Penning Ionization Research Articles

Penning ionization of cobaltocene by collision with He *(2 3S) metastable atoms

Penning ionization of cobaltocene (C 5H 5) 2CO upon collision with He *(2 3S) metastable atoms was studied by two-dimensional Penning ionization electron spectroscopy (2D-PIES). In the studied collision energy range (80–180 meV), the interaction potentials around cyclopentadienyl rings were found to be strongly attractive in the normal direction of the ring plane and were found to be repulsive around C H bonds. Assignments of the photoelectron bands of cobaltocene were discussed in detail by using 2D-PIES and the density functional theory (DFT).

Penning ionization electron spectroscopy of titanyl phthalocyanine ultrathin films: electronic state and molecular orientation

Penning ionization electron spectroscopy (PIES) was used to investigate the molecular orientation, aggregation of molecules, and electronic structure of ultrathin films of titanyl phthalocyanine (OTiPc) vapor deposited on graphite and MoS 2 substrates. For the monolayer film, the molecular orientation was remarkably different between the two substrates. At room temperature, molecules spread over and almost cover the substrate on graphite, whereas they aggregate and form islands on MoS 2. The PIES band due to the nonbonding orbital of the oxygen atom extending perpendicular to the molecular plane is extraordinary strong for the He *(2 3S) metastable atom but not for the He *(2 1S), indicating that the potential for the entrance channel is quite different between the two species.

Two-dimensional penning ionization electron spectrum of N 2 by collision with He *(2 3S) metastable atoms

The collision-energy/electron-energy-resolved two-dimensional Penning ionization electron spectrum forN 2/He *(2 3S) was observed in the collision-energy range 70–380 meV. A large peak shift to higher electron energies and band width broadening were observed for the A state with increase of collision energy in comparison to those for the X and B states. Branching ratios for the three ionic states of N 2 + as a continuous function of collision energy were investigated. The order of the branching ratios changes fromB > X > A toA > B > X in the 100–200 meV collision energy region.

Ion source development at GSI

Different ion sources are used at GSI to match the requirements for specific tasks at the accelerator. At the standard injector a Penning ionization gauge ion source is used (design ion U10+, m/q<24, 1 emA). Further development of this source is mainly aiming at an increase of the extractable currents and an increase of the lifetime of the source. The new injector produces the beam by means of an electron cyclotron resonance source (Caprice-type). Here the design ion is U28+, m/q<8.5, 5 eμA. Clearly, the oven technology is our main development goal. In addition we are trying to improve the extraction flexibility by a moveable accel–decel system. For our high current project ion sources are required which are capable to deliver a beam of several mA even for heavy ions. Design ion here is U4+, m/q<65, 15 emA. For that application two different types of ion source are used: the multicusp ion sources “cold or hot reflex discharge ion source” and “multicusp ion source” for gaseous ions and the MEVVA source for metal beams.

Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atom

Penning ionization of the CO2 molecule by Ne* (3 3P2,0) metastable atoms is studied by electron spectroscopy. The recorded electron energy spectrum is dominated by an important vibrational progression that is decomposed over the nν1 and nν1+2ν3 vibrational series up to n=5. In addition, we observe a low-energy broad hump extending from 0.5 to 1.5 eV. The presence of this new feature is attributed to a non-van der Waals interaction between the colliding particles. Making the hypothesis that an ion–pair state channel opens during the collision, we are in a position to interpret the entire set of experimental results. We believe that the opening of this channel is responsible for the production of the CO2 molecule in an autoionized Tanaka–Ogawa Rydberg state.

Dynamics of molecular Penning ionization: angle-energy distributions of N 2+ from He *(2 1S)+N 2

Angle-energy distributions resulting from the Penning ionization of N 2 by metastable singlet helium He *(2 1S) at collision energies of 2.5 and 4.4 kcal/mol have been measured using crossed supersonic molecular beams and a rotatable mass-spectrometric detector. Predominantly forward scattering of N 2 + is observed, and although substantial backscattering is also present at both energies, the collision intermediates are mainly short-lived. Both the sharpness of the forward-scattering peak and the mean recoil energy increase at the higher energy. The results are compared to quantum scattering calculations based on a recently proposed set of potential surfaces.

Excitation Transfer Process in Penning Ionization of the CO Molecule by Helium Atoms in Singlet Metastable State

Penning ionization electron spectroscopy of the CO molecule by He*(2 1 S) metastable atoms is studied. Vibrational populations are reported for the three energetically accessible states of the CO + ion (X 2 Σ + , A 2 Π and B 2 Σ + ). An obvious discrepancy is observed between the excited vibrational levels of the X 2 Σ + state and the calculated Franck-Condon factors. This result is attributed to an excitation transfer process via Rydberg vibrational progression converging to the CO + (D 2 Π) ionic state in agreement with the recent observations by fluorescence of neutral dissociate states. The presence of electrons coming from the well-known σ shape resonance is not excluded.

Deexcitation of Ne(3P1) and Ne(3P2) in collisions with Ar, Kr, and Xe

The cross sections for the deexcitation of Ne(3P1) and Ne(3P2) by Ar, Kr, and Xe have been measured as a function of the mean collisional energy in the range of 17–38 meV or in the temperature range from 133 K to 295 K using a pulse radiolysis method combined with time-resolved optical absorption spectroscopy. The deexcitation cross sections are in the range of 18–26 Å2 and 11–20 Å2 for Ne(3P1) and Ne(3P2), respectively, and increase slightly with increasing the collisional energy. The absolute experimental cross sections for the Ne(3P1) deexcitation are in relatively good agreement with the total Penning ionization cross sections obtained by an optical model calculation, which shows the importance of an electron-exchange interaction in the deexcitation of Ne(3P1) in collisions with Ar, Kr, and Xe. The absolute cross section values for the deexcitation of resonant Ne(3P1), which are only slightly larger than those for the deexcitation of metastable Ne(3P2), have also supported the discussion on the deexcitation mechanism of Ne(3P1) by Ar, Kr, and Xe.

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.

Collision-Energy-Resolved Penning Ionization Electron Spectroscopy of Nitriles: Conjugation Effects on Interactions with He*(23S) Metastable Atoms

Penning ionization of propionitrile (CH3CH2C⋮N), acrylonitrile (CH2CHC⋮N), and 3-butenenitrile (CH2CHCH2C⋮N) upon collision with He*(23S) metastable atoms has been studied by two-dimensional Penning ionization electron spectroscopy in which Penning electron intensity is measured as a function of both collision energy and electron energy. We have observed a strong negative collision energy dependence of the partial ionization cross sections for ionization from nonbonding and πCN orbitals of all samples studied, which indicates that the interaction potentials between He*(23S) and target molecules are strongly attractive around the cyano group. For π-conjugated acrylonitrile, strong attractive interaction was shown for the πCC orbital region, while the attractive interaction for the πCC orbital region became weak for nonconjugated 3-butenenitrile. The observed results are consistent with calculated model potential curves. The electron density difference for charge transfer was analyzed for the lone pair orbital region where the molecule acts as an electron donor, while the molecule acts as an electron acceptor for the conjugated π orbital region.

Penning Ionization of CH3SCH3, CH3SSCH3, and CH3CH2SH by Collision with He*(23S) Metastable Atoms

Penning ionization of CH3SCH3 (dimethyl thioether), CH3SSCH3 (dimethyl disulfide), and CH3CH2SH (ethyl thioalcohol) upon collision with He*(23S) metastable atoms was studied by two-dimensional (electron-energy/collision-energy-resolved) Penning ionization electron spectroscopy. A collision energy dependence of the partial ionization cross sections has indicated that the interaction potentials are anisotropic between He*(23S) and the target molecules. For CH3SCH3 and CH3CH2SH, the interaction potential around the sulfur atom was found to be more attractive for the out-of-plane direction vertical to the CSC or CCS plane than for the in-plane direction, while the opposite trend was observed around the oxygen atom of CH3CH2OH. Anisotropic potential for CH3SSCH3 was found to be similar to that for CH3SCH3. Strongly enhanced bands in Penning ionization electron spectra of sulfur compounds, which were assigned to ionization from molecular orbitals having sulfur 3s (S3s) atomic orbital character, were found to be associated with an intermolecular excitation transfer from He* to the target molecule and also with the following intramolecular Auger-like autoionization process.

Theoretical investigation of the autoionization process in molecular collision complexes: Computational methods and applications to He*(23S)+H(12S)

The first complete ab initio treatment is applied to the autoionization process in the He*(2s3S)+H(1s) collisional complex. The autoionizing resonance state is defined through Feshbach projection based on orbital occupancy, and the corresponding potential is determined from multireference–configuration interaction (MR-CI) calculations with an accuracy of about 10 meV. The energy-dependent coupling with the continuum is derived from a compact (L2) molecular orbital (MO) without any phase information being lost. This “Penning MO” is projected onto the states of the continuum electron for energies that comply with the resonance condition thus providing the l-dependent coupling elements in local approximation. The continuum electron functions are calculated within the static-exchange approximation for up to 25 coupled angular momentum channels. The nuclear dynamics calculation is based on a complex Numerov algorithm and uses a converged set of seven complex coupling matrix elements. Weighting with experimental collision energy distributions finally gives the angle-dependent, as well as the angle-integrated, electron spectra for Penning and associative ionization processes. The results are discussed with respect to previous, either partial or model studies, and are compared with the recent most detailed experimental study of the angular-dependent Penning ionization electron spectra. The close agreement of theory and experiment demonstrates the adequacy of the local complex potential approach, as well as the importance of electron angular momentum transfer so far neglected in theoretical treatments.

Autoionization process in Penning ionization of the CO molecule by helium atoms in triplet metastable state

In this paper, we investigate the Penning ionization of the CO molecule by helium atoms in metastable states. The energy distribution spectrum of electrons related to the ion state shows a deviation of the vibrational population from the predicted Franck - Condon factors. The angular distribution measurements exhibit an intensity increasing towards the backward direction for the v = 0 level, while the corresponding angular distributions of the higher vibrational levels (v = 2, 3 and 4) are isotropic. To explain these results, an excitation transfer process between the colliding particles is suggested. This gives rise to a CO molecule either in a doubly excited Rydberg state converging to or in the quasi-bound state (shape resonance). Autoionization of the doubly excited state leads to the population of vibrational levels which are forbidden by the Franck - Condon principle in the case of a direct processes.

Effect of HOMO Levels on Chemiionization of Substituted Ethylenes by Metastable Helium Atoms

Abstract Chemiionization reactivity (Penning ionization cross section) of substituted ethylenes in collision with helium metastable atoms was studied by velocity-resolved Penning ionization electron spectroscopy. Collision energy dependences of ionization probabilities of π orbitals were found to correlate with the HOMO levels. The higher the HOMO levels, the more effectively the reactive site of the ethylenes is hit by metastable atoms because of increased attractive interactions.

Improved ion beam deposition system with RF sputter-type ion source

A newly developed RF sputter-type ion source has been introduced into our ion beam deposition (IBD) system for high purity Fe film formation. Vacuum environment during deposition has been much improved compared with our previous IBD system, in which a conventional CCl 4 method was employed to produce Fe + ions in a microwave ion source. With no use of CCl 4 gas, an inert argon background has been achieved. The ion source is composed of an RF (13.56 MHz) Cu coil and an Fe sputter target located inside. An Ar discharge was generated at an RF power of 100–300 W and a negative DC bias voltage of 500–1000 V was applied to the Fe target. The tip of the target was heated almost up to the melting temperature and simultaneous sputtering and evaporation took place. Major ion species extracted were Ar + and Fe + ions. With an increase in the DC bias voltage up to 1 kV, a relative ratio of Fe + to Ar + ion intensities became one order larger. This efficient Fe + ion production is thought to be due to a Penning ionization effect of metastable Ar ∗ atoms. It is expected to form extremely high purity Fe films, in contrast to our former Fe films which contained C and Cl as impurities.

Collision-energy/electron-energy resolved two-dimensional study of Penning ionization of Ar by He metastable atoms 23Sand 21S

Selection of collision energies by a time-of-flight method has been combined with Penning ionization electron spectroscopy by utilizing a multichannel scaler and 2 MB random access memory. This technique provides a two-dimensional Penning ionization electron spectrum (2D-PIES) in which the Penning electron intensity is observed as a continuous function of both collision energies and electron energies. The observed 2D-PIES for Ar/He* (23S and 21S) was compared with calculated 2D-PIES obtained by classical trajectory calculations based on interaction potentials and transition probabilities.

Classical and quantal calculations for the Penning ionization system N2+He*(23S)→N+2+He+e−

Quasiclassical trajectory and quantal calculations are carried out for the Penning ionization system N2+He*(23S)→N+2(2Σg,2Πu,2Σu)+He+e− based on an ab initio resonance potential and energy widths which were obtained in previous work. Total and partial ionization cross sections are evaluated for the collision energy range of 0.1–1.0 eV. For the trajectory calculation, the collisional energy dependence of the cross section is in better agreement with a recent experiment on state resolved Penning ionization than calculations using the classical sudden approximation. The result in the high frequency rotation limit is significantly different from that for the sudden approximation, which is in contrast to the H2–He* system. The results for quantal calculations using the sudden and spherical-potential approximations confirm the reliability of classical treatments. The results obtained suggest that analyses with the widely used atomic-target models lose their validity for significantly anisotropic systems in which targets have large moments of inertia.

The long-range interaction between He and H+2: an ab initio and dynamical study

A multi-configuration self-consistent field, multi-reference configuration interaction (MCSCF-MRCI) calculation with extended atomic basis sets has been carried out on the electronic ground state of HeH2 + for intermediate-to-large intermolecular distances R between He and H2 + for both C∞v and C2v geometries at a fixed H2 + bond length of 2·0 a 0. All energies were corrected for basis set superposition error using the counterpoise method. The most stable geometry occurs in C∞v with a well depth of 7·16 kcal mol-1 (2504 cm-1) at an intermolecular distance of 2·98 a0 (1·576 Å), in good agreement with three previous high level calculations. The anisotropy of the interaction is strong, and is accounted for largely by the induced-dipole–quadrupole long-range force. The computed energy points were fitted to an analytical two-term Legendre expansion incorporating the appropriate long-range behaviour. This potential function was used to compute bound-state rovibrational energy levels of the HeH2 +complex variationally using a basis set method. The intermolecular zero-point energy of the complex was found to be 1020 cm-1, yielding a dissociation energy of 1484 cm-1 (4·24 kcal mol-1). The influence of the anisotropy on complex formation and ion-molecule reactions at thermal energies was assessed using close-coupling optical potential calculations of capture cross-sections; significant positive deviations from Langevin behaviour were found at sub-eV energies. The surface should be of value in interpreting the thermal energy Penning ionization of H2 by excited He.

Autoionization process in the Penning ionization of the CO molecule by Ne ∗ ( 3P 0, 3P 2) metastable atoms

Penning ionization electron spectroscopy of the CO molecule by Ne ∗ ( 3P 2, 3P 0) metastable atoms is reported. An extensive vibrational excitation of the electronic ground state X 2Σ + of the CO + ion is observed. The measured angular distribution of the electrons corresponding to the first vibrational states ( ν = 0−1) is anisotropic, while that of the higher levels is isotropic. A quasi-resonant excitation transfer process, leading to excitation of the molecular target into an intermediate autoionizing Rydberg state, is proposed to explain the discrepancy between the Franck-Condon factors and the observed excited vibrational levels of the X 2Σ + state.

Chemical influence of inert gas on the thin film stress in plasma-enhanced chemical vapor deposited a-SiN: H films

The growth of amorphous hydrogenated silicon nitride (a-SiN:H) films by plasma enhanced chemical vapor deposition (PECVD) of SiH4−NH3−N2 reactive gas mixtures has been studied. Films were deposited at low temperature (T < 250 °C) in a commercial PECVD system commonly used to grow a-SiN: H for semiconductor integrated circuit passivation. It has been observed that the stress of the a-SiN: H film can be controlled through dilution of the film precursors with an inert gas. Experiments indicate that the influence of the inert gas on the process extends from growth kinetics and plasma chemistry to hydrogen bonding, elemental composition, and biaxial elastic modulus. The stress in films deposited without dilution is tensile. When argon is added to the plasma, Si–Hx plasma chemistry and film hydrogen bond density change producing a reduction in the amount of tensile stress. Dilution with helium can be used to shift the film stress from tensile to compressive with minimum change in growth rate. The observed helium/film stress relationship is associated with helium-based Penning ionization processes, which create metastable reactive gas species. In turn, the metastables influence nitrogen and hydrogen incorporation into the film. Nitrogen incorporation produces volume expansion of the film, increasing the compressive character of the film stress. This effect is similar to that observed when the RF power is varied or when low or multifrequency plasma excitation is used during PECVD growth of a-SiN: H.