ALBERT

Notes: The interaction between charged surfaces bearing terminally attached polyelectrolyte chains is investigated using Monte Carlo computer simulations and a mean-field theory. In particular we focus on the role played by the addition of a simple electrolyte component. It is found that the force between the surfaces increases with increasing electrolyte concentration because of a repulsive osmotic pressure contribution due to the small ions, while the attractive bridging component is insensitive to added salt. Considering the approximate nature of the mean-field (polyelectrolyte Poisson–Boltzmann) theory it compares well with the computer simulations.

Notes: A detailed study has been undertaken on the infrared photofragmentation of SF6(NO)±n cluster ions. A line-tuneable CO2 laser has been used to excite the ν3 vibrational mode of the SF6 molecule which is followed by the observation of three separate fragmentation channels: –SF6, –NO, and –2NO. The relative intensities of the fragments are found to be sensitive to the sign of the charge on the ion, the cluster size, and whether n is either odd or even. Within clusters of the same charge, the most marked transitions in fragmentation pattern are found between odd- and even-sized cluster ions, with the decay channels favoring those processes which lead to the formation of even electron ions. There are also large differences in fragmentation pattern between the negatively and positively charged ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and their shapes confirm a pronounced difference in behavior between SF6 (NO)−n and SF6 (NO)+n. The results for the positively charged ions are interpreted in terms of a central (NO)+3 core which serves to fix the position of the SF6 molecule via an attractive ion-induced dipole interaction. In contrast, observations on the negatively charged ions, suggest that the interaction between the excess electron and the SF6 is predominantly repulsive and also sufficiently diffuse as to keep the molecule comparatively mobile even in large cluster ions. It is suggested that the excess electron in SF6 (NO)−n may occupy a surface state and that the state has associated with it approximately 15 NO molecules. © 1994 American Institute of Physics.

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: 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: 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: 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: 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.