Penning Ionization Cross Sections Research Articles

Calculation of Deexcitation Probability of Ne(3P2) by Ar for the Case E>>D

The deexcitaion probability calculation of the total Penning ionization cross section for Ne(3P2) by Ar has been made at collisional energy 38.1 meV. The comparison of deexcitationcross sections, polarizability, α/β , deexcitation probability, {IP(M)/IP(Ne*)}1/2 and the welldepth of interaction potentials for Ne(3P2) by Ar for the case E >> D has been made.DOI: http://dx.doi.org/10.3126/jncs.v30i0.9335Journal of Nepal Chemical Society Vol. 30, 2012 Page: 52-55 Uploaded date: 12/16/2013

Two-Dimensional Penning Ionization Electron Spectroscopy of Open-Shell Metallocenes: Outer Valence Ionic States of Vanadocene and Nickelocene

In order to investigate outer valence ionic states of open-shell metallocenes, we have applied two-dimensional collision-energy/electron-energy-resolved Penning ionization electron spectroscopy (2D-PIES) upon collision with metastable He*(2(3)S) excited atoms as well as a high level ab initio molecular orbital calculation (the partial third-order quasiparticle theory of the electron propagator (P3)) to ionization from neutral ground states of vanadocene ((4)A2g) and nickelocene ((3)A2g). Assignments of observed Penning ionization electron/He I ultraviolet photoelectron spectra were consistent with the P3 calculation results for ionization of α and β spin electrons except for electron correlation bands observed by PIES. Negative collision energy dependence of partial Penning ionization cross-sections (CEDPICS) indicate attractive interaction with He*(2(3)S) around the molecule. Results by model potential calculation utilizing Li(2(2)S) instead of He*(2(3)S) for interaction between He*(2(3)S) and open-shell metallocenes do not explain the strong negative CEDPICS of the bands observed in PIES.

Ionization in collisions between metastable hydrogen atoms

Associative and Penning ionization cross sections are calculated for collisions between metastable hydrogen 2s atoms at thermal energies. Cross sections for deuterium 2s collisions are also reported. The associative ionization cross sections behave as E −1 for collision energy E, in agreement with an existing experiment. The Penning ionization cross sections dominate for all energies and are found to follow the E −2/3 behavior that was predicted in previous work for the total ionization cross section. The magnitudes of our theoretical associative ionization cross sections for H(2s) + H(2s) collisions are between two and four times larger than the experimental data.

Light assisted collisions with cold metastable neon atoms

Control of the combined Penning and associative ionization cross section is demonstrated with cold (~1mK) metastable Ne (3s3P2) atoms in a magneto-optical trap (MOT). By illuminating the trapped atoms with a near resonant probe laser beam, increased ionization rates are observed at several detunings. The probe beam is swept through a region from +500MHz to −500MHz. The increase in the Penning and associative ionization cross section is observed in both the red and blue regions of the spectrum.

Collision-Energy-Resolved Penning Ionization Electron Spectroscopy of Glycine with He(23S) Metastable Atoms: Conformational Isomers in Collisional Ionization

Conformationally dependent ionization of the simplest amino acid, glycine, is studied by Penning ionization electron spectroscopy with velocity-resolved metastable He*(2(3)S) atoms. The observed He I ultraviolet photoelectron and Penning ionization electron spectra are reproduced by superimposed theoretical spectra, assuming thermal distributions of conformers. The conformations of amino acids are determined by analyzing the observed Penning ionization cross sections, peak shifts, and collision energy dependences of partial ionization cross sections (CEDPICS). The Penning ionization cross sections are governed by collisionally accessible exterior electron densities. When the amino and carbonyl groups are exposed to He* access, the nonbonding orbitals of N (n(N)) and O atoms (n(O)) give rise to strong bands. The observed negative peak shifts and negative CEDPICS for the n(N) and n(O) orbitals suggest the presence of attractive interactions around their electron distributions. The most attractive wells are estimated to be approximately 400 meV in the direction of the n(N) orbitals by ab initio model calculations. A conformer possessing dual hydrogen bonds contributes predominantly to the spectra.

Outer Valence Ionic States of Cr(CO)6and (η5-C5H5)Co(CO)2Observed by Two-Dimensional Penning Ionization Electron Spectroscopy

Outer valence ionic states of Cr(CO)(6) and (eta(5)-C(5)H(5))Co(CO)(2) were investigated by means of Penning ionization electron spectroscopy (PIES) upon collision with metastable He*(2(3)S) excited atoms as well as high-level ab initio molecular orbital calculations taking electron correlation effects into consideration. By the 2D measurement combining the collision-energy-resolved technique of the metastable atomic beam and electron spectroscopy, ionic-state-resolved measurement of collision energy dependence of partial ionization cross sections (CEDPICS) was carried out. Because partial Penning ionization cross sections can be connected to the spatial extent of corresponding molecular orbitals (MOs) outside the boundary surface of colliding He* atoms, different slopes of CEDPICS were related to anisotropic interaction around the MO region. The observed CEDPICS and PIES band intensity were consistent with the order of calculated ionic states.

Penning ionization electron spectra of pyrene, chrysene, and coronene in collision with metastable He(2 3S) atoms in the gas phase

Penning ionization electron spectra (PIES) of pyrene (C(16)H(10)), chrysene (C(18)H(12)), and coronene (C(24)H(12)) in the gas phase are recorded using metastable He(*)(2 (3)S) atoms. The assignments of PIES are presented based on the outer valence Green's function calculations with the correlation consistent polarized valence triple-zeta basis sets and the exterior electron density calculations of contributing molecular orbitals. The definite positions of all of the pi bands in the PIES are identified making use of the large PIES cross sections. Broad bands are observed in low-electron-energy regions for chrysene and coronene and are ascribed to ionization processes of non-Koopmans types from sigma orbitals. The anisotropic interaction potential energy surfaces for the colliding systems are obtained from ab initio model potentials for the related systems with similar outer valences Li(2 (2)S) + C(16)H(10), C(18)H(12), and C(24)H(12), respectively. The attractive well depths in the out-of-plane directions are found to be similar between these molecules, and the repulsive walls embrace the in-plane perimeters uniformly. Collision energy dependencies for partial Penning ionization cross sections and negative peak shifts in PIES for chrysene support these anisotropic interactions. Effects from thermal populations in low-frequency vibrational modes are estimated to be minor in one-electron ionization processes.

Stereodynamics and Outer Valence Ionic States of Ferrocene in Collisional Ionization with a He*(23S) Metastable Atom by Two-Dimensional Penning Ionization Electron Spectroscopy

Outer valence ionic states of ferrocene were investigated by means of Penning ionization electron spectroscopy upon collision with metastable He*(2(3)S) excited atoms. By two-dimensional measurement combining electron spectroscopy and collision-energy-resolved technique of the metastable atomic beam, ionic-state-resolved measurements of collision energy dependence of partial Penning ionization cross sections (CEDPICS) were carried out. Since the partial Penning ionization cross sections can be connected with spatial extension of corresponding molecular orbitals (MOs) outside the boundary surface for the collision with He* atoms, different slope values of CEDPICS were related with stereodynamics in Penning ionization as well as anisotropic interaction around the ionization region: attractive interaction around ligand pi orbitals and repulsive interaction around ligand sigma orbtials and the metal atom. The observed negative collision energy dependence of ionization cross sections for the first ((2)E(2)') and second ((2)A(1)') ionic states was consistent with configuration interactions suggested by ab initio MO calculations [J. Chem. Phys. 2002, 117, 6533], which is due to the strong electron correlation effects beyond the one-electron MO description.

Anisotropic Interaction and Stereoreactivity in a Chemi-Ionization Process of OCS by Collision with He*(23S) Metastable Atoms

Ionic-state-resolved collision energy dependence of Penning ionization cross sections for OCS with He*(2(3)S) metastable atoms was measured in a wide collision energy range from 20 to 350 meV. Anisotropic interaction potential for the OCS-He*(2(3)S) system was obtained by comparison of the experimental data with classical trajectory simulations. It has been found that attractive potential wells around the O and S atoms are clearly different in their directions. Around the O atom, the collinear approach is preferred (the well depth is ca. 90 meV), while the perpendicular approach is favored around the S atom (the well depth is ca. 40 meV). On the basis of the optimized potential energy surface and theoretical simulations, stereo reactivity around the O and S atoms was also investigated. The results were discussed in terms of anisotropy of the potential energy surface and the electron density distribution of molecular orbitals to be ionized.

Probing anisotropic interaction potentials of unsaturated hydrocarbons with He*(2S3) metastable atom: Attractive-site preference of σ-direction in C2H2 and π-direction in C2H4

State-resolved collision energy dependence of Penning ionization cross sections of acetylene (C2H2) and ethylene (C2H4) with He*(2 3S) metastable atoms was observed in a wide collision energy range from 20 to 350 meV. A recently developed discharge nozzle source with a liquid N2 circulator was employed for the measurements in the low-energy range from 20 to 80 meV. Based on classical trajectory calculations for the energy dependence of the partial ionization cross sections, anisotropic potential energy surfaces for the present systems were obtained by optimizing ab initio model potentials for the chemically related systems Li+C2H2 and C2H4. In the case of C2H2, the global minimum was found to be located around the H atom along the molecular axis with a well depth of 48 meV (ca. 1.1 kcal/mol). On the other hand, a dominant attractive well with a depth of 62 meV (ca. 1.4 kcal/mol) was found in the piCC electron region of C2H4. These findings were discussed in connection with orbital interactions between molecular orbitals of the target molecules and atomic orbitals of the metastable atom. It is concluded that sigma-type unoccupied molecular orbitals of C2H2 and a piCC-type highest occupied molecular orbital of C2H4 play a significant role for the attractive-site preference of sigma direction in C2H2 and pi direction in C2H4, respectively.

Development of a cooled He*(2S3) beam source for measurements of state-resolved collision energy dependence of Penning ionization cross sections: Evidence for a stereospecific attractive well around methyl group in CH3CN

A low-temperature discharge nozzle source with a liquid-N(2) circulator for He*(2(3)S) metastable atoms has been developed in order to obtain the state-resolved collision energy dependence of Penning ionization cross sections in a low collision energy range from 20 to 80 meV. By controlling the discharge condition, we have made it possible to measure the collision energy dependence of partial ionization cross sections (CEDPICS) for a well-studied system of CH(3)CN+He*(2(3)S) in a wide energy range from 20 to 350 meV. The anisotropic interaction potential energy surface for the present system was obtained starting from an ab initio model potential via an optimization procedure based on classical trajectory calculations for the observed CEDPICS. A dominant attractive well depth was found to be 423 meV (ca. 10 kcal/mol) at a distance of 3.20 A from the center of mass of CH(3)CN in the N-atom side along the CCN axis. In addition, a weak attractive well (ca. 0.9 kcal/mol) surrounding the methyl group (-CH(3)) has been found and ascribed to the interaction between an unoccupied molecular orbital of CH(3)CN and 2s atomic orbital of He*(2(3)S).

Probing the Shape and Stereochemistry of Molecular Orbitals in Locally Flexible Aromatic Chains: A Penning Ionization Electron Spectroscopy and Green's Function Study of the Electronic Structure of Biphenyl

We report on the results of an exhaustive study of the interplay between the valence electronic structure, the topology and reactivity of orbitals, and the molecular structure of biphenyl by means of Penning ionization electron spectroscopy in the gas phase upon collision with metastable He*(2(3)S) atoms. The measurements are compared with one-particle Green's function calculations of one-electron and shake-up valence ionization spectra employing the third-order algebraic diagrammatic construction scheme [ADC(3)]. Penning ionization intensities are also analyzed by means of the exterior electron-density model and comparison with photoelectron spectra: in contrast with the lines originating from sigma orbitals, ionization lines belonging to the pi-band system have large Penning ionization cross sections due to their greater extent outside the molecular van der Waals surface. The involved chemi-ionization processes are further experimentally investigated using collision-energy-resolved Penning ionization electron spectroscopy. The cross sections of pi-ionization bands exhibit a markedly negative collision-energy dependence and indicate that the interaction potential that prevails between the molecule and the He*(2(3)S) atom is strongly attractive in the pi-orbital region. On the other hand, the partial ionization cross sections pertaining to sigma-ionization channels are characterized by more limited collision-energy dependencies, as a consequence of rather repulsive interactions within the sigma-orbital region. A comparison of ADC(3) simulations with the Penning ionization electron spectra and UV photoelectron spectra measured by Kubota et al. [Chem. Phys. Lett. 1980, 74, 409] on thin films of biphenyl deposited at 170 and 109 K on copper demonstrates that biphenyl molecules lying at the surface of polycrystalline layers adopt predominantly a planar configuration, whereas within an amorphous sample most molecules have twisted structures similar to those prevailing in the gas phase.

Determination of outer shape of molecular orbitals based on two-dimensional Penning ionization electron spectroscopy for N 2 and CO by He*2 3S

Classical trajectory calculations for collision energy dependence of partial Penning ionization cross-sections (CEDPICS) of He*(2 3S) + (N 2,CO) were performed to obtain molecular orbital (MO) functions based on the observed data; coefficients in linear combination of atomic orbitals and orbital exponents for a minimal atomic basis set (STO-6G) were optimized in order to give good agreement between observed and calculated CEDPICS. Although a crude basis set was assumed, obtained MOs derived by experimental CEDPICS were found to be very similar in the outer characteristics to theoretically obtained SCF-MOs in terms of much larger basis sets (6-311+G*).

Probing branching mechanism through resonance-type oscillation in the collision-energy dependence of Penning ionization reaction of Ar (3P) with oriented CH3I

The collision energy and orientation-angle-resolved Penning ionization cross sections were determined for the Ar (3P)+CH3I reaction. A remarkable depletion was recognized in the collision- energy dependence of the orientation-angle-resolved Penning ionization cross section. The depletion could be interpreted as the competitive neutral dissociation from the Rydberg states formed by an electron exchange mechanism related to a 3a1 molecular orbital. The stereo selectivity for the Penning ionization and neutral dissociation were discussed. From the series of our studies on Ar (3P)+CH3X (X=Cl,Br,I) reactions, it was substantiated that the orientation angle and collision-energy-resolved Penning ionization cross section give an important information about the branching mechanism to the Rydberg states that are linked to the neutral dissociation channels.

Collision-energy-resolved Penning ionization electron spectroscopy of OCS with He *(2 3S) metastable atoms

Collision-energy dependence of partial Penning ionization cross-sections (CEDPICS) of OCS was obtained from collision-energy/electron-energy-resolved two-dimensional spectrum with a highly sensitive electron energy analyzer. The negative CEDPICS for X 2 Π and C 2Σ + states indicates attractive interaction around S and O atoms with He *(2 3S), where the corresponding 3π(S 3p–O 2p) and 8σ(σ CO) orbitals extend, respectively. Utilizing classical trajectory calculations for CEDPICS and optimization of model potential energy surface, a large ring-like attractive well with a depth of ca. 120 meV was found for perpendicular directions around CS axis and smaller interaction was found in the collinear direction of the CO axis with well depth of ca. 70 meV around the oxygen atom in the lone pair region by sp 2 hybridization.

Collision-energy resolved steric effect on the Penning ionization reaction of tert-butyl bromide and Kr( 3P)

Collision energy dependence of the Penning ionization cross-section for the Kr( 3P) + (CH 3) 3CBr → Kr + (CH 3) 3CBr + + e − reaction was determined under orientation-angle resolved condition using an oriented (CH 3) 3CBr beam and time-of-flight measurements. A remarkable resonance-type structure was found in the energy dependence of orientation-angle resolved Penning ionization cross-section. This resonance-type structure could be interpreted as a new-type ‘nuclear-excited Feshbach resonance’ in the formation of vibrational excited Rydberg states leading to a competitive dissociative exit channel.

Spin−Orbit Coupling Effect and Intramolecular Orbital Interactions: Penning Ionization of CH2BrCl, CHBrCl2, and CH2BrCN by Collision with He*(23S) Metastable Atoms

He*(23S) Penning ionization electron spectra (PIES) as well as He I ultraviolet photoelectron spectra have been measured for CH2BrCl, CHBrCl2, and CH2BrCN. For the lower ionization-potential bands, spin−orbit split states are assigned in the spectra for CH2BrCl and CH2BrCN, whereas valence-orbital, ionic states are assigned for CHBrCl2. These assignments are based on the fact that the intramolecular orbital interactions in CHBrCl2 are much stronger than those in the other molecules, whereas the spin−orbit coupling effects predominate for the other two molecules. Collision energy dependence of partial Penning ionization cross sections and the PIES indicate that the magnitudes of the attractive interactions in the electron distribution regions of the lone pair orbitals (n) of the Cl and Br atoms and the N atom in the CN group and πCN orbitals are in a sequence of nN ≫ nBr ∼ πCN > nCl.

Two-Dimensional Penning Ionization Electron Spectroscopy of CH2ClI and CH2ClCN

The electronic structures of CH2ClI and CH2ClCN and anisotropic interactions with the metastable He*(23S) atoms are studied by two-dimensional (electron-energy and collision-energy-resolved) Penning ionization electron spectroscopy as well as He I ultraviolet photoelectron spectroscopy. A broad band with the high ionization potentials in the Penning ionization electron spectrum of CH2ClI is proposed to be related to autoionizations of the Cl** or I** species produced through dissociations after the excitation transfer from the He* atom. The split bands having the characteristics of the Cl and I lone pair electrons (nCl and nI) are observed in the spectra, as being interpreted by the intramolecular orbital (nCl ↔ nI and nCl ↔ πCN) through-space/through-bond interactions rather than the spin−orbit coupling effects. The magnitude of intramolecular orbital nCl ↔ πCN interactions is much stronger than that of the nCl ↔ nI interactions. The collision energy dependence of partial Penning ionization cross sections is interpreted on the basis of the delocalized characteristics of the electron distributions for respective molecular orbitals and the model calculations of the interaction potential energies.

Intramolecular Hydrogen Bonding in 2-Chloroethanol and 2-Bromoethanol and Anisotropic Interactions with He*(23S) Metastable Atoms: Two-Dimensional Penning Ionization Electron Spectroscopy Combined with Quantum Chemistry Calculations

Five stable isomers of 2-chloroethanol and 2-bromoethanol have been found by the density functional theory with the hybrid density-functional B3LYP and the second-order Møller−Plesset perturbation method using 6-31++G(d,p) and 6-311++G(2d,2p) basis sets. Intramolecular hydrogen bonding plays an important role in the most stable isomers. Assignments, especially for the low ionization potential bands, in the He*(23S) Penning ionization electron spectra and He I ultraviolet photoelectron spectra were made on the basis of the characteristics of molecular orbitals, collision energy dependence of partial Penning ionization cross sections (CEDPICS), and the related calculations. The intramolecular hydrogen bonding (C−Cl···H−O and C−Br···H−O) leads to the significant steric shielding effects, which further results in distinctly different slopes of CEDPICS for the low ionization potential bands.

Nuclear-excited Feshbach resonance in the Ar(3P)+CH3Br Penning ionization reaction under orientation and velocity specified collisions

The collision-energy and orientation-angle-resolved Penning ionization cross section was determined for the Ar(3P)+CH3Br reaction by using an oriented beam and a time-of-flight measurement. A remarkable resonance-type structure in the Penning ionization cross section is recognized in the collision energy dependence at fixed molecular orientations, which is very similar to the one we previously observed in the Ar(3P)+CH3Cl reaction. This resonance-type structure could be interpreted as a new type nuclear-excited Feshbach resonance where the branching to competitive neutral dissociation channel is enhanced due to the formation of vibrationally excited CH3Br Rydberg states.