Partial Penning Ionization Cross-sections Research Articles

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.

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.

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*).

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.

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.

Classical trajectory calculations of collision energy dependence of Penning ionization cross-sections for N 2 and CO by He [formula omitted]2 3S; optimization of anisotropic model potentials

Anisotropic potential energy surfaces were optimized for classical trajectory calculations in order to reproduce collision energy dependence of partial Penning ionization cross-sections for N 2 and CO molecules with a He ∗ (2 3S) atom. Ab initio model potentials using a Li(2 2S) atom in place of a He ∗ (2 3S) atom based on the similarity of valence electronic configurations were found to be successfully corrected by addition of negative terms including an exponential function combined with Legendre expansions. Optimized sets of parameters demonstrate that the most important correction terms are related to attractive interactions due to electron transfer from He ∗ (Li) to unoccupied molecular orbitals.

Collision energy-resolved study of the emission cross-section and the Penning ionization cross-section in the reaction of BrCN with [formula omitted

The dissociative excitation of BrCN producing CN(B 2Σ + ) fragment by the collision of He *(2 3S) was investigated by the collision energy-resolved electron and emission spectroscopy using time-of-flight method with a high-intensity He * beam. The Penning electrons ejected from BrCN and the subsequent CN ( B 2Σ +– X 2Σ +) emission were measured as a function of collision energy in the range of 90–180 meV. The formation of CN ( B 2Σ +) is concluded to proceed dominantly via the promotion of an electron from Π-character orbital, by comparison between the collision energy dependence of the partial Penning ionization cross-sections and the CN ( B 2Σ +– X 2Σ +) emission cross-section.

Penning ionization of [2,2]-paracyclophane by collision with metastable He *( [formula omitted]) atoms

Penning ionization of [2,2]-paracyclophane (PC) with metastable He *( 2 3 S ) atoms has been studied by means of simultaneous detection of collision energy and electron energy. Negative collision energy dependences of partial Penning ionization cross-sections have been observed in the range of 115–283 meV. This indicates the presence of an attractive potential well around PC. Assignments of Penning ionization electron and HeI ultraviolet photoelectron spectra have been made by outer valence Green's function calculations. Molecular-orbital and model-potential calculations have also been performed, and the characteristic features of electron density and interaction potential are discussed in comparison with the case of benzene.

Two-Dimensional Penning Ionization Electron Spectroscopy of NNO, HCNO, and HNNN: Electronic Structure and the Interaction Potential with He*(23S) Metastable and Li(22S) Ground State Atoms

The electronic structure and Penning ionization of NNO, HCNO, and HNNN upon collision with He*(23S) metastable atoms were studied using He I photoelectron and two-dimensional Penning ionization electron spectroscopies (2D-PIES). From the peak shifts in PIES and collision energy dependence of partial Penning ionization cross sections, the interaction potentials between molecules and He*(23S) atoms were deduced. In the studied collision energy range, the interaction potential was found to be attractive around the nitrile oxide (−CNO) and azide (−NNN) groups, but no characteristic interaction was observed between NNO and He*(23S). Ab initio calculations on the similar interacting systems M−Li(22S) (where M = NNO, HCNO, and HNNN) at the CCSD/6-311++G** level revealed fine details of the anisotropy of the interaction potentials and were in good agreement with experimental results. The spectroscopic investigations predicted the existence of thermodynamically stable MLi radicals, and the structure and stability of HCNOLi and HNNNLi were calculated at the QCISD/6-311++G** level.

Collision energy resolved Penning ionization electron spectra of polycyclic aromatic hydrocarbons

Penning ionization of naphthalene and anthracene with He *(2 3S) atoms has been studied by means of simultaneous detection of collision energy and electron energy. The anisotropy of interaction potentials between He *(2 3S) atoms and the target molecules has been found on the basis of the collision energy dependences of partial Penning ionization cross-sections. Based on the analyses of collision energy dependences and electron correlation calculations, assignments of ultraviolet photoelectron spectra are discussed.

Penning Ionization of Cyclic Ethers by Collision with He*(23S) Metastable Atoms

Penning ionization of tetrahydropyran, tetrahydrofuran, 1,4-dioxane, and 1,3-dioxane upon collision with He*(23S) atoms was studied by collision-energy-resolved Penning ionization electron spectroscopy. In the studied energy range, the interaction potential around the oxygen atoms was found to be attractive and anisotropic depending on the in-plane and the out-of-plane access of He*, in good agreement with model potential calculations. Observed collision energy dependence of partial Penning ionization cross sections can be related to the molecular orbital of which ionization occurs and also to local interaction potentials where the electron distribution of the molecular orbital is mainly exposed to the outside. Effects of attractive interaction potentials around the oxygen atoms were found to depend on the stereochemical surroundings. Enhancement of PIES intensity was commonly found for C2s bands and some πCO bands of studied cyclic ethers. On the basis of these findings, assignments of ultraviolet photoelectron spectra were discussed.