Index of content:
Volume 72, Issue 5, 01 March 1980

Solvation forces in simple dense fluids. I
View Description Hide DescriptionThe grand canonical ensembleMonte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

Time resolved spectroscopy of xenon excimers excited by synchrotron radiation
View Description Hide DescriptionFormation and decay rates of the lowest 0_{ u } ^{+} and 1_{ u } states of Xe_{2}*, excited by monochromatized synchrotron radiation from the Stanford storage ring (SPEAR), have been measured in pure xenon and in xenon–argon mixtures over the pressure range 10^{2} to 10^{4} Torr. The results are interpreted to yield radiative lifetimes (4.6±0.3 and 99±2 nsec, respectively, for vibrationally relaxed 0_{ u } ^{+} and 1_{ u } molecules), vibrational relaxation rates [7×10^{−11} and 6×10^{−12} cm^{3}/sec for Xe_{2}* (1_{ u }) in collisions with xenon and argon, respectively], the Xe_{2}* (0_{ u } ^{+}) three‐body formation rate from Xe(^{3} P _{1}) (5.3×10^{−32} cm^{3}/sec), and rates for 0_{ u } ^{+}–1_{ u } mixing by collisions with xenon and argon.

UV laser photochemistry of CCl_{4} and CCl_{3}F
View Description Hide DescriptionThe ArF (193.3 nm) irradiation of CCl_{4} and CCl_{3}F has been investigated. The laser photolysis of CCl_{4} yields emission from CCl (A ^{2}Δ–X ^{2}Π), CCl_{2}(^{1} B _{1}–X ^{1} A _{1}), Cl_{2}[A ^{3}Π (0_{ u } ^{+})–X ^{1}Σ_{ g } ^{+}], and other systems. A dye laser was used to obtain the fluorescence excitation spectrum of the CCl radical near the 0–0 band center (A ^{2}Δ–X ^{2}Π). The CCl (A ^{2}Δ) radicals are formed by at least a three photon absorption process and laser fluence dependences are reported for various photofragments. The ArF laser photolysis of CCl_{3}F yields results very similar to those of CCl_{4}. Possible mechanistic channels are presented for both CCl_{4} and CCl_{3}F photolysis which are consistent with the observed photofragments, laser fluence dependences, and known thermochemical data.

Perfect pairing valence bond generalization of self‐consistent electron pair theory
View Description Hide DescriptionA generalization of the self‐consistent electron pairs (SCEP) method for correlated wave functions is presented which makes possible the use of multiconfiguration reference wave functions that have the form of a generalized valence bond, perfect pairing (GVB/PP) wave function. All singly and doubly substituted configurations relative to this reference are included, though the sets of configurations arising from a given orbital substitution are restricted to occur in the correlated wave function with the same relative importance as the corresponding configurations in the reference wave function. The computational manipulations of this new procedure are essentially the same as basic SCEP, but the generalization provides for the treatment of systems where a Hartree–Fock reference wave function is inappropriate. These would include i reactive systems that involve changes in bonding and also weakly interacting systems where higher order correlation effects are necessary.

Iodine at high pressures and low temperatures
View Description Hide DescriptionUsing a cryogenic clamp‐type high pressure apparatus, the electrical resistance behavior of iodine was studied for pressures from 80 to 280 kbar and temperatures from 300 to 2.6 °K. The resistivity of iodine approaches metallic values for pressures higher than 130 kbar. No superconductivity is observed in metallic iodine either in the molecular phase below 210 kbar or in the monatomic phase up to 280 kbar.

Diffusion‐controlled reactions on a two‐dimensional lattice
View Description Hide DescriptionWe discuss the kinetics of surface‐catalyzed reactions occurring by adsorption from a gas phase onto a square lattice and subsequent diffusion to fixed reaction sites. In the absence of other interactions between migrating species, the restriction of lattice sites to single occupancy does not greatly complicate the problem, so that saturatuion effects can be rigorously taken into account. At very low density of active sites, the combination of saturation and surface diffusion can generate an effective rate constant which varies inversely as the absolute logarithm of the reactant concentration in the gas. Steady state results valid at all reaction site densities are given for both periodic and random distributions of active sites, in the latter case as variational inequalities placing the reaction rate between closely spaced upper and lower bounds.

Spectra and modeling of laser‐induced emission from multiple‐photon (λ=248.4 nm) irradiation of UF_{6}
View Description Hide DescriptionThe laser‐induced emission from KrF‐laser irradiation of gaseous, room temperature UF_{6} has been studied as a function of time, wavelength, pressure, and laser fluence. Spectra in the 200–900 nm region are presented at several sampling times. One group of 17 emission peaks in the 650–900 nm range has been assigned to the 3Γ_{6}→1Γ_{7} transition in UF_{5}. A possible kinetic mechanism is presented that is qualitatively consistent with the time‐resolved emission data over the experimental pressures and laser fluences investigated.

High temperature equilibrium measurements of the yttrium–hydrogen isotope (H_{2}, D_{2}, T_{2}) systems.
View Description Hide DescriptionEquilibrium pressures as a function of concentration and temperature were measured for the systems, yttrium–hydrogen, yttrium–deuterium, and yttrium–tritium. Temperatures were varied from 700–1000 °C and concentrations in the solidyttrium phase ranged from H/Y or D/Y atom ratios of 0.01 to 2.0 and T/Y ratios of 0.03 to 1.1. Equilibrium isotherms have been plotted and Van’t Hoff isopleths calculated for these systems. Relative partial molal enthalpies and entropies have been tabulated for the reactions of the hydrogen isotopes with yttrium as a function of atom ratio in the solid. In the low concentration range (H/Y=D/Y=T/Y<0.3), the data obey Sieverts law where the concentration of hydrogen isotope in yttrium is proportional to the square root of the hydrogen isotopepressure. Sieverts constants have been derived. At constant atom fraction the tritium compound has the highest equilibrium pressure and the protium compound the lowest. Expressions for the isotopic hydrogen pressure ratios have been derived and agree well with a simple theoretical treatment in which the dissolved hydrogen atoms are regarded as independent oscillators held in tetrahedral potential wells in the yttrium lattice.

A Monte Carlo simulation of the diffusion of gases in porous solids
View Description Hide DescriptionA Monte Carlo simulation of the Knudsen diffusion of gases in a porous solid is described and results of such simulations presented. The solid was represented in the computer as an assemblage of spheres. Equations are presented which enable the prediction of a Knudsen diffusivity from the mean pore size (or, preferably, the mean size and standard deviation) without resort to a tortuosity factor.

Kinetic study of e‐beam excited Ar–CO_{2} mixture
View Description Hide DescriptionDissociation of the CO_{2} molecule by collision with metastable argon (^{3} P _{2}) was shown a few years ago to lead to the excitation of the first vibrational levels of the metastable (a ^{3}π) state of CO and the aim of the present work is to investigate the possibility of using this transfer reaction to obtain population inversions of the a ^{3}Π→X ^{1}Σ transition of the CO molecule. Because of the low CO* fluorescence intensity level, we only performed an indirect kinetic study of e‐beam excited Ar–CO_{2} mixture. Using N_{2}(c→B) (λ=337 nm) fluorescence emission as an Ar* population tracer in Ar–CO_{2}–N_{2} mixture, we measured the kinetic constants relevant to Ar–CO_{2} mixture. It is shown that for certain Ar–CO_{2} mixtures, 50% of the initial excitation created by the electron gun is transferred to Ar* atoms and then disappears in Ar*+CO_{2}collisions. A comparative study of the instantaneous fluorescence intensity of CO* (a ^{3}π) in Ar–CO_{2} mixture with that of N_{2}(C) in Ar*–N_{2} mixture showed that the branching ratio of the Ar*+CO_{2}→CO* (a ^{3}π) reaction was, at most, 10%. Consequently, the largest population efficiency (5%) does not permit laser oscillation in our experiment. Furthermore, the feasibility of such a laser depends on eventual direct population of ground state level by Ar*+CO_{2} reaction.

Exchange perturbation theory. IV. Calculations on H_{2} ^{+}
View Description Hide DescriptionWe have applied the two localized‐wave‐function (LW) exchange‐ perturbation‐theories (EPT) which we have proposed, to the 1sσ_{ g } and 2pσ_{ u } states of H_{2} ^{+} with the objective of verifying the insights gained from these EPT’s and of testing their accuracy. The one LW EPT determines a p r i m i t i v ewave function identical to that of the Eisenschitz–London EPT through first order, but which differs from it in all higher orders. The other determines a function identical to that of the Hirschfelder–Silbey EPT through first order and through infinite order, but which differs from it through all intermediate orders. We find that in terms of the perturbation expansion of the interaction energy through third order, our EPT’s are as accurate as the original EPT’s to which they are related. The LW EPT’s have the conceptual asset that their primitive wave functions are l e a s t d i s t o r t e d from the zero order wave function in a precisely defined sense. We have also calculated interaction energies using the integrals which define the interaction energies in terms of LW’s, and substituting the LW’s approximated by sums through first, second, and third order. The energies generally increase in accuracy as the LW is summed to higher orders. When each order contribution to the LW is multiplied by a weight which is determined to minimize the interaction energy, and the LW is summed through third order, the interaction energy is in error by 0.07% or less for nuclear separations ranging from 1.0 to 10.0 bohr. An examination of the LW’s shows how the optimization procedure works. Other quantities are calculated which show that the LW EPT’s are systematically refinable methods for the calculation of LW’s as well as for the calculation of interaction energies. This is important because LW’S may be used to calculate distinct ’’physical’’ contributions to interaction energies.

Statistical theories for molecular collisions: A maximum entropy derivation
View Description Hide DescriptionStatistical theories are particularly appropriate when one can define a strong interaction regime. We consider the distribution of classical trajectories which enter or exit from this regime. That distribution of trajectories which is of maximal entropy subject only to total conservation of flux is shown to lead to the familiar ’’phase–space’’ expression for the reaction probability. By including more refined conservation conditions as constraints one obtains improved statistical theories. As an example the ’’unified’’ statistical theory of Miller and the Hirschfelder–Wigner expression for the reaction probability are derived by imposing one more conservation constraint. Transition state theory is derived as a special case corresponding to a particular, extreme, numerical value of the constraint. Phase–space theory is obtained when the value of the constraint is at the other extreme (in which case the constraint is not informative). Essentially, exact results for the reaction probability in the collinear H+H_{2} reactive collision are obtained using two conservation conditions (beside the conservation of total flux). In general, it is shown that the procedure is variational, i.e., that including additional constraints can only improve the results.

Photodissociation and photodetachment of molecular negative ions. IX. Atmospheric ions at 2484 and 3511 Å
View Description Hide DescriptionA rare‐gas–halogen laser has been used with a drift‐tube mass spectrometer to extend measurements of the photodestruction cross sections of atmospheric negative ions to 2484 Å. Ions studied include O^{−}, O_{2} ^{−}, O_{3} ^{−}, O_{4} ^{−}, CO_{3} ^{−}, CO_{4} ^{−}, HCO_{3} ^{−}, NO_{2} ^{−}, O_{2} ^{−}⋅NO, and NO_{3} ^{−}; hydrates of many of these ions were also studied. As expected, the cross sections for most of the ions were substantially larger at 2484 Å than at wavelengths longer than 3500 Å.

Energy redistribution in low energy collisions between I_{2}* and He
View Description Hide DescriptionThe process of collision‐induced energy transfer between an excited I_{2} molecule and a He atom has been studied in a supersonic free jet expansion. The relative rates of vibrational relaxation as well as the extent of the rotational energy transfer were measured by observing the dispersed fluorescencespectrum of the I_{2}* molecule. Results show that the vibrational relaxation cross section is much larger in a supersonic free jet than in a hot static gas, and this indicates that the low energy colllisions prevalent in the jet are much more effective than are high energy collisions. Mechanisms for energy transfer and relaxation enhancement in cold collisions are discussed. A comparison is made between the collision‐induced energy redistribution in I_{2}* and the energy distribution in the photodissociation of the van der Waals molecule I_{2}He. Results show that in a collision more extensive rotational energy transfer occurs, and that the branching ratio of the Δv′=−2 to the Δv′=−1 vibrational relaxation processes is much larger than in the decomposition of the I_{2}He* van der Waals complex.

A theoretical method for determining particle distribution functions of classical systems
View Description Hide DescriptionAn equation which involves the triplet distribution function and the three‐particle direct correlation function is obtained. This equation was derived using an analogue of the Ornstein–Zernike equation. The new equation is used to develop a variational method for obtaining the triplet distribution function of uniform one‐component atomic fluids from the pair distribution function. The variational method may be used with the first and second equations in the YBG hierarchy to obtain pair and triplet distribution functions. It should be easy to generalize the results to the n‐particle distribution function.

Rotational predissociation of triatomic van der Waals molecules
View Description Hide DescriptionIn this paper we present analytic expressions for rotational predissociation of triatomic van der Waals molecules X...BC, where X is a rare‐gas atom and BC a normal diatomic molecule. The process considered is the decay X...BX(j) →X+BC(j′), where j and j′ designate rotational states of the BC molecule. ’’Body fixed’’ and ’’intermediate’’ rotational basis sets are used in the formulation. Application is made to the X...H_{2} (X=Ne, Ar, Kr) and Ar...N_{2} van der Waals molecules. It is concluded that rotational predissociationlinewidths for these systems should be amenable to experimental measurement in supersonic beams.

Electrostatic potential–electronic density relationships in atoms
View Description Hide DescriptionSome electrostatic potential (V) –electronic density (ρ) relationships have been investigated for a number of neutral ground‐state atoms, the largest being chlorine. The results provide further evidence of the physical significance of the outermost minimum in the radial density function, 4πr ^{2}ρ, which has previously been used to define the boundary between the core and valence regions in an atom. It was found, for all of the atoms, that the V–ρ behavior in the core regions agrees remarkably well with the Thomas–Fermi equation, V=4.785ρ^{2/3}. In the valence regions, on the other hand, the Thomas‐Fermi expression is no longer obeyed and the atoms show much less uniformity in their V–ρ behavior; a reasonable rough approximation, however, is V=αρ, where α depends upon the atom. By using this more realistic V–ρ relationship in the valence region, three correction terms can be derived which markedly improve the atomic energies obtained with the approximate formula E= (3/7) Z V _{0}, in which V _{0} is the electrostatic potential at the nucleus. For the atoms K–Kr, the new formula reproduces the Hartree–Fock energies to within an average 0.25%. Further, the density is now predicted to decrease exponentially at large distances from the nucleus, correcting a weakness of the Thomas–Fermi theory.

Multiphoton ionization of azulene and naphthalene
View Description Hide DescriptionMolecular beams of azulene and naphthalene are irradiated by the output of a tunable pulsed laser and the charged species produced by multiphoton ionization (MPI) are analyzed using a quadrupolemass spectrometer. A study of the ion fragmentation pattern as a function of laser power and laser wavelength permits these two C_{10}H_{8} isomers to be readily distinguished. Compared to electron impact (EI) ionization, the MPI process can produce more gentle ionization at low laser powers and more extensive fragmentation at high laser powers. The former permits the study of azulene and naphthalene dimers from which their heats of dissociation are estimated. In the case of azulene, two‐color excitation yields a mass spectrum that differs from the mass spectrum obtained by excitation with either color, alone. The ionization efficiency of MPI is very high, exceeding that of EI during the time the pulsed laser is on. A qualitative description involving autoionizing states is presented to account for this efficiency and the growth of the daughter ion yield with increasing laser power.

Use of the multiple‐scattering method for calculating the asymmetry parameter in the angle‐resolved photoelectron spectroscopy of N_{2}, CO, CO_{2}, COS, and CS_{2}
View Description Hide DescriptionCalculations based on the multiple‐scattering method have been performed for the asymmetry parameter (β) as a function of photon energy (from the ionization threshold to 2 Ry above threshold) for the gaseous molecules: N_{2}, CO, CO_{2}, COS, and CS_{2}. For the diatomic molecules N_{2} and CO we have investigated the use of an overlapping spheres transition state model. The calculations were critically compared with experimental data in the literature. In addition, we have carried out angle‐resolved photoelectron spectroscopy on the five molecules using a polarized source of He(i) radiation. Although the calculations show in general good predictive capability, there are also several instances where these calculations seem inadequate and suggestions are made for future work.

Recoil ranges of 2.73 MeV tritons and yields of ^{18}F produced by the ^{16}O(t,n)^{18}F reaction in neutron‐irradiated lithium compounds containing oxygen
View Description Hide DescriptionThe ^{16}O(t,n)^{18}F reaction induced by recoil tritons in neutron‐irradiated crystalline lithium compounds containing oxygen has been studied with emphasis laid upon the determination of recoil triton ranges. The physical parameters required for quantitative description of successive (n,α) – (t,n) processes have been enumerated. The yields of ^{18}F predicted from the calculation agreed well with the observed ones for a variety of lithium compounds. Mean cross sections, averaged over the triton recoil range, for the ^{16}O(t,n)^{18}F reaction in solids were rather constant [?_{F}= (9.87±0.35) ×10^{−26} cm^{2}] and were independent of the triton range. The triton ranges in polycrystalline substances can be determined from ^{18}F yield measurements. Ranges derived from the present results include 6.2±0.4 mg cm^{−2} [Li_{2}O(s)], 7.1±0.6 mg cm^{−2} [LiOH (s)], and 7.0±0.6 mg cm^{−2} [Li_{2}CO_{3}(s)].