Evidence recently accumulated 1 " 3 has demonstrated that simple atoms and molecules transmitted through thin solids at velocities V>v 0 , where v 0 is the Bohr velocity, can be the incident species with their original electron(s) as well as those reconstituted by the process of target electron capture after the loss of the incident electron(s). For convenience, we distinguish the two as the 0 (original) and (ft (reconstituted) transmission regimes, respectively. The 0 regime may be simply characterized by the survival of the incident projectile and this has been described in detail elsewhere. 1 " 3 The (ft regime may not be so simply understood except perhaps for the case of atomic projectiles. Indeed, data on molecular transmission in the (ft regime have been available 4 " 8 for a number of years and, to our knowledge, no quantitative account has previously been given for the yield of even the simplest case of H 2 + . We describe here a quantitative model for fast H 2 + which reproduces remarkably well the observed transmitted yield of H 2 + as well as the associated yield of H° breakup fragments.The experimental features of the H 2 + transmit-13 J. Sugar and V. Kaufman, Phys. Rev. A 21, 2096 (1980). 14 P. Shorer and A. Dalgarno, Phys. Rev. A lj>, 1502 (1977). 15 P. Shorer, Phys. Rev. A L8, 1060 (1978). This paper contains excitation energies for Zn-like Ga, Br, Mo, W, and U. Values for intermediate atoms were communicated privately by the author. 16 K. T. Cheng and Y.-K. Kim, At. Data Nucl. Data Tables 22, 547 (1978).ted fraction at 0.4, 0.8, and 1.2 MeV/amu through carbon foils 1-8 /ig/cm 2 thick are shown in Fig. 1 as a function of dwell time, t D , in the target. These new data were obtained at University Lyon-I using a procedure described elsewhere. 3 The 0 and (ft features can clearly be seen. There is the © region of t D % 1 fs which is observed to follow exp(-1 D /T) , with T = 0.17 f s, independently of the projectile velocity V. This translates into an "electron-loss" cross section o t cc 1/V which is a feature expected from binary collisions. 1 On the other hand, there is the other region of t D > 1 fs, which exhibits a strong dependence on V. Such a feature can only be reasonably understood in terms of a reconstitution process after the loss of the incident electron, and it is this process that is of principal concern here. To properly focus on the (ft regime, we subtract out the exponential 6 transmission yield from the data and display in Fig. 2(b) the (ft transmission yield F(H 2 + ) relative to twice the equilibrium neutral fraction $ 0 for incident H + of the corresponding velocity. In effect then we have made use of a previous observation 6 that the Transmission yield of fast H 2 + through carbon foils has been measured over a wide range of foil thicknesses for 0.4-1.2-MeV/amu H 2 + projectiles. A model is described which gives an excellent quantitative account of this yield as well as that of the associated H° production.
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