bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear susceptibilities and coherent and spontaneous Raman spectroscopy of polyatomic molecules in condensed phases
Authors: Deng Zhifang , Mukamel Shaul
Published: July 2002

A microscopic theory is developed for time-domain and frequency-domain four-wave mixing of polyatomic molecules in condensed phases. The nonlinear response function R(t3,t2,t1) is expressed in terms of the four-point correlation function of the dipole operator and is evaluated explicitly for a stochastic Langevin model of line broadening which holds for any correlation time of the solvent. Our results interpolate between the fast modulation limit, in which the optical Bloch equations are valid, and the static limit of inhomogeneous line broadening. Green function techniques are developed and used to perform the necessary summations over the vibronic manifolds required in the calculation of nonlinear susceptibilities in harmonic polyatomic molecules. Coherent anti-stokes Raman (CARS) line shapes are calculated for a simple model system and for the S0–S1 transition in azulene, which has seven Raman active modes. A detailed comparison with ordinary spontaneous Raman line shapes is made.
Theory of the order–disorder transition in hydrogen–niobium systems
Authors: Futran Mauricio , Hall Carol K.
Published: July 2002

A theoretical investigation of the high temperature phase change behavior of hydrogen in niobium is described. In an effort to understand the interplay between the hydrogen–hydrogen interactions considered and the appearance of the α, α′, β, and ε phases, a series of lattice-gas model calculations of the phase diagram is performed. It is shown that the pairwise interactions at the first three shells beyond the hard core must satisfy certain constraints in order to obtain the α→α′→β sequence of phases at high temperatures. A set of interactions which gives the best predictions for the α–α′–β phase boundary is described. It does not appear possible to produce a set of pairwise interactions of range <21/2a which simultaneously yields the α, α′, β, and ε phases. This indicates that multibody interactions are of nonnegligible significance at higher hydrogen concentrations.
Statistical mechanics of charged objects: General method and applications to simple systems
Authors: Rosenfeld Yaakov , Blum Lesser
Published: July 2002

A general variational approach to study systems composed of complex charged molecules is discussed. In this approach the variational trial functions for the free energy functionals are constructed from the asymptotic limiting (AL) forms of the direct correlation functions. A number of examples are discussed, and in each case the variational form of the direct correlation is given explicitly. The relation to Onsager’s procedure of immersing the system in an infinite conducting fluid for obtaining an energy bound is discussed in detail.
Approximate interaction energy in terms of overlap integral
Authors: Koga Toshikatsu , Umeyama Tatsuya
Published: July 2002

When the natural orbital expansion is applied to the total wave function, the (electronic) kinetic energy T is exactly expressible in terms of the overlap integral S between the natural orbitals. Then we can derive an implicit but rigorous relation between the total energy E and the overlap integral based on the integral form of the virial theorem. For a particular case of two interacting atoms with a large internuclear separation, an approximate but explicit relation is obtained between E and S, and its implication is discussed. An illustrative example is given for the H+2 system.
Information on the impact parameter dependence of the Ba+HI → BaI(ν=8)+H reaction
Authors: Noda Chifuru , McKillop John S. , Johnson Mark A. , Waldeck Janet R. , Zare Richard N.
Published: July 2002

Using selectively detected laser-induced fluorescence, the rotational state distribution of the BaI product has been measured for the beam–gas reaction Ba+HI → BaI(ν=8)+H. Owing to the highly constrained kinematics for this system, these measurements can be used to derive the reaction probability as a function of the impact parameter for this channel, called the ‘‘specific’’ opacity function, once the reaction probability as a function of velocity has been determined. Unfortunately, lack of knowledge of the exoergicity and the height of any energy barrier prevents a conclusive determination of the specific opacity function for this reaction. Instead, various approximate opacity functions are estimated based on different models of the velocity dependence of the reaction channel studied. If the reaction probability is the same for all relative collision velocities, then the BaI(ν=8) specific opacity function peaks strongly near 2.6 Å with a full width at half-maximum of 1.0 Å. However, the possible presence of a small energy barrier in the entrance channel causes a cutoff in the relative collision velocity distribution, and this type of velocity dependence would significantly affect the shape of the specific opacity function.
Theory of vibrational predissociation of van der Waals complex in the adiabatic approximation model
Authors: Kokubo T. , Fujimura Y.
Published: July 2002

A theory of the vibrational predissociation (VPD) of linear triatomic van der Waals (vdW) complexes in the adiabatic approximation model is developed aiming to evaluate the VPD rates from higher quantum states as well as the lowest one of the initially excited intramolecular vibrational mode. The adiabatic wave functions of the high frequency intramolecular mode and the adiabatic potential curves are constructed from a Morse-type interaction potential between the two neighboring atoms of the vdW bond. Using Fermi’s Golden Rule, expressions for the rate of the VPD are derived both in the Condon approximation and in the non-Condon scheme. Important contribution of the non-Condon scheme in the adiabatic approximation model is demonstrated by performing model calculations of the VPD rate constants of I2X vdW complexes. The magnitudes of the rate constant in the Condon approximation are smaller than those in the non-Condon scheme by about one to two orders. It is shown that there is a good correspondence in the vibrational quantum number dependence between the treatment in the adiabatic approximation model and that in the scattering theory based on the zero-order diabatic basis set derived by Beswick and Jortner, and that the magnitudes of the vibrational predissociation rate constant calculated by them exist between those in the Condon approximation and in the non-Condon scheme. An approximate expression for the potential displacements between two adiabatic potentials and that for the intramolecular frequency shifts are also derived.
Transitions to and three-limit, four-channel representations for the m
s
m
p3, m
p
n
d J=3° levels in Si i and Sn i
Authors: Ginter Dorothy S. , Ginter Marshall L.
Published: July 2002

A three-limit, four-channel representation of the msmp3, mpnd J=3° level structures for Si i and Sn i is presented which fits the known energy levels below the lowest ionization limit and the intensities of the ms2mp2 1D2→ and ms2mp2 3P2→msmp3, ms2mpnd J=3° transitions to within experimental uncertainties (m=3 and 5 for Si and Sn, respectively, and n≥m).
Time-resolved fluorescence polarization and four-wave mixing from an ensemble of free symmetric rotors
Authors: Myers Anne B. , Hochstrasser Robin M.
Published: July 2002

A theoretical treatment of time-resolved fluorescence polarization and two time-resolved four-wave mixing processes (polarization spectroscopy and transient gratings) from a thermal ensemble of free symmetric top rotors is presented. Time-dependent signals are calculated for both parallel and perpendicular transitions at low and high temperatures. At high  J, both the fluorescence anisotropies and the nonlinear signals exhibit transients near time zero which level out to essentially constant values within several average rotational periods. These transients, which represent rigid-body rotational motion in a classical picture, arise in a quantum mechanical treatment from interferences between different intermediate rotational states that are coupled to the same pair of initial and final states. The nonlinear signals are sensitive not only to rotational dynamics but also to the total dephasing rates between the final pairs of coupled levels and to effects of selective phase matching. The relevance of these short-time effects for designing and interpreting experiments to probe rotational dynamics of large molecules is discussed.
1
4N nuclear quadrupole resonance study of phase transitions of 1M and 2Or polytypes of potassium hexacyanoferrate (III)
Authors: Murgich Juan , Abanero Jose A. , Santana R. Magaly , Capparelli Mario V.
Published: July 2002

1 4N nuclear quadrupole resonance (NQR) frequencies were measured between 77–328 K for simple monoclinic (1M) and orthorhombic (2Or) polytypes of potassium hexacyanoferrate (III) (KFR). A reversible first order transition was found at 133 K for 1M and at 138 K for 2Or polytype. The transition was interpreted in terms of the coupling of a quasisoft rotatory mode with a temperature dependent tilt motion of the complex ion. An irreversible interpolytype transition produced by internal strains was observed in a sample formed by 1M and 2Or polytypes and repeatedly cycled between around 80–140 K and room temperature. Selective broadening of lines observed in only a pair of N atoms of each hexacyanoferrate ion was attributed to the occurrence of one dimensional disorder of neighboring K ions. The disorder disappeared after sample recrystallization. It was found that low crystallization rates produced only the 1M structure while high rates increased the occurrence of the 2Or over 1M polytype and even produced a third polytype. Annealing of KFR for two weeks at 160 °C produced minor changes in polytypic composition thus showing a high energy barrier for interpolytypic conversion. The results obtained in KFR are explained in terms of the theory of order–disorder structures in layered compounds.
Theory of exciton–photon interaction in polymers: Polariton spectra of polydiacetylenes
Authors: Suhai S.
Published: July 2002

A first principles method will be presented for the calculation of quasiparticle eigenstates in a polymer crystal strongly interacting with electromagnetic radiation. The matter field will be represented by delocalized excitons whose energy spectrum and wavefunction will be determined with the help of the ab initio Green’s function formalism of charge transfer exciton theory. The method has been applied for the computation of bulk polariton bands in highly conducting polydiacetylene crystals.
Isomerization and unimolecular dissociation channels of the oxalic acid monomer
Authors: Bock Charles W. , Redington Richard L.
Published: July 2002

The results of molecular orbital calculations performed for the oxalic acid monomer using the 6-31G, 6-31G*(5d), 6-31G**(5d), and MP2/6-31G*(5d) levels are reported. At the latter three levels of approximation the geometry with intramolecular hydrogen bonds is calculated to have the lowest energy, and the energies calculated for planar rotational conformers fall within about 2 kcal/mol of one another. The barrier hindering internal rotation of the carboxyl groups about the C–C bond for the nonhydrogen bonded conformer is calculated to be less than 1 kcal/mol, and secondary potential energy wells for gauche, rather than for the expected cis, carbonyl orientation are found about 300 cal/mol above the trans conformer at the 6-31G level. Vibrational frequencies calculated at the 6-31G level for the H-bonded conformer average about 9% higher than the observed values. On including electron correlation at the MP2 level the barrier calculated for the concerted, symmetrical, transfer of two protons between equivalent potential energy minima is 31.9 kcal/mol. This value is probably an upper bound. When electron correlation is ignored, calculations for the transfer of a single proton lead to a potential energy well that stabilizes an (HO)2CCO2 configuration with C2v symmetry at an energy 26–28 kcal/mol above that of the H-bonded ground state conformer. When electron-correlation is included at the MP2 level there is a saddle point in the PES instead. This is the transition state for proton exchange by successive transfers, and a search for its position at the MP2 level was not made. However, energy values of 25.7 and 36.6 kcal/mol are calculated using MP2/6-31G*(5d) at optimized 6-31G critical point geometries, and these values are taken as temporary estimates for the MP2 level transition state energies for two-step proton exchange and for decarboxylation, respectively. It remains to be determined whether the stepwise proton exchange channel will be present for higher level calculations. The results of these MO calculations are consistent with the experimental observations of Lapidus, Barton, and Yankwich

yielding first order kinetics, a low activation energy (31.5 kcal/mol), and complex kinetic isotope effects for the thermal decarboxylation of the oxalic acid monomer. Because of its importance in this reaction, calculations of the 1,2 H atom shift of dihydroxycarbene are also reported.
Accurate analytic model potentials for D2 and H2 based on the perturbed-Morse–oscillator model
Authors: Huffaker J. N. , Cohen Dennis I.
Published: July 2002

Model potentials with as few as 19 free parameters are fitted to published ab initio abiabatic potentials for D2 and H2, with accuracy such that rovibrational eigenvalues are in error by only about 10−2 cm−1. A three-parameter model is suggested for describing nonadiabatic effects on eigenvalues, with the intention that such a model might be suitable for all hydrides. Dunham coefficients are calculated from the perturbed-Morse–oscillator series expansion of the model, permitting a critical evaluation of convergence properties of both the Dunham series and the WKB series.
Mean field theory of polymer crossover behavior
Authors: Deutch J. M. , Hentschel H. G. E.
Published: July 2002

An argument is presented that the exponent previously employed to describe crossover behavior of either a linear or randomly branched polymer from the theta region to the swollen or collapsed state is inconsistent with the mean field description of the polymer free energy. In place of the conventional exponent φ(d)=[2−dνθ(d)], it is suggested that mean field theory compels use of the exponent ψ(d)=[νθ(d)d−1], where νθ(d) is the index which describes the dependence of polymer length on molecular weight in the theta region. The consequences of this different choice of crossover index are discussed for crossover behavior of the polymer length and for phase separation behavior. Comparison of the results of the different predictions with both Flory–Huggins theory and ε-expansion calculations is included. For linear chains in two dimensions striking differences are found for the predicted behavior of the phase separation curve at high monomer concentrations.
The effect of hydrostatic pressure on micelle size and crystallization
Authors: Fisch M. R. , Benedek George B.
Published: July 2002

The effects of hydrostatic pressure in the range 1–1200 atm on both the mean hydrodynamic radius and the critical micellar temperature of sodium dodecyl sulfate (SDS) micelles in aqueous NaCl solutions have been determined using quasielastic light scattering techniques. The mean hydrodynamic radius decreases monotonically with pressure for all temperatures investigated in the range 30–45 °C. The critical micellar temperature increases monotonically by approximately 13 °C over the same pressure range. The data on both micellar sizes and critical micellar temperatures are consistently interpreted entirely in terms of the effect of pressure and temperature on the fundamental chemical potentials.
The effect of a beam of electrons on a beam of alkali halide molecules effusing from a source
Authors: Glogovsky K. G. , Greene E. F.
Published: July 2002

Increased fluxes of neutral species in beams from an effusive source of alkali halide vapor observed when a beam of electrons is coaxial with the neutral beam are shown to be well accounted for by a model involving electron stimulated desorption of alkali atoms. The new model replaces an older one which attributed the increased flux to the electron beam acting as a weak trap for the neutrals thus serving to provide some controls of their motion.
Chemiluminescent reactions of second-row atomic ions. II. P+(1D)+H2, D2→PH+(A 2Δ), PD+(A 2Δ)+H,D
Authors: Müller B. , Ottinger Ch.
Published: July 2002

Chemiluminescent collisions of P+ ions in a beam with H2 and D2 molecules have been observed in the energy range of 2.3 to 12.2 eVCM. A spectral analysis at the peak of the cross section, 6.4 eVCM, by means of computer-generated spectra showed the PH+ and PD+ A 2Δ–X 2Π systems, which included some previously unobserved bands. The well-known A-state predissociation was confirmed, but appears to be incomplete above its onset at N=13 for PH+(A, v=0) and N=17 for PD+(A, v=0). The formation of PH+(A) has an activation barrier of 1–1.5 eV above the thermochemical threshold, which is analogous to the Al++H2 case and may be typical of second-row ion reactions.
Quantum beats in atomic fluorescence excited by molecular photodissociation
Authors: Diebold Gerald J.
Published: July 2002

When a diatomic molecule is photodissociated on a singlet molecular potential energy surface to give two singlet atoms, one of which is electronically excited, the atoms are left in pure angular momentum states referred to the molecular quantization axis. Transformation of such pure states to a laboratory frame of reference gives atomic wave functions that are described as coherent superpositions of states. Application of an external magnetic field to an ensemble of excited atoms causes quantum beats, or, alternately, a Hanle effect to appear in the fluorescence emitted by the atoms. Calculations show that the interference effects are observed only for certain geometrical arrangements of the electric vector of the photodissociating radiation, the external field, and the polarization direction accepted by the fluorescence detector. The expressions for the fluorescence intensity show that the phase and modulation depth of the quantum beat can be used to determine the asymmetry parameter for recoil of the photofragments.
Intermolecular potentials calculated by an extended geminal model: Theory
Authors: Ro/eggen I.
Published: July 2002

A model aimed at describing intermolecular interactions is formulated within the context of extended geminal models. The model has the following properties: It has a conceptual structure which facilitates interpretation. The model can be applied for any intersystem distances. The model is size extensive. Basis set superposition errors can be practically eliminated. The model is computationally efficient. A test calculation on the helium dimer yields a well depth of 10.64 K. The experimental determined value of the well depth is (10.79±0.22) K.
Pulsed Ge+n microcluster concentration spectra
Authors: Phillips J. C.
Published: July 2002

Bloomfield, Geusic, Freeman, and Brown have used the Smalley synchronized laser and supersonic quenching carrier gas pulsed method to obtain spectra for Ge+n similar to pulsed spectra obtained previously for Si+n. Their data are presented, compared to data obtained with electron beam ionization, and discussed extensively in order to separate effects due to cluster stabilization energies, growth kinetics, and ionization fragmentation. Differences in chemical bonding and growth kinetics between Si+n and Ge+n microclusters are discussed.
Theory of translation–rotation coupling in sodium azide
Authors: Sahu D.
Published: July 2002

A microscopic theory of softening of elastic modulii in β-NaN3, the high temperature phase of solid sodium azide, is given. The softening of the elastic modulii occurs due to coupling of the rotational degrees of freedom of an azide ion to the translational degrees of freedom of the neighboring ions. The softening of elastic modulus C44 is found to be more pronounced than that of C66, while C14 is found to be independent of temperature. The contribution of fourth order angular momentum terms l=4 to softening are found to be negligible for C44, but are appreciable for C66. A good fit to the Brillouin data of Kushida and Terhune (Ref. 1) is obtained by reducing both the electric charge on the end N atoms of N−3 and the strength of the Born–Mayer repulsive interaction from their values available in the literature. The experimental value of the extrapolated transition temperature Texpc =18 °C agrees well with the calculated transition temperature of Tc =24 °C.
Thermally induced nucleation. II
Authors: Buxbaum Susan P. , Haq Zia U. , Katz Joseph L. , Partch Richard
Published: July 2002

Nucleation was used to study the concerted unimolecular rearrangement reactions of low concentrations of different solutes. Those solutes that exhibited a polar transition state or a polarizable intermediate during thermal rearrangement caused a solvent, when supersaturated in the vapor phase in a thermal diffusion cloud chamber, to nucleate at supersaturations smaller than those needed for homogeneous nucleation. This thermally induced nucleation decayed over time. The solutes that did induce nucleation were: benzocyclobutenol, 2-methoxybenzocyclobutene, allyl vinyl ether, and an unknown decomposition product of allyl vinyl ether. However, 1,5-cyclodecadiene, whose transition state has no polar character, did not induce nucleation. Most of this research used benzocyclobutenol as the dopant. Evidence was found which confirms the suggestion made previously that the thermal ring opening of benzocyclobutenol to its E-enol leads to induced nucleation. Added 1,4-naphthoquinone successfully quenched nucleation by trapping the E-enol in solution. Activation energies of 30.9±0.6, 33.5±1.6, and 34.7±2.0 were obtained for the overall decomposition of benzocyclobutenol to o-tolualdehyde using bottom plates having copper, gold, and aluminum surfaces, respectively. Monitoring of the liquid phase concentration of benzocyclobutenol indicated a third order dependence of the nucleation rate on concentration.
Intramolecular conversions over low barriers. VII. The aziridine inversion—Intrinsically non-RRKM
Authors: Borchardt D. B. , Bauer S. H.
Published: July 2002

The rate of bond inversion at the nitrogen atom in aziridine (dimethylene-imine) was measured in the gas phase for a range of pressures and temperatures. 1H NMR spectra were recorded of samples at 5–1100 Torr, and from 298 to 388 K. Rate constants for inversion were derived by complete line shape analysis of the NMR spectra. Their temperature dependence at the high pressure limit (1100 Torr) gave Ea =15.8±0.4 kcal mol−1; ΔH‡ =15.2±0.4 kcal mol−1; ΔS‡=−5.5±3 eu; and ΔG‡298=16.8±0.5 kcal mol−1. These values are in excellent agreement with previously reported mangitudes obtained for solutions and in the gas phase at high pressures. However, at all temperatures we found no dependence of the first order rate constants on pressure down to 5 Torr, in sharp contrast to the fall-off curves calculated with standard RRKM codes, which indicated that for pressures below 100 Torr this system should be in the bimolecular regime. A less extended set of measurements was made with 2-methyl-aziridine which does show a small but clear fall off in kuni. These results can be rationalized in terms of the regional phase space model, previously proposed for methylnitrite. In systems such as aziridine, where the density of states at the barrier height is sparse (≈10/cm−1) the rate of inversion is limited by the relaxation time for intramolecular vibrational energy redistribution. Our results indicate that many nanoseconds are required to attain relaxation rather than picoseconds, estimated from the Marcus formula for k23(E)=N‡(E+)/hρ(E). In the methyl-substituted analog the barrier height is approximately 3 kcal mol−1 higher, and the density of states at this barrier is higher than in aziridine. Relaxation data for conformational changes in C6H12, H3CONO, SF4, and C5H10O were reanalyzed in terms of the regional phase-space model. At low pressures these also show departures from RRKM predictions.
Gated photochemical hole burning in photoadducts of polyacenes
Authors: Iannone Mark , Scott Gary W. , Brinza David , Coulter Daniel R.
Published: July 2002

Two-color, photon-gated photochemical hole burning (PHB) is observed in the absorption spectrum of a photoadduct of anthracene and tetracene (A–T) in a polymer matrix at 1.5 K. A–T undergoes PHB when irradiated with narrow band exciting light into the 0–0 band of the S1 ← S0 absorption. The efficiency of this PHB process is enhanced by simultaneous irradiation near the maximum of the Tn ← T1 absorption of A–T. Holewidths of less than 0.07 cm−1 have been observed for this photochemical cleavage of A–T. PHB was not observed for the related photoadduct dianthracene (A2) under similar irradiation conditions.
Observation of anomalous pulse duration dependent IR absorption spectrum in C3F7I
Authors: Mukherjee Pritish , Kwok Hoi S.
Published: July 2002

The collisionless infrared absorption spectrum of C3F7I was measured with picosecond and nanosecond CO2 laser pulses. The spectral width was found to decrease significantly as the measuring pulse duration was reduced. This data was interpreted with the multitier level scheme of the vibrational quasicontinuum. It was concluded that the measured absorption spectrum for any polyatomic molecule in a dense vibrational quasicontinuum should be dynamic in general and should depend on the interaction time. This is because the number of states interacting with the laser field indirectly via intramolecular coupling is time dependent. Therefore, the essentially static spectral measurement can yield dynamical information on the interaction of energy levels within the molecule.
A paramagnetic Cl–O defect produced by x irradiation of KBr:ClO−4
Authors: Sekhar Y. Ravi , Bill H.
Published: July 2002

Experimental results on KBr:ClO−4 single crystals, before and after x irradiation at 77 K, are reported. Infrared and polarized Raman spectra show clearly that the perchlorate molecules are isolated in the KBr matrix and possess Td symmetry. EPR spectra, at 9.5 and 36 GHz, of irradiated crystals at 4.2 K indicate the formation of ClO2−4 in trigonal symmetry. The g and 35Cl hyperfine tensors are found to be strictly axial and collinear. Temperature variation of the EPR spectra presented no motional averaging while uniaxial stress applied to the crystals yielded negative results. The structure of the ClO2−4 molecule is discussed in terms of a trigonal Jahn–Teller effect and a probable dissociation into (ClO−3–O−) complex.
The vapor pressures of solid and liquid ozone
Authors: Hanson David , Mauersberger Konrad
Published: July 2002

Vapor pressures of solid and liquid ozone have been measured over a temperature range 87 to below 66 K. The experiment was performed under flow conditions, the gas was analyzed by a precision mass spectrometer system. In the range of solid ozone two forms, supercooled and crystalline ozone, were found. A least square fit of the data for crystalline ozone resulted in the equation log P(Torr)=A+B/T where A=10.460 and B=−1021.6. The estimated uncertainty of the data is ±1.0%. A triple point temperature of 79.6±0.3 K was found where supercooled and crystalline ozone data intersect.
A temperature dependent simulation of the x-ray powder spectrum of ice Ih using Metropolis Monte Carlo sampling
Authors: Deutsch Peter W. , Stanik Tom D.
Published: July 2002

In this study we simulate the temperature dependence of powder x-ray diffraction for ice Ih using a model ice sample and Metropolis Monte Carlo simulations. We permit the water molecules to interact through the revised central force potentials of Stillinger and Rahman [J. Chem. Phys. 68, 666 (1978)]. The model ice sample is a periodic unit cell possessing 192 water molecules which are arranged in their initial configuration to yield very small point dipole and quadrupole moments [J. Chem. Phys. 78, 5103 (1983) and J. Phys. Chem. 87, 4309 (1983)]. We report the x-ray powder simulations for input temperatures of 100, 200, and 260 K, comparing them with earlier work at 20 K. A full set of the corresponding structure factors is computed for 300 K. The results suggest a monotonic decrease in the logarithm of the diffraction intensities with increasing temperature and an abrupt change between 260–300 K. The drastically decreased values at 300 K are consistent with previous related work [J. Phys. Chem. 87, 4309 (1983)]. The results are compared with the literature on related x-ray measurements [Nature 188, 1144 (1960)]. Here we comment on the qualitatively good comparisons between experiment and simulation, we note quantitative differences between experiment and simulation, and we discuss the differences in light of limitations in both experiment and theory.
Velocity modulation infrared laser spectroscopy and structure of the amide anion (NH−2 )
Authors: Tack Leslie M. , Rosenbaum Neil H. , Owrutsky Jeffrey C. , Saykally R. J.
Published: July 2002

The vibration–rotation spectrum of the amide anion (NH−2 ) has been measured in the 3 μm region by velocity modulation laser spectroscopy. The ν1 and ν3 bands have been simultaneously fit to a Watson S reduced Hamiltonian; the band origins are (in cm−1) 3121.9306(61) and 3190.291(14), respectively. The r0 structure has been determined: rNH=1.0367(154), θHNH=102.0(3.3). Under appropriate discharge conditions, NH−2 concentrations near 4×1011 molecules/cm3 can be produced in cells with metal-coated walls. Comparisons with condensed phase measurements and ab initio calculations are presented.
A comparison of the bonding in Cr(NO)4 and Ni(CO)4
Authors: Bauschlicher Charles W. , Siegbahn Per E. M.
Published: July 2002

The bonding in Cr(NO)4 is very ionic, with about 1.5 electrons donated from the Cr to the NO 2π orbitals. There is also a NO σ donation of about 0.5 electrons to the Cr, yielding a net charge on Cr of about 1. A large MCSCF expansion is needed to describe both the ionic and covalent contributions to the Cr 3d–NO 2π bonding. This bonding is compared to that in the isoelectronic Ni(CO)4.
Experiments on the influence of stereospecific interaction on the liquid density of some stereoisomers
Authors: van den Oord R. J. , Breunese J. N. , Hermans L. J. F. , Beenakker J. J. M.
Published: July 2002

The liquid density difference between optically pure stereoisomers and their racemic mixture, resulting from stereospecific interactions, is determined in a differential experiment. Results are reported for 2-chlorobutane, 2-butanol, 2-hexanol, and 2-octanol at 293 K. The largest effect is observed for 2-hexanol, the relative density difference being 1.3×10−4.
Coordination of Ag+ ions in AgI–Ag2O–B2O3 glasses by x-ray diffraction
Authors: Licheri G. , Musinu A. , Paschina G. , Piccaluga G. , Pinna G. , Magistris A.
Published: July 2002

X-ray diffraction measurements have been performed on glasses of the Ag2O–B2O3 and AgI–Ag2O–B2O3 systems. When silver iodide is present, the radial distribution function is dominated by a large peak at 2.85 Å, due to Ag–I interactions, with a shoulder at ∼2.40 Å, due to Ag–O pairs. The average number of Ag–I contacts has been calculated through analysis of the high angle structure functions si(s)’s, either by using the total si(s)’s or by using the partial si(s)’s, the latter obtained by subtracting the experimental structure function of the Ag2O–B2O3 glasses from that of the corresponding AgI-doped glasses. The compatibility of the results obtained with recently proposed models of AgI–Ag2O–B2O3 glasses is examined.
Fluids with internal degrees of freedom. II. Rayleigh–Brillouin scattering
Authors: López de Haro M. , Rodríguez R. F. , García-Colín L. S.
Published: July 2002

The extended thermodynamic treatment developed in part I for a fluid with internal degrees of freedom, is used to calculate its Rayleigh–Brillouin spectrum. A hydrodynamic model is first constructed which involves unspecified phenomenological coefficients. When these coefficients are identified in terms of experimental parameters, reasonably good agreement is obtained between the calculated and measured dynamic structure factor of CS2 at 20 °C with a density 1.262 g/cm3 and for k=8.608×10 cm−1. It is also shown that when the relaxation times of the trace and the nondeviatoric part of the stress tensor of the fluid tend to zero, the results previously derived by Mountain are recovered. The possible connections between this extended thermodynamic approach and Rytov’s hydrodynamic theory are also hinted at.
Transition state theory for photoisomerization rates of t
r
a
n
s-stilbene in the gas and liquid phases
Authors: Pollak Eli
Published: July 2002

Recently measured isomerization rates of trans-stilbene and diphenylbutadiene over a very large pressure range indicate that a medium can seemingly increase the rate beyond the gas phase high pressure limit. A model based on a generalized Langevin equation is proposed and solved using transition state theory. Rates obtained are in good agreement with experiment. The model incorporates a solvent shift which lowers the barrier to reaction. The model is based on the unification of two different approaches to the description of a dissipative harmonic bath.
Infrared spectra of the very weak H2--HF and O2--HF complexes in solid neon
Authors: Hunt Rodney D. , Andrews Lester
Published: July 2002

Very weak, hydrogen bonded complexes of molecular hydrogen and oxygen with HF have been prepared by condensing the neon diluted reagents at 4–5 K. Infrared spectra of the H2--HF complex revealed a νs(HF) mode at 3938 cm−1 and νc(H2) mode at 4155 cm−1 which are red shifted from isolated molecule values in agreement with theoretical predictions. Increasing the HF concentration produced a 1:2 complex, H2–(HF)2. Similar experimental results were obtained for molecular oxygen complexes using HF and DF; the oxygen fundamental in this complex, however, exhibited a 6 cm−1 blue shift. Spectra of these complexes in argon matrices showed that argon is not an effectively inert medium for very weak complexes where interaction with the solid argon environment is competitive with the H2–HF and O2–HF interactions.
Path integral approach to electrostatic problems
Authors: Gersten Joel I. , Nitzan Abraham
Published: July 2002

A theory that is able to account for electrostatic effects in microscopic situations is formulated in terms of the path integral method. The theory relates the solution of the Poisson equation to the propagator of the diffusion equation. Applications are made to some typical problems of interest, such as the solvation energy of an ion in a solution and to the electrical properties of a diffuse surface.
A helium diffraction study of the structure of the Ni(115) surface
Authors: Kaufman Donald S. , Aten Ralph M. , Conrad Edward H. , Allen Lynn R. , Engel Thomas
Published: July 2002

Hard wall and soft wall potentials have been used to analyze helium diffraction data from Ni(115). From this analysis, we have determined the equipotential surface which is simply related to the electron charge distribution at the surface. Corrugation functions for the hard wall as well as the repulsive and classical turning point corrugation functions for a corrugated Morse potential have been compared with electron density contours calculated from atomic superposition. The corrugation functions determined from soft and hard wall potentials have the same shape and differ only slightly in their maximum amplitudes. This shows that the hard wall model is suitable for structural studies even on strongly corrugated surfaces. The shape of the surface corrugation is not in agreement with electron density contours calculated for bulk termination. Large surface relaxations can bring the shapes of the corrugation function and the electron density contour into good agreement. However, we conclude that these relaxations are physically unreasonable and that electron delocalization rather than shifts in atom position primarily determine the shape of the electron density contour at the surface.
Concentration dependent fluorescence quenching with ionic reactants
Authors: Yang D. Y. , Cukier R. I.
Published: July 2002

A calculation of the quencher concentration dependence of the fluorescence quenching lifetime τ of a fluorophore is presented. The fluorophore and quenchers are assumed to be charged. An effective medium theory, which leads to deviations from linear Stern–Volmer behavior, is solved by numerical methods, when the ions interact by Coulomb’s law. For oppositely charged ions, whose contact reaction rate is not too fast, a regime can be reached where 1/τ appears to return to linear Stern–Volmer behavior with slope determined by this contact reaction rate. A second quenching scheme is introduced, which is often invoked in studies of electron transfer reactions, and analyzed by related techniques.
Effects of cooperative intermolecular interactions on the electronic delocalization in mixed-valence biferrocenium trihalide compounds
Authors: Kambara Takeshi , Hendrickson David N. , Dong Teng-Yuan , Cohn Michelle J.
Published: July 2002

Intramolecular electron transfer in the mixed-valence complexes in the solid state shows qualitatively different behavior from that in an isolated complex. The ‘‘extra’’ electron in a given mixed-valence complex in the solid state may be localized at lower temperature and delocalized at higher temperature, even though the extra electron may be delocalized in the isolated molecule at any temperature. The solid state environment has an essential effect on the intramolecular electron-transfer rate in mixed-valence complexes. In order to clarify the mechanism of any possible environmental effects, a model is proposed for binuclear mixed-valence biferrocenium trihalides. A binuclear mixed-valence biferrocenium cation has two localized electronic states, [FeIIAFeIIIB] and [FeIIIAFeIIB], which are coupled to an out-of-phase combination of symmetric ligand–metal stretching modes, one on each metallocene unit. An intramolecular electronic interaction α induces the electron transfer between the two vibronic states of the mixed-valence cation. Each trihalide counterion is also mixed valence, also has two electronic states,

[X−A- - -Y–XB] and [XA–Y- - -X−B], and an intramolecular electronic interaction β that induces charge oscillation in the trihalide anion. If the anion has only one molecular form, the charge oscillation due to the anion may be induced by another mechanism: Each anion may move between the two stable lattice positions as a whole. In addition to these two intramolecular interactions, cation–cation and cation–anion intermolecular interactions are considered in the context of a molecular field approximation. Energies and wave functions of the crystals are obtained. The electronic localization in the mixed-valence cation may be induced by the intermolecular interactions j, when j is large compared to α. The localized state becomes more unstable with increasing temperature, that is, the extra electron transfers more and more rapidly between the two iron ions in the cation, and the mixed-valence cation becomes delocalized at temperatures above a critical temperature TC. This electronic localization–delocalization transition occurs cooperatively and is a phase transition. The charge oscillation (X−A- - - Y–X−B ⇄ XA–Y - - -X−B) in the counteranions and/or the whole anion moving modulates the electron transfer rate in the cation. The present model consistently explains the various observed data from Mössbauer spectra, heat capacity measurements, x-ray crystallography, and infrared spectroscopy.
Interpretation of dispersive hole-burning kinetics
Authors: Richert R.
Published: July 2002

Nonexponential decay of an ensemble of first-order processes are modeled assuming distributions of rate-controlling parameters to be responsible for dispersive kinetics. Some problems of deriving microscopic properties from experimental decay data are illustrated focusing on hole-burning kinetics on the basis of the two-level system model. This approach is compared with reasonings leading to the logarithmic decay law.
Laser induced osmosis? Kinetic measurements in ethylene
Authors: de Hemptinne X.
Published: July 2002

The pressure effects of CO2-laser radiation in low pressure unbuffered ethylene are measured. The experimental results show two distinct and oppositely signed pressure excursions which the theory (irreversible thermodynamics) ascribes to osmosis or coherent diffusion (dominant at low pressure) and heating (higher pressure measurements). The two relaxation times and the intensities of the effects are measured.
Fractal-like triplet state energy transfer kinetics in doped polymer films
Authors: Lin Y. , Nelson M. C. , Hanson D. M.
Published: July 2002

The time evolution of the donor phosphorescence following pulsed laser excitation is used to monitor direct energy transfer at 16 K between the triplet states of donor molecules (4,4′-dibromobenzophenone) and acceptor molecules (1,4-dibromonaphthalene) doped into polymer films (polystyrene). The correspondence between the experimental data and the theoretical description provided by Inokuti and Hirayama is not satisfactory. Better agreement between theory and experiment is obtained by introducing an improved configuration average, which is valid at higher concentrations, and a nonuniform density of acceptor sites, which is a consequence of the polymer structure. The nonuniform density reveals a fundamental difference between the properties of a small molecule glass and a polymer glass. This difference can be quantified by the effective fractal dimension, which equals 3 for the small molecule glass and is found to equal 2.2 for this system.
Nonadiabatic solvation model for S
N2 reactions in polar solvents
Authors: Gertner Bradley J. , Bergsma John P. , Wilson Kent R. , Lee Sangyoub , Hynes James T.
Published: July 2002

An analytic theory for SN2 reactions in polar solvents in the nonadiabatic solvation limit is presented and used to interpret the computer simulation results of the preceding paper by Bergsma et al. The theory is based on the nonadiabatic solvation limit of previous studies by van der Zwan and Hynes and incorporates the solvent approximately but explicitly via a coordinate additional to the intrinsic reaction coordinate. Central results include: an explicit expression for the reaction transmission coefficient κ, the dependence of reaction probability on kinetic energy, the interpretation of κ in terms of nonequilibrium solvation entropy effects, and the deviation of the reaction coordinate from that assumed in the standard equilibrium solvation transition state theory view of the reaction.
Photodissociation, third harmonic interferences, polarization effects, and the MPI spectroscopy of chlorine
Authors: Li Leping , Wu Ming , Johnson Philip M.
Published: July 2002

We present a study of the multiphoton ionization spectrum of chlorine in the region of 2 1Πu←X 1Σ+g(1,0–0) three-photon resonances. Spectra at various pressures were obtained with different polarizations and counterpropagating beam experiments were carried out to restore the peaks quenched by third harmonic interference effects. Chlorine atoms, caused by photodissociation, produce prominent lines in the spectrum and are also responsible for third harmonic generation (THG) which in turn excites some spin-forbidden transitions by one-photon absorption. We discuss the change of polarization behavior in the presence of THG and the resultant effect on the symmetry assignments of the excited states. We also discuss third harmonic destructive interference effects in molecular systems and show that total suppression of three-photon transition to one-photon allowed states, common in atoms, will not usually occur in molecules although in general changes in polarization behavior should be observed at higher pressures. This conclusion is demonstrated in the chlorine spectrum.
Damped quantum mechanical oscillators with thermal fluctuations
Authors: Levine A. M. , Ozizmir E. , Schreiber W. M.
Published: July 2002

The solution for the relevant Heisenberg operators of a quadratic time-dependent quantum mechanical Hamiltonian is presented in convolution form suitable for the inclusion of thermal fluctuations. The approach to equilibrium of an oscillator in contact with a thermal bath is studied. An expression is obtained for the reduction of the random fluctuation width of the position coordinate as a consequence of the non-Markovian nature of a quantized thermal bath.
Product formation in ion–ion recombination reactions involving the molecular anions of SF6 and perfluoromethylcyclohexane
Authors: Valkenburg C. A. , Krieger L. A. , Grimsrud E. P.
Published: July 2002

Experiments are described in which resonance electron attachment reactions A+e→A−, where A is SF6 or perfluoromethylcyclohexane (C7F14), are made to occur under conditions in which the observed depletion of A is affected not only by the electron attachment reaction, but also by the detailed nature of the subsequent recombination of the molecular anions with positive ions, p++A−→neutrals. It will be shown that the composite rate coefficients thereby measured, along with knowledge of the electron attachment coefficients, allow the efficiencies of molecular regeneration by ion–ion recombination, p++A−→A+neutrals, to be determined for A−=SF−6 and C7F−14. It is shown that SF−6 is converted by recombination primarily to unknown neutral species which do not undergo further electron attachment. It is also shown that C7F−14 is converted by recombination primarily back to the molecular species, C7F14.
Transient properties of a constrained autocatalytic reacting system perturbed by external white noises
Authors: Leung H. K.
Published: July 2002

Stochastic properties of an autocatalytic reaction system constrained to have a carrying capacity are studied by considering the effects of white-noise fluctuations on various rate parameters. The transient behaviors and the subsequent steady state (SS) possibilities in these noisy systems are analyzed through the Stratonovich calculi together with the moment expansion approximation scheme. The results can thus be compared readily with those of the internal fluctuations. It is found that, among various noises, the noise in the carrying capacity is most fatal to the reaction processes while the additive noise is less significant. It is interesting to find that the multiplicative noise due to the fluctuating net growth rate may induce a slowing down phenomena when the noise intensity approaches a critical value. The stochastic SS structures are found to be similar to those found previously for the internal case. Though the stable SS are possible over a wide range of noise intensities, the transient behaviors demonstrate that a fluctuation catastrophe may happen long before the expected SS is established.
Feshbach-type calculation of autoionizing states of the BeHe4+ quasimolecule
Authors: Martín F. , Riera A. , Yáñez M.
Published: July 2002

In a recent article, we pointed out a fundamental difference between the molecular treatment of processes involving a multicharged ion and hydrogen or helium atoms, which is the autoionizing character of the molecular channels, and we reported a (new) implementation of the Feshbach method to calculate the molecular energies and couplings. In the present work we point out that for multicharged ions heavier than lithium exit autoionizing atomic channels are open, and the correlation diagram is considerably more complicated. As a prototype for these systems which have considerable experimental interest, we present the energies, radial, and rotational couplings, evaluated with our Feshbach method, for the molecular channels involved in Be4++He collisions, and discuss the correlation diagram in detail. Conclusions are also drawn as to the use of qualitative correlation rules.