Selective excitation of parabolic Stark states of He i by proton impact

Büttrich, S.; Oppen, Gebhard von

doi:10.1103/physrevlett.71.3778

Cited by 12 publications

(19 citation statements)

References 17 publications

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“…Initial experimental results obtained at proton energies E p = 12.5 keV [5] led to the conclusion that saddle dynamics [6] provides an adequate description of inelastic p-He collisions at intermediate projectile velocities. According to this model, the excited electron is promoted on the saddle of the two-centre potential of the collision system.…”

Section: Introductionmentioning

confidence: 99%

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

Buettrich¹,

Gildemeister²,

Thuy³

et al. 1998

J. Phys. B: At. Mol. Opt. Phys.

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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.

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Section: Introductionmentioning

confidence: 99%

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

Buettrich¹,

Gildemeister²,

Thuy³

et al. 1998

J. Phys. B: At. Mol. Opt. Phys.

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“…The relative importance of this mechanism has been discussed within the theoretical framework of classical mechanics [2][3][4] as well as in various quantum mechanical approaches [5][6][7][8][9][10][11][12]. Many experimental studies have sought evidence for it in ionization [3,[13][14][15][16] and excitation [17,18]. Recently Pieksma and co-workers [19] measured the velocity distribution of electrons emitted in 1-6 keV p-H collisions integrated over all emission angles and found a maximum of the cross section at the velocity of the saddle point.…”

mentioning

confidence: 99%

Imaging of Saddle Point Electron Emission in Slowp−HeCollisions

Dørner

Khemliche²,

Prior³

et al. 1996

Phys. Rev. Lett.

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The two-dimensional velocity distribution of electrons emitted in 5 -15 keV p-He collisions has been measured for completely determined motion of the nuclei, that is, as a function of the impact parameter and in a well defined scattering plane. The electrons are emitted preferentially in the scattering plane and in the forward direction. The velocity distributions show sharp structures that vary strongly with impact parameter and projectile velocity. The results are compared to classical trajectory calculations. [S0031-9007(96)01614-6] PACS numbers: 34.50.FaIn ion-atom collisions, where the velocity of the projectile is much slower than the classical Bohr velocity of the target electrons, the dominant process which ionizes the target is electron capture to bound states of the projectile. Compared to this capture transition, the emission of an electron into the continuum is extremely unlikely [1]. While in a fast ion collision direct ionization by light projectiles is rather well understood, there has been much discussion of which mechanism promotes electrons into the continuum in slow collisions.On the grounds of classical mechanics, Olson [2] suggested a "saddle point mechanism." The potential between the projectile and the residual target ion has a point (the saddle point) where no force acts on an electron moving between them. As the projectile and target separate, the potential rises and hence electrons traveling with the velocity of this saddle could be "left stranded" in the continuum between the target and projectile [2]. The relative importance of this mechanism has been discussed within the theoretical framework of classical mechanics [2-4] as well as in various quantum mechanical approaches [5][6][7][8][9][10][11][12]. Many experimental studies have sought evidence for it in ionization [3,13-16] and excitation [17,18]. Recently Pieksma and co-workers [19] measured the velocity distribution of electrons emitted in 1-6 keV p-H collisions integrated over all emission angles and found a maximum of the cross section at the velocity of the saddle point. Kravis and co-workers [20] obtained two-dimensional images of the longitudinal and transverse velocity of the continuum electrons, using a technique similar to that used here, but without impact parameter determination, for a wide range of impact velocities of p and C 61 projectiles. Only for proton impact below 1 a.u. (atomic unit velocity) did they find most of the electrons in the saddle point region. Within the hidden crossing theory [9] ionization at these slow velocities has been explained as a multistep promotion process in the quasimolecule formed during the collision, and characteristic electron velocity distributions in the scattering plane have been predicted for a quantum mechanical analog of the classical saddle point mechanism [11,12].In this Letter we present an experimental study of 5-15 keV p-He collisions, which, for the first time give two-dimensional images of the square of the final state electron wave function in velocity space with simul...

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“…Highly asymmetric charge distributions were also measured for the post-collisional target [3] and projectile states [9] resulting from H + -He collisions as well as for the excitation of helium atoms by highly charged ions [13]. Though these collision systems are asymmetric, a decomposition of the electronic wavefunction into an even and an odd component with respect to the saddle plane may be justified.…”

Section: Discussionmentioning

confidence: 99%

“…A scheme of the experimental set-up used for measuring the intensity functions I λ (F z ) of the thermal target atoms has been shown in [2]. As in the experiments on the asymmetric collision system H + -He performed previously [9,10], the ion beam (ion current I He + ≈ 10-20 µA) was crossed with a thermal helium atomic beam. Two tube-shaped electrodes (inner/outer diameter 5.5 mm/12 mm) concentric to the ion beam were placed up-stream and down-stream of the collision volume with a spacing s = 9 mm between the two electrodes.…”

Section: Experimental Set-upmentioning

confidence: 99%

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

Busch¹,

Drozdowski²,

Ludwig³

et al. 2004

J. Phys. B: At. Mol. Opt. Phys.

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By measuring the anticrossing spectra of the 1s4 and 1s5 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 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.

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Selective excitation of parabolic Stark states of He i by proton impact

Cited by 12 publications

References 17 publications

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

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

Imaging of Saddle Point Electron Emission in Slowp−HeCollisions

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

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Selective excitation of parabolic Stark states of He i by proton impact

Cited by 12 publications

References 17 publications

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

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

Imaging of Saddle Point Electron Emission in Slowp−HeCollisions

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

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