bluuebunny/crossref_metadata_2025 huggingface dataset:
Bend and splay elastic constants and the rotational viscosity of nematic decylammonium chloride and ammonium chloride water mixtures
Authors: Haven T. , Armitage D. , Saupe A.
Published: February 2003

Elastic and dynamic properties of two amphiphilic nematic mixtures consisting of decylammonium chloride and NH4Cl, molar ratio 2.76:1, and water, 90.5 and 89.7 mole %, respectively, were studied on thin surface aligned films. The data can be successfully analyzed on the basis of the Frank–Oseen and Leslie–Erickson theory. The magnitude of the curvature elastic constants is comparable to that of typical thermotropic nematics. The mixtures have a nematic range of 20 °C and transform in cooling to neat soap. The bend elastic constant and the rotational viscosity diverge at this transition while the splay elastic constant shows no critical behavior.
Further developments of the theoretical treatment of the time dependent Kerr effect following a sudden change of a homogeneous electric field
Authors: Watanabe Hiroshi , Morita Akio
Published: February 2003

The theoretical treatment of the time dependent Kerr effect following a sudden change of a homogeneous electric field in the previous paper [H. Watanabe and A. Morita, J. Chem. Phys. 73, 5884 (1980)] is extended to a more general form in which the transient of the Kerr effect is expressed in the closed form of a matrix equation. Numerical aspects of the equation are discussed and the equation is used to see how the transients of the Kerr effect at fixed ratios of the contribution of the permanent dipole moment and the anisotropy of the polarizability depend on the field strength. It is shown that the theory can be easily extended to the polydisperse system. The areas bounded by the normalized transients for the stepup and the reversing fields are expressed in a closed form and dependencies of areas on the field strength are demonstrated.
A continuum theory for quadrupole relaxation of ions in solution
Authors: Hynes James T. , Wolynes Peter G.
Published: February 2003

A simple continuum theory for quadrupole relaxation rates of ions in polar solvents is presented. The predicted rates agree fairly well with experimental rates determined for various ions in several solvents. The surprising success of the relatively crude continuum model is discussed and the implications for molecular theories of quadrupole relaxation are indicated.
Homogeneous nucleation of sulfur hexafluoride clusters in Laval nozzle molecular beams
Authors: Abraham Oommen , Kim Sang-Soo , Stein Gilbert D.
Published: February 2003

The condensation of SF6 in Ar and He carrier gases has been studied in diverging Laval nozzles with throat diameters in the range of 0.1 to 0.2 mm. The clustering is detected using laser scattering, molecular beam intensity measurements, and high energy electron diffraction. Although the nozzles have cooling rates in the 107 °C/sec range, the correction for nucleation lag time results in changes which are less than 15% from the steady state rate theory. For 3% SF6 in Ar the onset may enter the Ar supersaturation region with the attendant possibility for binary nucleation. Even though these expansions are not isentropic, due to nozzle wall boundary layers, they are nevertheless much more effective cluster sources than the ’’isentropic’’ free jet expansions. The onset of condensation is consistent with earlier results for much larger nozzles and agrees with the classical Volmer theory.
1H and 7Li NMR studies on the dynamics of water and cations in quasi-two-dimensional electrolyte layers of hydrated chalcogenides
Authors: Röder U. , Müller-Warmuth W. , Schöllhorn R.
Published: February 2003

A complete set of NMR wide line data for the monolayered chalcogenide hydrates of the type A+0.33(H2O)y(TaS2)0.33− with A = Li, Na, K, NH4, Rb, and Cs shows a linear correlation between the enthalpy of hydration of A+ and the temperature which governs the transition from the rigid lattice to the range of ordered mobility. Measurement of the angular dependence of the two-spin splitting has provided further support for the model of structure and motion recently proposed for these materials. There is no indication of proton exchange processes other than a strong influence of the pH value. For fully hydrated sulfides Ax(H2O)zMS2 with A = Li and Na and M = Ta and Nb with bilayers of water, 1H and 7Li NMR wide line studies as well as measurements of the proton relaxation rates have given information on the dynamic structure. In the case of Li+, the reversible removal of water from the interlayer space is connected with a marked change of the coordination. The relaxation behavior of the intercalated water is accounted for by an asymmetric distribution of correlation times.
Matrix-isolated bismuth. II. New fluorescence band systems and resonance Raman spectra of Bi2 and Bi4 in solid argon
Authors: Ahmed Fakhruddin , Nixon Eugene R.
Published: February 2003

For Bi2 molecules in argon matrices, a new laser-induced fluorescence system with origin at 16 893 cm−1 has been observed. The upper state A′ lies 826 cm−1 below the well-established A state. A progression of six bands of the resonance Raman spectrum of Bi2 is also reported. More concentrated matrices display an extremely weak fluorescence which is very probably the B–X system of Bi4, previously observed in absorption. A sequence of resonance Raman bands with an irregular intensity pattern is attributed to an unknown Bin molecule.
Electron paramagnetic resonance in CuTiF6⋅4H2O
Authors: Rubins R. S. , De D. K. , Black T. D.
Published: February 2003

Electron paramagnetic resonance in single crystals of CuTiF6⋅4H2O was studied between 77 and 485 K in the frequency ranges 23–25 and 34–36 GHz. The monoclinic lattice has two magnetically distinct Cu2+ sites, but (except at 4.2 K) only one resonance line was observed at all orientations. Assuming tetragonal symmetry at each Cu2+ site, the observed g values were used to determine the single ion parameters g∥ = 2.403±0.005 and g⊥ = 2.082±0.002 at 77 K. Linewidth data in the ac plane were explained in terms of the exchange narrowing of the line broadened by the dipolar and unresolved hyperfine interactions.
Energetics of molecular elimination in the infrared multiphoton dissociation of CF2Cl2, CF2Br2, CF2ClBr, and CFCl3
Authors: Morrison Richard J. S. , Loring Roger F. , Farley Robert L. , Grant Edward R.
Published: February 2003

In a series of scavenged product analysis experiments we have measured absolute conversions and branching ratios for simple bond scission versus diradical formation in the infrared multiphoton dissociation of methyl freons: CF2Cl2, CF2Br2, CF2ClBr, and CFCl3. From an analysis of the dependence of yield patterns on laser intensity, we conclude that diradical products are formed in CF2Cl2 and CF2Br2 by direct elimination of molecular halogen. For both systems the relative importance of molecular elimination is observed to depend sensitively on the pumping intensity. At optimum laser energy in each case the maximum yields of :CF2 were found to be 10% for CF2Cl2 and 30% for CF2Br2. Evidence for this channel was less conclusive in CF2ClBr, where the upper limiting contribution from molecular elimination was found to be 1%. No indication of a low energy Cl2 elimination pathway was found for CFCl3. Reasonable estimates of the thresholds and internal energy dependences for molecular elimination in CF2Cl2 and CF2Br2 have been obtained by means of a model calculation applied to the pump laser energy dependence of the elimination/simple scission branching ratios in these systems.
Lyman-α fluorescence from hydrogen photofragments of CH4 and H2O
Authors: Wu C. Y. R. , Judge D. L.
Published: February 2003

Lyman-α fluorescence has been observed from the atomic hydrogen photodissociation fragments of CH4 and H2O, and the fluorescence cross sections have been measured throughout the synchrotron radiation continuum from 175 to 780 Å. The maximum fluorescence cross sections are 4.6×10−20 cm2 at 568 Å and 1.72×10−18 cm2 at 703 Å for CH4 and H2O, respectively. The photodissociation is mainly a direct process following absorption of a photon into the continuum state(s). Structure in the excitation functions is attributed to predissociation of Rydberg series. It is noted that in H2O, but not in CH4, there is a strong competition between the predissociation and photoionization channels. It is shown in CH4, in particular, that the use of fluorescence detection of the dissociation fragments provides very useful information for interpreting photoabsorption processes in the high energy region. The Lyman-α emission resulting from the dissociation process CH4+hν→CH3+H(n = 2) has been observed in the present work although the threshold for this process has not been determined. However, the threshold corresponding to the process CH4+hν→CH2+H+H(n = 2) and/or CH+H2+H(n = 2) has been measured and found to agree with that determined from electron impact studies.
Four time density correlations around a dissolved HCl molecule in dense argon, krypton, and xenon as determined from linewidth data
Authors: Menger P. , van der Elsken J.
Published: February 2003

The far infrared pure rotational spectrum of HCl molecules in a medium of xenon at T = Tc and ρ = 1.6ρc has been measured. The result expressed in terms of linewidths is compared with earlier results obtained in krypton and in argon, also at Tc and 1.6ρc. Coefficients that connect linewidths with the power spectrum of the fourth order anisotropic density fluctuations were calculated and are given together with existing coefficients for the second order terms. A scaling procedure is used to connect the data obtained in argon, krypton, and xenon with molecular dynamics data on the second order anisotropic density fluctuations. This leads to fourth order density fluctuations for the three noble gases of which the power spectra do indeed obey the same scaling laws. The physical meaning of the power spectrum, H(ω,0,ω) of the four time anisotropic density correlation function is discussed.
Classical trajectory approach to photodissociation: The Wigner method
Authors: Brown Robert C. , Heller Eric J.
Published: February 2003

A time-dependent semiclassical method for generating energy-dependent photodissociation partial cross sections is presented. The method is based on the Wigner equivalent formulation of quantum mechanics with the semiclassical limit arising from one dynamical approximation: the replacement of the quantum Liouville operator by its h/→0 limit. The results of the present scheme for the collinear dissociation of ICN on a single dissociative surface are compared to those obtained from a distorted-wave analysis and a semiclassical wave packet propagation. A model calculation employing standard trajectory techniques indicates that the present method has several distinct advantages over the traditional quasiclassical approach.
Theoretical and experimental analysis of phase diagrams and related dynamical properties in the Belousov–Zhabotinskii system
Authors: De Kepper P. , Boissonade J.
Published: February 2003

It is shown on the basis of a semiquantitative dynamical analysis and full nonlinear computations that the irreversible Oregonator with flux conditions exhibits a characteristic cross-shaped phase diagram which expresses the relationships between bistability and periodicity. The results are in good agreement with a whole set of new experiments on the Belousov–Zhabotinskii reaction performed in a continuous stirred tank reactor.
Isotopic vibration–vibration energy exchange in hydrogen chloride molecules
Authors: Shin H. K.
Published: February 2003

Vibration–vibration energy exchange processes in the collision systems H35Cl(1)+H37Cl(1), H35Cl(1)+H35Cl(1), and H37Cl(1)+H37Cl(1) have been investigated over the temperature range 100–1000 °K based on a long-lived collision model. It is found that the isotopic energy exchange process H35Cl(1)+H37Cl(1)→H35Cl(2)+H37Cl(0) is the most efficient, whereas the production of H37Cl(2) from the same initial state is the least efficient. A negative dependence of energy exchange probability on temperature is found for all four processes over the temperature range.
Multiphoton ionization photoelectron spectroscopy of nitric oxide
Authors: Miller John C. , Compton R. N.
Published: February 2003

We have investigated the photoelectron spectrum (PES) of nitric oxide following four-photon ionization resonant with various vibrational levels of the two-photon-allowed A state. The process studied was       When the laser was tuned to each resonant vibrational state, two electron energy peaks were observed. For v = 0,1,2,3 the higher energy peak corresponds to direct ionization, leaving the positive ion in a single vibrational quantum state which is the same vibrational state as that of the initially excited nitric oxide Rydberg state. The origin of the lower energy peak representing near zero energy electrons is not understood at this time, but several possible mechanisms are discussed. The PES spectrum through the A2Σ+(v = 3) level indicates that two competing processes can occur—vibrational autoionization after three-photon absorption, or the ’’quasi-free–free’’ absorption of a fourth photon resulting in formation of the X 1Σ (v = 3) state of the ion. Results of multiphoton ionization PES resonant with the C state are also presented.
Semiempirical three-dimensional potential energy surfaces suitable for both reaction channels of the XH2 system (X = F, Cl)
Authors: Last (Lyast) Isidore , Baer Michael
Published: February 2003

Three-dimensional potential energy surfaces for Reactions (1) F+H2→HF+H, (2) H′+HF→H′F+H, (3) H+HCl→H2+Cl and (4) H′+HCl→H′Cl+H were calculated by a modified version of the diatomics-in-molecules (DIM) method. In this version a term which incorporates contributions of three-center molecular integrals neglected by the DIM method is added to the DIM energy. This is the first time that both reaction channels of all of these systems were considered simultaneously. The potential barriers of Reactions (1) and (2) and the difference between the potential barriers (3) and (4) were fitted by adjusting three parameters. The potential barrier of Reaction (3) was then predicted to be 4.9 kcal/mole. The dependence of the barrier heights, saddle points, and other features of the potential energy surfaces on the geometry were investigated. The transition state geometry was proved to be linear for Reactions (1), (3), and (4) and nonlinear for Reaction (2).
Polymers confined to thin slabs: Scaling law
Authors: Lax Melvin , Barr Ronnie , Brender Chava
Published: February 2003

A scaling method is applied to a polymer in a thin slab of thickness L. The results indicate that the component of the mean square end-to-end distance, in the direction parallel to the slab plane, <R2y(L)≳, behaves asymptotically as <R2y(L)≳∼L−1/4 N3/2. Results from data on self-avoiding walks generated on the SC and FCC lattices verify the scaling law.
A second order MCSCF method for large CI expansions
Authors: Lengsfield Byron H. , Liu Bowen
Published: February 2003

A second order MCSCF method is presented in a form which is suitable for large CI expansions. This methodology allows one to employ second order MCSCF theory to problems in which the CI expansion is an order of magnitude larger than could be addressed previously. Sample calculations are reported on the X 1Σ+ and 2 1Σ+ states of HF in which the MCSCF wave function was composed of 1436 CSFs. (AIP)
Rotational cooling and electronic relaxation in diazabenzenes
Authors: Jameson A. Keith , Okajima S. , Lim E. C.
Published: February 2003

Rotational cooling of fluorescence of diazabenzene is considered. (AIP)
A molecular model of the hydrodynamic stick boundary condition
Authors: Masters A. J. , Keyes T. , Madden P. A.
Published: February 2003

A molecular model of the hydrodynamic stick boundary conditions is attempted.(AIP)
Ion–matrix interactions: Optical spectrum of the Ca+ cation in solid argon
Authors: Bondybey V. E. , English J. H.
Published: February 2003

Ca+ cations are generated by photoionization of Ar matrices containing calcium atoms. They are detected and identified by their laser induced fluorescence spectrum. The observed spectra and lines shapes of the ions and those of the neutral alkali atoms are compared and discussed in terms of the guest–host interaction potentials.(AIP)
Triple quantum NMR on spin systems with I = 3/2 in solids
Authors: Vega S. , Naor Y.
Published: February 2003

Pulsed NMR experiments are developed for the excitation and the detection of triple quantum coherences in quadrupolar spin systems with I = 3/2. Two experimental methods are used: For systems with small quadrupolar frequencies direct triple quantum excitation and detection is accomplished by weak rf irradiation pulses. For large quadrupolar frequencies modulated rf pulses are applied to create and to monitor triple quantum coherences. Modulated rf pulse techniques are also used to search for quadrupolar frequencies and to excite selectively center frequency spectral lines of I = 3/2 nuclei. Applications of these techniques are demonstrated by 23Na(I = 3/2) NMR experiments on an oriented single crystal of sodium ammonium tartarate tetrahydrate.
An electron impact investigation of the forbidden and allowed transitions of norbornadiene
Authors: Doering J. P. , McDiarmid Ruth
Published: February 2003

In this electron impact investigation of the excited states of norbornadiene, variable scattering angle and incident energy have been employed to distinguish between different types of electronic transitions. Rydberg transitions have been observed to the first and second ionization limits. Forbidden valence transitions have been observed at 5.25 and 7.50 eV and allowed valence transitions at 5.95 and 6.65 eV. Based on the electron impact energy loss spectrum, the splitting of the occupied orbitals is estimated to be 0.95 eV and that of the excited states is 1.3 eV. Either little or constant electron rearrangement appears to accompany electron promotion. The unperturbed transition energy is estimated to be approximately equal to that observed for norbornene.
A theoretical study of resonance Raman scattering from molecules. III. Resonance Raman scattering and resonance fluorescence
Authors: Fujimura Y. , Kono H. , Nakajima T. , Lin S. H.
Published: February 2003

A theory for the resonance light scattering in a molecular system interacting with the heat bath is developed, taking into account the multilevel structure of the molecular system. An expression for the cross section of the resonance light scattering process is derived by using the Liouville operator method in the Markoff approximation; the cross section consists of three terms: resonance Raman scattering, resonance fluorescence, and their mixing. The mixing term vanishes in the limit of the zero vibrational dephasing constant. Analytical expressions for the resonance Raman scattering and resonance fluorescence cross sections are derived in the displaced harmonic oscillator model. Model calculations of the vibrational band structure in the resonance light scattering cross section are performed by using the expressions derived. Differences of the band intensity distribution in the resonance fluorescence from that in the resonance Raman scattering are illustrated.
Influence of higher order density correlation in a fluid on the linewidths of rotational transitions of a dissolved molecule
Authors: Spiekerman R. J. N. , van der Elsken J.
Published: February 2003

Time dependent perturbation theory gives the relation between the dipole correlation function of a rotating dipole molecule and the anisotropic density fluctuations in a fluid around that molecule. Second order terms give the width of the absorption line associated with a rotational transition as a linear combination of terms containing the values of the power spectra of the density correlations taken at the frequencies of virtual transitions from initial and final levels. Fourth order terms account for the phase relations that might exist between some of these second order terms and constitute negative contributions to the linewidths. It is shown how this opens the possibility for linewidth measurements to give information about the power spectra H(ω,ω,ω) of the three time eight point density correlation functions. The complete expressions for the calculation of the coefficients is given and relative importance of the terms of different order is estimated.
The surface de-excitation coefficient for a vibrationally excited gas
Authors: Chandra Ramesh
Published: February 2003

An expression for the surface de-excitation coefficient (γ) of a vibrationally excited gas has been derived. The functional dependence of γ on parameters such as pressure and temperature has been discussed.
Monte Carlo study of freely jointed ring polymers. III. The generation of undistorted perfect ring polymers
Authors: Chen Yi-der
Published: February 2003

In the first two papers of this series, we presented an efficient method of generating ring polymers of the freely jointed model and the study of statistical properties (such as the radius of gyration, the writhing numbers, etc.) of the ring polymers generated by this method. The drawback of this method is that the rings generated may be slightly distorted. In this paper, we present a slightly different dimerization procedure that generates perfect, undistorted rings. Differences between the two methods in terms of the statistical properties of the generated rings are discussed.
Dual fluorescence excitation spectra of methyl salicylate in a free jet
Authors: Heimbrook Lou Ann , Kenny Jonathan E. , Kohler Bryan E. , Scott Gary W.
Published: February 2003

Separate fluorescence excitation spectra of the blue- and UV-emitting forms of methyl salicylate cooled in a free-jet expansion are reported. This study represents the first observation of the detailed vibrational structure of these transitions. The two excitation spectra have no features in common, and their intensity patterns are very different. Many individual lines are ∼2 cm−1 wide (nearly laser limited), although in the excitation spectrum of the UV emission, spectral congestion persists at high energies despite the high degree of cooling. (AIP)
On the validity of the power gap model in the strong coupling case
Authors: Schinke Reinhard
Published: February 2003

The validity of the power gap model to fit rotationally inelastic integral cross sections is investigated for the systems He–Na2 and Ne–Na2 for which pronounced rotational rainbow features have been reported in previous studies. The theoretical data suggest that a two parameter model is necessary to describe the regions of low and high transitions simultaneously. The branching occurs at the transition from the classically allowed to the classically forbidden (E, ΔE) region.
Identification of higher order clusters in charge compensated materials using three-body energy transfer
Authors: Seelbinder Mark B. , Wright John C.
Published: February 2003

Alkaline earth fluoride crystals doped with trivalent lanthanide ions exhibit defect clustering at relatively low dopant concentrations. The ions within these clusters can efficiently exchange energy with each other. These energy transfer processes are used to correlate the sites observed for one ion with those observed for another. Three-body energy transfer has been isolated in CaF2:Tb3+, Yb3+ where up-conversion cannot be achieved with normal two-body energy transfer. Only trimers or higher order clusters contribute to this up-conversion and the spectral features corresponding to these clusters can therefore be identified.
Elastic and flow properties of biaxial nematics
Authors: Saupe A.
Published: February 2003

Elastic and hydrodynamic properties of biaxial nematics of D2 symmetry are theoretically studied. The treatment follows closely the classical formalism used for solids which is extended by inclusion of director fields. The resulting theory is a generalization of the well-established theory of uniaxial nematics. The elasticity and viscosity coefficients have similar physical meanings. The theory for incompressible biaxial nematics contains 12 relevant elastic constants and 12 viscosity coefficients. Three of the elastic constants can be assigned to twist deformations, six to bend deformations, and three to coupling between bend and twist. Nine of the viscosity coefficients can be determined in simple shear flow experiments. The remaining three describe the friction connected with the rotation of alignment axes.
Quantum statistical mechanical model for polarizable fluids
Authors: Ho/ye J. S. , Stell G.
Published: February 2003

The quantum statistical mechanics of a polarizable fluid model is considered using a path-integral approach. The quantum mechanical partition function associated with the internal degrees of freedom of each molecule is approximated by a classical partition function of a polymer ring, while the center-of-mass motion of each molecule is treated classically. The resulting system of particles can be described by an Ornstein–Zernike equation, which we solve analytically in the mean spherical approximation. We give the dielectric constant, free energy, and internal energy of our model in both its continuum-fluid and lattice-gas versions. (In the former we assume hard-sphere cores; in both versions we take harmonically oscillating dipole moments as characterizing the internal degrees of freedom, with ideal dipole–dipole intermolecular coupling.)
Statistics of polymer chains embedded on a diamond lattice. I. Exact results for the lattice version of the freely rotating chain
Authors: Thorpe M. F. , Schroll W. K.
Published: February 2003

A polymer of finite length is embedded on a diamond lattice where the angle between adjacent monomers is cos−1(−1/3) = 109°. We show that the characteristic function Cn(k) can be calculated in closed form for an n link chain if all configurations (i.e., trans and gauche) are given equal weights. It is necessary to do a spherical average to get rid of the cubic symmetry artifically imposed by the diamond lattice. This represents the only model, other than the freely jointed chain, for which the characteristic function is known in closed form. This model may be regarded as the lattice version of the freely rotating chain. From the characteristic function we extract the first few moments 〈R2〉n, 〈R4〉n, and 〈R6〉n. We show that the results for the second and fourth moments are identical to those for the freely rotating chain, but the sixth moment is different. Various approximations to the probability distribution function Wn(R) [the Fourier transform of Cn(k)] are tested against the exact result.
Electronic excitations of benzene, pyridine, and pyrazine adsorbed on Ag(111)
Authors: Avouris Ph. , Demuth J. E.
Published: February 2003

Using high resolution electron energy loss spectroscopy (EELS), we have investigated the electronic excitations of benzene, pyridine, and pyrazine adsorbed on an Ag(111) surface under ultrahigh vacuum conditions. The molecular orientation of the adsorbed molecules is also deduced from a study of their vibrational spectra. Several electronic excitations are observed for each molecule which can be assigned as metal-perturbed intramolecular transitions. Both singlet and triplet (π,π*) states are observed. The adsorption on the metal surface imparts minimal shifts on the transition energies of these excitations from their gas phase values. The observed energy shifts for these excitations are in dissagreement with the predictions of classical image field theory. In addition to the above ’’intramolecular’’ excitations, we find, in the case of pyridine and pyrazine, a low energy (∼2−2.5 eV), broad, onset-like feature, which does not have a free molecule analog. Our studies show that this transition is present only for molecules in direct contact with the surface and is assigned to a metal-molecule charge-transfer (CT) state. The possible involvement of these CT states in the surface enhanced Raman scattering (SERS) process is discussed. It is suggested that resonant excitation of these CT states enhances the Raman cross section of the adsorbed species. The short range enhancement in SERS thus could be accounted for by the enhancement resulting from CT state excitation. Finally, the excited state lifetime of adsorbed 1B2u pyrazine is deduced from a study of vibronic broadening as a function of the number of adsorbed molecular layers. Very short T2 ∼5×10−15 s (first layer) and ∼3×10−14 s (second layer) relaxation times are deduced. Although these lifetimes are comparable to those predicted by classical theory, other processes of energy transfer to the metal (such as electron–hole pair formation near the metal surface), which are not included in the classical theory, may contribute to this lifetime.
Photolytic spectroscopy of simple molecules. III. The selective photolysis of CsKr and Cs2Kr at visible wavelengths
Authors: Collins C. B. , Lee F. W. , Golnabi H. , Davanloo F. , Vicharelli P. A. , Popescu D. , Popescu Iovitzu
Published: February 2003

In this work a two-photon technique was used for the measurement of the relative cross sections for the selective photolysis of an equilibrium distribution of CsKr and Cs2Kr populations existing at 523 °K in the presence of 0.132 amagat Kr. The cross sections for photolysis that selectively produced the lowest four excited states of Cs were obtained. Over the wavelength range 420 to 660 nm, the resulting dispersion curves showed one major feature which could be attributed to CsKr, the 6sσ→ 5dσ band dissociating to produce Cs(5 2D5/2). Various considerations indicated that the other structures observed in the spectrum belonged to the trimer Cs2Kr formed from Cs2 through the attraction of a Kr by polarization forces. The occurrence in the spectra of both CsKr and Cs2Kr of relatively broad photolysis bands leading to extremely selective distributions of product state populations was found to be fairly common. As in the previous work with Cs2 it was observed that adiabatic and, hence, state-selective dissociation occurred if the photolysis was excited at a transition energy not too much in excess of the threshold for the process. An examination of the kinetics of the photolysis products gave rate coefficients for the mixing and quenching of the fine structure components of Cs (5 2D) of 7×10−12 and 3.7×10−13 cm3 sec−1, respectively. At the densities used in this experiment these implied that useful populations of single states of Cs could be photolytically pumped with broadband radiation for periods of the order of tens of nanoseconds, while populations of energy levels could be pumped for more than an order of magnitude longer period of time.
Rotation–vibration analysis of the B 0+u–a 1g and B 0+u–a′ 0+g electronic systems of I2 by laser-induced-fluorescence Fourier-transform spectroscopy
Authors: Churassy S. , Martin F. , Bacis R. , Vergès J. , Field R. W.
Published: February 2003

The spectrum of fluorescence excited by the 514.5 nm line of a multimode Ar+ laser is recorded by a Fourier-transform spectrometer. In the region 7000–7300 cm−1 (1.43 to 1.37 μm), transitions from the B 0+u state are recorded into two states, which had been previously only indirectly observed, as well as into the X 1Σ+ state. Vibrational levels 0–9 for the 0+g and 0–14 for the 1g states are sampled. The principal molecular constants determined are as follows:         where all constants are in cm−1 except when noted, with 2σ uncertainties in parentheses.
Electric dipole moment of H2O in the ν2 excited vibrational state
Authors: Kuze Hiroaki , Amano Takayoshi , Shimizu Tadao
Published: February 2003

The Stark effect of the 41,4←32,1 and 42,3←33,0 microwave transitions in the ν2 excited vibrational state of H2O has been measured. The analysis is made with the rotational correction terms brought by the centrifugal distortion in the Stark effect Hamiltonian. The dipole moment free from the effect of the molecular rotation is determined to be 1.827±0.005 D.
A coupled-channel study of electronic–vibrational– translational energy exchange in nonadiabatic collisions
Authors: Becker C. H. , Saxon R. P.
Published: February 2003

Fully quantum mechanical coupled-channel calculations of electronically nonadiabatic collisions in the presence of many vibrational states have been performed over a range of collision energies from 0.27 to 43.5 eV. The calculations were based on simplified model potentials for K+O2, considering the doublet potential curves arising from the ground electronic neutral K(2S)+O2(3𝒥g−) and ionic K+(1S)+O2−(2Πg) states, and assuming isotropic interaction. Comparison of calculated scattering properties and recent experimental results at hyperthermal energies for K+O2 indicates that the simplified model interaction nevertheless gives a good physical description. This work investigates the collision dynamics, while providing a reference for other more approximate theoretical approches. Processes leading to neutral and ionic products are discussed for this system where many vibronic channels are strongly coupled.
State selected ion–molecule reactions by a TESICO technique. III. H2+(v)+Ar→ArH++H, Ar++H2: Observation of enhanced charge-transfer cross sections at near resonance
Authors: Tanaka Kenichiro , Kato Tatsuhisa , Koyano Inosuke
Published: February 2003

Reactions of the H2+ ions with Ar, i.e., H2++Ar→ArH++H [proton transfer, Reaction (1)] and H2++Ar→Ar++H2 [charge transfer, Reaction (2)] have been studied for the individual vibrational states v (v = 0–4) of the reactant ion at collision energies of 0.48, 0.77, and 1.24 eV, utilizing the threshold electron–secondary ion coincidence (TESICO) technique. At 0.77 and 1.24 eV, the cross sections for Reaction (1) are almost independent of the vibrational energy or slightly increase with increasing vibrational energy, wheras those for Reaction (2) show a resonance-like enhancement at v = 2 . At the lowest collision energy studied (0.48 eV), however, the enhancement of the charge transfer cross section at v = 2 is accompanied by a concurrent decrease in the proton transfer cross section, indicating the competitive nature of the two channels at this collision energy. These results are consistent with the trajectory–surface hopping model of the ArH2+ system by Chapman and Preston. Reaction (2) has also been studied at a higher collision energy of 19 eV, where the endoergic reaction with v = 1 is found to be considerably enhanced and has a cross section close to that for the near-resonant reaction with v = 2.
Improved a
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i
t
i
o effective potentials for Ar, Kr, and Xe with applications to their homonuclear dimers
Authors: Christiansen Phillip A. , Pitzer Kenneth S. , Lee Yoon S. , Yates John H. , Ermler Walter C. , Winter Nicholas W.
Published: February 2003

Effective core potentials for the Ar, Kr, and Xe atoms derived from numerical Hartree–Fock and Dirac–Hartree–Fock wave functions are applied in SCF and CI calculations of homonuclear diatomic potential energy curves. Detailed comparisons are made with the all-electron calculations of Wadt for the ground and lowest positive states. Relativistic effects, excluding spin–orbit coupling, are seen to be relatively unimportant. Plots of the potential energy curves and computed spectroscopic constants show excellent agreement with the all-electron results. On the other hand, comparisons with results obtained using effective potentials derived using varients of Phillips–Kleinman procedures show dramatic differences for Xe2 and Xe2+. From SCF calculations on Xe2 and Xe2+ it was found that the explicit inclusion of the spin–orbit operator in the SCF procedure (using ω–ω coupling) results in essentially the same potential curves obtained by adding the spin–orbit correction as a final semiempirical perturbation.
Intrinsic convergence in closed-shell SCF calculations. A general criterion
Authors: Stanton Richard E.
Published: February 2003

It is shown that the intrinsic convergence of the classical SCF algorithm for closed shells is governed by the eigenvalues of a supermatrix Q whose elements involve orbital excitation energies and electron repulsion integrals over occupied and virtual orbitals. In general, the SCF process is intrinsically convergent if all eigenvalues of Q have absolute values less than unity, and intrinsically divergent if one ore more is greater than unity. It is also shown how the symmetry preserving features of the SCF algorithm enable one to avoid certain divergences provided one starts a calculation with symmetry adapted orbitals. In cases of intrinsic convergence it is proved that the ratio of successive energy increments produced by the SCF algorithm is equal to the square of the largest eigenvalue of Q. The implications of the theory are illustrated by calculations on linear H8, on the cyclic planar ion S4N3+, and on model Q matrices filled with random numbers over the interval (−0.5,0.5).
Effective core potentials for fully relativistic Dirac–Fock calculations
Authors: Ishikawa Yasuyuki , Malli Gulzari
Published: February 2003

A fully relativistic effective core potential method has been developed for use in the valence-only Dirac–Fock self-consistent-field calculations for atoms and molecules. The effective potentials are constructed so that the atomic valence-only Dirac–Fock calculations reproduce the energies and the outer region of the corresponding valence spinors obtained from the full Dirac–Fock SCF calculations.
Some topics in the molecular dynamics ensemble
Authors: Lado F.
Published: February 2003

The molecular dynamics method generates averages in an ensemble with constant particle number, volume, energy, and total momentum. For this ensemble, we obtain the exact ideal gas partition function and momentum distribution function, show that a ’’correction’’ applied in the past to equation-of-state data of hard core particles is not needed, and rederive a known relationship between potential energy fluctuations and the heat capacity.
On the efficiency of thermal engines with power output: Consideration of inertial effects
Authors: Fairén Victor , Ross John
Published: February 2003

We consider an idealized heat engine with nonzero power output, which receives heat from and rejects heat to reservoirs by linear heat conduction and produces work in the surroundings by providing kinetic and potential energy to a mass in the gravitational field. The working fluid of the engine is in internal equilibrium. For this engine we write the equations of motion including inertial effects, i.e., we include Newton’s equation for conservation of momentum. We define variables, controls, parameters, and boundary conditions in these equations in connection with the optimization of some performance (cost) functional. The conceptual neglect of inertial effects, as has been done previously, is shown to change the set of variables and controls, thus changing the optimization problem. Furthermore, the neglect of these terms leads to requirements of energy balance which must be incorporated as boundary conditions. We do not solve any optimization problem explicitely but show that appropriate inclusion of inertial effects in such engines precludes in the optimal cycle the existence of isothermal branches which are not preceded and followed by acceleration and deceleration branches.
On the efficiency of thermal engines with power output: Harmonically driven engines
Authors: Fairén Victor , Ross John
Published: February 2003

In a previous article we defined and studied a model of a heat engine with the purpose of emphasizing the role of inertial effects, particularly their importance in relation to optimization problems. Here we pursue our argument on this issue by comparing the performance of models of heat engines harmonically driven around a state of equilibrium. For the first model, with inertia, we define the model, calculate its nonlinear response with emphasis on the linear approximation, and discuss the issues related to the coupling of the thermal and mechanical driving forces. We study the influence of increasing values of the mechanical friction coefficient, and show that when this coefficient is small the work output displays subharmonic resonances that disappear when the friction coefficient increases. In the second model, without inertial effects, no such resonances appear as expected, since these are due only to the inertial terms.
The Fokker–Planck–Langevin model for rotational Brownian motion. III. Symmetric top molecules
Authors: McClung R. E. D.
Published: February 2003

The general series expansion for the angular velocity–orientation conditional probability density for a fluid composed of symmetric top molecules is derived, and expressions for the reorientational correlation functions, correlation times, spectral densities, angular velocity correlation functions, and the correlation times relevant to magnetic relaxation via spin–rotational interactions are presented. Numerical calculations of the reorientational correlation functions, memory functions, spectral densities, and correlation times indicate that these properties are sensitive to the anisotropy of the angular velocity correlations produced by an anisotropic friction tensor in the rotational Langevin equation.
Structure of bimetallic clusters. Extended x-ray absorption fine structure (EXAFS) studies of Os–Cu clusters
Authors: Sinfelt J. H. , Via G. H. , Lytle F. W. , Greegor R. B.
Published: February 2003

An investigation was conducted of the extended x-ray absorption fine structure (EXAFS) associated with the Liii absorption edge of osmium and the K absorption edge of copper in silica supported osmium–copper catalysts with a 1:1 atomic ratio of copper to osmium. The effect of the copper on the Liii x-ray absorption threshold resonance of the osmium was also investigated. Chemisorption and catalysis studies had earlier indicated a strong interaction between osmium and copper in such catalysts, and led to the proposal that bimetallic clusters of osmium and copper were present on the silica. Information on the structure of the osmium–copper clusters has been obtained from the EXAFS data of the present investigation. The osmium atoms in the clusters are coordinated to a large extent to other osmium atoms, whereas the copper atoms are coordinated extensively to osmium atoms as well as to copper atoms. The results are consistent with a model in which the copper in the clusters is present at the surface and are very similar in this regard to our earlier reported results on ruthenium–copper clusters. In addition to the structural information derived from the EXAFS data, some indication of electronic interaction between osmium and copper was obtained from a comparison of the osmium Liii x-ray absorption threshold resonances in osmium and osmium–copper clusters. The copper appears to contribute electrons to unfilled d states of the osmium.
A semiempirical estimate of the ground state polarizability of atomic iodine
Authors: Drühl Kai
Published: February 2003

We estimate the polarizability α of atomic iodine by a semiempirical method using the oscillator strength sum rule for the valence electrons and empirical term energies. Values obtained are 9.3×10−24 cm3 <α<11.6×10−24 cm3. Furthermore, we obtained a lower bound on α in terms of an average transition energy E≳ which is rigorous in the approximation where the effect of shell electrons on the core is neglected. This bound is α≳7.4×10−24 cm3 for E≳ = 6.9×104 cm−2. All values are significantly larger than results calculated previously.
Systematics of Rydberg structure in the spectra of the rare gas dimers between the atomic 2Po3/2 and 2Po1/2 fine-structure thresholds
Authors: Dehmer P. M. , Pratt S. T.
Published: February 2003

The relative photoionization cross sections for the rare gas dimers Ar2, Kr2, and Xe2 were measured in the energy region between the respective atomic 2P3/2o and 2P1/2o fine-structure thresholds at wavelength resolutions of 0.15 Å for Ar2, 0.28 Å for Kr2, and 0.45 Å for Xe2. Discrete autoionization structure was observed for all three systems and has been partially analyzed in terms of Rydberg series converging to the C 2Π1/2u state of the molecular ions. Vertical binding energies for the individual Rydberg states were estimated based on the assumption that the Rydberg states dissociate to a rare gas atom in an ns′[1/2]1o or nd′[3/2]1o excited state plus a rare gas atom in the 1S0 ground state. The binding energies thus obtained are in excellent agreement with the corresponding values for the C 2Π1/2u ionic states obtained from previously reported photoelectron spectra. Some of the molecular structure exhibits Beutler–Fano profiles similar to those observed in the nd′[3/2]1o atomic series. This is in contrast to the energy region near the first ionization threshold, where it is extremely difficult to find a simple correlation between the atomic and molecular Rydberg structure.
The free jet cooled, laser induced fluorescence spectrum of s
y
m-triazine
Authors: Heaven M. , Sears T. , Bondybey V. E. , Miller Terry A.
Published: February 2003

A laser induced fluorescence excitation spectrum has been obtained for sym-triazine seeded in a free jet expansion in He. Under these conditions, the translational, rotational, and (probably) vibrational temperature of the sym-triazine is found to be 1 °K. This low temperature yields a greatly simplified vibronic spectrum for the Ã 1E″↔X̃ 1A′1 transition, with implications for vibrational assignments and the interpretation of the Jahn–Teller and pseudo-Jahn–Teller effect in the Ã 1E″ state. Completely resolved R and P rotational structure has been observed for selected vibronic transitions. Lifetime and collisional quenching cross sections have also been obtained for selected vibronic levels of the Ã 1E″ state.
Raman study of molecular motions in liquid phosphorous trichloride
Authors: Amorim da Costa A. M.
Published: February 2003

Isotropic and anistropic Raman band shape analyses of the ν2(a1) mode (the Cl–P–Cl bending) of phosphorous trichloride are reported for the pure liquid and carbon tetrachloride solutions. The experimental values are compared with those calculated for the free gas molecule reorientation and the angular velocity correlation times and with those obtained from Stokes–Einstein theory. It is shown that the PCl3 molecule tumbling about the axes perpendicular to the C3 axis in the pure liquid proceeds by small-step Brownian diffusion and is associated with an Arrhenius activation energy of 7.1 kJ mol−1, in good agreement with reported NMR data. The vibrational linewidth of the ν2(a1) mode does not depend strongly on temperature and concentration. Intramolecular vibrational energy exchange with nearby states is assumed as the main fraction of the vibrational relaxation of this mode.
The structure of Ar–N2O
Authors: Joyner C. H. , Dixon T. A. , Baiocchi F. A. , Klemperer W.
Published: February 2003

The rotational spectra of two isotopes of the weakly bound complex Ar–N2O have been measured by molecular beam electric resonance spectroscopy. The following spectroscopic constants and structural parameters were derived from the observations:           In the average semirigid structure, the Ar is 3.460(5) Å distant from the center of mass of N2O and 7.4°(2) to either side of a line perpendicular to the N2O axis at the center of mass. Analysis of the moments of inertia of the two isotopic species and the measured dipole moments, with a classical harmonic bender model, indicates that the equilibrium Ar–N–O angle is 87±1.5° and that the sign of the N2O dipole moment is +NNO−.
Structure determination of SO2 by electron diffraction
Authors: Holder C. H. , Fink M.
Published: February 2003

A determination of the structural parameters of gaseous sulfur dioxide has been made with counting electron diffraction apparatus at an electron energy of 54.157 keV over the s range from 10 to 30 Å−1. The structure parameters are found to be ra(S–O) = 1.4343(3) Å, ra(O⋅⋅O) = 2.4718(28) Å, l(S–O) = 0.0359(10) Å, l(O⋅⋅O) = 0.0561(31) Å, and ∢ OSO = 119.5 °(0.3 °). The quoted uncertainties are 2σ. Since the experimental results agree very well with the spectroscopic and theoretical structure parameters, it is proposed that SO2 be accepted as the standard reference molecule in place of CO2.