Imaging of Saddle Point Electron Emission in Slowp−HeCollisions

Abstract

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...

Intermediate energy ionization of helium by proton impact

“…The recoil momenta have been selected to lie in the negative y-direction, so that these spectra represent pictures of the electron spectra viewed looking 'down onto' the collision plane from above, where the collision plane is defined by the beam axis and the direction of the recoil momentum. As discussed previously [17], since the electron velocities lie approximately in the collision plane, this view provides the maximum information on the character of the electron velocity distributions. The units are v e /v p where v e is the electron velocity and v p is the projectile velocity.…”

“…The broad aspect ratios predicted by quantal calculations, which extend only to finite vt are now well established. The aspect ratios for CTMC calculations, which do extend to infinite vt, are much narrower and closer to the experimental results [10,11,17]. It would be quite helpful to have a general procedure, which will analytically continue the results at finite vt to infinity.…”

“…This energy region bridges the gap between the non-perturbative 'saddle point' region and the perturbative region, and addresses the question of how the evolution from the low-energy picture to the high energy proceeds. In particular, we examine to what extent the structured behaviour observed by Dörner et al [17] is retained as the collision time decreases. We compare our results with those from a theoretical calculation recently developed to deal with ionization in one-electron collision systems by Sidky and Lin and show the successes and limitations of employing a model potential to apply this oneelectron theory to a two-electron collision system.…”

“…Much discussion has appeared in the literature [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] concerning the role of 'saddle point' ionization and associated classical and quantal treatments of this process. Over the past few years, the experimental side of the issue has advanced considerably with the use of imaging techniques which have allowed the full momentum space distribution of the very low-energy ejected electrons to be measured for fixed values of the vector impact parameter [17][18][19][20]. For singly and doubly charged projectiles on atomic targets, these electron momentum distributions have been found to centre in velocity space around the location of the saddle point formed in the potential of the two receding heavy fragments, and to display marked structure which is attributable to the structure of the dominant participating molecular orbitals promoted into the continuum during the collision.…”

“…Dörner et al [17] used the technique of cold target recoil ion momentum spectroscopy (COLTRIMS) in studying electron ejection in the protonhelium system for low impact energies (5-15 keV) using the recoil momentum of the target He + fragment to select the collision plane and impact parameter. They found that the ejected electrons lie mainly in the scattering plane.…”