bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational hyper-Raman excitation profiles: Further evidence of J-dependent subpicosecond dynamics of NH3
Authors: Chung Y. C. , Ziegler L. D.
Published: July 2002

Resonance hyper-Raman (RHR) excitation profiles of rovibrational transitions of NH3 are observed as the incident blue radiation is tuned through two-photon resonance with the v″2 =2 and v″2 =3 bands of the UV X→A Rydberg transition. The excitation frequency dependence of the rotationally resolved HREPs are fit by our previously derived intensity expressions [L. D. Ziegler et al., J. Chem. Phys. 87, 4498 (1987)]. These fits to theory reveal J-dependent photodissociation rates for both vibronic bands on the A state surface that correspond to subpicosecond lifetimes. The rotationally assisted dynamical effects are consistent with the previous results of linear rotational Raman excitation profile studies and with an adiabatic centrifugal mechanism which couples rotational motion with the photodissociative reaction coordinate.
Theoretical study of B2H+4 and B2H4
Authors: Curtiss Larry A. , Pople John A.
Published: July 2002

An ab initio molecular orbital study is presented of B2H+4 and B2H4 at the G1 level of theory, including correlation energy beyond fourth-order perturbation theory and large basis sets. The B2H+4 ion is predicted to have a nonplanar doubly hydrogen-bridged C2v structure [HB(H2)H]. For B2H4, two structures of nearly equal energy are found, confirming previous work by Mohr and Lipscomb. One is a nonplanar doubly hydrogen-bridged structure similar to the cation and the other is a D2d H2BBH2 structure. The theoretical results suggest that the recently measured appearance potential for B2H6→B2H+4+H2+e does not correspond to the lowest energy form of B2H+4, but rather to a triply bridged isomer closely related to the most stable form of B2H+5.
Vibrational energy relaxation in a molecular monolayer at a metal surface
Authors: Harris A. L. , Levinos N. J.
Published: July 2002

Picosecond time-resolved measurements of vibrational energy relaxation from ν=1 to ν=0 for C–H stretching modes of the terminal methyl group in 9 Cd stearate Langmuir–Blodgett monolayer on an evaporated silver film were made. The experiments used infrared–visible sum spectroscopy to dynamically probe vibrational level populations. To the authors’ knowledge, this is the first direct measurement of vibrational energy relaxation for molecular adsorbates at a bulk metal surface. Multicomponent decay processes with lifetimes of 3 ps to >1 ns indicate complex intramolecular vibrational energy transfer processes in these ordered monolayer films, which may be different than for similar molecules in liquids.
Conformational changes upon S1←S excitation in 4-dimethylaminobenzonitrile and some of its chemical analogs
Authors: Grassian V. H. , Warren J. A. , Bernstein E. R. , Secor Henry V.
Published: July 2002

One-color time-of-flight mass spectra (mass resolved excitation spectra) for jet-cooled 4-dimethylaminobenzonitrile (4-DMABN) and some of its chemical analogs, dimethylaniline (DMA), 3-dimethylaminobenzonitrile (3-DMABN), N, N-dimethyl-4-(trifluoromethyl)aniline (4-CF3-DMA), and 4-(d6-dimethylamino)benzonitrile (4-d6-DMABN), are presented and analyzed. Near the origin of the S1←S0 transition the low frequency modes can be assigned to motions of the dimethylamino group for this series of molecules. The inversion motion of the dimethylamino group and the dimethylamino group torsion about the Cipso–N bond (the twist coordinate) in S1 give rise to the most prominent peaks in this spectrum. The potential parameters for the twist coordinate of 4-DMABN and DMA are quite similar in S1: B=0.546, V2=175, and V4=525 cm−1 for 4-DMABN and B=0.546, V2=175, and V4=515 cm−1 for DMA. The V2 and V4 terms are slightly larger for 3-DMABN and 4-CF3-DMA. The inversion motion is also similar for these molecules but is more anharmonic for the para-substituted dimethylanilines, 4-DMABN and 4-CF3-DMA, than for the meta and unsubstituted molecules. A Franck–Condon intensity analysis for the dimethylamino twist in these molecules suggests that this group in 4-DMABN is displaced in the excited state by ∼30° with respect to its planar orientation in the ground state. In both solutions and monosolvate clusters of 4-DMABN with polar aprotic solvents, a low lying charge transfer (CT) state is identified in addition to the usual ππ* excited state of the bare molecule. The relation between the bare molecule 4-DMABN twisting displacement upon excitation and the low lying CT state is discussed.
Analysis of the heat capacity results of the methylpyridines
Authors: Smith David
Published: July 2002

The heat capacity due to the rotation of the methyl group on the monomethylpyridines is studied by subtracting the heat capacity of 3-methylpyridine from 4-methylpyridine and 2-methylpyridine from 4-methylpyridine. Both difference curves have a hump at about 20 K that is due to the very low barrier to rotation of the methyl group in 4-methylpyridine. The librational and tunneling frequencies of the methyl group in the monomethylpyridines are determined from the difference curves. A few other systems are also considered: 3-methyl-1-butanethiol(I) and 3-methyl-2-butanethiol(II), and 4,6-dimethylindan and 4,7-dimethylindan.
Accurate determination of a potential energy surface for CD3H
Authors: Iung Christophe , Leforestier Claude
Published: July 2002

We present in this paper an application of the Lanczos algorithm to the fitting of a potential energy surface from the experimental spectrum. This method presents two definite improvements on the conventional approach: (i) the Lanczos algorithm allows to treat ultra-large basis sets (120 000 states in this calculation) and (ii) it is possible to tune selectively the calculation to specific components of the spectrum (e.g., a given combination band nνi+mνj) thus reducing considerably the complexity of the fit. This method has been applied to the CD3H molecule, considering all the vibrational degrees of freedom. Converged line positions have been obtained for high overtones of the C–H stretching mode (up to 16 000 cm−1). The accuracy of the fitted surface is demonstrated by direct comparison of experimental and calculated spectroscopical parameters.
Comparison of small shear flow rate–small wave vector static structure factor data with theory
Authors: Ackerson Bruce J. , De Kruif C. G. , Wagner Norman J. , Russel William B.
Published: July 2002

Light scattering measurements determined the ratio of the nonequilibrium to equilibrium static structure factor for suspensions of ‘‘hard spheres’’ undergoing shear flow as a function of volume fraction of the suspended particles, the shear rate, and a limited number of scattering angles. These data are compared with existing theoretical predictions for the static structure factor ratio in the limit of small shear flow rates. We conclude that the existing theories, some which are considered exact at low volume fractions, give increasingly poor estimates of the true structure factor at small scattering angles as the volume fraction increases.
Arrangement channel approach to atom–diatom reactive systems: Theory and accurate three-dimensional probabilities for the H+H2 system
Authors: Baer Michael
Published: July 2002

In this work is presented an integral equation approach to exchange collisions between an atom and a diatomic molecule which result from a straightforward extension of the Lippmann–Schwinger equation originally devised to inelastic collisions. The extension to N-arrangement channels is done via a N×N matrix W which is responsible for the explicit coupling among the various arrangement channels. Different choices of the W matrix elements lead to different sets of integral equations; one of these, in which the W matrix elements are assumed to be dependent on the three orientation angles γα; α=λ, ν, k only, is discussed in detail. In addition to the theoretical derivations, we refer to the corresponding coordinate representation, discuss numerical aspects and present accurate results of the three-dimensional–three-channel H+H2 system.
Analysis of a four-variable model of coupled chemical oscillators
Authors: Hocker Craig G. , Epstein Irving R.
Published: July 2002

A simple model for the coupling of two chemical oscillators is proposed. The model consists of twotwo-variable subsystems, each of which gives rise to a ‘‘cross-shaped phase diagram’’ containing two different steady states plus regions of bistability and of oscillation. The subsystems are coupled by diffusion of both species. Extensive numerical simulations reveal that when one subsystem is in the bistable and the other in the oscillatory region of the parameter space, coupling can result in birhythmicity, period doubling, and chaos. When both subsystems are in the oscillatory region, the coupled behavior is even richer, including quasiperiodicity, entrainment, chaos, and phase death (cessation of oscillations), with various hysteresis phenomena between these modes of behavior. The relation of these observations to behavior found experimentally in coupled chemical oscillators is discussed.
The kinetics of CO dissociation on Ru(001): Time-resolved vibrational spectroscopy at elevated pressures
Authors: Hoffmann F. M.
Published: July 2002

Time-resolved Fourier transform-infrared reflection absorption spectroscopy (FT-IRAS) has been utilized to measure the kinetics of CO dissociation on a Ru(001) surface at elevated pressures (10−3 to 10 Torr) and temperatures (500–700 K). The reaction of CO with Ru(001) is found to be a nonsteady state and results in CO disproportionation, i.e., 2CO→C+CO2. The decrease in total CO coverage follows first order kinetics and exhibits Arrhenius behavior with an activation energy of 20.6 kcal and a preexponential factor of 102 s−1. Comparison of the overall reaction rate with that of CO2 formation (O+CO→CO2) confirms that CO dissociation is the rate-limiting step in the disproportionation reaction. The in situ reaction rate constant exhibits a weak dependence on CO pressure (<first order). However, the determination of local CO coverages during reaction reveals a linear dependence of the dissociation rate with CO coverage. This confirms that the chemisorbed state of the molecule is a precursor to dissociation and that a high pressure is required to maintain a steady state surface coverage of CO at reaction temperature. In situ vibrational spectra demonstrate the formation of carbon islands under reaction conditions which prevent further CO adsorption and result in a decrease in total CO coverage at constant local CO coverage. Post-reaction spectroscopy confirms the formation of two-dimensional islands of carbon whose reactivity toward oxidation is found to be between that of amorphous carbon and three-dimensional graphite.
Polarized Raman spectra for the full range of isotopic dilution for ice Ic and amorphous ice: Mixtures of intact H2O and D2O
Authors: Devlin J. Paul
Published: July 2002

The 90 K polarized Raman scattering of thin films of ice Ic and ice Ia containing mixtures of intact H2O and D2O have been measured using the interference-enhanced internal-reflection technique. The effects of isotopic dilution reported for samples with a D2O content ranging from 0% to 100% are consistent with the commonly held view that intermolecular coupling produces collective vibrational modes that determine the appearance of the Raman spectra in the stretching mode region of crystalline and amorphous ice. The convergence of bands produced by the collective modes on the isotopically decoupled mode frequencies of both ν1 and ν3 is displayed in this study. Bands at 3323 and 3209 cm−1 in the anisotropic scattering of protiated ice Ic are shown to converge on the decoupled antisymmetric stretch frequency of 3370 cm−1. The gradual shift of the dominant isotropic scattering at 3085 cm−1 towards the frequency of the decoupled ν1 mode is also revealed, with bands for the first stage in the evolution of the in-phase ν1 collective mode, i.e., the neighbor coupled mode, observed at 3180 (H2O) and 2340 cm−1 (D2O). The data also suggest that the shift of the in-phase coupled ν1 band begins with the first traces of added diluent, a behavior that indicates a macroscopic extension of the resonant coupling for both crystalline and amorphous ice. By contrast, the insensitivity of isotropic scattering near 3420 cm−1 to isotopic dilution suggests that the overtone of the bending mode is the likely source.
Laser-induced emission of CH3O in solid argon
Authors: Chiang Su-Yu , Hsu Yen-Chung , Lee Yuan-Pern
Published: July 2002

The A 2A1−X 2E fluorescence of CH3O in solid Ar in the wavelength range 310–420 nm has been studied either by simultaneous laser photolysis and excitation of CH3ONO in the Ar matrix or by laser excitation of the products deposited from the reactions of microwave-discharged CF4 with CH3OH diluted by Ar. The spectrum showed an extensive progression in C–O stretching (ν3). The zero-phonon lines of 12CH3O and 13CH3O yielded unambiguous vibrational assignments with ν00=31 291, ω″e=1051, and ω″ex″e=6.5 cm−1 for 12CH3O. Observation of several weak combination bands also yielded ν″2=1356, ν″4=2758, and ν″5=1406 cm−1. The laser excitation spectra in the 273–322 nm region also exhibited an intense progression in C–O stretching with ω′e=657 and ω′ex′e=4.4 cm−1 for 12CH3O. Additional wave numbers ν′2=1308 and ν′5=1410 cm−1 were also obtained from the combination bands.
The van der Waals equation of state around the van Laar point
Authors: Meijer Paul H. E.
Published: July 2002

The discovery of van Laar that the van der Waals equation of state for binary mixtures can be solved analytically at the double point on the critical line, provided one introduces the geometric-mean condition on the interaction parameters, is used to obtain the explicit expression for the critical line for this case. The critical line is expressed as a function of the density variables, and the origin is shifted to this double point: the van Laar point. By doing so it is possible to show that this double point is also a tricritical point. This is different from the lattice gas, where the double point is always a tricritical point. Small deviations from this point in parameter space induce very different phase diagrams. The influence of excursions from the van Laar point is expressed as a function of the state variables. Both the k factor (the deviation from the geometric-mean rule) and the ‘‘asymmetry’’ coefficient e (the deviation from the crossing point) are introduced. The results are given in the form of polynomials in local coordinates in density space. Conditions under which the double point is maintained are given and the differences between the Scott and van Konynenburg classes II, III, IV, and IV* are explained.
State-to-state dynamics of H+HX collisions. I. The H+HX→H2+X (X=Cl,Br,I) abstraction reactions at 1.6 eV collision energy
Authors: Aker Pamela M. , Germann Geoffrey J. , Valentini James J.
Published: July 2002

The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti-Stokes Raman scattering spectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H2 product vibrational excitation. For this reaction the H2 vibrational distribution is strongly inverted and peaks in v′=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H2 vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v′=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H2 rotation. All three reactions show a strong propensity to conserve the translational energy, that is the translational energy of the H2+X products is very nearly the same as that of the H+HX reactants. For the reactions with HCl, HBr, and HI the average translational energy of the products is 1.3, 1.7, and 1.7 eV, respectively, and the width of the translational energy distribution is only about 0.5 eV full width at half maximum. The energy disposal in all three reactions is quite specific, despite the fact that this high collision energy is well above the barrier to reaction in all three systems and a large number of product quantum states are energetically accessible. Only a few of these energetically allowed final states are appreciably populated. Although detailed theoretical calculations will be required to account completely for the state specifity, quite simple models of the reaction dynamics can explain much of the dynamical bias that we observe.
Statistical thermodynamics of flexible-chain surfactants in monolayer films. I. Theory of fluid phases
Authors: Cantor Robert S. , McIlroy Peter M.
Published: July 2002

A general statistical mechanical theory is developed to describe structural and thermodynamic properties of surfactant monolayer films at the interface between water and a hydrophobic solvent. It is assumed that the surfactants are comprised of a single head group and one or more flexible hydrocarbon tails, and that the head group serves only to constrain one end of the molecule to the aqueous interfacial plane. Each chain is characterized by the profile of volume it occupies perpendicular to the interfacial plane. Since the position of the maximum in the volume profile varies with conformation, the lateral excluded area of each conformation is approximated as an average over all pairs of conformations. By assuming ‘‘ideal’’ two-dimensional mixing of solvent with the chains, and of chains with each other, the entropy of the monolayer is then determined. For purposes of determining interaction energies, the surfactant chains are also characterized by the position and orientation of their surface area available for nearest-neighbor contact. The orientational distribution of chain surface may be highly anisotropic, particularly at high molecular surface densities when the chains are largely aligned, so the total area of intermolecular contact cannot be determined from the chain segment profile alone, as in regular solution theory. Interaction energies among chain, solvent, and water are reduced to two parameters, one related to the chain–solvent interfacial tension, and the other to the difference of chain–water and solvent–water interfacial tensions. The equilibrium chain probability distribution is obtained by minimizing the free energy with respect to the distribution, from which all structural and thermodynamic properties can be predicted. In the subsequent paper (part II), pressure-area isotherms are predicted using a modified cubic lattice model for the chains, and shown to be characterized by two first-order phase transitions.
Electron degradation and yields of initial products. IV. Subexcitation electrons in molecular oxygen
Authors: Ishii M. A. , Kimura Mineo , Inokuti Mitio , Kowari Ken-ichi
Published: July 2002

Electron slowing-down processes in molecular oxygen gas in the subexcitation domain (below the ionization threshold) are studied by using the Spencer–Fano (SF) equation and its simplification, the continuous-slowing-down approximation (CSDA), both in time-dependent and time-independent representations. Compared to the previously studied cases of N2 and CO2, O2 has the special features in its inelastic cross sections of (i) strong delta-function-like peaks in the vibrational excitation cross section below 1.3 eV and (ii) very low energy thresholds of electronic excitation channels. These features provide a stringent test for the CSDA. Indeed, our results clearly show for the first time that the CSDA fails even qualitatively to reproduce the electron degradation spectrum given by the exact SF method over the whole energy regime studied.
Correlation and scattering functions in a lattice model of oil–water–amphiphile solutions
Authors: Widom B.
Published: July 2002

The scattering function is calculated in mean-field approximation for two different thermodynamic states of a lattice model of oil–water–amphiphile solutions, one a disordered state and the other a one-dimensionally ordered (lamellar) state. The total scattering function for the former, and the fluctuation contribution to the scattering function for the latter, have maxima near q=1, where q is the magnitude of the scattering vector q in units of the reciprocal lattice spacing. For q up to and a little beyond the maximum the scattering functions and their spherical averages (averaged over the surface of a sphere of radius q in q space) are of the Teubner–Strey form I(q)=I(0)/(1−bq2+cq4) with b and c both positive, as in microemulsions. It implies a correlation function that is an exponentially damped sinusoid, of wavelength d and attenuation length, determined by b and c.
Femtosecond studies of electron photodetachment of simple ions in liquid water: Solvation and geminate recombination dynamics
Authors: Long Frederick H. , Lu Hong , Eisenthal Kenneth B.
Published: July 2002

Femtosecond studies of electron photodetachment in aqueous solutions of NaCl and NaOH have been performed. The decays observed in the solvated electron absorption signal we believe to be the first direct observation of geminate electron–atom recombination. The solvation dynamics of the electron originating from a Cl− ion are slower, 500 fs, than the electron originating from a neutral water molecule, 350 fs. The recombination and solvation dynamics strongly suggest that short-range molecular effects are important.
A realistic molecular dynamics simulation of the plastic crystalline phase of neopentane. I. The model and its static properties
Authors: Breymann W. , Pick R. M.
Published: July 2002

A molecular dynamics simulation of a realistic model of neopentane [C(CH3)4] in its plastic phase has been performed on a sample of 6×6×6 fcc unit cells (i.e., 864 molecules) at 135, 175, and 230 K. The molecules of the simulated sample interact through phenomenological exp-6, atom–atom potentials between all the atoms of nearest neighbor molecules. The orientational probability density function (opdf), the displacement probability density function (dpdf), and its second moment the Debye–Waller factor have been computed. We confirm the very large value and the important thermal variation of the Debye–Waller factor and the strong anisotropy of the opdf deduced from neutron diffraction experiments. The computed opdf is very well reproduced by a mean-field calculation making use only of the microscopic intermolecular potential and of the equilibrium position of the molecular centers of mass, a result in line with the isotropic character of the dpdf, but not valid for other plastic crystals made of molecules with different geometries.
A simplified proof of the Kosloff–Rice theorem: Intramolecular quantum dynamics cannot be chaotic
Authors: Manz J.
Published: July 2002

The title theorem follows from formally equivalent time evolutions of the bounded systems’ quantum wave function described by the Schrödinger equation, and regular classical trajectories described by corresponding Hamilton’s equations. Instructive model applications include dynamics of molecules with time-dependent Hamiltonians, e.g., intramolecular vibrational energy redistributions, and state selective vibrational transitions induced by periodic sub-ps IR laser pulses.
Electron attachment reactions of perfluoroalkyl transition metal carbonyls: Rate constants and product analysis
Authors: Marotta Christopher J. , Tsai Cheng-ping , McFadden David L.
Published: July 2002

Absolute rate constants for electron attachment reactions of CF3Mn(CO)5, CF3Co(CO)4, and C2F5Co(CO)4 have been measured in a gas flow system with electron cyclotron resonance (ECR) detection. Free electrons were generated by Penning ionization of argon gas by helium metastables produced in a low power radio-frequency discharge. The pressure was 2.3–2.5 Torr. The rate constants at 293 K in units of cm3 molecule−1 s−1 are (3.5±0.6)×10−7 for CF3Mn(CO)5; (2.0±0.4)×10−7 for CF3Co(CO)4; and (1.8±0.3)×10−7 for C2F5Co(CO)4. Negative ion mass spectra indicate that nondissociative attachment is important at thermal electron energy. The cross section for nondissociative capture falls rapidly with increasing electron energy. Dissociative product channels are also observed. Electron capture rate constants for several additional molecules are presented and compared with values from the literature. The rate constants in units of cm3 molecule−1 s−1 are 2.8×10−7 for SF6; 3.7×10−7 for CCl4; 3.1×10−7 for CFCl3; 1.8×10−9 for CF2Cl2; 1.5×10−8 for CF3Br; 3.8×10−9 for CHCl3; and 1.2×10−8 for C2F3Cl3.
Reactant states model: Predicted k(E,J) for NO2(2A1)→O(3P)+NO(2Π), based on spectroscopic data
Authors: Toselli Beatriz M. , Barker John R.
Published: July 2002

High-order spectroscopic data for the reactant are used exclusively to determine both the sum of open reactive channels and the density of states, which are used in a statistical theory to predict dissociation rate constants. Practical methods are introduced for calculating sums of reactive channels and densities of states, when couplings among all degrees of freedom are included. An empirical method is described for reconciling spectroscopic parameters with known dissociation energies (also determined spectroscopically). The predicted k(E,J)’s and thermal k∞(T) for NO2 dissociation are in good agreement with experimental data, especially when the effects of electronically excited states are included. The predicted low pressure thermal rate constants are generally in fair agreement with experiment, although a slightly different temperature dependence is calculated; this discrepancy is probably due to the absence of unknown higher order spectroscopic terms and to the crude corrections made for excited electronic states. When high order spectroscopic (or theoretical) data are available and when the effects due to excited electronic states are considered, this theory is useful for predicting, fitting, and interpreting unimolecular rate data.
Collisional electron detachment and decomposition cross sections for SF−6, SF−5, and F− on SF6 and rare gas targets
Authors: Wang Yicheng , Champion R. L. , Doverspike L. D. , Olthoff J. K. , Van Brunt R. J.
Published: July 2002

Absolute total cross sections for collisional electron detachment and collision-induced dissociation (CID) have been measured for binary collisions of SF−6 and SF−5 with rare gas and SF6 targets for laboratory collision energies ranging from about 10 up to 500 eV. The cross sections for electron detachment of SF−6 are found to be surprisingly small, especially for the SF6 target, for relative collision energies below several tens of electron volts. Specifically, detachment onsets are found to occur at around 30 and 90 eV for the rare gas and SF6 targets, respectively. The CID channel which leads to F− as a product is observed to dominate detachment for relative collision energies below 100 eV. The results for the SF−5 projectile are remarkably similar to those exhibited for SF−6. The role of long-lived excited states in the reactant SF6 ion beam is discussed.
Raman line shape analysis in isotopic mixtures of liquid hydrogen and deuterium chloride
Authors: Oehme Karl-Ludwig , Klostermann Klaus
Published: July 2002

Raman scattering in liquid (and in some cases in solid) isotopic mixtures of HC1 and DC1 is analyzed to prove recent theories by Bratos and Tarjus [Phys. Rev. A 32, 2431 (1985)], Logan [Mol. Phys. 58, 97 (1986)], and Knapp [J. Chem. Phys. 81, 643 (1984)] on vibrational line broadening in liquids. The concentration and temperature dependencies of isotropic [Ji(ω)] and anisotropic [Ja(ω)] line shapes have been studied between triple point (Tt) and critical temperature (Tc). It has been found that in accordance with the Bratos–Tarjus theory, Ji (ω) is much more sensitive of isotopic composition of the liquid than Ja(ω). An analysis of the concentration dependence of the broadening parameters near Tt illustrates the importance of cross correlations between the environmental broadening and the resonant intermolecular coupling. The spectral activity of three-particle resonant transfer also becomes significant. From the change of the maximum of Ji(ω) with isotopic dilution, which is a linear function of mole fraction, the dipole moment derivative δμ/δq is estimated to be more than twice that of its gas phase value. The asymmetry of the isotropic bands of both HC1 and DC1 changes with concentration at constant temperature. With increasing temperature, Ji(ω) of pure and diluted samples narrows as T−0.5 and T−0.3, respectively. Ji(ω) has been found to be intermediate between the slow and the fast modulation limit. From the high frequency wing of Ji (ω) the time constant of the zeroth order memory function was obtained. Its activation energy increases with increasing T. This is in qualitative agreement with the temperature dependence of the Enskog collision time and the spin–rotational correlation time. Taking into account the results of the Bratos–Tarjus theory, orientational correlation times τ(2) are determined from Ja (ω). The Raman method yields τ(2) values which are twice as long as those determined from NMR relaxation.
The exchange potential in path integral studies: Analytical justification
Authors: Hall Randall W.
Published: July 2002

We present analytical justification for our previously described exchange pseudopotential. We show how the fermi quantum partition function can be constructed from the Boltzmann (distinguishable particle) wave functions if the states that correspond to like-spin electrons occupying the same quantum state are excluded. A class of weighting functions that satisfy this constraint approximately is discussed. Our previous pseudopotential falls under this class. Essentially, our pseudopotential forces the unwanted states to have high energy and, hence, to make negligible contribution to the partition function. Exchange potentials of the form discussed in this article should be useful for studying systems where the (allowed) correlated Boltzmann wave functions have negligible amplitude for like-spin fermion–fermion distances less than the diameter of the individual particle wave packets. For example, in the case of two spin-up (or spin-down) fermions, if one fermion is located at r, then ‖Ψ(r,q)‖2 is negligible if q≂r. This should be the case for systems where a tight binding model is appropriate or for systems with strong interparticle repulsions.
Frequency and intensity analysis of the ν3, ν4, and ν6 bands of formaldehyde
Authors: Reuter D. C. , Nadler S. , Daunt S. J. , Johns J. W. C.
Published: July 2002

The infrared spectra of the ν3, ν4, and ν6 bands of formaldehyde in the region from 890 cm−1 to 1580 cm−1 have been obtained at high resolution using tunable diode laser (TDL) and Fourier transform-infrared (FT-IR) spectroscopy. The transition frequencies have been analyzed using a Hamiltonian including terms through sextic in centrifugal distortion and including five interstate vibration–rotation coupling terms. Excited state pure rotational transitions are also included in the data, and their frequencies are reproduced well. Individual measured line intensities are used to determine dipole derivatives and band strengths using the fully coupled, asymmetric top eigenvectors.
Analysis of the reaction He++He+He→He+2+He. III. Effects of resonant quasibound states
Authors: Russell J. E. , Shyu J. S.
Published: July 2002

Two previous theoretical investigations of the three body association reaction He++He+He→He+2+He, both of them concerned with molecule formation by a mechanism largely classical in nature, are extended to take into account two hitherto neglected quantum effects, each involving resonant quasibound states (RQS) of the He+2 molecular ion and each involving tunneling through the rotational barrier. One of the quantum effects is the effect that the discrete nature of RQS has on outward tunneling after quasibound states are formed by the classical mechanism; and the other, which was proposed originally by Dickinson, Roberts, and Bernstein, is collisional deexcitation of

RQS formed by inward tunneling. While it is found that taking into account the discrete nature of RQS in determining the effect of outward tunneling re− sults in only minimal changes in the rate coefficient, it is also found that the formation of such states by inward tunneling can account for the 10% to 20%

discrepancy between our previous calculations and the experimental results of Johnsen, Chen, and Biondi. As in previous work, computations were performed

using four assumed forms of the He+3 interaction; and while statistical accuracy now permits some of these interactions to be distinguished from one another, each gives a rate coefficient with magnitude and temperature dependence in agreement with experiment. A considerable amount of discussion is devoted to some of the approximations that have been made, principally our use of the JWKB approximation in identifying RQS and our neglect of collisional breakup of newly formed He+2 ions. It is argued that the inaccuracies introduced by these two approximations are likely to be of opposite sign and to have magnitudes not much greater than the experimental error. The uncertainties in our calculation notwithstanding, it is concluded that the inward tunneling mechanism surely plays a significant, though not dominant, role in He+ recombination. Our calculations of the rate coefficient due to inward tunneling are several times smaller than the very rough estimates accompanying the original suggestion of this mechanism, and it is noted that this difference may be due largely to our estimates of cross sections for collisional deexcitation of RQS, which are obtained using the Monte Carlo trajectory method and which have the appearance of being several times smaller than the simple gas kinetic estimates employed by Dickinson et al. Some other possible explanations of this difference are noted.
The A 3Σ−–X 3Π transition of the SiC radical
Authors: Brazier C. R. , O’Brien L. C. , Bernath P. F.
Published: July 2002

The 0–0 band of the A 3 Σ−–X 3 Π system of SiC, analogous to the Ballik–Ramsay system of C2, has been observed in emission near 4500 cm−1. The internuclear separations (r0 ) were found to be 1.813 56 and 1.721 87 Å for the A 3 Σ− and X 3 Π states, respectively.
A collocation approach for quantum scattering based on the S-matrix version of the Kohn variational principle
Authors: Yang Weitao , Peet Andrew C. , Miller William H.
Published: July 2002

A collocation approach to quantum scattering is presented. The method is based on the S-matrix version of the Kohn variational principle with a different linear expansion used for the two wave functions—one is a linear combination of basis functions and the other is a pointwise representation with proper asymptotic conditions imposed. The resulting equations are similar in structure to the usual version of the Kohn variational principle, however, in the present approach there are no integrals between the square integrable (L2) basis functions. In addition, the method does not require the knowledge of quadrature weights associated with the collocation points as was the case in a previous pointwise method for quantum scattering. This property means that the method is readily applicable to reactive scattering problems which use different sets of coordinates for reactants and products. Appliction to a simple inelastic test problem (collinear He–H2 vibrationally inelastic scattering) shows the accuracy of the approach to be comparable to that of the usual variatinal form of the S-matrix Kohn method.
On the inversion of atomic scattering data: A new algorithm based on functional sensitivity analysis
Authors: Ho Tak-San , Rabitz Herschel
Published: July 2002

A new iterative inversion scheme of atomic scattering data within the framework of functional sensitivity analysis is presented. The inversion scheme is based on the first order Fredholm integral equation δσ(θ)=∫∞0K(θ,R)δV(R)R2 dR, K(θ,R)≡δσ(θ)/δV(R), or symbolically, δσ=KδV, which relates infinitesimal functional changes in the elastic differential cross section δσ(θ) and in the underlying interatomic potential δV(R). This equation can be written equivalently, via integration by parts, as δσ(θ)=∫∞0K[n](θ,R) {Rn×δV(n)(R)}R2 dR, or δσ=K[n]{Rn×δV(n)}, under the a priori assumption that {R(2+n)×K[n](θ,R)×δV(n−1) (R)}‖∞R=0=0. Here K[n](θ,R)≡−R−(2+n) ×∫R0K[n−1](θ,R′) R′(1+n)dR′, δV(n)(R) ≡(dn/dRn)δV(R), with n=0,1,2,..., and K[0](θ,R)≡K(θ,R).

A choice of n corresponds to a particular level of additional stabilization inverting the scattering data. By invoking a least squares regularization procedure and singular system analysis, the new indirect inversion scheme solves the linear relation δσ=K[n]{Rn×δV(n)} and results in the approximate solution Rn×δV(n)(R), which can in turn be integrated, n times, to yield the potential correction δV(R). The new algorithm not only makes the inversion more stable and more efficient, but also increases the sensitivity of the large angle scattering data to the repulsive part of the potential, in comparison with a previous method that directly solves the relation δσ=KδV.

For illustration, the model system He–Ne is considered at both low- and high-collision energies, relative to the well depth of the potential. It is found that the indirect method based on the linear relation δσ=K[2]{R2×δV(2)} can more accurately determine both attractive and repulsive parts, including a large section of classically forbidden region, of the potential than its direct counterpart. Furthermore, the indirect method allows for substantial deviations of the initial reference potential from final result, not only in well depth, but in range parameter and asymptotic behavior. Finally, these observations are not significantly dependent on the magnitude of the collision energy.
Molecular integrals over spherical Laguerre Gaussian-type functions
Authors: Matsuoka Osamu
Published: July 2002

Fieck’s formulation of molecular integrals over the spherical Laguerre Gaussian-type functions, L(l+1/2)n(αr2) exp(−αr2)rlYlm(θ,φ) [where L(s)n(x) is the Sonine (or the generalized Laguerre) polynomial and Ylm(θ,φ) is the complex spherical harmonic], has been generalized to include four types of operators: Ynlm(r), Ŷnlm(∇), Ŷnlm(∇)(1/r), and Ŷnlm(∇12)(1/r12), where Ynlm(r) is the homogeneous solid harmonic and Ŷnlm(∇) its operator. These operators represent such various kinds of molecular properties as the energy (i.e., the overlap, the kinetic energy, the nuclear attraction, and the electron repulsion), the electric multipole moments and transitions, the electric field and gradient, the quasirelativistic interactions (e.g., the mass velocity, the spin–orbit, and the spin–spin), etc. Attention has been paid on the aspects of numerical computation of their molecular integrals so that the integral formulas are suitable for computation in the scheme of ‘‘shell structure’’ and over the contracted Gaussian-type functions.
Time dependence of condensation: A kinetic model
Authors: Lippmann David , Schieve W. C.
Published: July 2002

A set of rate equations describing condensation of a gas is derived. These equations are solved numerically for several combinations of intermolecular bonding functions, temperature, initial saturation ratio, and condensation regime. The rates of condensation are found to decrease to a minimum and then to accelerate. The minimum rates correspond to the steady state rates predicted by a simplified version of nucleation theory.
The structure of ammonia overlayers physisorbed onto the surface of single crystal graphite, determined by means of atomic beam diffraction
Authors: Rowntree P. , Scoles G. , Xu J.
Published: July 2002

The structure of NH3 monolayers and submonolayers physisorbed on graphite C(0001) has been determined by means of low energy helium atom beam diffraction experiments. The measured diffraction pattern is found to consist of diffraction ‘‘rings,’’ indicating a large degree of orientational disorder among the adsorbed domains. We observe up to fourth order diffraction and periodicities which can be generated from a rectangular 3.53×6.11 Å unit cell or by a 7.06×7.06 Å hexagonal mesh. Both of these possibilities are characteristic of a 3.53 Å nearest neighbor separation. These findings suggest that, because of the relatively strong molecule–molecule interactions, the degree of order within a given domain is relatively high, while the domain’s orientational disorder may be due to the insensitivity of the adlayer to the corrugation and anisotropy of the graphite substrate. This behavior is atypical, since HCl, CH3F, CH3Cl, and CH3Br all form well organized monolayers which have a single well defined orientation with respect to the graphite substrate.
Determination of the internal state distribution of NO produced from the H+NO2 reaction
Authors: Sauder Deborah G. , Dagdigian Paul J.
Published: July 2002

The internal state distribution of the NO product from the H+NO2 reaction was determined in a crossed-beam experiment. Rotational populations in the v=0 to 2 vibrational levels of NO were derived from laser fluorescence excitation spectra of the A2Σ+–X 2Π band system. The vibrational distribution decreases monotonically with v, and the rotational state distribution is peaked at low J (most probable J of approximately 11.5), but the tail of the distribution extends out to N≊50. After correction for the flux-density transformation, the vibrational population distribution is found to equal 1:0.17±0.04:0.05±0.02 for v=0,1,2, respectively. The lower F1(Ω=1/2) spin-orbit component is preferred over the F2(Ω=3/2) manifold by a ratio of 1:0.52±0.11, independent of vibrational level. At high J, a preference for the Π(A′) Λ doublet levels is observed. Approximately 9.5%±2% of the reaction exoergicity is found in NO internal excitation. The NO and previously determined OH internal state distributions are compared with statistical distributions calculated by phase-space theory. The energy disposal in OH is found to be greater than statistical, while the opposite is true for NO, as might be expected in a direct reaction of the type A+BCD→AB+CD for the ‘‘new’’ and ‘‘old’’ bonds, respectively. The Λ doublet preference observed here for the NO product, and previously for the OH product, can be explained by the dissociation of an HONO(X̃ 1A′) intermediate. A mechanism for the generation of unequal NO spin-orbit populations, involving nonadiabatic mixing in the exit channel, is proposed.
Infrared absorption spectroscopy of CO2–HX complexes using the CO2 asymmetric stretch chromophore: CO2HF(DF) and CO2HCl(DCl) linear and CO2HBr bent equilibrium geometries
Authors: Sharpe S. W. , Zeng Y. P. , Wittig C. , Beaudet R. A.
Published: July 2002

Infrared absorption spectra associated with the CO2 asymmetric stretch vibration have been recorded for weakly bonded gas-phase complexes of CO2 with HF, DF, HCl, DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15×38 mm2) that provides >20 MHz overall resolution. Results obtained with CO2–HF are in agreement with earlier studies, in which the HF-stretch region near 3900 cm−1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO2–DF, the natural linewidths are markedly narrower than with CO2–HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO2–HF. Both CO2–HF and CO2–DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigned to either a slightly asymmetric rotor with Ka=1, or a hot band involving a low-frequency intermolecular bend mode.

Results obtained with CO2–HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the vibrationally excited state is almost identical to the lower state. Like CO2–DF, linewidths of CO2–HCl and CO2–DCl are much sharper than those of CO2–HF, and in addition, CO2–HCl and CO2–DCl exhibited weak hot bands, as were also evident with CO2–HF and CO2–DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO2 underwent a blue shift relative to uncomplexed CO2. This can be understood in terms of the nature of the hydrogen bonds, and ab initio calculations are surprisingly good at predicting these shifts. Deuteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero-point energy, and in a highly anharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,CO2–HBr is asymmetric, with the Br–C symmetry line essentially perpendicular to the CO2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive to the location of the H atom in CO2–HBr, Bose–Einstein statistics require that odd K″a states are missing for C2v symmetry, as is observed with T-shaped CO2–(rare gas) complexes. However, we observe a full complement of odd and even Ka states, indicating that the H atom is not located symmetrically about the C2v axis on the time scale of the measurement. With CO2–HBr, the low gas-phase acidity of HBr and the high Br-atom polarizability encourage a qualitative change in the geometry relative to CO2–HCl and CO2–HF. This has valuable implications for photoinitiated reactions in such complexes.
Transition-metal diatomics: Mn2, Mn+2, CrMn
Authors: Cheeseman M. , Van Zee R. J. , Flanagan H. L. , Weltner W.
Published: July 2002

The Mn2 molecule, exhibiting antiferromagnetic exchange coupling between the 3d5 electrons on each atom, has been investigated further in cyclopropane matrices at temperatures ranging from 12 to 110 K. This allowed electron-spin-resonance spectra to be observed in the higher spin states S=4 and 5 and confirmed the previous deduction of van der Waals bonding with r≂3.4 Å. In addition, the derived anisotropic parameters DS suggest that exchange striction (as previously observed in solids) is occurring such that r varies from about 3.2 to 3.6 Å in the S=0 to S=5 states, with a corresponding variation in the exchange coupling constant J in these states. The removal of one electron to produce Mn+2 results in the formation of a single σ bond and high-spin coupling among the remaining 11 unpaired electrons to yield a 12Σ ground state. In contrast, its isoelectronic counterpart CrMn is found to have a 4Σ ground state and multiple bonding, more in accord with expectations for a diatomic between Cr2 and Mn2.
Relativistic linear combination of Gaussian-type orbitals density functional method based on a two-component formalism with external field projectors
Authors: Knappe Peter , Rösch Notker
Published: July 2002

A two-component scalar relativistic method which is based on the no-pair projection operator formalism of quantum electrodynamics has been studied within the framework of local density functional theory. This projection operator formalism, first proposed by Sucher for wave function based methods, provides a stable approach to the relativistic electronic structure problem without the well-known deficiencies of some variational procedures for the Dirac equation. Several approximations of increasing accuracy have been investigated. Besides the method based on free-particle projectors and on external field projectors for the singular nuclear potential, projectors on the full electronic potential are presented here for the first time. Atomic all-electron calculations within this framework give excellent results compared to a fully numerical solution of a scalar relativistic approximation to the corresponding one-particle Dirac problem. Calculations for cerium and for lead are analyzed in detail. About 96% of the relativistic shift of the one-electron energies and about 98% of the relativistic effect on the total energy are covered within this method. The method allows for a rather straightforward generalization to molecular systems with multinuclear potentials.