bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural properties of Langmuir–Blodgett films of charge transfer salts: Pristine and iodine doped conducting films of (N-docosyl-pyridinium)TCNQ
Authors: Richard Joel , Vandevyver Michel , Lesieur Pierre , Ruaudel-Teixier Annié , Barraud André , Bozio Renato , Pecile Cesare
Published: July 2002

We have investigated the molecular constitution and ordering of Langmuir–Blodgett films prepared by deposition of N-docosylpyridinium–TCNQ charge transfer salt onto CaF2 substrates and by their subsequent doping with iodine vapor to yield reasonably good (∼10−1 Ω−1 cm−1) conducting films. Optical and electron scanning microscopy of the precursor film show that it is made up of a mosaic of highly crystalline platelets lying flat on the substrate though with random orientation. Structural information at the molecular level is gained by measuring and analyzing the ESR spectra and their angular and temperature dependence as well as the UV-visible and infrared spectra and their linear dichroism. A triplet fine structure is observed in the ESR spectra of the precursor films with zero field splitting parameters ‖D‖=7.48×10−3 cm−1 and ‖E‖=1.2×10−3 cm−1 and singlet–triplet activation energy J=0.20 eV, markedly different from those of the N-docosylpyridinium–TCNQ powder salt. The UV-visible and infrared spectra show features typical of a system containing

weakly interacting (TCNQ−)2 dimers similar to those found in a water solution. The dichroic behavior of the observed infrared active modes of the TCNQ− anion and of N-docosylpyridinium as well as of the vibronically induced absorptions of the interacting TCNQ system are analyzed both for the precursor and for the iodine doped film. According to such an analysis and to that of the angular dependence of the ESR line shapes, the polar sheets in the precursor film consist of slipped (ring-bond overlap) (TCNQ−)2 dimers with their molecular planes almost parallel to the plane of the supporting slide. The spectroscopic and linear dichroism data allow us to monitor the changes induced by the iodination process and to show that, besides the partial oxidation of TCNQ− anions to TCNQ0, there is a complete change in the orientation of the TCNQ units. In many respects, the UV-visible and infrared spectra of the conducting film resemble those of mixed valence, intermediate conductivity salts like, e.g., (TEA) (TCNQ)2. The TCNQ molecules stand edge on with their molecular planes roughly perpendicular to the plane of the slide and with their long axes close to being normal to it.
Reply to: ‘‘Comment on ‘A critique of the Brownian approximation to the generalized Langevin equation in lattice dynamics’ ’’
Authors: Riley M. E. , Diestler D. J.
Published: July 2002

The comments of P. Mark Rodger ( ref.3) on the authors paper (ref.1) are answered. It is contended that the comments address a more general phenomenon than was analyzed in the authors article. (AIP)
Collisional relaxation among rotation–inversion levels in the highly excited vibrational state of NH3
Authors: Matsuo Yukari , Nakagawa Ken’ichi , Kuga Takahiro , Shimizu Tadao
Published: July 2002

Pressure broadening parameters of the visible overtone transitions to the v=5 state in the N–H stretching mode of 14NH3 are measured. It is found that the parameters show good similarity to those of the microwave region both in the rotational quantum number dependence and in the absolute values. Even in the high vibrational state, the collision induced transitions between inversion levels give dominant contributions to the broadening parameters. Well-defined rotational quantum number dependence obtained in the present analysis also gives confirmation of the previous assignments of rotational structure in the overtone transitions.
Nonlocal pseudopotential calculation of the surface tension of simple liquid metals
Authors: Lai S. K.
Published: July 2002

A full nonlocal pseudopotential calculation of the surface tension of simple liquid metals is presented. The basis of the theory is the direct perturbation expansion to second order in a weak electron–ion pseudopotential. By invoking the Born–Oppenheimer adiabatic approximation, an effective Hamiltonian is obtained. This effective Hamiltonian is then used, in conjunction with Gibbs–Bogoliubov inequality, to derive tractable expressions for the calculation of surface tension of simple liquid metals. It is found that, within the same approximation, our nonlocal pseudopotential calculations yield surface tensions of liquid metals much smaller than similar calculations obtained by Hasegawa and Watabe [J. Phys. C 15, 353 (1982)]. However, a review of the theory and a close examination of the various contributions to the surface tension show that the results of calculations by Hasegawa and Watabe are to some extent fortuitous. Various possible improvements and sources of discrepancies on the computation of surface tension will be checked and discussed in the text.
Energy-adapted basis sets for quantal scattering calculations
Authors: Staszewska Grażyna , Truhlar Donald G.
Published: July 2002

A simple technique for energy adapting translational basis functions for quantal scattering calculations is proposed. It is demonstrated to reduce the required number of basis functions by up to a factor of 5 for a test case.
Endpoint distribution for neighbor avoiding walks
Authors: Straley Joseph P.
Published: July 2002

In a recent paper, Ishinabe and Chikahisa enumerated neighbor avoiding walks, and showed that analysis of this data by a standard method led to an exponent value δ=5, in disagreement with the value δ=4 predicted by scaling. The purpose of this note is to extend the walk length, and to make some observations on the difficulties of the analysis of this problem. Even with greatly extended walk length, it is difficult to determine the value of δ for this problem by fitting the endpoint distribution.
Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in two-dimensional fluids
Authors: Korb J.-P. , Ahadi M. , Zientara G. P. , Freed J. H.
Published: July 2002

The dynamic effects of pair correlation functions (pcf) on spin relaxation by translational diffusion in infinite two-dimensional fluids are considered explicitly through a Smoluchowski equation, for the usual conditional probability, with appropriate boundary conditions, especially at the contact separation of the interacting pair of molecules. The solution of this equation by finite difference techniques permits the calculation of time correlation functions, spectral densities, and spin-relaxation rates associated with a dipolar relaxation mechanism between the spin-bearing molecules. Comparison of the two-dimensional spin-relaxation results obtained with different pcf is presented. The spectral densities and spin-relaxation rates are indeed found to be significantly altered by the pcf. For example, for a nonuniform pcf, the two-dimensional spectral densities, at (and above) the Larmor frequency ω0, are greater for translational correlation times τt that are an order of magnitude faster than τt=ω−10 which provides the maximum spectral density in bulk theory. This fast motion result is consistent with the two-dimensional dynamical results found by other techniques. Moreover for a nonuniform pcf and a single translational correlation time, two well-defined minima are found in the variation of the spin-lattice relaxation time with the diffusion coefficient. This could be very useful for interpreting the spin-relaxation data of diffusing complexes in clays, intercalation compounds, and bilayers.
Spectra and structure of small ring compounds. XLVIII. Conformational stability of methylcyclobutane from low frequency Raman data of the gas
Authors: Durig J. R. , Geyer T. J. , Little T. S. , Kalasinsky V. F.
Published: July 2002

The low frequency (500–80 cm−1) Raman spectra of gaseous methylcyclobutane, c-C4H7CH3, and methyl-d3-cyclobutane have been recorded. A series of Q branches beginning at 161 cm−1 for the light molecule and 154 cm−1 for the d3 compound with successive transitions falling to lower frequencies have been assigned to the ring puckering vibrations of both the low energy equatorial and high energy axial conformers. These data have been fitted to an asymmetric potential function of the form: V(cm−1)=(4.78±0.10)×105X4−(3.08±0.04)×104X2 +(2.18±0.1)×104X3 with an assumed reduced mass of 160 amu for the light compound. Utilizing this potential the difference between the puckering angles for the two conformers was calculated to be 4.3° with the equatorial conformer having the larger value of 20.7°. A similar potential was obtained for the d3 molecule. The energy difference between the equatorial and axial forms was found to be 247±20 cm−1 (706 cal/mol) and a barrier of 641±20 cm−1 (1.83 kcal/mol) was found for the interconversion. Experimental values for the ΔH of both the liquid (354±179 cm−1) and gas (295±75 cm−1) from relative intensities of different Raman band pairs over 85 and 37 °C temperature ranges, respectively, were found to be consistent with the value obtained from the potential function. These results are compared to the corresponding quantities for similar molecules.
Pure rotational spectrum of boron trifluoride–11B observed by microwave Fourier transform spectroscopy
Authors: Oldani M. , Bauder A.
Published: July 2002

Pure rotational transitions of 11BF3 in the ground vibrational state with Δk=±3 selection rule have been measured between 14 and 17 GHz. The extremely weak transitions have been observed with a pulsed microwave Fourier transform spectrometer. The Q subbranch between k=±1 and ∓2 has been assigned for transitions with J=11 to 50. In addition, an R and a P subbranch have been identified for transitions with J between 17 and 45 and k between 10 and 32. A total of 60 measured transition frequencies has been combined in a least squares fit of the molecular constants B, C−B, DJ, DJK, DK, HJ, HJK, HKJ, and HK.
EPR of VO2+ in Zn(antipyrine)2(NO3)2: On the orientation of vanadyl doped in a low symmetry crystal field
Authors: Prasad L. Sreeramachandra , Subramanian S.
Published: July 2002

Single crystal EPR of VO2+ doped in Zn(antipyrine)2(NO3)2 is reported. The orientation of substitutionally incorporated vanadyl in relation to the geometry of the ligand field in the host lattice is examined. The preferential orientation of vanadyl is analyzed in terms of factors that are known to effect the energy of the complex such as steric hindrance and molecular in plane bonding. Our results indicate that, for VO2+ in Zn(antipyrine)2(NO3)2, while the V–O bond is not oriented along any metal–ligand bond, it is close to the principal z direction of g for Cu2+ (with a formal dx2–y2 ground state) in the same host lattice. Based on this it is concluded that molecular in-plane bonding is probably the determining factor for the orientation of vanadyl in the present case.
Four-level microwave–microwave double resonance in NH3⋅He gas mixtures
Authors: Peterson Dean B. , Schwendeman R. H.
Published: July 2002

Four-level microwave–microwave double-resonance measurements have been carried out for a number of pump–probe pairs in the inversion spectrum of NH3 in dilute mixtures of NH3 in He. The results completely confirm corresponding measurements previously carried out by Oka with a different apparatus in a different laboratory [Adv. At. Mol. Phys. 9, 127 (1973)]. The original data had been questioned by Davis and Green on the basis of a comparison of observed and calculated results [J. Chem. Phys. 78, 2170 (1983)].
Direction of the dipole-moment derivative of the O–H stretching vibrations of coupled O–D--O–D pairs in H2O ice Ic
Authors: Whalley Edward , Klug D. D.
Published: July 2002

The relative intensity of the in-phase and out-of-phase vibrations of intermolecularly coupled O–D--O–D pairs in ice Ic shows that the dipole-moment derivatives for the O–D stretching vibrations in ice Ic are within about 3° of the tetrahedral lines between water molecules. In the gas phase the dipole-moment derivatives are directed 25° outside the O–D bonds.
Concentration effects on the Mullins–Sekerka instability
Authors: Rigos A. A. , Deutch J. M.
Published: July 2002

The Mullins–Sekerka linear stability analysis of a perturbation on a growing interface is extended to take into account concentration effects. The concentration dependence is included by considering the surrounding particles as an effective medium in the form of a sink term [x−κ2c] in the diffusion equation. This problem is analyzed without the effects of surface tension for d=2 and d=3 and with surface tension for d=3. The stability analysis is also applied to two particles in isolation as well as N particles arranged in a regular planar polygon to study directly the competition for the diffusing species between the N growing spheres. We conclude that the Mullins–Sekerka criterion for growth or decay of an instability is only valid for an isolated particle and that in the presence of an effective medium, the surrounding particles have the effect of increasing an otherwise negative growth rate to a positive value.
Mass-independent isotopic fractionation in nonadiabatic molecular collisions
Authors: Valentini James J.
Published: July 2002

Symmetry and parity constraints associated with nonadiabatic collisions are shown to result in selection rules that can produce mass-independent isotopic fractionation between molecular electronic states coupled by a nonadiabatic process. Nonadiabatic transitions from Π or Δ molecular electronic states to ∑ electronic states can result in isotopic fractionation for atoms occurring in equivalent positions in the molecule if the common isotope of those atoms is a spin-zero nucleus. The Π or Δ state becomes depleted of the rare isotopes of those atoms while the ∑ state is enriched in the rare isotopes. Chemical processes that distinguish between the Π or Δ and ∑ states can convert this isotopic fractionation between electronic states of the same chemical species to a fractionation between different chemical species. Such nonadiabatic-collision-induced isotopic fractionation might explain observations of mass-independent isotopic fractionation in electrical discharges, as well as in meteoritic samples. Nonadiabatic-collision-induced isotopic fractionation also may provide an explanation for observations of isotopic enrichments in the earth’s atmosphere.
Elastic constants of semirigid liquid–crystal polymers
Authors: Ten Bosch A. , Sixou P.
Published: July 2002

The Frank elastic constants of a polymer liquid crystal are calculated. The effects of chain flexibility and screened orientational interactions are considered. A quadratic dependence in the chain length is shown to occur under certain conditions.
Density fluctuation dynamics in a screened Coulomb colloid: Comparison of the liquid and bcc crystal phases
Authors: Cotter Lise K. , Clark Noel A.
Published: July 2002

Inelastic light scattering from particle number density fluctuations was carried out on a suspension of 0.109 μ diam sulfonated polystyrene microspheres exhibiting coexisting colloidal liquid and body-centered-cubic single crystalline phases. Comparison of the first cumulants of the decay of the density–density correlation function in the two phases reveals that the wave vector dependence in the liquid exhibits many features in common with that for longitudinal 110 lattice vibrations in the crystal.
Thermal expansion of a structure II hydrate using constant pressure molecular dynamics
Authors: Marchi Massimo , Mountain Raymond D.
Published: July 2002

Constant pressure molecular dynamics simulations were used to investigate the thermal stability of a structure II clathrate crystal containing krypton guest atoms relative to the empty hydrate. The temperature ranged from 70 to 280 K and the hydrate remained stable over this interval. The empty hydrate spontaneously transformed to an amorphous ice structure at about 240 K.
A
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n
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o Hartree–Fock energy band structure calculations on polyaniline
Authors: Otto P. , Dupuis M.
Published: July 2002

A b initio crystal orbital calculations on the oxidized form of polyaniline with alternating benzoid–quinoid structure are reported. For two possible configurations, the trans and the cis isomer, the twist angle between nearest neighbor rings is optimized with respect to the total energy per elementary cell. It turns out that the perpendicular orientation of the two rings leads to the most stable structure in agreement with 13C-NMR spectroscopic data. Analogous investigations are performed on the salt formed between oxidized polyaniline and hydrogen chloride. In these systems again the cis configuration with the perpendicular conformation of adjacent rings is the energetically preferred geometry. The protonation of one of the two nitrogen atoms in the elementary cell leads to a band representing the N–H bond at lower energy than the band standing for the lone pair orbital in the unprotonated form. The three highest filled bands can be attributed to the chloride ion. The energy gap in the protonated oxidized polyaniline is smaller than for the neutral form which may be responsible for the conductive properties of the salt form.
Relaxation time study of diammonium hydrazinium (+2) tetraperchlorate
Authors: Harrell J. W.
Published: July 2002

The relaxation times T1, and T1ρ, and T1D of protons in (NH4)2N2H6(ClO4)4 were measured in the temperature range from 77 to 430 K. T1 measurements were made at 36 and 18 MHz, and T1ρ measurements were made as a function of H1. The temperature and frequency dependence of T1 is explained in terms of dipole–dipole interactions within anisotropically reorienting NH3 groups and spin-rotational interactions within reorienting NH+4 ions. The value of the T1ρ minimum due to reorienting NH3 groups is unusually large with weak H1 dependence, possibly due to tunneling motions. The T1ρ measurements exhibit a strong H1 dependence on the high-temperature side of the T1ρ minimum. An analysis of the H1 dispersion in terms of cross relaxation between protons and nitrogen nuclei within the N2H++6 ions has yielded values for the nitrogen T1.
Electron mobilities, free ion yields, and electron thermalization distances in liquid long-chain hydrocarbons
Authors: Gee Norman , Freeman Gordon R.
Published: July 2002

At 296 K the electron mobility decreased in the liquids n-hexane>n-decane>n-dodecane >n-tetradecane in accord with the two state model of electron transport (e−il⇄e−qf); the percolation model was inadequate. The temperature dependences were determined for n-decane, n-dodecane, n-tetradecane to T>450 K. Free ion yields were also measured. At a given temperature Gofi was the same in all these liquids to within ±4%. The density-normalized thermalization ranges of secondary electrons, estimated from the extended Onsager model, were bGPd =(49±1)10−6 kg/m2 under all conditions.
Laser pulse induced n-photon absorption in an atom or molecule aligned by a static electric field
Authors: Thomas Gerald F.
Published: July 2002

Within the rotating-wave and Magnus approximations, the electrical anharmonicity induced in an atom or molecule aligned by a static electric field is shown to lead to the absorption of many photons delivered by a Gaussian-modulated laser pulse of finite intensity and duration. In the presence of the static field the effective area of the near-resonant pulse vanishes at prescribed intensity-dependent frequencies, resulting in the appearance of satellite fringes flanking the usual Gaussian line shape. The transition probability for excitation of the oriented system by a train of phase-coherent Gaussian pulses is calculated.
Quadrupole structure in the rotational spectrum of the v8=1 and 2 and ground vibrational levels of CH3C14N for the frequency range 17–56 GHz
Authors: An I. , Rhee W. M. , Roberts J. A.
Published: July 2002

The rotational spectra in the ground and v8=1 and 2 vibrational states of methyl cyanide have been determined in the frequency range 17–56 GHz. Molecular constants for these vibrationally excited states have been determined from more than 30 observed rotational transitions. Experimentally measured frequencies are presented and compared with those calculated using the results of basic perturbation theory. Constants were obtained for the v8=1 level and these, along with the ground constants, used to generate constants for components of the v8=2 vibration. A model was developed for the quadrupole splitting for the ground and v8=1 levels. From these values spectral components were predicted and assigned for the higher vibration v8=2.
Nonequilibrium kinetic energy distributions of energetic atoms with inelastic collisions
Authors: Gorensek M. B. , Kostin M. D.
Published: July 2002

Asymptotic solutions are derived for the kinetic energy distribution of energetic atoms produced by a high energy source in a system in which inelastic collisions occur. An energy balance is used to derive the normalization constants of the asymptotic solutions. Agreement between the asymptotic solutions and the corresponding numerical solutions is found to be excellent over the asymptotic range.
Spectroscopy of matrix-isolated methylnitrene
Authors: Ferrante Robert F.
Published: July 2002

The methylnitrene radical CH3N has been produced by the interaction of metastable molecular nitrogen and methylazide, and has been trapped in solid nitrogen at 10 K. The IR and UV absorption spectra are consistent with gas phase emission results, and also show transitions to several additional vibrational levels in the A 3E excited electronic state. The UV band system consists chiefly of a progression in the C–N stretching mode, possibly complicated by additional unresolved vibrational structure. These observations indicate that the absence of extensive hot band structure in the gas phase work is not due to rapid dissociation or isomerization from the upper state, which is bound by at least 3500 cm−1. The ESR signals previously attributed to CH3N cannot be produced under the same experimental conditions, but require additional photolysis. This observation suggests that a reexamination of previous ESR data is warranted.
Estimation of the amount of adjacent reentry in polymer crystallization. I. The basic equations
Authors: DiMarzio Edmund A. , Passaglia Elio
Published: July 2002

We develop a treatment of polymer chain folded crystallization in which stems are incorporated into the crystal by both adjacent and nonadjacent reentry. By defining certain stages of partial incorporation of a molecule as separate species, we have been able to adapt a kinetic transfer matrix technique derived previously to solve the problem of the growth of multicomponent chains to the solution of the present problem. A prescription is given for obtaining the relative amounts of adjacent and nonadjacent reentry as a function of the forward and backward rate constants for stem incorporation. The difficult problem of determining the rate constants as well as the actual numerical calculations are presented in a subsequent paper.
Hydrogen bonding and cation radical formation of methyl 4-(N,N-dimethylamino)phenyl carbamate, DMAPCMe
Authors: Laidlaw Richard K. , Miura Yozo , Grant Janice L. , Cooray Lalit , Clark Matthew , Kispert Lowell D. , Metzger Robert M.
Published: July 2002

The donor DMAPCMe is found by theoretical calculations and cyclic voltammetry to be a moderate electron donor (calculated adiabatic ionization potential 7.17 eV, E1/2=0.55 V vs SCE), which is intermediate between the experimental ionization energies of TMPD (6.25 eV), TTF (6.83 eV), and pyrene (7.41 eV). DMAPCMe is extensively hydrogen-bonded in solution, as seen by the growth of a shifted N–H infrared vibration band. The paramagnetic resonance spectrum of the DMAPCMe+ radical cation matches fairly well a simulated monomer radical spectrum, but there is evidence for H-bonded paramagnetic adducts. These same adducts reduce irreversibly at 0.37 V vs SCE in acetonitrile solution.
The modified Pitzer–Gwinn method for partition function of restricted internal rotation of a molecule
Authors: Hui-yun Pan
Published: July 2002

The Pitzer–Gwinn method is modified with the Wigner–Kirkwood perturbation expansion used instead of the classical partition function. This method is applied to the calculation of partition function of restricted internal rotation for various temperatures and potential barriers. Numerical results show that the modified Pitzer–Gwinn method provides remarkably accurate partition function.
The orientation of the NH3D+ ion in crystals: Phases, tunneling, and photophysics
Authors: Trapani Andrew P. , Gensler Steven W. , Strauss Herbert L.
Published: July 2002

The equilibrium orientation and the kinetics of reorientation of NH3D+ dilute in a number of Tutton salts and in ammonium sulfate have been studied. The kinetics of reorientation are very slow (hours or days) at 4 K and consequently it is difficult to obtain the equilibrium distribution. Our previous report of a maximum extent of orientation at about 30 K is incorrect. Instead the extent of orientation increases monotonically as the temperature is lowered. This can be modeled with NH3D+ ions interacting by means of their dipole moments. Although the salts behave as well defined phases at low temperatures, the possibility of glass-like behavior remains open for both the salts and the Monte Carlo simulation. The kinetic processes occur by tunneling, and the tunneling rates have been measured. These rates give reasonable values for the barrier to reorientation. The orientation rate is accelerated by broadband infrared irradiation of the sample. The radiation is probably absorbed by the N–D stretches and induces the rotation of the excited ions.
Mutual orientation of three magnetic tensors in a polycrystalline dipeptide by dipole-modulated 15N chemical shift spectroscopy
Authors: Hartzell C. J. , Pratum T. K. , Drobny G.
Published: July 2002

This study demonstrates the mutual orientation of three tensor interactions in a single NMR experiment. The orientation of the 15N chemical shift tensor relative to the molecular frame has thus been determined in polycrystalline L-[1-13C] alanyl-L-[15N] alanine. The 13C–15N and 15N–1H dipole interactions are determined using the 1H dipole-modulated, 13C dipole-coupled 15N spectrum obtained as a transform of the data in t2. From simulations of the experimental spectra, two sets of polar angles have been determined relating the 13C–15N and 15N–1H dipoles to the 15N chemical shift tensor. The values determined are βCN =106°, αCN =5° and βNH =−19°, αNH =12°. The experiment verifies, without reference to single crystal data, that σ33 lies in the peptide plane and σ22 is nearly perpendicular to the plane.
Fluorescence line-narrowing spectra of Zn-cytochrome c. Temperature dependence
Authors: Koloczek Henryk , Fidy Judit , Vanderkooi Jane M.
Published: July 2002

Fluorescence site selection spectra of Zn-substituted cytochrome c and Zn-mesoporphyrin were measured as a function of excitation energy and temperature within 5–60 K. The emission energies determined for Zn-cytochrome c were comparable with those of Zn-mesoporphyrin and mesoporphyrin in polar glassy matrices and with resonance Raman spectra of heme proteins. In the site selection spectra of both Zn-cytochrome c and Zn-mesoporphyrin in polar organic glass an increase in the intensity of zero phonon emission lines was observed with an increase of temperature between 5 and 30 K. The irregular spectral changes were interpreted on the basis of the temperature dependence of the linewidth as well as possible hole-burning phenomena.
Direct observation of inversion transitions in the v=5 state of NH3 by millimeter wave-optical double resonance
Authors: Matsuo Yukari , Endo Yasuki , Hirota Eizi , Shimizu Tadao
Published: July 2002

A powerful method for detailed studies of the static and dynamic properties of molecules in highly excited vibrational states is presented. NH3 molecule is excited to the v=5 state in the N–H stretching mode by a pulsed dye laser and a subsequent transient change in the millimeter wave absorption due to the inversion transition in the excited state is observed before the distribution returns to thermal equilibrium. The determined inversion transition frequencies are more accurate by more than two digits than those reported in the previous work.
Dynamics in BSA solutions at low ionic strengths as observed by holographic relaxation spectroscopy
Authors: Barish Amy O. , Gabriel Don A. , Johnson Charles S.
Published: July 2002

Holographic relaxation techniques (HRS) were used to study dynamics in solutions of bovine serum albumin (BSA) labeled with azobenzene-p-isothiocyanate (ABITC). The ionic strengths ranged from 0.5 to 100 mM and the protein concentrations were 3 to 50 g/L. A single diffusive component was observed above 25 mM salt, but at lower ionic strengths two components were resolved. Also, electrophoresis combined with holographic relaxation spectroscopy (EHRS) showed two components. A photoionization model, in which the net charge of the BSA-ABITC molecule is altered by the writing laser pulse, is proposed to explain the results. The coupled diffusion problem for the bleached and unbleached macroions and the counter and coions is solved to obtain the concentration and ionic strength dependences of the diffusion coefficients. Also, effective diffusion coefficients for the components in EHRS are obtained. Overall, there is good agreement between this simple model and experiment; however, the macroion charges required in the theory are roughly a factor of two lower than those found by titration and electrolysis.
Vibronic coupling in polyenes: High resolution optical spectroscopy of all-t
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s-2,4,6,8,10,12,14-hexadecaheptaene
Authors: Simpson Jeffrey H. , McLaughlin Lee , Smith D. Scott , Christensen Ronald L.
Published: July 2002

High resolution fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8,10,12,14-hexadecaheptaene have been obtained in n pentadecane at 10 K. The most prominent vibronic features, like those in spectra of shorter polyene hydrocarbons, are due to combinations of symmetric carbon–carbon stretching vibrations. These in turn are dominated by a double bond mode whose frequency increases from 1555 to 1782 cm−1 upon electronic excitation (1 1Ag→2 1Ag). This 227 cm−1 increase, the largest yet observed in polyene spectroscopy, can be explained by vibronic coupling between the ground and first excited singlets. The possible role of vibrational mixing due to mutual polarizibilities between carbon–carbon bonds also is discussed.
Theory of coherent polarization anisotropy in time-resolved two-photon ionization of isolated molecules. Effects of Coriolis couplings
Authors: Sato S. , Nomura Y. , Fujimura Y.
Published: July 2002

Effects of vibration–rotation (Coriolis) couplings on the coherent polarization anisotropy are theoretically studied in a time-resolved two-photon ionization of a symmetric top molecule. This polarization anisotropy originates from a coherent excitation of the resonant rovibronic molecular eigenstates (rovibronic coherence) whose zeroth order states are mixed through the Coriolis coupling. Expressions for the time-dependent degree of polarization after the coherent excitation of the rovibronic states produced by the Coriolis coupling are derived as a function of the delay time in the pump–probe two-photon ionization. Model calculations of the time-dependent degree of polarization as well as the probabilities of the two-photon ionization are performed to demonstrate the Coriolis coupling effects in the low excess energy regions of the resonant intermediate state. It is shown that oscillatory behaviors in the time-dependent degree of polarization should be observed as a result of the creation of the rovibronic coherence. It is demonstrated that oscillations of the degree of polarization involve both contribution of the purely rotational J-coherence and that of the rovibronic coherence in the resonant manifold when the rotational branches are coherently excited and the characteristic rotation–vibration interaction energy is larger than a typical free rotational energy under jet-cooled condition.
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4N nuclear quadrupole resonance and a
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o MO study of organic azides
Authors: Murgich Juan , Aray Yosslen
Published: July 2002

An ab inito SCF-MO calculation of the electric field gradient (EFG) using a double zeta basis set with polarization is reported for N−3, HN3 NCN3, and H3CN3. A similar calculation using a Dunning set with polarization is also reported for N−3, HN3, and H3CN3. Excellent correlation was found between the calculated and the components obtained from gas phase microwave data. The calculation showed that the terminal N atom had a positive nuclear quadrupole coupling constant and its pincipal z axis was found to be perpendicular to the NNN direction in substituted azides. The changes in the charge distribution of HN3 and NCN3 obtained from the analysis of the atomic multipoles was shown to be in general agreement with the variations found in the N EFG components calculated for these azides. The 14N nuclear quadrupole resonance frequencies of the terminal and the first N atom of the azide group have been measured at 77 K for phenyl, benzyl, triphenylmethane, and tosyl azide. An analysis based on the Townes and Dailey approach of the measured NQR data showed that the terminal N atom have lost more σ than π orbital population. A similar analysis for the first N atom of azides showed strong inductive and resonance interactions with the substituents.
A high pressure Raman study of 9-hydroxyphenalenone and pressure-induced phase transitions
Authors: Jayaraman A. , Kourouklis G. A. , Haddon R. C.
Published: July 2002

The pressure effect on 9-hydroxyphenalenone (9-HPLN) has been investigated using Raman scattering and optical absorption techniques in a diamond anvil cell up to 70 kbar hydrostatic pressure. A pressure-induced first-order phase transition occurs near 6 kbar and the Raman data shows that the transition is to the same phase that is obtained on cooling 9-HPLN below 255 K at ambient pressure. The phase boundary has a positive slope with dT/dP=7 K/kbar. The Raman data indicate that there may be another phase change near 50 kbar. A strong and sharp Raman frequency near 12 cm−1 is observed in all the phases up to the highest pressure investigated. The weak pressure dependence of this frequency and its presence up to the highest pressure suggests that it may be a molecular mode rather than an external mode frequency. The optical absorption associated with π–π* excitation exhibits a large red shift, as expected of an aromatic molecular crystal.
Longitudinal NMR relaxation studies of molecular structure and dynamics in PHO−23
Authors: Farrar Thomas C. , Locker Ilene C.
Published: July 2002

The NMR longitudinal relaxation of a coupled two spin system has been investigated for the case where two relaxation mechanisms, namely, dipole–dipole and chemical shift anisotropy, are present. A method for determining correlation times, as well as bond distances and chemical shift anisotropies for molecules in solution is presented. This method requires that a set of experiments, including inversion of one nucleus and observation of the other as well as ordinary inversion–recovery experiments, be performed at a single temperature. For PHO2−3 in solution, the P–H internuclear distance as found to be 1.48 Å, the phosphorous chemical shift anisotropy was 123 ppm and the proton chemical shift anisotropy was −5.8 ppm.
Nonequilibrium percolative c(2×2) ordering: Oxygen on Pd(100)
Authors: Evans J. W.
Published: July 2002

Dissociative adsorption of oxygen on certain (100) metal surfaces has been modeled as random dimer adsorption onto diagonally adjacent empty sites of a square lattice subject to the additional constraint that all six neighboring sites must be empty (the 8-site model). Here we adapt this model to analyze the nonequilibrium c(2×2) ordering recently observed for oxygen on Pd(100) at coverages up to saturation (>1/4 monolayer), under conditions of low temperature and high pressure where effects of diffusive mobility can be ignored. We do, however, propose that adsorption could be followed immediately by short range transient mobility to dissipate excess energy. We first show how exact master equations for this model can be used to obtain analytic expressions for various local quantities of interest: the probability of an empty 8-site configuration (which determines the sticking coefficient), the c(2×2) island edge or domain boundary densities, etc. They also provide a characterization of, e.g., the asymptotic decay of spatial correlations. Near-percolating (percolative) c(2×2) ordering is readily observed in computer simulations of the saturation state. Through a simple extension of the physical model, we provide a framework for analysis of the large scale characteristics of this ordering via correlated polychromatic percolation theory. Corresponding scaling relations and some real space renormalization group analysis are described. Simulation results for average sizes, the effective dimension, and perimeter length to size ratios, of c(2×2) islands, are also presented.
Translational diffusivity from the Zimm model
Authors: Öttinger Hans Christian
Published: July 2002

Various approaches to the translational diffusion of polymers in a dilute solution are discussed for the Zimm model. The whole discussion is carried out for an arbitrary definition of the center of a polymer, for all homogeneous steady flows, and for constant external forces. It turns out that the result for the diffusion coefficient differs from the well-known Kirkwood expression, however, the numerical difference is less than 2%. We point out which approximations in the various derivations of the Kirkwood expression for the diffusion coefficient lead to this difference. The expression for the diffusion coefficient given in this paper can be generalized to arbitrary bead-spring models. Furthermore, we give the configurational distribution function for the Zimm model with time-dependent external forces in an arbitrary homogeneous flow field.
Applications of filtering theory for the inversion of temporal chemical systems
Authors: Beumee J. G. B. , Rabitz H.
Published: July 2002

This paper presents a method for inverting temporal experimental data from chemical models to obtain estimates of unknown parameters. Most of the models under consideration are deterministic and we assume that the measurements obtained from experimental observations are represented as the solution of a differential equation containing the variables of the model. To incorporate any extraneous laboratory effects that are not included in the model, we assume that these equations are perturbed by a white noise process so that the measurements become time-dependent stochastic variables. A particular measurement is then equivalent to a realization of these variables and applying stochastic estimation theory this realization can be used to obtain estimates of the unknown parameters in the model. As an example of this estimation method, we consider chemical kinetics models with various observational equations and construct an estimator for the unknown reaction rate constants. We also show the estimators for the structural constants in a laser model depending on the representation of the experimental data. In some cases the observations are simulated numerically and we present the parameter estimates as a function of time. The efficiency of the estimation process is calculated as the ratio of the a posteriori variance of the parameter estimator and the Rao–Cramer lower bound. Some issues in the numerical implementation of the filtering equations are discussed and a comparison is made between the minimum least-square estimation method and the filtering method.
Ultrasonic study of fluctuations in polystyrene solutions
Authors: Fenner D. B.
Published: July 2002

Measurements of the pulsed-ultrasound absorption spectra (1 to 91 MHz) and velocity (5 MHz) observed in monodisperse polystyrene (PS) solutions with toluene (Tol) and diethyl malonate (DEM) are reported and discussed extensively. The PS+Tol (good solvent) samples with a low concentration (φ≂0.05) and higher temperature (T≥0 °C) were weakly absorbing with nearly single-relaxation spectra. At high concentration (φ≂0.27) the PS+Tol spectra became the critical-fluctuation type. All PS+DEM (poor solvent) solutions had a large excess absorption αex with spectra well described by fitting the (binary-mixture) critical-fluctuation absorption calculations of Kroll and Ruhland (KR) and of Ferrell and Bhattacharjee (FB). The KR approach gave characteristic frequencies fc which obeyed an empirical (nonuniversal) scaling fc∼ΔT ρ, with ΔT the temperature interval above segregation on each isochore. Results indicate a slowing down (ρ>0) and strengthening of the fluctuations as ΔT→0. A crossover in ρ was observed with PS+DEM near but below the theta temperature (ρ smaller below). For critical-isochore samples ρ approached 1 as the PS

molecular weight was increased toward 106. Near phase separations, spectra followed the approximate prediction of FB theory, αex∼f, f the sound frequency. The FB characteristic (half-attentuation) temperatures ΔT1/2 scaled with f, going from exponent 1/2 (for small molecules) to 1 as f was increased. Earlier data for PS in cyclohexane are reevaluated with the KR and FB methods with results similar to PS+DEM. These PS results are compared extensively with the work of Garland and Sanchez on a small-molecule binary-mixture critical point, where ρ was universal, fc two orders of magnitude higher at large ΔT, and ΔT1/2 tenfold smaller at low f. This study finds that fluctuation absorption occurs commonly in PS solutions and is due to energy fluctuations decoupled (nonuniversal exponents) from critical PS-composition fluctuations (known to be the binary-mixture type). Light scattering verified that the critical point was approached closely in PS+DEM. The sound velocity was independent of the PS molecular weight, but increased as a weak quadratic function of the concentration for all solvents. Mixture rules for the modulus predict this qualitative trend.
Optically detected spin coherent transients from organometallic diimine complexes in the phosphorescent triplet state
Authors: van Oort E. , Sitters R. , Scheijde J. H. , Glasbeek M.
Published: July 2002

Using optical-microwave coherence techniques, spin coherent transients in zero field are measured for Rh(bpy)3+3 and Rh(phen)3+3 in the phosphorescent 3ππ* state. Surprisingly long phase memory times (of a few microseconds at 1.2 K) are found. The importance of selective spin–orbit couplings for the pure spin dephasing process is emphasized.
Weak infrared bands in solid CH4-II
Authors: Baciocco G. , Calvani P. , Cunsolo S.
Published: July 2002

The spectrum of solid methane in its partially ordered phase II was studied between 1000 and 3500 cm−1 with improved sensitivity. The librational band of D2d molecules associated with the ν3 and ν4 mode could be observed in detail, and evidence of induced absorption was found in the region of the infrared inactive ν2 mode. The first infrared spectra in solid CH4 of the overtone 2ν4 and of the combination band ν2+ν4 are also reported.
Concentration dependent nondiffusive triplet energy transport on a disordered lattice in the chemically mixed crystal dichlorobenzene–dibromobenzene
Authors: Kirski T. , Grimm J. , von Borczyskowski C.
Published: July 2002

Nondiffusive triplet energy transport is observed in a p-dibromobenzene single crystal doped with p-dichlorobenzene (DCB) at variable concentrations. Phosphorescence kinetics of DCB monomers and aggregates are strongly nonexponential and can be described assuming a stretched exponential exp[−(t/τ)β], where β varies from 1 at low concentrations to 0.28±0.05 at concentrations above 0.05 mol/mol.
Rate constant for the reaction of atomic oxygen with phosphine at 298 K
Authors: Stief L. J. , Payne W. A. , Nava D. F.
Published: July 2002

The rate constant for the reaction of atomic oxygen with phosphine has been measured at 298 K using flash photolysis combined with time-resolved detection of O(3P) via resonance fluorescence. Atomic oxygen was produced by flash photolysis of N2O or NO highly diluted in argon. The results were shown to be independent of [PH3], [O], total pressure and the source of O(3P). The mean value of all the experiments is k1=(3.6±0.8)×10−11 cm3 s−1 (1σ). Two previous measurements of k1 differed by more than an order of magnitude, and our results support the higher value obtained in a discharge flow-mass spectrometry study. A comparison with rate data for other atomic and free radical reactions with phosphine is presented, and the role of these reactions in the aeronomy or photochemistry of Jupiter and Saturn is briefly considered.
The excluded-volume expansion in polymer chains: Evaluation of the Flory exponent in the Gaussian approximation
Authors: Allegra Giuseppe , Ganazzoli Fabio
Published: July 2002

The self-consistent excluded-volume chain-expansion problem is reexamined, in the Gaussian approximation. The integral equation resulting from minimization of the free energy is solved analytically on the basis of a conjecture that is shown to be logically consistent. The mean-square distance between atoms separated by k bonds (k≫1) within an infinitely large chain is 〈r2(k)〉∝k6/5{∏∞n=1Ln [ln(k/k̄)]}2/5, where Ln(x) =1+an ln Ln−1(x), L1=1+a1x; k̄ is a short-range cutoff and the ai’s are suitable constants, generally comprised between 0 and 1, while limn→∞ an =1. Accordingly, the divergence associated with the short-range cutoff is explicitly evaluated instead of being removed through renormalization techniques. For (k/k̄)→∞ the above function is independent of k̄ and we have ln〈r2(k)〉∝6/5 ln k, thus reobtaining Flory’s asymptotic exponent. Numerical results obtained from the integral equation in the nonasymptotic regime give support to the above analytic solution.
Tunable UV laser photolysis of NF2: Quantum yield for NF(a 1Δ) production
Authors: Heidner R. F. , Helvajian Henry , Koffend J. Brooke
Published: July 2002

The UV photodissociation of NF2 has been investigated from 240–270 nm, using tunable UV radiation from a frequency upconverted YAG pumped dye laser system. The absorption cross section of NF2, and the photolysis quantum yield for the fragment NF(a1Δ) were measured with 0.25 cm−1 resolution. The NF(a1Δ) quantum yield decreases at longer wavelengths and is only 1% at 260 nm. This suggests that the first long wavelength band in NF2 leads primarily to ground state NF(X 3∑), and that the existence of a new higher lying NF2 electronic state is responsible for the NF(a1Δ) production.
Fluctuation method for calculation of elastic constants of solids
Authors: Pratt Lawrence R.
Published: July 2002

This paper derives a method for calculation of the elastic constants of a solid from computer simulation data on atomic displacement correlations at long range. It is shown that principal axes of the elastic constant tensor can be found from a Fourier analysis of the second-rank tensor of displacement correlations together with a numerical search in the reciprocal Fourier k space. The full fourth-rank tensor of elastic constants then can be obtained by rotations after the principal values are calculated. The method is not limited to special crystal symmetries, to pair-decomposable forces, to low temperatures, to thermodynamic states of only slight distortion from the minimum energy configuration, or to harmonic systems.
Pure dephasing of a vibrational adbond
Authors: van Smaalen Sander , George Thomas F.
Published: July 2002

A theoretical study of the vibrational relaxation of the adbond between a physisorbed molecule and a crystalline substrate is presented. The bond between the admolecule and the substrate is described as a one-dimensional Morse oscillator. Using standard perturbation theory, expressions for the relaxation of the vibrational adbond due to its interaction with the substrate lattice vibrations are derived. Both energy relaxation and pure dephasing are considered. A numerical evaluation of the resulting expressions for the relaxation constants is given for a Debye spectrum for the phonons and for a surface phonon spectrum. The relative importance of pure dephasing and energy relaxation in determining the absorption linewidth is derived as a function of the fundamental frequency of the active mode.