bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron–phonon coupling in amorphous organic host materials as investigated by photochemical hole burning
Authors: Friedrich J. , Swalen J. D. , Haarer D.
Published: February 2003

The absorption line shapes of photochemical holes of quinizarin in alcohol glasses have been investigated and special attention is given to a detailed analysis of the experimentally observed, intense phonon sidebands at the low energy side of the zero phonon hole. An analytical expression for the shape of these photochemical holes was derived which can be integrated for short burning times. Computer modeling was performed to describe the line shape function in both the short and long burning time limit. These calculations show that there are two contributions to the observed phonon sidebands: One which is due to phonon induced photochemistry and has its maximum on the low energy side of the zero phonon hole and one which is due to zero phonon photochemistry at the laser frequency and has its maximum on the high energy side. It is shown that the various line shape contributions to the photochemical hole have a different burning time behavior. A remarkable feature is that the low energy ’’pseudo-phonon wing’’ dominates the direct phonon wing under all experimentally achievable conditions.
Radiative lifetimes and two-body collisional deactivation rate constants in argon for Kr(4p 55p) and Kr(4p 55p′) states
Authors: Chang R. S. F. , Horiguchi H. , Setser D. W.
Published: February 2003

The radiative lifetimes and collisional deactivation rate constants, in argon, of eight Kr(4p5[2P1/2]5p and [2P3/2]5p) levels have been measured by a time-resolved laser-induced fluorescence technique in a flowing afterglow apparatus. The measured radiative lifetimes are compared with other experimental values and with theoretical calculations. Radiative branching ratios of these excited states also were measured in order to assign the absolute transition probabilities of the Kr(5p,5p′–5s, 5s′) transition array from the radiative lifetimes. In addition to the total deactivation rate constants, product states from two-body collisions between Kr(5p and 5p′) atoms and ground state argon atoms were identified from the laser-induced emission spectra, and product formation rate constants were assigned. Two-body intermultiplet transfer from Kr(4p5[2P1/2]5p) to the Kr(4p5[2P3/2]4d) levels occurs with ease. Intermultiplet transfer from the lowest level in the (4p55p) configuration to the Kr(4p55s and 5s′) manifold was fast despite the large energy defect. However, this was the only Kr(5p) level that gave appreciable transfer to the Kr(5s or 5s′) manifold. Generally the favored product states are within a few kT of the entrance channel.
The UV photolysis of hydrazoic acid
Authors: Piper L. G. , Krech R. H. , Taylor R. L.
Published: February 2003

The UV photolysis of HN3 has been studied by observing the temporal decay in the number density of the NH(a 1Δ) product following photolysis and by observing the temporal behavior of the fluorescence from the secondary photolysis product NH2 (? 2A1), both as a function of HN3 pressure. The HN3 was photolyzed at λ?290 nm by pulses from a frequency-doubled, flashlamp-pumped dye laser while the NH(a 1Δ) was probed by laser-induced fluorescence on the NH(c 1π−a 1Δ) band system at 324 nm using a frequency-doubled, nitrogen-pumped dye laser. The data are analyzed to obtain the rate constants k2=18×10−11 cm3 molecule−1 s−1 for the reaction NH(a 1Δ)+HN3→ NH2(? 2A1)+N3 [Eq.(2)], and k4=9.3×10−11 cm3 molecule−1 s−1 for the reaction NH2(? 2A1)+HN3→NH2(? 2B1)+HN3 [Eq.(4a)]→ NH3+N3 [Eq.(4b)].
Electron-impact excitation of the singlet states of N2. I. The Birge–Hopfield system (b1πu–X1Σ+g)
Authors: Zipf Edward C. , Gorman Michael R.
Published: February 2003

The absolute emission cross sections for the excitation of the Birge–Hopfield bands (b1πu–X1Σ+g) by electron impact on N2 from threshold to 500 eV are reported. With the exception of the v′=1, 5, and 6 vibrational levels, all other b1πu levels are found to predissociate with a specific predissociation branching ratio, ≳0.99. This forbidden process is a major source of the nitrogen atoms produced by electron-impact dissociation of N2. Quite remarkably, the b1πu (v′=1) state is only slightly predissociated (<10%). The emission cross section for the v′=1 level has a maximum value of 2.3×10−18 cm2 at 35 eV. The absolute transition probabilities, emission cross sections, and the variation of the electric dipole moment with internuclear distance for 21 BH (1, v′) bands are given.
An improved phase shift approach to the energy correction of the infinite order sudden approximation
Authors: Chang B. , Eno L. , Rabitz H.
Published: February 2003

A new method is presented for obtaining energy corrections to the infinite order sudden (IOS) approximation by incorporating the effect of the internal molecular Hamiltonian into the IOS wave function. This is done by utilizing the JWKB approximation to transform the Schrödinger equation into a differential equation for the phase. It is found that the internal Hamiltonian generates an effective potential from which a new improved phase shift is obtained. This phase shift is then used in place of the IOS phase shift to generate new transition probabilities. As an illustration the resulting improved phase shift (IPS) method is applied to the Secrest–Johnson model for the collinear collision of an atom and diatom. In the vicinity of the sudden limit, the IPS method gives results for transition probabilities, Pn→n+Δn, in significantly better agreement with the ’exact’ close coupling calculations than the IOS method, particularly for large Δn. However, when the IOS results are not even qualitatively correct, the IPS method is unable to satisfactorily provide improvements.
The kinetics of free radicals generated by IR laser photolysis. III. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) induced by collisions with oxygen
Authors: Mangir M. S. , Reisler H. , Wittig C.
Published: February 2003

Reactive and energy transfer processes involving collisions of C2(X 1Σg+) and C2(a 3Πu) with molecular oxygen are reported. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) is induced very efficiently by collisions with O2. The triplet→singlet rate coefficient is (2.7+0.3−0.6)×10−11 cm3molecule−1 s−1 and the ratio between this and the singlet→triplet rate coefficient is ∼3 at 300 K. The rate coefficient for reaction is much less than that for intersystem crossing, making the measurement of separate singlet and triplet reaction rate coefficients with oxygen impossible. The previously measured reaction rate coefficient (3×10−12 cm3molecule−1 s−1 at 300 K) for both singlet and triplet C2 removal by O2 is reinterpreted as the rate coefficient for removal of equilibrated C2 molecules. Using hydrocarbon scavengers, it is shown that the source of the observed emissions from excited triplet states of CO is mainly reactions of C2(X 1Σg+) with O2.
The influence of rotational motion on intersystem crossing in isolated molecules
Authors: Novak Frank A. , Rice Stuart A.
Published: February 2003

We have calculated the effect of molecular rotation on the rate constant for intersystem crossing in a large isolated molecule by constructing a simple displaced oscillator model. The effects of the changes in moments of inertia, Coriolis coupling, and of intramanifold rotation–vibration coupling are included in the model. Because of the sum rule constraining sums of products of Clebsch–Gordon coefficients the above are the only mechanisms for rotation to influence intersystem crossing. These effects are seen to be small for physically reasonable values for the parameters of the theory. This is consistent with some recent experimental work by Michel and Tramer.
The hydrogen atom inside boxes with paraboloidal surfaces
Authors: Ley-Koo E. , Rubinstein Sergio
Published: February 2003

The Schrödinger equation for the hydrogen atom inside boxes defined by pairs of confocal, coaxial and mutually intersecting paraboloidal walls is solved exactly using parabolic coordinates. The energy eigenvalues and eigenfunctions for the lowest states of the system are determined numerically for such boxes, having different sizes and shapes. Also, the isotropic and anisotropic components of the hyperfine splitting are evaluated.
Polymer scaling theories for general interaction potentials
Authors: Hall Carol K.
Published: February 2003

The validity of scaling theories for polymer solutions which interact via a general pairwise segment–segment interaction is examined. In recent work, Kosmas and Freed have shown that scaling holds for potentials whose integrated strength over all space is finite. This restriction does not allow for a hard core or a soft core interaction which goes to infinity as the distance between segments goes to zero, features which are found in most commonly used potentials such as the Lennard-Jones potential. We show that the above restriction on range of potentials, for which scaling is valid, may be relaxed. Using the approach of Kosmas and Freed, we show that scaling behavior will be obtained for long chains provided that the Fourier transform of the Mayer f function exists. This transform will exist for reasonable short-range potentials even if they have a hard core or a soft core which goes to infinity as the separation between segments goes to zero. Thus our work extends the results of Kosmas and Freed to more realistic potentials. A major difference between our result and Kosmas and Freed’s result is in our respective choices of excluded-volume parameter. In our derivation, the excluded-volume parameter is defined as the Fourier transform of the Mayer f function at k=0 (which coincides with the often-used definition of the excluded-volume parameter as the second virial coefficient of the potential), whereas in Kosmas and Freed’s derivation, the excluded-volume parameter is chosen as the integrated strength of the potential over all space.
Surface segregation of a Pt–Au alloy: An atom-probe field ion microscope investigation
Authors: Tsong T. T. , Ng Yee S. , McLane S. B.
Published: February 2003

An absolute composition depth profile of a dilute gold alloy of platinum is obtained with a true atomic layer depth resolution. At 600±20 °C, the top {001} layer Au concentration is found to be as high as 99±20% for a sample with a bulk gold concentration of 4.1±0.7%. The Au concentration decreases monotonically into the bulk, with a characteristic depth of about three atomic layers.
The significance of Weinhold’s length
Authors: Salamon Peter , Andresen Bjarne , Gait Paul D. , Berry R. Stephen
Published: February 2003

An interpretation is given for the proposed metric of weinhold1 for the space of thermodynamic states. The proposed metric is nij=∂2U/∂Xi∂Xi where X’di and Xj are extensive variables. The physical significance of Nij is discussed. (AIP)
On conformational mobility in Δ6-bicyclo [3.2.0] heptene
Authors: Van-Catledge Frederic A.
Published: February 2003

Molecular mechanics calculations predict that Δ6-bicyclo [3.2.0] heptene exists as a mixture of chair and boat conformations. The chair form is predicted to lie 2.2 kcal/mole above the boat form. The transition state for chair–boat interconversion is estimated to lie at least 4.7 kcal/mole above the boat form. (AIP)
A uniform semiclassical sudden approximation for rotationally inelastic scattering
Authors: Korsch H. J. , Schinke R.
Published: February 2003

The infinite-order-sudden (IOS) approximation is investigated in the semiclassical limit. A simplified IOS formula for rotationally inelastic differential cross sections is derived involving a uniform stationary phase approximation for two-dimensional oscillatory integrals with two stationary points. The semiclassical analysis provides a quantitative description of the rotational rainbow structure in the differential cross section. The numerical calculation of semiclassical IOS cross sections is extremely fast compared to numerically exact IOS methods, especially if high Δj transitions are involved. Rigid rotor results for He–Na2 collisions with Δj≲26 and for K–CO collisions with Δj≲70 show satisfactory agreement with quantal IOS calculations.
Determination of linewidths and T1/T2 ratios for inversion transitions in NH3 broadened by H2
Authors: Amano Takayoshi , Amano Takako , Schwendeman R. H.
Published: February 2003

A computer-coupled microwave spectrometer has been used to record the line shapes of 11 inversion transitions in 15NH3 in mixtures of a small amount of 15NH3 in a large excess of H2. The steady-state absorptions were recorded as a function of frequency under conditions of low and moderate microwave powers. From the low power line shapes, values of Δν were obtained and used as input data for the analysis of power-broadened line shapes, from which values of (T1/T2)0 and qT1/T2 were obtained. The (T1/T2)0 values are all closer to 1 than the results previously reported for NH3⋅NH3 collisions. The difference in (T1/T2)0 ratios for the two sets of data decreases with increasing J and K and is explained as being the result of increased importance of collisions that change rotational states of NH3 molecules in states of low J and K in NH3⋅H2 mixtures. The pressure dependence of shifts in center frequencies of the transitions are reported and are very small for all of the transitions studied.
Vibrational energy transfer map for OCS
Authors: Mandich M. L. , Flynn G. W.
Published: February 2003

Fluorescence has been detected from the OCS ν1, ν2, ν3, 4ν2, and 2ν2+ν1 levels following initial excitation of the OCS bending overtone, 2ν2, with a pulsed CO2 laser. Analysis of each time-dependent fluorescence yields multiple exponentials which are consistent with a rather simple energy transfer path. The excess population placed in 2ν2 by the laser rapidly equilibrates through ladder climbing processes with the rest of the ν2 bending manifold at a near gas kinetic rate. Subsequently, the 4ν2 level couples to the asymmetric stretch, ν3, with a rate constant k4ν2→ν3=12±2 msec−1 Torr−1 (680 gas kinetic collisions); 2ν2 then fills the symmetric stretch ν1 with a rate constant k2ν2→ν1=3.3±0.5 msec−1 Torr−1 (2400 gas kinetic collisions). This intermode vibrational energy transfer process is followed closely by overall relaxation of the vibrational energy into rotations and translations with a rate constant kVT=2.3±0.5 msec−1 Torr−1 (3400 gas kinetic collisions). Vibrational relaxation from ν2 to ν1 may also occur but the maximum rate constant for this process is kν2→ν1?0.3 msec−1  Torr−1(?24 000 gas kinetic collisions). Theoretical probabilities derived from a short range repulsive force model are in reasonable agreement with experimentally derived probabilities when proper anharmonically mixed states are used in the transition matrix elements. A model calculation of the probability for the 4ν2→ν2 coupling based on long range attractive forces is about 3 times the measured value. The vibrational energy transfer mechanism and rates for OCS are compared with those for three other triatomics, CO2, N2O, and SO2, which have been extensively investigated. In all of these molecules, the vibrational relaxation of the lower states is consistent with a few, simple ’’propensity rules.’’
Ionization reactions of halogens and metal polyhalides with alkali atoms and dimers: Absolute cross sections
Authors: Wells Gregory J. , Reck Gene P. , Rothe Erhard W.
Published: February 2003

Absolute cross sections for the chemi-ionization of K2 and Cs2 with F2, Cl2, Br2, I2, SnCl4, MoF6, and UF6, and of K and Cs with UF6, are measured. A supersonic alkali beam passes through a gas cell and the product ions are measured. The relative error in the cross sections is about 30% while the absolute values are within a factor of two. For the halogens the cross sections (in Å2) range from 111 for Cs2–Br2 to 0.07 for Cs2–F2. An explanation is offered for these large differences. The dimer cross sections with the polyhalides range from 6 to 0.06. With UF6 the cross sections with K and Cs are much smaller than those for K2 and Cs2.
Hartree–Fock–Slater–LCAO calculations on the Cu(II) bis(dithiocarbamate) complex; magnetic coupling parameters and optical spectrum
Authors: Geurts P. J. M. , Bouten P. C. P. , van der Avoird A.
Published: February 2003

The electronic structure of the copper (ii) bis(dithiocarbamate) complex has been calculated by the nonempirical Hartree–Fock–Slater–LCAO method and, from the resulting molecular orbitals, the g tensor and the copper and sulfur hyperfine tensors have been obtained. The bonding between the Cu atom and the four ligand S atoms is mainly covalent and the unpaired electron is delocalized over these atoms. All the magnetic parameters are in fair agreement with the experimental EPR results and also the electronic excitation energies agree rather well with the optical spectrum.
Atomic electronegativity from density functional theory
Authors: Alonso J. A. , Girifalco L. A.
Published: February 2003

The atomic electronegativity, identified with the chemical potential of energy density functional theory, has been studied by using approximate functionals containing gradient corrections to the kinetic energy. The electronegativity has been found to be related to the asymptotic behavior of the electron density in the free atom. By requiring good agreement between the density functional electronegativity and the Mulliken scale we suggest that the Weizsäcker coefficient (λ=1/8) in the gradient term is more accurate than the Kirzhnits coefficient (λ=1/72) for the valence electrons and then for chemical binding studies.
Calculations of the spin–dipolar term and vibrational corrections to the spin–spin coupling constant of HD. An estimate of the value of the Fermi contact term
Authors: Schulman Jerome M. , Lee W. S.
Published: February 2003

Calculations of the spin–dipolar contribution to JHD are performed in the coupled Hartree–Fock approximation. Using these results and literature values for the other terms, best estimates of the ’’noncontact’’ contributions to JHD and a correction for vibrational averaging are made. Working backwards from experiment we obtain a value for the Fermi contact term at the equilibrium internuclear separation, with which theoretical values can be compared. This value is 40.95 Hz.
Proposed extensions to the Tanford–Kirkwood theory of protein titration
Authors: van Faassen Ernst E. , Mofers Frans J. M. , Casteleijn Gerard
Published: February 2003

A new formal procedure is presented for averaging a wide range of properties over all charge configurations occurring on a polyelectrolyte. In addition, self-energy effects arising from the Tanford–Kirkwood theory of protein titration are investigated. It is shown that reasoning involving essentially macroscopic concepts leads to physical absurdities when used in the calculation of self-energies. Finally, a method is proposed for avoiding the dielectric discontinuity in the Tanford–Kirkwood model. This method entails an orthogonal expansion of the Green function for the problem of a special class of spherical symmetric continuous variations of the dielectric constant.
Thermochemical properties of gaseous FeO and FeOH
Authors: Murad Edmond
Published: February 2003

The dissociation energy of FeOH, D00(Fe–OH), has been measured mass spectrometrically by passing H2O(g) and H2(g) over Fe2O3 in a Knudsen cell. The measurements lead to D00(Fe–OH)=76.9±4 kcal/mol (3.3±0.2 eV). The ionization potential of FeOH was measured and found to be 7.9±0.2 eV. An equilibrium involving gaseous FeO was also measured and the results of this study together with an analysis of previous data lead to D00(FeO)=93.0±3 kcal/mol (4.03±0.13 eV). The following data were derived from the measurement: D00(Fe+–O)=3.15±0.2 eV, D00(Fe+–OH)=3.3±0.2 eV, D00(FeO+–H)=4.7±0.2 eV, and PA(FeO)=D00(FeO–H+)=9.40±0.2eV. The similarity in bonding between D00(Fe–OH) and D00(Fe–Cl) is briefly discussed.
Infrared spectrum of formaldehyde in solid nitrogen. II. Dimer spectrum and dimer structure
Authors: Nelander Bengt
Published: February 2003

Infrared spectra of (H2CO)2, (D2CO)2, and H2CO⋅D2CO in solid nitrogen have been observed. In addition (H2CO)2 has been studied in argon matrices. On the basis of the results a noncentrosymmetric complex structure with equivalent dimer components is suggested.
Magnetic multipoles in time dependent fields
Authors: Sanctuary B. C.
Published: February 2003

The effects of time dependent magnetic vector fields on magnetic multipoles is presented. It is shown that a single rapid pulse causes the qth spherical component of the kth multipole in the rotating frame to evolve as the associated Legendre function Pkq(cosϑ) at resonance. The initial state for multispin systems which is prepared by such a pulse is given.
Pulsed spin locking in pure nuclear quadrupole resonance
Authors: Cantor R. S. , Waugh J. S.
Published: February 2003

The technique of pulsed spin locking in NMR has recently been applied to a pure NQR system, with results very similar to those obtained in the NMR case. However, the theoretical methods used to explain the NMR results are not applicable to the pure NQR case. Using a model of the NQR system, a method is developed for following its evolution, which leads to results which reproduce the experimentally observed short-time behavior, and which are in some agreement with the long-time behavior.
Kramers–Kronig relations and resonance Raman scattering
Authors: Hassing S. , Mortensen O. Sonnich
Published: February 2003

A relationship between an absorption spectrum and a corresponding Raman excitation spectrum has been derived from the optical theorem, the Kramers–Kronig relations and the well known recurrence relations for harmonic oscillator functions. The relations are given for both fundamentals, overtones and combination bands in the Raman spectrum. The relation for the fundamentals is similar to the relation proposed by Tonks and Page, but it is shown here that it is only strictly valid at zero temperature, contrary to their claim that it is valid at all temperatures.
Effect of surface magnetic fields on laser-induced predissociation of diatomic molecules
Authors: Bhattacharyya Dilip K. , Lam Kai-Shue , George Thomas F.
Published: February 2003

A semiclassical S-matrix formalism is used to demonstrate a possible laser surface synergistic effect.(AIP)
Resonances in the H2O photodissociation: A converged three-dimensional quantum mechanical study
Authors: Segev Eli , Shapiro Moshe
Published: February 2003

The photodissociation rate of H2O is calculated using the artificial channel algorithm.(AIP)
The identification of molecular NaPO3 and NaPO2 by matrix isolation IR spectroscopy
Authors: Ogden J. S. , Williams S. J.
Published: February 2003

Matrix isolation and irbrational spectroscopy has been used to identify and study NaPo3 and NaPo2 ions in high temperature alkali metal phosphate vapors. (AIP)
A combined dynamical–statistical approach to calculating rates of complex bimolecular exchange reactions: Asymmetric systems
Authors: Davis Jon P.
Published: February 2003

Trajectory calculations are reported for the systems H+CN, H+NC, and H+NSi.(AIP)
On the use of ladder operators in simultaneous V
V and V
T transitions at high collision energies
Authors: Ree Taikyue , Shin H. K.
Published: February 2003

We show the use of exponential forms of ladder operators in solving the time dependent wave equation for simultaneous vibration–vibration and vibration–translation transitions in diatomic molecules. A general form for such transitions involving one-quantum processes is obtained and applied to D2+D2. The VT steps strongly couple with VV processes at collision energies several times h/ω to reduce the overall VV energy exchange probability substantially.
Multiconfiguration self-consistent-field study of the importance of triply and quadruply excited electronic configurations in the water molecule
Authors: Laidig William D. , Saxe Paul , Schaefer Henry F.
Published: February 2003

The importance of triple excitations, configurations differing by three electrons from the Hartree–Fock reference configuration, is of considerable interest in electronic structure theory. A simple double zeta basis set O(9s 5p/4s 2p), H(4s/2s) has been used to address this problem for the water molecule. Configuration interaction (CI) and multiconfiguration self-consistent-field (MCSCF) wave functions including up to all single, double, triple, and quadruple (SDTQ) excitations (a total of 17 678 1A1 configurations) have been obtained for this purpose. An interesting result is that the MCSCF wave function including only single excitations yields 52.3% of the comparable correlation energy obtained with all single and double excitations. Using canonical SCF orbitals, triple excitations are found to contribute only 0.8% of the correlation energy. However, the MCSCF procedure increases this correlation energy fraction by more than a factor of 5. The MCSCF energies for the wave functions including SD and SDTQ excitations are less than 0.001 hartree lower than the comparable straight CI energies.
Potential energy surfaces for excited neon atoms interacting with water molecules
Authors: Bentley John
Published: February 2003

A substantial body of experimental data on interactions of metastable rare gas atoms with water molecules exists, but models of the interaction process are lacking. In order to interpret experiments involving collisions of Ne* with H2O, molecular orbital calculations with configuration interaction have been carried out for the six three-dimensional potential surfaces arising from interaction of Ne (2p53s 1,3P) with a rigid H2O molecule. Basis sets of roughly double zeta and double zeta plus polarization quality have been used. An attraction of 0.25 eV is found between Ne* and H2O at a neon–oxygen distance of 2.5 Å, in good agreement with the structure calculated for a Na–H2O complex. Multipolar expansions of the lowest triplet potential surface are reported. Interpretation of Penning ionization electron spectra and some excitation transfer results are aided by the present surface. An electrostatic model is proposed to construct potential surfaces of the type presented here.
Quantum theory of coherent hyper-Raman scattering from isotropic materials
Authors: Bjarnason Jón Örn , Andersen Hans C. , Hudson Bruce S.
Published: February 2003

The fully quantum mechanical theory of coherent hyper-Raman scattering is developed. In this scattering process, which has previously been proposed, two laser beams of wave vectors and frequencies k1 and ω1 and k2 and ω2, respectively, impinge upon the sample to produce a scattered beam characterized by a wave vector k3=4k1−k2 and a frequency ω3=4ω1−ω2. An expression for the rate of scattering from a gas is derived, and several polarization experiments are discussed. It is shown that the selection rules for coherent hyper-Raman scattering are identical to those for the spontaneous hyper-Raman process. A relation between the coherent hyper-Raman spectrum and a correlation function of the hyperpolarizability is obtained. The coherent hyper-Raman process, though not yet observed in the laboratory, may afford an improved way of obtaining hyper-Raman spectra. This process bears the same relation to spontaneous hyper-Raman scattering as the coherent Raman process does to spontaneous Raman scattering, and it should display similar advantages, viz., a higher signal to noise ratio and a resolution limited only by the laser linewidth.
The longitudinal spin lattice relaxation time of the uranyl phenanthraquinone radical ion complex
Authors: Hutchinson D. A. , Chen K. S. , Russell J. , Wan J. K. S.
Published: February 2003

The longitudinal spin lattice relaxation time T1 for the uranyl phenanthraquinone radical complex ion (UO2PQ)+. is much less than that for the phenanthraquinone radical anion PQ−.. A qualitative understanding of the effect is achieved by recognizing that the U atom has an enormous spin–orbit coupling. A calculation which includes the U 5f±2 (5fz,x2−y2 and 5fzxy) orbitals in the π system provides a semiquantitative understanding but the formal charge of U is closer to +1 than to +2. The state of the odd electron is described by a Huckel approach and the description of T1 includes G tensor modulation and spin–rotation interaction.
Self-consistent electronic structure of the refractory metal ZrB2, a pseudographite intercalation compound
Authors: Johnson David Linton , Harmon B. N. , Liu S. H.
Published: February 2003

The self-consistent band structure of ZrB2 has been evaluated using the KKR method. It is noted that a large charge transfer is not necessary to explain many of the experimental results which can be understood in terms of the band structure and the bonding nature of the wave functions. X-ray photoemission spectra and optical reflectance measurements are compared with the calculated density of states and joint density of states, respectively. The calculations are also discussed with reference to nuclear quadrupole experiments, Hall effect measurements, and the electronic specific heat. The similarities to intercalated graphite and related compounds are discussed and the strong bonding as reflected in the hardness and high melting point is considered.
Photoisomerization of polyenes: Potential energy surfaces and normal mode analysis
Authors: Ohmine Iwao , Morokuma Keiji
Published: February 2003

Photoisomerization processes of polyenes in the triplet mechanism are investigated theoretically. Global features of the lowest triplet and the ground state potential surfaces are evaluated for butadiene, pentadiene, hexatriene, and the protonated Schiff base of butadiene in an ab initio method and for longer polyenes in a semiempirical method. The steric relaxations along the isomerization are carefully examined. To understand the mechanism of radiationless transitions (intersystem crossings) concomitant with photoisomerizations, the normal mode analyses are performed and the overlap integrals of the Franck–Condon type are calculated. The torsional modes around the certain C–C bonds are found to be the main acceptor of the electronic energy in the deactivation process and some other modes such as H flappings act as minor but non-negligible acceptors.
The order parameter model of liquids and glasses with applications to dielectric relaxation
Authors: Lesikar Arnold V. , Moynihan Cornelius T.
Published: February 2003

The order parameter model is generalized to describe systems whose equilibrium states depend on other intensive variables, e.g., electric field E, in addition to temperature T and pressure P. The set of order parameters required to specify the state of a liquid or glass is shown to form an abstract Euclidean vector space in the vicinity of a particular equilibrium state. The results of relaxational experiments are then connected with geometric relations in this space of order parameters. Thermodynamic stability requires that certain angles in this space have real values. This leads to thermodynamic stability conditions (TSC’s), which include the well known Prigogine–Defay condition for systems with intensive variables T and P and analogs of it for systems with other sets of intensive variables. The order parameter model is applied to dielectric relaxation, and its predictions are tested against available data. It is shown that the addition of E as an intensive variable requires at least one more order parameter to specify the state of the system than the number needed when T and P are the only intensive variables.
Volumetric and derived thermal characteristics of liquid D2O at low temperatures and high pressures
Authors: Kanno H. , Angell C. A.
Published: February 2003

Using a refinement of a capillary technique described earlier precise PVT data on D2O between 0.1 and 140 MPa and down to −30 °C have been obtained, and the pressure dependence of the volumetric behavior in the vicinity of the well-known density maximum has been examined. The ’’sharpness’’ of the density maximum, defined by dα/dT at α=0 K−1, decreases with increasing pressure in a simple linear manner. Extrapolations suggest that at about 250 MPa the density maximum would have the same sharpness as that known for the tetrahedral network liquid SiO2, but would occur at temperatures too low to be observed. A relation between volumetric anomalies and bridge-bond angles in liquids with tetrahedral network structures is suggested. The volumetric data are used to derive the effect of high pressure on the constant pressure heat capacities. From these it is argued that, on extended supercooling at 2 kbar, water could pass normally into the glassy state at about 140 K without encountering the entropy paradox which exists for water at normal pressures.
Proton tunneling dynamics and an isotopically dependent equilibrium geometry in the lowest excited π–π* singlet state of tropolone
Authors: Rossetti R. , Brus L. E.
Published: February 2003

Fluorescence and fluorescence excitation spectra have been observed for tropolone isolated in neon matrices near 4 K. The fluorescence spectra are vibrationally analyzed showing that specific geometrical changes in the excited state are dependent upon isotopic substitution of the phenolic proton. The intramolecular tunneling barrier is found to be substantially higher in the ground electronic state. The gas phase data of Alves and Hollas are analyzed showing that the tunneling reduced mass is very close to that of a free proton (deuteron).
Stable pure sulfur discharges and associated spectra
Authors: Peterson D. A. , Schlie L. A.
Published: February 2003

Stable, self-sustained discharges in pure sulfur and He–sulfur mixtures have been achieved for the first time, at operating temperatures between 0 and 250 °C. Very intense S2 (B–X) band emission was observed. No other atomic or molecular emission was detected. This result indicates that S8 molecules are broken directly into S2 molecules and thus provides the potential for highly efficient discharge pumping of the (B–X) S2 laser transition at much lower operating temperatures (<300 °C) than previously thought. The S2 (B–X) band emission was examined in great detail and through the analysis of these spectra, the ground electronic state spectroscopic constants have been determined to the fourth order. These are w′′e=727.848 cm−1, wex′′e=3.0195 cm−1, w″ey″e=4.196×10−3 cm−1, wez″e =9.77×10−6 cm−1. The first observation of the B″ 3πu− −X 3Σg− system transitions was also made, and tentatively identified. Potential energy curves for both the X 3Σg− and B 3Σu− states are reported.
Theory of Fourier transform ion cyclotron resonance mass spectroscopy: Response to frequency-sweep excitation
Authors: Marshall Alan G. , Roe D. Christopher
Published: February 2003

Although all previous theoretical treatments of Fourier transform ion cyclotron resonance (FT–ICR) spectral line shape have been based on the ICR time-domain response to fixed-frequency pulsed excitation, experimental FT–ICR signals are produced by frequency-sweep excitation. This paper presents the first analysis of the FT–ICR line shape resulting from frequency-sweep excitation, including the effects of circularly polarized versus linearly polarized exciting radiation, continuous versus stepwise variation in swept frequency, variation in sweep parameters (e.g., frequency range, sweep rate, size of step, and number of steps), phase conventions, reliability of fast Fourier transform data reduction, and time delay between excitation and detection. In addition, we show how the nonlinear amplitude and phase variation of an FT–ICR spectrum obtained by direct Fourier transformation of a time-domain ICR response can be corrected by deconvoluting the response with the frequency-sweep excitation. Implementation of these results promises to enhance the appearance, reliability of peak height information, and resolution of experimental FT–ICR spectra.
Molecular beam study of the chemi-ionization reaction: Ba+Cl2→BaCl++Cl−
Authors: Burton Randolph H. , Brophy John H. , Mims Charles A. , Ross John
Published: February 2003

Angular and translational energy distributions have been measured for the BaCl+ and Cl− products of the reaction Ba+Cl2 in crossed molecular beams. A center of mass (cm) cross section was derived by fitting an assumed functional form of cm to the observed laboratory (lab) scattering data. The cm behavior suggests a strongly coupled collision which contrasts with the dynamics of the neutral product channel BaCl+Cl. The threshold energy for reaction is (0.15±0.05) eV. A reaction mechanism is proposed involving small impact parameter collisions with a crossing from the reactant to product surfaces occurring at the inner repulsive part of the potential.
Crossed beam studies of low energy proton transfer reactions: H2+(Ar,H) HAr+ from 0.4 to 7.8 eV (c.m.)
Authors: Bilotta R. M. , Preuninger F. N. , Farrar J. M.
Published: February 2003

We present a crossed beam study of the proton transfer reaction H2+ (Ar,H) HAr+ over an extended energy range. At the lowest collision energies (∼0.4 eV), the reaction appears to proceed with substantial interaction among all three atoms, while the dynamics become ’’direct’’ in the higher energy regime. Collision induced dissociation of the HAr+ product is observed for collision energies above ∼2 eV, but products with internal energy in excess of the HAr+ dissociation limit are observed in all cases. These observations are rationalized in terms of the metastability of HAr+ products with rotational energy in excess of the dissociation limit. A surprisal analysis on the product translational energy distributions is also presented.
Metastable excitations of large clusters of 3He, 4He, or Ne atoms
Authors: Gspann J. , Vollmar H.
Published: February 2003

Metastable electronic states of helium clusters of 106 to 108 atoms of either isotope, or of neon clusters of some 104 atoms, are observed to be excited by the impact of electrons of 30 to 100 eV energy. Being detected through the release of electrons which results from the impact of the excited neutral or ionized clusters onto the first dynode of an electron multiplier, these metastable excitations allow the time-of-flight spectrometry of speeds and sizes of large clusters. The energy dependence of the excitation probability of the helium clusters indicates the initial excitation of the metastable atomic triplet state while the observed flight times of about 10−3 s point to the metastable triplet molecular state as the final helium cluster excitation.
ESR of matrix isolated bromine atoms produced in the H+Br2 reaction
Authors: Bhat S. V. , Weltner W.
Published: February 2003

The products of the H(D)+X2 reaction, where X is Br, Cl, or F, have been trapped in solid argon at 4 °K and observed via ESR. With Br2 as reactant the observed spectra are attributed to Br atoms electronically quenched in an axial crystal field. The spectra obtained using the other halogens were not clearly attributable to quenched atoms. The ESR of matrix-isolated Br atoms has not been observed previously, {While this paper was being reviewed, H. Muto and L. D. Kispert observed the ESR spectrum of partially quenched Br atoms in x-irradiated N-bromosuccinimide single crystals [J. Chem. Phys. 72, 2300 (1980)]} but their magnetic properties are similar to those recently observed by Iwasaki, Toriyama, and Muto for I atoms quenched in solid xenon. For Br: g⊥=2.646(1), g∥=1.55(1), ‖A⊥‖=1937(20) MHz, ‖A∥‖=423(10) MHz, and Q∥=100(10) MHz. Comparison was made with crude axial crystal field predictions derived using the magnetic parameters of the gas-phase atoms.
Resonantly enhanced multiphoton ionization of pyrrole, N-methyl pyrrole, and furan
Authors: Cooper C. D. , Williamson A. D. , Miller John C. , Compton R. N.
Published: February 2003

The resonantly enhanced multiphoton ionization (REMPI) spectra of pyrrole (C4H5N), N-methyl pyrrole (C5H7N), and furan (C4H4O) have been measured in the wavelength region from 365 to 680 nm. New and previously observed Rydberg states are reported for pyrrole and furan. Vibrational constants are presented for most of the Rydberg series. Accurate ionization potentials are derived for pyrrole (8.207±0.003 eV) and N-methyl pyrrole (7.94±0.02 eV). A strong two photon allowed transition is observed in N-methyl pyrrole at 41 193 cm−1 (0,0) and is attributed to an 1A2 state. The corresponding state is not seen in pyrrole; however, it may be obscured by overlapping Rydberg series. Mass spectra following REMPI for benzene, pyrrole, and furan are reported. The degree of ionic fragmentation depends upon laser power density and wavelength.
Singlet energy transfer from the charge transfer excited state of tris (2,2′-bipyridine) ruthenium (II)
Authors: Mandal Krishnagopal , Pearson T. D. L. , Demas J. N.
Published: February 2003

The energy transfer efficiency from [Ru (bpy)3]2+ to the lasing state of Rh101 is determined in oxygenated methanol.(AIP)
A remark on the water dimer and its isomerism
Authors: Slanina Z.
Published: February 2003

The water dimer has been studied by SCF CI method of Matsuoka, clementi and Yoshimre. The isomerism was not found in water dimers. If is concluded that equilibrium description of water dimer is given by a single structure namely the linear one. (AIP)
Collision and encounter statistics from a molecular dynamics computation on a model of a fused salt
Authors: Sundheim Benson Ross
Published: February 2003

The analysis of collisions (defined as approach within a selected distance) between like and unlike pairs of ions in a computer model of molten KCl is described. The rates of collisions, distribution of encounter lengths, rates of first return and of second return, and the probabilities of escape are reported. These quantities bear on the transient term in diffusion calculations, cage models, fluorescence quenching, and geminate recombination reactions.
Rate constant for the reaction of hydroxyl radical with formaldehyde over the temperature range 228–362 K
Authors: Stief L. J. , Nava D. F. , Payne W. A. , Michael J. V.
Published: February 2003

Absolute rate constants for the reaction OH+H2CO have been measured over the temperature range 228–362 K using the flash photolysis–resonance fluorescence technique. The results were independent of variations in [H2CO], total pressure [Ar] and flash intensity (i.e., initial [OH]). The rate constant was found to be invariant with temperature in this range, the best representation being k1?0.11)×10−11 cm3 molecule−1 s−1 where the error is two standard deviations. This result is compared with previous absolute and relative determinations of k1. The reaction is also discussed from a theoretical point of view.
Collisional effects in the multiphoton dissociation of CF2CFCl
Authors: Stone James , Thiele Everett , Goodman Myron F. , Stephenson John C. , King David S.
Published: February 2003

We report results of a combined experimental and theoretical study of the effects of collisions with an inert buffer gas, on the CO2 laser induced MPD of CF2CFCl to form CF2 and CFCl. Rates of formation of the primary product CF2 have been determined, in real time using the laser excited fluorescence technique, at four IR laser intensities (Imax = 35, 47, 73, 220 MW/cm2) and a range of argon buffer gas pressures (0⩽PAr⩽500 Torr). The experimental data clearly show the effects of collisional hole filling at low pressures and V–T collisional deactivation at higher (≳100 Torr) pressures. We present a generally applicable theoretical model for collisional effects in MPD, in which two parameters (1/τ, ΔE) specify the collisional deactivation, two parameters (s, <ω≳) specify the density of states of the absorber, two parameters (A∞,Eact) specify the microscopic reaction rates and one parameter (δ) specifies the radiative pumping rates. In applying this model to CF2CFCl five of these seven parameters are readily estimated from independent kinetic or spectroscopic data. Treating only the remaining two parameters ΔE and δ as adjustable, an excellent theoretical fit of the high pressure experimental data is obtained. In this fit a value (ΔE=2.6 kcal/mole) is found for the mean energy transferred per collision to the buffer gas molecules. This value lies within the range expected for ΔE on the basis of chemical activation studies. The overall closeness of fit confirms the basic soundness of the ’’single quantum exchange’’ model for T2 relaxation, from which expressions are derived for the microscopic radiative pumping rates. Using only parameters determined in the MPD fit, we predict as a function of laser intensity and buffer gas pressure the energy absorbed from the light field. This prediction can be tested in an experiment that measures absorbed energy by the acousto-optic technique.
Theoretical characterization of the potential energy surface of the ground state of the HCO system
Authors: Dunning Thom. H.
Published: February 2003

Large scale, ab initio configuration interaction calculations are reported on the ground state potential energy surface of the HCO system. The calculated equilibrium geometry of HCO of RCH=1.12 Å, RCO=1.19 Å, and ϑHCO=126° is in good agreement with that determined experimentally, although the calculated H–CO bond strength De=16 kcal/mole is too small by ∼3 kcal/mole. A barrier of 7.5 kcal/mole is found for recombination of H+CO; the height of the barrier is estimated to be too large by 5–6 kcal/mole. The COH isomer, with a calculated equilibrium geometry of ROH=0.98 Å, RCO=1.29 Å, and ϑCOH=114°, is predicted to lie 40 kcal/mole above the HCO isomer, or 24 kcal/mole above the H+CO limit. For COH the barrier for dissociation of 16 kcal/mole is substantially less than that for isomerization (29 kcal/mole) in spite of the fact that isomerization is energetically more favorable. The COH isomer has not yet been identified experimentally. For the HCO system there seems to be no significant difference between the variational and perturbation calculations which have been reported, aside from a constant shift in the energy.
Calculation of the parity nonconserving energy difference between mirror-image molecules
Authors: Hegstrom R. A. , Rein D. W. , Sandars P. G. H.
Published: February 2003

The theory of energy shifts in mirror-image molecules arising from parity nonconserving weak neutral currents is developed. Its connection with the theory of optical activity is discussed. In order to demonstrate the existence of the energy shift, which produces an energy difference between an optically active molecule and its mirror image, an exact calculation is performed for a simple one-electron model (the Condon model). Then approximate LCAO-MO calculations are performed on two specific molecules, twisted ethylene and A-nor-2-thiacholestane, in order to obtain more reliable order-of-magnitude estimates of the energy shifts than available previously. According to our calculations, the former molecule has an energy shift two orders of magnitude larger than the latter, but both molecules have energy shifts which are appreciably smaller than the previous estimates.
A quadratically convergent multiconfiguration–self-consistent field method with simultaneous optimization of orbitals and CI coefficients
Authors: Werner Hans-Joachim , Meyer Wilfried
Published: February 2003

A quadratically convergent MC–SCF procedure is described which is based on the direct minimization of the energy. In comparison to the Newton–Raphson technique, which has previously been applied by several authors for orbital optimization, the convergence radius is much improved by taking into account in the energy expansion those parts of third and higher order terms which account exactly for the orthonormality constraints imposed on the orbitals. The nonlinear equations which define the improved orbitals are solved iteratively by a simple adaption of the Gauss–Seidel method. The coefficients of the configuration expansion can be optimized simultaneously with the orbitals, a necessary requirement for over-all quadratic convergence. The removal of redundant variables as well as useful approximations for the optimization of core orbitals are discussed. The convergence of the method is demonstrated to be much superior to classical Fock operator techniques and MC–SCF methods which are based on the generalized Brillouin theorem. The formalism is carried down to matrix operations and shows a simple structure.