bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution gas phase emission and laser induced fluorescence excitation spectra of 1,3,5-C6F3H3+ and 1,3,5-C6F3D3+: Critical bands in the Jahn–Teller effect analysis
Authors: Cossart-Magos Claudina , Cossart Daniel , Leach Sydney , Maier John P. , Misev Liubomir
Published: February 2003

Cooled gas phase laser induced fluorescence excitation (LIFEX) spectra of 1,3,5-C6F3H3+ (h3+) and 1,3,5-C6F3D3+ (d3+) ions were obtained between 4600 and 4110 Å at a resolution of 1 cm−1. Band intensities were corrected for laser power variations. Gas phase discharge emission spectra of the same ions were photographed at a resolution of 0.25 cm−1 from 4450 to 4800 Å and at 0.1 cm−1, between 4500 and 4700 Å. In emission, only the spectral region λ<4580 Å, corresponding to the 000 band and bands to lower wavelengths, is reported here. Particular regions of the LIFEX spectra were also recorded at a resolution of 0.25 cm−1. Vibronic analysis of the B̃ 2A″2–X̃ 2E″ transition is made on the basis of five (seven) e′, three a′1, and one a″2 excited state fundamental frequencies which were assigned in the h3+ (d3+) ions. A few low-lying ground state levels, among which the first j=3/2 (A1, A2) levels of mode 6 (e′), the most active Jahn–Teller (JT) mode, are particularly critical. Some discrepancies in band relative intensities and absolute wave numbers are found between the h3+ LIFEX spectrum obtained here and that previously published by Bondybey et al. Nevertheless, interpretation of our h3+ and d3+ spectra is more consistent with the high values of JT parameters derived by Bondybey, Miller, and co-workers, rather than the low values obtained in earlier work by Cossart-Magos and Leach. Some differences between observed and calculated JT features are rationalized in terms of model inadequacies.
Selective vibrational excitation of the ethylene–fluorine reaction in a nitrogen matrix. I
Authors: Frei Heinz , Pimentel George C.
Published: February 2003

Selective vibrational excitation of the cryogenic reactions between fluorine and C2H4, CH2CD2, trans-CHDCHD, and cis-CHDCHD have been studied in solid nitrogen at 12 K. For the C2H4⋅F2 reaction, quantum yields showed a general increase over five orders of magnitude as photon energy increased; ν (cm−1), φν: 953, <3×10−6; 1896, 6.6×10−4, 2989, 2.3×10−2; 3076, 7.0×10−2; 3105, 4.3×10−2; 4209, 3.1×10−1. Deviations from this general trend give evidence of mode selectivity. Similar deviations indicative of mode selectivity are found for trans-CHDCHD. At comparable photon energy, the quantum yields of the dideuteroethylenes decrease markedly in the sequence φ(t-CHDCHD)≳φ(c-CHDCHD) ≊φ(CH2CD2). These differences are attributable to a g, u selection rule in the phonon-relaxation mechanism. All of the results are consistent with a competition between chemical reaction of the vibrationally excited ethylene and phonon-assisted relaxation. The resulting cascading process accounts for the general rise in φ as ν increases and deviations from this general rise are attributable, at least in part, to mode selectivity.
A shock tube study of DF dissociation
Authors: Bott J. F. , Cohen N.
Published: February 2003

The dissociation rate for DF in argon has been studied with shock tube experiments at temperatures between 3600 and 7200 K. The data were best fitted by the rate coefficient equation k1=2.7×1022 T−2 exp (−137 130/RT) cm3/mol s. A few experiments performed at ∼5000 K indicated that DF dissociates more rapidly in a helium bath than in argon by a factor of 2.9–4.1. A comparison of recombination rate coefficients for several hydrogen halides at 4000 K reveals only small deviations from a value of 1014.4 cm6/mol2 s.
Analysis of periodic perturbations of limit cycles
Authors: Rehmus Paul , Ross John
Published: February 2003

An algorithm due to Loud is used to find asymptotically convergent series solutions for limit cycles subjected to weak periodic perturbations. If an exact or approximate solution to the unperturbed limit cycle is available near or far from marginal stability, then accurate predictions can be made for entrainment bands and the phase relationships between the various oscillatory chemical species and the perturbation. The utility of this method is shown for several model systems. In an appendix, the appearance and character of critical slowing down at the edges of entrainment bands is demonstrated.
Unimolecular and bimolecular reactions in the C4H6+⋅ system: Experiment and theory
Authors: Jarrold Martin F. , Bass Lewis M. , Kemper Paul R. , van Koppen Petra A. M. , Bowers Michael T.
Published: February 2003

New experimental data on the metastable reactions of C4H6+⋅ are presented. The bimolecular reactions between C2H2+⋅ and C2H4 and between C2H4+⋅ and C2H2 have been reinvestigated using a tandem ICR. Total rate constants and branching ratios were measured and isotopic scrambling investigated using deuterium labeling. The experimental data reported here and the PIPECO data of Werner and Baer [J. Chem. Phys. 62, 2900 (1975)] are compared with the predictions of the transition state switching model. Good agreement was found between the predictions of the model and the experimental data. The branching ratios were found to be strong functions of both energy and angular momentum. In the bimolecular reaction between C2H2+ and C2H4 charge transfer appears to occur predominently via a loosely bound intermediate which only samples a small volume of the available C4H+⋅6 phase space.
The unimolecular decomposition rates of energy selected methylnitrite and deuterated methylnitrite ions
Authors: Gilman Jerome P. , Hsieh Tacheng , Meisels G. G.
Published: February 2003

The fragmentation of methylnitrite ion (CH3ONO+) involves dissociation from noninterconverting electronic states characterized by the formation of CH3O+ (m/z=31) and NO+ (m/z=30) ions. The lifetimes of the precursors of these ions have been determined from ion time-of-flight curves obtained by threshold photoelectron–photoion coincidence (TPEPICO) mass spectrometry. The deuterated methylnitrite ion (CD3ONO+) was also studied to examine the kinetic isotope effects on ion lifetimes. The rate constants for the reactions leading to the formation of CH3O+ and CD3O+ were found to be independent of internal energy over ∼0.2 and 0.9 eV, respectively. The isotope effect increases with increasing internal energy, contrary to that expected from the quasiequilibrium theory (QET) of mass spectra. We suggest that methylnitrite ion fragmenting to CH3O+ and NO+ occurs from noncompeting noninterconverting electronic states involving a surface crossing.
Infrared photodissociation of sulfur dioxide ions in a fast ion beam
Authors: Stein H. , Erben-Russ M. , Kompa K. L.
Published: February 2003

A laser-ion beam apparatus for studying laser-induced photodissociation and fragmentation of fast molecular ions in both coaxial and crossed beams configurations has been constructed. IR photodissociation of highly internally excited SO+2 ions is reported using a low power cw CO2 laser. The fragmentation of SO+2 into SO++O and S++O2 has been observed and the relative dissociation yields as function of laser frequency and power dependence of the two dissociation channels have been measured. No IR photodissociation could be observed for SO+ and SO+3 at 934.9 cm−1.
The reaction Hg+I2→HgI+I revisited
Authors: Oprysko M. M. , Aoiz F. J. , McMahan M. A. , Bernstein R. B.
Published: February 2003

The crossed molecular beam study of Mayer et al. (1977) on the subject reaction is revisited. The present work employs a different beam configuration and thus kinematic framework, and a larger range of relative translational energies is covered (i.e., from the threshold of 1.15 to 3.75 eV). Measurements include in-plane angular distributions and relative values of integral reaction cross sections as a function of energy. At low energies, the results of the present experiments are in good agreement with the previous work. Starting at the threshold, the reaction proceeds through the formation of a long-lived complex, presumed to be IHgI. At higher energies, the c.m. angular distributions show a gradual increase of the so-called ‘‘backscattered component.’’ This is interpreted as the opening of a new reaction path: the direct-mode abstraction of I via collinear approach of the Hg atom to the I2 molecule. The overall dynamics of this reaction are interpreted in the context of the semiempirical potential energy surfaces and electronic state correlation diagrams of Muckerman et al. (1977). From the present experimental results, the height of the barrier in the exit channel for the collinear configuration can be estimated to be in the range 2.0–2.3 eV. The excitation function rises from threshold and reaches a maximum at collision energy of 2.6 eV, falling off monotonically thereafter.
Topography of potential energy surfaces: DIM surface for H+F2
Authors: Duggan J. J. , Grice R.
Published: February 2003

Potential energy surfaces have been calculated over the full range of nuclear configurations for the H+F2 reaction using the DIM method with the 14 polyatomic basis functions arising from the atomic states H(2S), F(2Pu), H+(1S) and F−(1S). The lowest potential energy surface agrees with the LEPS surface only in the collinear configuration. Qualitatively different forms are observed in strongly bent configurations which are determined by configuration interaction with low-lying surfaces corresponding to electronically excited states which are inadequately treated in the LEPS formulation. Trajectory calculations using the LEPS surface at higher collision energy or for vibrationally excited F2 are likely to give erroneous results when broadside configurations contribute to the scattering.
Transfer of state multipoles in open shell molecule–open shell molecule scattering
Authors: Whitaker B. J. , McCaffery A. J.
Published: February 2003

General expressions are derived for the transfer of state multipoles in open shell polyatomic–open shell polyatomic rotationally inelastic collisions. These are then used to write expressions for the circular polarizations of spin and rotationally resolved spectral emission lines. Using the distorted wave Born approximation (DWBA), circular polarization ratios are calculated for the specific case of inelastic H+NH2 collisions. The potential is decomposed into a spin-dependent term and an exchange term using tensor methods to facilitate the calculation. Alternatively, the scattering may be considered to take place on singlet or triplet surfaces. An interesting feature of the results is that reorientation appears to be faster on the triplet surface than on the singlet.
Microwave studies of collision-induced transitions between rotational levels. VIII. Collisions between NH3 and polar molecules
Authors: Fabris Allan R. , Oka Takeshi
Published: February 2003

The technique of four-level microwave double resonance has been applied to the study of rotation-inversion transitions of NH3 induced by collisions with various polar molecules. H2O, D2O, CH3OH, CH3X and CHX3 (X=F, Cl, Br, I), NO, CO, and OCS were used as collision partners. The values of η=ΔI/I observed for many four-level systems which are connected by dipole-type transitions (ΔJ=±1, ΔK=0, parity +↔−) are given and qualitatively explained taking into account the long-range dipole–dipole interaction and the pattern of rotational energy levels of the collision partners.
The line shape of a zero-field level-crossing effect in methyl fluoride
Authors: Sandholm S. T. , Schwendeman R. H.
Published: February 2003

The line shape of the zero-field level-crossing effect that occurs when 13CH3F is irradiated by the 9P(32) CO2 laser has been recorded by an infrared laser Stark spectrometer operating under computer control. The level-crossing effect, which occurs as a narrow decrease in the absorption due to superposition of the m components of the Doppler-broadened R(4,3) transition in the ν3 band, has been recorded at several pressures. Comparison of the experimental line shapes with theoretically calculated curves leads to a value of 13.9±0.9 MHz/Torr for the pressure broadening parameter for this transition. An algebraic solution is given for the linear system that occurs in the density matrix treatment of the steady state line shape of a level crossing or level anticrossing effect. The modifications of the solution required to treat three-level double resonance or two-photon effects are also given.
Centrifugal distortion and internal rotation analysis of the microwave spectrum of ethyl fluoride
Authors: Fliege E. , Dreizler H. , Demaison J. , Boucher D. , Burie J. , Dubrulle A.
Published: February 2003

The internal rotation fine structure of the microwave spectrum of ethyl fluoride CH3CH2F was resolved and analyzed in the ground state. The assignment of the high J lines was checked by a centrifugal distortion analysis. The barrier height V3 determined from the ground state is 3349±4 cal/mol.
The rotational spectrum and molecular structure of the furan–HCl complex
Authors: Shea J. A. , Kukolich S. G.
Published: February 2003

The microwave spectrum of the furan–HCl complex in the ground vibrational state has been measured and assigned using a Fourier-transform microwave spectrometer employing a Fabry–Perot cavity and a pulsed supersonic nozzle as a molecular source. Furan–HCl is planar, with the axis of the HCl subunit oriented along the a axis of furan, bisecting the oxygen–carbon angle. A hydrogen bond is formed between the HCl proton and the lone electron pair of oxygen. The spectroscopic constants for furan–H 35Cl in MHz are A″=9421.3(39), B″=1004.2001(27), C″=904.5526(26), τ1=−0.9392(12), τ2=−0.0751(2), τbbbb =−0.004 01(19), τcccc=−0.002 17(17), χaa=−52.803(17), χbb=25.626(54), and χcc=27.177(54). Rotational transitions for the isotopic species furan–H 37Cl and furan–D 35Cl were also measured and assigned. The oxygen–chlorine distance is 3.26(1) Å. The binding of the HCl to oxygen, rather than to the π bonds between the β carbon atoms, was confirmed by measurements on the 2-D furan–H 35Cl complex.
Microwave spectrum, structure, and quadrupole coupling for the ethylene–hydrogen cyanide complex
Authors: Kukolich S. G. , Read W. G. , Aldrich P. D.
Published: February 2003

Microwave spectra for the complexes ethylene–HCN and ethylene–DCN were observed with the pulsed beam, Fourier transform spectrometer. The complexes are ‘‘T’’ shaped with the HCN hydrogen atom adjacent to the C–C double bond of ethylene. The HCN molecule lies on a line perpendicular to the plane of ethylene and passing through the ethylene center of mass. The HCN carbon atom is 3.702(10) Å above the ethylene plane. The nitrogen quadrupole coupling strength is eQqaa(N)=−4.363(2) MHz for ethylene–HCN and eQqaa(N)=−4.396(3) MHz for ethylene–DCN. The deuterium quadrupole coupling strength is eQqaa(D)=0.178(7) MHz for ethylene–DCN. The anisotropy in the nitrogen quadrupole coupling eQ(qbb−qcc)(N) is 0.013(3) MHz for ethylene–HCN and 0.047(12) MHz for ethylene–DCN. The rotational constants are Centrifugal distortion constants DJ and DJK were obtained.
Beam maser spectroscopy on CD3CN
Authors: Murray Alice M. , Kukolich S. G.
Published: February 2003

The J=1→0 rotational transitions of CD3CN were measured using a two-cavity molecular beam maser spectrometer. This spectrometer employs the Ramsey method of separated oscillating fields to obtain very high resolutions and allows precise measurements of the hyperfine structure splittings. The nitrogen quadrupole coupling constant is eqaaQ(N)=−4229.2±0.6 kHz and the deuterium quadrupole constant is eqaaQ(D)=−55.1±0.4 kHz. Assuming cylinderical symmetry about the C–D bond axis we obtain the C–D bond-axis deuterium quadrupole coupling strength of eqzzQ(D)=169.6±4.5 kHz. The b-axis rotational constant, and nitrogen and deuterium spin-rotation interaction strengths are also reported.
Detection of a new trigonal bipyramidal copper species in Cu–CaX zeolite by electron spin resonance and electron spin echo modulation analysis
Authors: Narayana M. , Kevan Larry
Published: February 2003

Electron spin resonance and electron spin echo modulation analysis are used to identify the location and coordination of Cu2+ in hydrated and dehydrated CaX zeolite. A new, trigonal bipyramidally coordinated Cu2+ is found to be the dominant copper center in this zeolite. It is located in a six ring window (site SII) and coordinated to three six ring oxygens and two hydroxyls, one in the α cage and the other in the β cage of the zeolite. This Cu(Oz)3(OH)2 species is found to be stable for dehydration up to 180 °C. At higher temperatures the Cu2+ species seems to be reduced irreversibly. However, a Cu2+ species can be regenerated by oxidation and hydration but most of it is only trigonally coordinated.
Optical multiple pulse sequences for multiphoton selective excitation and enhancement of forbidden transitions
Authors: Warren W. S. , Zewail Ahmed H.
Published: February 2003

In this paper we present novel and simple pulse sequences for enhancing the intensity of forbidden or highly nonresonant optical transitions. These sequences provide a straightforward approach to circumventing the most serious limitation of optical coherent transient spectroscopy: Available laser intensities are often insufficient to excite a significant fraction of the ground state population into desired excited states, either because of large inhomogeneous broadening or, in the case of multiphoton absorption, large anharmonicities. Optical phase modulation (which can be produced by an acousto-optic modulator) or amplitude modulation (which can be produced by an interferometer) with carefully chosen pulse flip angles and delays can effectively remove even very large energy mismatches, thus permitting essentially complete population inversions arbitrarily far from resonance. Coherent averaging theory and computer calculations are used to derive particularly valuable sequences. Pumping enhancement of 103–105 (depending on individual molecular parameters) for these modulated pulse sequences are predicted. Specific applications to multiphoton pumping of local vibrational modes are discussed.
Laser vaporization of tin: Spectra and ground state molecular parameters of Sn2
Authors: Bondybey V. E. , Heaven M. , Miller Terry A.
Published: February 2003

Sn2 molecules are produced by YAG laser vaporization of tin metal in a stream of cold helium gas and studied by time resolved laser induced fluorescence. Several vibronic bands of an electronic transition with origin near 18 200 cm−1 are analyzed and the ground state vibrational frequency and equilibrium molecular constants are determined. The electronic structure of Sn2 is discussed and an assignment of the observed transition proposed.
Computer ‘‘experiment’’ for nonlinear thermodynamics of Couette flow
Authors: Evans D. J.
Published: February 2003

Nonequilibrium computer simulations reveal that the equation of state of fluids undergoing shear flow, varies with strain rate. This observation prompted the development of a nonlinear generalization of irreversible thermodynamics to describe steady planar Couette flow, very far from equilibrium. In this paper we use computer simulation to perform a quantitative test of a prediction of this thermodynamics. The prediction tested is: fluids which exhibit positive shear dilatancy for isothermal shear flow should also cool as the strain rate is increased while keeping the internal energy constant. To perform calculations of this effect a new nonequilibrium molecular dynamics algorithm was developed to simulate Couette flow at constant internal energy.
DNA–divalent metal cation interactions measured by electrophoretic light scattering
Authors: Rhee Kee Woo , Ware B. R.
Published: February 2003

We have used the laser Doppler technique of electrophoretic light scattering to study the electrokinetic and gross conformational manifestations of the binding of divalent cations to DNA in low-salt solutions. By electrokinetic assay, divalent ion affinities to DNA in the 5–500 μM range increase in the order Mn+2<Co+2<Mg+2<Cu+2. The ELS spectra show that Cu+2 denatures double-stranded DNA in the 50–200 μM concentration range, thereby augmenting the reduction in electrophoretic mobility. Counterion condensation theory correctly predicts the form of the electrophoretic mobility reduction for territorially bound ions such as Mg+2, but the absolute magnitudes of electrophoretic mobility predicted by an equation deduced from condensation theory overestimate our experimental values.
The effect of solvent environment on molecular electronic transition moment directions: Symmetry lowering in pyrene
Authors: Langkilde Frans W. , Thulstrup Erik W. , Michl Josef
Published: February 2003

Measurements of polarized fluorescence and phosphorescence of pyrene, 2-fluoropyrene, and 2-methylpyrene in 3-methylpentane glass at 77 K reveal significant differences among the three molecules. These are interpreted in terms of solvent-induced mixing of the Lb and La states. It is proposed that such symmetry-lowering effects of the solvent environment may well be important contributors to the often observed discrepancies between the theoretical limiting values of polarization ratios and the values actually observed.
Electric-field-induced differential rotational Raman scattering
Authors: Buckingham A. D. , Shatwell R. A.
Published: February 2003

An electrostatic field induces a linear change in the intensity of light scattered by a gas in circularly polarized incident radiation. Differential scattering intensities for O, P, Q, R, S lines are evaluated for a linear dipolar molecule. The weak R(0) line shows a strong differential scattering. In the classical limit, differential scattering comes from molecules initially in the nonrotating J=0 state.
Ground state vibrational randomization in alkylbenzenes
Authors: Hopkins J. B. , Langridge-Smith P. R. R. , Smalley R. E.
Published: February 2003

Measurements of intramolecular vibrational randomization (IVR) previously carried out on the S1 excited state of a series of jet-cooled alkylbenzenes have been extended to the ground electronic state. The ground state ring modes 6b, 1, and 12 were conveniently activatd by pulsed laser excitation of the 000 vibronic band of the S1←S0 ultraviolet absorption system, followed by fluorescence. The onset of IVR was then probed by resonant two-photon ionization (R2PI) of these vibrationally activated ground state alkylbenzenes. When the alkyl chain exceeded three carbon atoms in length, the hot band 1201 (Ev∼1010 cm−1) was found to be absent from the R2PI-probed spectrum, while the 6b01 hot band (Ev∼625 cm−1) remained easily observable through n-pentylbenzene. This overall pattern of IVR is essentially the same as that observed previously in the S1 excited state. In both electronic states, the only effective barrier to vibrational energy decay from the ring to the chain appears to be an inadequate density of states. Above the critical state density, IVR appears to be complete, and this critical state density appears to be about the same in both the ground and excited electronic states.
A Monte Carlo study of the electrostatic interaction between highly charged aggregates. A test of the cell model applied to micellar systems
Authors: Linse Per , Jönsson Bo
Published: February 2003

Monte Carlo simulations of electrolyte solutions under conditions corresponding to ionic micellar solutions have been performed. The cell model usually applied in the electrostatic theories of micelles has been tested and found to be valid for thermodynamic properties only at low micelle concentrations. However, the counterion distribution close to the micellar surface is correctly described within the cell model even at high concentrations. No dramatic difference in the accuracy of the cell model approximation of mono- and divalent counterions was found. The micelle–micelle and ion–ion radial distribution functions obtained in the simulations show a considerably structure even at low concentrations. The different radial distribution functions and thermodynamic properties calculated in the simulations have been compared with the results found from solutions of the hypernetted chain equation. The conclusion is that the hypernetted chain equations is a less useful approximation in miceller systems as compared to the Poisson–Boltzmann approximation.
The thermodynamics of systems in a steady statea)
Authors: Benofy Susan J. , Quay Paul M.
Published: February 2003

A rigorous thermodynamic theory of steady-state systems is developed by generalizing the methods which were used by Clausius and Kelvin in the development of classical thermodynamics (thermostatics). The zeroth law is extended to nonequilibrium situations and the concept of temperature generalized accordingly. The law of homogeneous circuits, shown to be complementary to Kelvin’s principle, can be fused with it to give a generalized second principle. The thermodynamic principles are applied to those conversions of heat to work that result from transitions between two or more steady states or from the activity of systems that remain in a single steady state. It is proved that these latter systems must be multiply connected if conversion is to be continuous. We prove the existence of both scalar and vector functions of state for all steady-state systems. Steady-state conversion coefficients can be defined as derivatives of the vector functions of state, and useful relations among these coefficients derived. These include not only generalized forms of Kelvin’s relations for the thermocouple but similar relations for an analogous fluid system and, indeed, for any system in which potentials can be defined governing the flow of conserved quantities.
Homogeneous nucleation rate measurements for water over a wide range of temperature and nucleation rate
Authors: Miller R. C. , Anderson R. J. , Kassner J. L. , Hagen D. E.
Published: February 2003

An expansion cloud chamber was used to measure the homogeneous nucleation rate for water over a wide range of temperature from 230–290 K and nucleation rates of 1–106 drops cm−3 s−1. The comprehensive and extensive nature of this data allows a much more detailed comparison between theory and experiment than has previously been possible. The expansion chamber technique employs continuous pressure measurement and an adiabatic pulse of supersaturation to give the time history of supersaturation and temperature during the nucleation. The resulting drop concentration is determined using photographic techniques. The experimental observations are presented in tabular form and from them an empirical nucleation rate formula is determined: J=S2 exp[328.124−5.582 43T+0.030 365T2−5.0319E−5T3 −(999.814−4.100 87 T+3.010 84E−3 T2)ln−2S], where J is the nucleation rate in units of drops cm−3 s−1. S is the supersaturation ratio and T is the temperature in K.
The structure and thermodynamics of a solid–fluid interface. II
Authors: Rull Luis F. , Toxvaerd Søren
Published: February 2003

The structure and thermodynamics of a fluid interface at a solid surface is analyzed by performing molecular dynamics calculations for fluid systems up to fcc(111), fcc(100), and bcc(100) surfaces. The calculations are carried out at the triple-point of a Lennard-Jonesium and show that the structure is given by the short-range strong repulsive part of the Lennard-Jones potential. The stratification decays exponentially and approximatively as exp[(−z/(ξΔz2)] where Δz is the spacing between fluid layers; Δz depends on the index and the lattice structure. Also the lattice order (bond orientation) in the layers decay exponentially with the distance z to the surface. The surface tension is estimated to be small.
Theory of surface enhanced Raman scattering
Authors: Arunkumar K. A. , Bradley E. B.
Published: February 2003

It is now generally accepted that more than one mechanism could be at work in enhancing the Raman signal from molecules adsorbed onto a rough surface. In this paper we present a simple surface coverage, surface roughness dependent model that accounts for one such mechanism. According to our model, each adsorbate is subjected to both polarizing and depolarizing fields from its oscillating neighbors. Competition between these two fields is shown to determine the magnitude of signal enhancement.
Magnetic resonance experiments and a new molecular property in NO2
Authors: Bylicki F. , Weber H. G.
Published: February 2003

We report on optical-radio-frequency double resonance and zero magnetic field level crossing and level anticrossing experiments with selective optical excitation of single fine structure levels of NO2 under molecular beam conditions. The experiments reveal novel properties of the excited state molecule: a characteristic dependence of the Landé g factor on the rotational quantum number Ka and a new fluorescence depolarization effect not present in atoms and diatomic molecules. We discuss these results.
Stark effects in the rotational spectrum of the dimer H2O⋅⋅⋅HF and the variation of the electric dipole moment with excitation of the low-frequency, hydrogen-bond modes
Authors: Kisiel Z. , Legon A. C. , Millen D. J.
Published: February 2003

The Stark effects in the J=2←1 rotational transitions of the nearly prolate asymmetric rotor H2O⋅⋅⋅HF have been analyzed in the vibrational ground state and in each of the states (1, 0, 0), (0, 1, 0), (0, 0, 1), and (1, 1, 0). The nonmenclature (vβ(o), vβ(i), vσ) refers to the vibrational quantum numbers associated with the out-of-plane bending mode vβ(o), the in-plane bending mode vβ(i) and the stretching mode vσ, respectively, which are the three lowest energy modes of the hydrogen bonded dimer. The electric dipole moment μ for each of these states determined via the Stark effect is as follows:
Infrared absorption spectra of nitrogen oxides in liquid xenon. Isomerization of N2O3 a)
Authors: Holland R. F. , Maier William B.
Published: February 2003

Infrared absorption spectra of nitrogen oxides present when NO and NO2 are dissolved in liquid xenon between 160 and 220 K have been measured. Spectral absorptions of NO, NO2, (NO)2, N2O4, and N2O3 are identified, and integrated absorption cross sections are given for several spectral features. The spectra suggest that symmetric N2O3 and asymmetric N2O3 are in equilibrium in solution and that the energy difference between these two isomers is 1.8±0.2 kcal/mol.
Unimolecular dissociation of N+4
Authors: Stephan K. , Märk T. D. , Castleman A. W.
Published: February 2003

The unimolecular dissociation process N+4 → N+2+N2 was investigated with a double focusing sector field mass spectrometer. N+4 was produced by electron impact ionization of N4 present in a free jet expansion. Besides collision induced dissociation, the existence of a metastable dissociation process with a decomposition rate of about 102 s−1 was demonstrated. The occurrence of this metastable process is interpreted using the results of quantum mechanically calculated potential energy curves of N2–N+2.
Excitation of the O2(a 1Δg) state by low energy electrons in O2–N2 mixtures
Authors: Yamabe C. , Phelps A. V.
Published: February 2003

Coefficients for the excitation of the a 1Δg state of O2 by low energy electrons in mixtures of O2–N2–Ar and O2–N2 and in pure O2 have been determined using a drift tube technique. The time dependence of the absolute intensity of 1.27 μm band emission was measured for 1% to 5% O2 and 1% to 10% N2 in Ar at total gas densities of 2 to 10×1024 m−3 and in 20% O2–80% N2 mixtures at a total density of about 2×1024 m−3. The measured decay constants for O2(a 1Δ) metastable densities in these mixtures are in agreement with previous data. Comparison of experimental and calculated excitation coefficients for the mixtures shows the importance of a 1Δg excitation by cascading from the states of the A 3Σ+u, C 3Δu, and c 1Σ−u group near 5 eV in O2 for mean electron energies between 1.5 and 5 eV. Using published a 1Δ deexcitation rate coefficients, the measurements of absolute intensity at 1.27 μm from pure O2 yield excitation coefficients that agree satisfactorily with theory at E/N values of 8–20×10−21 Vm2.
Spectral distribution of CO2 vibrational states produced by collisions with fast hydrogen atoms from laser photolysis of HBra)
Authors: Chu Jack O. , Flynn George W. , Weston Ralph E.
Published: February 2003

193 nm photofragmentation of HBr molecules in HBr/CO2 mixtures produces energetic (∼2.5 eV) hydrogen atoms which vibrationally excite CO2 molecules via an inelastic translation-vibration/rotation (T-V/R) energy exchange process. Prompt infrared emission in the 4.3 μm region has been wavelength resolved, and the spectral energy distribution associated with the excited CO2 product was measured. The emission profile is strongly red shifted from the spectral region of the ν3 (0001–0000) fundamental transition indicating that these energetic collisions preferentially produce combination and overtone levels with oscillator strength in the 4.3 μm region. Cold gas filter studies show that significant emission originates from CO2 bend–stretch excitation compared to pure stretching excitation. Approximately 15% of the emission is due to the 0111–0110 transition while the remaining fluorescence has contributions from levels above the 0111 state. The data are in qualitative agreement with theoretical calculations on O/CO2 collisions employing either quasiclassical trajectory or infinite order sudden methods, but do not agree with predictions based upon either (harmonic oscillator) breathing sphere theory or statistical theory.
An adiabatic analysis of the reactive infinite order sudden approximation
Authors: Jellinek Julius , Pollak Eli
Published: February 2003

A vibrationally adiabatic infinite order sudden approximation (IOSA) is formulated in terms of angle dependent adiabatic barriers. These barriers are determined by half-integer action periodic orbits. We show that in the F+HH reaction, the adiabatic theory describes quantitatively the exact IOSA total reactive cross section over a relatively large range of translational energies. A detailed analysis points out why in many cases transition state theory becomes more accurate as one goes from the collinear to the 3D world. Inversion of the quasiclassical IOSA provides a good estimate of the angular dependence of the adiabatic barrier parameters.
Rotation-vibration excitation using the infinite order sudden approximation for rotational transitions: Li+–N2
Authors: Pfeffer George Allen , Secrest Don
Published: February 2003

The rotational transitions in a collision of Li+ with N2 at 4.23 eV (center of mass) have been treated in the infinite order sudden approximation while treating the vibrational transitions in close coupling. A number of new problems were encountered and solved. The solution of the close coupling equations for the long range ion–molecule interaction was facilitated by use of an asymptotic ending technique. Due to the high energy of the collision and the extent of the potential, as many as 10 000 relative angular momentum states were required. Solutions for the high angular momentum states were obtained by means of a Born-like approximation. Intermediate angular momentum states were treated in the WKB approximation. Many low angular momentum states were obtained by solving the vibrationally close coupled equations, and a technique was developed by which it was possible to interpolate the magnitude and phase of the S-matrix elements between relative angular momentum states, permitting the solution of the close coupling equations for far fewer angular momentum states than would otherwise be required. Since the phase of the S matrix varies rapidly with diatom orientation, the integration of the S matrix over molecular orientation requires a large number of Gauss integration points. To avoid this problem we used Filon quadrature which allows integration over several periods of the phase with just a few integration points. A fit to an a priori interaction potential for Li+–N2 was made. The calculations of integral and differential cross sections were made over a range of angles which covered the experimentally measured cross sections. The results of the calculation do not agree with the experiment.
ESR of pseudorotating 6Li3 and 7Li3
Authors: Garland D. A. , Lindsay D. M.
Published: February 2003

ESR spectra assigned to pseudorotating 6Li3 and 7Li3 molecules have been produced by arc lamp photolysis of argon matrices doped with isotopically enriched 6Li or 7Li. The spectrum of 6Li3 consists of seven equally spaced first derivative transitions with relative intensities in good agreement with the expected values for three equivalent I=1 nuclei. The measured hf constant and g value are ā=12.21(1) G and go=2.002 82(1). The absence of a stationary trimer spectrum suggests that the ground vibronic state of Li3 is relatively nonlocalized. The average isotropic spin population of the trimer ρ̄=0.225 is in excellent agreement with the predictions of ab initio calculations. The total isotropic spin population 3ρ̄≂0.68 suggests an approximately 30% p-character for the unpaired electron wave function. A comparison of the orbital composition of M, M2, M3, M∞ (M=Li, Na, K) shows (i) a diminishing s-character with increasing cluster size, (ii) a remarkable similarity between Na and K for all cluster sizes, and (iii) a smaller s-character (greater hybridization) for Li clusters.
Radiative decay rates from deperturbed v=0–7 vibrational levels of CO A 1Π measured using synchrotron radiation
Authors: Field R. W. , d’Azy O. Benoist , Lavollée M. , Lopez-Delgado R. , Tramer A.
Published: February 2003

Fluorescence decay times from single vibronic levels and from restricted groups of rotational levels are measured for CO A 1Π under selective excitation by synchrotron radiation. The largest cause of variation of decay curves with v′ and J′ quantum numbers is singlet ∼ triplet perturbations. These perturbations cause measured, rotationally averaged decay rates to differ from the deperturbed, pure A 1Π decay rate by up to 20%. Failure to take perturbation effects into account could systematically affect transition moment functions derived from radiative lifetimes, but not those from absorption oscillator strengths or fluorescence branching ratios. The dependence of the CO A 1Π–X 1Σ+ transition moment on internuclear distance is evaluated from deperturbed lifetimes of A 1Π v=0–7 vibrational levels and found, assuming linear Re vs r̄ variation within the region 1.0≤r̄v′v′′≤1.6 Å, to be Re(r̄vA′vX′′)= [7.48+0.34−0.34][1−(0.683+0.006−0.008) r̄v′v′′]D, where r̄vA′vX′′ is the r centroid and the correlated uncertainties cited correspond to three standard deviations. However, the occurrence of strong A–X fluorescence bands with r centroids near 1.46 Å implies a decidedly nonlinear form for Re(r̄). Since the v=0–7 A 1Π lifetimes sample Re(r̄) very weakly at r̄≳1.3 Å, the transition moment function derived here is valid only for 1.0≤r̄≤1.3 Å. The Re(r̄) function determined here is nearly identical to that derived by Mumma et al. [J. Chem. Phys. 54, 2627 (1971)] from the radiative lifetimes measured by Hesser [J. Chem. Phys. 48, 2518 (1968)]. This is a coincidence, not evidence that perturbation effects are inconsequential.
Magnetocatalytic effect on rare earth oxides
Authors: Jugel J. , Schwab G.-M. , Voitländer J.
Published: February 2003

The rates of ortho–para hydrogen conversion on some self-supported rare earth oxides have been measured at room temperature. The change of the rates by application of a weak external magnetic field has been determined. Only on Pr2O3 and Yb2O3 a small decrease of the rates could be observed. On the basis of two coupled proton spins under the influence of a magnetic field—composed of an extrinsic and an intrinsic part—a simple theory is given which explains the occurrence, the sign and also the field dependence of the magnetocatalytic effect. It is found that the sign depends upon the mutual orientation of the H2 molecules and the magnetic moments of the active sites. Finally, the theory allows us to deduce the well-known formula for the ortho–para hydrogen conversion rate given by Wigner.
Comment on ab initio calculation of spin densities in hydrocarbon radicals
Authors: Chipman Daniel M.
Published: February 2003

An earlier calculation of hydrocarbon radical hyperfine coupling constants, using the spin-projected unrestricted Hartree Fock (PuHF) method is updated. Unprojected uHF calculations are performed for all the systems studied before. (AIP)
Reply to the comments by Morrison
Authors: ten Brinke G. , Karasz F. E.
Published: February 2003

The scaling theory is further explained in relation to the enthalpy of mixing for binary mixtures. The stability criterion is shown to explain certain qualitative features relating the signs of the enthalpy of mixing and the volume of mixing to the type of phase behaviour in accordance with Morrison’s conclusion. (AIP)
The cluster–lattice interaction in the calculation of the electronic structure of CrF3−6 in K2NaCrF6
Authors: Barandiarán Z. , Pueyo L. , Beltrán F. Gómez
Published: February 2003

The cluster–lattice interaction involving the CrF3−6 in K2NaCrF6 has been analyzed, in the way proposed by Pueyo and Richardson [J. Chem. Phys. 67, 3583 (1977)] by computing the lattice potential with the Ewald method and introducing an analytical representation of it in the cluster Fock matrix before self-consistency. Many different expressions have been tested in order to obtain the best representation of the Ewald results. All of them have been centered in the metal site. The partition of this interaction in core-lattice and valence-lattice portions reveals that the earlier procedure of centering the lattice potential function on the ligand site is inappropriate for improving the cluster in-vacuo calculation of the equilibrium metal–ligand distance. The new results go in the required direction, but the effects of the interaction are now uniformly small. These effects have been examined in several properties, including the 10Dq, the ground state vibrational frequency ν̄(a1g), the horizontal displacement of the 4T2g with respect to the 4A2g, the vertical Stokes shift associated with these two states and the ligand-to-metal charge transfer originated in the SCF process.
Fluids of hard diatomic molecules. II
Authors: Wertheim M. S.
Published: February 2003

We consider fluids of hard diatomic molecules composed of two fused hard spheres of unequal diameter. The orientation average of the Mayer f function and the second virial coefficient B2 are calculated exactly by analytic integration in closed form. The calculation uses a decomposition of f(12) into five terms. The only difficult term is integrated in tetrahedral coordinates. Agreement with previous numerical results for B2 is excellent. Two of the five terms make small to moderate contributions to B2, depending on the molecular parameters. This supports the view that the decomposition is a useful device in formulating approximate theories. The integrals evaluated for the diatomic case are used to obtain B2 and the orientation-averaged f(12) exactly for a class of linear symmetric triatomic molecules.
A molecular theory for the freezing of hard spheres
Authors: Haymet A. D. J.
Published: February 2003

Using statisical mechanics and a series of well-defined approximations we present a calculation of the equilibrium liquid to solid transition for hard spheres. No computer simulation results are used. The transition is located from structural information about the liquid using a first-principles order parameter theory of freezing. The order parameters are the coefficients of a Fourier expansion of the spatially varying single-particle density ρ(r) in terms of the reciprocal lattice vectors of the solid. The thermodynamic and structural properties are calculated from a perturbation expansion in ρ(r). The effects of second order terms in this expansion and intermediate wavelength properties are investigated for the first time. The theory predicts that the equilibrium freezing transition occurs from a liquid with density 0.976 to an fcc solid of density 1.035 (in units of σ3, where σ is the hard sphere diameter), in good agreement with Monte Carlo simulations which find empirically that the liquid at a density 0.939–0.948 has the same pressure and free energy as the solid at a density 1.036–1.045.
Variational model for micelle structure
Authors: Stillinger Frank H.
Published: February 2003

A free energy variational principle is proposed for the study of aggregation structures in amphiphile solutions. For nonionic amphiphiles the postulated free energy functional depends on the scalar concentration fields for solvophilic head groups (H) and solvophobic tail groups (T); it has a form appropriate for a binary mixture of head-group chains and tail-group chains supplemented by a term that enforces chemical bond connection between pairs of those chains. Numerical solutions to the Euler equations for the variational principle are presented for molecules with formal constitution (H)10 (T)10, under conditions of spherical aggregate symmetry. These solutions include globular micelles, hollow vesicles, and a type of point defect in a smectic A lamellar system.
Magic numbers for argon and nitrogen cluster ions
Authors: Friedman Lewis , Beuhler Robert J.
Published: February 2003

Evidence for the existence of magic numbers for cluster ions of argon atoms and nitrogen molecules has been found with high molecular weight mass spectra of their cluster ions. The observed numbers of 145, 303, 550, and 912 in mass spectra of cluster ions coincides within experimental error limit with the number of particles required for complete shell icosahedra. Failure to observe similar magic number structures in mass spectra of polyatomic molecule cluster ions and differences between polyatomic molecule cluster ion mass spectra and the argon and nitrogen cluster ion spectra are discussed.
The perturbation theories for the dipolar fluids
Authors: Lee Lloyd L. , Chung T. H.
Published: February 2003

We derive here four different perturbation equations for the calculation of the angular pair correlation functions of dipolar fluids; namely, the first order y-expansion, the modified Percus–Yevik (MPY) expansion, the modified hypernetted chain (MHNC) expansion, and the modified linearized hypernetted chain (MLHNC) equation. Both the method of the functional expansion and the method of the cluster integrals are utilized. Comparison with other perturbation theories (e.g., the Melnyk–Smith equation) is made. While none of the theories is exact, as shown by the cluster diagrams, the MLHNC and the MHNC contain more diagrams than, say, the MPY and y-expansion. The y-expansion equation can be improved by including the correction terms to the Kirkwood superposition approximation for the triplet correlation function. For example, the inclusion of the correction term ρℱd4h(14)h(24)h(34) in a formula given by Henderson, is shown to improve substantially the y-expansion equation. We examine the performance of two of the theories: the y-expansion and the MLHNC equation for a Stockmayer (dipolar) fluid with a reduced dipole moment μ*2 [=μ2/(εσ3)]=1.0. Comparison with Monte Carlo simulation results of Adams et al. and with other theories (e.g., the QHNC equation) shows that our results are reasonable. Further improvements of the equations are also pointed out.
Analysis of the Rouse model in extensional flow. I. A general solution of the distribution function in an arbitrary flow field
Authors: King D. H. , James D. F.
Published: February 2003

The statistical mechanics of the Rouse model for a macromolecule are analyzed by the use of Laplace transform techniques. The analysis leads to the known constitutive equation for a dilute solution of Rouse-type molecules and to the general solution for the distribution function ψ in an arbitrary flow field. The form of the general solution is different from that presented by Lodge and Wu and appears to be easier to use. In particular, the new solution makes it straightforward to calculate average molecular quantities like deformation because there is a simple relation between the quantity and the Laplace transform of ψ. This capability is shown by calculations of molecular size, shape, and elastic storage for a general flow field. The deformation of a submolecule unit is also calculated for an assessment of the validity of the Rouse model. The assessment shows that the model is valid per se even at high rates of pure extension.