Total cross sections for K + Br2 at relative energies between 0 and 8000 eV

Trajectory Surface Hopping (TSH) calculations have been applied to the non-elastic scattering in the K + Br₂ collision system over a wide range of relative kinetic energies from 0 to 8000 eV. Absolute total cross sections have been computed for the formation of various collision products with an accuracy of 5% with respect to statistical errors. The following non-elastic processes have been studied: chemical reaction, inelastic neutral scattering, neutral dissociation and ion pair formation, yielding atomic as well as molecular negative bromine ions together with PC ions. The absolute values of the respective total cross sections, obtained from the TSH calculations, are in close agreement with the available experimental data, both for chemical reaction and for ion pair formation, over the whole energy range considered. The three particle character of the collision system is important in describing the experimental results quantitatively at relative kinetic energies below 100 eV.     

Reaction dynamics of Na n + in collision with molecular oxygen

Reaction dynamics of sodium cluster ions, Na _n ⁺ (n = 2–9), in collision with molecular oxygen, O₂ was investigated by measuring the absolute dissociation cross sections and the branching fractions by using a tandem mass spectrometer equipped with several octapole ion guides. The mass spectrum of the product ions show that the dominant reaction channels are production of oxide ions, Na_kO_i (i =1, 2), and intact ions, Na _p ⁺ (p < n). With increase in the collision energy, the cross section for the production of the oxide ions decreased, while that for the production of the intact ions increased. The collision-energy dependences of the cross section for the oxide formation reveals that electron harpooning from the molecule to Na _n ⁺ preludes the oxideion formation. On the other hand, the collision-energy dependences of the cross sections for the intact ion formation is explained by a hard-sphere-collision model similar to the collisional dissociation of Na _n ⁺ by rare-gas impact.     

Charge transfer collisions between fullerenes: C 60 3+ + C60

Charge transfer collisions between C ₆₀ ³⁺ and C₆₀ are studied for collision energies between 400 and 3600 eV. Single and double electron transfers are observed, both occuring under single collision conditions. Absolute charge transfer cross sections are determined as a function of collision energy. The cross section for single electron capture of approx. 300 Ų is about two times larger than that for double electron transfer. For both processes the cross section increases slightly with increasing collision energy.     

Predissociation of the c3Πu state of H2, populated after charge exchange of H2+ with several targets at keV energies

A detailed study of the predissocitation of the c³Π_u state of H₂ has been made with a new, very sensitive, experimental technique. A resolution better than 1% is obtained in the measurement of the released kinetic energy of HH pairs after charge exchange of H₂⁺ with Ar, H₂, Mg, Na and Cs by detecting both fragments with a time- and position-sensitive microchannel-plate detector. Eighteen vibrational levels of the c³Π_u state can be clearly distinguished in the range of 7.2–10.2 eV. Detailed information is extracted from the spectra with the aid of a convolution procedure. The vibrational energy levels of the c³Π_u state as calculated by Ko?os and Rychlewski are found to be correct within the experimental accuracy of 5 meV. Predissociative lifetimes are measured, yielding 6.2±0.5 ns for the lowest rovibrational level (υ = 0, N = 1), which are in good agreement with theoretical calculations of Comtet and de Bruijn. The cross section for charge exchange is observed to increase gradually with the vibrational level and seems to follow the geometrical cross section of the molecule. Rotational excitation during the charge exchange is also found to increase considerably with the vibrational quantum number. The final rotational temperature further depends strongly on the target gas used and increases with the resonance energy defect ΔI in the charge exchange collision.     

Ground state correlations in H2 measured by the (e,2e) technique

The relative intensities for exciting the 1sσ_g, 2pσ_u, 2pπ_u, and 2sσ_g states of H₂ are measured in a 1200 eV noncoplanar symmetric (e,2e) experiment on H₂. Momentum distributions are obtained at separation energies corresponding to the various transitions. The ratio of transition probability to the excited states relative to the ground state is strongly dependent on the ion recoil momentum q, having a minimum value of approximately 2% at small q. The excited state cross sections are sensitive to electron correlation effects and the data are compared with calculated cross sections using a configuration interaction wavefunction for H₂.     

Complex formation in proton—D2 collisions

Classical trajectory calculations are used to compute the formation cross section (suitably defined) for strongly interacting collision complexes formed in H⁺ + D₂ collisions in the kinetic energy range from 0.1 to 4 eV. This cross section corresponds to the usual Langevin cross section only if the kinetic energy is less than 0.2 eV, and provided that little initial excitation is present, while for higher kinetic energies it drops exponentially. It is in much better agreement with absolute integral cross sections observed experimentally than the latter. Further study shows that it is the contribution from large orbital anglular momenta, which the Langevin cross section overestimates. Orbiting complexes (of H⁺ around D₂) play a negligible role, and are very short-lived. The lifetime of strongly coupled complexes is estimated to be 450 E^?1.3 fs, where E is the total energy in eV. The use of trajectory data to improve Light's phase space theory is discussed.     

Charge exchange in alkali cluster collisions

Collisional charge exchange between mass selected alkali cluster ions and Cs has been studied and cross sections have been determined for the processes Na _n ⁺ + Cs and K _n ⁺ + Cs, withn=1–21 andn=1–14, respectively. A strong dependence of the cross sections on the energy defect as well as on cluster size and collision energy is found. The results are analysed by a coupled two state density matrix model, taking account of the relaxation of electronic amplitudes due to interaction with the nuclear motion in the cluster.     

Collision spectroscopy with aligned and oriented atoms

Electron capture processes in the H⁺?Na(3s) and H⁺?Na(3p) collisions are experimentally investigated in the 0.3–3 keV energy range using a crossed beam experiment. The excited Na(3p) target is produced with a well-defined alignment using laser pumping. The time of flight technique enables the identification of all the H(n)+Na⁺ channels populated in the collision. Total cross section ratios σ_3p (n=2)/σ_3s (n=2),σ_3p (n=3)/σ_3s (n=2) and σ_3s (n=3)/σ_3s (n=2) for the production of H(n=2) and H(n=3) are measured in the H⁺?Na (3s) and H⁺?Na (3p) collisions. They reveal a strong dominance of the production of H(n=2) in the H⁺?Na(3p) collision, especially for energies below 1 keV.     

Ion pair formation in alkali-SF6 collisions: Dependence on collisional and vibrational energy

The dependence of ion pair formation in collisions of fast alkali atoms (K, Na and Li) with SF₆ on the initial relative kinetic energy and the internal energy of the target molecule has been studied by the crossed molecular beam method. Using a mass spectrometer we have measured total cross sections for negative ion formation as a function of translational and internal energy. Collision energies ranged from threshold up to 35 eV and SF₆ source temperatures were varied from 300 K to 850 K.By means of an inverse Laplace transform of the measured cross sections, we have determined total specific cross sections for each negative ion depending on the SF₆ vibrational energy and at fixed relative kinetic energy.The relative importance of both collisional and internal energy in promoting the electron transfer process is discussed for the various reaction channels in terms of a collision model. An essential feature of this model is the stretching of the S-F molecular ion bond during the collision. The product show complete relaxation in the threshold region, i.e., vibrational and collisional energy are equivalent: This holds for the SF⁻₆ formation only near threshold and for the SF⁻₅ and F⁻ formation up to about 2 eV above threshold. In the post-threshold region the effect of the internal energy on the cross section dominates over that of the translational energy.From these measurements the adiabatic electron affinity of SF₆ is inferred to be 0.32 ± 0.15 eV, T = 0 K. Some other thermodynamic data are deduced: EA(SF₅) > 2.9 ± 0.1 eV (T = 300 K) and D₀(SF⁻₅-F) = 1.0 ± 0.1 eV.     

Sputtered metal and silicon cluster ions: collision-induced fragmentation and neutralization

Mass separated metal and silicon cluster ion beams M _n ^{+, ?} are produced by sputtering and undergo fragmenting and/or neutralizing collisions at different kinetic energies (100–1800 eV) in Ar and SF₆. Fragment patterns induced by rare gas collisions open a way to determine ionization potentials and electron affinities of clusters. These values are compared to known experimental and theoretical data. For negatively charged clusters the absorption in gas targets is mainly due to neutralization, the cross sections varying with cluster material, number of atoms and collision partner from 10 Ų to about 50 Ų.     

Collision cross sections for protein ions

A method for the determination of cross sections for gas-phase protein ions, based on the energy loss of ions as they pass through a collision gas, is described. A simple model relates the energy loss to the number of collisions and hence the cross section. Results from a Monte Carlo model that support the validity of this approach are described. Experimental cross sections are reported for motilin, ubiquitin, cytochrome c, myoglobm, and bovine serum albumin. Cross sections range from approximately 800 Ų for motilin to approximately 14,000 Ų for bovine serum albumin and generally increase with the number of charges on the ion. Cytochrome c ions from aqueous solution show somewhat smaller cross sections than ions formed from solutions of higher organic content, suggesting that the gas-phase ions may retain some memory of their solution conformation.     

Absolute cross sections for electron impact ionization of Ag2

The previously measured relative cross section function for electron impact ionization (EII) of neutral Ag₂ has now been calibrated quantitatively by combining the electron impact ionization with in situ non resonant two photon ionization (NR2PI). By comparing the NR2PI saturation intensities measured for Ag ₂ ⁺ and Ag⁺ with the corresponding EII intensities, the ratio between the electron impact ionization cross sections (EIICS) of neutral Ag₂ and Ag was determined to be σ_Ag2/σ_Ag=1.53 for an electron energy of 46 eV. This result agrees well with the geometricn ^2/3-rule \((\sigma X_n \sim n^{2/3} )\) commonly proposed for the dependence of the EIICS of clustersX _n on the cluster sizen.     

Traveling Wave Ion Mobility Mass Spectrometry and Ab Initio Calculations of Phosphoric Acid Clusters

Positive and negative ion electrospray mass spectra obtained from 50 mM phosphoric acid solutions presented a large number of phosphoric acid clusters: [(H₃PO₄)_n?+?zH] ^z+ or [(H₃PO₄)_n – zH] ^z– , with n up to 200 and z up to 4 for positively charged clusters, and n up to 270 and z up to 7 for negatively charged cluster ions. Ion mobility experiments allowed very explicit separation of the different charge states. Because of the increased pressures involved in ion mobility experiments, dissociation to smaller clusters was observed both in the trap and transfer areas. Voltages along the ion path could be optimized so as to minimize this effect, which can be directly associated with the cleavage of hydrogen bonds. Having excluded the ion mobility times that resulted from dissociated ions, each cluster ion appeared at a single drift time. These drift times showed a linear progression with the number of phosphoric atoms for cluster ions of the same charge state. Cross section calculations were carried out with MOBCAL on DFT optimized geometries with different hydrogen locations and with three types of atomic charges. DFT geometry optimizations yielded roughly spherical structures. Our results for nitrogen gas interaction cross sections showed that values were dependent on the atomic charges definition used in the MOBCAL calculation. This pinpointed the necessity to define a clear theoretical framework before any comparative interpretations can be attempted with uncharacterized compounds.

Absolute partial cross sections and kinetic energy analysis for the electron impact ionization of ethylene

We present absolute partial electron impact ionization cross sections for ethylene in the electron energy range between threshold and 1000 eV measured with a two sector field double focusing mass spectrometer. Ion kinetic energy distribution functions have been measured at all electron energies by applying a deflection field method. Multiplication of the measured relative cross sections by the appropriately determined discrimination factors lead to accurate relative partial cross sections. Normalization of the sum of the relative partial cross sections to an absolute total cross section gives absolute partial cross section values. The initial kinetic energy distributions of several fragment ions show the presence of two or more contributions that exhibit different electron energy dependencies. Differential cross sections with respect to the initial kinetic energy of the ions are provided and are related to specific ion production channels. The electron threshold energies for the direct and numerous other dissociative ionization channels are determined by quantum chemical calculation and these allow the determination of the total kinetic energy release and the electron energy loss for the most prominent dissociative ionization channels.     

HeIα photoionisation cross-section determination of Br atoms

An extended electron modulation spectroscopy method is described which allows the accurate determination of photoionisation cross sections of transient species relative to those of precursor compounds. In this paper cross section at 584 Å for atomic and molecular bromine transitions from neutral ground to lowest ionic states have been measured relative to that of the HBr⁺ (X²Π_1/2,3/2)←HBr(X¹Σ⁺) ionisation. Using the cross section of this HBr transition as an absolute standard and with relative cross-section data for ionisations leading to the accessible excited ionic states of Br⁺ and Br⁺₂, absolute total angle-integrated cross sections for the valence shell ionisation process in Br⁺ and Br⁺₂ are presented.     

Theoretical approach for collisional depolarization of Rydberg atoms

A theoretical analysis of the collisional depolarization of Rydberg atoms is presented. Using the general formalism of irreducible tensors for collisional depolarization and an approximate expression of the collision matrix for the Rydberg-perturber scattering, simple analytical expressions for the depolarization cross sections of different multipoles are obtained. It is shown that depolarization cross sections may be expressed in terms of the universal cross section for the collisional broadening of Rydberg states. Explicit expressions for cross sections of the one-electronnP _3/2 andnp states are presented.     

Collision induced fragmentation of mass-selected (CO2) n + clusters

The collisional velocity dependence of the cross sections for fragmentation of mass-selected (CO₂) _n ⁺ (n+2...7) clusters in collisions with Ar atoms is presented. Interesting structure can be observed in the cross sections which indicate that the collision occurs between the Ar atom and one CO₂ molecule within the cluster. The results may be explained by assuming that the collision leads to either vibrational excitation of a loosely bound CO₂ monomer which then leaves the cluster or excitation of the entire cluster to a dissociative state.     

Absorption spectra of small niobium and gold clusters measured by photodepletion of their rare gas van der Waals complexes: some preliminary experiments

The optical absorption spectra of small niobium clusters have been determined over the wavelength range 260 – 740 nm by photodetaching Krypton atoms from the corresponding neutral van der Waals, vdW, complexes, Nb_nKr_m, n=5–15, m=1–3. Cross sections for small gold clusters were determined by photodetachment experiments oncharged vdW complexes [Au_nXe_m]⁺, m=1, 2. The absorption cross sections are observed to increase monotonically with decreasing wavelength. At the long wavelength end of the range, the cross section is practically independent of the cluster nuclearity, n; whereas, at the short wavelength end of the range, the cross section increases monotonically with n.     

Comparison of collisional activation spectra of some positive ions produced both by charge reversal of negative ions and by decomposition of positive ions

The charge reversal collision induced decomposition mass analyzed ion kinetic energy spectrum of allyl anion has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of allyl cation and found to be identical except for the presence of +2 ions formed by charge stripping in the spectrum of the [C₃H₅]⁺ ion. Likewise, the collision induced dissociation mass analyzed ion kinetic energy charge reversal spectrum of [CH₃Se]^? has been compared with the collision induced dissociation mass analyzed ion kinetic energy spectrum of [CH₃Se]⁺ and found to be identical. A study of the pressure dependence of the collision induced dissociation mass analyzed ion kinetic energy spectrum of [C₃H₅]⁺ and [C₃H₅]^? showed increasing fragmentation with increasing collision gas pressure, and suggests that a greater mean number of collisions converts more energy to internal modes in the collision induced dissociation mass analyzed ion kinetic energy experiment even at low pressures.