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Volume 71, Issue 12, 15 December 1979

Anisotropic hyperfine interaction in the electron spin resonance spectrum of the methyl radical trapped in CH_{3}COONaâ 3D_{2}O crystal at low temperatures
View Description Hide DescriptionThe electron spin resonance spectrum of the methyl radical produced by Îł irradiation of CH_{3}COONaâ 3D_{2}O single crystal at 77 K has been measured at low temperatures. The spectrum observed at below 24 K consisted of eight lines with approximately equal intensities, instead of the four lines (1:3:3:1) at higher temperatures, due to the quantum tunneling of the rotational states. The transitions at low temperatures were angular dependent upon the applied field. From the analyses of the angular dependence of the spectrum with the aid of the theoretical calculation of the transition field for the tunneling methyl radical, the following anisotropic hyperfine tensor for the proton was determined (MHz): A_{ x x }=â31.4, A_{ y y }=â93.7, A_{ z z }=â60.3, a (i s o) =â61.8. These parameters provide the first experimental assessment of the wellâknown theoretical results on the proton hyperfine anisotropy in the free methyl radical.

Timeâdependent theory of Raman scattering
View Description Hide DescriptionA timeâdependent picture of vibrational Raman scattering in the weak field limit is presented. From this viewpoint we can separate the static effects, due to the coordinate dependence of the electronic transition dipole, from the dynamic effects that arise from wave packet propagation on the BornâOppenheimer surfaces. Away from resonance, the energy uncertainty relation gives the propagation time necessary to obtain the cross section as being inversely proportional to the mismatch of the excitation frequency with the excited surface. The wave packet, given by the initial vibrational wave function times the transition dipole, hardly moves on the excited surface when the excitation frequency is far from resonance. As the excitation frequency is tuned closer to resonance, the propagated wave packet samples a larger portion of the surface. Using the short time approximation to the propagator, we obtain formulas for the cross section that are applicable for Raman scattering by polyatomics. The short time approximation is expected to be good away from resonance independent of the nature of the surface, and also on resonance with a repulsive surface. For an attractive surface, the approximation gives the average resonant cross section useful in the case when the vibrational structures cannot be observed.

A comparison of the vibrational structures of ethanol, acetic acid, and acetaldehyde adsorbed on alumina
View Description Hide DescriptionThe adsorption on aluminum oxide of acetaldehyde (CH_{3}CHO) and acetic acid (CH_{3}COOH) has been studied by utilizing inelastic electron tunnelingspectroscopy (IETS) to probe the vibrational structures of the surface species over the spectral range between 240 and 4000 cm^{â1}. Results for temperatures between 295 and 475 K are compared with our previous IETS results for ethanol (CH_{3}CH_{2}OH) adsorption on alumina up to 575 K. Although all three hydrocarbons form acetate complexes with the surface under certain conditions, slight differences in bonding and structure are evident in the IET spectra. The formation of some bulk aluminum hydroxide is observed also in certain cases. Observed variations in surface concentrations, bulk hydration, and the nature of surface hydroxyl groups are related to previous experimental results. Mechanisms are substantiated for the initial adsorption and subsequent reaction of each hydrocarbon with the surface.

Vibrational population relaxation of compressed ^{ n }H_{2} fluid in the 15â110 K range
View Description Hide DescriptionExperimental observations of the population relaxation time of the first excited vibrational level of ^{ n }H_{2} have been made in the fluid state over a very wide range of temperature and pressure (15â110 K, 0â500 atm). These extensive measurements yield new insight into the fundamental parameters governing energy relaxation in simple fluids.

A semiclassical model for vibrational energy relaxation in simple liquids and compressed fluids
View Description Hide DescriptionUsing a simple modelintermolecular potential, a density dependence of the vibrational population relaxation time in liquids is obtained which is in qualitative and quantitative agreement with experiment, over the whole range of density and temperature studied. This model is applicable to systems where the energy transfer involved is large with respect to thermal energies and describes the variation of the relaxation time as being essentially related to the variation of the collision frequency on the strongly repulsive part of the intermolecular potential. The formalism employed demonstrates clearly the role of excluded volume at high density and of bound states at low density and temperature.

New study of CO adsorption at low temperature (90 K) on Pt (111) by EELS
View Description Hide DescriptionWe have performed a new study of the vibrational modes of CO adsorbed at low temperature (90 K) on Pt(111). As in previous work a sequential occupation of top and bridge sites is observed. At a certain bridge site coverage, however, a new vibrational mode with a frequency of 720 cm^{â1} appears. The mode is not observed at room temperature since sufficiently high coverages are not achieved there. The appearance of the 720 cm^{â1} mode is attributed to the CâPt asymmetric stretching vibration becoming dipole active at higher coverages when, according to LEED results, an outâofâregistry CO overlayer is formed.

High pressure studies of poly (Nâvinylcarbazole) excimer emission in polymer films
View Description Hide DescriptionThe excimer fluorescence properties of poly (Nâvinylcarbazole) (PVCA) dissolved in a variety of polymer films were investigated. The existence of two interchain excimers emitting from frozen preformed sites in the film was observed. Large hydrostatic pressures were used in order to change the local environment of the two sites. Changes with pressure of the ratio of respective intensities are interpreted according to a hopping singlet excitondiffusion model with the two sites acting as competing traps. There are two distinct mechanisms, one predominant at low and the other at high pressures. The assumption that a small rearrangement between the two excimers is possible at low pressures is valid since both glass transition temperatures and overall intensity changes support it. The change of volume in the transformation between the two excimers was calculated for the different films and is equal to or less than 3% of the monomer molar volume.

Generalized sensitivity analysis in quantum collision theory
View Description Hide DescriptionThis paper considers the sensitivity of the scattering matrix with respect to variations of input parameters including those that persist asymptotically. The analysis shows that the first order sensitivity coefficients (i.e., the partial derivatives with respect to input parameters) of the scattering matrix may be expressed in terms of the available solution of the SchrĂ¶dinger equation. This work encompasses earlier results which appear as special cases. As an application sensitivity with respect to the total energy is considered and the relevance of the result to the energy behavior of observables is discussed. In a second application, we show how sensitivity analysis may be used to estimate corrections to effective Hamiltonian results and also to examine their accuracy. The coupled states and infinite order sudden methods are chosen to illustrate the approach. Finally, the scope of sensitivity analysis in quantum collision theory is discussed.

The generalized moment method for transport phenomena in dense fluids
View Description Hide DescriptionBased on the kinetic equation reported previously, the generalized moment method is presented for dense fluids in this paper. The equations of change are presented for stress tensor and heat flux in nonuniform dense fluids and by solving them up to second order by a perturbation method,constitutive relations and various linear and nonlinear transport coefficients are obtained. Comparison of the transport coefficients with the counterparts in the generalized ChapmanâEnskog (GCE) method shows that two results agree with each other if the first ChapmanâEnskog approximants are used for the GCE results and some potential energy contributions are neglected in the molecular expressions for fluxes. Discussions are given regarding the constitutive relation for the Stokes fluids and the nonâNewtonian behavior of real fluids.

A theory for magnetic resonance measurements of diffusion in anisotropic fluids
View Description Hide DescriptionElectron spin resonance(ESR) spin exchange and the enhancement of nuclear spinrelaxation frequencies due to the presence of paramagnetics can be used to study diffusion in liquid crystals and membranes. Previous analyses of the diffusion data obtained in this manner assumed isotropic diffusion. In this paper derivations are given for the dependence of the relaxation frequencies measured in these two experiments on the diffusion constants parallel and perpendicular to the director, D _{â„} and D _{â„}, respectively.

Ultrasonic measurement of the vibrational relaxation in liquid SO_{2}
View Description Hide DescriptionMeasurements of the absorption and velocity of ultrasound in liquid sulfur dioxide have been made over a wide range of temperatures, greatly extending the temperature range of the measurements of Bass and Lamb on the same system. The data are complete now from 203 to 323 K in the frequency range from 6 to 90 MHz. All the data have been fit satisfactorily to a singleârelaxation description, and give a temperatureâindependent relaxation frequency of 22.5Â±1 MHz. The relaxation amplitude has a strong temperature dependence of the Arrhenius type, with an energy of activation of âŒ3 kcal/mole, the temperature behavior of which can be satisfactorily calculated on the basis of reported vibrational frequencies of the SO_{2} molecule. The velocity of sound, as measured, is a linear function of the temperature. Measurements are also reported for dilute mixtures of cyclohexane in SO_{2} where no shift in the measuredrelaxation frequency is observed, contrary to the usual situation for vibrational relaxations.

HTFFR kinetics studies of the Al/SO_{2} reaction from 700 to 1600 K. Implications for D(AlâO)
View Description Hide DescriptionMeasurements of the kinetics of the bimolecular Al/SO_{2} reaction over the temperature range 700 to 1600 K have yielded a k (T) = (1.6Â±0.8) Ă10^{â10} exp[(=2000Â±300)/T]cm^{3}âmolecule^{â1}â s^{â1}. This k (T is significantly larger than that for the Al/CO_{2} reaction in the same temperature range. This temperature dependence indicates with 95% probability that the AlâO dissociation energy ?126 kcalâmol^{â1}, somewhat higher than most often thought. AlO laser fluorescence has been used to identify AlO+SO as a major, and probably the only, product channel. A k of 2Ă10^{â12} cm^{3} molecule^{â1}âs^{â1} is measured for the AlO/SO_{2} reaction near 940 K.

Transmission of 0â15 eV monoenergetic electrons through thinâfilm molecular solids
View Description Hide DescriptionThe transmission coefficient for monoenergetic electrons (0.04 eV FWHM) passing through thin films (âŒ100 Ă ) of molecular solids is measured as a function of electron energy in the range 0â15 eV. The visibility of sharp maxima in the transmission spectra is enhanced by measuring the negative value of the second energy derivative of the transmitted current with respect to electron energy. In benzene, fluorobenzene, pyridine, benzaldehyde, furan, cyclohexene and 1,3â and 1,4âcyclohexadiene, the spectra exhibit from six to eight maxima, whereas in pyrrole and thiophene four structures are resolved. With the exception of the first two features in fluorobenzene and the first feature for the other solids investigated, all other maxima correlate within at least Â±0.5 eV with the gasâphase electronic transitions. This correlation is achieved by increasing the energy scale of the transmission spectra relative to the gasâphase electronic energy levels. These findings corroborate previous experiments on solid organic films by Hamill and coâworkers who attributed peaks in the first energy derivative of the transmitted current to energyâloss events. It is shown that the transmission features can arise either from an increase in the overall transmission coefficient due to the production of inelastically scattered electrons or from the formation of an electronâexciton complex through the temporary capture of an injected excess electron by field of a Frenkel exciton. The first lowâenergy feature in each spectrum may possibly arise from an enhancement of vibrational excitation related to the energy dependence of the structure factor.

Mechanism of thermal electron attachment in O_{2}âC_{2}H_{4}, O_{2}âCO_{2}, and O_{2}âneopentane mixtures
View Description Hide DescriptionThermal electron attachment of O_{2} in O_{2}âC_{2}H_{4}, O_{2}âCO_{2}, and O_{2}âneopentane mixtures has been investigated at room temperature, using a microwaveconductivity technique combined with pulse radiolysis. The measurements have been extended to a higher pressure region (âŒ850 Torr) than previous observations in order to compare them with the results of an electron swarm method at very high pressures. From low pressure data, the values of (2.0Â±0.3) Ă10^{â30}, (3.2Â±0.3) Ă10^{â30}, and (7Â±1) Ă10^{â30} cm^{6}/molecule^{2}âsec are determined for the overall three body attachment rate constants of O_{2} with the stabilizing partners C_{2}H_{4}, CO_{2}, and neopentane, respectively. In each case, the effective rate constant continued to increase with increased density and exceeded those predicted by the BlochâBradbury mechanism by sizeable amounts. The excess attachment is suggested to involve preâexisting van der Waals complexes such as (O_{2}â C_{2}H_{4}). Some quantitative conclusions which follow from this mechanism are given.

Test of meanâfield behavior by light scattering in three phases of a fluid mixture near its tricritical point
View Description Hide DescriptionMeasurements are reported of the angular dependence of intensity and spectral width of light scattered quasielastically from three coexistent liquid phase Î±, ÎČ, Îł near a tricritical point. From the data, the susceptibilities Ï and correlation lengths ÎŸ in each of the phases were calculated. Two predictions from meanâfield theory were tested, namely ââGriffithsâ first sum ruleââ: Ï_{Î±} ^{1/2}+Ï_{Îł} ^{1/2} =Ï_{ÎČ} ^{1/2} or ÎŸ_{Î±}+ÎŸ_{Îł}=ÎŸ_{ÎČ}, and ââGriffithsâ second sum ruleââ: Ï_{Î±} ^{â1/2}+Ï_{Îł} ^{â1/2}âÏ_{ÎČ} ^{â1/2}â (TâT _{ tr}), or ÎŸ_{Î±} ^{â1}+ÎŸ_{Îł} ^{â1}âÎŸ_{ÎČ} ^{â1} â (TâT _{ tr}). The second sum rule is found to hold within the accuracy of our data. The first sum rule is violated for the correlation lengths. It is found to hold for the susceptibilities, but only after the scattered intensities are properly corrected for differences, between phases, in the derivative of the dielectric constant with respect to order parameter. This derivative was obtained from model calculations of Kaufman and Griffiths. The viscosities of the coexisting phases were measured; combined with the spectral width and correlation length data, they confirmed, with no adjustable parameters, Kawasakiâs equation for the Rayleigh linewidth of a critical fluid.

High magnetic field MĂ¶ssbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues: Theoretical interpretations
View Description Hide DescriptionMĂ¶ssbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (ii) 2âmethylimidazole mesoâtetraphenylporphyrin, and iron (ii) 1,2âdimethylimidazole mesoâtetraphenylophorphyrin, observed in high magnetic fields and over a wide range of temperature, have been examined theoretically in an effort to correlate them with known susceptibility data, and a search for a satisfactory model within the ^{5} D term has been carried out. It is shown in a general way that any solution within ^{5} D would demand the use of magnetic hyperfine coupling constants about half as large as the smallest previously observed. Even allowing this anomaly we have failed to find a solution within ^{5} D. An interesting result is that the myoglobin appears to have a singlet ground state while the 2âmethyl substituted model has a nearly degenerate ground doublet, approximately describable as S _{z}=Â±2. Yet the hyperfine fields of these materials are very similar at elevated temperatures. It appears that the introduction of low lying states of different spin multiplicity will be needed to accommodate this behavior and to provide the observed magnitude of magnetic hyperfine coupling.

Intramolecular rate processes in highly vibrationally excited benzene
View Description Hide DescriptionIntracavity cw dye laser techniques incorporating photoacoustic detection have been used to measure visible absorption spectra of gas phase benzene (C_{6}H_{6} and deuterated analogues C_{6}H_{5}D, pâC_{6}H_{2}D_{4}, and C_{6}HD_{5}). The prominent spectral features are attributable to CH stretch overtones that correspond to highly localized (ââbondâselectiveââ) excitations of individual CH oscillators. These features have broad (âŒ100 cm^{â1} FWHM) Lorentzian line shapes that narrow as a function of increasing vibrational energy for most isotopic species. A simple model is presented to interpret the broad line shapes and their energy dependence as a manifestation of intramolecular VâVâČ (vibrationâtoâvibration) energy redistribution and/or dephasing processes that occur rapidly (on 5Ă10^{â14} sec time scales), but nonstatistically, in isolated benzene and other polyatomic molecules.

A study of the ground states and ionization energies of H_{2}, C_{2}, N_{2}, F_{2}, and CO molecules by the variational cellular method
View Description Hide DescriptionSelfâconsistent calculations for the ground state potential curves and ionization energies are reported for the molecules H_{2}, C_{2}, N_{2}, F_{2}, and CO, using the recent proposed variational cellular method. For all these molecules the results are quite good. The calculated binding energies, equilibrium interatomic distances, and energy ionization spectra are in excellent agreement with the available experimental findings.

Studies of H^{+}, H^{+} _{2}, and H^{+} _{3} dissociative ionization fragments from methane, ethane, methanol, ethanol, and some deuterated methanols using electronâimpact excitation and a timeâofâflight method incorporating mass analysis
View Description Hide DescriptionKinetic energy distributions and relative intensities of energetic (âł1eV) H^{+},H^{+} _{2}, and H^{+} _{3}dissociativeâionization fragments from methane, ethane, ethanol, and methanol have been measured using pulsed electronâimpact excitation and a timeâofâfight method incorporating a new mass analysis technique. The various deuterated analogs of H^{+}, H^{+} _{2}, and H^{+} _{3} fragments were also observed from CH_{3}OD, CD_{3}OH, and CD_{3}OD. Results for these deuterated fragments show that energetic H^{+} _{2} and H^{+} _{3} fragments originate primarily from the âCH_{3} group, rather than from H atom extraction from both theâCH_{3} and âOH groups. Comparisons among the kinetic energy distributions of these light fragments from CH_{3}OH, CD_{3}OD, and CD_{3}OH reveal no major isotope effects for fragment energies exceeding 2 eV.

Resonance fluorescence as a probe to elucidate mechanisms of intramolecular relaxation processes
View Description Hide DescriptionDensity matrix formalism is used to study resonance fluorescence processes of isolated gaseous molecules. The WangsnessâBlochâFano theory is applied to the irreversible intramolecular relaxation process. In the present treatment, it is assumed that the rotational angular momentum J is a constant of motion in the zeroth order approximation. Thus intramolecular interactions can be classified as tensor operators with respect to J. The fluorescence decay patterns depend on excitation bandwidths and monitoring conditions. Under narrow band excitation, the population (monopole), orientation (dipole), and alignment (quadrupole) of the excited state density matrix Ï_{ e } can be prepared. For a sharply prepared angular momentum state under pulsed excitation, the population does not undergo radiationless decay and the resonance fluorescence intensity is a sum of three exponentials in general. For cw experiments, where lifetime limited linewidth or polarized fluorescence intensities are measured, the population can decay via an intramolecular relaxation channel providing the orientation component is initially prepared. Explicit J dependences of lifetimes are given for pulsed experiments. Under broad band excitation, quantum beats due to the coherence of Ï_{ e } are derived from the Liouville equation. Most important of all, the population can decay under pulsed, broadâband excitation conditions. The scalar part of the nonadiabatic coupling becomes an additional contributor to the intramolecular relaxation process in this last case.