bluuebunny/crossref_metadata_2025 huggingface dataset:
Recombination of radical pairs in high magnetic fields: A path integral–Monte Carlo treatment
Authors: Schulten Klaus , Epstein Irving R.
Published: February 2003

The magnetic field dependence of photoinduced geminate hydrogen (electron) transfer processes originates from a Brownian motion of radical pairs, a hyperfine coupling, and Zeeman-induced precession of the unpaired electron spins and a spin-selective recombination. In this paper we introduce a Wiener path integral description for these processes. This approach entails an integration over all possible diffusion trajectories of two functionals, one that weights the diffusion paths according to the laws of Brownian motion, and a second that accounts for the spin motion and recombination of the radical pairs. This formulation maintains a close connection with the actual physical processes and is suitable for microscopic radical reaction processes under quite general circumstances. For the path integration we employ a Monte Carlo method, and for the electron spin precession we adopt a semiclassical approximation. The theory is then applied to the magnetic field dependence of a photoinduced hydrogen transfer process involving a ketyl–hexyl radical pair recently observed with nanosecond time resolution at fields of a few kG. It is argued that the magnetic field dependence observed cannot be reconciled with a simple hyperfine-coupled Zeeman-induced spin motion. Finally, a simple analytical expression for the magnetic field dependence of geminate radical pair recombination at high magnetic fields is presented.
Relativistic calculation of the electronic structure of UF6
Authors: Boring Michael , Wood J. H.
Published: February 2003

Using a one-component relativistic theory we have obtained the self-consistent (SCF) one-electron levels of UF6 via the multiple scattering (MS) Xα method. The calculated ionization energies are found to be in good agreement with the photoionization spectrum. Our first electronic transition (dipole forbidden) energy at 3.34 eV agrees well with the onset of absorption and the experimentally assigned transition at 3.04 eV. An analysis of the spin–orbit parameters, calculated via perturbation theory, is given. It is found that the contributions from the 6p components are especially important. We conclude that the absorption spectrum can not be interpreted in terms of the spin–orbit splitting of the highest occupied MO (tlu). A comparison is made with other relativistic (and nonrelativistic) one-electron calculations of UF6. This comparison yields reasonable agreement on the relativistic one-electron structure of this molecule.
Direct calculation of the orientational distribution function of partially ordered ensembles from the EPR line shape
Authors: Friesner Richard , Nairn John A. , Sauer Kenneth
Published: February 2003

We have developed a method for the determination of the orientational distribution function of partially ordered systems from the EPR line shape that is an alternative to the usual one of expansion in the elements of the Wigner rotation matrices. The method can be applied to distribution functions whose expansion in the rotation matrix elements are slowly convergent or where the EPR intensity cannot be computed analytically. It can also be used to extract geometrical information relating the principal magnetic axis system to the preferred direction of orientation. An application to a partially ordered ensemble of triplets observed in the photosynthetic bacterium Rps. viridis is given, and it is suggested that our technique may prove particularly valuable in analysis of biological systems.
Vapor-phase fluorescence emissions of pyridine and its methyl derivatives: Excitation-energy dependence of nonradiative electronic relaxation
Authors: Yamazaki Iwao , Sushida Kazuyoshi , Baba Hiroaki
Published: February 2003

Vapor-phase fluorescence from the S1 (n, π*) states of pyridine-h5 and -d5, α- and β-picolines, and 2,6-lutidine has been studied. The fluorescence quantum yields of these compounds are, respectively, 5.9×10−5, 6.0×10−5, 3.5×10−5, 5.4×10−5, and 2.5×10−5 for excitation to the zero-point level of S1. The quantum yields of the first four compounds decrease sharply as the excitation energy is raised to the value corresponding to the S2 (π, π*) state, whereas the yield of 2,6-lutidine is fairly constant throughout the S1←S0 and S2←S0 absorption regions. The fluorescence quantum yields and their excitation-energy dependence undergo no significant change upon pressure increase from 0.5 to 20 Torr or addition of a foreign gas up to 760 Torr. The rate constants for the intersystem crossing (kISC) and the other nonradiative processes (kFQ) are evaluated as functions of the excitation energy for pyridine-h5 and α-picoline with the aid of the intersystem-crossing yield values which are available from the literature. The results show that kFQ increases rapidly with increasing excitation energy, while kISC is nearly constant. The fast nonradiative decay responsible for the sharp decrease of the fluorescence quantum yield is attributed to the S1→S0 internal conversion.
A note on SCF calculations of valence levels in heavy molecules
Authors: Boring Michael , Wood J. H.
Published: February 2003

Using a one-component relativistic theory we have recently developed, we illustrate some interesting features of self-consistent field (SCF) relativistic one-electron calculations. In particular, it is demonstrated that nonrelativistic calculations of valence levels in actinide atoms and in molecules containing actinide atoms in which only the core states are treated relativistically, do not agree with fully relativistic results. Also it is found that if only the heavy atom(s) of a molecule is treated relativistically one obtains excellent agreement with a fully relativistic calculation. Non-SCF relativistic calculations (using self-consistent nonrelativistic potentials) are shown to give results equivalent to those obtained from first order perturbation theory. The differences between effects due to the relativistic portions of the Hamiltonian (direct) and effects due to the SCF process (indirect) are given for both atoms and molecules. The three examples used are the uranium atom, the UF6, and UO22+ molecules. All calculations were performed using the local exchange approximation.
Vibrational assignment and potential function for t
r
a
n
s-diazene (diimide): Predictions for c
i
s-diazene
Authors: Craig Norman C. , Levin Ira W.
Published: February 2003

A revised assignment of vibrational fundamentals is developed for trans-diazene (HN=NH) and its −d1 and −d2 modifications that is consistent with all of the published spectra. The fundamental frequencies (in cm−1) for trans-diazene are: (Ag) 3128, 1583, 1529; (Au) 1286; (Bu) 3120, 1322. A seven-parameter, general valence type potential function is fit to the 18 observed frequencies of the three isotopic species with an average frequency error of 6.6 cm−1. When transformed into internal coordinate space, the theoretical potential function calculated by Perić, Buenker, and Peyerimhoff compares favorably with the empirical function. From empirical potential functions of trans-methyldiazene, trans-diazene, and cis-methyldiazene, a potential function for cis-diazene is constructed and used to predict the vibrational frequencies of this elusive substance.
Generalization of the direct configuration interaction method to the Hartree–Fock interacting space for doublets, quartets, and open-shell singlets. Applications to NO2 and NO2−
Authors: Handy Nicholas C. , Goddard John D. , Schaefer Henry F.
Published: February 2003

The direct configuration interaction method, including all Hartree–Fock interacting single and double replacements of a single root function, has been generalized to a wide class of systems. Programs capable of handling closed-shell singlets and open-shell doublets, triplets, quartets, and singlets in a routine fashion are described. The open-shell singlet presents some particular difficulties which have been overcome, and thus it appears that the direct CI method can be a fairly general and quite automatic approach to the electron correlation problem. The versatility of these programs is illustrated by applications to the vertical electronic spectra of NO2 (doublet and quartet spin states) and of NO2− (closed-shell singlet, open-shell singlet, and triplet spin states).
Polydisperse systems. I. Scattering function for polydisperse fluids of hard or permeable spheres
Authors: Blum L. , Stell G.
Published: February 2003

The scattering function I (k) for all k is given in the Percus–Yevick approximation for a polydisperse fluid of hard spheres. The extension to a simple model of interpenetrable particles introduced herein, the permeable-sphere model, is made in order to widen the applicability of our results to colloid and polymer problems.
A Bethe model for phase transitions in the hard sphere lattice gas
Authors: Cowley E. R.
Published: February 2003

A generalized form of the Bethe approximation is applied to the lattice gas with exclusion effects between near shells of neighbors. For short range interacting the results are in satisfactory agreement with other methods of calculation, establishing the high physical content of the approximation used. Longer ranged interactions are also considered and for the three dimensional case a new first order transition is identified. For sufficiently long ranged interactions no phase transitions are found in either two or three dimensions and reasons for this behavior are given.
Raman spectroscopy of the hydrogen bond in crystallized normal and deuterated glycerol: Influence of the anharmonic multiphonon interactions on νOH, νOD, and γOH vibrations
Authors: Beaudoin Jean Louis , Eloundou Jean Pascal
Published: February 2003

The polarized Raman spectra of oriented single crystals of glycerol (normal and 0-deuterated) have been recorded from 120 to 273 K; the spectra of super-cooled and glassy glycerol have been obtained within the same range of temperature. The behavior of the νOH, νOD and γOH bands of the crystal is analyzed according to the theory of anharmonic multiphonon processes. The interactions involving one or two low-frequency (optical and acoustical) phonons appear to be chiefly responsible for the damping of these modes. The thermal shift of the bands is mainly assigned to the effect of volume variation; the corresponding Gruneisen mode parameters are calculated.
Quenching of Ba (5d 1D) by N2, CO, H2, and D2
Authors: Eversole J. D. , Djeu N.
Published: February 2003

Metastable Ba (5d 1D) atoms have been prepared in a flow apparatus via laser pumping of the (6p′ 1P) level. The return of metastable atom density to its equilibrium value has been monitored with a cw laser beam in the presence of the gases N2, CO, H2, and D2 to yield the quenching rate constants of Ba (5d 1D) for these gases.
Coherence loss for vibrational and librational excitations in solid nitrogen
Authors: Abram Izo I. , Hochstrasser Robin M. , Kohl James E. , Semack Myron G. , White David
Published: February 2003

We have investigated the process of phase relaxation (loss of coherence) for the Ag vibrational exciton (vibron) and the Eg libron in crystalline α-N2 at liquid helium temperatures. The relaxation of the Ag vibron was found to be a nonexponential decay whose effective decay constant (1/T2) decreases with time. In the first 100 ns of the decay, the effective decay time of the observed signal (T2/2) ranged from T2/2<1 ns to T2/2=14.5 ns. The decay is due to a statistical distribution of crystal defects; its qualitative features are in agreement with simple theories of disorder scattering in crystals. The decay of the Eg libron was found to be at least 28 times faster than that of the Ag vibron (i.e., T2/2<36 ps during the first few ns) and could not be measured directly.
On the use of the iterative extended Hückel method and model Hamiltonians in chemisorption theory
Authors: van Santen R. A.
Published: February 2003

It is shown that current versions of the extended Hückel method that include self-consistency of charge have a principle defect, because screening of the intra-atomic electron repulsion due to interatom electron–electron interactions is neglected. A similar shortcoming exists for Anderson and Hubbard model calculations of covalent chemical bonding. Using the Roothaan equations it is demonstrated that the screening due to interatom electron-electron interactions depends on the bond order between the atoms. Hence transferability of effective intra-atomic repulsion integrals in calculations where chemical bonding changes considerably is not justified. CNDO and INDO semiempirical methods are shown to be consistent with first order in overlap approximations to the interatomic electron repulsion integrals.
Comparison of dielectric and refractive virial coefficients and collision induced absorption bands
Authors: Birnbaum G. , Bose T. K.
Published: February 2003

The second dielectric virial coefficient minus the second refractive virial coefficient is equal to the Kramers–Kroning contributions arising from the far-infrared and infrared pressure induced absorption bands due to bimolecular interactions. The usefulness in comparing such results is illustrated for CO2, C2H4, CH4, and SF6. It is shown that for SF6 there may be as yet unmeasured collision induced bands and for C2H4 the far-infrared collision induced absorption is anomalously large.
Measurement of the rate coefficients for the bimolecular and termolecular ion–molecule reactions of Ar+2 with selected atomic and molecular species
Authors: Collins C. B. , Lee F. W.
Published: February 2003

This work reports the measurement of second and third order ion–molecule reactions of diatomic argon ions with ten reactants selected because of their widely varying values of polarizability and dipole moment. In this study Ar+2 destruction frequencies have been experimentally determined from measurements of optical absorption at 280 and 290 nm in high pressure afterglows of mixed gases excited by intense electron beam discharges. Data have been obtained as functions of argon pressure over the range from 1000 to 2500 Torr and as functions of the partial pressure of reactant from 14 to 100 mTorr. From these data pressure dependent rate coefficients have been extracted and subsequently resolved into contributions from second order, bimolecular and third order, termolecular, components. The bimolecular components have been found to agree with tabulated values appearing in the literature. The sensitivity of the method has been sufficient to detect termolecular components as small as 2×10−30 cm6 sec−1 and values were found to range from less than 2×10−30 cm6 sec−1 for Kr and H2 to 44×10−30 cm6 sec−1 for NO. A classical approximation to the three-body capture rate has been evaluated that explains these values in terms of the rates at which third-body encounters change glancing collisions into inwardly spiraling orbits. The data reported here are in agreement with this model and indicate that the reaction probabilities are generally high. The sizes of these termolecular rates suggest their general importance in charge transfer reactions in plasmas at atmospheric pressures.
Calculation of radial distribution functions for molecules in excited vibrational states
Authors: Kacner M. A. , Bartell L. S.
Published: February 2003

It is shown how to reduce N-dimensional probability densities calculated from vibrational wave functions of polyatomic molecules to one-dimensional radial distribution functions. Such distributions are, in principle, derivable from diffraction experiments. In this initial treatment, anharmonicity and nonlinearities in transformations between normal coordinates and internuclear distances are neglected. Illustrative calculations are given for H2O, CF3Cl, and SF6 in various vibrational states or thermal distributions among vibrational states. Possible applications are briefly discussed.
Thermodynamics of the dimerization of gaseous UF5
Authors: Kleinschmidt P. D. , Hildenbrand D. L.
Published: February 2003

The gaseous dimerization equilibrium 2 UF5=U2F10 was studied by high temperature mass spectrometry over the range 600 to 870 K. Gaseous UF5 monomer and dimer were generated by the reaction of UF6(g) with a solid phase that was initially UF4(s), and the abundances of the two species were monitored with both the parent and major fragment ions. Measurements of the U2F10/UF5 ratio as a function of pressure established that gaseous equilibrium was achieved. The average of several second-law determinations yielded a value of −39.0±1.0 kcal/mole for the enthalpy of dimerization of UF5 at 750 K corrected to −40.0±1.0 kcal/mole at 298 K. From an absolute pressure calibration, the equilibrium constant was evaluated, leading to an entropy of dimerization of −40.0 cal/mole deg at 750 K. The results indicate that previously reported transpiration studies on UF5(s) should be interpreted in terms of vapor transport by U2F10 (g) rather than UF5 (g). The enthalpies of sublimation of UF5 (g) and U2F10 (g) from UF5 (s) at 298 K are calculated to be 39.0 and 38.5 kcal/mole, respectively, from the corrected transpiration vapor pressure data and the UF5 dimerization data reported here. The electron impact mass spectra of UF6 and UF5 at several ionizing energies are reported. A weak U2F+11 signal was observed at high UF6 flow rates, apparently the product of the secondary ion source reaction of UF+5 and UF6.
Critical behavior of the refractive index in a binary fluid
Authors: Beysens D. , Wesfreid J.
Published: February 2003

We have performed very accurate interferometer measurements of the refractive index n (precision ±6×10−7) in the critical mixture nitroethane–iso-octane. n depends on the density ρ and local field effects F. Even though this mixture is expected to show a very small anomaly in Δρ/ρ (exponent α) so that local field effects should have been observed, in fact we never saw a contribution due to F and the critical behavior of n is governed by the density anomaly alone. Using the matching of refractive index components and of thermal derivatives, the regular part of n can be evaluated and the value α=0.10±0.02 is found, in agreement with the group renormalization value 0.110. The corresponding amplitude is compatible with the value obtained from ξ0 and (∂Tc/∂p) V,xc measurements using universal amplitude relations.
Extrapolation in configuration interaction calculations using perturbation theory natural orbitals: Applications to H2O and HeH2+
Authors: Pendergast Phil , Morrell Glenn O. , Hayes Edward F.
Published: February 2003

A procedure for carrying out the Buenker–Peyerimhoff extrapolation using perturbation theory natural orbitals (PTNO’s) is presented. This procedure is then applied to the H2O and HeH2+ systems within the framework of a configuration space comprising all single and double excitations from a single configuration. The results indicate that significantly smaller secular determinants need to be solved when PTNO’s are used; the extrapolated CI energies are within 5×10−4 a.u. of energy obtained from the full calculation within the same configuration space.
On the force bias Monte Carlo simulation of simple liquids
Authors: Rao M. , Berne B. J.
Published: February 2003

Two recently devised Monte Carlo schemes, the force bias method and the smart Monte Carlo method are compared using a simple Lennard-Jones fluid as an example. Using diffusion in the configuration space as a criterion, the efficiencies of the two methods are evaluated. When the diffusion is optimized the FB method is found to be more efficient.
Debye–Edwards electron recombination kinetics
Authors: Hamill William H.
Published: February 2003

The decay of recombination luminescence intensity I with time t, given by the Debye–Edwards law I ∝ t−m, with m?1 and time independent, applies to Coulombically correlated charge pairs in rigid low-dielectric media. For observation times much greater than preparation times it is remarkably successful, e.g., for a range of 10−6–102 s for some media. These results provide a minimal experimental basis for examining improved procedures for measurement and presentation of results. A simple extension of the D–E law for electron decay or luminescence intensity describes results quantitatively for continuous excitation at low constant dose rate with in-phase measurement, extending the range and improving the precision.
Thermal effects on the optical properties of single crystals and solution-cast films of urethane substituted polydiacetylenes
Authors: Chance R. R. , Patel G. N. , Witt J. D.
Published: February 2003

The optical properties of single crystals and solution-cast films of two polydiacetylenes, poly3BCMU and poly4BCMU where the substituent group is– (CH2)3.4OCONHCH4COOC4H9, are characterized. Visible absorption and reflection spectra for the polymer crystals are typical of those observed for other urethane substituted polydiacetylenes. The optical properties of the polymer films are controlled primarily by intramolecular hydrogen bonding between the N–H and C=O of the urethane functionalities on adjacent substituent groups. Hydrogen bonding stabilizes the planar, fully conjugated conformation of the individual polymer chains in the films. Increased temperature causes a disruption of the hydrogen bond network and a destabilization of the planar polymer conformation. Dramatic color changes result because of the sensitivity of the optical properties to backbone conformation.
Brownian dynamics study of a polymer chain of linked rigid bodies
Authors: Pear M. R. , Weiner J. H.
Published: February 2003

A Brownian dynamics model for the backbone chain of a macromolecule is developed as a system of linked rigid bodies so that constraints on valence angles and bond lengths are satisfied exactly. For comparison, a corresponding flexible model is developed in which bond lengths and valence angles are held nearly constant by strong harmonic potentials. Equilibrium properties and barrier crossing rates are examined theoretically and by computer simulation of both models, with differences arising due to the presence of constraints in the rigid case. A compensating potential based on the metric determinant of unconstrained coordinates in the rigid model is found to eliminate the effect of constraints. Barrier crossing rates in the transition state approximation are studied when a force fixed in space is applied to the end atoms of the three-bond chain. An exact transition state rate formula developed for this case predicts curved Arrhenius plots of barrier crossing rates; this result is confirmed by computer simulation.
On the valence bond diatomics-in-molecules method. I. A projection operator reformulation
Authors: Faist Michael B. , Muckerman James T.
Published: February 2003

A rederivation of the diatomics-in-molecules (DIM) method for obtaining potential energy surfaces is presented in terms of projections onto fragment diatomic and atomic subspaces. This approach renders the approximations inherent in the DIM method transparent. The method is considered from both ab initio and semiempirical viewpoints. Nonadiabatic and spin–orbit coupling in the fragments is discussed. Comparison with previous DIM formulations is made.
A photoelectron spectroscopic study of the formic acid dimer
Authors: Carnovale F. , Livett M. K. , Peel J. B.
Published: February 2003

The He (I) photoelectron spectrum of the gas-phase dimer of formic acid, (HCOOH)2, has been measured using a high pressure nozzle inlet system operating at room temperature. Interpretation of the spectrum with the aid of molecular orbital calculations indicates that the dimer shows considerable interaction of the monomer-based states of σ symmetry, but little interaction of π states. The σ splittings arise mainly from through-space interactions with the bridging hydrogen atoms making a small contribution to the stability of the lower energy molecular orbital resulting in each case.
The description of the smectic A and C phases and the smectic A–C phase transition of TCOOB with a diffuse-cone model
Authors: de Vries Adriaan
Published: February 2003

A previous paper demonstrated that the orientational disorder present in all smectic A phases causes most molecules to make fairly large angles with the normal to the smectic plane. Thus, the molecular directions form a diffuse cone around the layer normal. It is now argued that even the preferred directions of the molecules in a smectic A phase will make a nonzero angle with the layer normal, and from the data examined, this angle, ϑm, appears to be about 10–15 ° for the compound TCOOB (trans-1,4-cyclohexane-di-n-octyloxybenzoate). The smectic A-to-C phase transition is found to have two aspects in the diffuse-cone model: the distribution of the molecular directions around the layer normal loses its rotational symmetry at the phase transition temperature TA–C, and the angle ϑm starts increasing sharply at a temperature Tt. In most cases these two temperatures are probably equal. The increase of ϑm with decreasing temperature in the smectic C phase of TCOOB can be described by a (Tt−T)0.35 dependency superimposed on a much smaller linear temperature dependency already present in the smectic A phase.
Study of hydracids trapped in monatomic matrices. I. Near infrared spectra and aggregate structures
Authors: Maillard D. , Schriver A. , Perchard J. P. , Girardet C.
Published: February 2003

The infrared spectra of H35Cl, D35Cl, HBr, DBr and their mixtures in solid argon and krypton in the range 10–30 K are presented. Studies have been made over a wide concentration range, for matrix/hydracid ratio from 1000 to 20 which, together with annealing experiments and H/D isotopic dilutions, allow a complete discussion of polymeric species spectra. In the monomer region, Q features are clearly identified at low M/R ratio and assigned to lattice distortion effects on isolated molecules. A new assignment for dimer bands is proposed, in which the two molecules are not equivalent and characterized by different values of force constants and moment derivatives. The trimer is found cyclic, a configuration already mentioned, the three molecules being equivalent and oriented along the sides of an equilateral triangle. After annealing, a new polymeric species associated with a strong band at lower frequency appears and is identified, in the light of H/D mixture spectra, as a cyclic planar tetramer. From accurate frequency measurements one concludes that the H/D isotopic substitution slightly changes the hydracid force constant, a DX molecule acting as a stronger perturber than a HX one on any partner belonging to the same aggregate. At last the analysis of mixed H35Cl–HBr and HCl–HI spectra enables a comparison between the perturbing power of these three molecules, the decreasing electron donor power being in the order HI, HBr, HCl.
The potential energy curve for the X 1Σg+ state of Mg2 calculated with coupled pair many electron theory
Authors: Purvis George D. , Bartlett Rodney J.
Published: February 2003

The effects of linked quadruple excitation and superposition errors on the MG2 potential curve are discussed.(AIP)
Low energy electron scattering from methane
Authors: Barbarito E. , Basta M. , Calicchio M. , Tessari G.
Published: February 2003

Measurements of the exponential attenuation of electron currents in a straight line collision chamber are reported using methane gas as the scattering medium. From the transmission spectra obtained at different methane pressure in the scattering chamber, the total cross sections for the energy range 0–16 eV are derived and compared to previous measurements. A small structure in the region of the Ramsauer–Townsend minimum is observed, confirming earlier interpretations of electron swarm experiments. A possible mechanism of excitation is discussed.
Conduction band energy in dense ethane fluid
Authors: Yamaguchi Yoh-ichi , Nakajima Toshitaka , Nishikawa Masaru
Published: February 2003

The conduction band energies in ethane were determined by measuring the work function change of zinc electrodes in vacuo compared to that when fluid was present. (AIP)
Far infrared reflection spectra of lead (II) chloride and iodide
Authors: Hiraishi Jiro , Tani Katsuhiko , Tamura Tadao
Published: February 2003

Far infrared reflection spectra of single crystals of PbCl2 and PbI2 have been measured using a Hitschi 070 for infrared interferometer. AIP
The excited states of ethylene
Authors: Mulliken Robert S.
Published: February 2003

New assignments are suggested for the well-established excited states of ethylene. The states are assigned 1B1g and 1B2g configurations. (AIP)
EPR spectrum of Sb4+ in a silicate glass
Authors: Schreurs J. W. H. , Davis D. H.
Published: February 2003

The EPR spectrum of polychromatic glass continuing Ce3+ and Ce4+ and after exposure to long wavelength UV light is reported. The EPR center is identified as Sh4+. (AIP)
Comment on ’’Depolarized Rayleigh spectroscopy from benzonitrile’’
Authors: Whittenburg S. L. , Wang C. H.
Published: February 2003

The effects of deconvolution on translation–rotation coupling parameter as measured by Rayleigh light scattering are discussed. The results of Alms and Patterson imply that the translation–rotation parameter in benzonitrile is temperature independent. This discrepancy is discussed in terms of convolution of true spectra with the instrumental spectrum. (AIP)
Reply to ’’Comment on ’Depolarized Rayleigh spectroscopy from benzonitrile’ ’’
Authors: Alms G. R. , Patterson G. D.
Published: February 2003

This reply will appear in the Journal of Chemical Physics in response to the above named comment on the previously published paper. (AIP)
Electronic spectroscopy in condensed media: The lowest n-π* transition of the solvated nitrite anion
Authors: Banerjee Ajit , Simons Jack
Published: February 2003

A general formalism developed in earlier publications for treating the electronic spectra of solvated anions is found to successfully described the spectra of the nitrite anion in solvents of differing polarity (H2O, EtOH, CH3CN, DMF). The analysis presented supports the conclusions that the first excited state (1B1) of the solvated nitrite anion is a bound state. The entire width of the spectra can be accounted for via a Frank-Condon progression alone. The predicted vibrational frequencies of the participating bending mode in the aqueous solution are found to be 1074 and 827 cm−1 in the ground (X 1A1) and the excited (1B1) states, respectively. These predicted frequencies have been further verified by ab initio calculations which simulate the gas-phase and solvated ion. The force constant for the ground state bending mode is found to be 9.8×10−11 dyne cm. in aqueous solution. The fitting procedure allows us to calculate an inhomogeneous broadening of 383 cm−1 for NO2− in water. These properties have been also calculated for the other solvents (EtOH, CH3CN, and DMF) and trends in the changes of these quantities have been established.
Mode-to-mode vibrational energy flow in S1 benzene. V–V resonant energy transfer, microscopic reversibility, and the role of level degeneracies
Authors: Atkinson G. H. , Parmenter C. S. , Tang K. Y.
Published: February 2003

The pattern of mode-to-mode vibrational energy flow from the level ν6′=522 cm−1 in the S1 state of benzene has been studied with OCS as a collision partner. OCS has a near resonant bending vibration ν2″=520 cm−1. V–V transfer to this bend of OCS is efficient, requiring only ten collisions. However, a T,R→V transfer from OCS adding ν16′=237 cm−1 to the ν6′ benzene fundamental is even more efficient. Since other V–T,R transfers are also fast, the V–V resonance does not dominate vibrational energy flow pattern from the initially excited benzene level. As with other added gases energy flow is governed by strong propensity rules which can be expressed simply in terms of final level degeneracies, vibration quantum changes, and energy exchange between V and T,R degrees of freedom. Measurements of V–T,R transfer rates from other S1 benzene levels with added CO have been used to demonstrate microscopic reversibility and to show the control of transfer rates by the degeneracies of the final level. OCS also quenches the S1 electronic state of benzene with a rate constant k= (1.1±0.1) ×106 Torr−1 s−1 for destruction of the S1 state with a 300 K Boltzmann distribution of vibronic levels.
A master equation for energy migration in multicomponent crystals
Authors: Keyes T.
Published: February 2003

A projection operator method is used to derive a master equation for energy migration in multicomponent crystals. The equation contains the effect of disorder in the excited state energies of the molecules, as well as the effect of the connectivity of the lattice and the concentrations. A discussion is given of existing work on the problem, with an eye to unification of contending points of view. The dynamical analogs of some important concepts in time-independent solid state physics are obtained.
ESR matrix isolation investigation of the aluminum hydride radical cation−AlH+
Authors: Knight Lon B. , Martin Richard L. , Davidson Ernest R.
Published: February 2003

The AlH+ radical cation has been trapped in a neon matrix at 4 K and studied via electron spin resonance spectroscopy (ESR). The magnetic parameters are Al: A∥=1009(3), A⊥=877(3) and H: A∥=A⊥=283(3) MHz; g∥=2.002(1) and g⊥=2.000(1). The isotropic and dipolar hyperfine components were calculated from ab initio CI and SCF type wave functions for comparison with experimental results. AlH+ is the first paramagnetic molecular cation studied via rare gas matrix isolation ESR spectroscopy.
Self-diffusion and viscosity of methylcyclohexane in the dense liquid region
Authors: Jonas J. , Hasha D. , Huang S. G.
Published: February 2003

The self-diffusion coefficients of liquid methylcyclohexane have been measured as a function of temperature and pressure over the density range 2.5⩽ρ/ρc⩽3.3 and temperature range 0.35⩽T/Tc⩽0.5 using the NMR spin echo technique with a fixed field gradient. Shear viscosities and densities have also been determined under the same experimental conditions. An approximate analysis of the experimental diffusion and viscosity data in terms of the rough sphere model of liquids yields effective hard core diameters and a parameter A that reflects the degree of coupling between the rotational and translational motion of methylcylohexane molecules. The strong temperature dependence of this coupling arises most likely from the nonspherical shape of the methylcyclohexane molecule. The behavior of the rotational–translational coupling and its possible connection to melting properties and to the experimental finding that methylcyclohexane does not form an orientationally disordered crystalline phase is discussed in a qualitative way.
Unimolecular processes in vibrationally highly excited cycloheptatrienes. II. Steady-state photoisomerization
Authors: Troe J. , Wieters W.
Published: February 2003

The steady-state unimolecular photoisomerizations of cycloheptatriene, 7-methyl-cycloheptatriene, and 7-ethyl-cycloheptatriene have been studied at wavelengths between 238 and 289 nm. The reactions proceed via vibrationally highly excited electronic ground state molecules. The specific rate constants k (E) for the unimolecular aromatizations and H-atom shifts are derived and compared with statistical calculations. Absolute values of the average step sizes <ΔE≳ for energy transfer of highly excited molecules are obtained via calibration of Stern–Volmer curves against specific rate constants from thermal isomerization experiments.
A classical trajectory study of the fragmentation of CO−2 2Σ+g
Authors: Sizun M. , Goursaud S.
Published: February 2003

The dissociation of the 2Σ+g CO−2 ions formed by electron attachment is studied with a classical trajectory method in which the initial conditions are represented by the Wigner probability density function and the electronic state 2Σ+g of CO−2 is described by a Wall–Porter potential energy surface. Since both the ground initial state of CO2 and the upper dissociative state of CO−2 have an equilibrium angle of 180°, the dissociation is studied through the collinear approximation. The experimental data (excess energy transferred to vibration, population inversion of the vibrational CO levels) are found to be related not only to the autodetachment rate as usual, but also to some details of the repulsive potential energy surface such as the position and height of the saddlepoint and to the inertial coupling between translation and vibration.
The infrared spectrum of ice IV in the range 4000–400 cm−1
Authors: Engelhardt H. , Whalley E.
Published: February 2003

The infrared spectrum of ice IV has been recorded in the range 4000–400 cm−1. The location and breadth of the bands confirm that the water molecules in ice IV are moderately strongly hydrogen bonded, as they are in the other phases of ice, and are orientationally disordered.
Rotational and vibrational analysis of the ?←? system of XeF as observed in a supersonic free jet
Authors: Monts D. L. , Ziurys L. M. , Beck S. M. , Liverman M. G. , Smalley R. E.
Published: February 2003

The fluorescence excitation spectrum of XeF cooled in a supersonic expansion with helium has been observed for the wavelength region 2860 to 3205 Å. The cold spectrum displays well-resolved rotational structure for seven of the XeF isotopic species. Analysis of this rotational structure is reported for v′=16–21 in the ? (Ω=1/2) state and for v″=0 and 1 in the ? 2Σ+ state. Vibrational intervals have been measured for individual XeF isotopes with an accuracy of ±0.50 cm−1 for vibrational levels v′=11–22 and ±0.37 cm−1 for v″=0–2.
Reorientational motions of dipolar solutes in glassy o-terphenyl
Authors: Crossley J. , Gourlay D. , Rujimethabhas M. , Tay S. P. , Walker S.
Published: February 2003

The low frequency (102–105 Hz) dielectric relaxation behavior of small dipolar molecules dispersed in supercooled o-terphenyl has been studied below the glass transition temperature. Broad asymmetric loss curves are obtained for both rigid and nonrigid solute molecules. The relaxation times and Eyring activation enthalpies for molecular and intramolecular group reorientation processes are compared with the analogous values for the same solutes in a polystyrene matrix.
Thermodynamics of the lanthanide trifluorides. III. The heat capacity of neodymium trifluoride, NdF3, from 5 to 350 °K
Authors: Lyon William G. , Osborne Darrell W. , Flotow Howard E.
Published: February 2003

The heat capacity of a well-characterized sample of NdF3 was measured by aneroid adiabatic calorimetry from 5 to 350 ° K. The following results were determined at 298.15 ° K: the heat capacity, Cp= (92.42±0.09) J ° K−1 mole−1; the entropy, S°= (120.79±0.12) J °K−1 mole−1; the enthalpy increment, [H °  (298.15 °K)−H°  (0)]=17 360±17 J mole−1; and the Planck function, −[G° (298.15° K)−H ° (0)]/T= (62.56±0.06) J ° K−1 mole−1. The values of the last three functions were adjusted for the removal of the degeneracy associated with the ground-state Kramer’s doublet of Nd+3. A table of recommended thermal functions of NdF3 from 400 to the melting temperature, 1650 °K is presented. The excess heat capacity due to the internal electronic energy levels of Nd+3 was calculated from spectroscopic data and also from calorimetric results and the two calculations were found to be in reasonable agreement.
Raman studies of ice VI using a diamond anvil cell
Authors: Abebe M. , Walrafen G. E.
Published: February 2003

Pressure dependence of the Raman OH−streching frequency of ice VI is reported over a pressure range of 9−21 kbar. The ice samples were examined in a diamond anvil cell and pressures were measured by the ruby fluoresence method. (AIP)
Improved a
b i
n
i
t
i
o effective core potentials for molecular calculations
Authors: Christiansen Phillip A. , Lee Yoon S. , Pitzer Kenneth S.
Published: February 2003

We have investigated the sources of error in bond lengths and dissociation energies computed from ab initio effective potentials derived from Phillips–Kleinman type pseudo-orbitals. We propose an alternate pseudo-orbital, effective potential treatment with the primary objective of agreement with all-electron molecular calculations. This new treatment forces the pseudo-orbitals to match precisely the Hartree–Fock orbitals in the valence region and thereby eliminates the major cause of error in the earlier calculations. Effective core potentials derived from these revised pseudo-orbitals were used to compute potential energy curves for the ground states of F2, Cl2, and LiCl and the results are compared with previous all-electron and effective potential calculations. Our effective potentials yield dissociation energies and bond lengths which are in excellent agreement with the all-electron values. Furthermore, in contrast to other procedures, our revised effective potentials result in an excellent description of the inner repulsive walls of the dissociation curves.
Structure and properties of the argon–ozone van der Waals molecule
Authors: DeLeon Robert L. , Mack Kenneth M. , Muenter J. S.
Published: February 2003

The Ar–O3 van der Waals complex is produced in a nozzle expansion and studied by radio frequency and microwave molecular beam spectroscopy. The rotational constants are A=12 686, B=1947, C=1716 MHz. Stark effect data produce the following dipole moment components: μa=0.1010, μb=0.0, μc=0.4643 D. These molecular properties are only consistent with a structure in which the Ar atom is 3.42 Å from the center of mass of O3 and the O3 plane is approximately perpendicular to the axis connecting Ar to the O3 center of mass. Large amplitude vibrational motion of O3 within the complex is observed.
Investigation of exchange processes by two-dimensional NMR spectroscopy
Authors: Jeener J. , Meier B. H. , Bachmann P. , Ernst R. R.
Published: February 2003

A new general technique for the investigation of exchange processes in molecular systems is proposed and demonstrated. Applications comprise the study of chemical exchange, of magnetization transfer by inter- and intramolecular relaxation in liquids, and of spin diffusion and cross-relaxation processes in solids.
Exothermic rate restrictions on electron transfer in a rigid medium
Authors: Beitz James V. , Miller John R.
Published: February 2003

Many highly exothermic (2–3 eV) electron transfer reactions are shown to be slower than moderately exothermic reactions by factors as large as 105. The decrease occurs in a regular way with increasing exothermicity, tending to confirm theoretical predictions of Franck–Condon restrictions on strongly exothermic electron transfer reactions. Deviations from the above trends occur if the reaction product, a molecular anion, has a low-lying electronic excited state into which the reaction may occur with more moderate vibrational exothermicity. Then greatly enhanced rates are found. The rates are enhanced to a lesser extent for acceptors likely to undergo configurational changes upon accepting an electron. These effects are found in measurements of rates of electron tunneling reactions between trapped electrons and 48 organic electron acceptors in rigid 2-methyltetrahydrofuran glass at 77 K. Electron tunneling rates were observed from 10−6 to 102 s. Measured tunneling distances were 15–40 Å. In most cases the observed kinetic decay curves are well simulated by a theory in which the only variable parameter is the effective Franck–Condon factor (F), which is a constant characteristic of the acceptor. For the various acceptors F ranges from 1 to 10−5, and scales the reaction rate at each distance. However in reactions of small vibration exothermicity, the Franck–Condon factors are expected to be very sensitive to small changes in reaction exothermicity caused by relaxations of trapped electrons, which deepen their trap depths with time and, possibly, a dispersion of trap depths. These effects cause F to change with time leading to changes in the shapes of the decay curves. The relationship between the shapes of the decay curves, Franck–Condon factors, and exothermicity allows a thorough and consistent semiquantitative interpretation of the present results and much of the earlier tunneling data on electron transfer reactions involving other solvents and solutes.
Vibronic intensity enhancement in the infrared spectra of heavily doped (CH)x and (CD)x
Authors: Rabolt J. F. , Clarke T. C. , Street G. B.
Published: February 2003

Fourier Transform IR transmission spectroscopy has been utilized to elucidate the nature of the polymer–dopant interaction in doped (CH)x. Significant changes in the IR spectrum occur after doping and several intense bands at 1385, 1295, and 832 cm−1 have been observed and their shifts studied upon deuteration. The origin of these intense features can be attributed to a vibronic activation and enhancement of the totally symmetric Raman active Ag modes in the IR spectrum.
Vibronic effects in the predissociation of the C 1Πu state of C2N2
Authors: Miller G. E. , Jackson William M. , Halpern J. B.
Published: February 2003

The partitioning of excess photochemical energy as a function of the vibrational energy of the C 1Πu state of C2N2 has been measured. Surprisal theory has been used to analyze the data and it shows that complete randomization does not occur before dissociation. The results are also inconsistent with the predictions of the quasidiatomic theory for photodissociation.
Vibrational relaxation in a structurally disordered organic solid: Temperature dependence of Raman active phonons in p-bromochlorobenzene and p-dichlorobenzene
Authors: Prasad Paras N. , Von Smith Randy
Published: February 2003

A theoretical discussion is presented to show that for a highly delocalized phonon motion, the line shapes derived from disorder and from vibrational relaxation due to anharmonic phonon–phonon scattering are both approximately Lorentzians. The linewidths obtained from disorder and vibrational relaxation are additive. A temperature dependence of the linewidth can be used to identify the mechanism and obtain the rate of vibrational relaxation. An experimental study is presented on the lowest frequency Raman active phonon of p-bromochlorobenzene which in the solid state shows an orientational disorder. The result is compared with that on the corresponding phonon in the isomorphic p-dichlorobenzene crystal which does not involve any orientational disorder. Experimental findings are in agreement with the theoretical predictions. The experimental study shows that in p-bromochlorobenzene at 2° K, the line broadening due to disorder is almost an order of magnitude larger than that due to anharmonic phonon–phonon scattering. Yet the overall line shape is a Lorentzian and the two linewidths are additive. The temperature dependence reveals that in both p-bromochlorobenzene and p-dichlorobenzene the mechanism of line broadening involves a T1-relaxation process in which the optical phonon decays into two phonons of half its frequency by cubic anharmonic interactions.
On the dynamics of exothermic triatomic exchange reactions: Semiclassical model for final state VRT distributions
Authors: Venzl Gerd , Fischer Sighart F.
Published: February 2003

A semiclassical theory for exothermic exchange reactions is presented which treats the subsystem of translational rotational motion classically and the vibrations quantum-mechanically. The rearrangement of bonds in a given configuration of atoms is described by Franck–Condon-type overlap factors which generalize a recently proposed collinear theory to bent configurations. In order to treat rotational excitation explicitly in a nonstatistical way the classical version of a transformation between reactant and product angular momenta due to a sudden bond exchange is introduced. Additional rotational excitation stems from the release of reaction energy via product repulsion which is taken into account in a simple impulsive model. Calculated product energy distributions P (v′, j′) show good agreement with experimental and quasiclassical trajectory results for two mass combinations with very different mechanisms of rotational excitation (L+HH→LH+H and H+LL→HL+L). Certain variations of the product energy distribution with the collision energy are reproduced.