bluuebunny/crossref_metadata_2025 huggingface dataset:
Dimensional interpolation for two-electron atoms
Authors: Herschbach D. R.
Published: July 2002

The ground-state electronic energy of helium-like atoms is calculated by interpolating between exact solutions for the limiting cases of one-dimensional and infinite-dimensional systems. In suitably scaled coordinates, the D=1 limit corresponds to replacing the Coulombic potentials by delta functions. With another choice of scaling, the D=∞ limit reduces to a classical electrostatic problem; the electrons take fixed positions that correspond to replacing wave functions with delta functions. The ground-state energy for arbitrary dimension D and nuclear charge Z can be represented as εD=ε∞[1+c1/D+c2/D2+F(D,Z)/D3]. The first three terms are obtained from a perturbation expansion about D=∞; these correspond, respectively, to the limiting rigid electronic structure (envisioned by G. N. Lewis!) and to harmonic and anharmonic vibrations about that structure. The interpolation function F(D,Z) is approximated as a geometric series, determined by fitting the accurately known energies for the D=1 limit and for D=5. This procedure yields an accuracy of 2 parts in 105 or better for D=3 and Z≥2. Study of the Z dependence reveals that at D=1 the perturbation expansion in powers of 1/Z has a self-similar structure. Also, for each D there exists a critical nuclear charge Z*, with magnitude near 12, for which F(D,Z*)=0; the first three terms of the expansion about D=∞ then give the exact energy.
On the multidimensional surface intersection problem and classical trajectory surface hopping
Authors: Mead C. Alden , Truhlar Donald G.
Published: July 2002

We comment on a paper [J. R. Stine and J. T. Muckerman, J. Chem. Phys. 65, 3975 (1976)] of the same title. We point out that their attempted reduction of a locally avoided intersection of two potential energy surfaces to a one-dimensional problem is incorrect.
Determination of the equilibrium structure of protonated nitrogen by high resolution infrared laser spectroscopy
Authors: Owrutsky J. C. , Gudeman C. S. , Martner C. C. , Tack L. M. , Rosenbaum N. H. , Saykally R. J.
Published: July 2002

We report the measurement of new high-J transitions (up to J=41) in the fundamental N–H stretch band and of the 1110–0110 bending hot band by velocity modulation infrared laser spectroscopy. These data were combined with measurements of the 100, 0110, and 001 fundamentals of both HNN+ and DNN+, and of the 1110–0110 hot band of DNN+, and of microwave ground state rotational transitions, made by several groups, in a simultaneous weighted least-squares analysis; from this analysis, the equilibrium structure of protonated nitrogen was determined to be: rNH =1.033 59(43) Å, rNN =1.092 766(92) Å. Several vibration–rotation and anharmonicity constants were determined; comparisons with high level ab initio calculations are presented.
New parametric analysis of the optical spectra of U4+ in ThSiO4
Authors: Khan Malek C. , Krupa J. C.
Published: July 2002

Crystals of ThSiO4–U4+ were grown by the flux method. Absorption and emission spectra were recorded from 4000 to 30 000 cm−1 at liquid helium and various higher temperatures. The transitions were assigned and fit to a semiempirical Hamiltonian with adjustable parameters via a least squares procedure. The crystal field parameters for U4+ in ThSiO4 were compared to those for U4+ in other hosts.
Vacuum ultraviolet laser spectroscopy. IV. Spectra of Kr2 and constants of the ground and excited states
Authors: LaRocque P. E. , Lipson R. H. , Herman P. R. , Stoicheff B. P.
Published: July 2002

Krypton dimers formed by supersonic jet expansion have been excited with tunable, and monochromatic VUV radiation leading to isotopically resolved fluorescence excitation spectra of three band systems at 125, 124, and 117 nm. Unambiguous upper-state vibrational numbering was possible, resulting in evaluation of the spectroscopic constants for the three lowest excited states. From system I at ∼125 nm, with bands v′=32 to 38 and v″=0, constants for the A 3Σ+u state of Kr2 were found to be T′e =74 303.9(8.0), ωe =244.60(43), ωex′e =2.6045(60), and De=5807.0(8.0) cm−1. Band system II at ∼124 nm, with bands v′=30 to 38 and v″=0, led to constants for the B 1Σ+u state, T′e =75 426.8(1.7), ωe =219.50(10), ωex′e =2.2325(15), and De =5629.1(2.5) cm−1. Improved values for constants of the C 1Σ+u state were obtained from analysis of band system III at ∼117 nm (with bands v′=0 to 5, and v″=0 to 2): T′e =85 520.60(10), ωe =43.82 (10), ωex′e =1.812(18), and De =465.3(2.0) cm−1. Vibrational constants for the ground state were obtained from bands with v″=0, 1, 2, namely ω″e =23.53(2) cm−1 and ωex″e =1.07(1) cm−1. Analyses of resolved rovibronic structure for all three band systems yielded values of rotational constants for the ground and three excited states, with the equilibrium internuclear distance r″e =4.017(12) Å for 84Kr2. These new data were used to derive potential energy curves.
Three-dimensional analytical model for the photodissociation of symmetric triatomics. Absorption and fluorescence spectra of ozone
Authors: Atabek O. , Bourgeois M. T. , Jacon M.
Published: July 2002

A full three-dimensional analytical Franck–Condon analysis is presented for dissociative spectroscopies of a symmetric triatomic system. Formulas derived for harmonic potential surfaces are applied to the theoretical calculation of ozone absorption spectrum in the Hartley continuum as well as to the determination of its fluorescence (or resonance Raman) spectrum when submitted to 266 nm wavelength excitation. The Hartley absorption line shape describing the 1 1B2←X 1A1 photodissociation is reproduced within very good agreement with experiment. The role of the so-called Duschinsky effect originating from the coupling of bending and stretching motions in the excited state is analyzed and comparisons are presented with previous more simple models neglecting this coupling. The influence of temperature is also taken into account through calculations dealing with vibrationally excited species. As for the photoemission fluorescence spectrum carrying a rich information content, the behavior is successfully predicted in its main lines, relative intensities of overtone progressions and of combination bands are fairly well reproduced.
Rovibrational analysis of an intermolecular hydrogen-bonded vibration: The ν16 band of HCN---HF
Authors: Wofford B. A. , Jackson M. W. , Bevan J. W. , Olson W. B. , Lafferty W. J.
Published: July 2002

The infrared spectrum of the intermolecular bending vibration, the ν16 band, of the heterodimer HCN---HF has been obtained with 0.010 cm−1 resolution, and the rotational structure of this band has been assigned. The spectroscopic constants of the ν16 state in cm−1 are: ν0=550.0285(2); B6=0.117 652 9(10); D6J =0.2791(5)×10−6; q6=0.579(8)×10−4; α6=−0.002 137(1), where the uncertainties cited are one standard deviation.
Conformational kinetics of methyl nitrite. IV. Bimolecular kinetics at low pressures
Authors: Chauvel J. Paul , Friedman B. Ri. , True Nancy S.
Published: July 2002

Rate constants and associated kinetic parameters for the methyl nitrite conformational interconversion process have been determined at low pressures (∼5–70 Torr) where the kinetics are very nearly second order. Comparisons with model calculations demonstrate that vibrational redistribution is statistical and very nearly ergodic in methyl nitrite molecules possessing ∼12 kcal mol−1 internal energy.
Measured variation in the autoneutralization lifetime of SF−6 by direct measurement of neutral intensity
Authors: Delmore J. E. , Appelhans A. D.
Published: July 2002

The autoneutralization lifetime of SF−6 has been studied by measuring the neutrals produced in a beam of SF−6 ions as a function of distance along the flight path. The measurement of neutrals (as opposed to ions) eliminated uncertainties encountered in previous investigators measurements. The beam was sharply focused, and the ion and neutral components were separated at various points along the flight path via electrostatic or magnetic fields. The neutral beam intensity was measured utilizing a multichannel plate array detector and photometer. The autoneutralization lifetime was measured as a function of pressure, acceleration voltage, and ion source temperature/photon flux. The effective lifetime increased from 15 to 20 μs as the ion source temperature/photon flux was reduced. Although the individual data sets exhibit a near single exponential behavior, composite analysis indicates multiple states (lifetimes) are active and that the process must involve a mixture of at least four states. The effect of temperature/photon flux on the effective single exponential lifetime implies that the distribution of energy states can be controlled to vary the composite autoneutralization rate.
Molecular jet study of solvation of pyrazine by small hydrocarbons
Authors: Wanna J. , Bernstein E. R.
Published: July 2002

Supersonic molecular jet 2-color time-of-flight mass spectroscopy studies are reported for pyrazine solvated by methane, ethane, and propane. The results parallel those obtained for benzene and toluene solvated by these molecules. The presence of nitrogen atoms and lone pair electrons in the aromatic system has only a small effect on the cluster structure. Lennard-Jones potential calculations are presented along with the experimental data. Cluster shifts and binding energies are reported and correlated with geometries. The nucleation process for cluster formation appears to involve solvent dimers, as was found for the aromatic hydrocarbon systems.
Temperature dependence of association of diatomic ions in diatomic gases
Authors: Johnsen Rainer
Published: July 2002

The temperature dependence of experimental rate coefficients for association of diatomic ions in diatomic gases is compared to model calculations in which the stabilization of the initially formed unstable association complex is assumed to occur by formation of a ‘‘super complex.’’ It is shown that acceptable agreement between the model and experimental data on four sample reactions can be obtained with a minimum of plausible assumptions. The agreement indicates that earlier theoretical models may provide an accurate description of the formation and destruction of the association complex, but failed to reproduce the experimental data, because the temperature dependence of the stabilization step was inadequately treated.
A hybrid approach to modeling the dynamics of macromolecules
Authors: Thacher Thomas , Ganesan Shridar , Askar Attila , Rabitz Herschel
Published: July 2002

A new approach to modeling macromolecular systems is presented which can be used to study both the low frequency collective motion of the entire macromolecule and the high frequency motion of local regions within the molecule. The approach combines the classical atomistic model of the molecule with a continuum model to form an overall classical hybird model. This hybrid model is developed by using a long wavelength limit to the discrete equations of motion to obtain a field of local elastic modulus tensors which depend on the intramolecular potential and molecular configuration. These tensors can then be used in a qualitative manner to correlate the elasticity of different regions in the macromolecule with the dynamics of the molecule as a whole. In addition, the tensors obey specific symmetry conditions which can be utilized to identify regions of the macromolecule that are displaced from equilibrium. The field of tensors can also be used in a quantitative manner to decrease the number of degrees of freedom for the macromolecule by the lumping of regions of uniform elasticity as dictated by the field. The symmetry properties may also be used to obtain equations of motion corresponding to an equilibrium configuration and an intramolecular potential which are consistent with each other. This latter result is important in the cases where the molecular configuration in the atomistic model does not properly correspond to the equilibrium configuration for the intramolecular potential. The applicability of the tensor as an analytic tool for investigating macromolecular systems is illustrated using several molecules and molecular analogs.
Long-lived high energy vibrational levels in benzene vapor
Authors: Chernoff Donald A. , Myers James D. , Pruett J. Gary
Published: July 2002

Single vibronic level fluorescence spectra from the 6112 level in 1B2u benzene and benzene-d6 in a free jet expansion include sharp transitions to S0 levels identified as high normal mode overtones of the C–C ring breathing mode up to 10 000 cm−1. For these levels, and for the reported method of state preparation, the upper bound for the rate of intramolecular vibrational redistribution is kIVR <1012 s−1. This rate is several times smaller than for C–H or C–D stretch local mode levels of similar energy when they are prepared via overtone absorption from the ground state.
A model potential for Rydberg states of alkali atoms in rare gas matrices
Authors: Schwentner N. , Chergui M.
Published: July 2002

The term values of the ground and excited Rydberg states of alkali metal atoms trapped in rare gas matrices are derived from an empirical model potential. The analytical potential smoothly connects the Coulomb potential near the core with the shielded Coulomb potential beyond the first shells of surrounding matrix atoms. The potential does not depend on the specific dopant and is determined for each rare gas matrix in a unique way from the polarization energy of the crystal by the ionic core P+ and from the electron affinity V0. The predicted transition energies agree with the experimental data within 0.1 to 0.2 eV.
ENDOR/ESR of Mn atoms and MnH molecules in solid argon
Authors: Van Zee R. J. , Garland D. A. , Weltner W.
Published: July 2002

Mn atoms and MnH molecules, the latter formed by reaction between metal and hydrogen atoms, were trapped in solid argon and their ESR/ENDOR spectra measured at 4 K. At each pumping magnetic field two ENDOR lines were observed for 55Mn(I=5/2) atoms, corresponding to hyperfine transitions within the MS =±1/2 levels. Values of the hyperfine interaction constant and nuclear moment of 55Mn were derived from the six sets of data. For MnH, three sets of signals were detected: a proton ‘‘matrix ENDOR’’ line, transitions in the MS =0,±1 levels involving MI (55Mn)=1/2, 3/2, 5/2 levels, and proton transitions corresponding to νH and νH±aH. Analysis yielded the hyperfine constant aH =6.8(1) MHz and the nuclear quadrupole coupling constant Q′(55Mn)=−11.81(2) MHz. The latter compared favorably with a theoretical value derived earlier by Bagus and Schaefer. A higher term in the spin Hamiltonian appeared to be necessary to fit the proton hyperfine data.
Dynamics of the laser-induced thermal desorption of nitric oxide from a platinum foil
Authors: Burgess D. , Mantell D. A. , Cavanagh R. R. , King D. S.
Published: July 2002

The internal and translational energy distributions of nitric oxide, which was desorbed from a cold polycrystalline platinum foil by laser-induced thermal desorption, were measured using a laser-excited fluorescence, time-of-flight technique. Under irradiation conditions which are estimated to produce a maximum surface temperature of 320 K, desorbed NO in quantum states with 25<ER<2162 cm−1 were represented by two distributions of molecules: a translationally energetic component with a mean kinetic energy 〈ET〉/2k which continuously increased from 1400 to 2650 K with increasing ER, a rotational temperature of TR=410 K, and a vibrational temperature of TV≈900 K; and a slower component with 〈ET〉/2k=300 K and TR=170 K.
Photodissociation dynamics of negative ion clusters: (SO2)−2
Authors: Kim Hyun-Sook , Bowers Michael T.
Published: July 2002

The photodissociation dynamics of (SO2)−2 to form SO2/SO−2 products is investigated over the wavelength range 656 (1.89 eV) to 458 nm (2.71 eV). Product angular distributions are obtained. An asymmetry parameter analysis indicates the lifetime of the (SO2)−2 photoexcited state is much less than a rotational period. Product kinetic energy distributions are obtained at all wavelengths. Both the overall shape of these distributions and comparison with statistical phase space theory calculations indicate the excited state assessed by the photon is repulsive consistent with the asymmetry parameter analysis. An impulsive model analysis suggests the bond between the two SO2 moeities in (SO2)−2 is probably between one oxygen atom on each moeity and the structure is quasilinear. Structure is also observed in the kinetic energy distributions. It is suggested this is due to selective photoexcitation of vibrational states of either the SO2 or SO−2 moeity in (SO2)−2. Hodges and Vanderhoff have reported a bimodel photodestruction cross section for (SO2)−2, with major peaks near 600 and 400 nm. We argue these peaks are due to photodissociation of (SO2)−2 not photodetachment, with the first excited doublet state (repulsive) leading to the 600 nm peak and the second excited doublet state (bound) leading to the second maxima. The repulsive state must correlate to ground state SO2/SO−2 products and the bound state to electronically excited SO2/SO−2 products. Predictions regarding the dissociation dynamics of the bound state are made.
Model calculation of PDA–PA heterostructures: Quantum wells in conjugated molecular chains
Authors: Ruckh R. , Sigmund E. , Kollmar C. , Sixl H.
Published: July 2002

The energy band structures and the corresponding eigenfunctions of polymer chain heterostructures are calculated on the basis of the Hückel theory. The model structure is given by a coupled conjugated polymer system consisting of polydiacetylene (PDA) and polyacetylene (PA) units. The PA–PDA ratio is varied from 0% to 100%. Discrete split-off states within the energy gap and at the exterior of the bands are analyzed with respect to their energies and localization of the corresponding wave function on the PA or PDA units. Criteria for one-dimensional molecular quantum well structures and molecular electronic devices are discussed.
Decay kinetics of the ultraviolet and visible luminescences emitted by electron-irradiated crystalline H2O ice
Authors: Trotman S. M. , Quickenden T. I. , Sangster D. F.
Published: July 2002

Electron pulse irradiated samples of high purity, crystalline H2O ice at 88 K showed three kinetically distinguishable regions of luminescence emission at 280–340 nm (band I); 320–600 nm (band II); and 450–600 nm (band III). Band II emission was assigned to the A 2∑+→X 2Π transition of OH, the gas phase peak being shifted from 306.4 to ∼385 nm by the ice lattice. The decay half-life of the band II emission resulting from a single, ∼0.05 Mrad electron pulse, was 25±3 ns and increased steeply to 210±10 ns for the second pulse and then steadily decreased to 140±10 ns after 20 pulses. Band II emission from the second or later pulses was resolved into a short lived component with a decay half-life of ∼30 ns and a longer lived component with a half-life of ∼400 ns. The latter decay fitted a second order homogeneous rate equation in which the initial concentrations of the two reactants were in the ratio (2.6±0.1):1 and was attributed to the formation of excited OH by electron–ion recombination in the bulk ice. The short lived band II emission was also attributed to excited OH and probably arose from a mixture of a fast intraspur recombination reaction with some other process of different reaction order. Dose accumulation (memory) effects were attributed to the accumulation of OH radicals and lattice vacancies in the irradiated ice. The band III emission had a half-life of 25±5 ns and its decay kinetics were consistent with emission from species such as excited OH− or H3O produced when electrons tunnel from a trapping site to a geminate partner.
Excitation spectra of the n
p 3Πg, n′p 3Σ+g ←5s 3Σ+u(1u,0−u) Rydberg series of Kr2
Authors: Ediger M. N. , Eden J. G.
Published: July 2002

Excitation spectra of the 5s 3Σ+u(1u,0−u) state of the Kr2 molecule have been obtained in the wavelength interval 335≤λ≲545 nm by monitoring atomic Kr emission at 758.7 nm (5p[1/2]0→5s[3/2]1) that accompanies dye laser excitation of the neutral excited molecule. The dominant peaks in the spectrum are identified as arising from np 3Πg←5s 3Σ+u Rydberg transitions where 7≤n≤18. The series limit and quantum defect δ for this series have been determined to be 28 471.9±3.9 cm−1 (351.12±0.05 nm) and 2.629±0.004, respectively, and the vibrational frequency ωe of the 5s 3Σ+u(1u,0−u) excimer has been estimated to be ∼195 cm−1. All of the observed Rydberg states have an A 2Σ+1/2u[1(1/2)u] ion core. Comparison of the np 3Πg←5s 3Σ+u series limit and the ionization potential of  Kr* 5s[3/2]2 shows that the A 2Σ+1/2u ground state dimer ion is ∼0.55 eV more deeply bound than is the Kr2 5s 3Σ+u neutral species. Also, dissociation energies of the np 3Πg

Rydbergs are shown to be nearly identical to that of the A 2Σ+1/2u dimer ion state. A second series of spectral peaks has also been identified and assigned to n′p 3Σ+g ←5s 3Σ+u transitions. Analysis of the series yielded a limit of 28 424±120 cm−1 and a quantum defect of 3.39±0.04. Two members of a third, unidentified series having a quantum defect of 3.24±0.01 and also converging to A 2Σ+1/2u (v′=0) have been observed. Absolute photoabsorption cross sections for the np 3Πg←5s 3Σ+u (7≤n≤18) transitions have also been determined.
Magnetic birefringence measurement in hydrogen and deuterium gases
Authors: Scuri F. , Stefanini G. , Zavattini E. , Carusotto S. , Iacopini E. , Polacco E.
Published: July 2002

The first measurement of the magnetic birefringence (Cotton–Mouton effect) in molecular hydrogen and deuterium has been performed. The Cotton–Mouton constant of H2 and D2 has been measured at room temperature, atmospheric pressure, and for λ=514.5 nm. Comparison with theoretical calculations shows very good agreement in both cases.
Studies of electron-molecule collisions: Applications to e-H2O
Authors: Brescansin Luiz M. , Lima Marco A. P. , Gibson Thomas L. , McKoy Vincent , Huo Winifred M.
Published: July 2002

We report elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120°. The implication of this feature for the determination of momentum transfer cross sections is described.
Effects of periodic perturbations on the oscillatory combustion of acetaldehyde
Authors: Pugh Spencer A. , Schell Mark , Ross John
Published: July 2002

Periodic perturbations are applied to the input rates of acetaldehyde and oxygen, one at a time, in the combustion of acetaldehyde in a CSTR. We measure periodic responses in five entrainment bands as we vary the frequency and amplitude of the external periodic perturbation. Outside of entrainment bands we find quasiperiodic responses. Next-phase maps of the experimental results are constructed in real time and used in the observation and interpretation of entrainment and quasiperiodic behavior. Within the fundamental entrainment band, we measure critical slowing down and enhancement of the response amplitude. As the bath temperature is increased, so that the oscillatory system approaches a Hopf bifurcation, we observe an increase in the amplitude enhancement. The predictions of a five-variable thermokinetic model agree with the experimental results.
Effects of two periodic perturbations on the oscillatory combustion of acetaldehyde
Authors: Pugh Spencer A. , DeKock Bruce , Ross John
Published: July 2002

Periodic perturbations are applied to the input rates of acetaldehyde and oxygen simultaneously in the combustion of acetaldehyde in a CSTR. With the two perturbations at the same frequency, we measure bistability in entrainment and hysteresis as a function of the phase shift between the two perturbations. The application of a perturbation in the flowrate of one reactant to the system already entrained to a perturbation of the flowrate of the second reactant can cause the system: to become quasiperiodic in both perturbations; to become entrained to both perturbations; to remain entrained, but not phase locked, to the first perturbation; or to become quasiperiodic in the first perturbation but entrained to the second perturbation. We measure the effects of frequency modulated and amplitude modulated acetaldehyde flowrate perturbations; the results compare well with predictions made by a five-variable thermokinetic model.
Time resolved study of the Fermi component Ω−(Ag) in solid CO2 at T≂5 K
Authors: Ouillon R. , Ranson P.
Published: July 2002

We have investigated the vibrational coherence relaxation for the long-living Ω−(Ag) hybrid bound state of the Fermi diad ω1:2ω2 in natural crystalline CO2 at T≂5 K by means of time resolved nanosecond CARS spectroscopy. The decay profile was found to be slower than exponential and fitting reasonably well to an exp{−(t/τ)1/2} time dependence with τ=2.3±0.2 ns. Our results are discussed in terms of weak diagonal structural disorder upon the vibrational relaxation of exciton states located near the band edges.
Rotational spectrum and structure of a linear B2H6–H/DF dimer
Authors: Gutowsky H. S. , Emilsson Tryggvi , Keen John D. , Klots T. D. , Chuang Carl
Published: July 2002

Rotational spectra have been observed at 4–18 GHz for several isotopic species of a diborane–hydrogen fluoride complex, using a Flygare, Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the molecular source. The dimer is a near symmetric prolate top (κ=−0.9995); it has a linear BB–H/DF equilibrium structure with the H/D end of the H/DF attracted axially to one of the diborane’s terminal BH2 groups. The B, C, DJ, and DJK rotational constants are: for 11B2H6–HF, 2111.601(1) and 2091.308(1) MHz, and 5.83(3) and 46.1(5) kHz; for 11B2H6–DF, 2092.991(2) and 2072.772(2) MHz, and 5.34(13) and 56.6(11) kHz; and for 10B11BH6–HF, 2176.086(1) and 2154.566(1) MHz, and 6.28(3) and 56.1(5) kHz, respectively. The spectra are insensitive to the value of A, which is assumed to be that of diborane itself (79.6 GHz). The hyperfine structure of the J=0→1 transitions for 11B2H6–HF and 10B11BH6–HF shows that the outer boron nucleus in the dimer has a very small quadrupole coupling constant (‖χaa‖ ≲15 kHz for 11B) while that for the inner boron is appreciable (χaa ∼−220 kHz for 11B). The implications of these results are considered. The hyperfine structure also gives an average torsional amplitude for the HF of 27° with respect to the a axis. In-plane torsional amplitudes of 13.5° were determined for the 11B2H6 and 10B11BH6 from the rotational constants for the HF dimers. With allowance for the torsional oscillations, the B⋅⋅⋅H distance is 2.5032 and 2.5038 Å in the dimers. A somewhat smaller B⋅⋅⋅D distance of 2.4955 Å is estimated for 11B2H6–DF. The attractive potential between B2H6 and HF is estimated from DJ to have a well depth ε of ∼530 cm−1, which is comparable with that reported for the N2–HF dimer. The novel structure of the B2H6–H/DF complex is discussed.
1H and 2H NMR study of pyridinium iodide. Disorder and molecular motion between inequivalent sites
Authors: Ripmeester John A.
Published: July 2002

1H NMR spin-lattice relaxation times and 2H NMR line shapes were measured in the low temperature phase of pyridinium iodide. The results are consistent with reorientation of pyridinium ions by 60° to populate secondary potential minima, 5.43 kJ/m above the favored orientation. This process, which constitutes the creation of orientational defects in the ordered phase, takes place by reorientation over a potential barrier of 14.6 kJ/m.
Spectral hole burning in semicrystalline polymers between 0.3 and 4.2 K
Authors: Thijssen H. P. H. , Völker S.
Published: July 2002

Optical dephasing in the S1←S0 0–0 transitions of organic guest molecules in semicrystalline polyethylene hosts has been studied via hole burning from 0.3 to 4.2 K and compared to amorphous systems. In contrast to the latter, a temperature dependence of the homogeneous linewidth much steeper than T1.3 was found, which approaches that of crystalline materials and varies with the degree of crystallinity of the polymer. Parameters which fit the equations of specific theoretical models to the experimental data are calculated.
Determination of population and alignment of the ground state using two-photon nonresonant excitation
Authors: Kummel Andrew C. , Sitz Greg O. , Zare Richard N.
Published: July 2002

A method is presented for determining the population A(0)0, the quadrupole alignment factors A(2)0, A(2)1, A(2)2, and the hexadecapole alignment factors A(4)0, A(4)1, A(4)2, A(4)3, A(4)4 for a (v,J) ground state distribution of a diatomic molecule probed by linearly polarized two-photon nonresonant excitation. General expressions are developed for the O, P, Q, R, and S branch transitions as a function of the rotational quantum number J. This treatment assumes that the resonant state reached by the two-photon transition is subsequently detected independent of its alignment. This can be achieved by 2+n multiphoton ionization in which the ionization steps are saturated, or by 2+1 laser induced fluorescence in which the fluorescence is collected independent of its polarization and spatial anisotropy. To extract the population and the eight alignment parameters the line intensities must be measured for several polarization settings of the laser beam. However, when the ground state distribution has cylindrical symmetry, only two alignment parameters are nonvanishing, A(2)0 and A(4)0, and they can be determined at a single polarization setting by comparing the line intensities of the different branches.
Molecular photoionization cross sections and asymmetry parameters by L2 basis functions calculations: H2O
Authors: Cacelli I. , Carravetta V. , Moccia R.
Published: July 2002

A K-matrix technique using a basis set of square-integrable functions is applied to the calculation of differential photoionization cross sections in molecules. Continuum orbitals are variationally determined in the static-exchange approximation of the ion field. Integrated photoionization cross sections and asymmetry parameters β of the three main valence ionization processes in H2O are calculated, in the independent channel approximation, for the photon energy in the range of 14–50 eV and compared with the available experimental data.
Dynamics of the 13C spin-exchange process in solids: A theoretical and experimental study
Authors: Henrichs P. Mark , Linder M. , Hewitt J. Michael
Published: July 2002

Spin exchange between rare nuclei such as 13C occurs readily when the rare nuclei are coupled to a reservoir of abundant spins such as 1H, but the mechanism by which the abundant spins enable the exchange process has not fully been clarified. Two different equations to describe the rate of spin exchange in terms of the magnitude of the dipolar coupling between the carbons and the nature of the carbon–proton interaction have been proposed. The derivation of one of the equations with the projection-operator formalism is described in this paper; the derivation of the other is outlined. Neither equation fully explains the results from a single crystal of glycine, 90% enriched at the carbonyl position. The discrepancies appear to result from the assumptions used in the derivations. Good agreement with experimental results can be obtained with an equation expressed in terms of the degree of overlap of the signals from each type of carbon in the normal nuclear magnetic resonance (NMR) spectrum if allowance is made for the fact that the surrounding protons affect the environments of both carbons in a pair undergoing spin exchange in a similar manner.
Fluorescence from extreme ultraviolet photoexcitation of CF4
Authors: Lee L. C. , Wang Xiuyan , Suto Masako
Published: July 2002

Photoabsorption and fluorescence cross sections of CF4 were measured in the 50–130 nm region using synchrotron radiation as a light source. Oscillator strengths for Rydberg states were determined from the absorption cross sections measured. The fluorescence spectra were dispersed to identify the emission species. Fluorescences appear in the excitation wavelength regions of 50–57.4, 75–80, and 85–95 nm, in which the emitters are attributed to CF+4, CF2, and CF3, respectively. The fluorescence quantum yield has been determined from the absorption and fluorescence cross sections measured simultaneously. The yield has a maximum of 26% at 55 nm.
Computational study and complex angular momentum analysis of elastic scattering for complex optical potentials
Authors: Connor J. N. L. , Mackay D. C. , Thylwe K.-E.
Published: July 2002

A systematic study of elastic scattering by complex valued optical potentials has been carried out. A Lennard-Jones (12,6) potential with an imaginary r−s term has been used in the calculations. The collision parameters are chosen to model the elastic scattering of K by HBr and Li by HBr. First-order semiclassical single turning point phase shifts are compared with accurate quantum phase shifts and close agreement is found. It is straightforward to calculate semiclassical phase shifts for an optical potential; it is unnecessary to introduce additional approximations (for numerical convenience) as is usually done in the literature. Up to 15 000 terms in the partial wave series are used to calculate elastic angular distributions. It is shown that a rich interference structure and a backward glory can occur in the large angle scattering, provided the transition from absorbency to transparency is sufficiently rapid. Different optical potentials can result in similar angular distributions. A complex angular momentum (Regge pole) analysis of the large angle scattering has been carried out. It is shown that the interference effects have a physical interpretation in terms of surface waves that propagate around the core of the potential and the directly reflected elastic scattering.
Electronic energy transfer between O2 and CO dopants in Ar crystals
Authors: Bahrdt J. , Schwentner N.
Published: July 2002

Excitation spectra of Ar crystals doubly doped with O2 and CO show the a 3Π, a′ 3∑+, d 3Δ, e 3∑−, A 1Π vibrational progressions of CO as well as the Schumann Runge bands ( B 3∑−u) and the following dissociation continuum of O2. In emission the Cameron bands (a 3Π) of CO and the Herzberg bands C 3Δu of O2 have been observed. No electronic energy transfer from CO to O2 or vice versa occurs. This rules out an intramolecular V–E conversion of high vibrational quanta (v=32) in the electronic ground state of CO to the a 3Π (v=3) state of CO and subsequent electronic energy transfer to O2 in the recently reported IR-induced UV-visible fluorescence in matrix-isolated CO thus supporting the other suggested intermolecular V–E and V–V transfer routes.
Random tempering of Gaussian-type geminals. I. Atomic systems
Authors: Alexander S. A. , Monkhorst H. J. , Szalewicz K.
Published: July 2002

We use a method inspired by Monte Carlo quadrature formulas to create a basis set of Gaussian-type geminals for the calculation of the second-order energy of the beryllium atom. This technique matches results obtained by the full optimization of all nonlinear parameters but requires considerably less computational effort. For calculations involving microhartree accuracy this reduction may be as much as a factor of 1000.
Vibrational energy transfer and migration processes in matrix isolated CH3F
Authors: Apkarian V. A. , Wiedeman Lawrence , Janiesch Willi , Weitz Eric
Published: July 2002

Following excitation of the ν3 mode in matrix isolated CH3F, population of 2ν3 is observed. Population of 2ν3 occurs via intermolecular vibration–vibration (V–V) energy transfer driven by the exothermicity of the anharmonic V–V step equilibrating these states. In the equilibration process, both resonant and nonresonant intermolecular energy transfer processes have been identified. The probabilities of each of these processes have been determined. The phonon assisted nonresonant V–V process has a probability of 3.8×10−35 and 6.7×10−35 cm6 s−1 in Xe at 10 and 20 K, respectively. These probabilities were obtained from fluence dependent studies of the rise rate of 2ν3 in concentrated matrices. V–V transfer has also been studied in dilute matrices and matrices of intermediate dilution. Signals obtained in these concentration regimes are not exponential but can be reproduced by a Monte Carlo model of V–V transfer. In these matrices there is no longer rapid vibrational energy migration on the time scale of vibrational deactivation. The consequences of the lack of rapid V–V transfer on this time scale are discussed. The difference in energy transfer behavior of CH3F when excited via ν3 vs excitation at 3000 cm−1 is also rationalized. The temperature dependence and fluence dependence of deactivation rates of ν3 excited CH3F are also reported in Ar, Kr, and Xe matrices. By overcoating Ar and Kr matrices with Xe, the temperature range of these measurements has been extended beyond that previously reported.
Reactive scattering from double minimum potentials: Energetics and mechanism of the gas phase dehydration reaction of lithium ion with t
e
r
t-butyl alcohol
Authors: Creasy W. R. , Farrar J. M.
Published: July 2002

We present a crossed beam study of the reactions of Li+ with tert-butanol, a reactive system hypothesized to proceed on a double minimum potential energy surface, over the collision energy range from 0.85 to 1.80 eV. We observe product energy and angular distributions for the dehydration products as well as for Li+ which is nonreactively scattered from a transient collision complex, thereby probing both wells on the surface. The direct observation of such nonreactive flux provides confirmation of the Brauman model for nonunity reaction efficiency as arising from significant nonreactive decay of the initial encounter complex back to reagents. We also measure the branching ratios for nonreactive to reactive scattering, and the branching ratios for the two dehydration products relative to one another, over the entire kinetic energy range of the experiments. The product angular distributions indicate that the collision complex lifetimes are under 1 ps. The product kinetic energy distributions are in reasonably good agreement with statistical phase space theory predictions, with larger deviations occurring for the nonreactive channel, and with the deviations for the dehydration channels increasing with increasing collision energy. The branching ratio for nonreactive to reactive scattering, in conjunction with statistical calculations, indicates that the intermediate barrier on the double minimum surface is equal to the energy of the asymptotic reagents within ±0.05 eV. The Li(H2O)+/Li(C4H8)+ branching ratios, when compared with statistical calculations including product dissociation, are consistent with products formed in statistical equilibrium. The data suggest that the reaction dynamics in both wells are in substantial agreement with statistical theories, with a reduced number of vibrational modes required to effect the dehydration process at high energies, resulting in significant energy transfer, but a dramatically reduced decay lifetime.
Potential energy determination by inverse perturbation analysis with local correction functions
Authors: Hamilton I. P. , Light J. C. , Whaley K. B.
Published: July 2002

Inverse first order perturbation analysis is used iteratively to correct a potential energy surface. With local Gaussian correction functions the analysis is shown to be accurate, flexible, and rapidly convergent for both on, e and two dimensional examples. The analysis is employed to obtain an improved two dimensional potential energy surface for the twist–pucker motion in cyclopentene.
Theoretical study of the X 1Σ+ states of the alkali hydrides NaH–CsH
Authors: Langhoff Stephen R. , Bauschlicher Charles W. , Partridge Harry
Published: July 2002

Theoretical potentials for the X 1∑+ states of NaH, KH, and RbH are reported using near Hartree–Fock quality Slater basis sets and incorporating electron correlation through the coupled-pair formalism (CPF). Excellent agreement is obtained with the available RKR potentials even though the CPF formalism is based on the SCF reference that dissociates incorrectly. Electric dipole moment functions are reported for NaH–RbH, as well as vibrationally averaged dipole moments, Einstein coefficients, and radiative lifetimes for the first ten vibrational levels. For KH, an extensive study is made of the computational requirements of obtaining an accurate permanent dipole moment. For CsH, re, ωe, De, and μe are determined.
Partial averaging approach to Fourier coefficient path integration
Authors: Coalson Rob D. , Freeman David L. , Doll Jimmie D.
Published: July 2002

The recently introduced method of partial averaging is developed into a general formalism for computing simple Cartesian path integrals. Examples of its application to both harmonic and anharmonic systems are given. For harmonic systems, where analytical results can be derived, both imaginary and complex time evolution is discussed. For two representative anharmonic systems, Monte Carlo path integral simulations of the imaginary time propagator (statistical density matrix) are presented. Connections with other Cartesian path integral techniques are stressed.
Interaction of atomic oxygen with copper clusters
Authors: Madhavan P. V. , Newton M. D.
Published: July 2002

The chemisorption of atomic oxygen on Cu(001) is treated using a many-electron embedding theory in which the copper lattice is modeled as a 25-atom cluster. Of the three high symmetry sites investigated—fourfold, bridge, and atop-atom—the fourfold site is found to be most stable, with a dissociation energy of ∼6.1 eV (r⊥=0.7 Å), while the atop-atom configuration corresponds to the least stable site for atomic oxygen adsorption. The atop-atom site yields a radical-like state, ‘‘O−’’, whereas the open sites involve an oxide-like state, ‘O2−’, for oxygen. The principal valence ionization from oxygen is found to occur ∼8–10 eV below the vacuum level. The results of calculations on smaller clusters, CunO, n=2–5, are found to be in qualitative agreement with the results for Cu25O.
Angular momentum relaxation in atom–diatom dilute gas mixtures
Authors: Evans Glenn T.
Published: July 2002

The angular momentum relaxation cross sections for a diatomic molecule in a dilute atomic gas are estimated subject to the assumption that the intermolecular torque is dominated by the hard, impulsive contribution (evaluated using Boltzmann kinetic theory for nonspherical molecules). For carbon monoxide in a variety of gases, the kinetic theory derived contribution to the angular momentum cross section is in qualitative agreement with the experimental results of Jameson, Jameson, and Buchi.
Nonequilibrium statistics of flexible macromolecules in dilute solutions. I. Macromolecular configuration
Authors: Saab H. H. , Bird R. Byron
Published: July 2002

For a macromolecule modeled as a Rouse chain in dilute solution, we give the exact nonequilibrium expression for the joint probability density for the configuration of any number of links of the chain, expressed in terms of the connector vector coordinates. From this expression we get the probability density for a single link and the joint probability density for two links. For these two functions we give the associated diffusion equations. We also give the length-independent probability density for the orientation of a single link. Finally we illustrate the application of these results in steady shear flow.
Spectroscopic studies of cryogenic fluids: Benzene in propane
Authors: Nowak R. , Bernstein E. R.
Published: July 2002

Energy shifts and bandwidths for the 1B2u↔1A1g optical absorption and emission transitions of benzene dissolved in propane are presented as a function of pressure, temperature, and density. Both absorption and emission spectra exhibit shifts to lower energy as a function of density, whereas no shifts are observed if density is kept constant and temperature and pressure are varied simultaneously. Density is thus the fundamental microscopic parameter for energy shifts of optical transitions. The emission half-width is a linear function of both temperature and pressure but the absorption half-width is dependent only upon pressure. These results are interpreted qualitatively in terms of changes occurring in the intermolecular potentials of the ground and excited states. Both changes in shape of and separation between the ground and excited state potentials are considered as a function of density. Classical dielectric (Onsager–Böttcher), microscopic dielectric (Wertheim) and microscopic quantum statistical mechanical (Schweizer–Chandler) theories of solvent effects on solute electronic spectra are compared with the experimental results. Calculations suggest limited applicability of dielectric theories but good agreement between experiment and microscopic theory. The results demonstrate the usefulness of cryogenic solutions for high pressure, low temperature spectroscopic studies of liquids.
Thermochemistry of the gaseous uranium bromides UBr through UBr5
Authors: Lau K. H. , Hildenbrand D. L.
Published: July 2002

High temperature gaseous equilibria involving the molecular species UBr, UBr2, UBr3, UBr4, and UBr5 were studied by mass spectrometry over wide temperature ranges; reaction enthalpy data were derived primarily from second-law analysis. From this information, the individual bond dissociation energies and the standard enthalpies of formation of the U–Br species were derived with an accuracy of about 8 kJ mol−1. The derived bond dissociation energies at 298 K in kJ mol−1 are as follows: D °(U–Br)=377; D °(BrU–Br)=435; D °(Br2U–Br)=430; D °(Br3U–Br)=348; and D °(Br4U–Br)=166. Trends in these values are similar to those found in the U–Cl system, but quite different from those of the U–F system. Absolute entropies evaluated from the equilibrium data indicate significant electronic contributions for the lighter species, decreasing markedly on approaching UBr5. Trends in the bond dissociation energies and electronic entropies of the fluorides, chlorides, and bromides are discussed.
The interaction of cesium with oxygen
Authors: Woratschek B. , Sesselmann W. , Küppers J. , Ertl G. , Haberland H.
Published: July 2002

Ultraviolet photoelectron spectroscopy (UPS) and electron spectroscopy by deexcitation of metastable noble gas atoms (MDS) were used to follow the continuous oxidation of Cs films. While UPS has a finite information depth, MDS probes only the properties of the outermost atomic layer. Small doses of O2 cause the formation of a ‘‘monolayer’’ of Cs11O3 in which the O2− ions are incorporated below the surface, while the surface itself consists merely of metallic Cs atoms. Further uptake of oxygen leads to a continuous transformation of the subsurface layer into Cs2O2, while a small concentration of adsorbed O atoms builds up on the surface which act as nucleation centers for subsurface oxidation. Finally, the surface layer is completely oxidized into CsO2 which then retards further oxygen penetration into the bulk. The chemical state of the surface can be directly correlated with the associated change of the work function. A monolayer of Cs adsorbed on a Cu(110) surface exhibits quite different properties.