bluuebunny/crossref_metadata_2025 huggingface dataset:
Supersonic molecular jet studies of toluene–helium and toluene–methane clusters
Authors: Schauer Mark , Law K. , Bernstein E. R.
Published: February 2003

The techniques of fluorescence excitation (FE), dispersed emission (DE), and one- and two-color time of flight mass spectroscopy (TOFMS) have been employed to study van der Waals clusters of toluene–helium (TolHe) and toluene–methane (TolCH4) formed in a supersonic molecular jet. Spectral shifts for the toluene 1B2←1A1 transition have been characterized for TolHe1,2 and Tol(CH4)1,2. The van der Waals stretching frequencies (V1) in the excited states of these clusters have been identified. The ground state stretch (V1) for TolHe has also been found; it is quite similar to the TolHe V1. The binding energies (D0) for Tol–He clusters lie between 75 and 90 cm−1 and those for Tol–CH4 clusters lie between 533 and 739 cm−1. The coordination of a third solvent molecule to those clusters generates a broad spectrum whose shift is close to the limiting value for a large (x>3) cluster. A comparison between these data and methane solution data is presented.
Divalent metal–acetate complexes in concentrated aqueous solutions. An x-ray diffraction and NMR spectroscopy study
Authors: Caminiti R. , Cucca P. , Monduzzi M. , Saba G. , Crisponi G.
Published: February 2003

Divalent metal–acetate solutions have been investigated by the x-ray scattering technique and 13C NMR spectroscopy. The aim of this work was to study the complex formation between the divalent metals and a typical organic complexing ligand in concentrated aqueous solutions. The correlation functions and the analysis of the structure functions agree with the literature information that the species present in solution are Me(H2O)2+6 , Me(H2O)6−z(CH3COO)2+−zz , and Me(H2O)6−2z(CH3COO)2+−zz . Good agreement with experimental data is achieved through a model in which the acetate ions are bonded as a monodentate ligand to the Mg, Co, Mn, and Zn cations and as a bidentate to the Cd cation. The cations also possess a second coordination shell of water molecules. Some indications have been obtained supporting the presence of hydration water around the acetate anions.
Increased amplitudes of electron spin echoes by phase shifted excitation
Authors: Tan Swee Lian , Waugh John S. , Orme-Johnson William H.
Published: February 2003

Excitation by two 90° pulses in phase quadrature lengthens the phase memory time of electron spin echoes. This prolongation enhances the accuracy with which modulations arising from hyperfine interactions can be measured in relatively concentrated spin systems. Comparison of these echoes with ones produced normally (without the phase shift) allow one to measure selectively the electron–electron spin coupling tensor.
Variation of nuclear magnetic shielding with intermolecular interactions and rovibrational motion. VII. 19F in CF2H2, CF2HCl, CFHCl2, CF2Cl2, and CFCl3
Authors: Jameson Cynthia J. , Jameson A. Keith , Oppusunggu D.
Published: February 2003

Variable density and temperature studies of fluorine-19 nuclear magnetic resonance spectra in the title molecules reveal the effects of intermolecular interactions, quantitatively expressed as a second virial coefficient of nuclear shielding σ1(T) and the effects of rovibrational averaging, expressed in terms of the temperature dependence of shielding in the molecules in the zero-pressure limit σ0(T). The quantities are reported as quadratic functions of temperature and compared with those of other molecules. In the four series of molecules: (a) CH3F, CF2H2, CF3H, CF4; (b) CFC13, CF2C12, CF3Cl, CF4; (c) CF2Cl2, CF2HCl, CF2H2; and (d) CFCl3, CFHCl2, CFH2Cl, CFH3, ‖dσ0/dT‖ has been found to decrease with H substitution and increase with Cl substitution. Gas-to-liquid shifts are also reported.
Infrared predissociation spectrum of the H+3 ion
Authors: Carrington Alan , Kennedy Richard A.
Published: February 2003

We have observed an infrared spectrum of the H+3 ion containing nearly 27 000 lines which span only 222 cm−1 from 872 to 1094 cm−1. A beam of H+3 ions at a potential of from 1.2 to 10.5 kV is aligned to be collinear with an infrared laser beam from a carbon dioxide cw laser. Photodissociation occurs to produce fragment H+ ions which are separated from the parent H+3 ions using an electrostatic analyzer. Doppler tuning is accomplished by scanning the H+3 ion beam potential and resonance lines corresponding to an increase in fragment H+ ion current are detected by means of a velocity modulation technique. The observed linewidths range from 3 to 60 MHz, with additional broader lines also being detected by chopping the laser beam. We believe that each resonance line arises from predissociation of H+3 to form H2 and H+. Pseudo-low resolution spectra constructed by computer convolution of the experimental data show well defined peaks which correspond closely in transition frequency to j=3–5 rotational transitions of H2 in its v=0, 1, 2, and 3 vibrational levels. It is therefore suggested that the H+3 ions studied by our technique are best regarded as H2⋅⋅⋅H+ complexes in which the vibrational and rotational states of the H2 are largely preserved. We believe that many of the observed resonance lines arise from H+3 ions with up to 2 or 3 eV internal energy above the lowest dissociation limit, and consequently that many metastable levels with a wide range of lifetimes exist. The vibration-rotation levels of the H2⋅⋅⋅H+ system are discussed in terms of the theoretical models which have been developed for van der Waals complexes and semiquantitative calculations using an ab initio H2⋅⋅⋅H+ interaction potential are described. Measurements of the H+ center-of-mass kinetic energy associated with individual resonance lines are described; they provide information about the energy of the predissociating H+3 level relative to its H2+H+ dissociation channel. Many of the resonance lines are associated with a relatively small energy release (10–500 cm−1), but energy releases of over 3500 cm−1 are also observed, which must arise from transitions between pairs of levels, both of which lie well above the lowest dissociation limit. This large energy release is almost certainly due to vibrational predissociation, while the smaller energy releases are associated either with rotational predissociation or tunnelling through a centrifugal barrier. Preliminary observations of similarly complex predissociation spectra of D+3, D2H+, and DH+2 have been made. A striking result is that spectra of D2H+ detected by monitoring either H+ or D+ photofragment ions are different. The results described have important implications for studies of reactive scattering processes and for our understanding of the potential energy surfaces for polyatomic molecules.
An analytic theory of photodissociation of linear symmetric polyatomic molecules
Authors: Chen C. J. , Osgood R. M.
Published: February 2003

An analytic theory of the direct dissociation of linear symmetric polyatomic molecules is developed. The theory is based on a power series expansion of the excited-state potential surface. It provides an analytical expression for the Franck–Condon integrals. Simple formula for the dissociation cross section as a function of wave number is given. To illustrate the immediate applicability of this approach, the formula is used to calculate the profiles of the absorption spectra of dimethylated metal alkyl molecules; good agreement with the measured spectra is seen, and molecular constants with clear physical meanings can be extracted from the curve fitting.
Polarized Raman spectra of oriented dipalmitoylphosphatidylcholine (DPPC). II. Scattering activities of phosphatidylcholine vibrations
Authors: Harrand Monique
Published: February 2003

The polarized Raman spectra of oriented phospholipids obtained from vesicle solution were studied with regard to the conformation of the phosphatidylcholine chain. Calculations of the Raman scattering activities were carried out, by the method previously described, and some information on the orientation of the phosphatidylcholine chain was obtained using the ν1(C–N) vibration of the choline head and the νs(PO−2 ) vibration. The chains slant more than in the DMPC crystals.
An investigation of the rotational spectrum of H2S⋅⋅⋅HF by pulsed-nozzle, Fourier-transform microwave spectroscopy: Determination of the hyperfine coupling constants χa
a (33S), χDa
a, and DH(D)Fa
a
Authors: Willoughby L. C. , Fillery-Travis A. J. , Legon A. C.
Published: February 2003

The high sensitivity of pulsed-nozzle, Fourier-transform microwave spectroscopy has been used to observe the rotational spectra of the isotopic species H234S⋅⋅⋅HF, H2 33S⋅⋅⋅HF, and H2 32S⋅⋅⋅DF of the hydrogen sulphide–hydrogen fluoride complex. The hyperfine coupling constants DHFaa =−236(7) kHz, χaa (33S)=32.785(9) MHz, and χDaa =274(6) kHz, respectively, have thereby been determined together with (B0+C0) and DJ values. The K−1=1, J=2←1 transitions of H2 32S⋅⋅⋅HF, which are very weak because of the low effective temperature of the supersonically expanded gas, have also been detected. Their frequencies, when combined with those of the 202←101 and 101←000 transitions, then allow the spectroscopic constants (B0−C0)=8.768(2) MHz, (B0+C0)=7672.413(2) MHz, DJ =15.10(22) kHz, and DJK =473.3 kHz to be obtained. The values of the various spectroscopic constants have been interpreted in terms of the geometry and internal dynamics of the complex.
The dynamics of photodissociation of cluster ions. II. Photodissociation of the (NO)+3 cluster in the visible wavelength range
Authors: Jarrold Martin F. , Illies Andreas J. , Bowers Michael T.
Published: February 2003

The photodissociation of the (NO)+3 cluster ion has been studied in the visible region of the spectrum (460–660 nm) using a crossed high energy ion beam/laser beam experiment. Total photodissociation cross sections, product branching ratios, product relative kinetic energy distributions, and information about the product angular distributions have been measured. The total photodissociation cross section peaks to the blue end of the visible spectrum with a maximum in excess of 5×10−17 cm2. The only ionic products observed were NO+ and (NO)+.2. The fraction of (NO)+⋅2 formed is 26±4% and is essentially independent of wavelength in the range studied. For (NO)+⋅2 the product kinetic energy distributions are narrow and characteristic of dissociation occurring on a repulsive surface. At 610 nm, 24% of the available energy appears as product relative kinetic energy, this fraction increases to 32% at 488 nm. The (NO)+⋅2 angular distributions are well described by an angular distribution of the form 1+βP2(θ) with β=1.5 indicating that dissociation occurs rapidly with respect to a rotational period. Product stability restrictions play a major role in the dynamics of stable (NO)+⋅2 formation and the mechanism of dissociation effectively channels energy into internal excitation of the neutral NO⋅ product. The NO+ kinetic energy distributions are bimodal for large photon energies suggesting that NO+ is generated by two mechanisms. The higher energy component was assigned to direct formation of NO+ on a time scale short compared to a rotational period and the lower energy component was assigned to unimolecular dissociation of excited (NO)+⋅2 photoproducts. The energy released in the unimolecular dissociation of (NO)+⋅2 forming the lower energy NO+ product is on the order of 0.1 eV which is consistent with statistical dissociation.
Interatomic potentials for Cd, Zn, and Hg from absorption spectra
Authors: Su Ching-Hua , Liao Pok-Kai , Huang Yu , Liou Shian-Shyang , Brebrick R. F.
Published: February 2003

The absorption coefficient has been measured over a 65 nm range in the red wing of the 213.8 nm line for Zn vapor at 1000 °C. It has also been measured in the blue wing and over a 60 nm range in the red wing of the 228.7 nm line for Cd vapor at five temperatures between 642 and 955 °C and over a 75 nm range in the red wing of the 253.7 nm line for Hg vapor at five temperatures between 460 and 860 °C. These data are analyzed in terms of the statistical theory of broadening. Oscillator strengths of 1.42±0.01 and 1.61±0.06 are obtained for, respectively, the Cd line and the Zn line. Pair potentials for both the ground and lowest excited state are also obtained in all three cases. For Cd this is done assuming no functional form and then assuming Lennard-Jones potentials. Both methods agree and give a ground state minimum of −47.5 meV at 0.482 nm separation and an excited state minimum of −1.06 eV at 0.410 nm. A functional form is required for the less extensive Zn data and the Lennard-Jones form leads to a range of possibilities including ground and excited state minima of −56 meV at 0.400 nm and −1.30 eV at 0.330 nm, respectively, which are in fair agreement with the theoretical calculations. For Hg the experiments indicate a single excited state and a ground state with a minimum of −55 meV. Assuming no functional form for the pair potentials, taking the excited state as doubly degenerate, and assuming the transition probability from the ground to excited state is one-sixth of the free atom value gives points along the ground and excited state potentials that join smoothly with other experimental results and agree well with the calculation of Baylis for the ground state.
Theory of fluorescence depolarization in macromolecules and membranes
Authors: Szabo Attila
Published: February 2003

A comprehensive formalism is developed to describe the decay of the fluorescence emission anisotropy r(t) in macroscopically isotropic systems where both excited state and orientational dynamics contribute to the depolarization. It is shown how energy transfer, heterogeneity, and interconversion of excited states with different emission characteristics as well as both overall and internal reorientation can be treated in a unified way. Limits when the state and orientation dynamics are uncoupled and when the interconversion of the states is either much slower or much faster than the irreversible decay rates, are considered. A systematic treatment of the influence of internal motions is presented. First, the geometry of the transition dipoles is explicitly ‘‘factored out’’ and general expressions for r(t) are obtained for several cases including when the motion occurs about a fixed axis and an axis which in turn can ‘‘wobble’’ about a director. The initial and long-time behavior of r(t) is examined and then, a variety of dynamical models (e.g., discrete jumps, free and restricted Langevin motion about an axis, diffusive motion of an axis in an orienting potential) are used to obtain the time dependence of the relevant correlation functions which appear in the above general expressions. In this way, one can obtain r(t) for a large class of models. Of particular interest is an approximate analytic expression for r(t), valid for any orientation of the transition dipoles and restricting potential, of a cylindrical probe in a membrane. The influence of collective (hydrodynamic) fluctuations of the membrane director are also considered.
Electron diffraction studies of laser-pumped molecules. IV. SF6, experiment and theory
Authors: Bartell Lawrence S. , Kacner Michael A.
Published: February 2003

A model of absorption of infrared radiation by supersonic jets proposed in paper III was tested experimentally. New nozzle designs permitted pumping the ν3 mode of SF6 at power densities in excess of 104 W/cm2. Vibrational excitation corresponding to the absorption of up to 3.6 photons/molecule was deduced from the increased amplitudes of vibration of the SF, FFcis, and FFtrans atom pairs and the lengthening of the SF bond. At high excitations, electron diffraction intensities were accounted for best if it was assumed that two subsets of molecules were produced, one much hotter than the other. Vibration–vibration relaxation from ν3 to the other stretching modes was too fast to be followed. Relaxation of stretching to bending could be monitored, crudely, at lower pressures where approximately 30 collisions were needed at the depressed temperatures in the jet. At higher pressures and excitations V-T/R relaxation was observed, corresponding to a transfer of perhaps one-tenth of the vibrational excitation in the course of 103 collisions. Excitation as a function of gas density, power density, and nozzle diameter was accounted for satisfactorily by the model of paper III.
Monte Carlo simulation of electron thermalization in gases. III. Epithermal electrons in molecular nitrogen
Authors: Koura Katsuhisa
Published: February 2003

The thermalization of epithermal electrons due to the rotationally inelastic and elastic collisions in molecular nitrogen is studied with the Monte Carlo simulation. The elastic and rotational cross sections are taken as the experimental momentum-transfer cross section and the Gerjuoy–Stein formula with the effective quadrupole moment, respectively, which are consistent with the swarm data. The accuracy of the approximate theory based on the assumption of the local Maxwell electron velocity distribution (MD) is examined, where the initial electron velocity distribution is taken to be the MD. The electron velocity distribution deviates significantly from the MD; consequently the degradation of the effective electron temperature 〈Te〉 (reduced mean electron energy), which is dominated by the rotationally inelastic collisions, is slower than that for the MD and the pressure normalized thermalization time τth p is about 20% larger than that for the MD. The 〈Te〉 degradation and τth p are compared with those calculated from the experimental energy exchange rate coefficient, relaxation time, or relaxation rate.
Structural and magnetic properties of a copper–amino acid salt: Copper (II) bis (α-amino isobutyrato)
Authors: Calvo R. , Mesa M. A. , Oliva G. , Zukerman-Schpector J. , Nascimento O. R. , Tovar M. , Arce R.
Published: February 2003

The copper salt of the amino acid α-amino isobutyric acid was synthesized and the crystal structure was determined by x-ray techniques. The crystals are monoclinic, space group P21/c (Z=2) with a=10.470(3) Å, b=5.335(1) Å, c=10.201(3) Å, and β=115.6(2)°. The magnetic susceptibility, measured between 1.4 and 12 K, shows a Curie–Weiss behavior with antiferromagnetic Curie temperature Tc =0.66 K. These data and a simple mean field argument which assumes equal superexchange pathways between Cu nearest neighbors gives J≂0.22 K (antiferromagnetic) for the exchange coupling constant between copper ions. The gyromagnetic factor g and the linewidth ΔH of the single exchange narrowed EPR line were measured as a function of the orientation of the applied magnetic field at room temperature. The gyromagnetic tensor reflects the molecular properties of the square planar Cu(II) complex and the orientation of these molecules within the unit cell of the crystal. The observed magnitude and angles variation of ΔH is analyzed in terms of the exchange narrowed dipolar and hyperfine linewidth. An estimate of the exchange coupling constant from the observed narrowing of the hyperfine broadening support the value obtained from the susceptibility data. Since the exchange parameter J obtained by assuming exchange narrowing of the dipolar linewidth is three times smaller than that obtained from the susceptibility data, we suggest the possibility of a quasi-two-dimensional magnetic behavior, which is supported by our crystallographic data.
UV-laser photodeposition from surface-adsorbed mixtures of trimethylaluminum and titanium tetrachloride
Authors: Tsao J. Y. , Ehrlich D. J.
Published: February 2003

The kinetics of surface photochemistry in adsorbed mixtures of titanium tetrachloride (TiCl4) and trimethylaluminum [Al2(CH3)6] is studied by UV-laser-induced photodeposition. Although both TiCl4 and Al2(CH3)6 individually undergo adsorbed-phase photoreactions, a new photoreaction channel is observed when they are simultaneously present. Pressure dependencies of the reaction rate indicate that the new channel is a two-component photoreaction in the adsorbed mixture. The data can be qualitatively understood in terms of a Langmuir–Hinshelwood mechanism in conjunction with competitive adsorption of the two components.
Pair and trio interactions between adatoms: Re on W(110)
Authors: Fink Hans-Werner , Ehrlich Gert
Published: February 2003

Quantitative observations in the field ion microscope have been made of rhenium dimers and trimers on the (110) plane of tungsten. For Re pairs, the radial distribution function under equilibrium conditions has been measured at 400 K on a plane for which the distance scale was first established by repeated observation of a single adatom diffusing over the surface. From these measurements it appears that at T≊400 K there is little likelihood of two rhenium adatoms being closer than 8 Å to each other; interactions are repulsive out to an interatomic separation of ≊10 Å, in agreement with previous studies. Despite repulsive pair interactions, three rhenium atoms deposited on W(110) readily form a trimer at T≊400–450 K. Linear as well as triangular trimers have been identified, with the latter proving slightly more stable. The binding of Re atoms in these trimers has been studied in two ways. From measurements of the lifetime for dissociation at 420 K, we infer an activation energy to trimer dissociation of 1.29 eV. This leads to a binding energy of 0.25 eV for Re in a trimer after allowing for a barrier to diffusion of 1.04 eV. The equilibrium between adatoms and trimers at T=435 K has also been studied. Quantitative examination of the thermodynamics yields a standard free energy difference of 0.27 eV between adatoms and triangular trimers, in good agreement with the results of the kinetic studies. In rhenium trimers on W(110) cohesion appears due entirely to trio interactions. Although this situation may prove to be unique, trio effects should play a substantial role in other adlayers as well.
Effect of molecular reorientation on excitation decay due to incoherent energy transfer
Authors: Knoester J. , Van Himbergen J. E.
Published: February 2003

A complete study is presented of the excitation decay ρ(t) due to incoherent energy transfer in a system of randomly distributed donor molecules, following a flash excitation at t=0. In particular the dipole–dipole interaction is considered, which causes the well-known anisotropic Förster transfer, and thus makes the transfer process sensitive to molecular reorientations. The general case of a finite reorientation time scale τa and an arbitrary orientational distribution is considered here, from which the static and dynamic limits are obtained as limiting cases for τa →∞ and τa →0, respectively, for arbitrary degree of orientational order. In the isotropic case, these reduce to well-known results. In our approximation the influence of reorientational motion on log ρ(t) can be fully contained in a time dependent factor g(t/τa). From a general expression, g(t/τa) is evaluated for the isotropic case by numerical techniques for different models of reorientation, viz. reorientation through large uncorrelated steps and diffusion type reorientation. By means of a rigorous scaling procedure of t, based on the exact expression of g(t/τa) for small t/τa, it is shown, that in principle, experimental determination of g(t) can be used to estimate τa and determine the amount of diffusive behavior in the reorientational motion.
VUV laser-induced photofragmentation of NH3: Internal energy distribution of NH(A 3Π)
Authors: Hellner L. , Grattan K. T. V. , Hutchinson M. H. R.
Published: February 2003

Ammonia is photolyzed with a xenon excimer laser at 172 nm. A weak emission observed between 539–572 nm is assigned to high vibrationally excited states of NH2(Ã 2A1), formed through a single photon process. However, the main feature of the emission spectra is due to NH(A 3Π), formed by a two-photon process and detected through the identification of the A 3Π → X 3Σ− transition between 328 and 345 nm. A simulated spectrum for Trot=3000 K and Tvib=10 000 K was found to give the best fit with the experimental spectrum for the observed relative populations of v′=0, v′=1 and their rotational levels. The measured natural lifetime τ0=400±70 ns should be considered as an average value for v′=0 and v′=1 populated to J′≂25. The quenching rate constant by NH3 was found to be kq=3.5±0.7×10−10 molecule−1 s−1 cm3. The different mechanisms for the formation of NH (A 3Π) are discussed.
Current instability, limit cycles, and entropy production surface
Authors: Ali Jaleel , Eu Byung Chan
Published: February 2003

A phenomenological model for the evolution equations for charged carriers in semiconductors is proposed which can give rise to a limit cycle of current as the field strength passes through a critical point. The model is a modification of the previously reported model for carrier mobility. We have studied the macroscopic dynamics of the model and its relation to the structure of the entropy production surface corresponding to the model, by solving the evolution equations by a numerical method. It is found that when a limit cycle appears as a solution the average entropy production over a cycle tends to decrease, compared with that of the unstable steady state from which the limit cycle has bifurcated. Based on the study made, we propose a conjecture on a relationship between the trajectories of a macroscopic evolution and the structure of the entropy production surface.
Predissociation and collisional depopulation of the Cs2(E) state
Authors: Wu Z. , Huennekens J.
Published: February 2003

We report here an experimental study of depopulation mechanisms of the Cs2(E) state. By combining ratios of atomic to molecular fluorescence with E state lifetimes obtained by the phase shift technique, all studied as a function of Cs density, we were able to obtain absolute values for predissociation, radiative, and collisional depopulation rates as well as the total quenching rates for the Cs2(E) state. The results are discussed in relation to those of other experiments.
Nonadiabatic processes involving quantum-like and classical-like coordinates with applications to nonadiabatic electron transfers
Authors: Marcus R. A.
Published: February 2003

Nonadiabatic processes may involve both classical-like and quantum-like coordinates. A semiclassical analysis is used to treat the contribution of the former to the Franck–Condon factor in the reaction rate expression, thereby avoiding the usual harmonic oscillator approximation. Microcanonical and canonical rate constants are calculated, yielding an expression which includes contributions from both types of coordinates. The results are applied to nonadiabatic electron transfer reactions in solution, and show how ΔG0 enters the final rate expression, even though ΔE0 is present in the initial Golden Rule nonadiabatic formula. This result avoids an approximation which has arisen in the nonadiabatic electron transfer literature.
Reactive (dissociative) diatom–solid surface collision: A quantum mechanical approach
Authors: Baer M.
Published: February 2003

In this work a quantum mechanical approach to the study of reactive diatom (stationary) surfacae collisions is presented. Following the general theory the infinite order sudden approximation (IOSA) is introduced and the corresponding S matrix elements derived. The case of noncorrugated surfaces is treated in detail and a new quasiselection rule is found which governs the reactive process.
The vapor phase far-infrared spectra of 1H-indene, benzo[b]furan, 1,3,2-benzodioxaborole, and indole: Evidence for planar skeletons and kinetic anharmonicity in the out-of-plane ring deformations
Authors: Smithson T. L. , Shaw R. A. , Wieser H.
Published: February 2003

The far-infrared spectra of the vapors of 1H-indene, benzo[b]furan, 1, 3, 2-benzodioxaborole (catechol borane), and indole are reported in the region of 100–500 cm−1. The spectra of the first three compounds exhibit series of C-type Q branches for the three out-of-plane ring deformations, establishing planar skeletal configurations and indicating that the aromatic skeletons are not truly rigid. The lower of these two sequences are assigned as interlocking transitions arising from the deformation of the five-membered ring and the butterfly mode, and are analyzed by means of a two-dimensional Hamiltonian which includes for each mode a quadratic potential constant and a quadratic dependence of the reduced mass on each displacement coordinate, as well as a potential constant coupling the two modes.
A picosecond holographic grating approach to molecular dynamics in oriented liquid crystal films
Authors: Eyring Gregory , Fayer M. D.
Published: February 2003

The picosecond transient grating technique offers a new approach to the characterization of rotational dynamics and mechanical properties of thin liquid crystal films. Sample excitation by two crossed 100 ps pulses having parallel polarization results in two kinds of phase gratings: one due to the optical Kerr effect, and the other to a standing longitudinal acoustic wave. Rotational reorientation times are calculated from the relaxation of the Kerr grating, while the ultrasonic velocity and absorption are obtained by monitoring the acoustic response. If the excitation pulses are perpendicularly polarized, no longitudinal acoustic waves are generated, so that the signal is due exclusively to the Kerr effect. Whereas previous workers using ∼20 ns excitation pulses observed a single exponential Kerr relaxation in the isotropic phase, we are able to resolve the decay into a fast nonexponential component followed by a slow exponential component. While the slow component disappears below the isotropic→nematic transition, we can detect the fast component even below the nematic→smectic A transition in CBOA. An explanation of the fast component is proposed involving individual rather than collective molecular reorientation. The nature of the polarization grating resulting from perpendicularly polarized excitation pulses is described. An unusual property of this grating is that it acts like a half-wave plate for the diffracted signal. Thus the polarizations of the incoming probe and the outgoing diffracted pulses can be made orthogonal. The theory and implications of this result are discussed, and the extension of the transient grating technique to the study of model biological membranes is outlined.
Spectroscopic manifestation of intramolecular relaxation of azulene in supersonic jets
Authors: Amirav Aviv , Jortner Joshua
Published: February 2003

In this paper we report on the line profiles in the absorption spectra for the S0 → S1 electronic origin and for the 659 and 1394 cm−1 vibrational excitations in the S1 manifold of azulene in pulsed, planar, supersonic expansions. The S1 electronic origin exhibits a Lorentzian line shape, whose homogeneous linewidth results in the lifetime τ=0.8±0.2 ps for interstate electronic relaxation in the isolated molecule.
Critical phenomena and aggregate structure in an oil-in-water microemulsion: A small-angle neutron scattering study
Authors: Magid L. J. , Triolo R. , Johnson J. S.
Published: February 2003

Small-angle neutron scattering measurements were performed on a five component oil-in-water microemulsion under conditions of composition and temperature close to a coexistence curve. The microemulsion aggregate population displays a modest polydispersity; the mean aggregate size is only weakly dependent on the coexistence curve’s proximity. Close to the coexistence curve, the scattering at low angle is dominated by critical fluctuations of correlation length ξ, consistent with the inferences drawn previously by Nicoli and co-workers from their light scattering measurements.
FT-IR evidence for neutral and cationic species resulting from the intercalation of pyridine into FeOCl
Authors: Salmon A. , Eckert H. , Herber R. H.
Published: February 2003

The reaction of pyridine with the high-spin Fe(III) layer compound FeOCl gives rise to well defined intercalation compounds which have previously been examined by detailed 57Fe temperature dependent Mössbauer effect studies. These studies have shown the presence of Fe(II) in the lattice resulting from electron transfer from the guest to the host species. The presence of pyridinium ion and 4,4′ bipyridine, postulated as originating from the redox intercalation process, as well as of the parent pyridine, in the host matrix, is inferred from the observation of characteristic absorptions in the infrared spectra of the intercalation compounds. Fourier transform IR spectra at 2 cm−1 resolution have been obtained at room temperature for a number of pyridine intercalates of FeOCl, and the IR spectroscopic characterization of these samples is found to be in good agreement with the results obtained from the 57 Fe Mössbauer effect studies in the temperature range 78≤T≤300 K.
Substrate signal suppression in Raman spectra of sputter deposited TiO2 films
Authors: Exarhos G. J.
Published: February 2003

Polarized Raman scattering measurements of thin sputter deposited TiO2 films on silica substrates are reported and demonstrate supression of substrate Raman scattering when one specific component of the scattered radiation is analyzed.
Picosecond coherent Raman investigation of the relaxation of low frequency vibrational modes in amino acids and peptides
Authors: Kosic Thomas J. , Cline Raymond E. , Dlott Dana D.
Published: February 2003

Picosecond Coherent Raman Scattering (ps CARS) is used to study vibrational dynamics in low temperature hydrogen bonded crystals including various amino acids and peptides. At low temperature this technique can be used to determine the vibrational lifetime T1 when the ps CARS decay is exponential, and a lower limit to T1 when it is not. Vibron (Ω=150–1600 cm−1) lifetimes are less than 10 ps, whereas librons (Ω=30–120 cm−1), which are torsional oscillations of the amino acid or peptide chains, have long lifetimes (10 ps<T1<5 ns). The theory of anharmonic processes is used to calculate the frequency dependence of T1 for low temperature librons. Most crystals show T1∝Ω−4, which is characteristic of spontaneous decay to two counterpropagating acoustic phonons at Ω/2. The crystal l-alanyl-l-tyrosine ⋅ 3H2O shows T1∝Ω−2, which is characteristic of decay to one libron and one acoustic phonon. This mechanism dominates when the unit cell is large and complex. The temperature dependent ps CARS data is discussed and is consistent with a mechanism involving absorption of acoustic phonons. Finally the relationship between the crystal dynamics and protein dynamics, and preliminary results on crystalline lysozyme are discussed.
Vibrational relaxation of highly excited diatomics. VII. DF(v=9–12) and HF(v=5–7)+HF(v=0), DF(v=0) in all combinations
Authors: Dzelzkalns L. S. , Kaufman F.
Published: February 2003

Vibrational relaxation rate constants are measured for DF(v=9–12) and HF(v=5–7) by ground state HF and DF using the fast-flow infrared chemiluminescence technique. The 14 rate constants range from (1.4 to 5.5)×10−10 cm3 s−1, i.e., probabilities per gas-kinetic (Lennard–Jones) collision from 0.47 to 1.72. The rate constants are independent of the magnitude or sign of the vibrational (V→V) energy defect which indicates that these are V→R,T processes. They are slightly faster for HF than for DF quencher. These results are discussed in the context of theoretical trajectory calculations and comparisons with other highly excited diatomics as well as other quenchers.
Coupled channel distorted wave method of atom–molecule reactive scattering: Application to p
a
r
a to o
r
t
h
o hydrogen molecule conversion
Authors: Choi B. H. , Poe R. T. , Tang K. T.
Published: February 2003

We present a three dimensional coupled channel distorted wave approach of the atom–molecule reactive scattering. The full entrance channel wave functions are obtained from the inelastic vibrational and rotational close-coupling approximation, and reactive cross sections are evaluated with those wave functions employing the transition matrix (T matrix) method. Therefore, in contrast to the previous adiabatic distorted wave model of the reactive scattering, the present method allows for the target molecule to be dynamically distorted following the motion of the incident atom. The formulation of the approach and efficient computational procedures for obtaining the reactive T matrix elements are presented. The reactive scattering cross sections and the rate constants of the H+H2 para to ortho hydrogen molecule conversion have been evaluated to illustrate the computational aspects of the present method. The wave function obtained from the rotational close-coupling approximation yields better results on the reactive cross section than that from the pure vibrational coupling. The wave function computed from the simultaneous vibrational and rotational close-coupling approximation results in the most accurate cross section. This indicates that in order to obtain accurate results with the T matrix method, the simultaneous vibrational and rotational distortions of the target molecule should be considered when approximating the full entrance channel wave function. The differential cross section and the rotational distribution of the product molecule calculated from the present coupled channel distorted wave scheme are qualitatively similar to those obtained from the previous adiabatic distorted wave method; however, the magnitude of the cross sections from the present theory is larger than the previous ones. This might arise from the fact that the vibrational distortion of the target in the previous adiabatic distorted wave model is not sufficiently large. The present approach is shown to account for all major features of the reactive scattering in the H3 system.
Basis quantum Monte Carlo theory
Authors: Öksüz Iskender
Published: February 2003

A quantum Monte Carlo theory with built-in Fermion antisymmetry is developed. The correct exchange symmetry is guaranteed by realizing the abstract iteration equations of quantum Monte Carlo using an antisymmetric position basis. The resulting basis quantum Monte Carlo method is readily applicable to many-electron atoms and molecules. Calculations for the Be atom and the LiH molecule yield energies accurate to within a relative error of a few parts per thousand.
Rotational diffusion in the rotator phase of n-alkanes
Authors: Doucet J. , Dianoux A. J.
Published: February 2003

From quasielastic neutron scattering experiments performed in the rotator phase of n-alkanes (CnH2n+2), as a function of both n and temperature, two types of rotational diffusion have been detected. Type B corresponds to a uniform rotational diffusion, whereas type A corresponds to a hindered rotation, probably limited to large amplitude oscillations. It is the first time that type A rotational diffusion, given by the orthorhombic symmetry phase RI, is observed. On increasing the temperature it rapidly evolves towards the type B diffusion. Correlation times τ which seem to be rather independent of the phase type are of the order of 5×10−12 s.
The role of Coulomb forces in the properties of ionic liquids
Authors: Lantelme Frédéric , Turq Pierre
Published: February 2003

To examine the role of Coulomb forces in the properties of liquids two ensembles of charged and neutral particles have been studied by molecular dynamics calculations. It is shown that the long range forces have a strong structuring effect and lead to high values of the internal energy. The differences between the two systems are very large at low pressure. At high pressure the confinement effect acts as a cohesive force and thus reduces the difference between charged and uncharged ensembles. The presence of Coulomb forces enhances the lifetime of local structures; this is principally apparent for small particles for which the cooperative anion–cation motions become important particularly at low density. This behavior results from the strong attractive forces and can be considered as the very beginning of an association process.
Structures and transitions in solid O2 to 13 GPa at 298 K by x-ray diffraction
Authors: Olinger Bart , Mills R. L. , Roof R. B.
Published: February 2003

The structures and phase transitions in solid O2 were studied at 298 K from 6 to 13 GPa using powder x-ray diffraction in an anvil cell. Pressures were determined from the compression of in situ NaF. X-ray photographs at various pressures showed patterns from five different phases of O2, two of which were previously unreported. Values of the bulk modulus and volume were derived up to 11 GPa where transition to ε-O2 occurs. A compromise fit to all available data indicates that ε-O2 may be an orthorhombic distortion of δ-O2.
Generalized Langevin theory for many-body problems in chemical dynamics: Formulation for molecular solvents, equilibrium aspects
Authors: Adelman Steven A. , Balk M. W.
Published: February 2003

This paper deals with equilibrium statistical mechanical aspects of the extension of the MTGLE approach for chemical reaction dynamics in liquids [S. A. Adelman, Adv. Chem. Phys. 53, 611 (1983)] so that it is applicable to molecular as well as monatomic solvents. This extension is necessary in order to conveniently treat energy exchange between the solute molecules and the solvent vibrational degrees of freedom. The analysis yields a separation of equilibrium solvent density fluctuations into translation-rotational (TR) and vibrational (V) contributions. The TR fluctuations are treated within the rigid solvent molecule model. The V fluctuations are treated within the harmonic oscillator approximation. The analysis is carried out in terms of a set of generalized solvent phase space coordinates S=pvvpww where v and w are, respectively, the V and TR coordinates and where pv and pw are the corresponding conjugate momenta. In this coordinate system, canonical ensemble distribution function for the solvent given that the solute is fixed at configuration point r0, denoted by fCA[S;r0], may be factorized as fCA[S;r0] =fCA[ pvv]  fCA[ pw w;v0r0] where fCA[ pww;v0r0] and fCA[pvv] are respectively: the rigid solvent model approximation to fCA[S;r0]; a vibrational phase space probability distribution function which governs equilibrium mean field V fluctuations in the pure solvent. This vibrational probability distribution function may be written as fCA[ pvv] =Z−1 exp[−β(Tv+W(v))] where Z=pure solvent partition function, where Tv =pure solvent kinetic energy, and where W[v] is a vibrational potential of mean force. The above factorization of fCA[S;r0] holds if: (i) Coriolis coupling between the v and w coordinates is ignored; (ii) The solvent vibrational frequencies are sufficiently high. Within the harmonic approximation to W[v], fCA[ pvv] =[2πkβT]−rNs det ω2 exp [−β(Tv+ 1/2  yTω2y)] where y are the mass-weighted displacements of the v coordinates from equilibrium, where rNs =number of solvent normal modes, and where ω2 is an (rNs×rNs) -dimensional dynamical matrix which determines the vibrational frequency spectrum of the pure solvent within the harmonic approximation. This dynamical matrix may be constructed from the canonical ensemble distribution function of the pure rigid molecule solvent. Thus given the harmonic approximation to W[v] the canonical ensemble distribution function of the nonrigid solvent fCA[S;r0] may be constructed from the equilibrium properties of the rigid molecule solvent.
The Ã 1B2–X̃ 1A1 two-photon fluorescence excitation spectrum of benzotrifluoride
Authors: Doraiswamy S. , Periasamy N.
Published: February 2003

Using the technique of two-photon fluorescence excitation, the Ã 1B2–X̃ 1A1 electronic system of benzotrifluoride (α-trifluorotoluene) was recorded and analyzed employing both linearly and circularly polarized laser beam. The band system is comprised of two groups, one group being unaffected and the other diminishing in intensity under circular polarization of the laser beam. These behavior patterns can be rationalized on the basis of the two-photon tensor patterns. The intensity ordering of the bands in the two-photon spectrum is 1410≫110>000 (Wilson numbering convention) while in the one-photon spectrum the origin band is the strongest. The spectral features of benzotrifluoride are compared with fluorobenzene and toluene.
Systematic estimate of binding energies of weakly bound diatomic molecules
Authors: Vahala Linda , Havey Mark D.
Published: February 2003

There is often insufficient spectroscopic data for a full RKR inversion to yield a potential for weakly bound diatomic molecules. In these cases, parametrized functions such as the Morse or Thakkar potentials may be used to obtain estimates of the binding energy. The Thakkar potential is more flexible, and has been used successfully on some weakly bound systems. In the more usual case, the Thakkar parameter p, which determines long-range behavior R−p, is chosen by p=−a1−1, where a1 is the first Dunham coefficient; p is usually noninteger. We present an alternative choice for p which makes systematic use of the determinable Thakkar coefficients en( p); we choose p to be the minimum integer necessary to obtain monotonically decreasing positive values for the en( p). This approach, which yields good estimates of known ground and excited state binding energies for numerous diatomic molecules, also produces physically meaningful R−6 long-range behavior for the known NaAr and NaNe potentials.
Dimers of aromatic molecules: (Benzene)2, (toluene)2, and benzene–toluene
Authors: Law K. , Schauer Mark , Bernstein E. R.
Published: February 2003

The optical absorption spectra of the first excited singlet states of the benzene, toluene, and toluene–benzene dimers, created in a supersonic molecular jet, are reported. The absorption spectra are detected through two-color time of flight mass spectroscopy; this method eliminates fragmentation of dimers and higher clusters and the dimer spectra are uniquely observed. The benzene dimer observed in this experiment is suggested to have a parallel stacked and displaced configuration of C2h symmetry. Both the toluene and toluene–benzene dimers have two configurations: parallel stacked and displaced [based on (benzene)2] and perpendicular. (Benzene)2, (toluene)2, and toluene–benzene form excimers in the excited state for the parallel stacked displaced configurations. The transformation of (benzene)2 to the excimer takes place at the 00 with a ∼0 cm−1 barrier while the excimer is formed for toluene–benzene with a ∼900 cm−1 barrier. An exciton analysis of the (benzene)2 000 and 610 yields M12, the excitation exchange interaction, equal to ∼1.6 cm−1.
Thermal positive ion production from KCl impinging upon Re: Comparison between theory and experiment
Authors: Kawano Hiroyuki , Kenpō Tsutomu
Published: February 2003

On the basis of our theory on thermal ionization, the emission current of K+ ions produced from KCl molecules incident upon a polycrystalline rhenium surface is calculated as a function of surface temperature in the range 950–2100 K. Theoretical results thus obtained are in good agreement with experimental data. This agreement gives an additional evidence for our previous conclusions that below ∼1800 K the work function of the surface is increased from 5.5 up to ∼7 eV mainly by adsorption of residual gas molecules at ∼2×10−7 Torr and also that the dissociation of incident sample molecules is greatly promoted by the work function increase.