bluuebunny/crossref_metadata_2025 huggingface dataset:
Luminescence and excitation line narrowing of [Ru(2,2′-bipyridine)3−x (2,2′-bipyridine-d8)x]2+ (x=0–3) in [Zn(2,2′-bipyridine)3](CIO4)2. Unequivocal evidence for localized lowest-excited states
Authors: Riesen Hans , Krausz Elmars
Published: July 2002

Broadband and narrowed luminescence and excitation spectra at liquid helium temperatures of the title series (abbreviated as h24, d8, d16, and d24 for x=0, 1, 2, and 3, respectively) are reported. The luminescence spectra of the h24, d8, and d24 materials are found to be very similar except that the inhomogeneous broadening varies and a deuteration shift of 37 cm−1 to higher energy is observed for the d24 system. The luminescence of the d16 material is a superposition of the h24 and d24 spectra. Two distinct luminescence lifetimes can be measured in this material. The excitation spectra of the d8 and d16 materials are well represented as superpositions of the h24 and d24 spectra. The crystal structure establishes that there is only one spectroscopic site with two equivalent ligands. The third ligand has a different environment. The lowest-excited metal-to-ligand charge transfer states of the two equivalent ligands are localized on an electronic time scale but very fast intramolecular excitation energy transfer occurs.
Cross sections and transport numbers of Li+–CO
Authors: Grice Stephen T. , Harland Peter W. , Maclagan Robert G. A. R.
Published: July 2002

Transport cross sections for the collisions of positive lithium ions with carbon monoxide molecules have been computed from theoretical Li+–CO interaction potentials. These cross sections have been combined with the kinetic theory of atomic ions in polyatomic gases to give the mobility and diffusion coefficients parallel and perpendicular to an external electric field. Comparison of the calculated values with experimental data show that theory and experiment agree within their mutual uncertainties.
Microwave measurements and ab initio dynamics of the large amplitude ring puckering motion in 2-sulpholene
Authors: McCarthy W. J. , Roehrig M. A. , Chen Qi-Qi , Henderson G. L. , Adamowicz L. , Kukolich S. G.
Published: July 2002

Microwave measurements were made on the rotational spectrum of 2-sulpholene using a modified Flygare–Balle pulsed beam Fourier transform spectrometer. Analysis and calculations provided information on the large amplitude ring puckering vibration of this system. Twelve and six rotational transitions were measured for the v=0 and v=1 states of the ring puckering vibration, respectively. The transitions for each vibrational state were fitted to a Watson’s A reduced Hamiltonian including terms for quartic distortion yielding for v=0 the values B=2125.96(6), C=1983.28(8), ΔJK=0.664(4), ΔK=−0.34(4) MHz, and for v=1 the values A=3995(26), B=2128.3(1), C=1984.6(1), ΔJK=−0.8(1), ΔK=−32(6) MHz. Subsequently, ab initio calculations were performed at the self-consistent-field (SCF)/3-21G*, MP2/6-31G*, and MP4/6-31G* levels of theory to determine the barrier to inversion. The MP4/6-31G* barrier was ΔE=116 cm−1, and can be considered to be the most accurate barrier value calculated in this study. An ab initio potential energy curve was calculated at the SCF/3-21G* level in terms of a single parameter (ω) describing the large amplitude motion of the ring puckering. Vibration-coordinate dependence of the effective reduced mass associated with this large amplitude motion and the resultant kinetic energy expression was determined. The solutions of a one-dimensional Schrödinger equation solved within this double well potential yield a separation between the v=0 and v=1 large amplitude motion vibrational states of 8 cm−1 when the effective reduced mass was assumed constant, and a separation of 9 cm−1 when the effective reduced mass was expressed as a function of the ω coordinate. The v=0 and v=1 eigenfunctions for the SCF ring puckering potential were found to give vibrationally averaged rotational constants in good agreement with those obtained from the microwave spectrum.
Spin-dependent unitary group approach to the Pauli–Breit Hamiltonian. II. First order energy level shifts due to spin–spin interaction
Authors: Gould M. D. , Battle J. S.
Published: July 2002

This paper is a continuation of a previous investigation of the Pauli–Breit Hamiltonian in the framework of the graphical spin-dependent unitary group approach to many electron systems. The SU(2) tensor form for the spin–spin interaction and its corresponding zero spin-shift component are determined explicitly and applied to investigate the first order energy level splitting due to the spin–spin interaction, entirely within the context of the unitary group approach. Our results are also discussed in terms of the unitary group density matrix formalism.
Rigorous bounds on the termination densities of hard disk and hard sphere fluids
Authors: Baram Asher , Fixman Marshall
Published: July 2002

Rigorous lower bounds for the termination activities and densities of the fluid phase of classical hard disks and hard spheres are obtained. The bounds for the termination densities are larger by a factor of approximately two than the best previously known bounds.
Measurement of the molecular content of binary nuclei. Use of the nucleation rate surface for ethanol–hexanol
Authors: Strey R. , Viisanen Y.
Published: July 2002

We measured the homogeneous nucleation rates J of mixed droplets in supersaturated ethanol–hexanol vapor mixtures in argon as functions of the vapor phase activities a1 and a2 of ethanol and hexanol, respectively. Measurements ranged from pure ethanol to pure hexanol at T=260 K. The experimental nucleation rate curves shape a nucleation rate surface in the three-dimensional J−a1−a2 space. We develop a theory for obtaining the average size and composition of a nucleus from this surface determined experimentally. Following this procedure experimental nucleus compositions are determined. This theory is then combined with our measurements in order to determine the makeup of the binary ethanol–hexanol nuclei. Results are compared with the predictions of classical binary nucleation theory. Qualitatively similar trends are observed for the nearly ideal ethanol–hexanol system studied.
Potential energy surfaces for the interaction of CH(X 2Π,B 2Σ−) with Ar and an assignment of the stretch-bend levels of the ArCH(B) van der Waals molecule
Authors: Alexander Millard H. , Gregurick Susan , Dagdigian Paul J. , Lemire George W. , McQuaid Michael J. , Sausa Rosario C.
Published: July 2002

New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the CH radical in its ground (X 2Π) and second excited (B 2Σ−) electronic states. These potential energy surfaces are then used in an adiabatic analysis of the rovibronic levels of the ArCH(X) and ArCH(B) van der Waals complexes. A qualitative discussion of the expected features in the B̃←X̃ electronic spectrum of ArCH is presented, and these are compared with the experimental spectrum reported earlier by Lemire et al. [J. Chem. Phys. 99, 91 (1993)].
Low-energy electron-stimulated production of molecular hydrogen from amorphous water ice
Authors: Kimmel Greg A. , Orlando Thomas M. , Vézina Christian , Sanche Léon
Published: July 2002

We have observed, via quadrupole mass spectrometry (QMS), stimulated production of D2 (H2) during low-energy (5–50 eV) electron–beam irradiation of D2O (H2O) amorphous ice. The upper limit for the D2 (H2) production threshold is 6.3±0.5 eV; well below the first excited state of condensed water at 7.3 eV. The D2 (H2) yield increases gradually until another threshold is reached at ∼17 eV and continues to increase monotonically (within experimental error) up to 50 eV. We assign the 6.3 eV threshold to D− (H−)+D2O (H2O)→D2 (H2)+OD− (OH−) condensed phase (primarily surface) reactions that are initiated by dissociative attachment. We associate the yield below ∼11 eV with the dissociation of Frenkel-type excitons and attribute the yield above ∼11 eV mainly to the recombination of D2O+, or D3O+, with quasifree or trapped electrons. Exciton dissociation and ion–electron recombination processes can produce reactive energetic D (H) atom fragments or D2 (H2) directly via molecular elimination. The importance of D+ (H+) interactions increases at ∼17 eV (dipolar threshold) and at energies ≥21 eV where multihole and multielectron final states are energetically accessible.
Potential energy surfaces for the interaction of BH(X 1Σ+,A 1Π) with Ar and a theoretical investigation of the stretch-bend levels of the ArBH(A) van der Waals molecule
Authors: Alexander Millard H. , Gregurick Susan , Dagdigian Paul J.
Published: July 2002

New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the BH radical in its ground (X 1Σ+) and first excited (A 1Π) electronic states. These potential energy surfaces are then used with an adiabatic bender model for the calculation of the vibrational energy levels of the ArBH van der Waals complex in its ground and first excited singlet electronic states. Comparison of vibrational energies calculated using this adiabatic bender model with computed exact vibrational energies indicates that the former provides a very useful description of the bound levels of the ArBH complex. A qualitative discussion of the expected features in the A 1Π−X 1Σ+ electronic spectrum of ArBH is also presented, to facilitate comparison with the experimental ArBH spectrum reported in the following paper [E. Hwang and P. J. Dagdigian, following paper, J. Chem. Phys. 101, 2903 (1994)]. The most strongly bound ArBH(A) levels, with Ar–BH separations less than in the ground state ArBH(X) complex, correspond to motion described primarily by the more attractive VA″ potential energy surface and to a helicopterlike internal motion of the BH moiety. For the more weakly bound states supported by higher bender curves, the vibrational motion cannot be described as occurring on either the VA″ or VA′ potential energy surfaces separately.
Study of halogen–carbon dioxide clusters and the fluoroformyloxyl radical by photodetachment of X−(CO2) (X=I,Cl,Br) and FCO−2
Authors: Arnold Don W. , Bradforth Stephen E. , Kim Eun H. , Neumark Daniel M.
Published: July 2002

Photoelectron spectra have been measured for the anions X−(CO2), with X=I, Br, Cl, and F. The vibrationally resolved spectra show that I−(CO2), Br−(CO2), and Cl−(CO2) are primarily electrostatically bound clusters, although the charge-quadrupole interaction is strong enough to distort the CO2 molecule by as much as 10° [in Cl−(CO2)]. Ab initio calculations and electrostatic models are used to describe the geometry and bonding of these clusters. The photoelectron spectrum of FCO−2 is qualitatively different and shows transitions to both the X̃ 2B2 ground and the Ã 2A2 first excited electronic states of the covalently bound FCO2 radical. The previously unobserved Ã 2A2 state is measured to lie 0.579 eV above the ground state. Vibrational frequencies are assigned with the assistance of ab initio calculations. The FCO2 heat of formation is determined to be ΔfH0298(FCO2)=−85.2±2.8 kcal/mol. While both FCO−2 and FCO2 are more strongly bound than the other halide–CO2 clusters, the C–F bonds are very weak relative to C–F bonds found in other halocarbon compounds.
Two-dimensional kinetics of binary nucleation in sulfuric acid–water mixtures
Authors: McGraw Robert
Published: July 2002

Homogeneous nucleation theory for binary mixtures is developed as a two-component extension of the classical multistate kinetics rate theory. A matrix formulation, based on the stochastic model of Shugard and Reiss [J. Chem. Phys. 65, 2827 (1976)], provides the framework for solving the strongly coupled two-dimensional flux network associated with tracking the evaporation and growth kinetics of each component for several thousand binary clusters, of varying composition, throughout the region of critical size. This approach avoids the assumption of a single nucleation path, e.g., through the saddle point of the binary free-energy surface, and considers, instead, all possible paths whose current density contributes to the nucleation rate. Calculations are presented for the steady-state nucleation rate and, by a new method based on the negative eigenvalue theorem, for the distribution of relaxation times in sulfuric acid–water mixtures. At water relative humidities typical of the atmosphere, quantitative agreement with the predictions of a closed-form rate expression due to Shugard et al. is found. At high supersaturations, typical of diffusion cloud chamber operation, the nucleation current is found to bypass the saddle point, due to kinetic forcing, resulting in a higher-than-expected nucleation rate.
Unitary group based state specific open-shell-singlet coupled-cluster method: Application to ozone and comparison with Hilbert and Fock space theories
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

Recently introduced state-specific coupled-cluster method, which exploits the formalism of the unitary group approach to the many-electron correlation problem and enables a properly spin-adapted treatment of open-shell states, is applied to several low lying singlet and triplet electronic states of ozone at the double-zeta plus polarization level of approximation. The method employs a nonstandard cluster Ansatz, based on a single spin-free reference built from either the ground state restricted Hartree–Fock orbitals or from the restricted open-shell Hartree–Fock orbitals, specific for each excited state. The results are compared with available experimental data and with other ab initio calculations, particularly with those employing spin-orbital based, spin nonadapted multireference coupled-cluster approaches of both state universal and valence universal types, as well as equation-of-motion coupled-cluster method. The general agreement is satisfactory except for the B2 states, where the difference between the multireference spin nonadapted and our state specific, but spin adapted, approaches amounts to as much as 0.64 eV. It is shown that this difference arises due to the spin contamination that is present in the spin-orbital based multireference approaches.
Molecular dynamics of model compounds of polymers with chlorocyclohexyl groups in their structure
Authors: Saiz Enrique , Riande Evaristo
Published: July 2002

The conformational mobility of 2-chlorocyclohexyl isobutyrate (CCHI), a model compound for the repeating unit of vinyl polymers containing chlorocyclohexane residues as side groups, is analyzed employing molecular dynamics (MD) procedures. Close to room temperature (ca. 300 K), the interconversion between axial (i.e., both chlorine atom and ester group in axial positions) and equatorial (both substituents in equatorial orientations) is not observed within the total time of 5 ns allowed to the MD trajectories. The analysis was then performed at temperatures in the range 1000 to 1500 K and the results extrapolated to lower temperatures. These extrapolations give energetic barriers of 5.72 and 8.15 kcal/mol, respectively for axial→equatorial and equatorial→axial transformations, with life times of τax≊9.6 and τeq≊46.3 ns for these two conformations at 300 K. The same procedure applied to unsubstituted cyclohexane gives an energetic barrier of 10.6 kcal/mol for the chair to chair interconversion, in excellent agreement with literature values. Further extrapolation to the temperatures at which the β subglass relaxation processes take place indicate that this interconversion is practically forbidden and therefore could not be invoked to explain the absorptions exhibited by this kind of polymers. The dipole moment of CCHI is also measured and calculated. Concordance between experimental (2.9±0.1 D) and calculated (2.7 D) values is very good.
Theory of friction: Dynamical phase transitions in adsorbed layers
Authors: Persson B. N. J.
Published: July 2002

When an external force acts on an adsorbate structure, the structure may slide or flow relative to the substrate. The mechanism behind this sliding motion is of fundamental importance for the understanding of friction and lubrication between two flat macroscopic surfaces, and is also related to the question of what boundary condition should be used for the velocity field at a solid–liquid interface when solving the Navier–Stokes equations of fluid dynamics. Here I study the friction which occurs when adsorbate layers slides on surfaces. I present results of simulations based on Langevin or Brownian-motion dynamics, which illustrate the dependence of the nonlinear (in the external driving force F) sliding friction η̄ on the coverage θ, and on the microscopic friction η. The sliding friction (as a function of F) exhibit three dynamical phase transitions, namely (commensurate solid→incommensurate solid), (fluid→commensurate solid) and (fluid↔incommensurate solid) transitions. The role of static imperfections (pinning centers) on the sliding dynamics is also discussed.
An extended basis set ab initio study of alkali metal cation–water clusters
Authors: Feller David , Glendening Eric D. , Woon David E. , Feyereisen Martin W.
Published: July 2002

Ionic clusters comprised of a single alkali metal cation and up to eight water molecules were studied at the Hartree–Fock and correlated levels of theory using the correlation consistent sequence of basis sets. Estimates of the degree of convergence in the computed properties with respect to the complete basis set limit were facilitated by the underlying systematic manner in which the correlation consistent sets approach completeness. In favorable cases, improved property values could be obtained by fitting finite basis set results with a simple analytical expression in order to extrapolate to the complete basis set limit. The sensitivity of structures and binding energies were analyzed with regard to the inclusion of valence and core-valence correlation recovery at the MP2, MP4, and CCSD(T) levels of theory. The replacement of metal core electrons and the introduction of relativistic contributions via effective core potentials was compared to corresponding all-electron results.
Vibrational force constants and anharmonicities: Relation to polarizability and hyperpolarizability densities
Authors: Hunt K. L. C.
Published: July 2002

In this work, the derivatives of molecular potential energy surfaces V({R}) with respect to nuclear coordinates RK are related to derivatives of the electronic charge density with respect to applied electric fields. New equations are obtained for second, third, and fourth derivatives of V({R}) in terms of the charge density, the nonlocal polarizability density α(r,r′), and the hyperpolarizability densities β(r,r′,r″) and γ(r,r′,r″,r‴). In general, the nth derivative of the potential V({R}) depends on electrical susceptibility densities through (n−1)st order. The results hold for arbitrary nuclear coordinates {R}, not restricted to the equilibrium configuration {Re}. Specialization to {Re} leads to a new result for harmonic frequencies in terms of α(r,r′), and to new results for vibration–rotation coupling constants and anharmonicities in terms of α(r,r′), β(r,r′,r″) and higher-order hyperpolarizability densities. This work provides a simple physical interpretation for force derivatives obtained by use of analytic energy differentiation techniques in ab initio work, or in density functional theory: The charge reorganization terms in harmonic force constants give the electronic induction energy in the change of field δF due to an infinitesimal shift in nuclear positions.

Cubic anharmonicity constants depend on the hyperpolarization energy of the electrons in the field δF, on the induction energy bilinear in δF and the second variation of the field δ2F, and on the gradients of the field from the unperturbed charge distribution. The results are derived by use of the Hohenberg–Kohn theorem or the electrostatic Hellmann–Feynman theorem, together with a chain of relations that connects the derivative of an electrical property of order n to the susceptibility density of order n+1. These derivatives are taken with respect to the nuclear coordinates RK, in contrast to the well known relations for derivatives with respect to an applied electric field. Analytic expressions are compared for the property derivatives that depend on susceptibility densities through γ(r,r′,r″,r‴). This includes the derivatives of V({R}) listed above; first, second, and third derivatives of the dipole moment; first and second derivatives of the polarizability; and the first derivative of the β hyperpolarizability with respect to the nuclear coordinates RK.
Angular momentum coupling in the exchange energy of multielectron systems
Authors: Kleinekathöfer U. , Tang K. T. , Toennies J. P. , Yiu C. L.
Published: July 2002

The exchange energy between two multielectron atomic systems is shown to be a product of an angular momentum factor and the energy of the triplet-singlet splitting of a single pair of electrons. The angular momentum factor accounts for the coupling of the angular momentum of the valence electrons and was first given by Duman and Smirnov [Opt. Spectrosc. (USSR) 29, 229 (1970)]. Here it is rederived and in the cases of interactions between hydrogen, rare gas, alkali and alkaline earth atomic systems the new corrected expressions are shown to reduce to a simple physical model. The angular momentum factors are listed for all these interacting systems. The important factors in the analytic expression for the distance dependent asymptotic exchange energies are also given for all the homonuclear alkali, alkaline earth, and rare gas dimers.
Electric birefringence relaxation spectroscopy of linear micelles in aqueous cetyltrimethylammonium bromide solutions with sodium salicylate
Authors: Oizumi Junichi , Kimura Yasuyuki , Ito Kohzo , Hayakawa Reinosuke
Published: July 2002

The influence of sodium salicylate (NaSal) concentration on the linear micelle formation in an aqueous solution of cetyltrimethylammonium bromide (CTAB) is investigated by the frequency-domain electric birefringence relaxation spectroscopy (FEB method). The contour length L of linear micelles is found to increase with increasing NaSal concentration Cs but for further increase of Cs in excess of the CTAB concentration, L decreases, i.e., linear micelles break into shorter pieces. The FEB result indicates also that the diffusion constant of salicylate ions bound to a linear micelle is nearly independent of Cs with an average value smaller than that of free salicylate ions.
Aluminum monocarbonyl and aluminum isocarbonyl
Authors: Wesolowski Steve S. , Crawford T. Daniel , Fermann Justin T. , Schaefer Henry F.
Published: July 2002

Ab initio studies of the aluminum monocarbonyl species AlCO and AlOC have been performed to predict the geometries, fragmentation energies, and harmonic vibrational frequencies. Both species were optimized at the self-consistent field, configuration interaction, and coupled-cluster levels of theory with large basis sets. At the highest level of theory, AlCO was found to be 22 kcal/mol more stable than the isocarbonyl, AlOC. Al–CO was found to have a dissociation energy of 9 kcal/mol, with no barrier to dissociation to Al and CO fragments. Al–OC was found to have a dissociation energy of −13 kcal/mol with an energy barrier to dissociation of less than 5 kcal/mol. The dipole moment of AlCO is found to be small (around 0.1 D), while that of AlOC is significantly larger (around 2.8 D). The C–O harmonic vibrational frequencies were evaluated at all levels of theory. For AlCO at the highest level of theory, the C–O frequency was 1914 cm−1 compared to the experimental value of 1868 cm−1, a 2.5% difference which may be attributed largely to anharmonic effects. The C–O frequency for AlOC is remarkably close to a tentative and disputed experimental spectral feature.
Electron thermalization in rare gases and their mixtures
Authors: Bronić Ines Krajcar , Kimura Mineo
Published: July 2002

The time evolution and temperature dependence of electron energy distribution functions (EDFs) are studied in pure rare gases (He, Ne, Ar, Kr, Xe) as well as in their mixtures by using solutions of the Boltzmann equation. A clear difference between the gases having the Ramsauer–Townsend (RT) minimum in the momentum-transfer cross section, (RT gases: Ar, Kr, and Xe), and those without the RT minimum (non-RT gases: He and Ne) is pointed out. The influence of the position and the depth of the RT minimum on the EDF and time evolution is studied for three different initial electron energies. A formula proposed for describing thermalization time in a mixture is tested on (i) a non-RT–non-RT gas mixture, (ii) a RT–non-RT mixture and (iii) a RT–RT gas mixture. The linear combination of the reciprocal thermalization times in gas mixture with the component concentrations as weighting factors is found to be valid for gases with a similar energy dependence of the momentum-transfer cross section, σm, and also for all rare-gas binary mixtures if the initial electron energy is sufficiently below the RT minimum. Conspicuous deviations from the linear relationship are observed in mixtures of gases whose energy dependence of σm (or the stopping cross section) are different, and theoretical rationales for these findings are provided.
Molecular dynamics study of energy transfer in binary collisions of water molecules
Authors: Ming Liu , Davidsson Jan , Nordholm Sture
Published: July 2002

Collisional energy transfer between two water molecules, one highly energized (reactant) and another thermally equilibrated (medium) molecule, has been studied by classical molecular dynamics simulation over a range of excitation energies and medium temperatures. The focus is on the dependence of the energy transfer efficiency on the excitation energy, the medium temperature, and the gross features as well as the details of the interaction between the molecules. High quality interaction potentials based on experimental data or quantum chemical calculations are used and the results are compared with those obtained by simpler potentials constructed from Lennard-Jones pair potentials and point charges. The dipolar contribution to the interaction is varied and the molecules are partially or fully deuterated. The strong electrostatic interaction is found to yield efficient energy transfer for small impact parameters but also a large cross section for water collisions. The energy transfer efficiency is sensitive to the detailed form of the interaction. However, if somewhat lower accuracy can be accepted then simple potentials can be used. The energy transfer can be well fitted by a conditional probability density based on a statistical model of equilibration among subsets of the degrees of freedom in the colliding molecules. Rotational energy transfer is far more efficient than vibrational energy transfer.
A simple but accurate ‘‘core-tail’’ pseudopotential approach to the calculation of the conduction-band energy V0 of quasifree excess electrons and positrons in nonpolar fluids
Authors: Plenkiewicz B. , Frongillo Y. , Lopez-Castillo J.-M. , Jay-Gerin J.-P.
Published: July 2002

The conduction-band minimum energy V0 (relative to vacuum) of quasifree excess electrons and positrons in fluid rare gases, methane, and silane is calculated as a function of fluid density n. The calculations are performed within the framework of the Wigner–Seitz mean-field approximation for nonpolar fluids, using a simple ‘‘core-tail’’ pseudopotential to model the excess electron (positron)–atom/molecule interactions. This pseudopotential is determined only by the atomic/molecular polarizability and the electron (positron) scattering length. Despite the simplicity of our approach, the present V0(n) results, which contain no adjustable parameters, show close agreement with those obtained using more elaborate potentials and with available experimental data.
A pulsed source for Xe(6s[3/2]1) and Xe(6s′[1/2]1) resonance state atoms using two-photon driven amplified spontaneous emission from the Xe(6p) and Xe(6p′) states
Authors: Alekseev V. A. , Setser D. W.
Published: July 2002

A new, simple method for the generation of Xe(6s[3/2]1) and Xe(6s′[1/2]1) atoms is described. The method involves resonant two-photon excitation of Xe(6p[1/2]0 and 6p′[3/2]2) states followed by amplified spontaneous emission (ASE) to the Xe(6s[3/2]1 and 6s′[1/2]1) states. The vacuum ultraviolet transitions, Xe(6s[3/2]1→5p6(1S0)) at 147 nm and Xe(6s′[1/2]1→5p6(1S0)) at 129.6 nm, were used to monitor the time dependence of the resonance state atom concentrations. The quenching rate constants of these resonance atoms with ten molecules were measured at 300 K. The quenching cross-sections of the Xe(6s and 6s′) resonance atoms are compared to the cross-sections of the metastable Xe(6s[3/2]2) atoms and Xe(6p[3/2]2) atoms. The correlation between quenching cross-sections and photoabsorption cross-section of the molecules predicted by the resonance dipole–dipole energy transfer model is discussed. The applicability of the two-photon driven ASE method for the generation of other resonance state atoms is considered.
A simple relationship between the first-order gradient correction to the exchange-energy functional and the average electron density for atomic systems
Authors: Tao Jianmin , Li Guobao , Li Jianmin
Published: July 2002

A simple accurate lower bound for all the bound-state systems, for the first-order gradient correction to the exchange-energy functional, viz., |K2[ρ]|=β∫[|∇ρ(r)|2/(ρ4/3(r))] dr⩾27(π/2) 4/3β〈ρ(r)〉1/3≡0.271〈ρ〉1/3 has been derived where 〈ρ(r)〉 is the average electron density. The result is numerically analyzed. An empirical relationship between |K2[ρ]| and 〈ρ〉 for neutral atoms has also been proposed.
Wave packet dynamics and photofragmentation in time-dependent quadratic potentials
Authors: Mo/ller Klaus B. , Henriksen Niels E.
Published: July 2002

We study the dynamics of generalized harmonic oscillator states in time-dependent quadratic potentials and derive analytical expressions for the momentum space and the Wigner phase space representation of these wave packets. Using these results we consider a model for the rotational excitation of a diatomic fragment produced in the photofragmentation of a triatomic molecule and we highlight the signatures of classical mechanics in the final product distribution of this process.
Vibrational couplings and energy flow in complexes of NH3 with HCN, HCCH, and HCCCCH
Authors: Hilpert G. , Fraser G. T. , Pine A. S.
Published: July 2002

A tunable color-center laser and a molecular-beam electric-resonance optothermal spectrometer have been used to record the infrared spectra of the C–H stretching vibrations of HCCH–NH3, HCCD–NH3, NCH–NH3, and HCCCCH–NH3. The hydrogen-bonded C–H stretching vibrations of NCH–NH3, HCCH–NH3, and HCCCCH–NH3 are redshifted by 200.88126(30), 75.1042(38), and 127.4(1) cm−1 from the respective free monomer modes. The non-hydrogen-bonded C–H stretches are less perturbed by complexation, being blueshifted by 0.2992(3) cm−1 in HCCCCH–NH3 and redshifted by 1.179(1) cm−1 in HCCD–NH3. Consistent with the much larger perturbation of the monomer vibration for the bonded C–H stretch, the B rotational constants increase by 1%–2% for the bonded C–H stretch excited NCH–NH3 and HCCH–NH3 complexes, but change by less than 0.1% for the nonbonded C–H stretches in HCCCCH–NH3 and HCCD–NH3. The decoupling of the two C–H stretches in HCCH–NH3 is not sufficient to allow the observation of the nonbonded C–H stretch in the complex, which correlates to the Raman-active symmetric C–H stretch of acetylene. Also, no spectra were observed for the weaker N–H stretching vibrations of the complexes, consistent with the very weak intensities of these modes in the monomer. The homogeneous linewidths of the transitions, assumed to be a measure of the vibrational predissociation rate, are approximately two orders of magnitude larger for the bonded C–H stretches than for the nonbonded C–H stretches. The similarity in homogeneous widths for the nonbonded C–H stretches in HCCD–NH3 and HCCCCH–NH3, of 7–12 MHz, suggests that the rate of vibrational energy flow along acetylene chains is only weakly dependent on chain length.
Dimensional perturbation theory for Regge poles
Authors: Germann Timothy C. , Kais Sabre
Published: July 2002

We apply dimensional perturbation theory to the calculation of Regge pole positions, providing a systematic improvement to earlier analytic first-order results. We consider the orbital angular momentum l as a function of spatial dimension D for a given energy E, and expand l in inverse powers of κ≡(D−1)/2. It is demonstrated for both bound and resonance states that the resulting perturbation series often converges quite rapidly, so that accurate quantum results can be obtained via simple analytic expressions given here through third order. For the quartic oscillator potential, the rapid convergence of the present l(D;E) series is in marked contrast with the divergence of the more traditional E(D;l) dimensional perturbation series, thus offering an attractive alternative for bound state problems.
The photodissociation of SiO
Authors: Jolicard G. , Zucconi J.-M. , Drira I. , Spielfieldel A. , Feautrier N.
Published: July 2002

The photodissociation of the SiO molecule, which is of considerable astrophysical interest, is studied within the framework of the Floquet theory. Only the one-photon transition process between the two lowest EΣ+1 states is considered. The computation of the electronic energy surfaces is performed with the MOLPRO code. The wave operator theory is used to select the active space related to this process, and to build the Floquet resonance states which constitute this active space. Particular attention is paid to the role of the shape resonance states induced by the presence of a barrier on the E 1Σ+ surface.
Structure of the liquid–vapor interface of a Sn:Ga alloy
Authors: Lei Ning , Huang Zhengqing , Rice Stuart A.
Published: July 2002

The atomic distributions along the normal to, and in the plane of, the liquid–vapor interface of a dilute (9:91 at. %) Sn:Ga alloy have been studied by x-ray specular reflectivity and grazing incidence x-ray diffraction at 57 °C, 41 °C, 26 °C, and 12 °C. Surface tensions, calculated from sample shape measurements, have also been determined at the same temperatures. The results of these experiments show that the atomic distribution in the liquid–vapor interface of the Sn:Ga alloy studied has both similarities and differences with the atomic distribution in the liquid–vapor interface of a dilute Bi:Ga alloy. In both alloys the atomic distribution along the normal to the liquid–vapor interface is stratified, and the composition of the outermost layer is, sensibly, 100% of the constituent with lower (pure liquid) surface tension, respectively, Sn and Bi. In both alloys the in-plane structure of the outermost layer is that of a liquid. In the homovalent Bi:Ga alloy the excess Bi segregates into a monolayer which forms the outermost layer of the stratified liquid–vapor interface, with no excess Bi in the second or deeper layers of the interface. In the heterovalent Sn:Ga alloy, the excess Sn segregates into both the outermost and the second layers of the stratified liquid–vapor interface. The outermost layer of the interface is found to be pure two-dimensional liquid Sn, in which the atomic diameter is smaller than that in normal liquid Sn by about 6%. In the second layer of the interface the Sn atomic concentration is found to be 22.3±1.6% at 57 °C and 22.7±1.7% at 41 °C. Within the limits of our experimental sensitivity, the Sn concentration in deeper levels of the liquid–vapor interface cannot be distinguished from that in the bulk liquid alloy. The surface tension of the (9:91 at. %) Sn:Ga alloy is found to be 671±42 dyn/cm at 57 °C, 573±58 dyn/cm at 41 °C, 587±50 dyn/cm at 26 °C, and 527±40 dyn/cm at 12 °C. The differences between the structures of the Sn:Ga and Bi:Ga liquid–vapor interfaces are interpreted, qualitatively, in terms of their electron density distributions.
The effects of electron correlation on the degree of bond alternation and electronic structure of oligomers of polyacetylene
Authors: Ho Choi Cheol , Kertesz Miklos , Karpfen Alfred
Published: July 2002

Full geometry optimizations on oligoenes have been performed with Hartree–Fock and density functional theory in combination with double zeta and triple zeta quality basis sets with primary focus on the degree of bond length alternation and on the energy gap. Monitoring the dependence of the computed properties on the oligomer size provides new insights into the reliability of the calculations, which are analyzed in terms of dynamical and nondynamical electron correlation. Our theoretical bond length alternation values for the oligomers of polyacetylene extrapolate to significantly smaller values than what has been established by experiments and earlier theoretical predictions. The exact exchange mixing to the exchange-correlation functional not only improves the agreement of the theoretical gap of oligoenes with experimental excitation energies but also increases the computed bond length alternations. Based on a newly proposed one parameter functional of Becke, the effect of the exact exchange mixing has been further identified, and a practical measure of nondynamical correlation energy has been suggested. The popular Becke–Lee–Yang–Parr density functional produces bond alternation values that are unacceptably small.
The ultrafast photodissociation of Fe(CO)5 in the gas phase
Authors: Bañares L. , Baumert T. , Bergt M. , Kiefer B. , Gerber G.
Published: July 2002

The photodissociation dynamics of Fe(CO)5 in a molecular beam have been investigated with femtosecond time resolution. In single pulse experiments, the parent ion Fe(CO)5+ and all the fragment ions Fe(CO)n+, n=0–4 could be observed in linear and reflectron time-of-flight (TOF) spectrometers. Ladder switching is suppressed by the use of femtosecond laser pulses. The TOF spectra show that the fragmentation patterns strongly depend on the laser wavelength, the laser intensity, and the laser pulse duration. Femtosecond pump–probe experiments were performed for the parent and every fragment molecule. We present a photodissociation model for the neutral Fe(CO)5. After the absorption of two 400 nm photons, Fe(CO)5 looses four CO ligands in about 100 fs. The subsequent dissociation of the fragment Fe(CO) takes place on a longer time scale of about 230 fs. The measured transient ionization spectra of the Fe(CO)n, n=2–4 fragments represent within the proposed model the fingerprints of the evolution of the [Fe(CO)5]‡ transition state on the way to dissociation. We also report on the observation of a metastable ionic fragmentation mechanism.
Molecular dynamics simulations of organically modified layered silicates
Authors: Hackett E. , Manias E. , Giannelis E. P.
Published: July 2002

Molecular dynamics (MD) simulations are used to study the static and dynamic properties of 2:1 layered silicates ion exchanged with alkyl-ammonium surfactants. These systems are in the form of oligomeric alkanes grafted by cationic groups on atomically smooth crystalline layers 10 Å thick and several microns wide. The organically modified layers self-assemble parallel to each other to form alternating, well-ordered organic/inorganic multilayers. By studying the systems at the experimentally measured layer separations, computer modeling directly provides the structure and dynamics of the intercalated surfactant molecules. The grafted-chain conformations are also expressed through the trans-gauche conformer ratios and transition frequencies which compare well with Fourier transform infrared spectroscopy (FTIR) experiments.
Comment on “Theory of rotational energy levels of open-shell complexes containing the O2 molecule” [J. Chem. Phys. 107, 7651 (1997)]
Authors: Fawzy Wafaa M.
Published: July 2002

An effective Hamiltonian was reported several years ago [Fawzy, J. Mol. Spectrosc. 160, 84 (1993)] on the effects of electron-spin on rotational energy levels of an open-shell complex containing a diatomic radical (in a 2S+1Λ electronic state, where Λ=0 for a Σ state, Λ=1 for a Π state, etc.; S⩾1/2) and a closed-shell partner. Recently, a paper was published [Qian, Low, Seccombe, and Howard, J. Chem. Phys. 107, 7651 (1997)] on rotational energy levels of an open-shell complex consisting of the O2 radical (in the 3Σg− state) and a closed-shell molecule. Even though the effective Hamiltonian of a complex containing oxygen can be easily obtained by simply setting L=Λ=0, S=1 in the 1993 model, the authors of the recent paper completely ignored these earlier results. Here, we present a comparison between the results of our least-squares fits of the reported infrared spectrum of the O2–N2O complex and those published by Qian et al. [Qian, Seccombe, and Howard, J. Chem. Phys. 107, 7658 (1997)]. The comparison shows the equivalence of the 1993 model and the recent model.
Ab initio calculations with a nonspherical Gaussian basis set: Excited states of the hydrogen molecule
Authors: Detmer T. , Schmelcher P. , Cederbaum L. S.
Published: July 2002

A basis set of generalized nonspherical Gaussian functions (GGTOs) is presented and discussed. As a first example we report on Born–Oppenheimer energies of the hydrogen molecule. Although accurate results have been obtained, we conclude that H2 is too “simple” to allow for a substantial gain by using nonspherical functions. We rather expect that these functions may be particularly useful in calculations on large systems. A single basis set of GGTOs was used to simultaneously calculate the potential energy curves of several states within each subspace of Σg,u1,3 symmetry. We hereby considered the entire region of internuclear distances 0.8⩽R⩽1000 a.u. In particular the results for the fourth up to sixth electronic states show a high accuracy compared to calculations which invoke explicitely correlated functions, e.g., the relative accuracy is at least of the order of magnitude of 10−5 a.u. Energies for the 4 1Σu+ and 4–6 3Σu+ were improved and accurate data for the 6 3Σg+, 5 1Σu+, and 6 1Σu+ state are, to the best of the authors’ knowledge, presented for the first time. Energy data for the seventh up to the ninth electronic state within each subspace were obtained with an estimated error of the order of magnitude of 10−4 a.u. The 7 1Σg+ and the 6 1Σu+ state were found to exhibit a very broad deep outer well at large internuclear distances.
High-spin→low-spin
relaxation in [Zn1−xFex(6-mepy)3−y(py)ytren](PF6)2
Authors: Schenker Sabine , Hauser Andreas , Wang Wei , Chan I. Y.
Published: July 2002

The thermal spin transition in the diluted mixed crystal [Zn1−xFex(6-mepy)3tren](PF6)2 (x=0.00025, (6-mepy)3tren=tris{4-[(6-methyl)-2-pyridyl]-3-aza-3-butenyl}amine) is studied at 1 bar and 1 kbar by temperature-dependent absorption spectroscopy. From thermodynamic analysis of the high-spin (HS) fractions, values for ΔHHL0 and ΔSHL0 of 1551(50) cm−1 and 7.5(5) cm−1/K and a molecular volume of reaction, ΔVHL0, of 22(2) Å3 result. Reconsideration of the cooperative effects in the neat [Fe(6-mepy)3tren](PF6)2 from Adler et al. [Hyperfine Interact. 47, 343 (1989)] result in a lattice shift, Δ, of 208(15) cm−1 and an interaction constant, Γ, of 109(15) cm−1. Temperature-dependent laser flash photolysis experiments in the spin-crossover system [Zn1−xFex(6-mepy)3tren](PF6)2 and the LS system [Zn1−xFex(py)3tren](PF6)2 in the pressure range between 1 bar and 1 kbar are presented. Above ≈100 K the HS→LS (low-spin) relaxations behave classically, whereas they become almost temperature independent below 50 K. At ambient pressure, the low-temperature tunneling rate constant in [Zn1−xFex(py)3tren](PF6)2 is more than three orders of magnitude larger than the one in [Zn1−xFex(6-mepy)3tren](PF6)2. External pressure of 27 kbar accelerates the low-temperature tunneling process by almost nine orders of magnitude. The kinetic results are discussed within the theory of nonadiabatic multiphonon relaxation.
Metastable states and the kinetics of colloid phase separation
Authors: Soga K. Geoffrey , Melrose John R. , Ball Robin C.
Published: July 2002

We report on the results of extensive Brownian dynamics simulations of colloid phase separation due to depletion flocculation. We study in detail the effect of potential variation at fixed volume fraction of colloid and fixed range of interaction. We find a variety of nonequilibrium behaviors for quenches into the colloid fluid+crystal phase coexistence regime. We present clear evidence of metastability, of homogeneous nucleation, and of a kinetically arrested gel state. We also find evidence that suggests a density instability in the metastable colloid fluid preceding crystal nucleation. Our findings are consistent with a previous proposal that nonequilibrium behavior is determined by a metastable vapor+liquid binodal hidden in the fluid+crystal phase coexistence regime.
Fine-structure spectrum of the FO radical, observed by far-infrared laser magnetic resonance
Authors: Tamassia Filippo , Brown John M. , Evenson Kenneth M.
Published: July 2002

The fine-structure transition Π1/22←2Π3/2 of the free radical FO has been detected by far-infrared laser magnetic resonance. All the observed transitions are magnetic dipole in character. The spin–orbit constant A0 has been determined experimentally; its value of −196.108 686(50) cm−1 is consistent with previous estimates. The analysis of a set of 290 transitions leads to the determination of a number of molecular parameters including rotational, centrifugal distortion, spin–orbit, lambda-doubling, magnetic hyperfine, and Zeeman terms. All four magnetic hyperfine structure constants a, bF, c, d for the F19 nucleus have been determined and are discussed in terms of the expectation values of the appropriate operators over the electronic wave function.
Prediction of electron paramagnetic resonance g-tensors of transition metal complexes using density functional theory: First applications to some axial d1MEX4 systems
Authors: Patchkovskii S. , Ziegler T.
Published: July 2002

We applied the recently developed density-functional (DFT) formulation of the electron paramagnetic resonance (EPR) g-tensor to a series of axially symmetric d1 transition metal complexes (MEX4z−, where M=V, Cr, Mo, W, Tc, and Re; E=O and N; X=F, Cl, and Br). Values for the g-tensor components are determined by an interplay between three contributions arising due to magnetic field-induced coupling between the following orbitals: (a) The singly occupied α b2 (“dxy”) molecular orbital (α-SOMO) and a metal-based vacant d orbital [either b1 (“dx2−y2”) or e1 (“dxz”,“dyz”) depending on the tensor component]; (b) the bonding counterparts of the metal’s b1/e1-type d orbitals and the vacant β-SOMO; and (c) ligand-based occupied MOs (molecular orbitals) of the appropriate symmetry and the β-SOMO. The first contribution (which is the only term accounted for in the simple ligand field theory) is usually negative, and decreases the g-tensor components relative to the free electron value, while contributions (b) and (c) are positive. Either of the three terms may dominate, so that values both below and above the free electron are obtained naturally. Calculated g tensors exhibit only a moderate dependence on the molecular geometry. Quasi-relativistic VWN (Vosko–Wilk–Nusair) LDA (local density approximation) geometries are in a good agreement with the available experimental data, and are satisfactory for calculation of g tensors. Tensor components obtained with VWN LDA and gradient-corrected BP86 functionals are essentially identical, and always too positive compared to experiment. The residual errors in both components exhibit strong correlation with the position of the transition metal center in the periodic table. Trends in g-tensor components within the same transition row are correctly reproduced by both functionals, so that a simple additive correction brings g∥ and g⊥ results into a good agreement with experiment. The deficiencies in the calculated g values may be traced back to the overestimation of the covalent character of bonds formed by metal d orbitals in popular approximate functionals. Calculations of EPR g-tensor thus provide a very stringent quality test for approximate density functionals.