Resonances at slow electron collisions with zinc atoms and ions

The optical excitation functions of four spectral lines corresponding to the transitions from the 4¹ D ₂, 5³ S ₁, 4³ D _j, and 6¹ S ₀ levels of atomic Zn were investigated with an electron spectrometer of a new construction. For the first time, elastic scattering of slow electrons from the Zn ions at an angle close to 180° was studied. In the energy range under investigation (0–7 eV), both the optical excitation functions of atomic spectral lines and the differential cross section of elastic scattering manifested the resonant structure caused by the contribution of autoionization states of the atom.     

Precision measurements of optical excitation functions for the mercury atom

We describe a computer-based facility for studying the excitation of atoms by ultramonochromatic electrons and give optical excitation functions for the 12 mercury spectral lines that originate from the n ¹ S ₀, n ¹ P ₁, n ¹ D ₂, n ³ S ₁, n ³ P _j , and n ³ D _j levels. We detected about 100 features in the energy dependences measured from the excitation threshold to 15.5 eV. The previously found positions of the features on the energy scale are in good agreement with our results. Most of the resonant features are shown to be mainly attributable to the decay of short-lived states of the negative mercury ion. We detected a postcollision interaction effect in the optical excitation functions of the lines that originate from the n ¹ S ₀ levels at energies of about 11 eV.     

Precision measurements of optical excitation functions for the cadmium atom

An experimental setup and a technique for the investigation of excitation of atoms by ultramonoenergetic electrons are described. The optical excitation functions are given for 14 spectral lines of the cadmium atom originating from the n ¹ S ₀, 5¹ P ₁, n ³ S ₁, 5³ P ₁, and n ³ D _j levels. More than 150 specific features are found in the energy dependences of the effective excitation cross sections measured from the excitation threshold to 16 eV. The most pronounced of these features are in good agreement with the fine structure observed previously. The main mechanisms of the initial-level population, namely, the direct transition of an electron from the ground atomic state to the initial level of a spectral line, the population of the initial levels due to the decay of short-lived states of a negative ion, and the cascade population, are separated. In the excitation functions of the lines originating from the n ¹ S ₀ levels, in the energy range from 10.9 to 12.4 eV, we observed for the first time an effect of postcollision interactions of emitted and scattered electrons in the vicinity of the thresholds of four autoionization states of the cadmium atom.     

Resonances at collisions of slow electrons with cadmium atoms and ions

Using the crossed-beam method and a hypocycloidal electron spectrometer, the energy dependence of the ionization cross section for the cadmium atom has been studied in the near-threshold region and the elastic scattering of slow electrons at an angle close to 180° by Cd⁺ ions was studied for the first time. Within the region under study (0–7 eV above the first atomic ionization potential), a resonance structure determined by the contribution of atomic autoionization states is revealed in both the ionization curve and differential cross section of elastic scattering. The structure in the measured curves has been analyzed with the use of data on the ejected-electron spectra obtained under the excitation of autoionization states of Cd atoms, as well as the data on the optical excitation functions for the atomic spectral lines at λ=430.7 nm (5¹ P ₁-8¹ S ₀), 515.5 nm (5¹ P ₁-7¹ S ₀), 298.0 nm (5³ P ₂-6³ D _j ), and 361.0 nm (5³ P ₂-5³ D _j ).     

Postcollision interaction upon electron excitation of 4sns1S0, 3S1 and 4snd1D2, 3DJ levels of zinc atom

The optical excitation functions (OEFs) for four series of spectral lines of the Zn atom at the transitions 4sn ₁ s ¹ S ₀ → 4s4p ¹ P ₁^∘ (n ₁ = 6–10), 4sn ₂ s ³ S ₁ → 4s4p ³ P ₂^∘ (n ₂ = 6−8), 4sn ₃ d ¹ D ₂ → 4s4p ¹ P ₁^∘ (n ₃ = 4−6), 4sn ₄ d ³ D _{1, 2, 3} → 4s4p ³ P ₂^∘ (n ₄ = 4−7) excited by a monoenergetic (0.04–0.08 eV) electron beam with energies from the excitation threshold to 18–19 eV are measured. In the energy dependences of the excitation cross sections, the effect of post-collision interaction of slow (scattered) and fast (emitted after the autoionization state decay) electrons is observed and described. This interaction leads to the capture of a slow electron into the listed discrete levels and the appearance of maxima in the excitation functions. The energies of the maxima are used to determine the energies and widths of single autoionization states or their groups by approximate formulas of the classical approach. The autoionization levels of the zinc atom responsible for the maxima observed are determined. Possible transfer of the orbital angular momentum between electrons is analyzed.     

Postcollision interaction upon electron excitation of the 5sns 1 S 0 and 5snd 3 D j levels of the cadmium atom

The optical excitation functions (OEFs) for two series of spectral lines of the Cd atom originating from the 5sns ¹ S ₀ (n = 6?11) and 5snd ³ D _{1, 2, 3} (n = 5 and 6) levels excited by an ultramonoenergetic (ΔE _1/2 < 0.05 eV) electron beam with energies exceeding the single ionization threshold are presented. In the energy range from 10.8 to 12.9 eV, the energy dependences of the excitation cross sections of the studied spectral transitions exhibit the effect of postcollision interaction of slow scattered and fast emitted electrons. This process leads to an additional population of the initial levels of the spectral transitions due to the capture of a scattered electron into an excited atomic level. The energies and widths of the electronic decay of autoionizing states are estimated in the classical approach by two methods, namely, by the least squares method and by direct calculation. Calculations are performed using approximate formulas valid for different relations between the postcollision shifts of the OEF maxima and the binding energies of the atomic levels. The terms of the autoionizing atomic states responsible for the maxima observed in the OEFs of the spectral transitions are determined.     

Excitation of doublet <Emphasis Type="Italic">D</Emphasis>-levels of a lutetium atom by electron impact

The electron-impact excitation of doublet D-levels of the lutetium atom is studied by the method of extended crossing beams. At the exciting electron energy of 50 eV, 64 excitation cross sections are measured. In the electron energy range of 0–250 eV, nine excitation optical functions are recorded. The occurrence of perturbations in the spectral series 6s ²(¹ S)6p ² P ₁^2/○ − 6s ²(¹ S)nd ² D _3/2 is discussed.     

Excitation of a singly charged Sr Ion by e-Sr collisions

Excitation of spectral lines of SrII, including laser lines, is studied experimentally in collisions of slow electrons with strontium atoms. Twenty-one excitation cross sections are measured at an energy of electrons of 30 eV. Five optical excitation functions are recorded at an energy ranging from 0 to 200 eV. Direct-excitation cross sections for the 5p ² P°_{1/2, 3/2} levels and the contribution to their population by cascade transitions are calculated. Excitation cross sections of two laser transitions in the IR spectral region are determined using known branching ratios. The results obtained are compared with data of previous experiments.     

Electron-impact excitation of the S°, P°, and D° doublet levels of cobalt atoms

The excitation of transitions from the ² S ^∘, ² P ^∘, and ² D ^∘ levels of the cobalt atom is studied by the method of extended crossing beams. Seventy-six excitation cross sections are measured at an exciting electron energy of 50 eV. Nine optical excitation functions are recorded in the electron energy region of 0–200 eV. Possible channels of excitation of levels under study are discussed.     

The role of the configuration interaction in excitation of the Cd(II) principal series at electron-atom collisions

The excitation functions of Cd(II) spectral lines, namely, the lines of the 4d ¹⁰5s ² S _1/2?4d ¹⁰ np ² P ₃ ^2/0 (n≤10) principal series and the line corresponding to a transition from the Beitler level 4d ⁹(5s5p ¹ P ⁰)² P ₃ ^2/0 , are analyzed under the conditions of electron-cadmium atom collisions in the electron energy range from excitation thresholds to 400 eV. It is found that the excitation functions of the spectral lines attributed to transitions from the Beitler level are similar to those of the principal series lines corresponding to transitions from the 4d ¹⁰ np ² P ₃ ^2/0 (n=9, 10) levels nearest to the Beitler level. This similarity is explained by the significant admixing of the Beitler level to the initial levels of the principal series lines. It is shown that the admixing substantially affects the excitation functions of this series for the states more distant from the Beitler level (up to n=6).     

Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 2³ S ₁ metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn ^?5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 2³ S?n ³ P series was verified. It is shown that the cross sections for a triplet level excitation from the 2³ S ₁ metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 10³ for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.     

Optical properties of laser-produced plasmas from Cd and layered crystalline CdAl2Se4 targets

An experimental investigation into emission by plasmas from Cd and crystalline CdAl₂Se₄ targets irradiated with a pulse-periodic YAG: Nd³⁺ laser is presented. The laser operates at a pulse width of 20 ns and provides 1−2×10⁹ W/cm² at the focus. The spectral and time characteristics of the emission are examined. The main findings are as follows: (1) The strongest lines correspond to the transitions from the 6³ S ₁ and 5³ D ₂ levels of Cd I. (2) With the Cd target, the recombination bottlenecks are 5² D _5/2 of Cd II (E _up=20.11 eV) and 8³ D _{1, 3} of Cd I (E _up=8.60–8.65 eV). (3) The average electron temperature outside the core of the plume is 0.64 eV. (4) For the 508.6-nm line of Cd I, the high filling rate of the 6³ S ₁ upper energy level may be related to the rapid recombination of cadmium ions with electrons via the intermediate levels of Cd⁻ at 6.82 and 7.24 eV.     

On resonance processes in near-threshold excitation of the resonance lines of a Zn+ ion in electron-ion collisions

The excitation of the components of the 4p ² P ₁ ^2,3/2/0 resonance doublet of the Zn⁺ ion by monoenergetic electrons in the interval 4–10 eV is investigated by a spectroscopic method in crossed beams. Resonances are found in the energy dependences below and above the excitation thresholds of resonance lines. The subthreshold resonances are satellite of the lines investigated and are excited in the process of dielectronic recombination. The main competing process here is the electronic decay of autoionizing states, which is manifested in a resonance excitation of the ions starting at the thresholds for the excitation of the levels. Autoionizing states lying between the levels of the doublet splitting of the 4p ² P ₁ ^2,3/2/0 state decay to the 4p ² P ₁ ^2/0 level in a Koster-Kronig process. Above the excitation energy of the 4p ² P ₃ ^2/0 level the the dominant contribution to resonance excitation is from autoionizing states with configurations 3d ⁹(² D _5/2,3/2)4s ² np,(n−1)f. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 8, 614–617 (25 October 1998)     

The effect of the electric field on the spectral line shape

The effect of the external electric field on the spectral line shape under conditions of a uniform population of magnetic sublevels is studied in the density matrix formalism. The line shapes of spontaneous radiation for the atomic 4¹P₁-2¹S₀, 4³D₁-2³P₀, 4³S₁-2³P₀, 4³P₁-2³S₁, 4³D₁-2³P₁, and 4³S₁-2³P₁ transitions ofHe are calculated. The electric field is shown to be responsible for the spectral line shape asymmetry. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 23–28, November, 1999.     

Lasing conditions in recombining helium plasmas

Lasing conditions for He have been evaluated numerically. We have used a collisional radiative model to calculate overpopulation densities Δn_ij, which are defined as differences between population densities per unit statistical weight of the upper and lower excited levels i and j, respectively. Laser oscillations for the level pairs 2¹P-3¹S, 2¹P-4¹S, 2¹P-3¹D, 2¹P-4¹D, 3¹D-4¹F, 3¹P-4¹S, 3¹P-4¹D, and 3³P-4³S are possible when the electron densities are within well defined limits at low electron temperature (T_e = 0.1 eV). For level pairs of the singlet state, the inversion mechanism for He is the same as for H. Only collisional processes produce population inversion in the triplet level pair 3³P-4³S.     

Two-photon spectroscopy of ytterbium

Two-photon laser spectra of the Yb vapor have been obtained. Transitions to highly excited 4f¹⁴ 6sns¹S₀ and 4f¹⁴ 6snd ¹D₂ states are seen in direct two-photon excitation. Hybrid resonances involving 4f¹⁴ 6s6p ¹P⁰₁ and 4f¹⁴ 5d6s ³D₂ intermediate states lead to transitions to 4f¹⁴ 6sns¹S₀, 4f¹⁴ 6snp ³P⁰_2,1 and 4f¹⁴ 6snd ¹D₂ levels.     

The dissociative excitation of singlet levels of the singly charged lanthanum ion in collisions of electrons with lanthanum bromide molecules

The method of extended crossing beams was used to study inelastic collisions of slow electrons with LaBr₃ molecules. Dissociative excitation caused the appearance of spectral lines of the singly charged lanthanum ion. 38 dissociative excitation cross sections of LaII spectral lines (upper singlet levels) were studied at a 100 eV exciting electron energy. Three optical excitation functions were recorded over the electron energy range 0–100 eV. The cross sections obtained are compared with the data on dissociative excitation cross sections of the LaCl₃ molecule.     

Excitation of low-lying even levels of the holmium atom in the electron-atom collisions

In an experimental study of excitation of the holmium atom by electrons with an energy of 50 eV, the excitation cross sections are measured for 102 transitions from even levels of Ho I. Radiative transitions to the 4f ¹¹6s ^{2 4} I°_{13/2,11/2,9/2} levels of the ground state term are considered. Twenty-nine optical excitation functions are recorded in the exciting electron energy range of 0–250 eV. An classification of a number of Ho I lines located in the UV and visible spectral regions is proposed using the known energy levels.     

Study of postcollision interaction upon electron-impact excitation of the magnesium atom

The excitation functions of the spectral lines of the MgI principal series in the VUV region (for the principal quantum numbers n = 4–7) are studied in experiments on collisions of electrons with magnesium atoms in the range of electron energies from the excitation thresholds to ～25 eV. The scatter in the electron energy was ～0.7 eV. Above the ionization potential, some singularities attributed to autodetachment and autoionization states were found in excitation functions. Shifts to higher energies with increasing n, which take place for the most significant singularity (maximum) relative to the autoionization state 3p4s ¹ P ₁ ^o , are explained by the important role played by this state in the population of the initial levels through the mechanism of post-collision interaction. The theoretical analysis of the experimental results based on the classical model of the postcollision interaction estimates the width of the autoionization state as ～0.45 eV, which is close to the data of other researchers.     

Die differentiellen Anregungsfunktionen dern+2-Zustände von Helium

Energy and angular dependences for the electron impact excitation of the Helium 2³ S, 2¹ S, 2³ P, and 2¹ P states have been measured using monochromatic (0.05 eV) electrons in the energy range from 19 to 23 eV and for scattering angles ranging from 0 to 110°. The structures near the 2³ S-threshold have been analyzed in terms of resonant and non resonant i.e. direct scattering phase shifts. The first peak in this excitation function at 19.95 eV is mostly due to the rapid change with collision energy of the directs-wave phase of the inelastic scattering, which in turn is a consequence of the existence of the 2² S-resonance at 19.3 eV. The energy position and behaviour with collision energy of the first peak at 20.7 eV in the 2¹ S-channel is in agreement with the predictions for the existence of a virtual resonance state ofBurke et al. A term sceme of the He^? resonances with assignments of configurations is presented.