bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient solution of Poisson’s equation in linear combination of atomic orbitals (LCAO) electronic structure calculations
Authors: Feibelman Peter J.
Published: February 2003

An accurate and efficient method is described for the evaluation of electrostatic contributions in LCAO electronic structure calculations. The charge density  ρ(r) is decomposed into  ρ(1)(r), a component whose rapid variation near any nucleus reproduces that of  ρ(r) to a very good approximation, and a remainder density δρ(r)≡ρ(r)−ρ(1)(r), which is thereby guaranteed to be slowly varying in space. The power of the decomposition resides in the fact that  ρ(1)(r) can be expressed exactly as a sum of one-center densities, without the use of any fit procedure. Because  ρ(1)(r) is a sum of one-center multipolar densities, the Hartree potential is a function with a simple one-dimensional integral representation, and its matrix elements can be obtained by performing one-dimensional integrals over it. Since δρ(r) is spatially slowly varying, the Hartree potential to which it corresponds and the matrix elements of this potential can accurately be evaluated on a relatively coarse coordinate space mesh, using fast Fourier transforms. The method is illustrated via molecular structure calculations for N2 and NH3. The calculations are accurate to a few percent when the required integrals over δρ(r) and δV(r) are performed on a mesh of spacing 0.4 a.u. The N–N bond length and stretch frequency are found to equal 2.10 a.u. and 2.3×103 cm−1, respectively. The equilibrium N–H bond length and H–N–H angle are calculated to be 1.93 a.u. and 105°, respectively, while the NH3 inversion barrier turns out to equal 0.25 eV. These results are in good agreement with earlier calculations.
The use of alpha hyperfine coupling tensors as a measure of unpaired spin density and free radical geometry
Authors: Bernhard William A.
Published: February 2003

The dipolar elements (Bx, By, Bz) of the α-hydrogen hyperfine coupling tensor (αA) are used to calculate unpaired spin density (ρ) and bond lengths (RH) by using the two center dipole equations derived by Gordy (Ref. 1). Bz is used as a measure of ρ in the equation Bz=Qdipz ρ with Qdipz =38.7 MHz. The αH-C bond length of electron gain π radicals and alkyl radicals are found to be 0.05–0.10 Å shorter than in comparable nonradicals while in electron loss π radicals the bond lengths are longer by 0.04–0.09 Å. The two center dipole formalism appears to be useful as an analytical method for distinguishing electron loss π radicals from electron gain π radicals. It is suggested that the isotropic component of αH hfc is affected by spin density β to the αH of interest, accounting for a significant portion of the variability in QHCH in the McConnel (Ref. 2) equation, αA =QHCHρ.
A unified algebraic model description for interacting vibrational modes in ABA molecules
Authors: van Roosmalen O. S. , Benjamin I. , Levine R. D.
Published: February 2003

A simple yet realistic model Hamiltonian which describes the essence of many aspects of the interaction of vibrational modes in polyatomics is discussed. The general form of the Hamiltonian is that of an intermediate case between the purely local mode and purely normal mode limits. Resonance interactions of the Fermi and Darling–Dennison types are shown to be special cases. The classical limit of the Hamiltonian is used to provide a geometrical content for the model and to illustrate the ‘‘phase-like’’ transition between local and collective (i.e., normal) mode behavior. Such transitions are evident as the coupling parameters in the Hamiltonian are changed and also for a given Hamiltonian as the energy is changed. Applications are provided to higher lying vibrational states of specific molecules (H2O, O3, SO2, C2H2, and C2D2).
Multichannel interactions in the (1σg)2(1σu) n
pλ(3Πg, 3Σ+g) Rydberg structures of He2
Authors: Ginter D. S. , Ginter M. L. , Brown C. M.
Published: February 2003

Minimal (six to nine) parameter models based on coupled channels are shown to reproduce quantitatively the known Rydberg structures associated with npλ(3Σ+g, 3Πg), v=0 in He2 over wide ranges of n* and N values. The dependence of the eigen quantum defects for npλ-based channels on level and rotational energies is found to be of the form μα(E, EN) =μ(0)α[1+μ(1)αE] [1+AαN(N+1)], where E is the energy of a level and N is its rotational quantum number. Optimized values of the multichannel parameters and models which include perturbative channel couplings from channels outside a minimal basis are presented. A number of perturbations are shown to scale with channel separation in (n*1, ..., n*L)mod1 spaces (i.e., in ‘‘Lu–Fano’’ hyperspaces) and simplified procedures for isolating the principal contributors to such interactions are included.
Structure of ND3 solids I and II at high pressure by neutron diffraction
Authors: Eckert J. , Mills R. L. , Satija S. K.
Published: February 2003

A preliminary investigation of two solid phases of deuterated ammonia has been performed by single-crystal neutron diffraction. At 5.1 kbar and 240 K, the newly discovered solid II was found to consist of orientationally disordered molecules in a hexagonal close-packed lattice with space ground P63/mmc and lattice constants a=3.588(2) Å and c=5.870(2) Å. The primitive cubic structure (space group P213) of solid I was confirmed up to a pressure of 5.1 kbar at 215 K, where the lattice constant is 5.035(2) Å.
Simulation of the initiation of detonation in an energetic molecular crystal
Authors: Tsai D. H. , Trevino S. F.
Published: February 2003

A molecular dynamical study of the detonation process in a dense system is presented. The model is a filament of a molecular crystal capable of undergoing exothermic dissociation. When the model is heated at one end, dissociation reactions start at that end and propagate along the filament. The accompanying expansion of the heated region drives a shock wave into the filament, causing further reactions due to shock heating. The results thus obtained provide a molecular description of the initiation of detonation.
The effect of collision energy and vibrational excitation on the reaction of C2H+2+H2
Authors: Turner T. , Lee Yuan T.
Published: February 2003

The relative efficiencies between the excitation of C–C stretching vibration and collision energy on the promotion of the H atom transfer reaction of C2H+2 +H2→C2H+3 +H have been examined. At low collision energies the reaction is strongly vibrationally enhanced in agreement with previous thermal energy studies. Translational energy was found to be not as effective as exciting the C–C stretching vibration in promoting the reaction. The results indicate that the H atom transfer from H2 to the C atom should have a late barrier, but the C–C stretching vibration of C2H+2 is apparently coupled to the reaction coordinate. Because the C–C bond in C2H+3 is expected to be longer than that of C2H+2, vibrational energy added to the C–C bond might reduce the barrier for H atom transfer from H2 to a C atom in C2H+2.
Is there resonance enhancement of the cross section in very low energy collision-induced vibrational relaxation?
Authors: Gentry W. Ronald
Published: February 2003

The molecular vibrational relaxation which occurs in free jet expansions at very low translational temperatures has been attributed previously to an orbiting resonance mechanism, with the assumption that ‘‘ordinary’’ mechanisms for vibrational relaxation have vanishingly small rates at low temperatures. This assumption is examined from three perspectives: (1) The kinetic energy dependence of the v=1→0 cross section for I2(X 1Σg+) + He collisions, as derived from the experimentally determined v=0→1 cross section and microscopic reversibility; (2) a semiclassical correspondence-principle description of the deexcitation process at zero kinetic energy; and (3) the quantum-mechanical threshold laws for exoergic collisions of two neutral particles. It is concluded that nonresonance mechanisms may plausibly result in relaxation rates which are comparable to those which have been attributed to the resonance mechanism, even at very low translational temperatures, and that therefore the question posed in the title should still be considered an open one.
Failure of the Born–Oppenheimer and Franck–Condon approximations for long distance electron transfer rate calculations
Authors: Beratan David N. , Hopfield J. J.
Published: February 2003

Quantum mechanical and semiclassical formulations of nonadiabatic electron transfer theory are usually implemented within a Born–Oppenheimer regime. Calculations on real weakly interacting systems are so difficult that this approximation is rarely questioned. The Born–Oppenheimer approximation becomes qualitatively wrong for electron transfers at very large distances. A model vibronic problem is exactly solved and compared with the Born–Oppenheimer result. Rate expressions are derived from the wave functions using the ‘‘golden rule’’ approximation. Electron propagation is intimately correlated to nuclear motion so that the vibrational energy left on the donor critically affects the electronic decay length. Several deviations from the usual predictions appear for transfers over very large distances.
Laser induced fluorescence study of Xe(5p56p, 5p56p′, 5p57p, and 5p56d) states in Ne and Ar: Radiative lifetimes and collisional deactivation rate constants
Authors: Inoue Gen , Ku J. K. , Setser D. W.
Published: February 2003

The radiative lifetimes and the collisional deactivation constants by Ne and Ar for all 6p, 6p′, 7p, and 6d states of Xe, except two, were studied by single or two-photon laser induced fluorescence in the afterglow of a pulsed discharge [the prime symbol denotes the Xe+(2P1/2) core rather than the Xe+(2P3/2) core]. The radiative lifetimes, 30–48 ns for 6p and 6p′ states and 68–172 ns for 7p states, agree well with theoretical values; but, the agreement between experiment and theory is less satisfactory for the 6d states. The collisional deactivation constants for Ne and Ar are similar if there is a nearby product level. However, the deactivation constants by Ar are much larger than for Ne, if there is a large energy defect for the product state. The product distributions also were measured; Ar collisions give products with larger energy defect than collisions with Ne, which tend to favor nearly isoenergetic product states. The number and type of product channels are related to the locations of crossings in the Rg–Xe potential curves and the more attractive nature of Ar–Xe* vs Ne–Xe*. There is no clear favoring between intra- and intermultiplet product states of Xe*. The radiative cascading (6p′,7p)→(7s or 4d)→6p also was studied.
A simple sum rule for total radiative decay rates: Comparison of quantal and classical methods for diatomics
Authors: Tellinghuisen Joel , Julienne Paul S.
Published: February 2003

Total radiative decay coefficients for electronic transitions in diatomic molecules are evaluated by means of an initial state expectation value 〈V3(R)μ2e(R) 〉 (V= difference potential, μe=transition moment function) in place of the usual sum over final states. This expectation value is calculated by quantal and classical methods and compared with the exact expression as a function of reduced mass and initial vibrational state for several combinations of initial and final potentials and transition moment functions. The errors in both the sum rule and its classical implementation seldom exceed 1%, and then only for hydrogenic reduced mass.
Accurate Hartree–Fock wave functions without exponent optimization. II. Basis functions with correct asymptotic exponents and powers
Authors: Davis C. L. , Monkhorst Hendrik J.
Published: February 2003

Results of Hartree–Fock calculations for He, Be, and Ne using basis functions of the form rn−1 e–αrM(a,b;-cr), where M(a,b;-cr) is Kummer’s function, are reported. These functions allow the trial orbitals to attain the correct irrational asymptotic power and exponential decay at large r, without introducing spurious singularities. Parameters in the basis functions were chosen analytically by matching the asymptotic forms of the basis functions with terms in the asymptotic expansions of the exact orbitals. The results for He and Be surpass the accuracy of existing optimized basis sets of similar size without the need for any nonlinear optimizations, and a prediction that accurate estimation of orbital moments 〈rn〉 requires the inclusion of correct asymptotic powers when n is greater than the number of basis functions has been verified. However, our results for Ne imply that routinely including correct asymptotic powers in basis sets will only be feasible for very small systems.
The molecular beam spectrum and the structure of the hydrogen fluoride dimer
Authors: Howard B. J. , Dyke Thomas R. , Klemperer W.
Published: February 2003

The molecular beam spectrum of the hydrogen fluoride dimer has been investigated in detail. Microwave and radiofrequency transitions of (HF)2, HFDF, and (DF)2 are reported for both the Ka =0 and Ka =1 rotational levels. The results provide information on the structure of the dimers and on the nature of the intermolecular potential energy surface. The average F–F distance is shown to be approximately 2.78 Å but with a probably significantly shorter equilibrium distance. The nonhydrogen bonded hydrogen atom is bent 63°±6° from the F–F axis. In addition, analysis of the perpendicular electric dipole moment indicates a nonlinear hydrogen bond. The large centrifugal distortion effects and the unusual quantum mechanical tunneling provide a crude model for the potential surface associated with the hydrogen bond; the effective bond stretching force constant is (1.36±0.03)×104 dyn cm−1.
Effects of arbitrary laser or NMR pulse shapes on population inversion and coherence
Authors: Warren Warren S.
Published: February 2003

We present a new perturbation expansion for calculating the effects of arbitrary pulse shapes in two-level systems, even when the effects are grossly nonlinear. The first two terms have simple physical interpretations. This expansion converges rapidly for all values of resonance offset with simple shapes, and for any pulse shape far from resonance. We generate very simple, symmetric, single phase pulse shapes which produce uniform inversion or polarization and which can be combined into multiple pulse sequences. We also show that pulse shape modification is superior to construction of composite pulse sequences, since such sequences must become erratic far from resonance.
A study of the rare earth ion sites in Gd3+ and Eu3+ doped CaSO4
Authors: Danby Robert J. , Manson Neil B.
Published: February 2003

The principal Gd3+ centers in CaSO4:Gd3+ single crystals have been studied using ESR and compared with the main Eu3+ centers evident in the optical spectra of CaSO4:Eu3+. The results indicate that the rare earth ion occupies the same four structural centers in each case. Structural models are proposed for these centers based on the symmetry of the measured crystal field at the principal Gd3+ sites, all of which were found to exhibit either orthorhombic or monoclinic behavior. ESR spin-Hamiltonian parameters have been calculated for these Gd3+ centers and accurate measurements made of the principal optical transition wavelengths for the corresponding Eu3+ centers. Analysis of the crystal field symmetry at the dominant Gd3+ site shows that it is not consistent with either an electrostatic model or a covalent ligand model, but that it can be well described by a combination of these two interactions in which each contributes approximately equally to the measured second order parameters. A pseudosymmetry analysis of this site’s measured fourth order parameters shows that the ligand contribution to the total crystal field possesses approximte D2d symmetry and that its major contribution is along the crystallographic c axis, in agreement with the predictions of the ligand superposition model of Newman and Urban.
A rigid rod normal coordinate analysis, and van der Waals force field of acetylene–HCl
Authors: Lee Rebecca , Henderson Giles
Published: February 2003

A normal coordinate analysis appropriate for two weakly bound rigid rods is developed. Asymmetric top centrifugal distortion constants and published vibrational frequencies are employed in the normal coordinate analysis to obtain the intermolecular force field and refinements in the structural parameters. The analysis permits a more accurate interpretation of the observed Cl quadrupole coupling constant suggesting that the hydrogen bonding in this complex causes a decrease in the magnitude of the electric field gradient at Cl by 8±1% from that in the free HCl monomer. These methods also suggest that hydrogen bonding decreases the electric field gradient at deuterium in C2H2⋅ ⋅ ⋅DF by 10% and increases the corresponding H–F bond length by 1%.
Counting vibrational quanta with a diode laser probe: Bending and stretching excitation in CO2 caused by collisions with hot atoms from excimer laser photolysis
Authors: Chu J. O. , Wood Carol F. , Flynn G. W. , Weston Ralph E.
Published: February 2003

A high resolution (10−3 cm−1) cw diode laser probe technique has been developed and used to determine the number of CO2 vibrational quanta of each kind (ν3 antisymmetric stretch, ν2 bend, and ν1 symmetric stretch) produced as a result of collisions between translationally hot hydrogen atoms and CO2 molecules. The experimental method relies on probes of the CO2 vibrational transitions mnlp → mnl( p+1) all of which ‘‘ride’’ the large oscillator strength of the fundamental 0000 → 0001 antisymmetric stretching transition. Transitions with different values of m, n, l, and p are easily separated due to the narrow spectral characteristics of the diode laser and the small anharmonicities associated with different vibrational quantum numbers. The probability for excitation of a CO2 ν3 quantum by collisions with hot hydrogen atoms produced by 193 nm excimer laser photolysis of H2S is about 1% per gas kinetic collision. Bending (ν2) quanta are produced about 5–6 times more efficiently than (ν3) antisymmetric stretching quanta. A precise value for the excitation probability of symmetric stretching (ν1) quanta cannot be obtained due to rapid equilibration between ν1 and 2ν2, but the number of ν1 quanta is found to be roughly one third the number of ν3 quanta. The thermalization rate for cooling hot hydrogen atoms below threshold for excitation of a CO2 ν3 quantum corresponds to two H atom/H2S collisions or 16 H atom/CO2 collisions.
Qualitative exact diagnostic tests for two common relaxation kinetic models
Authors: Clegg Robert M.
Published: February 2003

Qualitative general characteristics are derived which pertain to the spectrum of relaxation times for two common chemical reaction mechanisms. These reaction models are often invoked to interpret results of chemical relaxation experiments, such as temperature-jump. The results are generally applicable for two reaction models for any number of elementary steps and involve no approximations. These qualitative, but necessary, conditions are very useful guidelines when several relaxation processes are observed. Thus one can determine if the data is compatible with the models before undertaking a complete quantitative analysis which may be difficult or very involved.
State-to-state vibrational excitation of I2 in collisions with He
Authors: Hall Gregory , Liu Kopin , McAuliffe Michael J. , Giese Clayton F. , Gentry W. Ronald
Published: February 2003

Cross sections for v=0→1, 2, 3 excitation in I2+He collisions have been measured over the kinetic energy range from threshold to 0.4 eV by means of a new experimental technique in which pulsed molecular beams are crossed at a variable intersection angle and scattered products are state selectively detected by laser induced fluorescence. The values of the measured cross sections range from about 0.2 Å2 for v=1 at 0.4 eV to about 10−5 Å2 for v=3 at the low-energy detection limit. The kinetic energy dependences of the cross sections for exciting v=1, 2, and 3 are approximately linear, quadratic, and cubic, respectively. The experimental cross sections are compared with those predicted by a simple classical model and by the detailed quantum calculations reported by Schwenke and Truhlar in the accompanying article. It appears that vibrational excitation in this system comes primarily from nearly impulsive collisions with the steeply repulsive part of the interaction potential in approximately collinear geometries. The most probable rotational excitation is small, but even very high J states are excited with small probability.
Orientational ordering and site structure of SiF4 trapped in rare gas solids
Authors: Jones Llewellyn H. , Swanson Basil I. , Ekberg Scott A.
Published: February 2003

The infrared spectrum of the ν3 mode of SiF4 shows only one site with marked site-symmetry splitting accompanied by pronounced (>95%) orientational ordering. The results indicate a site symmetry of C3v with the SiF4 molecule occupying a four-atom substitutional site with a cubic-close-packed environment. In krypton and xenon matrices, however, SiF4 occupies a three-atom substitutional site. The fact that SiF4 shows only one dominant site when present in argon matrices of UF6 indicates that one of the two UF6 sites has a local hexagonal close packed environment induced by short range stacking faults.
Determination of ESR inhomogeneous line broadening for nitroxide radicals arising from heterogeneous environments of amorphous substances
Authors: Lee Sook , Ames D. P. , Brown I. M.
Published: February 2003

It has recently been shown that the ‘‘single-crystal’’ linewidths of anisotropic magnetic resonance centers in amorphous or polycrystalline samples can be measured with high accuracy by means of second-derivative powder spectra. Based on this second-derivative-powder-spectrum method, a new approach is presented for reliably determining the inhomogeneous electron-spin-resonance (ESR) line broadening of nitroxide radicals arising from heterogeneous environments of amorphous substances. The experimentally measured values of this inhomogeneous line broadening can be employed for characterizing structural deviations of nitroxide spin labels and probes due to heterogeneous environments of nitroxide host matrices.
A study of the ν1 fundamental and bend-excited hot band of DNN+ by velocity modulation absorption spectroscopy with an infrared difference frequency laser
Authors: Nesbitt David J. , Petek Hrvoje , Gudeman Christopher S. , Moore C. Bradley , Saykally Richard J.
Published: February 2003

The combined application of the ac discharge velocity modulation technique with cw tunable difference frequency infrared generation and dual beam subtraction of laser amplitude noise produces an extremely sensitive (10−6 absorbance/Hz1/2) direct absorption method for observing vibrational spectra of molecular ions. In this study we report observation of the fundamental ν1 (D–N stretch) of DNN+ in both P and R branches from J=0 to 24, as well as the 30-fold weaker hot band transitions originating from the (0110) bending state. l-doubling in these latter transitions is measured interferometrically with a precision of ±10 MHz. Spectroscopic constants for (0000), (1000), (0110), and (1110) states are reported and compared with ab initio calculations and previous microwave measurements for DNN+.
Torsional potential and intramolecular dynamics in the C2H+4 photoelectron spectra
Authors: Pollard J. E. , Trevor D. J. , Reutt J. E. , Lee Yuan T. , Shirley D. A.
Published: February 2003

The 584 Å photoelectron spectrum of ethylene was reexamined at higher resolution using a supersonic molecular beam source for rotational cooling. The vibrational frequencies of the C2H+4X̃ 2B3 state were measured with improved accuracy, which permitted the torsional potential energy curve to be characterized. The measured equilibrium torsional angle is 27°*2°, and the inversion barrier is 270±150 cm−1. The Ã 2B3 state is shown to be in reasonable agreement with a recent theoretical calculation which takes into account strong nonadiabatic effects. The correlation function calculated for this state is consistent with this calculation and recent calculations on the unimolecular dissociation of this state. The B̃ 2A state correlation function indicates a rapid decay mechanism is operative. The broadened vibrational structure in the fourth band results from a saddle point in the C̃ 2B2 potential energy surface from which the ion relaxes to the X̃ state by moving along a repulsive degree of freedom, which in this case is internal rotation. The C̃ state vibrational broadening as well as the decay of the correlation function yield a lifetime for this process of 7 fs.
The high-resolution visible overtone spectrum of CH4 and CD3H at 77 K
Authors: Scherer G. J. , Lehmann Kevin K. , Klemperer W.
Published: February 2003

We have obtained visible overtone spectra with Doppler-limited resolution of methane and trideuteromethane in the vicinity of six quanta of C–H stretch. At room temperature, the methane spectrum is unresolved. Upon cooling to 77 K in a specially designed photoacoustic cell, methane shows a complicated but rotationally resolved spectrum. The widths of all features in the spectrum are consistent with Doppler broadened linewidths at 77 K. Efforts to assign this spectrum are in progress. The overtone spectrum of CD3H has been recently studied by other workers at a resolution of 0.5 cm−1 [Perry, Moll, Kupperman, and Zewail (preprint)]. The spectrum in this region consists of two parallel bands, one at 16 156 cm−1 and another at 16 230 cm−1. These are assigned as arising from a Fermi resonance between the pure C–H overtone 6ν1 and a combination with the degenerate C–H bend, 5ν1+2ν5. A high resolution spectrum taken at 77 K shows nearly completely resolved K-subband structure for both bands. The rotational constants of both states cannot be rationalized purely on the basis of the above vibrational assignment, although it is the most reasonable explanation of the observed vibrational spectrum. Our rotational analysis of the bands indicates that both of these states are in Coriolis resonance with another state or states which are not seen in our spectrum.
The rotational spectra, molecular structures, and 21Hg nuclear quadrupole coupling constants of HgHCl and HgDCl
Authors: Shea J. A. , Campbell E. J.
Published: February 2003

High-resolution rotational spectra of 16 isotopic species of the gas phase van der Waals complex HgHCl have been measured using pulsed Fourier-transform microwave spectroscopy carried out in a Fabry–Perot cavity. The Hg–Cl distance is 4.097 Å in HgHCl and 4.112 Å in HgDCl, with the acid proton located between the two heavy atoms, analogous to the structures of previously studied rare gas-hydrogen halide complexes. Based on this structure, an effective van der Waals radius of 1.99 Å for mercury in HgHCl is determined. 201Hg nuclear quadrupole coupling constants were measured for two complexes: χHg (201HgH 35Cl)=+5.993 (165) MHz, and χHg(201HgD 35Cl) =+7.789 (165) MHz. From these constants, and from a similar measurement for 201HgHC 15N, we obtain the first experimental estimate of the Sternheimer shielding constant in the mercury atom γ∞ =−47(7). Force constants and normal frequencies for the van der Waals vibrational modes were also calculated and compared with those of rare gas-hydrogen chloride complexes.
Nitrogen K-shell photoemission and Auger emission from N2 and NO
Authors: Lindle D. W. , Truesdale C. M. , Kobrin P. H. , Ferrett T. A. , Heimann P. A. , Becker U. , Kerkhoff H. G. , Shirley D. A.
Published: February 2003

The first gas-phase photoelectron measurements near the nitrogen K edges of N2 and NO are reported. Shape-resonance behavior is exhibited in the cross sections for both N 1s photoemission and N KVV Auger emission. The measured cross sections agree well with absorption and electron-energy-loss results, and with Stieltjes–Tchebycheff moment-theory calculations (for N2) except for a small energy shift. The measured asymmetry parameters for N 1s photoemission also exhibit changes in the shape-resonance region, but not as pronounced as those predicted by the multiple-scattering method (MSM). Comparison of all of the results with the MSM calculations indicates that the shape-resonance effects predicted by the MSM are higher in energy, and are narrower and more intense than those actually observed.
Chemical exchange effects in holographic relaxation spectroscopy
Authors: Johnson Charles S.
Published: February 2003

Holographic relaxation spectroscopy combined with electrophoresis (EHRS) is suggested as a method for determining reaction rates in solution. This method is inherently more sensitive than electrophoretic light scattering (ELS) for small molecules and is selective since the molecules of interest are labeled. Equations are derived for the EHRS frequency spectrum in the presence of chemical exchange reactions, and the two-site problem is worked out in detail as an illustration. Simulations of spectra are presented for intra and intermolecular exchange. It is shown that, under favorable conditions, electrophoretic mobilities of macromolecules in equilibrium with photochromic dyes can be determined without the necessity of covalent labeling.
Internal field correction for infrared band intensity measured in solutions
Authors: Akiyama Minoru
Published: February 2003

The Polo and Wilson equation has been widely used to eliminate the effect of the internal electric field from the observed infrared absorption intensities in the condensed phase. However, the validity of this equation for solution states has not been confirmed yet. This paper gives a new equation for the internal electric field correction in solution states according to the two kinds of molecular models. The equation obtained is identical to that of Polo and Wilson to the limit of pure liquid.
Orientational dynamics and disorder of solid adamantane as studied by infrared spectroscopy
Authors: Corn Robert M. , Shannon Victoria L. , Snyder Robert G. , Strauss Herbert L.
Published: February 2003

The CH stretching and bending vibrations of isotopically dilute C10D15H in a host crystal of perdeuterated adamantane have been monitored by infrared spectroscopy as a function of temperature from 7 to 300 K. The infrared bands depend upon both intramolecular and intermolecular interactions. The intramolecular splittings of the bands have been analyzed and compared with normal coordinate calculations. The intermolecular splittings of the CH stretches in the tetragonal phase are found to collapse and broaden with increasing temperature due to librational motion. A Redfield theory analysis of the temperature dependent line shifts and linewidths yields a correlation time of 2 ps for the adamantane libration. In the disordered plastic phase of adamantane, the intermolecular and intramolecular splittings of the CH stretches disappear and the rotational disorder yields a broad inhomogeneous band.
Visible and ultraviolet emission from pulse irradiated amorphous and polycrystalline H2O ice
Authors: Freeman C. G. , Quickenden T. I. , Litjens R. A. J. , Sangster D. F.
Published: February 2003

Luminescence peaking at 405 nm was observed when thin films of amorphous or polycrystalline ice at 97 K were irradiated with a pulsed beam of 0.53 MeV electrons. These emissions differed from the luminescence emitted by crystalline ice in that memory effects were not observed; the peak wavelengths were red shifted by ∼20 nm; and the half-lives were 6–9 ns instead of ∼400 ns. The emission spectra of polycrystalline ice samples produced by rapid deposition or by annealing amorphous ice were similar, but both had substantially lower intensities than amorphous ice spectra.
Ultrasonic sound velocities and elastic constants of liquid and crystalline CD4 and C6H12
Authors: Marx S. V. , Simmons R. O.
Published: February 2003

Measurements, by Schaefer–Bergmann scattering, of sound velocity at 10 MHz in both liquid and crystal tetradeuteromethane (CD4) and cyclohexane (C6H12) are described. The elastic constants of CD4 at 85.57 K are found to be C11=2.056±0.015, C12=1.542±0.015, and C44=0.939±0.005 GPa. For cyclohexane at 278.6 K the results are C11=2.580±0.015 and C12+2C44=2.97±0.03 GPa. It is argued that the separate elastic constants of cyclohexane are C12=2.25 and C44=0.36 GPa. By comparison with published Brillouin scattering data, the presence of dispersion in the case of cyclohexane and its absence in the case of methane is noted.
Theory of chemical reactions of vibronically excited H2 (B 1Σ+u). III. Formation of bound excited states of the (H2)2, (H2)3, and (H2)5 clusters
Authors: Nicolaides C. A. , Petsalakis I. D. , Theodorakopoulos G.
Published: February 2003

We present a simple theory of bonding in certain van der Waals clusters and results of configuration-interaction calculations which demonstrate the existence of bound excited singlet states of the (H2)2, (H2)3, and (H2)5 clusters. The hypersurfaces of these bound states have minima close to avoided crossings with the lowest 1A′ state repulsive hypersurfaces. Similar results are expected for a variety of other clusters. Apart from their importance in the theory and spectroscopy of molecular structure and dynamics, these findings may find application in problems of excitation and energy transfer in liquids and solids, whose properties are supposed to be approximated by large clusters.
Nonadiabatic semiclassical scattering. II. Solution of two-dimensional models and comparison with quantum results
Authors: Herman Michael F.
Published: February 2003

The generalized surface-hopping procedure for semiclassical nonadiabatic scattering is applied to a two dimensional model for comparison with quantum results. The model consists of two adiabatic electronic potential energy surfaces with a radial nonadiabatic coupling between the electronic states. Both adiabatic potential surfaces are taken to be constant. The coupling is assumed to be weak. This model is sufficient to explore many of the important features of the generalized surface-hopping procedure. Comparison of the semiclasssical and quantum results demonstrates good agreement, and the first order quantum scattering amplitude is analytically found to be the ℏ→0 limit of the first order semiclassical scattering amplitude. The behavior of the wave function near singularities in the transition amplitudes is explored in a general context, independent of these simplified models. These singularities are found to be necessary to produce the correct behavior in the wave function.
Laser assisted charge transfer reactions in slow ion–atom collisions: Coupled dressed quasimolecular-states approach
Authors: Ho Tak-San , Chu Shih-I , Laughlin Cecil
Published: February 2003

Semiclassical coupled dressed quasimolecular states (DQMS) approaches are presented for the nonperturbative treatment of charge transfer reactions at low collision velocities and high laser intensities. The DQMS are first obtained via the Floquet theory. The laser assisted collision process can then be treated as the electronic transitions among the DQMS driven by the nuclear motion only. The expansion of the total electronic wave function in a truncated DQMS basis results in a set of coupled adiabatic equations. The adiabatic DQMS and their associated quasienergies (depending parametrically upon the internuclear separation R) exhibit regions of avoided crossings, where the electronic transition probabilities are large due to strong radial couplings induced by the nuclear movement. By further transforming the adiabatic DQMS into an appropriate diabatic DQMS representation, defined via the vanishing of the aforementioned radial couplings, we obtain a new set of coupled diabatic equations which offer computational advantage. The method is illustrated by a case study of the laser assisted charge exchange process He+++H(1s)+ℏω→He+(n=2)+H+, in a two-state approximation, for the velocity range from 1.5×105 to 2×107 cm/s and for the laser intensity in the range of 0.4 to 4.0 TW/cm2. Results of exact coupled diabatic DQMS calculations are presented along with several approximation calculations, using first order perturbation theory, the Magnus approximation, and the average cross section.
Hydrogen atom abstraction from aldehydes: OH+H2CO and O+H2CO
Authors: Dupuis Michel , Lester William A.
Published: February 2003

The essential features of the potential energy surfaces governing hydrogen abstraction from formaldehyde by oxygen atom and hydroxyl radical have been characterized with ab initio multiconfiguration Hartree–Fock (MCHF) and configuration interaction (CI) wave functions. The results are consistent with a very small activation energy for the OH+H2CO reaction, and an activation energy of a few kcal/mol for the O+H2CO reaction. In the transition state structure of both systems the attacking oxygen atom is nearly collinear with the attacked CH bond.
The calculation of interaction energies using the pseudopotential Hartree–Fock–Slater–LCAO method
Authors: Ravenek W. , Baerends E. J.
Published: February 2003

This paper describes an extension of Ziegler’s transition state formalism for the calculation of interaction energies in order to include the pseudopotential Hartree–Fock–Slater-LCAO method developed by Snijders and Baerends. Perturbation corrections to the expressions based on averaged pseudopotentials are obtained within a new, self-consistent scheme. Test calculations on a variety of systems are reported. It is found that the pseudopotential method reproduces results obtained with the frozen-core HFS-LCAO method quite well. For first and second row diatomics equilibrium distances agree within 0.02 Å, dissociation energies within 0.2 eV, and vibration frequencies within 20 cm−1. The spectroscopic constants are also in fair agreement with experiment. For systems containing the transition metal Cu, where the binding energy curves have rather shallow wells, dissociation energies are equally accurate, and deviations in equilibrium distances and vibration frequencies are larger.
Dielectric behavior and multibody orientational correlations of dipolar molecular liquids
Authors: de Leeuw S. W. , Quirke N.
Published: February 2003

Computer simulation studies of dipolar two center Lennard-Jones molecular liquids are reported. The dielectric constant, Kirkwood factor, two and three body orientational correlation functions are given as a function of molecular elongation for a series of approximately corresponding states. The packing of linear molecules in dense dipolar liquids is found to reduce the dielectric constant by between 50% and 60% (l>0.2) from that of an equivalent Stockmayer fluid. The Kirkwood factor is found to be <1 for elongated molecules as suggested by Buckingham. These changes are explained in terms of a local structure which also predicts the qualitative features of the two and three body dipole correlation functions.
IR and Raman spectra of matrix isolated Cs2XO4 (X=S, Cr, Mo, W) molecules. The use of the isotopic shifts and IR band intensity relationships in the evaluation of the bond angles in the D2d Cs2XO4 molecules
Authors: Nagarathna H. M. , Bencivenni L. , Gingerich K. A.
Published: February 2003

IR and Raman studies have been carried out on matrix isolated Cs2SO4, Cs2CrO4, Cs2MoO4, and Cs2WO4. The S, Cr, and Mo isotope shifts and the IR band intensity ratios of the E and B2 stretching modes have been used to evaluate the bond angles in these molecules. 18O substitution experiments have been carried out for Cs2WO4 in order to definitively establish the assignment of the ν(W–O) modes in this molecule.
Brillouin scattering in p-polyphenyls. III. The improper ferroelastic phase transition of p-terphenyl
Authors: Ecolivet C. , Toudic B. , Sanquer M.
Published: February 2003

The elastic properties of p-terphenyl were studied in its high and low temperature phases. The 13 elastic constants were determined in the monoclinic phase, whereas 16 elastic constants were found sufficient to describe the elastic properties of the triclinic phase. In this phase the main change in the Brillouin spectra is a splitting of a quasilongitudinal line. This splitting varies like the square of the order parameter. A quasielastic component is observed in the vicinity of the transition temperature. Deuterated and hydrogenated samples revealed the same behavior. Elastic anomalies are tentatively interpreted inside the frame of the Landau theory.
Fractal to Euclidean crossover and scaling for random walkers on percolation clusters
Authors: Argyrakis P. , Kopelman R.
Published: February 2003

Simulations of random walkers on two-dimensional (square lattice) percolation clusters were performed for a range of occupation probabilities from critical to unity. The number of distinct sites visited, over 2×105 steps, shows the conjectured scaling, crossover and superuniversaility (ds=4/3, within 1%) behavior over a wide range of site occupation probabilities. Possible deviations from superuniversality and/or scaling are discussed.
Laser photoelectron spectroscopy of vibrationally relaxed CH−2: A reinvestigation of the singlet–triplet splitting in methylene
Authors: Leopold D. G. , Murray K. K. , Lineberger W. C.
Published: February 2003

The photoelectron spectrum of CH−2 has been reinvestigated using a new apparatus that incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Earlier photoelectron studies [J. Chem. Phys. 74, 5460 (1981)] employing gas discharge and sputter ion sources were plagued by hot CH−2 ions whose vibrational population distribution, for reasons not yet explicable, could not be detectably altered by modification of source conditions. In contrast, the spectrum of cooled CH−2 displays a markedly changed vibronic band intensity profile for the CH−2 (2B1) →−e− CH2(3B1) transition. These results enable several previously observed spectral features to be positively identified as vibrational hot bands. The new CH−2 photoelectron spectrum is consistent only with the methylene singlet–triplet splitting of approximately 9 kcal/mol determined by numerous recent experimental and theoretical studies.
Microwave spectrum, dipole moment, and conformation of ethanedithiol
Authors: Nandi R. N. , Su Chun-Fu , Harmony Marlin D.
Published: February 2003

The microwave spectrum of 1,2-ethanedithiol has been investigated. Measurements yield rotational constants having values A=9292.451±0.006, B=2239.194±0.002, C=1935.557±0.002 MHz; and dipole moment components μa=0.683±0.011, μb=1.749±0.044, μc=0.774±0.034 D. Analysis of these data shows that the molecule adopts an unsymmetrical non-hydrogen-bonded conformation.
Broadband population inversion in solid state NMR
Authors: Tycko R. , Schneider E. , Pines A.
Published: February 2003

We present theory, simulations and experimental demonstrations of composite π pulses for population inversion in coupled spin systems such as occur in solid state NMR. The composite π pulses are phase-shifted rf pulse sequences designed to invert spins over a larger range of dipole or quadrupole couplings than a conventional π pulse, for a given rf power. We discuss a previously proposed theory for constructing composite pulses, in the specific context of solids. Two particular sequences 450180909018018090450 and 18001801201800 are examined in detail. Their performance in coupled spin systems of various sizes is evaluated in simulations. Experiments are performed on two solid compounds, Ba(ClO3)2⋅H2O and C4O4H2. The results reveal markedly less spectral distortion after composite pulse inversion than after conventional π pulse inversion at low rf powers.
Fluorescent and dynamic properties of optically excited dysprosium trifluoride
Authors: Xu Li-Wen , Crosswhite H. M. , Hessler Jan P.
Published: February 2003

Fluorescent, excitation, and absorption spectra of DyF3 are reported. The energies of the electronic states of the ground level are significantly shifted compared to those of the dilute system DyxLa1−xF3 and are consistent with recent specific heat measurements from 5 to 350 K. The fluorescent decay rate K of the (4F9/2)1 state follows the equation K(μs−1)=1.600+0.0307 T(K), where T(K) is the absolute temperature. At 0 K the quantum efficiency is approximately 4.5×10−4. The decay rate is determined by the donor-to-acceptor transfer rate, where an acceptor is a pair of coupled dysprosium ions which deactivate the (4F9/2)1 state. Cross relaxation of the form 4F9/2+6H15/2→6F3/2+6H5/2 is calculated to be the dominant dipole–dipole decay channel. Comparison of high-resolution absorption line shapes, measured above 4.2 K, and below the ferromagnetic transition 2.53 K shows a shift of the line centers, and a reduction, by a factor of 6 in the linewidths. These results are consistent with analogous measurements performed on the Ising antiferromagnet dysprosium aluminum garnet and reflect the short-range order of the system.
The UV spectra of primary, secondary, and tertiary alkyl radicals
Authors: Wendt H. R. , Hunziker H. E.
Published: February 2003

Gas phase absorption spectra have been measured for one primary, three secondary, and two tertiary alkyl radicals in the 195 to 370 nm wavelength range. Radicals were generated by Hg-photosensitized reactions and observed by modulation spectroscopy. The pattern of bands observed agrees well with ab initio predictions for the 3s, 3p, and 3d Rydberg transitions and can be reproduced by a Rydberg formula using experimental vertical ionization potentials and normal quantum defects. Oscillator strengths and condensed phase blue shifts are consistent with the Rydberg assignment. A similarity with the spectra of primary, secondary, and tertiary amines is noted.
Translational energy of products in the K+CH3COCl→KCl+CH3CO reaction
Authors: Pauluth M. , Rotzoll G.
Published: February 2003

Time-of-flight distributions of product KCl from the K+CH3COCl reaction have been measured over limited angular ranges at the collision energies 0.16 and 0.70 eV (cm). A simplified analysis assuming separability of cm angle and velocity yields the products’ mean translational energy Ē′T. ĒT is very low for the lower collision energy, but increases with increasing collision energy. This finding is in accord with the photodissociation model of Herschbach, that predicts low repulsive energy release for this system.
H ligand charges in HMn(CO)5, H2Fe(CO)4, and HCo(CO)4
Authors: Eyermann Charles J. , Chung-Phillips Alice
Published: February 2003

The Xα method has been used to calculate the charges carried by the H ligands in some transition metal carbonyl hydride complexes. Employing a H 1s STO with an exponent of 1.0, Mulliken population analyses of the PXα LCAO-MOs, which represent specifically those SCF-Xα-SW MOs with significant H ligand character, give respective H atomic charges of −0.26, −0.27, and −0.33 for HMn(CO)5, HCo(CO)4, and H2Fe(CO)4. These numerical values are consistent with previous theoretical estimates of H ligand charges and observed 1H chemical shifts.