Volume 71, Issue 8, 15 October 1979
Index of content:

The effect of phonon scattering upon resonance Raman scattering: The use of monochromatically excited resonant light scattering as a probe of ultrafast dephasing and relaxation in the condensed phase
View Description Hide DescriptionThe effects of excited state relaxation and dephasing upon resonance Raman scattering from a molecule in the condensed phase are considered from the perspective of a simple scattering event model, in the weak field limit. The model contains the minimum interactions necessary to produce dephasing of monochromatically excited resonant light scattering (MERLS). Emphasis is placed on how the different relaxation and dephasing processes determine the coherenceproperties of MERLS, from localized electronic states of a molecule perturbed by the environment. We consider how MERLS can be used to probe the line broadening mechanisms of vibronic absorption bands and associated phonon side bands. A comparison is made between the dephasingeffects observed by the time dependent coherent spectroscopic techniques (e.g., photon echoes) and those detectable by spectral analysis of MERLS.

Order–disorder phase transitions in NH_{4}Br_{ x }Cl_{1–x } crystals
View Description Hide DescriptionSingle crystals of NH_{4}Br_{ x }Cl_{1–x } have been investigated ultrasonically over a wide range of pressure, temperature, and composition. Using the elastic constant c _{44} as a convenient probe, we have established the p–T–xphase diagram in the region where the orientations of NH_{4} ^{+} ions undergo long‐range ordering. Two critical surfaces exist: one corresponding to ’’ferro’’ ordering and one corresponding to ’’antiferro’’ ordering. These surfaces intersect in a line of triple points closely related to the bicritical points in magnetic spin–flop systems. The adiabatic elastic constants c _{11} and C′ (as well as the acoustic absorption associated with these waves) have also been measured along several isobaric paths. The results are consistent with the existence of a master p–Tphase diagram valid for all values of x.

Hydrogen bonding and order–disorder phenomena in ammonium halide crystals
View Description Hide DescriptionThe ordering of NH_{4} ^{+} ions in CsCl‐type ammonium halide crystals is analyzed in terms of a model involving Ising pseudospin–phonon coupling. Direct interactions between NH_{4} ^{+} ions stablize parallel (’’ferro’’) ordering, and indirect interactions due to spin–phonon coupling stabilize antiparallel (’’antiferro’’) ordering. The strength of this coupling depends on the mass of the halide ion X, the frequency of an M‐point transverse acoustic lattice mode, and the gradient ΔV′ in the potential for the hydrogen bond between NH_{4} ^{+} and X^{−}. All available data for ordering in NH_{4}Cl, NH_{4}Br, NH_{4}I, ND_{4}Cl, ND_{4}Br, NH_{4}Br_{ x }Cl_{1–x }, (NH_{4})_{ y }M_{1–y }Cl, and (NH_{4})_{ y }M_{1–y }Br are used to obtain empirical values of the potential parameter ΔV′. The magnitude of this quantity and the variations in its value with changes in halide ion, pressure, deuteration, and substitution of NH_{4} ^{+} by alkali ions are completely consistent with expectations for systems with weak N–H⋅⋅⋅X hydrogen bonding.

Investigation of the dielectric constants and the diamagnetic anisotropies of cyanobiphenyls (CB), cyanophenylcyclohexanes (PCH), and cyanocyclohexylcyclohexanes (CCH) in the nematic phase
View Description Hide DescriptionWe have measured the dielectric constants of cyanobiphenyls (CB), cyanophenylcyclohexanes (PCH), and cyanocyclohexylcyclohexanes (CCH) using a capacitance bridge with a dielectric cell having a guard ring. The ratio Δχ/Δε was also measured for these materials using a new technique which is applicable when Δχ is small or even negative. As a surprising result we have found that this ratio is almost independent of temperature. The temperature dependence of the diamagneticanisotropies and the dielectric constants of these compounds are given and the results are discussed in terms of the molecular properties.

Study of electronic properties of the SF_{6} molecule by (e,2e) spectroscopy
View Description Hide Description(e,2e) spectroscopy has been used to obtain information on the electronic properties of the SF_{6} molecule, in particular on the valence states. The ordering of the orbitals given by ESCA spectroscopy has been confirmed. The measured electron momentum distributions are compared with the ones calculated from literature wave functions and show a poor agreement. Namely, it is shown that the minimum energy criterion that is successful in calculating the energy eigenvalues is not always sufficient to determine a good wave function basis set suitable to reproduce other chemical‐physics properties. Bonding character of the three outermost orbitals is evidenced well by their momentum distributions; two configuration interaction peaks following the valence orbitals are characterized as well.

Sub‐nanosecond time resolved transient photoconductivity in anthracene
View Description Hide DescriptionObservations are reported of current, time resolved to 800 psec, in a measurement of transient photoconductivity in anthracene. These measurements indicate that the transient currents associated with photogeneration, polaron formation, and hot carrier motion are over within 800 psec of the light illumination. These measurements also have revealed a new transient photoconductivity effect, a current peak whose duration equals the duration of the laser illumination. Possible origins of the current peak are considered.

Alkali halide photofragment spectra. I. Alkali iodide bond energies and excited state symmetries at 266 nm
View Description Hide DescriptionPhotofragment spectroscopy studies of the five alkali iodides have been done at 266 nm. For the four heavier iodides exclusive production of excited I^{2} P _{1/2} atoms is found; for LiI, both ground and excited I atoms are produced, with ground state atoms favored two to one. From the measured alkali atom recoil time‐of‐flight distributions the following bonddissociation energies (D ^{0} _{0}) in kcal mol^{−1} are determined: LiI 81.6±1.0; NaI 71.8±0.5; KI 76.2±0.5; RbI 75.3±0.5; CsI 79.7±0.5. No evidence of dimer photodissociation is found. From recoil angular distributions the percentage parallel character of the transitions at 266 nm is found to be 60±4, 54±2, 51±2, and 51±2 for NaI,, KI, RbI, and CsI, respectively. The transition in LiI leading to ground state I atoms is 29%±3% parallel. Implications of these and other results for the nature of the first and second excited states of the alkali iodides are discussed.

Shock compression data for liquids. I. Six hydrocarbon compounds
View Description Hide DescriptionDynamic high pressure equation‐of‐state data are reported for 1,3‐ and 1,4‐cyclohexadiene, cyclohexene, cyclohexane, toluene, and n‐hexane initially in the liquid state. Plane shock waves generated by high explosives produced pressures of 0.5 to 43.0 GPa and densities to twice normal density. Toluene and n‐hexane exhibit transformations at 12.6 and 19.0 GPa, respectively, but cyclohexadiene, cyclohexene, and cyclohexane do not. Decomposition of the molecule is the probable cause for the transition. Results indicate that reducing the number of double bonds by adding hydrogen pairs to the benzene molecule causes the transition to disappear. The shock velocity−particle velocity (u _{ s } −U _{ p }) data for toluene, toluene, cyclohexadiene, and n‐hexane are best represented by one or more line segments while cyclohexene and cyclohexane are fit by a quadratic in particle velocity. An extrapolation of the U _{ s }−U _{ p } curves to zero particle velocity result in a U _{ s } value for each liquid that is approximateley 40% higher than the bulk sound speed. The specific volume at a particular pressure for these liquids is ordered such that the hydrocarbon with the larger hydrogen‐to‐carbon ratio has a larger specific volume. This behavior is similar to that observed for other hydrocarbons.

Theory of the Rydberg spectrum of triatomic hydrogen
View Description Hide DescriptionEnergies and properties of the fifteen lowest Rydberg states of triatomic hydrogen are computed using Koopmans theorem with a large Gaussian basis set constructed to represent both core and Rydberg orbitals. Tests indicate that computed virtual orbital energies have converged to within a few wave numbers of the s p d SCF limit. All excited electronic states are computed to have geometries and vibrational force constants close to those of H^{+} _{3}. Jahn–Teller effects are weak. Small quantum defects and nearly integer values of electronic angular momenta indicate atom‐like character for the Rydberg states. Predicted vibronic transitions are in remarkably close agreement with rotational band spectra recently observed by Herzberg. Results using our model are in complete accord with Herzberg’s assignment of the strong emission bands observed near 5600 and 7100 Å, and offer a reasonable interpretation of the previously unassigned, weak emission bands observed near 6200 and 8500 Å.

Band structure in the C–H stretching region of the Raman spectrum of the extended polymethylene chain: Influence of Fermi resonance
View Description Hide DescriptionThe C–H stretching region of the Raman spectrum of the extended polymethylene chain is unexpectedly complex. This complexity is derived in large part from Fermi resonance interaction between the symmetric C–H stretching fundamental and a near continuum of overtones of the methylene &HCH scissors fundamentals. The Fermi resonance interaction equation is set up and solved for this system. Two unknown parameters are involved: the Fermi resonance interaction constant for an isolated methylene group and the frequency of the unperturbed symmetric C–H stretching fundamental. With adjusted values for these parameters and a reasonable form for the &HCH scissors dispersion curve, the observed positions and relative intensities of bands associated with the stretching fundamental can be accounted for quantitatively.

Vibrational correlation splitting and chain packing for the crystalline n‐alkanes
View Description Hide DescriptionVibrational correlation‐splitting patterns for the infrared active methylene rocking–twisting fundamentals of the orthorhombic and monoclinic n‐alkanes have been determined and analyzed.Polarization measurements show that the splitting patterns are different for the two structures. A simple two parameter coupled oscillator model accounts for the observed shapes of the c axis dispersion curves. It is suggested that the results of this analysis may be useful in establishing the type of packing involved in crystals containing long‐chain hydrocarbon molecules.

Generalized Langevin theory for gas–solid processes: Continuum elastic treatment of surface lattice dynamics
View Description Hide DescriptionModel studies of the systematics of elementary atom–solid energy exchange processes are presented. The studies are based on the generalized Langevin equation (GLE) classical trajectory method [S. A. Adelman and J. D. Doll, J. Chem. Phys. 64, 2375 (1976)] and on a full isotropic continuum elastictreatment of both bulk and surface solid atom velocity response functions ? (t). Within both bulk (BEM) and surface(SEM)elasticmodels, the solid dynamics is parameterized by two bulk properties, a Debye frequency ω_{ D } and a transverse to longitudinal sound velocity ratio R _{ s }. The Debye model used in earlier GLE simulations is a specialization of the BEM,R _{ s }=1.0, and thus does not include surface effects, e. g., Rayleigh waves, accounted for in the SEM. The main results of the trajectory studies are as follows: Gas–solid energy transfer efficiency within the BEM depends very sensitively on R _{ s } (for fixed ω_{ D }). For physical R _{ s } values ∼0.2–0.6, BEMenergy transfer is often much larger than Debye energy transfer.SEMenergy transfer is relatively insensitive to R _{ s } and is, moreover, dominated by the Rayleigh mode contribution to ? (t). The SEMenergy transfer is often fortuitously of comparable magnitude to Debye energy transfer.

Reorientational motions in compressed viscous fluids: Selectively deuterated glycerol
View Description Hide DescriptionThe pressure and temperature dependence of the deuteron NMR spin–lattice relaxation timeT _{1} have been measured in liquid glycerol‐d _{4}, (D_{2}COH)_{2}CHOH, and glycerol‐d _{3}, C_{3}H_{5}(OD)_{3}, within the pressure range 1 bar to 5 kbar from −10° to 125 °C. The experimental deuteron relaxation data which reflect intramolecular reorientational motions are interpreted in terms of a Cole–Davidson distribution of correlation times. Within experimental error the distribution width β=0.44 is found to be independent of temperature and pressure. A single correlation time model and a model allowing for internal and overall reorientation fail to explain the experimental data without introducing an arbitrary adjustable parameter. The reorientational motions of the carbon backbone and of the hydroxyl groups are strongly correlated reflecting the effect of hydrogen bonds. By comparing the magnitudes of the T _{1} values at the minimum of the T _{1} vs T plots in glycerol‐d _{4} and glycerol‐d _{3} the deuteron quadrupole coupling constant is found to be 202 kHz for the hydroxyl deuteron. The dependence of the average correlation time ?_{2} upon viscosity and the ?_{2} variation with temperature and density is explained on the basis of a Debye type two term phenomenological equation. A discussion of the experimental data in terms of this equation suggests that the extent of the hydrogen bond network in liquid glycerol is relatively insensitive to density changes but very much reflects the temperature effects. The temperature and pressure dependence of the average correlation time ?_{2} suggests that the reorientation of the glycerol molecule proceeds via a large‐angle jump diffusion mechanism. A detailed comparison of the present results with the conclusions of other NMR studies is presented.

Correlation function modeling via the third‐order memory function: Application to ethane
View Description Hide DescriptionIn order to understand the vibrational correlation function of the ν_{3} band of liquid C_{2}H_{6}, it has been necessary to include third‐order memory functions in the modeling procedure. The analysis indicates that the vibrational dephasing process may be a complex one involving both the spinning motion around the threefold symmetry axis and the tumbling motion of the axis itself. Information about the molecular dynamics in the temperature range 93–168 K has been obtained out to 7 ps.

Mixtures of hard spheres in the Percus–Yevick approximation. Light scattering at finite angles
View Description Hide DescriptionIn a previous paper (I), a new equation for the light scattering (or small angle x‐ray or neutron scattering) of a concentrated p‐component mixture of spherical (colloidal) particles in a low‐molecular weight solvent was derived. Use was made of Baxter’s factorization of the direct correlation matrix. It was found that the light scattering intensity can be formulated in factorized form as well. The formalism was applied to a multicomponent system of hard spheres treated in the Percus–Yevick approximation. For zero scattering angle, a rather simple, exact expression was obtained. In this paper it is proved that a closed expression can also be obtained for finite scattering angles. It contains at most 18 (averaged) functions of the scattering angle for any number of hard sphere components. This makes it possible to apply the equation to a continuous distribution of hard sphere diameters. A series expansion is given for small scattering wave numbers.

Potential energy curves for NO^{+}
View Description Hide DescriptionIn recent years, high‐resolution photoelectron spectroscopy and a b i n i t i o calculations have considerably revised and enlarged the understanding of the electronic structure of the NO^{+} ion. Based on these results, new potential energy curves for the electronic states of NO^{+} below 24 eV have been constructed and are given here. RKR potentials were calculated from new molecular constants for the eight experimentally well‐defined states. For the other states, the main features of the potentials were estimated from theoretical calculations and from isoelectronic analogies. A brief review is given of the information on which each potential energy curve is based.

Rate constants for the reactions of metastable NO^{+} (a ^{3}Σ^{+}) ions with SO_{2}, CO_{2}, CH_{4}, N_{2}, Ar, H_{2}, D_{2}, and O_{2} at relative kinetic energies 0.04–2.5 eV
View Description Hide DescriptionRate constants for the reactions of NO^{+}(a ^{3}Σ^{+}) ions with SO_{2}, CO_{2}, CH_{4}, Ar, N_{2}, H_{2}, D_{2}, and O_{2} have been measured for relative kinetic enegeies 0.04–2.5 eV in a flow‐drift tube. Most of these reactions are fast and show little kinetic energy dependence. The reaction with Ar is slightly endothermic and its rate constant increases substantially with increasing energy. For the reactions with SO_{2}, CO_{2}, CH_{4}, and Ar, the reactive channel, largely charge transfer, predominates over quenching. For N_{2}, the reactive and quenching channels are appriximately equal. Lastly, for H_{2}, D_{2}, and O_{2}, quenching predominates, except at higher energies for O_{2}, where the two channels are about equal.

On the product ratios of the reactions of Ar^{+} with NO and NO^{+} (a ^{3}Σ^{+}) with Ar at 300 K
View Description Hide DescriptionThe reaction of Ar^{+} with NO to yield the possible ground‐state NO^{+}(X ^{1}Σ^{+}) and metastable‐state NO^{+}(a ^{3}Σ^{+}) product ions has been studied near 300 K in a flow‐drift tube using helium and argon buffer gases separately. The ’’apparent’’ rate constant obtained in the argon buffer is an order of magnitude smaller than the rate constant obtained in the helium buffer. The difference in the rate constants is due to the ’’back’’ reaction of the NO^{+}(a ^{3}Σ^{+}) product ions with the argon buffer‐gas atoms to yield the possible product ions NO^{+}(X ^{1}Σ^{+}) and Ar^{+}. The ratio of the rate constants places precise limits on the product ratios in both cases; namely, the Ar^{+} + NO reaction yields greater than 85% NO^{+}(a ^{3}Σ^{+}) ions and the ratios NO^{+}(a ^{3}Σ^{+}) + Ar reaction yields greater than 85% Ar^{+} ions. The data also show that the rate constant for the quenching of NO^{+}(a ^{3}Σ^{+}) ions by NO is (8.0±4.0) ×10^{−10} cm^{3}/s at 300 K.

Production of NO^{+}(a ^{3}Σ^{+}) ions in the reaction of N^{+} ions with O_{2} at 300 K
View Description Hide DescriptionIt is well known that NO^{+} ions constitute about 45% of the product ions of the ionospherically important reaction of N^{+} ions with O_{2}. However, NO^{+} ions in both the ground X ^{1}Σ^{+} state and the metastable a ^{3}Σ^{+} state are energetically possible. The present study uses a selected‐ion flow tube to determine the percentage of NO^{+}(a ^{3}Σ^{+}) product ions. It is known from separate studies that NO^{+}(a ^{3}Σ^{+}) ions react rapidly with N_{2} to form N_{2} ^{+}; therefore, the addition of N_{2} and the occurrence of N_{2} ^{+} ions provide a monitor for the possible NO^{+}(a ^{3}Σ^{+}) product ions from the N^{+} reaction with O_{2}. It is found that NO^{+}(a ^{3}Σ^{+}) production accounts for less than 2% of the total rate constant at 300 K.

Molecular dynamics study of a polymer chain in solution
View Description Hide DescriptionThe behavior of a polymer chain in solution has been simulated by means of molecular dynamics techniques. The polymer itself is represented by a freely linked chain of hard spheres and the solvent by a hard sphere fluid. Both the equilibrium configurational properties of chains of various lengths and their time‐dependent autocorrelation functions are studied. The equilibrium behavior agrees with the results of previous work. The decay of the autocorrelation functions is exponential over a considerable range and the relaxation times, thus defined, appear to scale with the chain length. The effect of the solvent density is also examined.