Observation of electron-electron scattering in electron capture by fast protons from He

Abstract

The angular distribution of electrons ejected from He with about the projectile velocity in coincidence with the capture of the other electron by the projectile in 1-MeV proton-He collisions has been measured. The signature of the proton-electron-electron scattering leading to transfer ionization has been observed for the first time as a peak at about 90° in the angular distribution of the ejected electrons. The effect is in qualitative agreement with theoretical predictions. PACS numbers: 34.50.Fa, Considerable experimental and theoretical effort has been devoted recently to study the intriguing details of atomic collision processes involving the correlated motion of two active electrons. One of the conceptually simplest of these processes is electron capture from He by a fast proton through projectile-electron-electron ip-e-e) scattering. In this process the captured electron first collides with the proton and then scatters into a bound state of the projectile through a second collision with the other target electron, which is ionized due to this scattering. The simultaneous capture and ionization of two target electrons is called transfer ionization (TI) and can occur also through two independent interactions between the projectile and two target electrons. The p-e-e scattering mechanism leading to TI is very similar to the process of electron capture through nuclear double scattering. In that process, first discussed by Thomas* in classical terms, the second scattering of the captured electron occurs on the target nucleus. In the quantummechanical descriptions^'^ these double-scattering processes are represented by the second-order terms in the Born expansion. At very high collision velocities, where capture cannot be facilitated by the high-momentum components of the bound states, these double-scattering processes are the dominant mechanisms of electron capture.The experimental signature of the nuclear doublescattering mechanism is a sharp peak at 0.47 mrad in the angular differential scattering cross section of the charge-changed projectiles, reflecting the fixed kinematics of the first collision of the projectile with the target electron. This signature has been found"* in p-He, and more clearly in p-H collisions.^ Similarly, the p-e-e scattering should result in a peak at 0.55 mrad in the angular differential scattering cross section of chargechanged projectiles, which produce He^"^. Recently Horsdal, Jensen, and Nielson^ found a peak at around 0.55 mrad in the ratio of the differential scattering cross sections of charge-changed 0.3-0.7-MeV protons producing He^"^ and He"'", respectively. This result could, however, be described^ as simultaneous capture and ionization of independent electrons (i.e., without including the electron-electron interaction in the scattering ampli-tude).Briggs and Taulbjerg^ suggested that a signature of the /7-e-^-scattering-mediated capture in p-He collisions would be a peak at Ve = Vp (electron velocity equals the projectile velocity) in the energy spectrum of electrons ejec...

Transfer excitation reactions in fast proton-helium collisions

“…Transfer ionization, where one electron is captured into a bound state of the projectile and a second one is released into the continuum, have been studied in great detail [2][3][4][5][6][7][8][9][10][11][12][13][14] and have led to a better understanding of initial and final state correlation [15][16][17]. Instead of being lifted to the continuum, the second target electron can also be promoted to a bound but excited state.…”

Dynamics of ionization processes studied with the COLTRIMS method—new insight into e–e correlation

“…The electron transfer to the projectile can proceed via different reaction channels: (a) electron-electronThomas TI (eeTTI) Palinkas et al, 1989;Briggs and Taulbjerg, 1979;Ishihara and McGuire, 1988;McGuire et al, 1989McGuire et al, , 1995Shakeshaft and Spruch, 1979); (b) nucleus-electron-Thomas TI (nTTI) (Thomas, 1927, Horsdal-Pedersen et al, 1983Vogt et al, 1986); and (c) kinematical capture (KTI) (Brinkman-Cramer type). While eeTTI always leads to a transfer ionization where the second electron is ejected, the KTI and nTTI processes are accompanied by ionization of the second electron either by shake-off via e-e correlation or by an independent binary collision of the proton with the second electron.…”

Revealing the non- s 2 contributions in the momentum wave function of ground-state He

“…The TuTI process competes with other transfer ionization (TI) channels [6,7,8,10,11,12,13,15,16,17]. Each channel can be identified by a characteristic pattern in the H 0 and He 2+ final-state momentum phase space.…”