Collisional excitation of helium atoms by protons of intermediate energies (v p ∼ 1 au) 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 electron's motion on the saddle by the Paul-trap mechanism.

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

Ionization of hydrogen atoms with principal quantum number n =32, 40, and 51-74^by a 9.92-GHz electric field F(/) = z/ocoscof was studied with a superimposed static electric field F s = 0, 2, 5, and 8 V/cm. The measured field strengths Fo(\0°/o) at which 10% of the atoms were ionized are in excellent agreement with classical calculations in both one and two spatial dimensions. Covering finer detail as well as gross structure of the n dependence of FoOOo/o), the agreement supports the application of classical dynamics to the analysis of this strongly perturbed quantum system.

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