Multiple-electron processes in 1.4 MeV/u ion-atom collisions

Simultaneous multiple-electron capture and multiple ionization is studied for collisions of highly stripped ionsA ^q+ with rare gas atomsB=He, Ne, Ar, Kr and Xe. At a specific energy of 1.4 MeV/u coincidence measurements were conducted distinguishing between pure ionization, stripping and capture of up to four electrons by projectiles in charge states fromq=6 up toq=48. The coincident charge-state distributions of target recoil ionsB ⁱ⁺ range fromi=1 up toi=19 (in few cases). For highly charged projectiles the relative fractions of recoil ions for concomitant electron capture and ionization are found to be nearly independent of projectile charge or species. Average charge states 〈i〉 of the recoil ions produced by loss respectively capture ofk electrons (k=?2, ?1, 0, 1, 2, 3, 4) from/into the projectile ion were determined. Their systematic dependences onk, on the target atomic number and the projectile ion charge are discussed. A calculation of partial cross sections for multi-electron collision processes in the He target atoms using unitarized first order perturbation theory for impact parameter dependent probabilities and an independent-electron picture is presented and discussed on the basis of the experimental data.     

Ion attachment to Van der Waals clusters

Mixed ionized clusters have been produced in a supersonic nozzle beam experiment by attachment of stagnant cations (i.e. NO⁺ and Xe⁺) to neutral van der Waals clusters (i.e.Ar _n ) within a Nier type ion source. This new ionization technique leads to less fragmentation than electron impact ionization and the measured cluster distributions exhibit icosahedral shell and subshell closures which have not been detected in the case of electron impact of Ar _n -clusters ionization so far. Additionally, the obtained appearance energies and metastable fractions give insight into the production mechanism and the stability of the resulting ions.     

Substituent effects on ion abundances and energetics in substituted acetophenones

Based on the quasi-equilibrium theory of mass spectra it is shown that the intensity ratio [A]⁺/[M]⁺, where [A]⁺ is a fragment ion and [M]⁺ is the molecular ion, is given by [A]⁺/[M]⁺ = f′ (k₁/k_t) ((1/f) ? 1), where f is the fraction of molecular ions with insufficient energy to fragment, f′ is the fraction of [A]⁺ ions with insufficient energy to fragment, and k₁/k_t is the fraction of fragmenting molecular ions which form [A]⁺. For substituted acetophenones it is shown that f depends on the substituent present and that f′ k₁/k_t is also substituent dependent for formation of both [CH₃CO]⁺ and [YC₆H₄CO]⁺. It is also shown that no direct information concerning the effect of a substituent on the rate of a particular fragmentation reaction can be obtained from intensity studies. The ionization potentials of the parent molecules and the appearance potentials of the [YC₆H₄CO]⁺ fragment ions have been measured for fifteen substituted acetophenones and the correlations with substituent constants are discussed.     

Metastable dissociation dynamics of molecular cluster ions

Metastable uni-cluster dissociation for several hydrogen-bonded and van der Waals cluster ions are observed via resonance-enhanced two-photon ionization reflectron time-of-flight (TOF) mass spectrometry. All of the cluster ions studied show evaporation of a single molecule from the respective parent cluster ions as dominant metastable decay processes. Furthermore, the averaged metastable evaporation rate constants (k _evap) of these cluster ions in a fixed time domain of 0.2–50 µs are obtained by analyzing the relative intensity of metastable ion peaks due to evaporation in the acceleration and the field-free drift regions of the TOF mass spectrometer. An intensity anomaly in some of the observed metastable ion peaks, indicative of magic number stability of the cluster ion, is also presented.     

The calculation of excited state and ground state properties of conjugated heteroatomic molecules using a single SCF-LCAO-CI method including σ-polarization

An SCF-π method including variable π-electronegativity and σ-polarization is described and applied to the calculation of electronic transitions and ionization potentials of a large variety of heteroatomic molecules containing boron, nitrogen, oxygen, fluorine, chlorine, and sulfur. The necessary atomic parameters are the Slater effective nuclear charges and published ionization potentials, electron affinities and σ-orbital electronegativities for trigonally hybridized atoms. The program automatically adjusts the initial atomic parameters to reflect the molecular environment without the intervention of the user. The agreement between calculated and observed transition energies, oscillator strengths and ionization potentials is very good.     

Shannon information entropy of fractional occupation probability as an electron correlation measure in atoms and molecules

A new method to determine electron correlation energy is presented for atoms and molecules. This method is based on Shannon information entropy that is obtained by fractional occupation probabilities of natural atomic orbitals. It is indicated that the Shannon entropy increases as the number of electrons increases and thus can be considered as a possible measure for the electron correlation in atomic and molecular systems. For neutral atoms and singly charged positive ions we proposed an expression for correlation energy with explicit dependence on the Shannon entropy and atomic number. The obtained correlation energies have been used to compute the first ionization potentials of the ground state of the main group elements from hydrogen through krypton. The calculated ionization potentials are in reasonably good agreement with their corresponding experimental values.We also developed the additivity scheme to find a connection between Shannon entropy and molecular correlation energy. The estimated molecular correlation energies show an excellent agreement with those obtained by elaborate G3 method with R² = 0.990.     

Formation of negatively-charged cluster ions in thermal energy collisions with state-selected rydberg atoms

Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO₂) _k ^? (4≦k≦25) and (O₂) _k ^? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO₂) _m and (O₂) _m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s ³ P _{2, 0}) Penning ionization. For both CO ₂ ^? and O₂-clusters, the negative and the positive ion mass spectra are very different. For (CO₂) _k ^Emphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF ₆ ^? formation in collisions with SF₆. For (O₂) _k ^? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O ₄ ^? as dominant species).     

The photoionization of atomic Eu in the vicinity of its giant resonance

It is demonstrated that the partial photoionization cross sections of outer subshells of atomic Eu in the giant resonance region are determined by the action of the 4d-electron excitations. The cross section for photoionization of the semifilled 4f ⁷ subshell is also entirely dominated by the interaction with 4d ¹⁰ electrons.     

Ionization and appearance potential measurements in arene chromium tricarbonyls

The ionisation potentials and the appearance potentials for ions arising from the loss of carbonyl groups as well as certain other low-energy fragmentation ions have been measured for a series of substituted arene chromium tricarbonyls. The values obtained are found to correlate very well, for the loss of one and two carbonyl groups, with both the CO force constant k and the Hammett σ_p function and substituent on the benzene ring. The appearance potentials of the ions arising by the loss of three carbonyl groups do not exhibit a high correlation. Arene⁺ ions are found to occur at energies intermediate between those for the loss of two and three carbonyl groups.The various effects observed are discussed in terms of a model which involves definite contributions from both σ- and π- bonding of carbonyl groups in the gaseous cations.     

Threshold photoionization behaviour of (Li2O)Lin clusters produced by a laser vaporization source

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify Li_kO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of Li_kO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O^2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li₂O)Li_n clusters, clear shell closure effects are present at n = 8 and n = 20.     

Core polarization and relativistic effects competition in the first ionization potentials for some systems in the Cu,Ag, and Au isoelectronic sequences

Single-configuration relativistic Hartree–Fock values of the first ionization potentials for Cu through Kr⁷⁺, Ag through I⁶⁺, and Au through Pb³⁺ are computed in “frozen” and “relaxed core” approximations with and without allowance for core polarization. Effects of polarization of the atomic core by the valence electron are included by introducing a polarization potential in the one-electron Hamiltonian of the valence electron. The core polarization potential depends on two parameters, the static dipole polarizability of the core α and the cut-off radius r₀, which are chosen independently of the ionization potential data. It is demonstrated that by including the core polarization potential with α and r₀ parameters, which are simply chosen instead of being empirically fitted, it is still possible to account, on the average, for at least 70% of the discrepancy between the single-configuration relativistic Hartree–Fock ionization potentials and the experiment, a discrepancy usually ascribed to the contribution of valence-core electron correlations, and to bring the theoretical ionization potentials to an average agreement with experiment of around 1%. It can be concluded from this study that for low and medium Z elements the core polarization dominates for neutral systems or systems in low ionization stages, whereas for highly ionized systems the relativistic effects prevail. For heavy elements, however, the core polarization influence is comparable to the relativistic one only for neutral systems, whereas for ions the relativistic effects are overwhelmingly predominant.     

Ionization and appearance potentials in organic chemistry. II—Appearance potentials and enthalpy differences for the stereoisomers of methylbicyclo[4.4.0]decanes

The ionization potentials for the stereoisomers of trans-fused 2- and 3-methylbicyclo[4.4.0]decanes and the appearance potentials for the ions at m/e 137 [M–CH₃]⁺, 109, 96, 95 and 82 were measured by the electron impact method. The ionization potentials and appearance potentials of the [M–CH₃]⁺ ions appeared to be equal for each of the epimers. Appearance potentials of the other ions were always lower for the less stable epimer. No quantitative correlation was observed between the difference in the appearance potentials for any ions and the differences in enthalpies of the ground states.     

Appearance-ionization potentials differences as a guide to the relative stabilities of the cis- and trans-A/B-steroids

The ionization potentials of cis- and trans-A/B-steroids of the androstane and pregnane series, as well as the appearance potentials of the M⁺-Me ions have been determined.The approximate activation energies (obtained as the AP-IP differences) for the loss of an angular Me from the molecular ion were found to be higher in the case of the more stable trans-A/B (5α)-steroids as compared to their cis (5β)-isomers.     

Multiply ionization of diethyl ether clusters by 532 nm nanosecond laser: The influence of laser intensity and the electron energy distribution

The formation mechanism for multiply charged ions (C^q+ and O^q+ (q = 2–4)) were investigated using a dual polarity time-of-flight mass spectrometer when diethyl ether clusters interacted with nanosecond laser pulse. The signal intensity of multiply charged ions and electron energy was measured experimentally. It was shown that the intensity of multiply charged ions increased about 50 times when laser intensity increased from 7.6 × 10⁹ to 7.0 × 10¹⁰ W/cm², then saturated as laser intensity increased further. It is interesting that the evolution of the mean value of electron energy was same to that of multiply charged ions. The theoretical calculation showed the ionization potential of atomic ions could be significantly decreased due to the effect of Coulomb screening especially at low laser intensity. It indicated that the electron ionization combined with Coulomb screening effect could explain the production of multiply charged ions in nanosecond laser field.     

Fast calculation of excited-state potentials for rare-gas diatomic molecules: Ne2 and Ar2

A fast method for obtaining excited-state potentials of rare-gas diatomic molecules is described. Two types of excited orbitals are used: molecular orbitals calculated in the field of a singly charged molecular ion, and atomic orbitals (properly symmetrized) obtained in a similar atomic system. The RPA equations are solved within the manifold of excitations from the highest occupied orbital in each symmetry to the lowest excited orbital of either type in each symmetry. A simple model for estimating the dynamic correlation correction to excitation (and ionization) energies is given. Applications to excited states of Ne₂(^1,3Σ⁺_{g, u}, ^1,3Π_{g, u}) and Ar₂(^1,3Σ⁺_{g, u}) are described. Two-electron integral transformations involve only three orbitals of each symmetry, and the RPA matrices are four-dimensional. The computational effort required for all excited-state potentials adds less than one-tenths (in terms of computer time) to the effort involved in the preliminary ground state Hartree—Fock calculations. The resulting potentials compare favorably with more elaborate CI calculations and give good agreement with spectroscopic and scattering data. Potential curves for the molecular ions are also given.     

Improved valence basis sets for divalent lanthanide 4f-in-core pseudopotentials

Improved energy-optimized (6s5p4d) and (7s6p5d) primitive valence basis sets have been derived for energy-consistent scalar-relativistic 4f-in-core pseudopotentials of the Stuttgart-Cologne variety modeling divalent lanthanides with a $4\hbox{f}^{n+1}$ occupation (n = 0?C13 for La?CYb). Segmented contracted basis sets covering the range of polarized double-, triple-, and quadruple-zeta quality, augmented by 2f1g correlation sets, were created for use in molecular calculations. The basis sets contain smaller (4s4p3d) and (5s5p4d) primitive subsets, which are designed in particular for solid state calculations of crystals containing divalent lanthanide ions. Hartree?CFock, density functional theory and coupled cluster results obtained with the new basis sets for lanthanide atomic ionization potentials as well as of geometry optimizations of various test molecules, i.e. selected lanthanide mono- and dihydrides, mono- and difluorides, and monooxides, show a satisfactory agreement with experimental data as well as with corresponding scalar-relativistic all-electron results. Core-polarization potentials are found to improve the results, especially for the atomic first and second ionization potentials.     

The influence of the electron wave function on the PtL I andL III ionization probabilities by 3.6 MeV he impact

For 3.6 MeV He impact theL _I andL _III subshell ionization probabilities of Pt have been measured. Due to relativistic effects in the electron wave functions, theL _I subshell ionization probabilityI _LI (b) is strongly enhanced at small impact parameters exceeding evenI _LIII (b) in nice agreement with the SCA theory.     

Rate constants for the reaction of the chloromethyne radical with alkynes

Absolute rate constants were measured for the gas phase reactions of the CCl(X?²Π) radical produced in the flash photolysis of CHBr₂Cl with a representative series of alkynes. The rate of addition to the triple bond follows the trend established for the addition CCl(X?²Π) to olefinic double bonds and the acetylenic triple bond by the latter reagent. A linear correlation between log k and ionization potentials of the alkynes reflects the electrophilic nature of the reaction.     

Effects of target gas in collision-induced dissociation using a double quadrupole mass spectrometer and radiofrequency

Collision-induced dissociation (CID) of polyatomic ions sampled from an rf-powered glow discharge is examined by using three target gases including atomic (Ar and Xe) and molecular species (N2). Collisions with these targets in the first quadrupole of the double quadrupole system result in the loss of discharge species by dissociation, symmetric and asymmetric charge exchange, and scattering, each to varying degrees. These processes are seen to be a function of the relative mass, size... and ionization potentials of the target species, as well as the collision center-of-mass energies. In light of the comparisons, xenon appears to be the best collision target for both CID and charge exchange because of its relatively low ionization potential and high dissociation efficiency of polyatornic species. Evidence for both symmetric and asymmetric charge exchange is presented for Ar and Xe target gases.     

Magnetic circular dichroism and absorption spectra of the mononegative and dinegative ions of [16] annulene

The magnetic circular dichroism (MCD) and absorption spectra of the mononegative and dinegative ions of [16] annulene have been measured in the 16000–30000 cm^?1 region. The results are interpreted using a PPP molecular orbital calculation. It is found that the D_4h symmetry, using Slater type atomic orbitals excellent agreement between experiment and calculations is obtained. The excited ²E_g and ¹E_u states of the mono- and dinegative ions have experimental averaged angular monomenta of 1.0 h and 2.3 h respectively and we conclude to planar ring structures for the ions. Variations in the parameters for the PPP calculations, i.e. B, do not influence the MCD values to any great extent.In addition, from temperature dependent measurements between 20°C and ?150°C, the thermodynamic parameters ΔH and ΔS for the equilibrium observed between molecule and ions in solution have been determined.