Charge transfer in slow collisions of C with H below 1 keVÕamu

Charge-transfer cross sections for slow C1H collisions have been measured in many experiments since the earlier 1980s. Using a source of slow ions from a laser-produced plasma and a hydrogen furnace as a target, Phaneuf et al. measured the total electron-capture cross sections in the energy range of 15–387 eV/amu @1#. Using the photon emission spectroscopy, absolute state-selected electron-capture cross sections have been measured by Hoekstra et al. in the impact energy range of 0.05–1.33 keV/amu @2#. Both of these early measurements have quite large error bars. More recently, Bliek et al. used the state-of-the-art merged-beam techniques to determine absolute total electron-capture cross sections in the energy range of 6–1000 eV/amu @3#. This collision system has also attracted considerable interest and stimulated much theoretical work, partly due to the persisting discrepancies between experimental measurements and theoretical predictions @4–12#. Various quantal and semiclassical calculations were carried out based on molecular-orbital ~MO! expansion method and the adiabatic Born-Oppenheimer approximation. However, since the molecular orbitals do not satisfy the correct boundary conditions, modifications through electron translation factors ~ETFs! or reaction coordinates have to be introduced to account for electron translation effects. The earlier calculations are less reliable due to neglecting some radial and angular couplings or not having enough basis functions. The pioneering work of Gargaud et al. @4,8,9# was based on a quantal formalism using reaction coordinates. They improved their results later by adding more basis functions and including rotational couplings in the calculations. Saha @10# used a semiclassical approach and plane-wave-type ETFs. An alternative approach is to perform semiclassical calculations using atomic orbitals ~AOs! on the two collision centers as basis functions. This has been used by Fritsch and Lin @13#, and later by Tseng and Lin with improved basis functions and an ad hoc method was used to account for trajectory effects @14# at lower energies. Most recently, Errea et al. @11# carried out both quantal and semiclassical calculations based on MO expansion and reaction coordinates that are different from those used by Gargaud et al. @9#. Their results are in good agreement with those of Gargaud et al. @9# and the rectilinear trajectory AO results of Tseng and Lin @14#. In spite of these experimental and theoretical efforts, dis-