Excitation of He atoms by 50-500 keV proton impact: from Paul-trap promotion to the high-energy limit

Abstract

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 quasimolecular evolution of the collision system. States with dominant 1snp 1 P 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.

Coulomb excitation of helium atoms in collisions with highly charged ions

“…For measuring the intensity functions I λ (F z ) after excitation of He atoms by HCI impact, basically the same experimental technique was employed as for the investigations on proton impact excitation [1,2]. The experiments were performed at the cyclotron accelerator VICKSI of the Hahn-Meitner-Institut (HMI) Berlin.…”

“…In that case, radiative transitions to the 2 1 P level are possible within the whole electric field range; however, the triplet transitions occur only at electric field singlet-triplet anticrossings of the 1snl 1 and 3 Stark sublevels with l 2 [10], where singlet and triplet states are strongly mixed. The electric field range of the present experiments includes not only the 1s5l 1 × 3 anticrossings as in [1,2], but also all 1s4d and 1s4f 1 × 3 anticrossings. Due to the high signal-to-noise ratio of the 1s4l anticrossing signals, a detailed analysis of the collisionally excited n = 4 states becomes possible.…”

“…Recently we studied the excitation of helium atoms by 10-500 keV proton impact [1,2] covering the energy range from almost the effective excitation threshold to the high-energy limit, where Born's approximation applies. The results of these measurements were explained within the framework of saddle dynamics [3] and the Paul-trap model [4,5].…”

“…Due to the high signal-to-noise ratio of the 1s4l anticrossing signals, a detailed analysis of the collisionally excited n = 4 states becomes possible. For comparison with proton-impact excitation, for which measurements have so far [1,2] only been performed in weaker electric fields (F < 20 kV cm −1 ), some supplementary measurements are also performed with 200 keV protons within the extended electric field range used here. The results confirm that the JP scaling law is also approximately valid with respect to the charge distributions of states resulting from Coulomb excitation.…”

Electric dipole moments of He atoms excited to the 1s5l(l⩾ 2) states by He+-ion impact at intermediate energies

“…Favourable conditions for applying this experimental technique are given in the case of impact excitation of He atoms in collisions with protons [6,7] and highly ionised Ar atoms [8]. Due to the conservation of the total electron spin in atomic collisions, only singlet states of He atoms are excited.…”