Absolute cross sections for transfer ionization in H + -+-H ~ collisions have been measured in the center-of-mass energy range 50 eV to 40 keV, with the use of merged-and crossed-beam techniques, together with coincident detection of the products. A mechanism is proposed for the process in the lowenergy region. The cross section, calculated accordingly, is found to be in satisfactory agreement with the experimental data.PACS numbers: 34.70.+e Transfer ionization (TI) is a two-electron process in which one of the target electrons is captured into the projectile and the other one is ejected. In atommultiply-charged-ion collisions, this process has been extensively studied in the last years (see, e.g., the review by Salzborn and Muller 1 ). It is strongly exothermic, and its mechanisms are rather well understood, at least at low energy, in terms of the different Auger relaxation channels of a superexcited quasimolecular state. 2 In this Letter, we report on a fundamental investigation of transfer ionization in simple, endothermic conditions, namely, in collisions between H + and H ~ ions. This is, to our knowledge, the first work on TI in ion-ion collisions. With only two electrons, the H + -H~ system is simple enough hopefully to be treated theoretically without major uncertainties regarding the electronic structure, so that attention can be focused on the dynamical aspects. On the other hand, it is complex enough to offer intricate rearrangement possibilities. Whereas mutual neutralization 3 " 6 and electron detachment 7 " 10 in this system have been successfully studied in the past, there are, to our knowledge, no data available yet for the transfer-ionization process.In the present work, the cross section of the transferionization reaction H fl + + H*--*H fl + H* + +e has been measured over a wide energy range extending from 50 eV to 40 keV. Furthermore, a two-step mechanism has been considered and a model calculation carried out to estimate the cross section. The absolute cross section at center-of-mass energies from 880 eV to 40 keV has been measured in Giessen by means of the crossed-beam technique and a coincidence method in the detection of the reaction products H° and H + . Apart from some minor changes for the present measurements, the experimental arrangement used has been previously 11 described in full detail. In short, two well-collimated and momentum-analyzed ion beams of adjustable energies are arranged to intersect at an angle 0=45° in ultrahigh vacuum of -10 ~n mbar. The collision products formed in both beams are charge analyzed after the interaction region by electrostatic deflection. Both H° atoms and H + ions are counted individually by identical single-particle detectors. In order to reduce the background, both ion beams are cleaned, shortly before intersection, by electrostatic deflection of particles in other charge states which result from charge-changing collisions in the residual gas. In spite of this precaution and the prevailing ultrahigh vacuum in the interaction region, a coinciden...
