Metastable ions in chemical ionization

The ion [C₃H₅]⁺ generated in a chemical ionization source by a variety of methods, including protonation and charge exchange, exhibits a metastable peak for H₂ loss which is two orders of magnitude weaker than that formed in an electron impact source. The stable [C₃H₅]⁺ ions generated by electron impact and chemical ionization undergo collision-induced dissociation to a comparable extent, both losing H₂ by only one of the two competitive mechanisms observed for metastable ions. In contrast to the behavior of [C₃H₅]⁺, the molecular ions of p-substituted nitrobenzene, generated by charge exchange at high source pressure, yield composite metastable peaks for NO loss which are very similar in shape and intensity to those generated by electron impact. The contrasting behavior of the metastable ions extracted from high pressure ion sources in the two systems may be due to differences in the efficiencies of quenching of the ionic states responsible for fragmentation as metastable ions. It is noteworthy that the NO loss reactions require considerably lower activation energies than does the H₂ loss reaction.     

Molecular states formed by ion beam neutralization of [C2H2]+ and [C2H3]+ on metal targets

A combination of charge-stripping and beam-scattering techniques has been used to study the molecular states formed when a fast beam of [C₂H₂]⁺ and [C₂H₃]⁺ in several isotopic forms are neutralized by electron transfer from metal target atoms (K, Na, Mg and Zn). For [C₂H₃]⁺ the isotopic compositions and relative abundances of product states were found to be insensitive to the method of ion preparation (electron impact and chemical ionization). Ground state neutrals are formed in partial abundance when Mg or Zn is used as a target atom. With low ionization potential targets (K and Na) excitel dissociative states of C₂H₂ and C₂H₃ are formed as major beam constituents. For these states decomposition products have been identified and fragmentation energies measured. The excited states of C₂H₂ and C₂H₃ lie alout 6.8 eV and 2.9 eV, respectively, above their stable ground states. The discussion focuses on the possible identity of the excited states and their structural relations to the precursor ions.     

Electron impact on laser excited barium atoms

We have investigated the cross section for inner (5p) shell ionization of barium by high energy (6 keV) electron impact. The target atoms were in the ground state and in the laser excited metastable 6s 5d (^1,3 D) state, respectively. Comparison of the relative ionization cross sections yields information on the configuration mixed wave functions of neutral and ionic barium.     

Experimental and theoretical cross sections for photoionization of metastable Xe*. (6s 3 P 2, 3 P 0) atoms near threshold

Using high resolution laser photoelectron spectrometry we have determined absolute cross sections σJ ₀ J ₁ and the electron angular distribution parameter for one photon ionization of metastable Xe^*(6s ³ P _J0, J ₀ = 2, 0) atoms to the resolved Xe₊ (² P _J1, J ₁ = 3/2, 1/2) ion states at several wavelengths near threshold. For comparison with the present and future experimental data we have calculated partial cross sections and ß-parameters for photoionization of Xe^*(6s ³ P _J0, J ₀ = 2, 0) and of the analogous alkali atom Cs(6s) over the photoelectron energy range (0–5) eV. We have used both a term-dependent Pauli-Fock (PF) approach and a configuration interaction method involving Pauli-Fock atomic orbitals (CIPF). Through the PF method we include relativistic effects on the atomic orbitals; the CIPF method was designed to take into account the important electron correlation effects which are found to be essential for obtaining good agreement between the theoretical and experimental results.     

Theoretical study of the vertical electron affinity and ionization potentials of C3

The electron affinity and first three ionization potentials of C₃ are calculated using the multiconfigurational SCF and configuration interaction methods and by Möller-Plesset perturbation theory. Whereas Koopmans' theorem and SCF calculations indicate that the first cation state is ²Π_u, upon inclusion of correlation effects both the ²Σ_u and ²Σ_g cation states are found to lie lower in energy. CI calculations indicate that the ground state (²Π_g) anion is stable by 1.74 eV. Allowing for the error in the calculated electron affinity of the carbon atom, C₃^? is estimated to be stable by 2.0 eV, in excellent agreement with the 2.05 eV value determined from recent photodetachment measurements. No excited anion states are found to be bound at the equilibrium geometry of the neutral molecule.     

The absolute cross sections for the quenching of cadmium 5 3P1 and 5 3P0 atoms by carbon monoxide,carbon dioxide and nitric oxide

Absolute cross sections for the quenching of Cd(5 ³P₁ and Cd(5 ³P₀) by carbon monoxide, carbon dioxide and nitric oxide have been determined by a phase-shift method. It was found that carbon monoxide deactivates the Cd³P₁) atoms not only to the ground state, Cd(¹S₀), but also to the metastable state, Cd(³P₀), and that the cross section for the latter process is ten times larger than that for the former one. On the contrary, carbon dioxide quenches Cd(³P₁) atoms to the ¹S₀ state without producing Cd(³P₀) atoms. The quenching efficiency by nitric oxide was considerably large compared with those of carbon mono- and di-oxides. These quenching cross sections were compared with those of other excited metal atom states.     

Ionisation of small molecules by state-selected Ne* (3P0, 3P2) metastable atoms in the 0.06 < E < 6 eV energy range

The velocity dependence and absolute values of the total ionisation cross section for the molecules H₂, N₂, O₂, NO, CO, N₂O, CO₂, and CH₄ by metastable Ne* (³P₀) and Ne* (³P₂) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*H₂, N₂, CO, and CH₄, and the systems Ne*O₂, NO, CO₂, and N₂O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (π or σ^b) of the orbital of the electron that is removed from the target molecule. For the molecules H₂, N₂, CO, and CH₄ an electron from a σ^b orbital is removed from the molecule, whereas for O₂, NO, N₂O, and CO₂ an outer π-ortibal electron is involved. For the systems Ne* (³P₀, ³P₂)H₂ we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state NaH₂ potential of Botschwina et al. The resonance width Γ(r) as a function of the internuclear distance r shows a saturation at small r (r < 2.8 Å) for both the Ne*(³P₀)H₂ and the Ne*(³P₂)H₂ interaction. This supports previous conclusions of Verheijen et al. and Kroon et al. Reliable values for the absolute value of the total ionisation cross section have been obtained by performing a careful calibration of the density—length product of the supersonic secondary beam. The results are in good agreement with the values of West et al. for experiments without state selection. The total ionisation cross sections for molecules with π-type ionisation orbitals, with their larger spatial extent, in general are larger than those for molecules with σ^b-type ionisation orbitals.     

Improved study of the ionization of hydrogen atoms in thermal energy collisions with metastable He(23 S) atoms

The first electron spectrometric study of the ionizing reaction of metastable He(2³ S ₁) atoms with ground state hydrogen atoms has been carried out with sufficiently high resolution to partially resolve the rotational structure due to formation of rovibrationally excited HeH⁺ (v, J) ions at two different beam source temperatures (300 K and 90 K). The electron energy spectrum has been reproduced in model quantum calculations, using a new large scale ab initio calculation of the He(2³ S)+H(1² S)²Σ-potential. The imaginary part has been adjusted to yield a satisfactory fit to the measured spectrum. The collision energy dependence of the associative ionization electron spectra and of the total and partial ionization cross sections is discussed in some detail. No significant signs for limitations of the used local complex potential method, indicated by results of an earlier study of the He(2³ S)+H(1² S) system, have been found in the present work, in which the calculations were carried out with an improved and corrected program.     

Stationary states and dissociation of H<Subscript>3</Subscript>O radical in water clusters

The ground and low-lying excited states of H₃O(H₂O) _k radicals are studied. The character of the unpaired electron localization in the systems is analyzed, and the relative probability of the radical dissociation onto a water cluster and atomic hydrogen is estimated. Reaction coordinates of the dissociation are constructed and conditions of metastable existence of an H₃O radical are determined. Structures, in which H₃O can spontaneously dissociate, are found. Lifetimes of H₃O(H₂O) _k clusters before the hydrogen atom detachment at the initial conditions of two kinds, namely, upon the vertical attachment of an electron to H₃O⁺(H₂O) _k cations and upon the vibrational excitation of metastable neutral H₃O(H₂O) _k systems, are estimated.     

The interaction of metastable helium atoms with alkaline earth atoms: He*(23 S,21 S)+Mg,Ca, Sr and Ba

We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(2³ S) and He*(2¹ S) atoms with ground state alkaline earth atoms X(X=Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40–70 meV, are reported; relative populations of the different ionic X ⁺ (ml) states are presented and well depths D*_e for the He*+X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(2³ S)+Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(2¹ S) +Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X ⁺(² S), X ⁺(² P) and X ⁺(² D) and reveal that transitions to a repulsive potential — attributed to He+X ⁺(² P)² Σ formation — are mainly involved for the X ⁺(² P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(2³ S)+Ca and He*(2¹ S)+Ca. The respective well depths D _e ^* are calculated to be 243(15) meV and 544(15) meV; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 Ų and 201 Ų, respectively. Very good overall agreement with the experimental electron spectra is observed.     

An Investigation of the Positive Column of a Cd–Ne Glow Discharge. I: Steady State

An investigation of cadmium-neon plasma in cylindrical axially homogeneouspositive column of a dc glow discharge at low and intermediate pressure ispresented. Electron impact excitation cross sections, heavy-particlecollision rate coefficients, and radiative decay rates for Cd–Nemixture have been assembled from the literature and summarized. Aself-consistent collisional-radiative model based on numerical solution ofthe electron Boltzmann equation coupled with a system of particle balanceequations for the electrons, excited atoms and ions, as well as the electronenergy balance equation, is developed. By this model, the electron energydistribution function, mean energy, electron density, electron mobility, anddiffusion coefficient, populations of Cd and Ne excited states and theapplied dc electric field are calculated. The populations of the excitedCd(5p³P_0,1,2) atoms are measured using opticalabsorption spectroscopy. From these measurements the effective lifetime ofthe resonance state Cd(5p³P₁) and the diffusioncoefficients of both metastable states Cd(5p³P₀) andCd(5p³P₂) are determined. The electron density isderived by probes measurements. The influence of the Cd vapor pressure anddischarge current on the main plasma characteristics is studied. Modelpredictions compare favorably with measured electron density and populationsof excited states in a wide range of discharge conditions.     

Crossed-beám chemiluminescence. II. Kinetics and mechanisms for reactions of group IIA Metals in ground and metastable states with fluorine

Visible chemiluminescence technique under crossed-beam conditions has been applied to the study of the reactions of the group IIA metal atoms Mg, Ca, Sr and Ba in their ground state (¹S₀) or in an excited metastable state (³P_i, ³D_i or ¹D₂) with F₂. The monofluoride emission bands are the most prominent features in the chemiluminescence spectrum. Using higher fluorine densities radiation is observed from excited alkaline earth difluorides, which are shown to principally originate from secondary reactions. Formations of MF* are determined to be first order with respect to both the metal atom and the fluorine molecule. Total cross sections for removal of ground state metal atoms from the beam by F₂ are 115 ± 15Ų for Ca, 125 ± 15Ų for Sr, and 160 ± 15Ų for Ba, which is consistent with an electron jump model. Chemiluminescence cross sections are reported for the reactions involving electronically excited reactants M*. Photon yields of 12 ± 3% for Mg*, 18 ± 5% for Ca*, 20 ± 5% for Sr*, and 15 ± 8% for Ba* reacting with F₂ are measured. These high photon yields are remarkable when compared with absolute photon yields for the ground state reactions which indicate that less than 2% of the products are MF* molecules. It was possible to obtain vibrational state distributions for some of the excited monofluorides which are found to be populated in a non-thermal manner. This strongly suggests that the dynamics of the reactions are governed by a direct mechanism. From the crossed-beam chemiluminescence spectra the dissociation energies of the ground state monofluorides are estimated. In addition, improved spectroscopic constants, dissociation energies and dissociation products of some of the excited electronic states of MF are given.     

Reactions of metastable calcium 3P: quenching by Ca(1S), Mg1S) and inert gases

Rate constants have been measured (by laser induced fluorescence) for the quenching of metastable Ca(³P) by the ground states of Ca and Mg and by the inert gases. Rate constants for the metals were considerably larger than for the inert gases. The cross sections for the inert gases are well correlated by an orbiting model.     

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.     

Electron ionization induced metastable decompositions of isomeric trimethylsilylmethanol, (CH3)3SiCH2OH, and methoxytrimethylsilane, (CH3)3SiOCH3

The metastable decompositions of trimethylsilylmethanol, (CH₃)₃SiCH₂OH (MW: 104, 1) and methoxytrimethylsilane, (CH₃)₃SiOCH₃ (MW: 104, 2) upon electron ionization have been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectroscopy and D labeling. The metastable ions of 1 ^·+ decompose to give the fragment ions m/z 89 (CH ₃ ^· loss) and 73 (^·CH₂OH loss), whereas those of 2 ^·+ only yield the fragment ion m/z 89 (CH ₃ ^· loss). The latter fragment ion is generated by loss of a methyl radical from the trimethylsilyl group via a simple cleavage reaction as shown by D labeling. However, the fragment ions m/z 89 and 73 from 1 ^·+ are generated following an almost statistical exchange of the original methyl and methylene hydrogen atoms in the molecular ion as shown also by D labeling. This exchange indicates a complex rearrangement of the molecular ion of 1 ^·+ prior to metastable decomposition for which as key step a 1,2-trimethylsilyl group migration from carbon to oxygen is suggested. A different behavior is also found between the source-generated m/z 89 ions from 1 ^·+ which decompose in the metastable time region to give ions m/z 61 by loss of ethylene and those from 2 ^·+ which decompose in the metastable region to yield ions m/z 59 by elimination of formaldehyde.     

Absolute detection of metastable rare gas atoms by a cw laser photoionization method

A novel, accurate method for the absolute detection of metastable rare gas atoms is described and demonstrated. It involves a direct in situ determination of the electron emission coefficient γ for impact of the respective metastable atom on a conducting surface. γ is reliably obtained by a cw two-photon ionization — depletion technique: the reduction ΔI _S in electron current from the detector surface due to efficient photoionization removal of the metastable flux is compared with the photoelectron current ΔI _P (γ = ΔI _S/ΔI _P). The principle of the method, possible realization schemes for the different metastable rare gas atoms and the apparatus are described in detail. The method has been applied so far to metastable Ne* (3s ³ P ₂), Ar* (4s ³ P ₂), and Kr* (5s ³ P ₂) atoms, and corresponding results for γ, obtained with five different chemically clean, polycrystalline surface materials and at two surface temperatures (300 K, 360 K) are reported. Whereas for Ne*, the value of γ (≈0.35) showed only a rather weak dependence on the surface material and temperature (as also found for a mixed He* (2³ S, 2¹ S) beam), strong variations in γ, especially at 300 K, were detected for Ar* and Kr* (values between 0.25 and 0.003). Some applications of the described method, especially with regard to the determination of absolute reaction cross sections involving metastable rare gas atoms, are discussed.     

Lifetimes and perturbations of the 3d9 4s(3D) 4p structure in the spectrum of copper I

Natural radiative lifetimes have been measured of the²P,²D,²F and⁴D_5/2 terms in the 3 d⁹ 4s(³D) 4p structure of copper I. A pulsed hollow cathode was used to generate 3d⁹ 4s^{2 2}D metastable atoms. From these metastable levels the states investigated were populated by a pulsed dye laser pumped by a Nd:YAG laser. A comparison with theoretical and experimental literature values is given.     

Dissociation patterns in N2O following electron impact

A number of dissociation channels in N₂0 have been observed by time-of-flight spectroscopy following electron impact excitation. The metastable atoms and molecules produced were directly detected. Excited N₂ molecules were produced in the A³Σ_u⁺, B³Π_g and B′³Σ_u^? states in conjunction with ground state oxygen atoms. A number of additional dissociation channels were monitored by observing the production of oxygen and nitrogen atoms in Rydberg states. The results indicate the existence of potential minima in some of the repulsive surfaces involved.     

Lifetime measurements of the 3d 94s(1 D) 4p configuration of Cu I

The radiative lifetimes of the levels in the 3d ⁹4s(¹ D)4p configuration of Cu I are measured. The levels are excited from the metastable 3d ⁹4s ^{2 2} D _3/2,5/2 levels. The metastable Cu atoms are generated in a pulsed hollow cathode discharge. The levels investigated are populated with a 35-ps laser pulse at wavelengths around 220 nm. The laser induced fluorescence signal is detected. The lifetime of the 3d ⁹4s(³ D)4p ⁴ D _1/2 level is also determined by direct excitation from the ground state. A comparison with calculated literature values is given.     

Accurate comparison of Hel,NeI photoionization and He(23, 1 S), Ne(3s 3 P s,3 P 0) penning ionization of argon atoms and dimers

Using crossed beams and mass spectrometric ion detection, we have investigated the ionization of argon atoms and dimers in a skimmed supersonic beam by HeI (58.4 nm) and NeI (73.6, 74.4 nm) photons and by He(2^3,1 S) and state selected Ne(3s ³ P ₂,³ P ₀) metastable atoms. The cross section ratioq ₂₂/q ₁ (i.e. the cross sectionq ₂₂ for formation of Ar ₂ ⁺ ions from Ar₂ divided by the total ionization cross sectionq ₁ for Ar atoms), arbitrarily normalized to 1 for HeI impact, is found to vary weakly as follows: HeI/NeI/He(2^{3, 1} S)/Ne(³ P ₀)Ne(³ P ₂)=1/1.136(9)/0.893(4)/1.034(12)/0.985(9). The results are qualitatively interpreted using available information on the intermolecular potentials and the two different ionization processes. The observation thatq ₂₂/q ₁ is 5% larger for Ne(³ P ₀) than for Ne(³ P ₂) is attributed to anomalies in the respective branching ratios for formation of the Ar+(² P _3/2)/Ar⁺(² P _1/2) ion states in conjunction with differences in the stability of the formed Ar-Ar⁺(² P _3/2) and Ar-Ar⁺(² P _1/2) molecular ions.