Relative Total Cross Sections Research Articles

Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O2-) Collisions with Benzene (C6H6) Molecules.

In this study, novel experimental total electron detachment cross sections for O2− collisions with benzene molecules are reported for the impact energy range (10–1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160–900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39–42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.

Ionization and electron capture for H+ collisions at low keV energy with CCl4 molecules

Relative total ionization cross sections and fragment target ions have been measured for proton impact on CCl4 molecules for the first time, namely associated with the formation of CCl3+, CCl2+, CCl+, Cl2+, Cl+ and C+ ions. The experiment was conducted by crossing a proton beam of 2.5 to 10 keV with a CCl4 vapor target where the fragment ions were measured employing the time of flight technique. Relative fragment cross sections were added leading to the total relative cross section. All relative cross sections seem to be independent of the collisional energy. A qualitative comparison was conducted with previous measurements of electron impact experiments. Ratios of CCl3+/CCl2+, CCl3+/CCl+, CCl3+/C+ and CCl3+/Cl+ were compared to those obtained from electron impact experiments.

Electron impact ionization of the gas-phase sorbitol

Ionization and dissociative ionization of the sorbitol molecule by electron impact have been studied using two different experimental methods. In the mass range of m/z = 10–190, the mass spectra of sorbitol were recorded at the ionizing electron energies of 70 and 30 eV. The ion yield curves for the fragment ions have been analyzed and the appearance energies of these ions have been determined. The relative total ionization cross section of the sorbitol molecule was measured using monoenergetic electron beam. Possible fragmentation pathways for the sorbitol molecule were proposed.

Electron transfer processes of atomic and molecular doubly charged ions: information from beam experiments

Single-electron transfer reactions in collisions of atomic and molecular doubly charged ions, with atoms and molecules, were investigated in a series of crossed-beam scattering, translational spectroscopy and product luminescence experiments. Investigation of a series of atomic dication-atom electron transfer at collision energies of 0.1–10 eV provided data on differential and relative total cross sections of state-to-state processes. Populations of electronic and vibrational states and rotational temperatures of molecular product ions were obtained from studies of non-dissociative electron transfer in systems containing simple molecular dications and/or molecular targets. The product electronic states populated with highest probability were those for which the translational energy release was 3–5 eV, indicating that the ‘reaction window' concept, based on the Landau–Zener formalism, is applicable also to molecular systems. Population of the vibrational states of the molecular products could be described by Franck–Condon factors of the vertical transitions between the reactant and product states, especially at higher (keV) collision energies. Rotational temperature of the product molecular cations was found to be surprisingly low, mostly 400–500 K, practically the temperature of the ion source.

State-to-state differential and relative integral cross sections for rotationally inelastic scattering of H2O by hydrogen

State-to-state differential cross sections (DCSs) for rotationally inelastic scattering of H2O by H2 have been measured at 71.2 meV (574 cm−1) and 44.8 meV (361 cm−1) collision energy using crossed molecular beams combined with velocity map imaging. A molecular beam containing variable compositions of the (J = 0, 1, 2) rotational states of hydrogen collides with a molecular beam of argon seeded with water vapor that is cooled by supersonic expansion to its lowest para or ortho rotational levels (JKaKc = 000 and 101, respectively). Angular speed distributions of fully specified rotationally excited final states are obtained using velocity map imaging. Relative integral cross sections are obtained by integrating the DCSs taken with the same experimental conditions. Experimental state-specific DCSs are compared with predictions from fully quantum scattering calculations on the most complete H2O-H2 potential energy surface. Comparison of relative total cross sections and state-specific DCSs show excellent agreement with theory in almost all details.

K-shell and total ionization cross sections following electron-molecule collisions: An empirical scaling law

Collisions between electrons and various molecular targets (H{sub 2}O, CH{sub 4}, C{sub 3}H{sub 4}, N{sub 2}) at projectile energies above the K-shell ionization threshold of the molecule have been investigated experimentally. From electron emission spectra, relative total ionization cross section {sigma}{sub t} and K-shell ionization cross section {sigma}{sub K} are determined. The ratio {sigma}{sub K}/{sigma}{sub t} is then deduced for each target as a function of the projectile energy and compared with those evaluated in the case of atomic targets. Strong differences between atomic and molecular targets are observed in the slope of the ratio at the highest projectile energies. These differences are explained using the well-known Kim-Rudd formula developed for atomic targets. In the projectile energy range we explored, we develop a simple empirical scaling law for the ratio {sigma}{sub K}/{sigma}{sub t} as a function of the projectile energy.

Fragmentation and internal steric rearrangement of C60- in electron transfer dynamics

The crossed molecular beam technique is used for producing C60 - species through potassium-buckyball collision processes, and studying the collision dynamics in an energy ranging from 10 eV up to 500 eV. At low collision energies only the negative parent ion is formed. As long as the collision energy is increased the fragmentation pattern of the fullerene negative ion could be identified and relative total cross-sections could be measured. Surprisingly, some satellite contributions have been observed in the band structure of C60 - parent ion time-of-flight spectrum, which points to the existence of distinct conformational C60 - isomers, which are likely to be formed during the collision, favoured by the strong polarization of the C60 - in the presence of the K+ projectile ion. Such presumed detection of different spheroidal metastable C60 - conformers was made possible due to an electric effect operating at the nanoscale.

Above-threshold ionization near the3p4dFo1autoionizing state in magnesium

Two-photon above-threshold ionization (ATI) relative cross sections from the $3\phantom{\rule{0.2em}{0ex}}^{1}P$ state of Mg have been measured using two-color ionization in the focus of a magnetic bottle spectrometer and have been calculated using the Green's-function method in the Feshbach formalism and an ${L}^{2}$-integrable close-coupling approach, with a basis of ${L}^{2}$-integrable $B$-spline functions. We report these cross sections in the region of $3d4p$ $^{1}F^{o}$ autoionizing state, with photon energies of $3.3--3.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This is one of the few direct comparisons between ab initio theory and experiments in ATI in the vicinity of a Feshbach resonance. A good agreement between theory and experiment is found in the relative total cross sections.

State selected reactions of krypton ions with methane

Using a photo-electron product-ion coincidence method, the reactions of Kr +( 2P 1/2,3/2) ions with methane molecules are studied in the relative kinetic energy range between 0.1 and 1 eV. The relative total cross-sections and the branching ratios for production of CH 4 + and CH 3 + are determined selectively for the two krypton ions. Using available absolute reaction rates for unselected krypton ions, absolute selective rates are obtained. In addition, a simple model is presented that describes the reactions quantitatively in terms of two distinct mechanisms: (1) reactions via “close collisions”, and (2) reactions via charge exchange in “distant collisions”. Based on this model, evidence for breakdown of the “orbiting transition state theory” for the description of the close collision complex at collision energies above about 0.4 eV, is obtained.

Dynamics of chemical and charge transfer reactions of molecular dications: beam scattering and total cross section data on CF 2D + (CF 2H +), CF 2+, and CF + formations in CF 22+ + D 2(H 2) collisions

Dynamics of formation of the chemical rearrangement product CF 2D +, the charge transfer product CF 2 +, and the dissociative products CF + and CFD + in collisions of the molecular dication CF 2 ++ with D 2 was investigated in crossed beam scattering experiments over the collision energy range 0.3–1.0 eV (center of mass). The scattering data show that coulomb repulsion between two singly charged products, CF 2 + + D + and CF 2 + + D 2 +, plays a dominant role in the nondissociative processes. A large fraction of the energy available (about 6 eV in the chemical reaction, about 4 eV in the charge transfer) goes into relative translational energy of the products. Relative total cross sections for formation of the nondissociative and dissociative products in collision of CF 2 ++ with D 2 and H 2 were determined over the collision energy range of 0.2–3.6 eV. The shape of the relative velocity dependence of the cross section for CF 2 + formation can be described by a simple model based on the Landau-Zener formalism. The data suggest that the dissociative product CF + is formed prevailingly in a subsequent dissociation of the charge transfer product CF 2 +. A potential surface model is described which accounts for competition of various processes in dication–neutral collisions.

Photodetachment of theCa−ion at the3Pkpthreshold

A crossed laser-ion beam apparatus has been used to spectroscopically analyze electrons detached from an 80-keV beam of Ca{sup {minus}} ions at a photon energy of 1.903 eV. At this energy only the {sup 1}Sthinspks,d photodetachment channel is open. The photon flux dependence and the angular dependence of the measured electron yield were used to determine the photodetachment cross section {sigma}=48(11) Mb and the asymmetry parameter {beta}=0.90(3), respectively. The measured cross section is in significant disagreement with a recent {ital R}-matrix calculation. The present measurement has been used to establish an absolute scale for previous relative total cross-section data over a photon energy range 1.1{endash}3.1 eV. thinsp {copyright} {ital 1999} {ital The American Physical Society}

Structure and dynamics of the4p→ns,mdautoionizing resonances between the3Pand1Sthresholds in atomic bromine

The relative partial photoionization cross sections ${\ensuremath{\sigma}}_{i}$ and photoelectron angular distribution parameters ${\ensuremath{\beta}}_{i}$ are measured for all possible final ionic states of ${\mathrm{Br}}^{+}$ between the ${}^{3}{P}_{2}$ and ${}^{1}{S}_{0}$ thresholds. The decay patterns of the autoionizing ${4p}^{4}{}^{3}{P}_{1,0}n\mathcal{l}$, ${4p}^{4}{}^{1}{D}_{2}n\mathcal{l}$, and ${4p}^{4}{}^{1}{S}_{0}n\mathcal{l}$ Rydberg series arising from the $4\stackrel{\ensuremath{\rightarrow}}{p}ns,md$ excitations are observed at the fine-structure level in all available channels. For each Rydberg series, the energies, quantum defects, and photoelectron angular distribution parameters are determined, as well as the widths and shape parameters of the low-lying members. Relative total cross sections are derived from the partial cross sections and from ion-yield measurements over the lowest members of the ${}^{3}{P}_{1,0}$ and ${}^{1}{D}_{2}$ series. All major spectroscopic and dynamic properties of these series are reported for this open-shell atom. Conclusions are drawn from the comprehensive data sets in comparison with other halogen atoms and the neighboring closed-shell rare gas atoms. Our results, encompassing the entire autoionization regime, are compared with other experimental data and theoretical calculations, where available.

Auto-ionization of the collisional complexes of metastable neon and H2, D2, or HD

Abstract The ionization of H 2 , D 2 , and HD molecules by metastable neon atom collisions is studied in a crossed beam experiment. The relative total ionizatin cross sections and the product branching ratios been measured as a function of the collision energy in the laboratory range 0.04–0.3 eV. Penning (H 2 + , D 2 + , and HD + , associative (NeH 2 + , NeD 2 + , and NeHD + ) and rearrangement (NeH + and NeD + ) ionization occur in all the three isotopic variants, Penning ionization being the dominant channel. For the rearrangement it is observed that NeD + is produced with more efficiency than NeH + . In the case of Ne ∗ -HD, an NeD + /NeH + branching ratio of ∼1.5 is observed. This ratio is below the statistical expectation for the isotope effect. The total ionization cross sections are analyzed in terms of an optical potential. The opacity function, provided by this potential, is then used for a first approach to the description of the collisional auto-ionization dynamics through a two step model.

Dynamics of charge transfer and chemical reactions of doubly-charged ions at low collision energies

Abstract Results of beam scattering studies on the dynamics of single charge transfer and chemical reactions of doubly-charged ions carried out in Prague are reviewed. Investigation of several atomic ion-atom charge transfer processes at collision energies 0·1-10 eV provides data on differential and relative total cross-sections of state-to-state processes. Populations of electronic and vibrational states of molecular product ions are obtained from experiments on molecular dication-atom and atomic ion (He2+)-molecule systems. Studies of chemical reactions of the dication CF2+ 2 with D2 show that Coulomb repulsion between the products (CF2D+ + D+) governs the dynamics and energy partitioning.

Photodetachment of Li- from the Li 3s threshold to the Li 6s threshold.

Eigenchannel R-matrix calculations (including effects of long-range multipole interactions beyond the reaction volume) for ${\mathrm{Li}}^{\mathrm{\ensuremath{-}}}$ photodetachment partial cross sections from the vicinity of the Li 3s threshold to the Li 6s threshold (3.8 eV \ensuremath{\le}\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\le} 5.65 eV) are presented. Excellent agreement with the relative total cross section measurements of U. Berzinsh et al. [Phys. Rev. Lett. 74, 4795 (1995)] in the vicinity of the Li 3s and Li 3p thresholds is found. The calculated resonance structures are analyzed in detail. In particular, the energy region between the Li 4s and Li 5p thresholds (for which there are as yet no experimental measurements) is shown to have types of doubly excited resonances which are prominent only because of the nonhydrogenic ${\mathrm{Li}}^{+}$ core; such types are weak or absent in higher-energy regions as well as in ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ photodetachment spectra. \textcopyright{} 1996 The American Physical Society.

Experimental Identification of Saddle-Point Electrons

Experimental and theoretical velocity distributions of electrons ejected in 1--6 keV ionizing H[sup +-]H collisions are presented. Both the experimental velocity distributions and the relative total ionization cross sections can be explained by the theory in a fully consistent way. Strong evidence is found for the existence of a saddle point ionization mechanism at collision energies of 4 keV and higher.

Steric effects in total integral reaction cross sections for Sr+HF(v=1,j=1,m=0)→SrF+H

In a crossed beam study we have investigated the reactive scattering of Sr atoms from state prepared HF molecules. All measurements have been performed for two different directional distributions (alignments) of the molecular axis of HF favoring either end-on or side-on attacks. Using a surface ionization detector we have measured angular distributions of the total SrF flux at various collision energies ranging from Etr=230 to 626 meV. We found that the total product flux as well as the relative total integral reaction cross sections deduced from these data are markedly influenced by the prepared alignments (steric effect). The energy dependence of the steric effect has been quantitatively rationalized using the angle-dependent line-of-centers model. The resulting anisotropic height of the potential energy barrier features a minimum at a bent transition state. Ab initio potential energy surfaces calculated for the homologous systems Mg+HF and Ca+HF substantiate this result.

Excitation of the nitro group in nitromethane by electron transfer

In this paper we report on complementary measurements on ion-pair formation in collisions between K atoms and CH3NO2 molecules. The experiments were performed in a c.m. energy range from 20 up to 300 eV. Double differential cross sections were obtained by measuring the K+ ion yield as a function of the scatter angle and as a function of the post-collision laboratory energy. On the other hand, relative total partial cross sections for the formation of CH3NO−2, NO−2, and O− were measured in the same energy range. The experimental results lead to the conclusion that in this energy range electron transfer takes place to three ionic states of CH3NO−2, a dominantly repulsive 2A1 state and two 2B1 states with relatively deep potential wells.

Observation of multiphoton detachment of the H- ion.

We have observed nonresonant multiphoton electron detachment of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions in moderately intense (a few tens of GW/${\mathrm{cm}}^{2}$) laser fields. A well-collimated beam of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions with an energy of 581 MeV was intersected by focused 10.6-\ensuremath{\mu}m radiation from a pulsed ${\mathrm{CO}}_{2}$ laser. The center-of-mass photon energy was tuned using the relativistic Doppler shift so that the minimum number of simultaneous photons required for electron detachment ranged from three to sixteen. Definite signals were observed for the minimum photon number ranging from three to eight. Our preliminary results show evidence for structure in the relative total cross section.

Crossed-beam reactive scattering of F2 plus C6H6: Heat of formation of i p s o-fluorocyclohexadienyl radical

Angular distributions and relative total cross sections were measured for products of the collisions of F2 with C6 H6 at relative collision energies of 14 to 26 kcal mol−1 . The most conspicuous product is the ipso-fluorocyclohexadienyl radical i-C6 H6 F ̇, which displays a rather narrow peak very near the center-of-mass angle at all collision energies studied. Product C6 H5 F is masked at M/e=96 by dissociative ionization of C6 H6 F ̇ near the center-of-mass angle, but becomes observable at smaller angles to which little or no C6 H6 F ̇ is scattered. This means that at least one reaction C6 H6 +F2 →C6 H5 F+HF occurs, and does so in such a way that the products acquire high translational energies. The dependence on collision energy of the total cross section for the production of C6 H6 F ̇ was probably measured, and displays a well-marked onset at 13.9±0.3 kcal mol−1. The center-of-mass system means final state translational energy as a function of collision energy was obtained from the angular distributions and goes to zero at 13.9±0.5 kcal mol−1 . This probably means that the onset is a true threshold, from which the heat of formation of the ipso-fluorocyclohexadienyl radical is calculated to be 14.8±0.4 kcal mol−1. The mean final state translational energy asymptotically approaches 3.7 kcal mol−1 as the collision energies are increased, which is so large that only a fraction of the vibrational modes of C6 H6 F ̇ can be involved in the energy partitioning. This conclusion is confirmed by the center-of-mass angular distribution, which favors forward scattering of product F atoms, but only by about 10%–30%. This work shows that the crossed-beam method can be used to make precision thermochemical measurements of gaseous polyatomic free radicals. It also reports the investigation of a polyatomic–polyatomic reaction in crossed beams, for which type of study there are still very few examples.