bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser correlation spectroscopy of amorphous polymethylmethacrylate
Authors: Cohen Claude , Sankur Vega , Pings C. J.
Published: February 2003

We have studied the light scattered from amorphous polymethylmethacrylate (PMMA) using the correlation technique for a range of temperatures from 6 to 165 °C encompassing the glass transition temperature (Tg∼120 °C). The data were analyzed in terms of two exponential decays, and the angular dependence of each of the corresponding relaxation frequencies was examined. The results for the high frequency relaxation mode are angular independent and fall reasonably well on two straight lines of different activation energy (∼8 kcal/mole at high temperatures and ∼1 kcal/mole at low temperatures), indicating the presence of two coupled relaxation mechanisms. The low frequency relaxation results are quite sensitive to the inhomogeneities of unannealed samples and have in this case irregular angular dependences. The angular dependence disappears for samples close to Tg and for annealed samples. Below Tg, this relaxation process has a fairly constant frequency of about 3 Hz independent of temperature. Above Tg, its frequency increases very rapidly with temperature to reach 130 Hz at 165 °C and follows the backbone main-chain relaxation frequency measured by other techniques.
Normal coordinate analyses of the neutral CO3 and SO4 molecules
Authors: LaBonville P. , Kugel R. , Ferraro J. R.
Published: February 2003

Normal coordinate analyses for the neutral molecules CO3 and SO4 were made. For the CO3 molecule a trans Cs structure was found to be more suitable than the three-membered ring C2v bridged structure, based on frequency and isotopic fit and force constant transferability. Three structures were considered for the SO4 molecule: C3v, C2v bridged structure, and Cs. The Cs structure was preferred because of the agreement between observed and calculated frequencies and reasonable values for the force constants.
Infrared absorption and the Born–Oppenheimer approximation. I. Vibrational intensity expressions
Authors: Walnut Thomas H. , Nafie Laurence A.
Published: February 2003

A formalism is presented which permits the expression of corrections to the Born–Oppenheimer approximation without direct reference to the excited electronic states of the molecule. This is accomplished by the introduction of a vibronic gauge function which is the solution of a separate, known differential equation. With this formalism the exact equivalence of the vector potential and dipole moment expressions for infrared vibrational intensities in the Born–Oppenheimer theory is established. The perturbation terms for the corrections to the Born–Oppenheimer approximation are discussed and their meaning is illustrated with the example of a translating hydrogen atom. The vibronic gauge formalism is extended to molecular orbitals where the separability of the problem in terms of individual orbitals is demonstrated. Simple examples of the solution of the vibronic gauge function are presented for inner shell orbitals.
Hidden potentials in classical theorems
Authors: Nakatsuji Hiroshi
Published: February 2003

Some exact equations are derived which clarify some potentials that are hidden in the classical theorems such as the Hellmann–Feynman (H–F) and the integral Hellmann–Feynman (I-H–F) theorems. The differential form of the density equation given previously includes not only the classical force operator but also the force operator associated with the quantum-mechanical potential introduced by Bohm. The latter arises essentially only from the noncommuting property of coordinates and momenta in quantum mechanics. However, after integration only the classical force term survives and results in the H–F theorem. The important role of the quantum force term is completely hidden in the H–F theorem. This fact would be closely related to the nondeterminicity and the classical interpretation of the H–F theorem. A similar role of the quantum potential is also shown for the I-H–F theorem. We have also investigated the origin of force density (the integrand of the H–F theorem) and isolated the roles of the generalized exchange and correlation effects which are also hidden in the H–F theorem.
On the dynamics of exothermic triatomic exchange reactions
Authors: Fischer Sighart , Venzl Gerd
Published: February 2003

Analytic expressions are derived for the population of vibrational states for triatomic exchange reactions. The predictions are in good accord with the so-called ’’Polanyi rules.’’ The results are expressed in terms of the masses, frequencies, exothermicity, interaction length, and the attractive part of the potential A⊥E*. Comparison with exact quantum-mechanical calculations is made for a series of light-heavy-heavy (LHH) reactions. An almost linear relation between the attractive part of the potential and the product vibrational energy is obtained. For the LHH-mass combination the slope is almost one. For other mass combinations like HHL, HLL, or HLH the final vibrational energy is relatively insensitive to A⊥, showing always strong population inversion. The width of the vibrational energy distribution is smallest for the HLH mass combination.
Carrier photogeneration from a higher excited singlet state produced via triplet–triplet annihilation in N-vinylcarbazole–1-vinylnaphthalene copolymer
Authors: Yokoyama Masaaki , Hanabata Makoto , Tamamura Toshiaki , Nakano Takeshi , Mikawa Hiroshi
Published: February 2003

The carrier photogeneration mechanism has been investigated in N-vinylcarbazole(VCz) –1-vinylnaphthalene(VNa) copolymers. The photocurrent as well as the delayed fluorescence of carbazole (Cz) chromophore are enhanced in the copolymer, which contains a few mole percent of VNa units, and the delayed fluorescence of naphthalene (Na) chromophore is observed with selective excitation of Cz in its lowest π–π* transition, i.e., with lower energy than the first excited singlet state of Na. The photocurrent in this copolymer exhibits a quadratic dependence upon the excitation light intensity in a high field. Detailed investigations of the emission spectra and the photocurrent in this VCz–VNa system lead to the ’’intrinsic’’ carrier photogeneration via the triplet state of carbazole moiety as follows: Czhν→1Cz*→3Cz* →3Na*T–Tannihilation→1Na**→1Cz**→carriers. Here, the double asterisk indicates the higher excited state. As evidenced by the existence of this mechanism in the copolymers, also in the VCz homopolymer is suggested the possibility of the intrinsic photocarrier generation from higher singlet via multiphotonic process involving a triplet state.
Selective excitation and detection in multilevel spin systems: Application of single transition operators
Authors: Wokaun A. , Ernst R. R.
Published: February 2003

An alternative definition of single transition operators is given for the description of selective excitation and detection experiments in multilevel spin systems. This definition has the virtues of a simple physical interpretation and easy application to arbitrarily complicated systems. Some applications to the excitation and detection of multiple quantum transitions in spin 1 and spin 3/2 systems as well as in coupled spin systems are described.
Photoelectron angular distribution and branching ratios for atomic oxygen at 304 Å
Authors: Dehmer J. L. , Dehmer P. M.
Published: February 2003

The angular distribution of electrons for the production of the 45° state of O+, and the HeII branching are reported. (AIP)
Rate of population of CO(v) in a CS2/O2 flame
Authors: Vetter A. A. , Culick F. E. C.
Published: February 2003

The rate of vibrational population of carbon monoxide in a steady CS2/O2 flame has been determined from CO overtone emission. A steady-state analysis indicates that, for a flame, the fifteenth vibrational level has the largest rate of population.
Photodissociation of NO+(NO) and NO+(H2O)
Authors: Vanderhoff J. A.
Published: February 2003

A drift tube mass spectrometer coupled with ion laser and ion laser pumped tunable dye laser photon sources has been used to measure the thermal energy (300 K) photodissociation cross sections for two positive ion clusters, NO+(NO) and NO+(H2O). Measurements were made spanning a photon wavelength range from 799 to 350 nm. The absolute photodissociation cross sections for NO+(NO) were found to vary smoothly with photon wavelength attaining a maximum value of 2×10−17 cm2 at ∼650 nm decreasing at both longer and shorter wavelengths. In addition, coupling of clustering reactions into the photodissociation cross section appeared under a variety of conditions. Photodissociation cross sections for NO+(H2O) were found to be small or zero. Upper limits of ∼1×10−19 cm2 were obtained for these cross sections.
Charge distributions and chemical effects. XVI. Valence electron energies in atoms and ions
Authors: Fliszár Sándor , Henry Hervé
Published: February 2003

Valence region energies of first-row atoms and ions of nuclear charge Z, with N1 core and N valence electrons are calculated from the formula EN=−(3/7) (Z−N1) N〈r−12〉, where 〈r−12〉 is the average inverse distance of the N electrons from the nucleus. The loss of an electron is described in terms of a shrinking of the valence region, 〈r−12〉N −〈r−12〉N−1= (∂〈r−12〉/∂N)0 [N/(N−1)], where the derivative β = (∂〈r−12〉/∂N)0 can be treated as a constant for the removal of any p electron. In this approximation it is 〈r−12〉N=〈r−12〉N=2 +β[N/(N−1)+(N−1)/(N−2)+⋅⋅⋅] where 〈r−12〉N=2 is the value for systems with two valence region electrons. Finally, since the ratio α=〈r−12〉N=2/(Z−N1) is remarkably constant, we have all the elements for evaluating 〈r−12〉N and, consequently, the valence region energy EN of the N valence electron ion or atom.
Optical properties of trivalent chromium in silicate glasses: A study of energy levels in the crossing region
Authors: Brawer Steven A. , White William B.
Published: February 2003

A theoretical and experimental study of the optical properties of trivalent chromium in silicate glasses is described. New experimental results presented are (1) the temperature dependence of the absorption, (2) determination of the dependence of emission on the excitation frequency, (3) determination of the 2E→4A2 emission. These results are interpreted in terms of the structure of the Cr3+ sites in the glasses. Of theoretical interest is the fact that for Cr3+ in silicate glasses, Dq is in the crossing region of the 4T2 and 2T1 levels which interact strongly by spin–orbit coupling. To explain the absorption line shape of such systems, it is necessary to include vibronic effects. Calculations of the absorption are made and compared with experiment.
Simple procedure for calculations of relaxation and correlation corrections to Koopmans’ theorem
Authors: Ohmichi Naoto , Nakajima Takeshi
Published: February 2003

We proposed a simple procedure for calculating the ionization energy which to some extent takes into account the relaxation and correlation effects. The procedure is based on the perturbation treatments of both the closed-shell ground state the open-shell ionized state. In order to obtain the unperturbed wavefunction.for the ionized state, we present a simple method. The relation among the Koopman’s theorem. Δ SCF-like method- and the purturbation theory is discussed.
Stochastic theory for molecular collisions in the perturbation limit
Authors: Augustin Stuart D. , Rabitz Herschel
Published: February 2003

Previous stochastic theory research has described individual molecular collisions by a master equation or a Fokker–Planck equation. The perturbation (weak coupling) limit of this theory is examined more carefully in this paper. It is shown how the formulation reduces to exact quantum mechanics in the perturbation regime. The methods are applied to the collinear forced quantum oscillator model for purposes of comparison with other theories. The results of this work and previous research show that stochastic collision theory firmly connects up to both the weak and strong coupling limits. It is suggestive that computationally attractive schemes which afford reasonable accuracy can now be developed for the intermediate coupling regime as well.
An interacting segment model of molecular electric tensor properties: Theory and application to electric dipole moments of the halogenated methanes
Authors: Miller C. K. , Orr B. J. , Ward J. F.
Published: February 2003

A model is developed to describe trends in molecular properties such as electric dipole moment, polarizabilities, and hyperpolarizabilities for series of structurally related molecules. The model takes account of intramolecular electrostatic interactions, which give rise to induced electric moments in the various segments of the molecule. For this purpose each molecular segment is ascribed a set of ’’bare’’ electric tensor properties, which describe the hypothetical situation where intersegment interactions are absent, and a corresponding set of ’’dressed’’ properties, which include the effects of the intramolecular field. Any overall molecular electric tensor property is then obtained by summing the appropriate dressed segment properties. After theoretical development the model is used to fit electric dipole moment data for the 12 halogenated methanes CXnY4−n(X,Y=H,F,Cl;n=0–4), for which a simple bond additivity model is known to be inadequate and which severely test any theory of induced dipole moments. The results of such an analysis are highly satisfactory and indicate that the model should also give an adequate description of higher-order electric tensor properties.
Analysis of torsional spectra of molecules with two internal C3v rotors. VI. Barrier to internal rotation of dimethylphosphine
Authors: Durig J. R. , Groner P. , Li Y. S.
Published: February 2003

Torsional splittings of the rotational spectrum in the ground state and the first two torsional excited states of (CH3)2PH have been analyzed by using the semirigid rotor model. The following potential coefficients were obtained for (CH3)2PH: V03 =V30=811.2 cm−1 (2.32 kcal/mole) and V′33=29.1 cm−1 (83 cal/mole). With the aid of these coefficients attempts were made to assign the torsional transitions for both the (CH3)2PH and (CD3)2PH molecules.
Theoretical studies of photoionization cross sections and angular distributions for the hydrogen molecule
Authors: Dutta C. M. , Chapman Frank M. , Hayes Edward F.
Published: February 2003

Theoretical studies of photoionization cross sections and angular distributions are reported for the ground state of the hydrogen molecule in the photon energy range 20–40 eV. Results are obtained using an integral equation technique recently developed by Chapman and Hayes. The method employs a partial wave expansion for the continuum wavefunction to generate a set of coupled equations which are solved exactly using an integral equation technique. Single center expansions are used for the initial and final electronic wavefunctions and exchange is treated iteratively. The results agree rather well with earlier studies of Kelly, of Ritchie, and of Hirota. Studies of the dependence of the differential cross section on molecular orientation are reported. These studies are of potential interest as a simple model for photoionization of a molecule adsorbed on a surface.
A study of viscoelastic properties of butanediol-1,3 using optical digital correlation spectroscopy
Authors: Drake P. W. , Dill J. F. , Montrose C. J. , Meister R.
Published: February 2003

A temperature study of viscoelastic relaxation in butanediol-1,3 was made using digital correlation and Brillouin scattering spectroscopic methods. The digital correlator provided measurement of the relaxation function from which average relaxation times were calculated. The Brillouin scattering method allows a more accurate determination of the high frequency longitudinal modulus than older ultrasonic methods that require extrapolation of data over a large temperature range for the determination of this modulus. From the light scattering data, it has been concluded that, contrary to previous studies, the relaxations observed in the real and imaginary parts of the complex longitudinal modulus are the same. Under these circumstances the use of a ’’hysteresis’’ effect to interpret the nature of the relaxation at high frequencies is unnecessary. A theoretical derivation has been given that allows the average relaxation time obtained via correlator studies to be compared directly to the measured shear viscosity.
Stochastic reduction for dynamics of reactions with complex formation
Authors: Zvijac David J. , Mukamel Shaul , Ross John
Published: February 2003

We present a method for evaluating the distribution of products in chemical reactions which proceed by complex formation. The method consists of separating the degrees of freedom into strong modes, which are correlated directly with the reaction dynamics and weak modes. We then treat the dynamics of the strong modes explicitly and perform a statistical averaging over the weak degrees of freedom. Our final result [Eq. (5)] for the distribution of products is in the form of a product of three matrices whose sizes are determined by the number of relevant strong modes. The first matrix accounts for the preparation of the complex, the second for the energy redistribution within the complex, and the third for the dissociation of the complex. As one possible course of procedure we evaluate the first and third matrices by applying a normal coordinate transformation of the strong modes from reactants to complex and then to products and then use Franck–Condon factors between the strong states of the complex and fragments (reactants and products); the second matrix is evaluated using a step ladder model. We then apply the formulation to the system F+C2H4 for which deviations from statistical behavior were observed. Nonstatistical behavior may occur in our model from two distinct sources: (1) the Franck–Condon factors which are associated with the dynamics and (2) the finite energy redistribution rate within the complex (relative to the dissociation rate). We discuss the influence of these two effects in F+C2H4 and conclude that the first one is dominant in this case.
Effects of magnetic field on triplet state of Pt–phthalocyanine single crystal
Authors: Kaneto K. , Yamamoto J. , Yoshino K. , Inuishi Y.
Published: February 2003

Zeeman effect of the first excited triplet state in Pt–phthalocyanine single crystal is studied in the magnetic field up to 100 kG by observing the phosphorescence emission at 4.2 K. Phosphorescence peaks are observed at the wave numbers 10 289.1 and 10 297.6 cm−1 in the zero magnetic field at 1.7 K (zero field splitting). The zero field splitting coefficients of the triplet state are estimated from the Zeeman study to be D=−7.8±0.2 cm−1 and ‖E‖=0.7±0.2 cm−1. The decay time of the emission peak of 10 289.1 cm−1 decreases with increasing magnetic field from 7 μsec at 0 G to 3 μsec at 100 kG. Z component of spin level has the largest decay rate of 3.3×105 sec−1 at zero magnetic field. Dependence of the phosphorescence intensity and the energy shift of their peaks on the direction of the magnetic field referred to the crystal axis are also measured and compared with the theoretical calculations.
A hydrodynamic theory of light scattering in ionic solutions
Authors: Pailthorpe B. A. , McQuarrie D. A.
Published: February 2003

A linearized hydrodynamic description of concentration fluctuations in ionic solutions is formulated and is used to provide a phenomenological theory of light scattering. Our formulation takes ionic effects into account and using model calculations we are able to show that Rayleigh scattering from charge fluctuations in simple 1–1 ionic solutions should be negligible in the usual light scattering experiments. This then justifies the previous treatment of an electrolyte as a single nonionic solute species in comparisons with light scattering and explains the failure to detect Rayleigh scattering from charge fluctuations in these experiments. This approach is extended to calculate Rayleigh-to-Brillouin intensity ratios and we find that for NaCl the neglect of charge fluctuations is within experimental error. We expect this to be less satisfactory for LiCl. For 2–2 salts the neglect of ionic effects is clearly inadequate and our study indicates the appropriate theoretical description. Our estimates of the ionic contributions exhibit some improvement in comparisons to experiment but more detailed calculations await theoretical characterization of appropriate model electrolyte systems.
Vibronic coupling and spin sublevel decay rates
Authors: Freed Karl F. , Kanamaru N. , Lim E. C.
Published: February 2003

The rate expression for nonradiative decay is reformulated to provide a representation in which the initial state wavefunction most faithfully reflects the states that are actually prepared in experiment. The expression is given in an exact golden-rule form in terms of dressed final states. The Schrödinger equation for the dressed states determines those couplings among final basis functions which have a significant effect on the rates. We consider applications to the T1 →S0 radiative and nonradiative decays in nitrogen-heterocyclic compounds to show how the individual spin sublevel decay rates can be altered as a result of changes in T1−T2(3nπ* −3ππ*) vibronic coupling.
Ionization of phosphine and deuterated phosphine by electron impact from threshold up to 180 eV
Authors: Märk T. D. , Egger F.
Published: February 2003

Electron impact ionization has been studied in PH3 and in a mixture of PH3 and PD3 as a function of electron energy up to 180 eV, with a double focusing mass spectrometer. Absolute partial ionization cross sections have been obtained in PH3 for the production of the parent ion PH+3, for the fragment ions PH+2, PH+, P+, H+2, and H+ and for the doubly charged ions PH++3, PH++2, PH++, and P++ by normalization against the Ar ionization cross section. From the measured partial ionization cross sections the total ionization cross section function of PH3 has been deduced. In addition, it has been possible to obtain the cross section ratios q (PD++3)/q (PD+3), q (PD++2)/q (PD+3), q (PD++)/q (PD+3), and q (P++)/q (PD+3) as a function of electron energy. The ion D++3, produced by the process PD3+e, has been found for the first time and an estimate of the ionization cross section is given. Using nth root extrapolation the following ionization and appearance potentials have been derived from the low energy cross section functions: I.P. (PH+3) =10.1±0.1 and 12.5±0.2 eV; I.P. (PD+3) =10.15±0.1 and 12.75±0.2 eV; A.P. (PH+2) =14.2±0.2 eV; A.P. (PD+2) =14.3±0.2 eV; A.P. (PH+) =13.0±0.2 eV; A.P. (PD+) =13.3±0.2 eV; A.P. (P+) from PH3=17.2±0.3, 18.8±0.3, and 23.5±0.3 eV; A.P. (P+) from PD3=17.7±0.3, 19.5±0.3, and 22.0±0.3 eV; A.P. (H+2) =14.8±0.2 and 24.8±0.5 eV; A.P. (D+2) =14.9±0.2 and 24.8±0.5 eV; A.P. (H+) =25.3±0.5 eV; A.P. (D+) =25.3±0.5 eV; I.P. (PH++3) =30.0±1.0 eV; I.P. (PD++3) =29.8±0.5 eV; A.P. (PH++2) =35.0±0.5 eV; A.P. (PD++2) =34.9±0.5 eV; A.P. (PH++) =33.0±1.0 eV; A.P. (PD++) =34.4±0.5 eV; A.P. (P++) from PH3=45.6±0.5 eV; and A.P. (P++) from PD3=46.7±0.5 eV. In all double ionization cases a square law has been found from threshold up to some 5–10 eV above threshold. The ionization and appearance potentials obtained are compared with previous results where available. It is possible to resolve a yet unexplained discrepancy of the ionization potentials of the doubly ionized ions of PH3.
Force constant corrections in redundant molecular coordinates
Authors: Van Zandt L. L. , Lu K. -C.
Published: February 2003

A new expression for effective force constant corrections in molecular systems described by redundant, curvilinear coordinates is derived. It is useful for multiply redundant coordinate systems, achieving great formal and computational simplicity. Explicit reference to constraint equations among coordinates is completely eliminated. Specific application is made to a system of four compounds, A-DNA, B-DNA, A-RNA, and tetrahydrofuran. The correction terms are found to be comparable in size to torsion force constants about single bonds, quite small. Qualitative arguments are adduced that this result is of wide generality.
Raman scattering of benzene in the ultraviolet,
Authors: Ziegler L. , Albrecht A. C.
Published: February 2003

The excitation frequency dependence of intensities of the Raman active fundamentals of benzene have been measured in the uv spectral region approaching the low lying forbidden electronic states (the 1B2u and 1B1u). In accordance with theory, these forbidden states do not cntribute to the scattering cross section of any fundamental in this preresonance region. Furthermore, all but one scattering mode (the e1g vibration) are found to derive their intensity from state in the deep vacuum uv (120 nm or above) and not the familiar strongly allowed ππ* 1E1u state at 185 nm. By contrast Raman scattering of the e1g mode (848 cm−1) appears to be derived from vibronic coupling between this 1E1u state and a weakly allowed nearby 1A2u state.
Approximate evaluation of the dielectric constant for hard nonspherical molecules from Monte Carlo simulations
Authors: Martina Esteban , Deutch J. M. , Streett W. B.
Published: February 2003

Alternative exact expressions for the dielectric constant of a rigid polar fluid are evaluated approximately employing new Monte Carlo results for the pair distribution function of hard heteronuclear diatomics. The primary innovation of this work is to proceed beyond formal theories of the dielectric constant to explore possible approximation schemes that make use of Monte Carlo data on asymmetric molecules to obtain numerical results for dense molecular fluids. Each of four exact expressions are evaluated, ignoring the presence of dipoles and including only the effects of the asymmetric core. In this approximation, it is found that the Kirkwood–Onsager and Nienhuis–Deutch formulas yield the most reliable and consistent results. An interpretation of these results and a brief comparison with experiment are given.
187Re, 14N, and 2H nuclear quadrupole couplings in NH4ReO4: Evidence for a possible phase transition
Authors: Brown R. J. C. , Segel S. L.
Published: February 2003

The nuclear quadrupole couplings of the 187Re, 14N, and 2H nuclei in NH4ReO4 have been measured over a wide range of temperatures. The 187Re nuclear quadrupole resonance has been observed throughout the previously reported ’’wipeout’’ range of temperature, showing that disappearance of the resonance is due to gradients of temperature and strain in the sample. The pressure dependence of 187Re quadrupole resonance has been measured as a function of temperature between 77 and 362 K. The slope of the graph of (∂ν/∂P)T versus temperature is indicative of a continuous phase transition at this temperature. The nature of the phase transition and its relationship to other phenomena in NH4ReO4 is unclear.
A theory for site–site pair distribution functions of molecular fluids
Authors: Johnson Elijah
Published: February 2003

A theory for site–site pair distribution functions of molecular fluids is presented. This theory is analogous to the RISM theory, but it is not restricted to interaction site pair potentials. It was derived directly from the Ornstein–Zernike equation using Fourier–Wigner series. Results of the theory for homonuclear diatomic pair potentials are compared with corresponding results obtained from the RISM theory and from computer simulation studies. The relationship between the RISM equation for homonuclear hard sphere diatomic pair potentials and the theory presented is given.
High pressure luminescence studies of mixed ZnS–CdS phosphors doped with Cu+ or Ag+
Authors: House G. L. , Drickamer H. G.
Published: February 2003

The effect of pressure has been measured on the emission peak location and emission intensity for a series of phosphors of the form (ZnxCd1−x)S:Ag:Cl and (ZnxCd1−x)S:Cu:Cl. The results are qualitatively consistent with the donor–acceptor model. The emission peaks shift to higher energy with increasing pressure but the magnitude of the shift decreases with increasing fraction of Cd. The pressure shift of the absorption edge of CdS is also smaller than that for ZnS. The emission peak shift is always less than the shift of the edge for any given fraction of Cd. Large intensity decreases are observed with increasing pressure for each of the phosphors. The observed intensity changes can be related to changes in the donor binding energy with increasing Cd concentration.
Bulk viscosity of polymer solutions
Authors: Metiu Horia , Freed Karl F.
Published: February 2003

The theory of the bulk viscosity of polymer solutions is considered for cases of low and moderate concentrations of phantom chains under ϑ conditions. It is shown that the familiar preaveraging approximation for the hydrodynamic interactions imposes the constraint that the polymer contribution to the bulk viscosity is minus one-third of its contribution to the shear viscosity, thereby providing an experimental means for testing this widely used approximation. Particular application is made to the theory of sound attenuation in polymer solutions as one potential means for measuring the bulk viscosity. At higher frequencies the longitudinal (sound) modes of the solvent introduce an additional exponentially decaying contribution to the hydrodynamic interactions which leads to a renormalization of the relaxation times of the polymer.
Collision-induced ion-pair formation of the thallium halides TlF and Tl2F2
Authors: Parks E. K. , Kuhry J. G. , Wexler S.
Published: February 2003

Absolute cross sections have been determined for collision-induced ion-pair formation (polar dissociation) of TlF and Tl2F2 from collisions with Xe and Kr atoms. A crossed-beam method was used in which the Xe or Kr atoms were aerodynamically accelerated to energies up to 17 eV (in the laboratory frame). Time-of-flight mass spectrometry permitted the determination of partial cross sections for each positive and negative ion channel in the collision. The cross sections for both monomer and dimer thallium fluoride were considerably smaller than those previously reported for thallium chloride, the difference most likely resulting from the poor energy transfer into the molecular target due to the light F atom. The cross section for the reaction Xe(Kr)+TlF→Xe(Kr)+Tl++F− exhibited, near threshold, a power-law rise with increasing energy σ =const.× (Etot−E0)n/Erel, where Etot is the total collision energy (kinetic plus internal), E0 is the threshold energy, Erel is the relative kinetic energy and n=1.85(1.95). The cross section for rearrangement ionization Xe +TlF→TlXe++F− exhibited a step behavior. Collisions of Xe with the dimer Tl2F2 resulted in both Tl2F++F− and Tl++TlF−2 ion products, the cross sections for the latter ion pair being about one third those for the former. With new values of the heats and entropies of formation of Tl2Cl2, Tl2Br2, and Tl2I2 the absolute cross sections for the previously reported reactions of the dimers Xe(Kr)+Tl2X2→Xe(Kr)+Tl2X++X− have been recalculated. The threshold behaviors of these reactions have also been analyzed in terms of the above power law, giving values of the power n in the range 2.1–2.7. Model calculations have been performed to determine bond energies of the (Tl–X–Tl)+ and (X–Tl–X)− ions, which were used to determine theoretical thresholds for the formation of these ions. The difference between the experimental and theoretical thresholds then gives the internal excitation of the molecular ions at threshold (assuming the inert gas atom carries away negligible energy). Tl2F+, in particular, is formed at threshold with considerable internal excitation (∼1.8 eV). The low cross sections for both TlF and Tl2F2 and the high internal excitation of Tl2F+ at threshold are consistent with a collision model in which only very dynamically constrained collisions result in the extraction of the light F− ion from either TlF or Tl2F2.
Accurate characterization of the transition state geometry for the HF+H′→H+H′F reaction
Authors: Wadt Willard R. , Winter Nicholas W.
Published: February 2003

The potential surface for the exchange reaction HF+H′→H+H′F has been investigated with various multiconfiguration self-consistent field (SCF) and configuration interaction (CI) wavefunctions. For the first time, nonlinear geometries have been considered. The calculations demonstrate the importance of diffuse functions on the fluorine for describing nonlinear geometries. A qualitative model is presented to explain the need for diffuse functions. The energy barrier for the exchange reaction is calculated to be ∼45 kcal/mole, which is comparable to the values obtained in previous ab initio calculations on the collinear reaction surface (Refs.1 and 2). More importantly, the calculations show that the saddle point region is very flat, the barrier changing by only 1–2 kcal/mole between collinear (180°) and perpendicular (90°) geometries. The optimum angle for the transition state geometry is calculated to be 106°. Dynamics calculations on the HF+H′→H+H′F reaction have employed semiempirical London–Eyring –Polanyi–Sato (LEPS) surfaces that had artifically small energy barriers, as has been pointed out previously. The present calculations show that not only were the barriers for exchange too low, but that the basic shape of the LEPS surfaces was inappropriate. The implications of this result for future dynamics calculations are discussed.
Raman scattering study of the protonic order–disorder transition in SnCl2⋅2H2O
Authors: Wang C. H. , Tatsumi M. , Matsuo T. , Suga H.
Published: February 2003

Raman spectra of anhydrous, monohydrated and dehydrated SnCl2 solids have been measured. The hydrogen bond vibrations in the two dimensional intervening H2O layers in SnCl2⋅2H2O (SCD) have been identified. Polarized Raman spectra of SCD in several scattering geometries and at several temperatures above and below Tc have been obtained. The results of spectral intensity near the phase transition have been discussed on the basis of proton population at various sites. The peak frequency and spectral linewidth of a hydrogen bond vibration are discussed in terms of anharmonicity in the hydrogen bond potential. No soft optical phonon has been observed in the Raman spectra of SCD, despite the fact that several bonds are affected by the phase transition. A relaxational rather than resonant soft mode may be important in SCD.
Diamagnetic contribution to thermal fluctuations of the magnetization
Authors: Titulaer U. M. , Felderhof B. U.
Published: February 2003

A recently proposed macroscopic theory of thermal fluctuations in polarization, magnetization, and the associated electromagnetic fields is extended to include diamagnetic contributions. Since the diamagnetic susceptibility does not vanish at high frequency, the magnetization contains a part that follows the magnetic induction instantaneously. Only the noninstantaneous part of the magnetization is treated as a separate fluctuating field. This leads to expressions for the correlation functions, which possess the required symmetry and positivity properties. The theory yields formal expressions for arbitrary spatially nonuniform media; explicit results for spatially uniform, but not necessarily isotropic, media are also given.
Temperature dependence of the vibrational relaxation of CO (v=1) by NO, O2, and D2, and of the self-relaxation of D2
Authors: Doyennette Lucien , Mastrocinque Giuseppe , Chakroun Annie , Gueguen Henri , Margottin-Maclou Monique , Henry Lucien
Published: February 2003

Rate constants for the vibrational de-excitation of CO(v=1) by NO, O2, and D2 have been measured as a function of the temperature using the laser-induced vibrational fluorescence technique. In the temperature range from 100 to 700 K considered in our experiments, far-from-resonance V–V energy transfers occur between CO and the collision partners. In CO–NO and CO–O2 samples, the CO fluorescence decay curves are single exponentials; only the total de-excitation rates χCO–M=kCO–M+kMCO of CO(v=1) by M=NO or O2 may be experimentally deduced, kCO–M being V–V transfer rate and kCOM the V–TR de-excitation rate. In the CO–D2 system, the fluorescence exhibits a double exponential decay; the V–V transfer rate kCO–D2 is found to be much greater than the V–TR de-excitation rate kCOD2, and the self-relaxation rate kD2 of deuterium has also been deduced from measurements. The V–V transfer rates kCO–M calculated from a semiclassical method derived from that of Shin, and using a Morse potential, have been compared to the experimental results.
Charge transfer and Penning ionization of N2, CO, CO2, and H2S in proton excited helium mixtures
Authors: Chen C. H. , Judish J. P. , Payne M. G.
Published: February 2003

In this work we report on studies of the kinetics of He–N2 mixtures excited by a proton beam. The rate of charge transfer from both atomic and molecular helium ions to N2 was studied using a time-resolved technique to measure the fluorescence of N+2(B 2Σ+u→X 2Σg). The population of N+2(B2Σ+u) through a Penning ionization process from metastable helium atoms or molecules is observed, and the Penning ionization and charge transfer process which populates this state in a He–N2 laser is discussed. The charge transfer rate constants of He+–CO, He+–CO2, He+–H2S, and He+2–CO are also measured.
Application of intensity correlation spectroscopy to the measurement of continuous distributions of spherical particles
Authors: McCally R. L. , Bargeron C. B.
Published: February 2003

This paper extends the formalism whereby one can obtain the parameters describing specific size distributions of spherical particles from the cumulants, which are commonly measured in intensity correlation spectroscopy, to include the Pearson III, Pearson V, Schulz, and Schulz (molecular weight) number fractions of particle radii. We demonstrate explicitly that the reduced second cumulant K2/K21 has distinct upper bounds for each of the distributions. The range of scattering angles and distributional parameters for which the Rayleigh–Gans corrections (to second order in the scattering vector q) are negligible is specifically calculated. The method is applied to data from human serum very low density lipoproteins and from narrowly distributed polystyrene latex spheres.
Soft phonon and central peak scattering in a molecular crystal system: C6Cl4O2 (chloranil)
Authors: Ellenson W. D. , Kjems J. K.
Published: February 2003

Elastic and inelastic neutron scattering has been used to study the continuous structural transition in chloranil at Tc=90.3 K. The transition results in a doubling of the monoclinic unit cell along the c axis and the molecular displacements correspond to a staggered rotation, φ, about axes perpendicular to the molecular planes. The temperature dependence of the order parameter, φ, has been determined from superlattice Bragg intensities with the result φ∝ (Tc−T)0.33±0.02 for 0.005< (1−T/Tc) <0.5. The response from the zone boundary phonon, whose eigenvector corresponds to the distortions below Tc, shows a pronounced softening in the temperature range 100–300 K. At temperatures below 140 K the spectrum also contains a central component diverging at Tc which is resolution limited in energy, but shows considerable width and anisotropy in momentum space.
Accurate Compton profiles for H2 and D2 including the effects of electron correlation and molecular vibration and rotation
Authors: Smith Vedene H. , Thakkar Ajit J. , Henneker W. H. , Liu Jang W. , Liu Bowen , Brown Richard Edwin
Published: February 2003

Accurate isotropic Compton profiles J (q,R) for H2(X 1Σ+g) are calculated for 22 internuclear separations from the 126 configuration wavefunctions of Liu. These profiles, supplemented by the united atom and separated atom profiles, are then rigorously averaged over vibration–rotation wavefunctions computed by direct numerical solution of the radial Schrödinger equation using nearly exact potential energy curves including adiabatic and relativistic corrections. These averages are performed for a large number of vibration–rotation states of H2 and the ground vibration–rotation state of D2. It is shown that the effects of averaging Compton profiles over vibration–rotation states are significant and cannot be neglected. The isotope effect is shown to be smaller than the vibration–rotation effect. The peak of the calculated H2 Compton profile for the ground vibration–rotation state is found to be in excellent agreement with the very recent high energy electron impact measurements of Lee. A number of expansion techniques for vibration–rotation averaging, including a new, very simple, and reasonably accurate delta approximation, are presented.
Theoretical study of the influence of resonance relaxation on infrared band shapes
Authors: Weidemann E. G. , Hayd A.
Published: February 2003

The resonance coupling between an active and an inactive ir transition and the influence of this mechanism on relaxation of the vibrational energy is discussed. An explicit expression is derived for the band shape in terms of a one-dimensional integral which is valid for arbitrary profiles of the uncoupled bands. Assuming Lorentzian profiles of these bands, a simple analytical formula is obtained, comprising the phenomenon of transmission windows as a special case. The band shape is discussed in terms of its Fourier transform, the autocorrelation function of the electric dipole moment. Two relaxation times are obtained which may be interpreted by resonance exchange of excitation energy between the coupled oscillators. A sharp separation of weak and strong coupling exists at exact resonance. The correlation function which oscillates with the frequency of the allowed transition is exponentially damped in the weak coupling case, whereas a multiple energy exchange occurs with strong coupling, resulting in a modulation of the correlation function with the exchange frequency. The theory is applied to the asymmetric OCO stretching band of acetate ions in methanol.
Molecular motion in solid odd-numbered paraffin C19H40: Proton spin relaxation spectroscopy from 5.8 kHz to 86 MHz
Authors: Stohrer M. , Noack F.
Published: February 2003

We describe the experimental setup of a continuously variable pulsed NMR spectrometer for solid-state investigations, based on a fast magnetic field cycling technique (Zeeman-field range H=0–1700 Oe, spin–lattice relaxation time range T1?10 ms), and report on measurements of the proton relaxation time T1 in solid odd-numbered paraffin C19H40 (Larmor frequency range ωL/2π=5.8 kHz–86 MHz, temperature range ϑ=−195 to +32°C). As a result of the extended frequency range one can identify three temperature regimes (I, II, III), where different relaxation mechanisms predominate. (I) In the low temperature regime the relaxation is determined by thermally activated rotational jumps of CH3 end groups with uniform hindrance. (II) In the prerotator phase just below the rotational phase transition the relaxation is essentially caused by the additional mobility of CH2 groups in thermally disordered molecules (kink defects); the T1 dispersion can quantitatively be described by the one-dimensional diffusion of kink defects in combination with competing rotational reorientations of the entire molecule. (III) In the rotator phase we observe T1 contributions of two different CH2 motions, supposed to be fast rotational reorientation of entire molecules and slow-self-diffusion or fluctuation of paraffin bundles, respectively. Our results agree for the prerotator phase with refined ideas of the Pechhold/Blasenbrey kink theory, but not for the rotator phase, where our analysis supports the Fischer/Strobl model of a premelting structure.
An electron spin density matrix description of nuclear spin–lattice relaxation in paramagnetic molecules
Authors: Gottlieb Hans P. W. , Barfield Michael , Doddrell David M.
Published: February 2003

A density matrix treatment applicable to molecular electronic systems is formulated and used to derive an expression for the spin–lattice relaxation rate (T−11) of ligand nuclei in paramagnetic transition-metal complexes. In this way the unpaired spin density is introduced into the nuclear relaxation expression. The two-center dipolar integrals which occur in the expression for T1 are evaluated by a method based on solid spherical harmonic expansions whereby the Hamiltonian operator is reexpressed in terms of coordinates with respect to the orbital’s origin. The theory is used to treat 13C and 1H spin–lattice relaxation in ruthenium acetylacetonate. Good agreement is found between the calculated and experimental ratio TC1/TH1 whereas the Solomon–Bloembergen point-dipole approach yields results significantly in variance with the experimental. The theory is used to treat proton spin relaxation in a representative low-spin Fe(III) complex to demonstrate under what conditions the point-dipole approximation is invalidated.
Spin–spin coupling constants between carbon-13 and bromine in bromomethanes
Authors: Yamamoto Osamu , Yanagisawa Masaru
Published: February 2003

The dominant relaxation mechanism of the 13C nucleus in bromomethanes is the scalar coupling interaction owing to the fact that the resonance frequency of 13C is close to that of bromine and that bromine has its relaxation mechanism by the quadrupole interaction. The relaxation curves for bromomethanes show the nonexponential decay because of the presence of two bromine isotopes with the ratio of about 1:1. By analyzing the relaxation curves the spin–spin coupling constants between 13C and 79Br are found to be 95, 127, and 151 Hz for CD2Br2, CDBr3, and CBr4, respectively. From the bromine relaxation times obtained simultaneously in the analysis, the reorientational motion of bromoform is discussed and compared with that of chloroform.
Coherent one-dimensional exciton transport and impurity scattering
Authors: Dlott D. D. , Fayer M. D. , Wieting R. D.
Published: February 2003

A model is developed which describes the transport of coherent one-dimensional excitons in molecular crystals containing a low concentration impurity having excited state energy above that of the host crystal’s exciton band. An expression is obtained for the exciton mean-square displacement and the exciton flux. The model is then employed to determine the effect of impurity scattering on an experimental observable, the time dependence of trap emission after impulse optical excitation of a one-dimensional crystal. The resulting formula is used to calculate, without adjustable parameters, the time-dependent intensity of phosphorescent trap emission from crystals of 1,2,4,5-tetrachlorobenzene (TCB) containing the naturally occurring scattering impurity, hd-TCB, at 1.25 °K. The results are compared to experiment and found to be in good agreement. Although the question remains open, the results are consistent with a coherent mode of triplet exciton transport in TCB and indicate the importance of naturally occurring isotopic impurity scattering in this type of system.
Improved calculation of electronic excitation energies of transition-metal compounds by a correlation energy correction: Application to CrF63− and NiF64−
Authors: Pueyo L. , Richardson J. W.
Published: February 2003

Correlation energy as an empirical correction is introduced into otherwise ab initio-type calculations of the electronic structure of trnasition metal complexes. The strong field MO electronic states are expanded in a series of free-ion LS terms and cluster correlation energies for the SΓ states are calculated from the coefficients of such expansions and atomic spectral data. Numerical results are presented for NiF64− and CrF63−. The effects of this correction on calculated spectra in these two cases are discussed in comparison with the limited (dn) configuration interaction. In both clusters the calculated spectra are significantly improved: the mean deviations (in cm−1) from all observed transitions are reduced from 3420 to 680 in NiF64− and from 2980 to 450 in CrF63−. An analysis of the influence of the electronic delocalization on the calculated spectra suggests that the charge-transfer states might be not very significant in an extended CI treatment of the correlation energy problem in these clusters.