ALBERT

Notes: Forces between planar surfaces immersed in a polymer solution are compared with those of the pure solvent in order to establish the effect of adding polymer. We use an off-lattice model consisting of hard-sphere polymers in a hard sphere (athermal solvent). The model is analyzed using a recently introduced density-functional theory. Due to the typically high density of liquids, the forces in both solution and solvent are oscillatory, leading to a more complicated picture than the classical description of polymer depletion forces. Rather than a well-defined attractive depletion regime in the surface forces, the effect of polymers is better described as a damping of the oscillatory force in the pure solvent. It is only when one goes to low (solvent and polymer) density that an attractive depletion regime can be identified.

Notes: Electrostatic interactions in and among reverse micelles in the microemulsion are studied by the grand canonical ensemble Monte Carlo simulation (GCEMC) and by the linear response/Poisson–Boltzmann approximation (LPB). The latter theory is found to reproduce the ion-correlation effects among the micelles seen in the earlier canonical Monte Carlo approach. The open ensemble simulation and the Poisson–Boltzmann (PB) equation are used to study solubilization of simple electrolyte in the microemulsion. In systems with monovalent ions, the PB approximation agrees well with the simulation. Since individual ions are allowed to enter or leave the micelle in our GCEMC simulation, the deviations from the electroneutrality of the droplets are also studied. A good agreement with the LPB theory and with the simple charge fluctuation models considering only the Born solvation energy of charged droplets is observed. For large reverse micelles, the LPB theory and the continuum approach of Eicke and co-workers [J. Phys. Chem. 93, 314 (1989)] proved satisfactory while only the discrete model of Hall [J. Phys. Chem. 94, 429 (1990)] agrees with the simulation for small droplets. The GCEMC results lend support to recent interpretations of electric conductivity of microemulsions. The intermicellar forces due to the correlated charge fluctuation on adjacent droplets are discussed. An approximate expression for the charge fluctuation contribution to the long-ranged pair potential among large droplets with the radius Rm of the form w(r)/kT(approximately-equal-to)−(1/2)(Rm/r)2 is derived.

Notes: In the Poisson–Boltzmann theory of macroion screening in electrolyte solutions the chief simplifying assumption is the neglect of correlations within the ionic atmosphere. Using a generalized van der Waals density functional analysis incorporating ion–ion correlation in a local approximation we obtain a simple correlation corrected Poisson–Boltzmann theory. For point charges in the salt free case, this local correlation approximation leads to asymptotic instability (a "structuring catastrophe'') though the correction is well behaved to low orders in perturbation theory. Results in zeroth order and in a zeroth order field approximation are compared with an extended series of Monte Carlo simulations within a cell model (and a planar electrode model). An excellent agreement is found over a wide range of coupling strengths.

Notes: An integral equation theory is developed for nonuniform polymer fluids. It is based on an exact formulation of the density functional theory for polymers, which is essentially identical in form to the popular self-consistent-field approximations. A nonuniform site–site Ornstein–Zernike equation is obtained. As well, the Triezenberg–Zwanzig–Wertheim–Lovett– Mou–Buff equations are generalized to polymer fluids. The Percus–Yevick and mean spherical approximations are suggested as plausible closures to these equations.

Notes: Argon dimer ions have been generated via three different techniques: (1) autoionization; (2) vertical ionization of neutral Ar2; (3) ionization and subsequent fragmentation of argon cluster ions. In experiments (2) and (3) the dimers and clusters are formed via the adiabatic expansion of argon in a supersonic beam. In each case Ar+2 ions have been mass selected and subjected to single-photon infrared excitation (912–1094 cm−1) using a line-tunable carbon dioxide laser in a crossed-beam arrangement. Only those Ar+2 ions with internal energies within 1000 cm−1 of a dissociation limit yield Ar+ photofragments, the kinetic energy spread of which has been measured using an electrostatic analyzer. The photofragment kinetic energy spectra of dimer ions formed by autoionization do not exhibit any dependence on the angle of laser polarization; it is proposed that such behavior is due to the presence of a high thermal rotational temperature (500 K). In contrast, the corresponding spectra of Ar+2 formed via vertical ionization, exhibit two quite distinct features, one of which shows a strong dependence on laser polarization angle. Calculations show that the latter behavior is most probably due to photodissociation out of an excited spin–orbit state of Ar+2. A very pronounced increase in Ar+2 infrared photodissociation signal is observed as a function of increasing nozzle stagnation pressure. To account for such behavior it is proposed that, following ionization, argon cluster ions fragment to give dimer ions in excited vibrational/rotational levels both in the electronic ground and an excited spin–orbit state.

Notes: Metastable Ar+2 (Ar+2 →Ar++Ar) has been observed in a double-focusing mass spectrometer from ions created by 70 eV electron bombardment of an Ar cluster beam. New ground and excited state potential energy curves have been calculated for Ar+2, and these have been used to show that metastability is due to radiative decay from the II(1/2)u state of the ion. It is shown that vertical (FC) ionization from neutral Ar2, with a vibrational temperature of approximately 30 K, results in a significant fraction of the ions occupying the II(1/2)u state. Detailed pressure dependent measurements show that collision-induced dissociation does not contribute to the observed Ar+ signal. The mean kinetic energy released to the Ar+ has been measured as 44 cm−1 in the center-of-mass frame, and calculations show that this value is consistent with the proposed mechanism.

Notes: Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time-of-flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption of a photon, all cluster ions up to Ar+25 were found to eject one/two neutral atoms with comparatively high kinetic energies. Any remaining internal energy appears to be dissipated through the loss of further neutral atoms with low kinetic energies. An analysis of the laser polarization dependence of these events, shows that those atoms identified as having high kinetic energies are ejected on a time scale which is short compared with the rotation period of a cluster ((approximately-equal-to)10 ps).These experimental observations are consistent with the results of recent molecular dynamics simulations of excited states in rare gas clusters by Landman, Jortner, and co-workers [J. Phys. Chem. 91, 4890 (1987); J. Chem. Phys. 88, 4273 (1988)]. Kinetic energy releases calculated from the TOF spectra exhibit marked fluctuations as a function of cluster size, with Ar+15 showing a minimum and Ar+19 a maximum. It is suggested that such behavior is part of a dynamic response to changes in structure as the cluster ions increase in size. A qualitative explanation is provided through the assumption that the cluster ions take the form of solvated Ar+2 structures. Considerations of the energy available from the photon and the relative contribution each TOF feature makes to the total signal, places an upper limit of two as the number of high kinetic energy atoms ejected by the larger cluster ions.

Notes: We derive, from statistical mechanical arguments, exact expressions for the osmotic pressure in a system involving two charged, hard walls in a solution containing polyelectrolytes. Relations are given appropriate to the polyelectrolyte molecules being either dispersed, i.e., free in solution or grafted to the surfaces. Our results appear as direct generalizations of existing osmotic pressure equations, well known for simple electrolyte systems. In the case of free polyions the corresponding equation is shown to take precisely the same form as found for ordinary electrolytes. We show also that these expressions, which appear in simple form, are satisfied by an appropriate mean-field theory for polyelectrolytes. Thus the relations provide for consistent comparisons of this mean-field approximation with exact results.

Notes: The principal simplifying assumptions in the description of Coulomb fluids are the neglect of correlations within the ionic atmosphere and the disregard of ion size effects. In order to account for short range repulsions, a straightforward heuristic approach is considered where a Debye–Hückel charge density is augmented with a central hard sphere and the possibility of an electrostatic exclusion zone is included to prevent an unphysical negative contact density. This approach leads to an analysis describing the relative competition between hard sphere and electrostatic mechanisms, which undergoes a well-defined interchange in the (γ,η) parameter space. While the structural information is, by construction, limited, thermodynamic properties agree remarkably well with both the rescaled mean spherical approximation and Monte Carlo simulations, up to moderate densities.

Notes: A double-focusing mass spectrometer in conjunction with a cluster beam source has been used to measure the average kinetic energy released following the unimolecular and collision-induced fragmentation (CID) of argon cluster ions. Measurements on unimolecular decay have been made for clusters in the range Ar+5–Ar+60, and for the CID studies the range was Ar+2–Ar+30. Within the observation time window, the kinetic energy release results for the loss of a single argon atom via unimolecular decay are consistent with internal energy being partitioned statistically. Three separate CID routes are identified: (i) loss of one Ar atom; (ii) rapid (〈10−7 s) loss of two Ar atoms within the confines of a collision cell; (iii) sequential loss of two Ar atoms on a time scale 〉10−7 s. It is proposed that the CID of small cluster ions proceeds via electronic excitation; but that as the clusters increase in size (n〉4) vibrational excitation predominates. A simple spectator model of collisional excitation accounts for the experimental CID results in cluster ions beyond Ar+15.