Doubly autoionizing capture resonances ine + Mg+ collisions

Fine details in the cross section for electron impact ionization of Mg⁺ ions (configuration 1s ² 2s ² 2p ⁶ 3s) have been measured with an energy resolution of 0.3 eV. Structure on top of a smooth direct-ionization “background” reflects the presence of indirect ionization mechanisms. Such contributions in a Na-like ion involvenon-resonant excitation of a 2p electron to singly autoionizing intermediate states subsequently decaying into the channel of net single ionization of the parent ion. We observe even stronger indirect contributions fromresonant excitation of a 2p electron with simultaneous dielectronic capture of the projectile electron into doubly autoionizing states which decay by successive emission of two electrons.     

Stark broadening and regularities of ionized neon and argon spectral lines

Stark widths of five Ne III, five Ne IV, one Ar III and nine ArIV spectral lines have been measured in a linear-pinch discharge plasma. The results were compared with existing experimental and theoretical results, and used to establish several types of regularities. Electron densities determined with single-wavelength laser interferometry were 2.18·10²³ m^?3 and 2.80·10²³ m^?3 in neon and argon plasma, respectively. The electron temperatures determined from the Boltzmann slope of several Ne III spectral lines, and ratios of Ne III to Ne IV or Ar III to Ar IV spectral lines were 59 000 K and 42 000 K in neon and argon plasma, respectively. The investigated spectral lines originate predominantly, from 3s–3p and 3s′–3p′ Ne III and Ne IV, and from 4s–4p and 4s′–4p′ Ar III and Ar IV transition arrays. The emphasis is on the Stark width (θ) dependence on the upper level ionization potential (I), the emitter core net charge (z) and electron temperature (T) for a given electron density. This dependence was found to be of the form: θ=az ² T ^?1/2 I ^?b , wherea andb are constants within(i) several stages of ionization of neon or argon and(ii) within nitrogen like (NI, O II, F III and Ne IV) 3s–3p or phosphorus like (P I, S II, Cl III and Ar IV) 4s–4p transition arrays. The established overall trends were used to predict the Stark widths of univestigated spectral lines originating from the given transition arrays.     

Collision spectroscopy with aligned and oriented atoms

Neutralisation processes in 0.15–1.5 keV collisions of H^? with Na atoms in the 3s ground state or in the excited 3p state have been investigated by means of time-of-flight analysis of the neutral H atoms produced. The H^? - Na(3p) system, investigated here for the first time, is particularly interesting since the entrance channel is embedded in the [H - Na(3s)] +e ^? continuum, enabling Penning detachment to occur. The measured relative neutralisation cross section ratios σ(3p)/σ(3s) decrease from 3 to 1.6 with increasing energy. Based on earlier published results for σ(3s), σ(3p) total cross sections exceeding 100 Ų are estimated.     

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.     

Electron impact and single photon ionization cross sections of neutral silver clusters

Neutral silver atoms and small clusters Ag _n (n=1...4) were generated by sputtering, i.e. by bombarding a polycrystalline silver surface with Ar⁺ ions of 5 keV. The sputtered particles were ionized by a crossed electron beam and subsequently detected by a quadrupole mass spectrometer. In alternative to the electron impact ionization, the same neutral species were also ionized by single photon absorption from a pulsed VUV laser (photon energy 7.9 eV), and the photoionization cross sections were evaluated from the laser intensity dependence of the measured signals. By in situ combining both ionization mechanisms, absolute values of the ratio σ _e (Ag _n )/σ _e (Ag) between the electron impact ionization cross sections of silver clusters and atoms could be determined for a fixed electron energy of 46 eV. These values can then be used to calibrate previously measured relative ionization functions. By calibrating the results using literature data measured for silver atoms, we present absolute cross sections for electron impact ionization of neutral Ag₂, Ag₃ and Ag₄ as a function of the electron energy between threshold and 125 eV.     

The large effects of electron correlation on the multiplet generalized oscillator strengths of non-closed shell systems: Be 1s22s21S → 1s22s2p 1P0 and B 1s22s22p 2p0 → 1s22s2p22D

Correlation effects are shown to change generalized oscillator strengths and inelastic form factors by large factors as large as 10, from the Hartree-Fock values. The effects are fully included in the charge wavefunctions of the non-closed shell many-electron theory by Sinano?lu. We calculate the charge wavefunctions and obtain from the generalized oscillator strength F_0η(q) versus ln q² and the integrated cross section σ_0η(E) versus lnE₁ for BeI 2s²¹S → 2s2p ¹P⁰ and BI 2s²2p ²P⁰ → 2s2p²²D cases, exemplifying the method.     

Valence-bond calculations on N2 and isoelectronic species

Valence-bond calculations are reported for the isoelectronic series of molecules and ions: N₂, CO, BF, NO⁺ and CN^?. The most important structures are N?N, C?O, B_π? F, N⁺?O and C?N. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ-orbital or gains a π-orbital are favored. π-bonds tend to be favored over σ-bonds. The bond in NO⁺ resembles that in CO, whereas that in CN^? resembles the bonding in N₂.     

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.     

Multiphoton association reaction He + H+ → HeH+ steered by ultra‐short laser pulse

The multiphoton association reaction He + H⁺ → HeH⁺ in the electronic ground state is investigated using the time‐dependent quantum wave packet method. It is shown that the collision pairs He + H⁺ in continuum state transfer into ν = 0 state and then produce stable molecules HeH⁺ through emission of two or three photons. The multiphoton transition takes place via intermediate states, and the transfer probability is determined by the collision energy and the intermediate states. The populations of the intermediate states and ν = 0 state can be controlled by the laser duration. The three‐photon transition is more efficient than the two‐photon transition for producing the molecule HeH⁺ in ν = 0 state. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Effect of the exchange integral K2s2p on the 2p-orbitals of the 1P and 3P states of the beryllium isoelectronic sequence arising from the configuration (1s)2(2s)(2p)

The ¹P and ³P states arising from the configuration (1s)²(2s)(2p) of the Be isoelectronic sequence are investigated. In the single configuration approximation, the energies of the two states are expressed as E⁰ + K_2s2p and E⁰ - K_2s2p, respectively. K_2s2p is the exchange integral between the 2s and 2p electrons and E⁰ is the energy of a model in which K_2s2p is deleted. First we calculate the 2s- and 2p-orbitals in this model. Second, by taking account of K_2s2p in this model, effects of this term on the 2p-orbitals in the ^1,3P states are investigated. In this manner, an explanation is given for the following facts which are obtained from a minimal Slater-type orbital set; (1) for Be and B⁺, the 2p-orbital of the ¹P state is broader than that of the ³P state; (2) for C²⁺, the extension of the 2p-orbital in the two states is almost the same; (3) for O⁴⁺ and Ne⁶⁺, in contrast to Be and B⁺, the 2p-orbital of the ¹P state is tighter than that of the ³P state.     

Dielectronic recombination of lithium-like Ar15+

Dielectronic recombination (DR) of Ar¹⁵⁺(1s ²2s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3? (Δn=1) core excitations and even individual terms in the 1s ²3?3?′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2 _{p 1/2}? and 2 _{p 3/2}? Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s ²3?n?′) Rydberg series.     

Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

Empirical relations between σI, σoR and taft σ* values of alkyl,acyl, benzoyl and substituted phenyl groups

The X(X) values¹ of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ_I and σ^o_R according to the regression equation

and η*_R=η(X)?η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*]^X_R=αη^X_R where α equals ?10.6 and ?10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*]^X_Rand η^x_r represent Taft σ* and [σ_I(X)?σ_oR(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ_I(X) and σ^o_R(X) according to the equations v(OH) = ?423.0 σ_I(X) + 3654.7 v(OH) = ?270.0 σ⁰_R(X) + 3586.7 where X = Ph, PhCO, CH₂=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp² hybridized carbon atom are proportional to σ⁰_R according to the equation σ*(X) = 3.97 σ⁰_R(X) + 1 New σ_I σ^o_R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.     

CO inner-shell excitation studied by electron impact spectroscopy

The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσ_c)⁻¹(2pπ) ³Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσ_c)⁻¹(2pπ) ³Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO⁺ quartet final states has been observed.     

Measurement of state-to-state rotational transfer rates in collision of I2*(B 0u+, υ = 15, j) with 3He, 4He,Ne, Ar,H2, D2 and I2

The selective laser excitation and induced fluorescence observation technique has been used to study rotationally inelastic collisions of I₂^*(B 0_u⁺, υ = 15,j) with I₂, ³He, ⁴He, Ne, Ar, H₂ and D₂. For each collision partner, several initial rotational levels ranging from j_i = 12 up to j_i = 146 have been excited. For purely rotational transfer within the υ = 15 level, our data are perfectly consistent with energy sudden (eventually corrected) scaling laws. Thus, any thermally averaged rate constant, k(j_i → j_f), can be expressed as a function of the basis rate constants k(l → 0). Furthermore, these k(l → 0) are found to follow simple empirical fitting laws. Consequently any k(j_i → j_f) can be predicted given a set of two or three fitting parameters. Collisions with relatively heavy particles (I₂, Ar and Ne) are well described by using the inverse power fitting law k(l → 0) = b[l(l+1)]^?γ, where b = 1.7, 1.2 and 1.2×10^?10 cm³ s^?1 and γ = 1.08, 1.02 and 1.17 for I₂^*-Ne, I₂^*-Ar and I₂^*-I₂ collisions respectively. For collisions with light particles (³He, ⁴He, H₂ and D₂), k(l → 0) shows a sharp decrease with l which can be accounted for by a hybrid power-exponential fitting law k(l → 0) = b[l(l+1)]^?γ exp[-l(l+1)/l^* (l^*+1)], where b = 0.84, 0.71, 2.77 and 2.78×10^?10 cm³ s^?1l⁺ = 20.6, 23.1, 18.8 and 31.4, and γ = 0.66, 0.66, 0.78 and 0.91 for I₂^*-³He, I₂^*-He, I₂^*-H₂ and I₂^*-D₂ collisions, respectively. We confirm that the rotational transfer dynamics in heavy molecules is mainly governed by angular momentum exchange.     

Oscillator strengths in first row neutral,singly and doubly ionized atoms: Comparison of recent theoretical and experimental values

Using correlated wave functions, oscillator strengths for transitions of the type 1s²2s²2pⁿ → 1s²2s2pⁿ⁺¹ in neutral, singly and doubly ionized B, C, N, O and F atoms are calculated. Such oscillator strengths are extremely sensitive to the details of electronelectron interactions. Comparison with results for other many-body calculations and beam-foil, phase-shift, emission and Hanle spectroscopies shows an overall agreement in the case of ionized atoms but an occasional discrepancy in the case of the neutrals. It appears that, assuming experiment is correct, in these cases one still needs a better understanding of electron correlation and its effect on oscillator strengths.     

Chlorine K shell photoabsorption spectra of gas phase HCl and Cl2 molecules

High resolution photoabsorption spectra of HCl and Cl₂ have been measured near the chlorineK edge in the 2810–2850 eV photon energy range. Below the ClK edge, the strongest resonance is interpreted as a simple core excitation into the unoccupied σ* valence orbital for both molecules, leading to a markedly repulsive state. Higher resonances due to low lying Rydberg states, are observed in both systems, but with a larger oscillator strength for HCl as compared to Cl₂. In Cl₂, the σ* orbital is deep enough to avoid any mixing with Rydberg orbitals. In HCl, we observe the dipole forbidden Cl 1s → 4s transition which denotes a strong 4s–4p hybridization. Above the ClK edge, the multiplet features seen for HCl are analysed in terms of double-core-valence excited vacancy states. In Cl₂, their counterpart are found very close to the ionization threshold because of the deep σ* orbital and possibly because the excited core and valence electrons originates either from the same atomic site or from different ones.     

Determination of Erbium by Two-Color Three-Photon Resonance Ionization Mass Spectrometry

Excited states and autoionization states of the erbium atom were investigated by the use of multicolor resonance ionization mass spectrometry. Among the observed first excited states, a level [4f¹²(³H₆)6s6p(³P₁)] located 17,348 cm⁻¹from the ground state is regarded as the most efficient state for excitation within the wavelength range investigated (560–600 nm), while a level located 17,080 cm⁻¹from this first excited state (E= 34,458 cm⁻¹) is identified as the best second excited state for the optimal photoionization scheme. Many ionization schemes adopting an autoionization state are also investigated, and the most efficient scheme is identified as 4f¹²6s²(³H₆) → 4f¹²(³H₆)6s6p(³P₁⁰), 17,348 cm⁻¹→ 34,458 cm⁻¹→ continuum state, which corresponds to the two-color (ω₁+ ω₂+ ω_1,2) scheme. Various concentrations of standard solutions for erbium are determined and the minimum amount detectable by two-color three-photon ionization was determined to be 20 pg.     

Quantum chemical ab initio calculations for excited states of F IV

Quantum chemical ab initio calculations have been performed for the ground state and for several excited states of the F³⁺ ion (F IV). Three levels of accuracy have been used: Frozen-core SCF calculations (FRC-SCF) to determine orbital energies ε _nl and quantum defects δ _l for excited Rydberg orbitalsnl; frozen-core SCF followed by CI calculations (FRC-CI) which account for multiplet splittings and configuration mixings, and multi-configuration coupled-electron-pair approximation (MC-CEPA) calculations which include dynamic correlation effects. The accuracy of the calculated excitation energies is in the order of 5000 cm^?1 at the FRC-CI level and in the order of 500 cm^?1 at the MC-CEPA level. This latter error amounts to about 0.1% for excitation energies in the range of 400000 to 600000 cm^?1. The MC-CEPA calculations have been performed for 17 experimentally known states and for 14 experimentally unknown states, in particular for the configurations 2s2p ² (² D)3s, 2s 2p ²(² S)3s, 2s ² 2p 4p, and 2s ² 2p 5p.