R. Drozdowski

Stark effect of atomic helium second triplet series in electric fields up to 1600 kV cm−1

We present experimental and theoretical investigations of the spectral series 2 3 P‐n 3 Q (n = 3‐10, Q = S, P, D,...,n 1) in electric fields up to 1600kVcm 1 . Such fields cause—for n >6—shifts of the upper levels of the observed transitions which are larger than the separation between levels with different principal quantum numbers. The patterns belonging to a certain principal quantum number become similar to hydrogen patterns; they are nearly symmetric and show a nearly linear Stark shift in higher electric fields. The applied fields were high enough that patterns belonging to neighboring principal quantum numbers begin to overlap, which leads to interesting level-anticrossing effects. The experimental results are compared with numerical calculations taking into account mixing between states of different principal quantum numbers and also between singlet and triplet states. The agreement between experimental and theoretical line shifts is quite good.

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.

Stark effect of atomic Helium singlet lines

We present experimental and theoretical investigations of the He spectral series 2S1−nQ1 (n=3÷9, Q=S,P,D,…,n−1) and 2P1−nQ1 (n=3÷9, Q=S,P,D,…,n−1) in electric fields up to 1635  kV/cm. Apart from the allowed transitions with |ΔL|=1, the transitions with |ΔL|=0,2,3,…—without field strictly forbidden—were observed. Several He patterns become similar to hydrogen patterns, which means they are nearly symmetric and show in higher fields nearly linear Stark shifts. The applied fields are high enough that patterns belonging to neighboring principal quantum numbers (for n>6) begin to overlap, which leads to interesting level-anticrossing effects. The experimental results are compared with numerical calculations, taking into account mixing between states of different principal quantum numbers and between singlet and triplet states. The agreement between experimental and theoretical results is quite good.

Anticrossing effects in Stark spectra of helium

In the spectral range between 480 nm and 630 nm the Stark effect of the transitions n 1Q-2 1S, n 1Q-2 1P and n 3Q-2 3P (n=3÷10, Q=S, P,...) was studied using electric field up to 1500 kV/cm. For such a high field the Stark splitting becomes greater than the simple structure of the atom. Hence anticrossings of the Stark components of the same magnetic quantum number occur. The experimental results have been compared with the theoretically determined shifts. The results of calculations show good agreement with observation.

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.

Anticrossing spectra of 3He and 4He atoms excited by He+–He collisions

Singlet–triplet anticrossing spectra of the helium isotope 3He have been measured for the first time. These anticrossing spectra differ strongly from those of the isotope 4He due to the hyperfine interaction. We exploited this difference for distinguishing between the excitation of target atoms and the formation of excited projectile atoms by electron capture in experimental investigations on the symmetric collision system He+–He.

Anticrossing spectroscopy of He atoms excited by 30–300 keV He+ impact

By measuring the anticrossing spectra of the 1s4l and 1s5l He I levels, we investigate single-electron excitation in 30-300 keV He + -He collisions. The post-collisional He I states are highly coherent superpositions of spherical states with large electric dipole moments. These superposition states vary gradually with the impact energy from purely parabolic states at 30 keV to superpositions of l ≤ 2 states at 300 keV. The results indicate that for the whole energy range single-electron excitation should be considered as a process mainly localized in the saddle region of the two-centre Coulomb potential of the (He + ) 2 molecular core. A theoretical model based on the symmetry of the collision system is discussed.

Stark effect in atomic helium in a very high electric field up to 1500 kV/cm

In the spectral range between 480 nm and 630 nm the Stark effect of the transitions n1Q-21S, n1Q-21P and n3Q-23P(n=3÷9, Q=S, P,...) was studied using electric field up to 1500 kV/cm. For such a high field the Stark splitting becomes greater than the simple structure of the atom. Hence, anticrossings of the Stark components of the same magnetic quantum number occur. The experimental results have been compared with the theoretically determined shifts. The results of caluclations show good agreement with observation not only for low field values, but also in high fields and in case of level anticrossings.

Paul trap promotion of electrons in atomic collisions

Electron promotion on rotating potential saddles is proposed as an important and efficient excitation mechanism taking place in atomic collisions at intermediate energies. Measurements on the excitation of atoms by positively charged particles, in particular, excitation of He I states in p–He collisions provides experimental evidence for this ‘Paul trap’ promotion in two-centre Coulomb potentials.

Charge distribution of He I states excited by 10-50 keV proton impact: a study of electron promotion in an atomic Paul trap

Collisional excitation of helium atoms by protons of intermediate energies is investigated experimentally. By measuring the intensities of various spectral lines as functions of an electric field applied parallel or antiparallel to the proton beam, we found that the excited He atoms are left after the collision in transient states with large electric dipole moments directed upstream. The experimental results are explained by assuming that during the collision one electron is promoted on the saddle of the two-centre Coulomb potential of the projectile and target. At intermediate energies saddle dynamics is particularly effective due to the stabilization of the electrons motion on the saddle by the Paul-trap mechanism.