Coherent excitation of H(n=2) in H+, H - He collisions

The coherent excitation of H(n=2) in H⁺, H - He collisions was investigated at incident energies of 5–25 keV. From a polarization analysis of the emitted Lyman-α radiation as a function of an external electric field, the partial cross sections for excitation to the H(2s) and the H(2p _m) magnetic substates and the real part of thes ?p ₀-coherence were extracted. For H⁺-He collisions, the measured partial cross sections are in fair agreement with previous two-electron calculations by Kimura and Lin; the agreement with one-electron calculations of Jain et al. is, particularly at the lower incident energies, less satisfactory. For both collision systems, an energy-dependent forward-backward asymmetry corresponding to a shift of the center-of-charge relative to the center-of-mass (dipole moment) was observed. In H⁺ - He collisions, the measured dipole moment was positive; it thus corresponds to an electron trailing behind the proton. The same analysis applied to the H - He system showed the electron riding in front of the proton.     

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.     

Analysis of impact-excited HeI states by applying magnetic and electric depolarization techniques

The magnetic depolarization of the impact radiation at λ(4¹ D?2¹ P)=492 nm and λ(3¹ D?2¹ P) = 668 nm as well as the splitting of the magnetic depolarisation signals in electric fields were investigated for 10–25 keV H ₂ ⁺ (D ₂ ⁺ )-He collisions (only 492 nm line) and for 5-22.5 keV He-He collisions (both lines). The results are compared with analogous measurements for He⁺- He and Ne⁺ - He collisions and the dynamics of the excitation process are qualitatively discussed. In particular, we emphasize the importance of inertia for the evolution of the electron cloud during the final phase of the collision process.     

Translational energy distributions and production mechanisms of the excited hydrogen atom (n = 3,4) produced in e-NH3 collisions

The Balmer α and β lines produced in e-NH₃ collisions have been measured precisely with the use of a Fabry-Perot interferometer. These lines are not polarized. The translational energy distributions of (H^*(n = 3,4) were determined from analysis of Doppler lineshapes and have five components; their peaks lie at 1, 3, 2, 4–5 and 8–12 eV. The excitation function [H^*(n = 4)] has five thresholds at 22.5, 29.0, 33.3, 38.9 and 41.7 eV, and indicates that five dissociation processes contribute to the formation of H^*. Excitation to the Rydberg states converging to the (2a₁)^?1 state of NH₃⁺ is a major process for the formation of the first and the second components. Doubly excited Rydberg states play important roles in the dissociative excitation of NH₃.     

Orientation and alignment of Mg+ (3p) states excited in 1–60 keV collisions with He and Ar

The orientation and alignment of Mg⁺ (3p) states created in 1–60 keV Mg⁺ (3s)-He, Ar collisions have been studied by the polarized photon-scattered particle coincidence technique at scattering angles corresponding to impact-parameter ranges of 0.5–1.0 a.u. (He) and 1.2–2.0 a.u. (Ar). Referring to the standard geometry, in the region of maximum excitation probability (～ 35 keV), a strong propensity for population of the Mg⁺ (3p _?1) state is observed. The propensity breaks down when going towards lower energies, and for collisions with He even a transient reversal of the angular momentum occurs. The alignment angle varies little in the entire range of impact parameters and energies investigated. These observations compare well with recent general predictions of Andersen and Nielsen.     

H+2 correlation diagram from finite element calculations

A two-dimensional, fully numerical approach to the electronic Schrödinger equation for linear molecules by the finite element technique is employed. For low-lying σ, π, δ and φ states of H⁺₂ and HeH²⁺ an accuracy of about 6 figures for the orbital energies with 211 grid points is found. Up to 10-figure accuracy is obtained with 496 grid points. An H⁺₂ correlation diagram, including states up to n = 5 of the united atom (He⁺), is given.     

Orientation of Na(3p) states excited in Na-He collisions

The orientation of Na(3p) states created in 3–13 keV Na(3s)-He collisions has been studied by the polarised photon-scattered particle coincidence technique at scattering angles corresponding to the impact-parameter range 1–2 a.u. In the standard geometry, at large impact parameters the excitation process exhibits a very high degree of orientation with about 90% of the Na(3p) states havingm ₁=?1. Strong reduction in this propensity is observed at impact parameters smaller than about 1.3 a.u., where a molecular curve crossing causes simultaneous He(n=2) excitation. In this region, also ionisation becomes important.     

Symmetry of the dissociation of hydrogen: angular dependence of the doppler profile of the excited hydrogen atom (n = 4) produced in e-H2 collisions

The angular dependence of the Doppler profile of the Balmer-β line indicates that the asymmetry parameter, b, is positive and the polarization of the electric vector, J_p, is 0.7 ± 0.1 for the formation of H¹(n=4) from H₂. Thus, H¹(n=4) is produced in a parallel transition, and the transition moment of H¹ lies along the dissociation axis. This result suggests that the intermediate states for the fast and anisotropic H¹(n=4) atoms should be of the type ¹Σ⁺_u(2pσ_u)(n/σ_g).     

Quantal close-coupling calculation of the cross sections for the j 1 m 1 → j 2 m 2 transitions in the 4p 2P and 3d 2D excited states of Ca+ induced by collisions with helium

Cross sections for the j ₁ m ₁ → j ₂ m ₂ transitions in the resonance 4p ²P and metastable 3d ²D states of the singly charged calcium ion induced by collisions with the ground-state He atom have been calculated using the quantal close-coupling method. The calculations are based on the earlier obtained Ca⁺-He pseudopotential SCF potential energy curves. The calculated cross sections are discussed in the energy range from threshold to 1.5 eV. Satisfactory agreement with other theoretical results has been found for the 4p ²P state. However, relatively large discrepancy between theory and available experimental data still exists for both the Ca⁺ states.     

Theoretical studies on structures and electronic spectra of linear carbon chains C2nH+ (n = 1−5)

The density functional theory (DFT) and the complete active space self‐consistent‐field (CASSCF) method have been used for full geometry optimization of carbon chains C_2nH⁺ (n = 1–5) in their ground states and selected excited states, respectively. Calculations show that C_2nH⁺ (n = 1–5) have stable linear structures with the ground state of X³Π for C₂H⁺ or X³Σ^? for other species. The excited‐state properties of C_2nH⁺ have been investigated by the multiconfigurational second‐order perturbation theory (CASPT2), and predicted vertical excitation energies show good agreement with the available experimental values. On the basis of our calculations, the unsolved observed bands in previous experiments have been interpreted. CASSCF/CASPT2 calculations also have been used to explore the vertical emission energy of selected low‐lying states in C_2nH⁺ (n = 1–5). Present results indicate that the predicted vertical excitation and emission energies of C_2nH⁺ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Multiple H 3 + fragment production in single collision of fast H n + clusters with He atoms

The production of H ₃ ⁺ ions resulting from single collisions of mass-selected ionic hydrogen clusters, H _n ⁺ (n=9, 25, 31), with helium at high velocity (1.55 times the Bohr velocity) has been studied. A strong double H ₃ ⁺ ion production resulting from one incident cluster is observed. Moreover, evidence for a triple H ₃ ⁺ fragment production is presented forn=25 and 31. Thus, in this energy range, the collision gives rise to multifragmentation processes. The formation of H ₃ ⁺ ions takes place in the fragmentation of the multicharged cluster resulting from the collision.     

Luminescence in near-thermal charge exchange. II. He+ + H2O and He+ + D2O

An ICR spectrometer fitted with synchronous photon counting equipment is used to study the emission produced by near-thermal (? 0.1 eV) collisions between He⁺ and H₂O (D₂). Within the investigated wavelength region, 185 to 500 nm, the only significant emission features are the A³Π (υ' ? 3) → X³Σ^? bands in OH⁺ and OD⁺, and the A²Σ⁺ → X²Π(0.0) band in OH and, possibly, in OD. The corresponding excitation rate constants represent only ? 2% of the total He⁺/H₂O (D₂O) charge transfer. The resonant electron-jump model for thermal-energy charge exchange is discussed in the light of recent information on the He⁺/H₂O reaction and on the excited states of H₂O⁺ and their excitation by electron and photon impact on H₂O (D₂O).     

Polarisation effects in electron transfer reactions with laser excited Na(3P) at medium energies

We report new experimental data for the investigation of the role of electronic orbital alignment and orientation in charge transfer processes, in the medium energy range where the collision velocityv _c and the velocity of the active electronv _e are of the same order of magnitude. The results obtained for the H ₂ ⁺ -Na(3p) and He⁺-Na(3p) collisions are discussed in comparison with the experimental and theoretical findings obtained for the H⁺-Na(3p) system. Recent time-of-flight measurements for charge transfer in Li⁺-Na(3s and 3p) collisions are also presented.     

Magnetic properties of lanthanide organometallic sandwich complexes produced in a molecular beam

The magnetic properties of gas-phase terbium–cyclooctatetraene multi-decker sandwich complexes, Tb_n(C₈H₈)_n+1 were measured using a Stern-Gerlach type magnetic deflection approach. Beams of Tb_n(C₈H₈)_n+1 complexes displayed one-sided deflection toward high field – indicating that fast spin relaxation occurs within the complexes as they pass through the magnetic field. The magnetic moment for Tb_n(C₈H₈)_n+1 (n = 1−5) was evaluated using the Langevin model. Evolution of magnetic moment with the complex size is discussed with electronic structures for oxidation states of Tb^3+/2+ ions, implying the possibility of antiferromagnetic interaction of two adjacent Tb²⁺ ions.     

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.     

Stable shell structures in hydrogen-bonded complexes

Neutral clusters (NH₃) _n ((CH₃)₃N) _m and (H₂O) _n ((CH₃)₃N) _m , prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time-of-flight mass spectrometry technique. The observed mixed cluster ions display a maximum intensity atm=2(n+1) whenn ≦ 5 for (NH₃) _n ·((CH₃)₃N) _m H⁺ andm=n+2 whenn ≦ 4 for (H₂O) _n ((CH₃)₃N) _m H⁺ indicating that the cluster ions with these combinations have a stable closed shell structure. However, the pattern begins to break down whenn>5 for ammonia system andn>4 for water system. Thereupon, the most intense peaks are reached with one molecule less than the pattern required, i.e.m=2(n+1)?1 whenn=6 for (NH₃) _n ((CH₃)₃N) _m H⁺ andm=(n+2)?1 whenn=5 for (H₂O) _n ((CH₃)₃N) _m H⁺. These findings strongly suggest the onset of hydrogen-bonded ring structures from chain-like ones at critical cluster sizes. This is also supported by the studies of the metastable decomposition.     

Measurement and calculation of the fine structure changing collision cross sections in the Mg+ and Ca+ resonance states by helium

Using laser ablation of solid samples in a buffer gas atmosphere, free magnesium and calcium ions are produced within a confined volume of a laser produced plasma above the sample surface. The fine structure transfer cross sections of the Mg⁺ and Ca⁺ resonance states by collisions with helium are measured at different gas temperatures by laser induced fluorescence. The experimental results are compared with theoretical cross sections of fine structure changing collisions taking into account radial and angular mixing of the relevant molecular terms. Good agreement between theory and experiment is obtained in the case of Mg⁺-He. For Ca⁺-He, the theoretical and experimental data do not agree. Possible systematic errors in the fine structure transfer measurements of Ca⁺ by helium are discussed.     

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.     

Ion—molecule reactions in GeH4/SiH4 systems studied by ion trap mass spectrometry

Gas phase ion—molecule reactions occurring in GeH₄/SiH₄ systems under different partial pressures and their mechanisms have been investigated by ion trap mass spectrometry (ITMS). SiH⁺_n (n=0–3) and GeH⁺_n (n = 0–3) are the main ionic species at zero reaction time when the GeH₄: SiH₄ ratio is in the range 1:1 to 1:12. Self-condensation sequences are observed at increasing reaction times. Moreover, formation of ions containing GeSi bonds, such as GeSiH⁺_n (in = 2–5) and GeSi₂H⁺_n (n = 4, 5), occurs by reactions of Si₂H⁺_n (n = 2–5) and Si₃H⁺_n (n = 4, 5) with GeH₄. At longer reaction times, further substitution of silicon with germanium in GeSiH⁺_n (n = 2–5) ions has been observed, to give Ge₂H⁺_n (n = 2–5).     

Reactivity of positively charged cobalt cluster ions with CH4, N2, H2, C2H4, and C2H2

Reactivity of positively charged cobalt cluster ions (Co _n ⁺ ,n=2?22), produce by laser vaporization, with various gas samples (CH₄, N₂, H₂, C₂H₄, and C₂H₂) were systematically investigated by using a fast-flow reactor. The reactivity of Co _n ⁺ with the various gas samples is qualitatively consistent with the adsorption rate of the gas to cobalt metal surfaces. Co _n ⁺ highly reacts with C₂H₂ as characterized by the adsorption rate to metal surfaces, and it indicates no size dependence. In contrast, the reactions of Co _n ⁺ with the other gas samples indicate a similar cluster size dependence; atn=4, 5, and 10?15, Co _n ⁺ highly reacts. The difference can be explained by the amount of the activation energy for chemisorption reaction. Compared with neutral cobalt clusters, the size dependence is almost similar except for Co ₄ ⁺ and Co ₅ ⁺ . The reactivity enhancement of Co ₄ ⁺ and Co ₅ ⁺ indicates that the cobalt cluster ions are presumed to have an active site for chemisorption atn=4 and 5, induced by the influence of positive charge.