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Volume 72, Issue 12, 15 June 1980

Recoil ^{18}F chemistry. XI. High pressure investigation of 1,1‐difluoroethane
View Description Hide DescriptionNuclear recoil ^{18}F reactions in CH_{3}CHF_{2} have been investigated throughout the effective pressure range 0.3–190 atm. The principal reaction channel is F‐to‐HF abstraction for which the combined yield from quasithermal and energetic processes in the presence of 5 mole% H_{2}S additive is 83.4%±0.2%. A reaction mechanism is proposed that involves the organic product forming channels F‐for‐F, F‐for‐αH, F‐for‐βH, F‐for‐ĊH_{3} and F‐for‐ĊHF_{2}. The results are compared with those reported for the ^{18}F+CH_{3}CF_{3} system.

Thermodynamics of sulfur chemisorption on metals. I. Alumina‐supported nickel
View Description Hide DescriptionSulfur chemisorption isosteres have been measured for nickel in powdered form and for nickel supported on two different alumina powders. The experiments were conducted in a closed‐loop gas recirculation system containing one atmosphere hydrogen. Isotherms were determined by stepwise injection of H_{2}S aliquots into recirculating hydrogen gas and analyzing for the H_{2}S concentration as equilibrium was approached. Isosteres were measured by varying the sample temperature and monitoring the H_{2}S/H_{2} ratio in the gaseous environment. A gas chromatograph and a photoionizationdetector was used to measure the H_{2}S concentration to levels below 1 ppb. As monolayer coverage is approached the H_{2}S/H_{2} ratio attains the equilibrium values reported for the bulk sulfide, Ni_{3}S_{2}. Adsorbed sulfur is very strongly bound to the surface of nickel. The heat of formation of chemisorbed sulfur with respect to 1/2 S_{2}(g) at 800 K is 247 kJ mol^{−1} more negative than the heat of formation of Ni_{3}S_{2}. The heat of segregation exceeds 190 kJ mol^{−1}. The results demonstrate that the equilibrium sulfur coverage exceeds one half monolayer on Ni/Al_{2}O_{3} unless the H_{2}S/H_{2} ratio is less than 1 ppb even at temperatures as high as 940 K.

Picosecond study of electron ejection in aqueous phenol and phenolate solutions
View Description Hide DescriptionThe photoionization of aqueous phenol and phenolate solutions was investigated using picosecond absorption spectroscopy. Phenol and phenolate were excited in the first singlet excited state using a single picosecond pulse of 27 ps duration, at 265 nm (4th harmonic of a Nd–YAG laser). Hydrated electron formation was ascertained by following the kinetics of formation and disappearance at 630 nm with a picosecond pulse obtained by stimulated Raman scattering(SRS). Similarly to the ferrocyanide ion, the excited phenolate ion undergoes a very fast electron ejection process. The lifetime of this process is shorter than the time resolution of our experimental device. In the excited phenol molecule, hydrated electron formation is retarded, the delay between the half‐rise times of the exciting pulse and the electron absorption signal being (12±1) ps. This delay is not reduced at low pH. These observations and the high yield of ̄e _{aq}formation are best explained by a consecutive two photon process, the second photon being absorbed by the excited singlet state S _{1} and the deprotonation occuring after ejection of the electron. The kinetics of hydrated electron disappearance with NO_{3} ^{−} as scavenger, are in excellent agreement with literature values. Using H_{3}O^{+} and Cd^{++} to scavenge the hydrated electron, its initial formation yield remains constant, showing that Cd^{++} does not react with any hydrated electron precursor. This difference between our results and those obtained in picosecond pulse radiolysis shows that the precursor of the hydrated electron is not the same in the two techniques. The precursor obtained in pulse radiolysis appears to be more reactive with scavengers. A possible explanation is that after phenolate or phenol excitation, the resultant internal state of the aromatic molecule is capable of undergoing electron transfer to the adjacent solvent molecules, thus producing at least partially hydrated electrons directly.

Surface enhanced Raman spectra: A critical review of the image dipole description
View Description Hide DescriptionRecent experiments have demonstrated enhancements of several orders of magnitude for Raman scattering of pyridine and other molecules adsorbed on a silversurface. One explanation which has been proposed is an image dipole model, in which a point dipole is mirrored in the metal, with the mirror image inducing an additional dipole in the adsorbed species and thus changing its effective polarizability α_{eff}. In this paper we suggest that the large enhancements obtained from such a point dipole description result from the use of an symptotic theory at unphysically short distances. We demonstrate this through a model calculation of α_{eff} of a hydrogen atom near a mirrorsurface. A point dipole approximation gives a divergent α_{eff} near a critical distance R _{ c } from the surface, while a more exact self‐consistent Hartree calculation (including the finite extent of the hydrogen charge cloud) gives only a slight enhancement or even a de‐enhancement near R _{ c }.

Convergence of a perturbation technique for evaluating isotopic partition function ratios
View Description Hide DescriptionSchwinger perturbation theory for partition functions is developed in a form which makes practical the numerical evaluation of higher order terms. The vibrational Hamiltonian of a molecule in the harmonic approximation, H=(1/2)∑(g_{ i j } p _{ i } p _{ j }+f _{ i j } q _{ i } q _{ j }), has been partitioned into an unperturbed Hamiltonian (the diagonal part, terms with i≠j) and a perturbation term (the off‐diagonal part, terms with i=j). The perturbation theory technique up to fourth order is used to calculate vibrational partition functions for a number of molecules. Vibrational partition functions so calculated are employed to calculate reduced partition functions and reduced isotopic partition function ratios. The results obtained by perturbation theory are compared with those of exact calculations carried out by actually obtaining the normal mode vibrational frequencies of the vibrational Hamiltonian. The previously observed good agreement, especially for the reduced isotopic partition function ratios, between perturbation theory in second order and exact calculations, becomes usually even better in fourth order perturbation theory.

Molecular dynamics simulation of submonolayer krypton films on graphite
View Description Hide DescriptionMolecular dynamics simulations of submonolayer krypton films on graphite show a first order transition from a low temperature √3×√3 registered phase to a disordered one which is fluid and not localized in the graphite wells (not lattice–gas‐like). This contrasts with simulations in the absence of a substrate ordering potential. There the transition is continuous.

The electronic structure of the lithium trimer anion and cation
View Description Hide DescriptionSCF and SCF–CI calculations have been performed on Li_{3}, its anion and cation. We predict a value of 1.1 eV for the adiabatic electron affinity. The Li^{−} _{3}bond energy is found to be 0.9 eV versus the 0.4 eV energy required to dissociate Li_{3} to dimer and atom. Thus, the bond strength of the anion considerably exceeds that of the neutral parent. The difference in the nature of electron binding to Li_{2} and Li_{3} can be understood through modification of simple MO concepts. The energy required to dissociate Li_{3} ^{+} to Li_{2} ^{+}+Li is found to be 1.28 eV. Hence, the bond strength of the cation also greatly exceeds that of the neutral. The Li_{3} ^{+}bond energy is comparable to the Li_{2} ^{+}dissociation energy (1.24 eV). The adiabatic ionization potential is found to be 3.95 eV. The combination of the present study and those of previous researchers indicates that the Li_{3}surface is weakly varying with bond angle and may be characterized by multiple minima. The current results for Li_{3} and Li_{3} ^{+} are discussed in the light of recent mass spectrometric studies of the trimer.

Distribution function of the end‐to‐end vector for short polymer chains
View Description Hide DescriptionA method is presented for the evaluation of the end‐to‐end vector distribution function for short polymer chains. The method is based on the expansion of this quantity in terms of a spherical harmonic series and is related with a very efficient algorithm recently proposed by Fixman e t a l. for the calculation of high moments. Numerical results are reported for a rotational isomeric representation of short polymethylene chains. The results are compared to those obtained earlier by other authors with an alternative procedure and the advantages of the new scheme are stressed.

Highly excited vibrational states of molecules by thermal lensing spectroscopy and the local mode model. I. CHCl_{3}, CHBr_{3}, CH_{2}Cl_{2}, CH_{2}Br_{2}
View Description Hide DescriptionVisible absorption spectra (14 700–19 100 cm^{−1}) measured with the thermal lensing spectrometer and near‐infrared absorption (5 600–14 000 cm^{−1}) measured with a conventional spectrophotometer are reported for CHCl_{3}, CHBr_{3}, CH_{2}Cl_{2}, and CH_{2}Br_{2}. Relatively strong absorption peaks are identified as overtones of the C–H stretching vibrations in these compounds. The overtone spectra are analyzed in terms of the local mode (LM) model, which treats the molecule as a set of loosely coupled anharmonic oscillators localized on individual C–H bonds. Relatively less intense peaks are observed and are assigned as combinations of a local mode C–H vibration and some lower frequency (normal) mode of the molecule. Fermi resonance is seen to occur in the spectra of CHCl_{3} and CHBr_{3}, resulting in anomalously high intensities for the combination bands involved. Significant anharmonic local–normal coupling constants are seen for these same molecules. These coupling constants appear to indicate the presence of strong physical coupling of the two motions involved. The importance of these coupled motions in the intramolecular relaxation of vibrational energy is suggested.

A b i n i t i o study of the vibrational dependence of hyperfine coupling constants in the methyl, silyl, and formaldehyde anion radicals
View Description Hide DescriptionThe influence of vibrational effects on the isotropic hyperfine coupling constants is studied by a b i n i t i o methods for the CH_{3}, SiH_{3}, and H_{2}CO^{−} radicals. The calculations are carried out using double‐zeta plus polarizationbasis sets of contracted Gaussian orbitals. A spin‐restricted SCF plus perturbative configuration interaction method including all spin adapted configurations with three uncoupled electrons is employed. The perturbation treatment is extended to second order which yields a definitive improvement of the results over previous first order calculations. A quantum mechanical treatment of the vibrational problem shows that the influence of the nuclear motion depends on the shape of the potential. The vibrational effects are found necessary to account for the experimental findings and the overall results are in very good agreement with experiment. It is found that the usual classical approach fails to reproduce the correct behavior of the splittings, especially for low temperature experiments.

Bethe surface and Compton profile of NH_{3} obtained by 35 keV electron impact
View Description Hide DescriptionComplete inelastic electron impact spectra have been obtained for NH_{3} for a momentum transfer range from 0.4 to 12.5 a.u. using 35 keV electrons. These spectra were converted to relative generalized oscillator strengths (GOS), were placed onto an absolute scale by the Bethe sum rule, and higher order sum rules were used to test the accuracy of the GOS. The Compton profile (CP) of NH_{3} was determined from the GOS by means of the impulse approximation (IA) and it was found that the CP’s were asymmetric and shifted in respect to the free electron theoretical CP. This Compton defect was analyzed in detail and was shown to be due to the failure of the IA. The area under the CP is discussed in terms of the x‐ray incoherent scattering factor and experimental results for the valence, inner shell, and total CP’s are compared to several theoretical profiles.

Vibrational study of phase transition of solid malononitrile
View Description Hide DescriptionSolid phases and phase transitions of crystals of malononitrile, CH_{2}(CN)_{2}, have been studied by Raman and infrared spectroscopy. Among twelve Raman active lattice modes of T′ phase a soft mode has been identified and assigned to a R′_{ b } libration. The T′–F–Ttransitions can be considered as second order and of displacive type since the soft mode frequency decreases continuously to zero at T _{ c } when the transition temperature is approached from either above or below. Critical exponent β∼1/2 has been determined from the soft mode frequency versus T _{ c }‐T plot. Half‐width of the soft mode band has also been studied as a function of temperature and relaxation times have been extracted from the corresponding curves. The stable low temperature phase L has been characterized spectroscopically: unlike T′, F, T phases it contains CH...N hydrogen bonds. The behavior of its lattice frequencies with temperature indicates that L–Fphase change is a first order transition.

CN(A ^{2}Π_{ i }→X ^{2}Σ^{+}) and CN(B ^{2}Σ^{+}→X ^{2}Σ^{+}) yields from HCN photodissociation
View Description Hide DescriptionThe cross sections for the production of CN(A ^{2}Π_{ i }→X ^{2}Σ^{+}) and CN(B ^{2}Σ^{+}→X ^{2}Σ^{+}) emissions from HCN photodissociation are measured using synchrotron radiation in the 1050–1550 Å region. The photoabsorption cross section of HCN is also measured, and the quantum yields for the emissions are determined. The spectra for the photoabsorption and photoemission cross sections are interpreted with the known excited states of HCN. Three new excited states are found from the CN emission quantum yields. It is observed that the vibrational population of the CN(B ^{2}Σ^{+}) photofragments produced by photodissociating HCN at Rydberg states are more vibrationally excited than those produced at other dissociative states. The photodissociation processes for producing CN emissions at various excited states are discussed.

Some relations connecting volume and enthalpy relaxation in the order parameter model of liquids and glasses
View Description Hide DescriptionA general inequality connecting linear enthalpy and volume relaxation is derived in the context of the order parameter model of liquids and glasses. This inequality is shown to yield a wide variety of relationships among the quantities characterizing these relaxations.

Intermolecule vibrational energy transfer dynamics in IR laser pumped SO_{2}/^{18}O_{2} mixtures
View Description Hide DescriptionIntermolecule vibrational energy sharing in SO_{2}/^{18}O_{2} mixtures has been observed by monitoring fluorescence emission from the SO_{2} ν_{3} state following pulsed CO_{2} laser excitation of the ν_{1} mode. The rate constant for the vibrational energy crossover step from the SO_{2} ν_{3} state to the v=1 level of ^{18}O_{2} has been determined to be 5.74 (±0.91) msec^{−1} (Torr ^{18}O_{2})^{−1} or approximately 1400 gas kinetic collisions. The present results are compared with an earlier ultrasound investigation of vibrational relaxation in the SO_{2}/^{16}O_{2} system. Theoretical probabilities of intermolecule energy transfer due to short‐range repulsive forces and long‐range dipole–quadrupole forces have been calculated for the SO_{2}/^{18}O_{2} and SO_{2}/^{16}O_{2} systems. Comparison of experimental and theoretical results suggests that both short‐range and long‐range forces contribute to the intermolecule vibrational energy process in SO_{2}/^{18}O_{2} mixtures while only short‐range forces appear to be important in the SO_{2}/^{16}O_{2} exchange. A general technique has been developed for determining the error in kinetic rate constants from the experimental uncertainty of the measured fluorescenceeigenvalues.

Lifetimes and transition moments among excited electronic states of BaO
View Description Hide DescriptionThe lifetime of the BaO C ^{1}Σ^{+} state is investigated for several vibrational and rotational levels at 1 Torr and under molecular beam conditions. A collision free radiative lifetime of 10.5±1 nsec is observed for the v=0 level. Dispersed fluorescence from v=0, 1, and 2 to various low lying electronic states is measured and used to obtain electronic dipole transition moments of : μ^{2} _{C–X}=9.9 D^{2}, μ^{2} _{C–A}=16.8 D^{2}, μ^{2} _{C–A′}=9.8 D^{2}, μ^{2} _{C–b}=3.1 D^{2}, μ^{2} _{A–X}=3.2 D^{2}, μ^{2} _{A′–X}=0.14 D^{2}.

Rotationally inelastic collisions of LiH with He. I. A b i n i t i o potential energy surface
View Description Hide DescriptionThe diagrammatic many‐body perturbation theory is applied through third order in the correlation energy to the interaction potential between He and a rigid LiH molecule. The a b i n i t i o calculations are used to derive an analytic representation of the potential surface in terms of orthogonal polynomials. Several different basis sets are employed to demonstrate the sensitivity of the energies to the computational techniques. The resulting potential surfaces are highly anisotropic with respect to the LiH center‐of‐mass and allow for a weak binding (∼7 meV) of the He to the Li end of the LiH axis.

Rotationally inelastic collisions of LiH with He. II. Theoretical treatment of the dynamics
View Description Hide DescriptionIntegral cross sections for the rotational excitation of LiH (j=0,1, and 2→j′) in collisions with He have been determined at collision energies of 0.2057, 0.3057, and 0.4057 eV. The rigid‐rotor collision dynamics were solved within the coupled states (CS) approximation. The a b i n i t i opotential surface described in the preceding article was used. Sizeable inelastic flux occurs even for large changes in the rotational quantum number, which is probably a reflection of the extreme anisotropy in the potential surface. The dependence of the cross sections on j′, the final rotational quantum number, is not monotonic but displays a pronounced oscillatory structure which is a sensitive function of collision energy. Use of the infinite order sudden approximation results in a disappearance of this oscillatory structure as well as a pronounced lowering in the actual magnitudes of the inelastic cross sections. By contrast excellent agreement with the CS j=1,2→j′ cross sections is provided by sudden limit scaling relations with the CS j=0→j′ cross sections as input. Neither the exponential‐gap nor power‐law parametrizations provide an adequate fit to the CS integral cross sections. The rotationally inelastic cross sections obtained with an electron‐gas potential surface are significantly smaller, because the surface is less repulsive. Thermally averaged cross sections are determined for comparison with the results of the experiment described in the following article.

Rotationally inelastic collisions of LiH with He. III. Experimental determination of state‐to‐state cross sections
View Description Hide DescriptionState‐to‐state rotationally inelastic cross sections for the scattering of ^{7}LiH with helium have been determined by the use of electric quadrupole rotational state selection and dye laserfluorescence detection. Results for the incident j=1 state and final states j′=0, 2–15 at an average collision energy of 0.32 eV are reported. As a result of the finite angular acceptance of the laser detection zone and the observed dependence of the cross sections on the scattering path length l, the present values are lower limits to the absolute integral σ_{ j=1→j′} values. Because of the theoretical simplicity of the LiH–He system, comparison with accurate quantum calculations for the cross sections (see Papers I and II of this series) has been possible. Calculations using the coupled states dynamical approximation with an a b i n i t i opotential surface reproduce well the qualitative behavior of the experimental cross sections. Comparison with results from simpler theoretical models suggests that the accuracy of the potential energy surface employed is crucial for an adequate theoretical description of the dynamics.

A method to reduce the number of two electron integral transformations in a second order multiconfigurational Hartree–Fock procedure
View Description Hide DescriptionIn standard multiconfigurational Hartree–Fock (MCSCF) theory, a two electron integral transformation is performed before application of the equation defining the MCSCF approach in each iterative cycle. It is shown how the two electron integral transformation may be replaced by an approximate orbital transformation introduced directly into the equation that defines the second order MCSCF approach. In this way, the number of two electron integral transformations required to obtain a set of MCSCF orbitals is reduced considerably. Numerical examples for the ^{3}Σ^{−} _{ g }, ^{1}Δ_{ g }, and ^{1}Σ^{+} _{ g } states of O_{2} indicate that an accuracy of 10^{−6} a.u. in the total energy may be obtained by carrying out 2–3 two electron integral transformations, which is about half the number of transformations required to obtain the same accuracy in the second order MCSCF approach. An accuracy of 10^{−10} a.u. is obtained after one further iteration is carried out with a second order MCSCF scheme.