Filip Heijkenskjöld

Wavelengths and energy levels of the 4d95s-4d95p transition array of Xe IX

The spectrum of Xe IX in the 400-1200 ? wavelength region has been obtained by observing the radiation emitted following electron capture by Xe9+ and Xe8+ ion beams incident on a He gas target. The xenon ions were produced by the Uppsala University ECR ion source. Many of the stronger lines were found to originate from 4d95s-4d95p transitions. All levels of the 4d95s and 4d95p configurations have been established. The analysis was assisted by MCDF calculations and isoelectronic comparison.

1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation

2,3,5,6-Tetraethyl-1,4-disilacyclohexa-2,5-dienes with either four chloro (1a), methyl (1b), or trimethylsilyl (TMS) (1c) substituents at the two silicon atoms were examined in an effort to design rigid compounds with strong neutral cross-hyperconjugation between π- and σ-bonded molecular segments arranged into a cycle. Remarkable variations in the lowest electronic excitation energies, lowest ionization energies, and the first oxidation potentials were observed upon change of substituents, as determined by gas phase ultraviolet (UV) absorption spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and cyclic voltammetry. A particularly strong neutral cyclic cross-hyperconjugation was observed in 1c. Its lowest electron binding energy (7.1 eV) is distinctly different from that of 1b (8.5 eV). Molecular orbital analysis reveals a stronger interaction between filled π(CC) and π(SiR2) group orbitals in 1c than in 1a and 1b. The energy shift in the highest occupied molecular orbital is also reflected in the first oxidation potentials as observed in the cyclic voltammograms of the respective compounds (1.47, 0.88, and 0.46 V for 1a, 1b and 1c, respectively). Furthermore, 1,4-disilacyclohexadiene 1c absorbs strongly at 273 nm (4.55 eV), whereas 1a and 1b have no symmetry allowed excitations above 215 nm (below 5.77 eV). Thus, suitably substituted 1,4-disilacyclohexa-2,5-dienes could represent novel building blocks for the design of larger cross-hyperconjugated molecules as alternatives to traditional purely cross-π-conjugated analogues, and could allow for design of molecules with properties that are not accessible to those that are exclusively π-conjugated.

Channels of energy transfer to atomic nitrogen in excited argon-nitrogen mixtures

The vacuum ultraviolet (VUV) and ultraviolet (UV) emissions of gaseous mixtures of argon with a small amount of nitrogen were investigated in the afterglow of a pulsed transverse discharge. The time dependences of the intensity of the VUV argon continuum at 126.0 nm, the UV nitrogen molecular band at 337.0 nm, and the VUV nitrogen atomic lines at 149.47 and 174.52 nm were analysed in the total pressure range 150–600 hPa and the range 0.125–1.5 hPa of the admixture concentrations. The results show that all these emissions have a common source of excitation energy, namely the atomic metastable 3P2 state of argon. However, atomic nitrogen also receives energy from the atomic 3P1 state of argon and from the excited Ar2* (1u) molecules.

Measurements of absolute transition probabilities in Ba ii through optical nutation: erratum

We have developed a method to measure absolute transition probabilities for ions by recording optical nutation. Ions in an accelerated beam are brought into resonance with a counterpropagating laser beam by a rapid Doppler switch. From the time evolution of the population of the upper level in the resonant transition, we determine the Rabi frequency of the transition. An accurate measurement of the electric field strength then yields the transition probability. We report measurements for five transitions in Ba ii. For three of these transitions our values are more accurate than those of earlier published results.

Isotope shifts and hyperfine structure of 134-138Ba II by fast ion beam-laser spectroscopy

We have measured the isotope shifts of the 5d to 6p transitions in 134-138Ba II, of which the 5d3/2 to 6p1/2 transition has not been measured earlier. We have also obtained values with improved accuracy of the magnetic dipole and electric quadrupole hyperfine coupling constants of the 6p 2Po1/2,3/2 levels in 135,137Ba II. The measurements are performed in the red and orange parts of the optical spectrum using fast ion beam-laser spectroscopy with an improved procedure which avoids calibration of the ion velocity. A comparison with theory is made.

Wavelengths and energy levels of Xe VII and Xe VIII obtained by collision-based spectroscopy

The Xe VII and Xe VIII spectra have been investigated by collision-based spectroscopy. The radiation emitted following electron capture by 10q keV Xe-q+ ions (q = 6-8) impinging on a He (Ar) gas target has, with some exceptions, been recorded in the 350-8

Collision-based spectroscopy of Xe viii Rydberg states

The photon emission from 60-keV Xe6+ colliding with Na or Ar was recorded in the 35–800-nm wavelength region. Several of the observed lines were classified as transitions from singly excited states in Xe5+. The dipole polarizability of Xe6+ and the ionization energy of Xe5+ were determined from a fit with the polarization formula to the experimentally determined energy levels of nonpenetrating states. The analysis was assisted by Hartree–Fock calculations. All Xe5+ lines in the range 35–800 nm, reported in this work or previously, are tabulated.The photon emission from 60-keV Xe6+ colliding with Na or Ar was recorded in the 35–800-nm wavelength region. Several of the observed lines were classified as transitions from singly excited states in Xe5+. The dipole polarizability of Xe6+ and the ionization energy of Xe5+ were determined from a fit with the polarization formula to the experimentally determined energy levels of nonpenetrating states. The analysis was assisted by Hartree–Fock calculations. All Xe5+ lines in the range 35–800 nm, reported in this work or previously, are tabulated.

Hyperfine structure of the lower configurations in Y II by fast ion beam-laser spectroscopy

The hyperfine structure (hfs) of levels in the configurations 4d5s, 4d2 and 4d5p in Y II has been recorded using fast ion beam-laser spectroscopy. The levels in the 4d5s configuration were excited with frequency doubled laser light to levels in the 4d5p configuration, whereas the levels in the 4d2 configuration were excited by visible laser light. All hfs components could not be observed in the spectra, and therefore the magnetic dipole coupling constants A were evaluated in a parametrization of the peak separations in the framework of the effective operator theory.

Studies of the collision-based Xe vi and Xe vii spectra

Collision-based spectroscopy of a Xeq+(q=6, 7) beam with a target T (T=Na, Ar) has been performed in the 35–800-nm wavelength region. Twenty-five new Xe vi lines and twenty-two new Xe vii lines were classified, twelve new energy levels of Xe vi and nine new energy levels of Xe vii were established, and one energy level of Xe vi was revised from the classified transitions. The analysis was supported by Hartree–Fock calculations.

Double photoionization of thiophene and bromine-substituted thiophenes.

We report the double photoionization spectra of thiophene, 3-bromothiophene, and 3,4-dibromothiophene using a coincidence spectroscopy technique based on electron time-of-flight measurements. Spectra have been recorded between the onset and 40.814 eV using He IIalpha radiation. The He I photoelectron spectrum of 3,4-dibromothiophene has also been measured. All the spectra have been analyzed and interpreted in detail on the basis of theoretical simulations from accurate Greens function calculations.