bluuebunny/crossref_metadata_2025 huggingface dataset:
Metastable hydrogen molecules. IV. Vibrational analysis and spin–rotation interaction
Authors: Lichten W. , Wik T.
Published: February 2003

This paper presents new experimental data which improves by an order of magnitude the precision of our understanding of the fine structure of the hydrogen molecule. By means of the molecular beam, magnetic resonance method, we find that the v=0, 1, and 2 vibrational levels of parahydrogen in the c 3Πu state are metastable. Measurements of four fs intervals in the N=2 rotational level of these three vibrational levels complete the set of six such measurements. In addition, we report eleven more intervals in the N=1 rotational level of the v=1 vibrational state, to comple the 13 possible measurements in that state. The four fine structure constants, including the spin-rotation constant C, are determined for all the v=0 and v=1, N=1, and N=2 levels. The linear combinations of constants, A+5C and B0+√6 B2, are found for N=2, v=2 also. Completion of vibrational assignments and inclusion of spin-rotation interaction now makes experimentally determined constants interpretable to a fraction of a percent in most cases. A method is given for using spectroscopic constants to find the vibrational average of an operator.
Measurements of molecular hyperpolarizabilities for ethylene, butadiene, hexatriene, and benzene
Authors: Ward J. F. , Elliott D. S.
Published: February 2003

Third-order polarizabilities are measured in the gas phase using dc electric-field induced optical second- harmonic generation and are found to be positive in each case.
Some exact results and the application of the mean spherical approximation to charged hard spheres near a charged hard wall
Authors: Henderson Douglas , Blum Lesser
Published: February 2003

An inconsistency within one of the Henderson–Abraham–Barker equations which fails to conserve local charge neutrality when applied to ionic mixtures near charged walls is discussed and is resolved by a careful analysis of the limit in which the wall particle is allowed to grow to infinite radius and zero curvature. This gives rise to a correction term which is needed only for the case of electrostatic interactions. The resulting equation is used to obtain exact asymptotic forms for the wall–fluid correlation functions at large positive and negative distances from the interface. Also, an exact relation for the contact value of the density profile is given. By means of a simple argument, results for both the total and direct wall–fluid correlation functions are given in the mean spherical approximation for charged hard spheres near a charged wall. These results are used to determine the capacitance of the interface in the mean spherical and exponential approximations. A modified form of the exponential approximation agrees closely with the Gouy–Chapman theory whereas the mean spherical approximation does not. On the basis of the exact expression for the contact value of the density profile it is argued that, for large charge on the wall, the Gouy–Chapman theory may give a reasonable capacitance even at high concentration.
Structure of a simple fluid near a wall. I. Structure near a hard wall
Authors: Sullivan D. E. , Stell G.
Published: February 2003

The wall-particle Ornstein–Zernike equation is used to obtain the density profile near a hard wall for a fluid with pair potential consisting of a hard-sphere core and Lennard–Jones attractive tail. Several closure approximations are compared: these include the wall-particle analogs of the PY, HNC, EXP, and RHNC approximations. We also examine a new class of approximations motivated by the special properties of surface structure, where attractive interparticle forces have much greater influence than in the case of bulk structure. These new approximations are of a mean-field (MF) nature, treating the fluid as equivalent to a hard-sphere fluid in a certain effective potential. The numerical results suggest that two new approximate closures, the MF–PY and MF–EXP approximations, bracket the true behavior of the fluid near a hard wall. There are striking differences between these approximations, however, as there are among all those that we have investigated (especially in the region close to the wall). Despite the relative sophistication of our approximation techniques, when viewed in terms of analogous bulk-fluid techniques, more ’’exact’’ computer-simulation results are needed before it will be clear which of our approximations is the most reliable.
High-field Mössbauer studies of reduced protocatechuate 3,4-dioxygenase
Authors: Zimmermann R. , Huynh B. H. , Münck E. , Lipscomb J. D.
Published: February 2003

We have studied the Mössbauer spectra of reduced protocatechuate 3,4-dioxygenase (3,4-PCase) from P. aeruginosa in the temperature range from 1.5 to 200 K in applied magnetic fields up to 55 kG. The entire data set was fitted to a spin Hamiltonian pertinent to the high-spin ferrous ion. By judiciously choosing the experimental conditions, the multiparameter problem could be solved rather unambigously. The iron sites of reduced 3,4-PCase are characterized by a negative zero-field splitting parameter D=−(6±1) cm−1 and a large rhombicity E/D=0.25±0.05. The data show clearly that the electric field gradient tensor is rotated relative to the zero-field splitting, suggesting a point symmetry of monoclinic or lower. The electronic spin relaxation rate, at 4.2 K, is slower than 107 s−1.
Brillouin scattering in single crystals of plastic cyclohexane
Authors: Ahmad S. F. , Kiefte H. , Clouter M. J.
Published: February 2003

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants of oriented single crystals of plastic cyclohexane for temperatures near and at the triple point. The values obtained at 279.2 K were c11 =2.8(57), c12=2.4(72), and c44=0.35(8) (in units of 109 N m−2) with an estimated uncertainty of ±2%. There is evidence for the occurrence of considerable dispersion in the solid, especially near the triple point.
Quasielastic light scattering study of charged polystrene particles in water
Authors: Dalberg P. S. , Bo/e A. , Strand K. A. , Sikkeland T.
Published: February 2003

We have measured intensity correlation functions for a water suspension of charged polystyrene particles in the ranges: 0.84×107–3.13×107 m−1 for the scattering vector q, 1 μs–3 ms for the delay time, and 0.15–2.1 mg/cm3 for the concentration. At low ionic strength the functions show an initial decay as well as a long time decay, characterized by the effective diffusion coeficients Deff and DL, respectively. Both Deff and DL have a minimum at the same q value and Deff scales with the inverse static structure factor. The data are analyzed using a model in which a particle is diffusing within a diffusing cage yielding values for their diffusion coefficients and for the ’’free’’ transit time of the particle in the cage. Attempts are made to correct the data for the effect of multiple scattering.
Classical path study of nonadiabatic transitions in atom–molecule scattering: Quenching of F(2P1/2) by H2
Authors: Thomas Lowell D. , Lester William A. , Rebentrost Frank
Published: February 2003

Classical path (CP) calculations of transition probabilities and scattering cross sections for quenching of F(2P1/2) by H2 using recently computed ab initio extended-basis-set SCF interaction potentials are reported for relative kinetic energies up to 2.4 kcal/mole. They are found to be in reasonable agreement with previous coupled-channel (CC) results obtained using the same surfaces and with Tully’s CP cross sections computed employing diatomic-in-molecules surfaces. Two extensions of the CP method, the APCP and AEMCP, are tested and found to yield slightly larger cross sections and improved agreement with CC results over the usual CP method.
Radiationless decay of vibronically coupled electronic states
Authors: Penner A. P. , Siebrand W. , Zgierski M. Z.
Published: February 2003

Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and two harmonic modes of vibration. The effect of the upper excited state on the nonradiative decay properties of the lower excited state is investigated for systems in which these states are coupled through the same non-totally-symmetric mode that couples the lower excited state to the ground state. If only this mode is considered, the model is exactly solvable and allows one to test the assumption that the initially prepared state is an adiabatic Born–Oppenheimer state. This assumption is found to be accurate unless the zeroth-order adiabatic vibrancy state from which the transition originates is very close to, e.g., within one vibrational quantum of, a zeroth-order state of the upper excited state manifold. Strong nonadiabatic mixing occurs when a vibrationally excited level of the lower excited state is in resonance with a level of the upper state. In general, the proximity of the two excited states increases the ability of the coupling mode to act as an energy accepting mode for radiationless decay to the ground state. This is shown by comparison with a totally symmetric, displaced oscillator in the adiabatic approximation. As a result vibronic coupling between two excited states may affect the energy gap law in that the roughly exponential decrease of the nonradiative rate constant with increasing energy gap is reduced or even reversed when the excited state approaches a second excited state to which it is vibronically coupled by the inducing mode. The model is also used to test, both analytically and numerically, the validity of approximate formulas for the calculation of matrix elements of the nuclear kinetic-energy operator. It is shown that the corresponding integrals are not normally separable into inducing and accepting mode integrals and that, when separation is possible, the accepting mode integrals are not simply overlap integrals. Treatments based on the Herzberg–Teller expansion and either Rayleigh–Schrödinger or Tanaka–Fukuda perturbation theory are shown to give rise to very large errors. These differences are traced back to differences in the diabatic basis sets used to expand adiabatic wavefunctions.
Radiative decay constants of the H2 Fulcher bands
Authors: Day Ray L. , Anderson Richard J. , Sharpton Francis A.
Published: February 2003

The radiative decay constants corresponding to the Fulcher bands of the H2 molecular spectrum are obtained using pulsed-electron beam techniques. Data in the range ∼1.8 to 2.8×107 s−1 are obtained at 25 eV incident electron energy for the d 3πu →a 3Σ+g (v′, v″) Q (K) and k 3πu →a 3Σ+g (v′,v″) Q (K) rotational lines with K=1,2,3 and v′=0,1,2,3,→v″=0,1,2,3. Additional measurements of the d 3πu→a 3Σg+(0,0) Q1 rotational line intensity indicate the presence of a two-mode decay over the energy range 50–300 eV. The long-lived decay component, identified as radiative cascade from upper levels, contributes ∼15% to the total d 3πu(v′=0) population.
The efficiency of engines operating around a steady state at finite frequencies
Authors: Richter Peter H. , Ross John
Published: February 2003

Linear response theory is applied to a study of the performance of engines which operate harmonically around a stable reference state which is either an equilibrium or a nonequilibrium steady state. Two kinds of quantities are relevant for efficiency and power output: those which are integrals over a semicycle (e.g., the heat input in a heat engine or the heat uptake in a refrigeration machine), and net quantities of the whole cycle, like the work exchanged with the mechanical surroundings, the net dissipative losses, and certain refractive quantities which, via Kramers–Kronig relations, correspond to the absorptive ones. We discuss their dependence on the frequency of operation and on the amplitudes of the driving forces. The general analysis is applied to three examples: The first is completely dissipative and corresponds to chemical reactions near equilibrium; the second has an inertial term in the mechanical operation; and the third is designed to have similar properties as the second example but with first-order time derivatives, to correspond to a chemical system far from equilibrium. In the last two examples oscillatory behavior may occur; if the frequency of oscillation is slow compared to the rate of heat conduction, then, depending on the amplitudes of the driving forces, there may be one or two power maxima. In case of two maxima, optimization from the point of view of power generation, or of efficiency, leaves a choice between fast and slow operation. This may have a bearing on our understanding of metabolic systems which often do exhibit oscillatory behavior.
A thermodynamic study of the palladium–hydrogen system at 245–352 °C and at pressures up to 34 atm
Authors: Picard C. , Kleppa O. J. , Boureau G.
Published: February 2003

The thermodynamic properties of the solutions of hydrogen in palladium at 245, 297, and 352 °C have been studied by a combined calorimetric–equilibrium method at pressures up to 34 atm. For hydrogen/palladium ratios up to x?0.35, the following equation within experimental precision represents the experimental data at 297 °C, i.e., near the critical temperature: Δ?H(cal mol−1) =Δ?H−TΔ?H =−1750−8250x+12 000x3+T{11.52+R ln[x/(1−x)]}.

Using this expression we obtained the following values of the critical parameters: Tc=567 K; Pc=19.35 atm; xc =0.2414; we have also calculated the limits of the (α+β) two-phase region xα(max) and xβ(min) at 245 °C to be 0.131 and 0.362, respectively. These calculated values are in good agreement with approximate values obtained directly from discontinuities in the observed partial enthalpies. Calorimetric measurements of the partial enthalpy in absorption and desorption measurements at 245 °C showed little evidence of hysteresis, and yielded Δ?H=−4.6 kcal mole−1 for the partial heat of absorption. This result is in good agreement with the average of earlier derived data from quasiequilibrium measurements.
Kinetics of transport of CO2 through porous CaO and BaO formed by carbonate decomposition
Authors: Roberts James A. , Jacobson Nathan S. , Searcy Alan W.
Published: February 2003

The decomposition of single crystals of CaCO3 and BaCO3 has been studied. The transport of CO2 through porous solids is studied by knudsen flow. (AIP)
Deuterium NMR study of molecular order and reorientation in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline
Authors: Dong Ronald Y. , Lewis J. , Tomchuk E. , Bock E.
Published: February 2003

The deuterium quadrupole doublet splittings and spin–lattice relaxation times (T1) are measured as a function of temperature in the nematic phase of p-methoxy-d3-benzylidene-d1-p-n-butyl-d9-aniline (MBBA-d13). It is shown that a single order parameter tensor is sufficient to fully describe the orientational order of the molecule even though the molecule is nonrigid. The molecular conformational averages, which are different for various C–D bonds, result in resolved deuterium NMR spectra from which the T1 for each deuteron site can be measured. While the simplest approach in treating order director fluctuations, in which molecular reorientations and collective fluctuation modes are assumed to be uncoupled, fails to explain our T1 data for all deuterons, Freed’s treatment of spin relaxation in liquid crystals does give a qualitative explanation. Using this theory, molecular reorientation times for different deuteron sites are estimated.
Pulsed NMR studies of matrix isolated HCl and (HCl)2
Authors: Kohl James E. , Semack Myron G. , White David
Published: February 2003

A proton free probe suitable for pulsed NMR studies of matrix isolated molecules has been used to obtain the proton spectrum of HCl in solid 40Ar for HCl mole fraction 0.003<x<0.02. The spectrum consists of a central Lorentzian associated with the HCl monomer superimposed on a well-defined Pake doublet due to the dimer (HCl)2. The Pake doublet is a result of the proton–proton dipolar interactions between the dimer HCl units. A structural parameter of the (HCl)2 ground state configuration is calculated from the doublet splitting. For the linear hydrogen bond the angle between the HCl unit containing the non-hydrogen-bonded proton and the axis containing the H bond is approximately 122°. An upper limit of 150 μsec for the chlorine spin–lattice relaxation time is calculated from the observed broadening of the Pake line shape. The linewidth of the HCl monomer is roughly an order of magnitude larger than that calculated from model systems used to account for the infrared absorption of matrix isolated HCl. It is suggested that this discrepancy is the result of a much more anisotropic matrix environment than previously assumed.
The unimolecular isomerization of methyl isocyanide to methyl cyanide (acetonitrile)
Authors: Redmon Lynn T. , Purvis George D. , Bartlett Rodney J.
Published: February 2003

The unimolecular isomerization of CH3NC→[CH3 NC] →CH3CN is studied by many-body (diagrammatic) perturbation theory. Using double-zeta plus polarization (DZP) contracted Gaussian basis sets (63 CGTO’s), correlated calculations are made by evaluating all second-, third-, and fourth-order diagrams that occur due to single, double, and quadruple excitations [SDQ–MBPT(4)]. These diagrams involve ∼107 CI type configurations. The SDQ–MBPT(4) model predicts the enthalpy of isomerization to be −22.7 kcal/mole [exp(−23.7 ±0.14)]. The activation barrier is found to be ∼41 kcal/mole (exp 38.4). The inclusion of quadrupole excitation diagrams has little effect on the enthalpy of isomerization but increases the activation barrier by 1 kcal/mole.
Molecular beam-laser induced fluorescence studies on the chemical reactions H+NO2→OH+NO and H+ClO2→OH+ClO
Authors: Mariella R. P. , Lantzsch B. , Maxson V. T. , Luntz A. C.
Published: February 2003

Molecular beam-laser induced fluoresence studies on the chemical reactions H+NO2→OH+NO and H+ClO2→OH+ClO have probed primary energy disposal into the OH product. In both cases, the gross results are interpreted in terms of linear rotational and vibrational surprisals. These experiments also allowed measurements of partitioning into the fine structure levels of OH. In each reaction, spin doublets are produced statistically, but there is a definite preference for one of the Λ-doublet components. For H+NO2→OH+NO, the effects of reactant rotation and translation on the energy disposal have also been investigated. These experiments indirectly yield the rotational excitation of the NO fragment and the orientation of this rotational vector relative to the OH rotation vector. The results reported here and previous dynamics experiments suggest a simple qualitative model for this reaction involving a planar HONO complex. The results for H+ClO2 →OH+ClO also suggest reaction through complex formation. We postulate that the complex HOClO, although never observed, is a thermodynamically stable molecule and is probed in this reactive potential surface.
The pair and direct correlation functions of an inhomogeneous fluid
Authors: Davis H. T. , Scriven L. E.
Published: February 2003

Functional expansions are given relating the pair and direct correlation functions of an inhomogeneous fluid to the many body direct correlation functions of a homogeneous fluid. The exact expression for the pair correlation function to second order in density gradients contains a coupling between particle correlation and density inhomogeneity. In contrast to several local density correlation models, typical of those used recently in the theory of fluid interfaces, the exact theory predicts such coupling even for particles lying in the plane perpendicular to the direction of the density gradient. This means that an interface can never be viewed strictly as a stack of thin homogeneous phases. In the low density limit, the three local density approximations considered agree with the exact theory to first order in density gradients but not to second order.
On the bifurcation analysis of the liquid–solid instability in hard-sphere systems
Authors: Müller-Krumbhaar H. , Haus J. W.
Published: February 2003

Recent bifurcation analyses of the BBGKY hierarchy for hard-sphere systems are shown to be equivalent to Kirkwood’s linear response analysis. They suffer, therefore, from the same inconsistency which has been pointed out by Kunkin and Frisch. The decoupling scheme used also leads to an inconsistency in the stability analysis of the dynamic BBGKY hierarchy.
Crossed molecular beam study of the reactions of methyl bromide with potassium and rubidium
Authors: Pang H. F. , Wu K. T , Bernstein R. B.
Published: February 2003

Using the crossed molecular beam method, the yields of the alkali halide product MBr from the exoergic reactions CH3Br+M→MBr+CH3 (M≡K, Rb) have been measured as a function of relative translational energy up to 0.9 and 1.3 eV for K and Rb, respectively. Supersonic seeded beams of CH3Br are crossed with thermal alkali beams and the in-plane angular distribution of MBr measured at different average relative translational energies ?tr. The reactions are found to have appreciable energy thresholds, 0.24±0.06 and 0.20±0.06 eV for the K and Rb-reactions, respectively. The product yields increase monotonically with ?tr above threshold. The postthreshold energy dependence of the cross sections has been obtained by deconvoluting these data from the crossed beam velocity distributions. The MBr angular distributions are characteristic of a direct, rebound mechanism, with a large fraction of the available energy going into product translation. The average recoil energy ?′tr of the product MBr increases linearly with ?tr (d?′tr/d?tr?0.73). The present data for the M+CH3Br systems are compared with previous results for the analogous CH3I reactions and with predictions of several theoretical models. The significantly higher activation barriers for the CH3Br reactions account for their smaller thermal reaction rate constants relative to the analogous CH3I reactions (from early flame experiments).
Elastic scattering and rotational excitation in ion-molecule collisions. II. Li+–H2 and H+–H2 collisions
Authors: McCann K. J. , Flannery M. R.
Published: February 2003

A general semiclassical treatment of elastic scattering and of rotational excitation in ion–molecule collisions is presented. When the orbits associated with the different channels corresponding to the internal modes do not differ significantly, simplification occurs and the internal degrees of freedom can then be coupled to the relative motion via the introduction of an optical potential (which in turn depends on the transition amplitudes). Total energy is consequently conserved. An expression is derived for the inelastic scattering amplitude which acknowledges various interference effects and possible rainbow scattering. With all phase-information supressed, the procedure, when compared with the full quantum-mechanical results, reproduces the background elastic and inelastic scattering in Li+–H2 and in H+–H2 collisions. Restoration of the phases, particularly of the eikonal or action phases associated with the different classical paths that contribute to a specified scattering angle, produces the interference oscillations present in the differential cross section for scattering angles less than the rainbow angle. The method, when compared with the full quantal procedure, is remarkably efficient and accurate.
Light scattering studies of phase separation in isobutyric acid + water mixtures
Authors: Wong Ning-Chih , Knobler Charles M.
Published: February 2003

Measurements are reported of light scattering by isobutyric acid + water mixtures that have been quenched into the two-phase region by a pressure-jump technique. The conditions of the jump method are discussed in detail. Examples are given of the behavior of the scattered intensity I (k,t) as a function of the time t and scattering vector k. Power-law expressions of the form km =A′t−a′ are found to represent the time dependence of the angle of maximum scattering. The intensity at the maximum can be represented by power laws as well: I (km) =A″ta″. For mixtures of the critical composition both a′ and a″ depend on quench depth; for other mixtures a′?0.3 and a″?1.1. Two measurements made at the cloud point show an initial t2 growth in I (km) followed by a slower growth ∝t1/2.
Single vibronic level fluorescence emission spectroscopy and radiative lifetimes of H2CO(?1A2):4 and 41
Authors: Shibuya Kazuhiko , Harger Robert A. , Lee Edward K. C.
Published: February 2003

Intensity distributions of the fluorescence emission bands from the two lowest vibrational levels of H2CO(?1A2) have been determined in the wavelength range of 360–460 nm. All of the emission bands observed are well characterized by the Δν2 progressions built on Δν4= odd, and the intensity distribution maxima of the 20m40n and 20m41n progressions occur at m =1, n=1 and m=1, n=2, respectively. From the thermalized intensity distribution data extrapolated to the high pressure limit of n-C4H10 as a buffer gas, the ratio of the radiative lifetimes for the two single vibronic levels, 41 and 40, has been obtained as 0.68±0.12, which agrees well with the theoretical values calculated by either Barnhart and Strickler or Miller and Lee.
Kinetic study of the reactions of Cl atoms with HNO3, H2O2, and HO2
Authors: Poulet G. , Le Bras G. , Combourieu J.
Published: February 2003

The kinetics of the reactions of Cl atoms with HNO3, H2O2, and HO2 have been studied by the discharge flow–mass spectrometric technique. The following rate constants were obtained (in cm3 molecule−1⋅s−1): Cl+HNO3→HCl+NO3, k=1.5 ×10−11 exp(−8700/RT) in the range 298–633 K; Cl+H2O2 →HCl+HO2, k= (4.0±0.4) ×10−13 at 298 K; Cl+HO2→HCl +O2, k= (6.8±2.5) ×10−11 at 298 K. The stratospheric implication of these data is discussed, and it is estimated that the Cl+HO2 reaction could have a comparable effect to the Cl+CH4 reaction as a sink of chlorine atoms in the stratosphere.
Spectra of the collision-induced emission from S(1S)
Authors: Black G. , Sharpless R. L. , Slanger T. G.
Published: February 2003

The spectra of collision-induced emission from S(1S) in collisions with He, Ar, N2, H2, Kr, and Xe have been measured. The S(1S) was made by OCS photodissociation with both a xenon resonance lamp (1470 Å) and a CO 4+ lamp (1500–1600 Å). With all gases except xenon, the most obvious effect on the S(1S0→3P1) transition at 4590 Å is the appearance of a neighboring ’’line’’ at 4508 Å corresponding to the S(1S0→3P2) transition. No evidence of the S(1S0→3P0) transition at 4628 Å is seen. Xenon does not produce the 4508 Å emission but results in the appearance of a blue degraded band extending from the 4590 Å line. All of the gases produce a blue degraded band on the S(1S0→1D2) transition at 7727 Å. The implications of these results for the potential curves of XeS are discussed.
A kinetic theory analysis of photodissociation
Authors: Jhon Myung S. , Dahler John S.
Published: February 2003

Kinetic theory is applied to a dilute gas in which photodissociation is occurring. The distribution function for the fluorescing dissociation products is governed by a linearized Boltzmann equation. When the term associated with moderating collisions is approximated by the BGK model this equation becomes exactly solvable. Steady-state and time-dependent solutions are presented and a formula is derived for the Doppler spectrum of the electronically excited product species. Explicit results are obtained for two simple models of the photodissociation process.
Potential constants of CF4
Authors: Jones Llewellyn H. , Kennedy Craig , Ekberg Scott
Published: February 2003

The infrared spectra of the 12C, 13C, and 14C isotopic species of CF4 have been observed at a resolution of 0.06 cm−1. In addition to the fundamentals ν3 and ν4 a number of combination bands have been observed. Using these results, combined with Raman data in the literature, we have calculated the quadratic valence force field, in terms of force constants as well as compliance constants, with considerably better precision than previously obtained. Interaction displacement coordinates have been calculated and show that stretching one CF bond leads, for minimum energy near equilibrium, to opening up of the angles between the other three bonds as well as to their contraction.
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o calculation of the vibrational energy transfer rate of H2 in Ar using Monte Carlo classical trajectories and the forced quantum oscillator model
Authors: Blais Normand C. , Truhlar Donald G.
Published: February 2003

We calculate the rotationally equilibrated vibrational excitation rate for parahydrogen in argon from 1620 to 4500 °K. We use the most accurate available potential energy surface and the well tested classical-trajectory forced quantum oscillator method. We find the temperature dependence is not well fit by the popular Landau–Teller functional form. We find a large effect of initial rotational excitation which is not necessarily associated with rotational to vibrational energy transfer. To the extent that our rates can be used to predict experimentally measured relaxation rates they are about a factor of 3 too large. We also studied two other (less accurate) potential energy surfaces to show the sensitivity of the calculated results to the assumed interaction potential.
Theory of vibrational energy transfer among diatomic molecules in inert matrices
Authors: Blumen A. , Lin S. H. , Manz J.
Published: February 2003

The purpose of the present paper is to develop a theory for the transfer of vibrational energy from a vibrationally excited diatomic molecule (donor) to another diatomic molecule (acceptor) in an inert matrix. The vibrational–vibrational (V–V) energy transfer is observable when the concentrations of the donors and acceptors are high enough so that the time for direct relaxation to the phonon bath and the radiative lifetime of the excited donor are comparable to or longer than the V–V transfer time. We calculate the microscopic, i.e., site-dependent, V–V transfer rate constant through the long range phonon-modulated interaction between the donor and the acceptor using a canonical transformation. Förster’s spectral overlap relation between the emission spectrum of the donor and the absorption spectrum of the acceptor is derived as an approximation to the present theory. Some numerical results demonstrating strong dependence of the V–V transfer rate constant on the corresponding vibrational energy mismatch are presented. It is shown that the resonant (zero-phonon) and exothermic (phonon-assisted) V–V transfer rate constants depend only weakly on the temperature of the matrix. The microscopic, i.e., site-dependent, master equation associated with the present model, which is important in describing the time-dependent vibrational populations of the molecules involved in the V–V transfer, is also obtained.
Optical free induction decay of molecules undergoing radiative and radiationless relaxation
Authors: de Vries Harmen , Wiersma Douwe A.
Published: February 2003

In this paper we examine the effect of radiative and radiationless processes on optical free induction decay (OFID) in singlet–singlet transitions of molecules in the condensed phase. The main conclusion is that the applicability of OFID to molecular singlet–singlet transitions is severely limited by the presence of an intermediate triplet state, which quite often forms a bottleneck in the optical pumping cycle. Using the theoretically derived formulas for the OFID signal, we examine the feasibility of OFID experiments on the mixed crystals of porphin in n-octane and naphthalene in durene and conclude that such experiments, if at all fruitful, would be difficult to execute. We re-evaluate the reported OFID results of pentacene in p-terphenyl and obtain for the lowest 1B2u←1A1g transition a dipole moment of 0.9±0.3 D.
On the use of various scaling theories in the deconvolution of rotational relaxation data: Application to pressure-broadened linewidth measurements
Authors: DePristo Andrew E. , Rabitz Herschel
Published: February 2003

The inversion of pressure-broadened spectral linewidth data to yield state-to-state rotationally inelastic rate constants is considered. Deconvolution procedures are presented based upon the rate relationships given by the information theory (IT), infinite order sudden (IOS), and dynamic-coupling theory (DCT) formalisms. In addition, a statistical analysis for the IOS and DCT inversion methods is developed. The three scaling theories are applied to the inversion of experimental data for the CO–He, CO–Ne, HCl–He, and HCl–Ne systems. The results indicate that the IT formalism is inappropriate for deconvoluting linewidth data. For the IOS method the extreme sensitivity of the rates to uncertainties in the measurements can often make the extracted rates unreliable. The DCT inversion procedure is shown to be stable and to yield reliable rates. The application of the DCT method to simulated linewidth data for the He–HCN system yields rates in excellent agreement with ab initio calculated values and correctly predicts the propensity for even rotational changes in this near-homonuclear system. The necessary input for an inversion is the widths as a function of rotational level. We conclude that accurate linewidth measurements contain a wealth of detailed state-to-state collisional information which can now be obtained directly by use of the DCT inversion technique.
The thermodynamic equation of state for insoluble monolayers. I. Uncharged films
Authors: Gaines George L.
Published: February 2003

The applicability of a thermodynamic equation of state to the Π–A curves of insoluble monolayers is examined. The equation of state is based on the Gibbs dividing surface formalism and takes explicit account of the subphase water. It is found that the entire course of the Π–A isotherm can be accounted for qualitatively with an activity coefficient for the water in the surface region. In particular, the expanded film Π–A curves of many compounds can be quantitatively reproduced with a surface-pressure independent activity coefficient which is evaluated from the measured value of the surface vapor pressure. This result implies that compression in the expanded film regime corresponds only to a reduction in the water content of the film. When the equation of state is applied to mixed-surfactant monolayers, it is shown that only if the activity coefficient of water is the same in both single component films and the mixed film do we obtain the surface area additivity, which has been used as a criterion of ideality of mixing in the past.
The electronic structure of Ni– and Ni2–ethylene cluster complexes
Authors: Basch Harold , Newton M. D. , Moskowitz J. W.
Published: February 2003

The electronic structure of metal cluster–ethylene complexes has been investigated by carrying out ab initio bonding pair-correlated, self-consistent field, and configuration interaction (CI) calculations on the NiC2H4 and Ni2C2H4 species. The π-NiC2H4 and π-Ni2C2H4 cluster complexes are found to be bound, the former only with CI, while diσ-Ni2C2H4 has only a repulsive Ni2–C2H4 ground state potential curve. The bonding in the π-type cluster complexes can be described as follows: The metal atom configuration is 3d94s1 with the 4s hybridized (by the metal 4p) away from the ethylene molecule, thereby allowing the π orbital to form a dative σ bond with the metal atom. The bonding interaction is promoted by the presence of a second nickel atom behind the first one, leading to a 4s orbital electron deficiency of the bonded nickel atom and thus making this nickel atom a better electron acceptor. Back donation from the occupied metal 3d into the ethylene π* molecular orbital also takes place to some extent, and thus both features of the classical Dewar–Chatt–Duncanson model are observed. The π-Ni2C2H4 species is analyzed in terms of the addition of a bare nickel atom to a π-NiC2H4 cluster complex, with concomitant stabilization of the orbitals of the bonded nickel atom. A study of the excited electronic states of π-NiC2H4 shows that low-lying 4s→π* and 3d→π* (M→L) charge transfer transitions are predicted. The former is not observed experimentally, probably due to the diffuse nature of the 4s orbital. The relationship between small cluster–ethylene complex systems and ethylene chemisorption on a nickel metal surface is discussed.
Effect of molecular anisotropy on beam scattering measurements
Authors: Goldflam Rudolf , Green Sheldon , Kouri Donald J. , Monchick Louis
Published: February 2003

Within the energy sudden approximation the total integral and total differential scattering cross sections are given by the angle average of scattering cross sections computed at fixed rotor orientations. Using this formalism we have examined the effect of molecular anisotropy on scattering of He by HCl and by CO. Comparisons with accurate close coupling calculations indicate that this approximation is quite reliable, even at very low collision energies, for both of these systems. Comparisons are also made with predictions based on the spherical average of the interaction. For HCl the anisotropy is rather weak and its main effect is a slight quenching of the oscillations in the differential cross sections relative to predictions of the spherical averaged potential. For CO the anisotropy is much stronger, so that the oscillatory pattern is strongly quenched and somewhat shifted. It appears that the sudden approximation provides a simple yet accurate method for describing the effect of molecular anisotropy on scattering measurements.
Quenching and reactions of laser-excited I(5 2P1/2) atoms with halogen and interhalogen molecules
Authors: Hofmann Hubert , Leone Stephen R.
Published: February 2003

Rate constants for the collisional deactivation of spin–orbit excited I(5 2P1/2) atoms with the halogens I2, Br2, Cl2, and interhalogens IBr, ICl, and BrCl have been determined using laser-excited, time-resolved infrared fluorescence techniques. The measured rate constants are (cm3 molecule−1⋅sec−1) 3.1 ±0.5×10−11, 5.2±0.3×10−11, 1.7±0.2×10−12, 6.6±0.3×10−11, 2.3±0.2×10−11, and 2.7±0.2 ×10−11, respectively. These results are discussed in terms of a collision complex formation model in which reactive-type collisions may predominate in the deactivation process under investigation. An excited Br* product is observed in the reactive collision systems I*+Br2→IBr+Br* and I*+IBr→I2+Br*. With an appropriate analysis of the kinetics involved, the fractions of the total deactivation rate constants attributed to these reactive channels are 15%±5% and 13%±5%, respectively.
Exciton migration and energy transfer in crystalline pyrene
Authors: Lebovitz Z. , Mansour S. , Weinreb A.
Published: February 2003

The temperature dependence of the rate of energy transfer from pyrene to rubicen and to coronen in multicrystalline pyrene films is investigated. The activation energy of the transfer process depends strongly on the nature of the guest molecule. A two stage model is described, consisting of a hopping process of the excimeric exciton with successive transfer of the excitation energy to the guest molecule which lies close to one of the excimers. For the hopping process a semiclassical and a quantum mechanical treatment are given. The temperature dependence of the second stage is treated assuming a dipole–dipole interaction. The experimental results are compared with those predicted by the model.
The spin-optimized SCF general spin orbitals. II. The 2 2S and 2 2P states of the lithium atom
Authors: Takatsuka K. , Fueno T.
Published: February 2003

The spin-optimized SCF general-spin–orbital (SO–SCF–GSO) method, which has previously been proposed by us, is applied to the 2 2S and 2 2P states of a lithium atom. The energies obtained are −7.448522 and −7.381053 hartree, respectively, which account for as much as 99.7% (2 2S) and and 97.7% (2 2P) of the radial limits of electron correlation. However, the Fermi contact terms calculated [2.750 (2 2S) and −0.1953 (2 2P)] are not necessarily improvements over the values obtained by hitherto-known orbital theories. This latter result is ascribable to the functional forms of the orbitals, which cannot fully account for the electron correlation without unduly reducing the weight for the configurations of spin polarization. The situation has been clarified for the case of the 2 2P state through comparative analyses of various existing orbital theories.
On the theory of dipolar fluids and ion–dipole mixtures
Authors: Chan Derek Y. C. , Mitchell D. John , Ninham Barry W. , Pailthorpe Bernard A.
Published: February 2003

The Stillinger–Lovett condition for the second moment of ion–ion distribution functions is obtained from first principles for an ion–dipole mixture. The Debye–Hückel form for the asymptotic behavior of the potential of mean force between ions is shown to emerge for arbitrary density of the dipolar solvent in the limit of low ionic concentration.
Predissociation photofragment spectroscopy of O+2 quartet states
Authors: Tadjeddine M. , Abouaf R. , Cosby P. C. , Huber B. A. , Moseley J. T.
Published: February 2003

The technique of predissociation photofragment spectroscopy is applied to a study of the photodissociation of O+2, primarily in the 5750 to 5870 Å wavelength region. The major part of this predissociation is shown to be due to the transition b 4Σ−g←a 4Πu, with predissociation of levels N′=9–23 of a (v′=4) and N′=7–15 of a (v′=5) observed. The N′=9 level of a (v′=4) is shown to be 1.1±1.1 meV above the O(3P2)+O+(4S0) dissociation limit, providing, in conjunction with other previously measured spectroscopic quantities, improved dissociation energies for the O+2(a 4Πu) and (b 4Σ−g) states, as well as for the previously reported O2(X3Σ−g). The lifetimes of the N′=9–15 levels are shown to be (70±15) ×10−11 sec for the F ′1 and F ′4 components, and (42±8) ×10−11 sec for the F ′2 and F ′3 components, with the lifetimes of the F ′1 and F ′4 components at least 1.5 times greater than the lifetimes of the F ′2 and F ′3 components.
Reactivity bands in atom–molecule collisions. IV. Coplanar and 3D studies of T+HT
Authors: Wright James S.
Published: February 2003

The system T+HT, which showed collinear reactivity bands for T+HT→TH+T, is studied in 1D, 2D, and 3D for all possible orientations. The energy range is 0–2 eV (c.m.). Two and three dimensional Monte Carlo studies show no evidence of bands in the total reaction probability. Coplanar reactivity maps are analyzed to explain why the bands disappear. It is shown that the region leading to multiple collisions occupies a very narrow portion of the total configuration space available to reactants. This region is masked by the large reactivity for nonlinear orientations. Some systems which may show more persistent banding effects are proposed.
Dissociation energies of the molecules RhTh and RhU from high temperature mass spectrometry and predicted thermodynamic stabilities of selected diatomic actinide–platinum metal intermetallic molecules
Authors: Gingerich K. A. , Gupta S. K.
Published: February 2003

Gaseous RhTh and RhU have been observed in a Knudsen effusion mass spectrometric investigation of a thorium–uranium-rhodium–graphite system at high temperatures (2400–2700 °K). Thermodynamic treatment of the experimental data yielded the atomization energies of RhTh and RhU as D0298(RhTh) =513±21 kJ mole−1 or 122.6±5 kcal mole−1 and D0298(RhU) =519±17 kJ mole−1 or 124.0±4 kcal mole−1. These values were derived from the third law enthalpies of the following reactions. The known experimental bond energies for the ligand-free gaseous intermetallic compounds between the actinides and the platinum metals have been interpretated in terms of a previously developed model based on the Brewer–Engel approach. Also, calculations are presented for the dissociation energies of certain selected actinide–platinum metal diatomic molecules which have not yet been experimentally observed.
Perpendicular vibrational predissociation of T-shaped van der Waals molecules
Authors: Beswick J. A. , Jortner Joshua
Published: February 2003

In this paper we present a numerical quantum mechanical study for vibrational predissociation on a single electronic potential surface of a T-shaped van der Waals molecule B X⋅⋅⋅‖ C where X is a rare-gas atom while BC is a conventional diatomic molecule. The rare-gas atom is constrained to move on a line perpendicular to the interatomic axis of the BC molecule. The interaction between X and atoms B and C is represented by a sum of Morse atom–atom potentials. The close-coupling scattering equations were solved numerically and the vibrational predissociation rates were related to the widths of the resulting resonances. The method is applied to the HeI2 (B) van der Waals complex. We also present numerical results for the vibrational relaxation of linear X⋅⋅⋅B–C molecules where the three atoms are restricted to move on a single line. Good correspondence with the approximate (distorted wave) analytical results previously derived by us for ArI2, NeI2, and HeI2 is obtained.
Matrix reactions of magnesium atoms with ozone. Infrared spectra of MgO, MgO2, and MgO3 in solid nitrogen
Authors: Andrews Lester , Prochaska Eleanor S. , Ault Bruce S.
Published: February 2003

The magnesium–ozone reaction has been studied by condensing the reagents with nitrogen at 14 K. Isotopic substitution with 26Mg, 16,18O3, and 18O3 has identified infrared absorptions due to two different Mg+O−3 species, MgO2 and MgO. Six isotopic MgO molecules support this infrared detection of MgO which shows a blue matrix shift. Analogous argon matrix reactions produced only Mg+O−3 and two isomeric MgO3 species.
Dielectric friction and molecular reorientation
Authors: Hubbard Joseph B. , Wolynes Peter G.
Published: February 2003

We present a theory of the effect of dielectric friction on molecular reorientation. This theory is based on a rotational Smoluchowski equation with fluctuating torques due to polarization fluctuations. This theory predicts several interesting consequences of dielectric friction which should be susceptible to experimental test: In contrast to previous theories, dielectric friction is shown to have a decreased effect for higher l rotational correlation functions. The theory also leads to unusual results for molecular reorientation in polar mixtures.
Rate constants for the deactivation of O(1D) by Xe, Kr, and Ar over the range 110–330 K
Authors: Davidson J. A. , Schiff H. I. , Brown Terry J. , Streit G. E. , Howard Carleton J.
Published: February 2003

The rate constants for the deactivation of O(1D) by Xe, Kr, and Ar were measured over the range 110–330 K. Only Xe had a significant temperature dependence. Its Arrhenius expression is { (8.5±1.7) ×10−11 exp[(−103±52)/RT] cm3/molecule⋅s} (E in cal/mole). The rate constants for Kr and Ar are (6.4±1.3) ×10−12 and (5.0±1.5) ×10−13 cm3/molecule⋅s, respectively. The observed trend in deactivation efficiency, Xe≳Kr≳Ar, and the observed temperature effects support the results and the spin–orbit interaction mechanism for deactivation given by Husain and co-workers.
Vibrational relaxation of water molecules near room temperature
Authors: Shin H. K.
Published: February 2003

The vibrational de-excitation process H2O(010)+H2O(000) →H2O(000)+H2O(000) has been investigated to determine its temperature dependence at low temperatures recognizing the manifestation of strong molecular attraction at such temperatures and considering the transfer of vibrational energy to both the oscillatory motion of weakly-bound complexes and the free rotational motion. The model predicts large values of the de-excitation probability with a strong negative temperature dependence in 200–400 °K; the probability varies from 0.46 at 200 °K to 0.16 at 400 °K.