G. von Oppen

Laser spectroscopy on the Stark effect of the level of He I

In electric fields , shift and splitting of the line of He I were measured spectroscopically by crossing a beam of metastable He atoms with a frequency-doubled cw Ti:sapphire laser beam. We deduced the difference of the scalar polarizabilities, the tensor polarizability and the energy separation of closest approach of the Stark sublevels with m = 0 and negative reflection parity at . The polarizabilities agree with calculated values. By comparing the measured energy separation with theory, we deduced that the scalar polarizabilities of the and differ slightly by .

Magnetic depolarisation studies on the polarisation of He I lines impact excited by 10-35 keV Ne+ ions

The polarisation of the 492 (1s4d1D to 1s2p1P) and 668 nm (1s3d1D to 1s2p1P) fluorescence lines of He I impact excited by 10-35 keV Ne+ ions has been investigated. Using magnetic depolarisation techniques, the contribution of cascade processes to the polarisation fraction could be separated. For both spectral lines the authors find a strong but approximately energy-independent cascade contribution to the polarisation fraction. Therefore the significant energy dependence of the total polarisation fraction reflects mainly an energy dependence of the direct excitation process. This result confirms the lack of correlation between emission cross sections and polarisation data of He I lines previously observed by Andersen et al. (1977) for Ne+-He collisions.

On shape and amplitude of level-crossing signals

Abstract In view of measurements of signs of atomic splitting constants we rediscuss shape and amplitude of level-crossing signals with special regard to Δμ = 1 crossings. Simple relations between the geometry of the experimental setup and the shape of level-crossing signals are derived in this study. Whether the scattered-light intensity increases or decreases at a crossing depends, under special conditions, on the state vectors of the crossing sublevels. Therefore, through the sign of such signals one obtains additional information valuable in connection with the determination of signs of atomic splitting constants. Further, some relations between the eigenstates of crossing sublevels are derived. These relations yield some new prospects for the detectability of Δμ = 1 crossings.

Coulomb excitation of helium atoms in collisions with highly charged ions

Collisional excitation of He atoms by 65 and 80 MeV 40Arq+ ions (q = 6,13,14) was investigated by measuring the intensities I(Fz) of He I spectral lines at 401 nm and 447 nm as functions of an electric field -30 < Fz < +30 kV cm-1 applied parallel and antiparallel to the ion beam. Well resolved signals of singlet-triplet anticrossings with n = 4 and 5 were measured for both field directions in addition to smooth intensity variations of the lines with n = 7 and 5. The asymmetric signal structure indicates a highly coherent excitation of states with different angular momenta. For comparison, some supplementary measurements of I447 for proton impact are also reported. The results are discussed within the framework of saddle dynamics and the Paul-trap model. They confirm the Janev-Presnyakov scaling law valid for electric dipole Coulomb excitation, according to which the excitation process is essentially determined by the scaled energies E0 = Eion/(A q) of the projectile.

Determination of signs of atomic splitting constants using level-crossing techniques.

Abstract By measuring the positions of level-crossing signals the ratio of atomic splitting constants (hfs constants, g J values, tensor polarizabilities, etc. ) can be investigated. However, due to the time-reversal properties of the atomic hamiltonian these measurements often yield no information about the sign of these constants. Nevertheless, the sign of tensor polarizabilities and other splitting constants can be determined by use of circularly polarized light if the shape of Δμ = 1 crossing signals is studied. A convenient experimental setup is proposed here and applied to the of the sign of the tensor polarizability α ten ( 1 P 1 ) of the first excited 1 P 1 level of the Sr-I spectrum, where a negative sign was found.

EXCITATION OF HE ATOMS BY 50-500 KEV PROTON IMPACT : FROM PAUL-TRAP PROMOTION TO THE HIGH-ENERGY LIMIT

Excitation of helium atoms by 50-500 keV proton impact to singly excited states was investigated experimentally. By measuring the intensities of He I spectral lines as functions of an axial electric field applied to the collision volume, we analysed the electric charge distributions of collisionally excited states with principal quantum numbers n = 4, 5 and 7. The experimental results reflect the transition from intermediate- to high-energy processes. States with large electric dipole moments were found in the lower part of the investigated energy range indicating a quasi-molecular evolution of the collision system. States with dominant components and accordingly very small electric dipole moments were found for 400 and 500 keV proton impact, signifying that the high-energy limit is reached.

Fine-structure effects in relativistic calculations of the static polarizability of the helium atom

We use the relativistic configuration-interaction method and the model potential method to calculate the scalar and tensor components of the dipole polarizabilities for the excited states 1s3p3P0 and 1s3p3P2 of the helium atom. The calculations of the reduced matrix elements for the resonant terms in the spectral expansion of the polarizabilities are derived using two-electron basis functions of the relativistic Hamiltonian of the atom, a Hamiltonian that incorporates the Coulomb and Breit electron-electron interactions. We formulate a new approach to determining the parameters of the Fuss model potential. Finally, we show that the polarizability values are sensitive to the choice of the wave functions used in the calculations.

Determination of excitation matrices of n1D levels of HeI for Ne+-He collisions

Using level-crossing techniques, the authors measured relative excitation cross sections sigma m of Zeeman sublevels of the 1snd 1D level (n=3,4) in He I excited by 10-35 keV Ne+ ion impact. The results are free of influences of cascades. A pronounced energy dependence has been observed. Even in the 25-35 keV energy range, where the polarisation fraction of the observed fluorescence light is approximately energy independent, the relative values of sigma m vary strongly with energy.

Electric-field singlet-triplet anticrossings of He I

With regard to experimental investigations of electric-field anticrossings, the Stark splitting of 1snl levels of He I is evaluated for n=3, 4 and 5. Anticrossings of 1snl 1 Lambda and 3 Lambda Stark sublevels with l>or=2 and Lambda = mod ML mod <or=2 occur at fields F approximately 80 kV cm-1, 20 kV cm-1 and 7 kV cm-1 for n=3, 4 and 5, respectively. The electric-field positions and the energy separations of closest approach of the anticrossings are calculated. Using the Liouville matrix formalism, a theory of anticrossing signals is formulated allowing a systematic analysis of signals arising from anticrossings of twofold degenerate Stark sublevels. The signal amplitudes are calculated as functions of the excitation parameters. The investigations show that these anticrossing signals are well suited for determining relative cross sections for the excitation of singlet and triplet substates by atom-atom and ion-atom collisions.

Proton-impact excitation of HeI triplet levels

We investigated the excitation of the λ(1s3d3D−1s2p3P)=588 nm line of atomic helium by proton and deuteron impact for projectile energies 10 keV≦Ep≦25 keV. In apparent contradiction to Wigners spin conservation rule, the emission cross section does not vanish. By measuring the intensity of the impact radiation as a function of homogeneous magnetic and electric fields applied to the collision volume, it has been shown thatp- andd-impact excitation of the 1s3d3D level of HeI proceeds via 1snl states withl≧3, which populate the 33D states by cascade decays. The well-known strong singlet-triplet mixing of these 1snl states enables a population of triplet states in accord with Wigners rule. Accordingly, we determine the excitation cross section of the 1s4f multiplet from the measured emission cross section of the 588 nm line. The field-dependent signals give evidence that predominantly substates with |mL|≦1 are excited.