperature range above 130 K. It has been established from the various Arrhenius plots that in this mixed crystal the temperature dependence of the phosphorescence lifetime is controlled by the thermal processes                       The kinetic scheme described satisfactorily explains the experimental results.
Raman spectra of supercooled H2O from 0 to −21 °C
Authors: Yeh Y. , Bilgram J. H. , Känzig W.
Published: February 2003

Polarized and depolarized Raman spectra have been obtained from supercooled water down to a temperature of 251 K (−21 °C). The spectral ranges investigated include the intramolecular OH stretch (3000→3800 cm−1) region, the 175 cm−1 region which represents asymmetric stretch of intermolecular hydrogen bonds, and the <50 cm−1 region which contains information on the hydrogen bond fluctuations and formation of clusters. The results indicate that as the temperature is lowered, the number of hydrogen-bonded polyhedra of water molecules increase. The Raman intensity change with temperature led to a determination of the enthalpy for hydrogen bonding of 3.1±0.5 kcal/mol. There is also a suggestion of decreased hydrogen bond fluctuation and an increase in the number of larger polyhedra clusters as the temperature is lowered.
Vibronic absorption spectra of phenyl alkyne cations in solid argon at 20 K
Authors: Andrews Lester , Kelsall Benuel J. , Harvey James A.
Published: February 2003

Phenyl alkyne samples at high dilution in argon were subjected to argon resonance radiation during condensation at 20 K. Sharp band origins at 579.0 and 586.0 nm agree with the differences between sharp first and fourth photoelectron bands for phenyl acetylene and 1-phenyl-1-propyne, which identifies the molecular cation absorptions. Vibronic assignments are supported by isotopic substitution and comparison to spectra of phenyl acetylene. Substitutent vibrations dominate the red transition, which is predominantly π(ring) ← π(alkyne), whereas ring vibrations dominate the UV transition for 1-phenyl-1-propyne, which is localized on the aromatic ring.
The influence of CT states and energy transfer on the luminescence of Ln2[Pt(CN)4]3⋅yH2O single crystals
Authors: von Ammon W. , Gliemann G.
Published: February 2003

Time resolved emission measurements of Ln2[Pt(CN)4]3⋅yH2O single crystals (LnCP) at T = 1.9 K are reported. The time dependence of the E⊥c polarized emission of these crystals—with the exception of LuCP—deviates considerably from that of other tetracyanoplatinates (MCP, M≠Ln). The results can be explained assuming an admixture of (Ln, 4f) orbitals to the emitting Pt-complex states (Pt, 5dz2).
Electric field perturbative calculations of vibrational intensities. II. Orbital polarizability model for Raman optical activity and simultaneous evaluation of Raman optical activity, Raman scattering, infrared circular dichroism, and infrared absorption intensities
Authors: Polavarapu Prasad L.
Published: February 2003

A procedure for evaluating Raman optical activity (ROA) intensities using molecular orbital (MO) schemes is presented. In this procedure, molecular electric dipole polarizability is considered to be the sum of electric dipole polarizabilities of localized MOs. Then these orbital polarizabilities are treated in a manner similar to bond or atomic polarizabilities to formulate the molecular magnetic dipole and electric quadrupole polarizability tensors. In this model, termed as an orbital polarizability model, calculation of ROA intensities involves the evaluation of orbital polarizabilites, orbital centroids, and their normal coordinate gradients. These quantities, along with molecular structural parameters and atomic displacement vectors during normal modes of vibration, are noted to be sufficient to evaluate Raman scattering, vibrational infrared circular dichroism (VCD), and IR absorption intensities, also. Then an orbital polarizability model calculation of ROA intensities also provides, simultaneously, Raman, VCD, and IR intensities. Based on the finite electric field perturbative approach, which was originally utilized by Komornicki and McIver for evaluating Raman and IR intensities [J. Chem. Phys. 70, 2014 (1979)], ROA intensities have been evaluated for the first time using MO schemes. In this procedure, the gradients of orbital centroids and polarizabilities, with respect to nuclear coordinates, are expressed in terms of the gradients of orbital contributions to forces with respect to an applied electric field. It is pointed out that in this approach, one normal MO and force calculation, and 18 electric field perturbative calculations all at equilibrium molecular geometry, are sufficient to obtain all aforementioned vibrational intensities for all (3N-6) normal modes. We have also tested the validity and soundness of this efficient procedure by performing calculations on a simple optically active molecule NHDT and comparing the results with those obtained in an alternate procedure. In the alternate scheme, the normal coordinate gradients of orbital centroids and polarizabilities are evaluated from the MO calculations repeated at displaced nuclear geometries. In the comparison we noted that the former scheme provides better and sufficient numerical accuracy for ROA and Raman intensities, while the latter does not. We have also noted that while the IR intensities predicited by both procedures are identical, the VCD intensities match within a factor of 2. An explanation for this difference is provided.
Determination of the ground-state potential energy curve of LiH6 up to dissociation
Authors: Verma K. K. , Stwalley W. C.
Published: February 2003

An ultraviolet argon ion laser (3336 Å) has been used to excite the A 1Σ+–X 1Σ+ system of the LiH6 molecule. A long progression of R–P doublets is observed in the range 0⩽v′′⩽21. This is the first time ground-state levels above v′′=12 have been observed for the lithium hydride molecule. Based upon these results, we have constructed a Rydberg–Klein–Rees (RKR) potential energy curve which corresponds to over 99% of the ground state potential well. This experimental curve is compared with theoretical ab initio calculations for the X 1Σ+ state of LiH. We find that Docken and Hinze’s X state potential energy curve [J. Chem. Phys. 57, 4928 (1972)] is probably the most accurate among the published ab initio calculations in the region of curve crossing of the zero order curves representing the ionic and covalent configurations of LiH, although some more recent calculations are of comparable accuracy.
A nonideal, nonautocatalytic polymer reaction model giving rise to stable limit cycles
Authors: Li Ru-Sheng , Nicolis G.
Published: February 2003

A simple nonautocatalytic polymer solution reaction model which can lead to instabilities as a result of nonideality is put forward. A stable limit cycle is shown to exist under certain conditions. The effect of entropy contributions in the free energy of mixing on the nonequilibrium transition phenomena is discussed.
An electron impact study of carbon dioxide dimers in a supersonic molecular beam: Appearance potentials of (CO2)2+, (CO CO2)+, and (Ar CO2)+
Authors: Stephan K. , Futrell J. H. , Peterson K. I. , Castleman A. W. , Märk T. D.
Published: February 2003

Electron impact ionization of (CO2)n clusters (1⩽n⩽5) was investigated in a molecular beam experiment coupled with a double focusing mass spectrometer. A special ion optical system and operating mode (deflection method) was used to discriminate between beam species and background species. For both neat and seeded (He, Ar, Kr) beams we have studied, ion beam profiles extracted from the ion source in order to obtain information about neutral cluster beam properties. All appearance potentials reported in this work have been referenced to rare gases in either the static gas or seeded beam mixtures. The following appearance potentials were obtained: (CO2)2+e→(CO2)2+, AP=13.1±0.2 eV, (CO2)2+e→(CO CO2)+, AP=19.1±0.5 eV, Ar CO2+e→(Ar CO2)+, AP=13.1±0.5 eV. The exothermicity of the reaction of CO2+ with CO2 to form (CO2)2+ was found to be 17.1±5 kcal/mol in good agreement with previous high pressure mass spectrometry determinations.
Dynamics of molecules using semigroup techniques: Vibrational line shapes in exciton models of mixed valency
Authors: Kosloff Ronnie , Ratner Mark A.
Published: February 2003

The vibrational line shapes expected for a case of ’’excitation mixed valency’’ (defined, in analogy, to ordinary electronic mixed valency, as one exciton moving between two equivalent electronic sites) is studied using a semigroup formalism to include relaxation effects. We include the quadratic electron-vibration coupling, corresponding to frequency changes. The results demonstrate the line narrowing expected in the rapid exchange limit, but also show that similar behavior may be caused by rapid electronic excitation. The simple model is readily extended to include transfer rates and ordinary (electronic) mixed valency. The relaxation process introduced a new time scale into the problem; when this becomes of the order of other relevant time scales (transfer, vibration, barrier residence) interesting line shape phenomena may be observed.
Analysis of NMR line shapes of rigid-lattice multispin systems. IV. 19F shielding tensors of trifluoromethyl halides
Authors: Garg S. K. , Ripmeester J. A. , Davidson D. W.
Published: February 2003

The principal elements of the fluorine chemical shift and spin-rotation tensors and their directions in the molecular coordinate system have been determined for CF3X molecules (X = F, Cl, Br, I) from analysis of low-temperature rigid-lattice NMR line shapes of these molecules enclathrated in D2O. In general, the shielding tensors are not axially symmetric, in disagreement with the assumption of axial symmetry commonly used to interpret liquid crystal results. The shielding tensor values in trifluoromethyl halides appear to be dominated by the electron currents induced by the mixing of the highest occupied nonbonding 2p orbitals on the fluorine atoms with low-lying unoccupied orbitals of the C–X bond under the influence of the magnetic field.
Determination of dipole coupling constants using heteronuclear multiple quantum NMR
Authors: Weitekamp D. P. , Garbow J. R. , Pines A.
Published: February 2003

The problem of extracting dipole couplings from a system of N spins I = 1/2 and one spin S by NMR techniques is analyzed. The resolution attainable using a variety of single quantum methods is reviewed. The theory of heteronuclear multiple quantum (HMQ) NMR is developed, with particular emphasis being placed on the superior resolution available in HMQ spectra. Several novel pulse sequences are introduced, including a two-step method for the excitation of HMQ coherence. Experiments on partially oriented [1−13C] benzene demonstrate the excitation of the necessary HMQ coherence and illustrate the calculation of relative line intensities. Spectra of high order HMQ coherence under several different effective Hamiltonians achievable by multiple pulse sequences are discussed. A new effective Hamiltonian, scalar heteronuclear recoupled interactions by multiple pulse (SHRIMP), achieved by the simultaneous irradiation of both spin species with the same multiple pulse sequence, is introduced. Experiments are described which allow heteronuclear couplings to be correlated with an S-spin spreading parameter in spectra free of inhomogeneous broadening.
Piecewise analytical integration of chemical rate equations. II. Error analysis, implementation, and applications
Authors: Bienstock Sergio , Gordon Roy G.
Published: February 2003

The accuracy and stability of the piecewise analytical method of our previous work for the integration of chemical rate equations are discussed for piecewise constant and linear polynomial approximants. In both cases, the solutions are shown to have excellent stability properties, even for stiff systems of differential equations. Thus, the step size can be controlled by the truncation error alone. The piecewise constant method is shown to have a local truncation error varying as the cube of the step size, while the more accurate linear approximation has an error which is fifth power in the step size. These results are used to produce a successful algorithm for control of step size in a variable step, variable order method. This new method has several advantages over previous stiff integrators, such as Gear’s method: (1) The results are defined by analytical functions at all times, so that integrals, derivatives for other properties of the solutions may be obtained analytically. (2) The solutions are strictly non-negative. (3) Considerably reduced computer storage is required. (4) Savings in computer time are projected for large calculations involving many species and reactions, and for ones with variable temperature.
Diode laser spectra of the torsional splittings in the ν9 band of ethane: Torsion-vibration-rotation interactions and the barrier to internal rotation
Authors: Susskind J. , Reuter D. , Jennings D. E. , Daunt S. J. , Blass W. E. , Halsey G. W.
Published: February 2003

Diode laser spectra of most of the Q branches of the ν9 band of ethane from RQ8–PQ15 have been recorded. The Q branches RQ0–RQ4 were deconvoluted to yield an effective resolution of (0.5–1.0)×10−3 cm−1 FWHM. Torsional splittings were observed for most lines. In contrast to predictions based on first order theory, the splittings which range from (2–53)×10−3 cm−1, have a marked J and K dependence. A second order theory of torsion-vibration-rotation interaction between ν9 and 3ν4 is developed, which fits the splittings with an rms error of 0.0006 cm−1, using only three adjustable parameters: the barrier to internal rotation in ν9, the energy difference between ν9 and 3ν4, and an effective coupling constant. The barrier to internal rotation in ν9 is found to be 1123±10 cm−1.
Sublevel phosphorescence spectra of pyrazine-d4. Vibrational assignments and the mechanisms of radiative and nonradiative transitions
Authors: Kokai Fumio , Azumi Tohru
Published: February 2003

The phosphorescence spectra from the three individual triplet spin sublevels of pyrazine-d4 are obtained. The radiative rate constants are determined for individual vibronic bands of the individual sublevel phosphorescence spectra. Based on the radiative characteristics of the individual vibronic bands, the vibrational analysis is made anew. The assignments thus accomplished substantially differ from previously reported results. Anomalous behavior of the radiative rate constant ratios for the totally symmetric vibrations ν1 and ν9a is interpreted from the standpoint of the displacement of the potential energy surfaces. The nonradiative decay rate constants are found nearly identical to those of pyrazine-h4 for any sublevels. The absence of the deuterium effect is discussed in relation to the nature of the accepting mode.
Critical behavior in annealed and unannealed crystals of benzil
Authors: Yoshihara A. , Bernstein E. R. , Raich J. C.
Published: February 2003

Simultaneous Brillouin-correlation light scattering results obtained under an external applied stress pulse are presented for annealed and unannealed (as grown) crystals of benzil. Critical fluctuations are characterized for the LA a-axis mode governed by the elastic constant c11 and are shown to contribute to this elastic anomaly in the high temperature phase. Both a central peak intensity anomaly and a critical relaxation time anomaly are characterized for unannealed crystals. These effects disappear in well-annealed crystals and without the external stress pulse even in as grown crystals. The critical relaxation behavior in benzil is coupled to the external stress compression wave generated by the mechanical refrigerator used to cool the sample. The latter interaction and relaxation anomaly are observed in polarized scattered light with the stress applied along the b (or a) axis as required for a c11 coupling. These observations are qualitively analyzed employing the theory of anelastic solids with higher order fluctuation terms incorporated into the free energy expansion. The relationship of this description to the Halperin–Varma theory of defect induced central peaks is outlined.
A kinetic theory calculation of the orientational correlation time of a rotorlike molecule in a dense fluid of spheres
Authors: Evans Glenn T. , Cole Robert G. , Hoffman David K.
Published: February 2003

The collective and single-particle orientational correlation times are calculated using Boltzmann–Enskog kinetic theory for N rotorlike molecules in a bath of spheres. By assuming that the rotors and bath are hard convex bodies, the collision integrals can be reduced exactly to one- and two-dimensional integrals, which are then readily amenable to numerical quadrature. The calculated correlation times depend on density and temperature in the same way as do the rough hard sphere results. However, the particle shape anisotropy plays the role of the roughness parameters. Comparison of the collective and single-particle orientational correlation times with experiment (including molecular dynamics) indicates that the kinetic theory consistently neglects part of the frictional drag. In all cases studied, the calculated times were a factor of 2 to 4 below the experimental values.
Particle distribution functions of lattice gases
Authors: Johnson Elijah M. C.
Published: February 2003

An apparently exact variational method for obtaining the three-particle distribution function of classical, pairwise additive potential energy function systems from the pair distribution function is used to treat two- and three-dimensional lattice gases. The results obtained strongly support the view that the variational method is exact. This method is also used in conjunction with the integrated Buff–Brout equation to obtain the pair and the three-particle distribution functions of two-dimensional lattice gases from the interparticle interactions.
Efficiency of light-energy conversion in photogalvanic cells and water cleavage systems
Authors: Dung Mei Hsu , Kozak John J.
Published: February 2003

In this paper we assess the thermodynamic efficiency of models for a well-known photogalvanic cell and a recently developed water cleavage system in order to determine, for comparable light-flux conditions, their relative effectiveness as light-energy conversion devices. The photogalvanic cell considered is the Ru(bipy)3+2/Ru(bipy)3+3–Fe+2/Fe+3 system and we base our calculations on the experimental work of Creutz and Sutin, and Lin and Sutin, and the theoretical analysis of Albery and Archer. The cyclic water cleavage system considered is the Ru(bipy)3+2/Ru(bipy)3+3–MV+1/MV+2 system (using a dispersed metal/metal oxide catalyst) discovered by Kalyanasundaram and Grätzel, and our calculations are based on their experimental work and the recent stability analysis of this system reported by Dung and Kozak. Although superficially different, the analysis draws attention to the common (photochemical, electrochemical, and mathematical) features of these two systems and, where possible, this correspondence is stressed and implemented in our simulations. Although other examples of each type of device are known, for this pair of systems (as described by the models presented herein) we find that the efficiency of conversion of light energy into electrical energy via a photogalvanic cell is similar to the efficiency of conversion of light energy into chemical energy via the (cyclic) water cleavage system, if the concentration of the dispersed metal/metal oxide catalyst in the system is <10−4 M. However, one finds a pronounced increase in the efficiency of the water cleavage system (under the same light-flux conditions) when the concentration of catalyst is increased, and this enhancement is quantified in our simulations. Alternatively, simulations also show that the efficiency of the photogalvanic system can improve markedly upon suppressing the back reaction between Ru(III) and Fe(II).
Voronoi polyhedra and cluster recognition
Authors: David Elaine Eisler , David Carl W.
Published: February 2003

Voronoi polyhedra are studied as an aid to understanding the local solvent structure about a solute molecule an angorithm is designed to determine voronoi polyhedra. (AIP)
Quantum dynamics of molecular multiphoton excitation in intense laser and static electric fields: Floquet theory, quasienergy spectra, and application to the HF molecule
Authors: Chu Shih-I , Tietz James V. , Datta Krishna K.
Published: February 2003

The multiphoton excitation dynamics of vibration-rotation states in diatomic molecules in intense laser and static electric fields is investigated. The Floquet matrix method is used to calculate the quasienergy and multiphoton absorption spectra of the HF molecule as functions of field strengths and frequency. Nonlinear effects such as power broadening, dynamic Stark shift, Autler–Townes multiplet splitting, hole burning, and S-hump behaviors, etc., are observed and discussed in terms of quasienergy diagrams. Many of the salient features in the spectral line shapes may be qualitatively understood in terms of an analytical three-level model. The addition of a dc electric field removes the restriction of the rotational dipole selection rule and causes significant intermixing of the bare molecular vibrator states. Due to the greater number of strongly coupled nearby states in the dc field, nonlinear effects such as those mentioned above appear at a much lower ac field strength than they would in the absence of the dc field. The introduction of an external dc field, therefore, strongly enhances the multiphoton excitation probabilities and results in a much richer spectrum.
The molecular structure of 1,1-dichlorocyclopropane by gaseous electron diffraction using rotational constants as constraints and by a
b i
n
i
t
i
o gradient computation
Authors: Hedberg Lise , Hedberg Kenneth , Boggs James E.
Published: February 2003

The molecular structure of gaseous 1,1-dichlorocyclopropane has been investigated using room-temperature electron-diffraction data with ground state rotational constants from published work introduced as constraints. The models of the structure were specified by parameters generating geometrically consistent distance sets of the type rz = rα0. The vibrational corrections necessary for the conversion of these to rg and to the ra set suitable to the diffraction data, and for the conversion of the observed B0 rotational constants to Bz, were calculated from a quadratic force field very similar to that deduced by others. The results leave no doubt that the unique C–C bond is significantly longer than the other C–C bonds as predicted from theoretical calculations. The important distances (rg), angles (&α), and rms amplitudes of vibration (l) with associated uncertainties estimated as 2σ are r(C–H) = 1.109(8) Å, 〈r(C–C)〉 = 1.511(3) Å, Δr(C–C) = 0.041(11) Å, r(C–Cl) = 1.759(2) Å, &Cl–C–Cl = 112.6(2)°, &H–C–H = 118.7(20)°, l(C–H) = 0.077(8) Å, l(C–C)opp = l(C–C)adj+0.001 = 0.055(3) Å, and l(C–Cl) = 0.053(3) Å. Features of this structure and of similar molecules are discussed. In addition, the geometry has been calculated ab initio by Hartree–Fock level gradient calculations with double-zeta basis sets augmented with polarization functions on the heavy atoms. The computed structure is very similar to the one derived from the electron-diffraction experiment except that Δr(C–C) is found to be only 0.020 Å and the computed &H–C–H is slightly smaller than the experimental value.
Time-dependent self-consistent field approximation for intramolecular energy transfer. I. Formulation and application to dissociation of van der Waals molecules
Authors: Gerber R. B. , Buch V. , Ratner Mark A.
Published: February 2003

The time-dependent self-consistent field method (TDSCF) is formulated and applied to the study of intramolecular dynamics and unimolecular decomposition processes. The method is illustrated by calculations on vibrational predissociation in van der Waals molecules, such as I2(v)Ne→I2(v′)+Ne. The TDSCF has the advantage of achieving formal separability of the modes by associating a time-dependent Hamiltonian with each mode, while permitting (possibly extensive) energy transfer among the modes via the time-dependent mean potential which acts on each mode. We present quantal, semiclassical, and classical versions of the method; a proper classical limit of the quantum TDSCF replaces the averages over wave functions by averages over self-consistently obtained bundles of trajectories. In all three versions, considerable computational economy is retained in comparison with full dynamics calculation. A detailed study is made of those properties which can be correctly obtained in such a time-dependent mean field theory. Attention is drawn to problems such as the occurrence of spurious states in the asymptotic region, and a simple method for avoiding them is suggested. We find for the I2(v)Ne vibrational predissociation that the TDSCF compares well with corresponding full dynamics calculations for average single mode properties such as dissociation lifetimes and the translational energy release. Moreover, comparison with classical trajectory calculations shows that the TDSCF method reproduces the essential dynamical mechanism of the dissociation (in-phase impulsive I⋅⋅⋅Ne collision following several ineffective vibrations). The self-consistent bundle trajectories and the time-dependent mean fields are analyzed, and provide insight into the process dynamics. It is concluded that the TDSCF approach in both quantal and quasiclassical versions is a potentially powerful tool in the study of intramolecular energy transfer and unimolecular dissociation.
Partial ionicity, cohesion, and charge correlations in narrow-band solids
Authors: Soos Z. G. , Ducasse L. R. , Metzger R. M.
Published: February 2003

A many-electron, correlated-state representation is introduced for narrow-band solids of donors and acceptors. The electrostatic energy for partial ionicity 0⩽q⩽1 is related to zero bandwidth Wigner lattices and to wide-band Hartree–Fock results for both simple orthorhombic lattices of point charges and for the organic conductor TTF–TCNQ at q = 1/2. Partial ionicity requires repulsive as well as attractive Coulomb contacts in the first coordination sphere, a condition that is met by organic conductors. The configuration interaction among correlated crystal states washes out the fixed neutral and ionic sites of a Wigner lattice, while retaining most of its electrostatic binding. Short-range charge correlations in partly ionic lattices substantially improve the Hartree–Fock cohesion and rationalize the shallow minimum (relative to neutral solids) in representative partly ionic organic solids.
The incomplete basis-set problem. V. Application of CIBS to many-electron systems
Authors: McDowell Keith , Lewis Lynn
Published: February 2003

Five versions of CIBS (corrections to an incomplete basis set) theory are used to compute first and second corrections to Roothaan–Hartree–Fock energies via expansion of a given basis set. Version one is an order by order perturbation approximation which neglects virtual orbitals; version two is a full CIBS expansion which neglects virtual orbitals; version three is an order by order perturbation approximation which includes virtual orbitals; version four is a full CIBS expansion which includes orthogonalization to virtual orbitals but neglects virtual orbital coupling terms; and version five is a full CIBS expansion with inclusion of coupling to virtual orbitals. Results are presented for the atomic and molecular systems He, Be, H2, LiH, Li2, and H2O. Version five is shown to produce a corrected Hartree–Fock energy which is essentially in agreement with a comparable SCF result using the same expanded basis set. Versions one through four yield varying degrees of agreement; however, it is evident that the effect of the virtual orbitals must be included. From the results, CIBS version five is shown to be a viable quantitative procedure which can be used to expand or to study the use of basis sets in quantum chemistry.
Quantum chemistry in Fock space. I. The universal wave and energy operators
Authors: Kutzelnigg Werner
Published: February 2003

The Fock space Hamiltonian H has a simpler structure than its projection Hn to n-particle Hilbert space. It is therefore recommended to diagonalize H—to the extent that this is possible—before one specifies n. Diagonalization of H is possible if one defines the diagonal and nondiagonal parts of an operator appropriately. It is shown that the diagonalized Fock space Hamiltonian L, called an energy operator, contains all information about the eigenvalues of H in a simply coded form. For a spinfree Hamiltonian, spin can be completely eliminated and all interesting quantities are expressible in terms of spinfree excitation operators (generators of the unitary group). The construction of the wave operator W and the energy operator L is formulated in terms of perturbation theory both in the strictly degenerate and the quasidegenerate version. Three variants are discussed that differ in the normalization of W, namely, a intermediate normalization, b and c unitary normalization with two additional conditions, b: WD=WD+, c:σD=(ln W)D=0. The transformation of operator products to normal product form is achieved by means of a generalization of Wick’s theorem to spinfree quantum chemistry. The contributions to the various orders of L, W, and σ are illustrated by means of diagrams. Only variant c guarantees a connected-diagram expansion of the Fock space energy operator L. None of the three variants satisfies a generalization of the Wigner (2n+1) rule, but with a slight change in the normalization from b to b′ or c to c′ (that does, in the case of c′, not affect the connectedness of the diagram expansion) L(2n) and L(2n+1) are expressible in terms of the W(k), k⩽n. A generalization of the variation principle for the Fock space energy operator is given and its consequences are discussed. Generalized Fock space Brillouin conditions are derived and a variational scheme for the construction of the energy operator is proposed. We finally compare the present Fock space approach with the traditional degenerate perturbation theory in n-particle Hilbert space. If the diagonal part ΩD of an operator Ω is defined in the spirit of the theory of the universal wave and energy operators, then the two approaches lead to the same results (for the same normalization), but the Fock space approach looks simpler and more transparent.
Force in SCF theories. Test of the new method
Authors: Nakatsuji H. , Kanda K. , Hada M. , Yonezawa T.
Published: February 2003

A new force method reported previously, which may be called the (r, r′) method, is tested for several di- and triatomic molecules, CO, N2, LiF, H2O, and HNO, with the use of several basis sets. It is shown that by the addition of the first derivatives of the basis set, the Hellmann–Feynman (H–F) theorem is essentially satisfied. The present force method is shown to be useful for calculations of equilibrium geometries and force constants. The basis set of double zeta accuracy such as 4-31G, 6-31G, and [3s2p/2s] sets are well suited for the present method. The STO-3G set seems to be less suitable. Further it is shown that by the present method the other one-electron properties are improved at the same time. We have discussed the features of the present method as compared with the energy gradient method.
Molecular beam-laser induced fluorescence experiments on the reaction of O(3P) with CH3CHO
Authors: Kleinermanns K. , Luntz A. C.
Published: February 2003

Crossed molecular beam laser induced experiments are reported on the reaction of O(3P) with supersonic nozzle beams of Ch3CHO seeded in H2, He, or SF6. (AIP)
Adsorption studies of H2 isotopes on ZnO: Coverage-induced IR frequency shifts and adsorbate geometry
Authors: Griffin Gregory L. , Yates John T.
Published: February 2003

The coverage dependence of the IR stretching frequencies for dissociative type I adsorption of H2 and D2 on ZnO powders has been measured using transmission IR spectroscopy. By comparing the frequency shifts observed when the isotopic composition of the adsorbate is varied at constant total coverage with the shifts observed when the total coverage of either pure component is varied, we can separate the dynamic and static contributions to the coverage-induced frequency shifts. The ZnH and ZnD shifts are due primarily to electrodynamic interactions. The observed shifts are in good agreement with the Hammaker model for dynamic dipole–dipole interactions, if adsorption is assumed to occur on (2×2) reconstructed ZnO(0001) surface planes. In contrast, the OH and OD shifts are due to electrostatic and inductive interactions. The electrostatic contribution can be estimated using a modification of Buckingham’s treatment of local environment effects. A qualitative model based on electron localization effects is presented to explain the observed inductive contribution.
The chemical dynamics of hydrogen atom abstraction from unsaturated hydrocarbons by O(3P)
Authors: Kleinermanns K. , Luntz A. C.
Published: February 2003

By molecular beam-laser induced fluorescence experiments, we have probed the nascent internal state distribution and dynamic energy thresholds of OH formed by the abstraction of hydrogen atoms from alkyl substituted olefins and alkynes. Low thresholds are obtained, especially for olefins, suggesting that this is an important mechanism in high temperature combustion. OH rotational excitation is small and implies a collinear transition state similar to that observed previously for O(3P) reaction with saturated hydrocarbons. OH vibrational excitation is also low for the olefins and suggests that considerable internal excitation of the allyl or methyl allyl radicals occur in the reactions.
The multiphoton absorption and decomposition of fluoroform-d: Laser isotope separation of deuterium
Authors: Evans D. K. , McAlpine Robert D. , Adams H. M.
Published: February 2003

Multiphoton absorption (MPA) studies of fluoroform-d, a molecule of interest for potential laser based hydrogen isotope separation processes, are reported for CDF3 pressures 0.2–1.3 kPa, and for a variety of 10 μm CO2 laser lines with pulse widths of 2 or 6 ns and fluences within the range 10−3–70 J/cm2. Unlike SF6, no red shift of the MPA spectrum relative to the small signal spectrum was observed at high fluence. Selective multiphoton decomposition (MPD) experiments using the 10R(26) line, 6 ns pulse to excite the CDF3 component in natural-abundance CHF3 (∼ 150 ppm D/H) at a pressure of 13.3 kPa resulted in the recovery of water enriched up to 30% in deuterium—a measured isotope enrichment of ⩾2000 fold. This demonstrates that a product, highly enriched in deuterium, can be recovered from the selective MPD of fluoroform.
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o calculations of low lying states of the BH+ and AlH+ ions
Authors: Klein R. , Rosmus P. , Werner H. J.
Published: February 2003

For the X 2Σ+, A 2Π, and B′ 2Σ+ states of the BH+ and AlH+ ions potential energy, electric dipole and transition dipole moment functions have been calculated from MC–SCF wave functions. For the X and A states the MC–SCF results are compared with those obtained from highly correlated PNO–CEPA wave functions. All emission processes in the X, A, and B′ states have been investigated. The absolute emission intensities for the most intense bandheads of the A→X and B→X transitions have been calculated, and the band shapes are compared with the experimental emission spectrum of BH+ obtained in the reaction B+ (1–10 eV/cm)+H→BH++H. A so far unobserved part of the B′–X emission system is predicted between 2000 and 2500 Å. The Franck–Condon factors and the energies for the ionizations of the AH molecules into the AH+ ions are given.
Nonlinear optical properties of organic crystals with hydrogen-bonded molecular units: The case of urea
Authors: Zyss J. , Berthier G.
Published: February 2003

The validity of simpler additive models, accounting for nonlinear optical properties of van der Waals crystals in terms of independent molecular contributions is questionable in the case of more associated molecular structures such as examplified by the hydrogen-bonded crystal lattice of urea. A number of one-electron properties of single isolated urea molecules are first considered by means of a finite-field perturbated LCAO Hartree–Fock model implemented at both semiempirical (INDO) and ab initio levels of approximation with different basis set extensions. Charge distribution, dipole (μ), multipoles, linear (α), quadratic (β), and cubic hyperpolarizabilities γ are computed and analyzed. The α tensor is shown to be quasi-isotropic in the molecular plane while the β tensor exhibits a stronger anisotropy and a prevailing βzzz coefficient (z is the molecular symmetry axis). The negative value of the product βzzzμz is interpreted by a decrease of the dominant excited state dipole as compared to that of the ground state dipole. A mutually consistent field approach coupled with a finite-field dipolar perturbation is developed at the LCAO Hartree–Fock ab initio level to account for the influence of the crystalline environment on molecular properties. Coulombian point-charge potentials are located around each molecule of the crystal, at specific atomic sites taking part in the lattice hydrogen bonding. The translational invariance of the crystalline charge distribution in the presence of the external field is ensured by a fast convergent iterative procedure. This model evidences, in the case of urea, a stronger anisotropic influence of the local crystalline environment on the lowest order nonlinear susceptibility (βzzz going from −44 to −74 a.u.) than on the polarizability (αzz, going from 27 to 31 a.u.). This noticeable enhancement of ‖βzzz‖ is linked to a possible cooperative influence of intramolecular charge-transfer and intermolecular bonding in the general case of a linear chain of hydrogen-bonded charge-transfer molecules interacting through the donor and acceptor groups.
Diagrammatic perturbation theory: An application to the LiH and FH molecules using a universal even-tempered basis set
Authors: Wilson Stephen , Silver David M.
Published: February 2003

Diagrammatic many-body perturbation theory is applied to the ground states of the LiH and FH molecules at their respective equilibrium nuclear geometries. The same universal even-tempered basis set of Slater exponential functions is employed in each calculation. 94.0% and 82.3% of the correlation energy is recovered in LiH and FH, respectively.
Self-consistent calculation of excitation spectra in simple molten salts
Authors: Sjögren L. , Yoshida F.
Published: February 2003

The dynamical properties of simple molten salts are studied through the mass and charge density correlation functions as well as through the autocorrelation functions for the velocity and for the total electric current. Employing the Mori memory function approach and following an earlier treatment of Hansen and Sjögren concerning the mass and charge fluctuations, the problem reduces to calculating the memory functions for the velocity and total electric current autocorrelation functions. Here, these memory functions are split into a binary collision part and a collective recollision part and the latter is expressed in terms of certain mode–mode coupling integrals. The resulting nonlinear equations are solved self-consistently by numerical means. Comparison is made with molecular dynamics results of Hansen and McDonald. It is found that the present theory is in semiquantitative agreement with the computer data. The reasons for some of the discrepancies are also discussed and explained.