Excitation and fragmentation of highly charged metal clusters in collisions with ions

We discuss the Coulomb fragmentation of highly charged metal clusters. The analogy with a classical conducting liquid drop is assessed from molecular dynamics calculations. Experimentally, the highly charged metal clusters are formed in collisions with highly charged ions (Xe²⁰⁺, Ar¹¹⁺, Ar⁸⁺, Ar³⁺, and O⁵⁺) at low velocity. We show new experimental data on the rate of emitted light charged particles that indicate an as yet unobserved fragmentation regime. Collisions of ions with metal clusters also offer a unique method to strongly excite the conducting electron gas within a short time of a few fs opening the possibility to study large amplitude electron dynamics and relaxation in microscopic systems.     

Charge transfer in collisions involving multiply charged C60 molecules

Electron capture in collisions of C ₆₀ ²⁺ and C ₆₀ ³⁺ molecular ions with atomic and molecular gases has been studied at impact energies around 100 keV. The cross-section dependence on the target-ionization potentials is discussed, and a simple over-barrier model is evoked to explain the energy dependences. The cross sections for endothermic processes are discussed in the light of a simple two-state model, and a general understanding of their behaviour is obtained.     

Vertical triple ionization of ethyne molecules in triple-electron-transfer collisions with O beam ions

Triple-electron-transfer (TET) collisions of 6 keV O²⁺ beam ions with gas-phase ethyne molecules were studied with mass spectrometric techniques. Measurement of the kinetic energies of the O⁻ ions produced yielded triple-ionization energies (TIEs) of ethyne to states of its triply charged positive ion, once those transitions corresponding to the transfer of three electrons in a single collision were identified: eight such peaks were found between 65.1 and 80.1 eV. Possible O²⁺ states in the beam include ³P, ¹D, ¹S; according to spin conservation the singlet ions should populate only doublet states of C₂H₂³⁺, while projectiles in triplet states would populate both its doublet and quadruplet states. Using ab initio propagator calculations for the triple-ionization transitions, the TIEs and quantum numbers for the states of C₂H₂³⁺ associated with each peak observed in the TET spectrum could be identified. The overall match between the theoretical and experimental energies was good, so that it could be established that the population of quadruplet states was comparable in intensity with that for doublets. This is consistent with previous evidence for O²⁺(³P) being the dominant state in the projectile beam used.     

Dissociative excitation of the singly charged lead ion in electron collisions with PbCl2 molecules

Inelastic collisions of slow electrons with lead dichloride molecules yielding excited lead ions in a single encounter event were studied by the extended crossed-beam technique. At an incident electron energy of 100 eV, 67 cross sections for dissociative excitation of PbII spectral lines were measured. Three optical excitation functions were determined in the electron energy range 0–100 eV. The obtained results are compared with data on excitation cross sections of PbII in electron-atom collisions.     

Electrochemically active films of negatively charged molecules,surfactant and synthetic clays

The electrochemical behavior of anionic molecules such as hexacyanoferrate and vitamin B₁₂ hexacarboxilate intercalated into clay–surfactant films is investigated. The results indicate that composites of surfactant cetyltrymethylammonium bromide (CTAB) and synthetic clay facilitate the permeation of the charged molecules through the film. By heating the modified electrode at 120°C for 1 h, the charged molecules, depending on hydrophobic and coulombic forces, are able to accommodate inside the film. Coulombic forces retain the anionic molecules in the clay–CTAB composite. The films are stable with good electroactivity.     

A semiclassical model for the vibrational excitation of spherical-top molecules in collisions with ions

A simple theory is presented to explain the previously observed single-mode vibrational excitation of the spherical-top molecules CH₄, CF₄ and SF₆ in collisions with H⁺ and Li⁺. The theory is based on a three-dimensional forced-oscillator model which has been modified to take account of many independent harmonic oscillators. For small-angle collisions the linear driving forces are the dipole-, polarizability- and quadrupole-derivatives taken from IR, Raman spectroscopy and simple estimates, respectively. To explain the results at larger angles near and beyond the rainbow it has been necessary to introduce short-range repulsive forces between the ions and the outer atoms of the molecule. For small angles both the predicted first moment of the energy transfer and the time-of-flight spectra agree quantitatively with the experimental results. At large angles, for which only the first moment of the energy is available, good qualitative agreement is obtained after a slight adjustment of the potential parameters. The energy transfer as a function of time is calculated and shows a different oscillatory behavior for the proton and Li⁺-ion systems. Also the effect of intra-mode coupling is investigated and shown to have only a small effect on the overall energy transfer. The paper closes with a discussion of the implications of these experiments and the possible role of rotational excitation. The field strengths in these ion scattering experiments are shown to be greater than in the strongest focused Q-switched laser pulses.     

Dissociation of hydrogen molecules in collisions with light alkali ions. II. Li+-H2

The dissociation of a ground state H₂ molecule in single collisions with a Li⁺ ion has been studied using a time of flight technique over a large range of center of mass scattering angles (30° ? υ ? 180°) and collision energies (16 eV < E_cm < 55.5 ev).The results have been transformed into the center of mass system to obtain inelastic differential cross sections (contour maps). In contrast to most other scattering experiments on collision induced dissociation, the results at high energies (E_cm > 40 eV) cannot be explained by a two-step mechanism. Instead dissociation appears to occur in a time comparable to the collision time. The results are consistent with several collision models. Of these the spectator model in which only one of the atoms of the molecule is struck by the incident ion is favored since it is in good agreement with the differential cross sections for backward scattering.     

Low-temperature inelastic collisions between hydrogen molecules and helium atoms

Inelastic H(2):He collisions are studied from the experimental and theoretical points of view between 22 and 180 K. State-to-state cross sections and rates are calculated at the converged close-coupling level employing recent potential energy surfaces (PES): The MR-PES [J. Chem. Phys. 100, 4336 (1994)], and the MMR-PES and BMP-PESs [J. Chem. Phys. 119, 3187 (2003)]. The fundamental rates k(2-->0) and k(3-->1) for H(2):He collisions are assessed experimentally on the basis of a master equation describing the time evolution of rotational populations of H(2) in the vibrational ground state. These populations are measured in the paraxial region of supersonic jets of H(2)+He mixtures by means of high-sensitivity and high spatial resolution Raman spectroscopy. Good agreement between theory and experiment is found for the k(2-->0) rate derived from the MR-PES, but not for the BMP-PES. For the k(3-->1) rate, which is about one-third to one-half of k(2-->0), the result is less conclusive. The experimental k(3-->1) rate is compatible within experimental error with the values calculated from both PESs. In spite of this uncertainty, the global consistence of experiment and theory in the framework of Boltzmann equation supports the MR-PES and MMR-PESs, and the set of gas-dynamic equations employed to describe the paraxial region of the jet at a molecular level.     

Quenching of metastable hydrogen atoms by low energy collisions with hydrogen molecules: comparison of experiments with theory

The experimental velocity dependence of the quenching cross section of metastable H(2S) atoms in low energy collisions with hydrogen molecules is compared with theoretical calculations using the formalism of Gersten. This formalism takes into account possible changes of the molecular rotational energy and is in good agreement with experiments at low energies where other theories fail. A dependence of this cross section on the temperature of the molecular gas is predicted which is due to the temperature dependent population of the molecular rotational levels.     

Polarizability of negatively charged helium‐like ions interacting with Coulomb and screened Coulomb potentials

The dipole and quadrupole polarizabilities (both static and dynamic) of negatively charged helium‐like ions are investigated. The mass dependence of the polarizability is studied by changing the mass of the positively charged particle from one unit of electron mass to infinitely heavy. The calculations are carried out in the framework of the pseudostate summation method using exponential correlated wave functions having pseudorandomly generated nonlinear variational parameters. The dipole and quadrupole polarizabilities in terms of frequency and nuclear mass are reported for the first time. The effect of screened Coulomb potentials on the polarizabilities of D^–, T^–, ¹H^–,Pi^–, Mu^–, and Ps^– are also presented.     

Protein interactions with negatively charged inorganic surfaces

Protein adsorption on charged inorganic solid materials has recently attracted enormous interest owing to its various possible applications, including drug delivery and biomaterial design. The need to combine experimental and computational approaches to get a detailed picture of the adsorbed protein properties is increasingly recognised and emphasised in this review. We discuss the methods frequently used to study protein adsorption and the information they can provide. We focus on model systems containing a silica surface, which is negatively charged and hydrophilic at physiological pH, and two contrasting proteins: bovine serum albumin (BSA) and lysozyme (LSZ) that are both water soluble. At pH 7, BSA has a net negative charge, whereas LSZ is positive. In addition, BSA is moderately sized and flexible, whereas LSZ is small and relatively rigid. These differences in charge and structural nature capture the role of electrostatics and hydrophobic interactions on the adsorption of these proteins, along with the impact of adsorption on protein orientation and function. Understanding these model systems will undoubtedly enhance the potential to extrapolate our knowledge to other systems of interest.     

Reactivity of niobium-carbon cluster ions with hydrogen molecules in relation to formation mechanism of Met-Car cluster ions

It is known that a niobium-carbon Met-Car cluster ion (Nb 8C 12 (+)) and its intermediates (Nb 4C 4 (+), Nb 6C 7 (+), etc.) are selectively formed by the aggregation of the Nb atoms in the presence of hydrocarbons. To elucidate the formation mechanism, we prepared Nb n C m (+) with every combination of n and m in the gas phase by the laser vaporization technique. The reactivity of Nb n C m (+) with H 2 was examined under the multiple collision condition, finding that Nb n C m (+) between Nb 2C 3 (+) and Nb 8C 12 (+) are not reactive with H 2. On the basis of the H 2 affinity of Nb n C m (+) experimentally obtained, we propose a dehydrogenation-controlled formation mechanism of niobium-carbon Met-Car cluster ions.     

The behaviour of water molecules associated with structural changes in negatively charged phosphatidyl-glycerol assemblies as studied by DSC

Variation of the thermotropic behaviour of both lipid assemblies and associated water molecules with an increase in water content was investigated for negatively charged phosphatidyl-glycerol (PG)-water system up to 90 wt.% water by DSC. The number of water molecules existing in interbilayer regions of the present gel phase was estimated from a deconvolution analysis of ice-melting DSC curves. On the basis of a result of the calorimetric analysis, a water-distribution diagram was constructed over the water content range from 0 to 90 wt.%. The diagram presented a continuous incorporation of interlamellar water up to 90 wt.% water, related to unilamellar-vesicle forming properties of charged lipids. Furthermore, similarly to a result for neutral lipid systems previously reported by us, the present diagram also showed the existence of a specific water content region (i.e., pre-region) where a structural change of planar to curved bilayers for multilamellar structures proceeds with the aid of bulk-like water before finally reaching unilamellar vesicles.     

Vibronic interactions in negatively charged polyacetylene

Electron-phonon interactions in the monoanions of polyacetylenes such as C2H4 (2tpa), C4H6 (4tpa), C6H8 (6tpa), and C8H10 (8tpa) are studied and compared with those in the monoanions of polyacenes. The C-C stretching A(g) modes around 1500 cm(-1) the most strongly couple to the lowest unoccupied molecular orbitals (LUMO) in polyacetylenes. The estimated total electron-phonon coupling constants for the monoanions (l(LUMO)) are 0.579, 0.555, 0.463, and 0.401 eV for 2tpa, 4tpa, 6tpa, and 8tpa, respectively. The l(LUMO) values for polyacetylenes are much larger than those for polyacenes. Furthermore, the l(LUMO) value for polyacetylene with C(2h) geometry is estimated to be 0.254 eV, and is larger than that (0.024 eV) for polyacene with D(2h) geometry. The phase patterns difference between the LUMO of polyacenes localized on the edge part of carbon atoms, and the delocalized LUMO of polyacetylenes is the main reason for the calculated results. The single charge transfer through the molecule in polyacetylenes are also discussed. The reorganization energies between the neutral molecule and the corresponding monoanion are estimated to be 0.164, 0.144, 0.125, and 0.113 eV for 2tpa, 4tpa, 6tpa, and 8tpa, respectively. Such reorganization energy decreases with an increase in molecular size. The conditions under which the attractive electron-electron interactions are realized in the monoanions of polyacetylenes and polyacenes are discussed. In terms of the electron-phonon interactions and the reorganization energies, the relationships between the normal and possible superconducting states are briefly discussed. We find that the monoanions with smaller molecular size cannot easily become good conductors, however, the conditions under which the interactions between two electrons are attractive are more easily realized in the monoanions with smaller molecular size than in the monoanions with larger molecular size.     

Tests of semiclassical treatments of vibrational-translational energy transfer collinear collisions of helium with hydrogen molecules

Three kinds of semiclassical theory are tested against quantum mechanical results for vibrational transition probabilities and average vibrational energy transfers in collinear collisions of atoms with harmonic and Morse vibrators for the He-H₂ mass combination. The interaction potential is assumed to be a repulsive exponential function with an exponential parameter which is realistic for He-H₂ collisions. The energy range studied is total energies of 2–8 in units of ?ω_e. The uniform semiclassical approximations of classical S matrix theory are tested only for classically allowed transitions, i.e., for transition probabilities greater than about 0.2. They are accurate quantitatively for both harmonic and Morse vibrators. The integral expressions of classical S matrix theory are found to be quantitatively accurate for classically allowed and weakly classically forbidden transitions, i.e., for transition probabilities greater than about 0.01–0.05, and to be unreliable for strongly classically forbidden transitions. Quasiclassical trajectory methods yield qualitatively accurate results only for classically allowed transitions but the phase-averaged energy transfer in quasiclassical collisions may be accurate even when classically forbidden transition probabilities are important for the calculation of the average energy transfer. Forced quantum oscillator methods using a classical path whose initial velocity is the average of the initial and final velocities corresponding to the transition of interest are accurate for transition probabilities as small as 4 × 10^?8 for harmonic vibrators but do not seem to accurately account for the effect of anharmonicity.     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

Slow electron collisions with polar molecules in plasma

Considering a polar molecule in a plasma medium, the electron-molecule dipole potential is modified to incorporate the screening effect of the plasma, which is characterised by the inverse Debye-shielding-length λ ina ^?1 ₀. Born differential cross-sections (DCS) for the rotational excitations of target molecules like HCl, H₂O and HCN are calculated for various low incident electron energies as well as for various values of λ. The effect of plasma screening is not only to reduce the DCS but also to alter the angular distribution. In general the DCS near the forward direction are considerably reduced. The present Born results are not expected to be reliable for strongly screening plasmas.