Ionisation and electron transfer in He 2 + + H ( 1 s ) collisions

Ionisation in collisions of alpha particles with hydrogen atoms has been investigated for impact energies lying between 2 and 900 keV amu-l. The multistate semiclassical impact parameter model with a two-centre expansion in travelling atomic orbitals was employed. The computed ionisation cross sections are, in particular in the low-energy region, in good agreement with the recent experimental data of Shah er al. The total and partial charge transfer cross sections have also been calculated and found to be in harmony with the available theoretical and experimental data. Cross sections for the ionisation process He2++ H( Is) + He2++ Ht + e(1) in the simplest heteronuclear He2++ H( 1s) collision system have been calculated in the impact energy range 2-900 keV amu-I. This study was motivated by the discrepancies found in recent high precision measurements (Shah and Gilbody 1981, Shah et a1 1988) of the cross section for the process (1) and the available theoretical results. The standard semiclassical multi-state impact parameter model with an expansion in two-centre travelling atomic orbitals was employed. This basis set contained exact atomic states augmented by some bound and positive energy pseudo-states of the separated atomic species. The 22 atomic states given in table 1 of Shingal er a1 (1985) were placed on the target (hydrogen atom) centre. The Het ion was represented by 6s, 4p and 2d Slater-type orbitals. The parameters along with the energies are listed in table 1. The highest positive energy pseudostate for the s wave was excluded from the final calculation. The total ionisation cross section for the process ( l ) , computed by summing over all the partial cross sections for positive energy pseudostates, is displayed in figure 1 along with the experimental data of Shah and Gilbody (1981) and Shah et a1 (1988). In general, our calculated ionisation cross sections are in harmony with the experimental data over the energy range displayed. At higher impact energies, our computed ionisation cross section falls below the experimental data. Similar underestimation of the ionisation cross section in the corresponding energy region had been observed in p-H collisions using similar basis expansions (Fritsch and Lin 1983). In this energy region direct ionisation is dominant and pseudostates with higher partial waves such as f and g orbitals should be included in the basis expansion. We note that comparison with other high energy theories has already been made by Shah et a1 (1988). Our major interest in ionisation cross sections is for the energy region below 40 keV amu-'. In this region, we also show the 34-state triple-centre atomic-orbital 09S3-407S/89/150445 +OS$02.50 @ 1989 IOP Publishing Ltd L445 L446 Letter to the Editor Table 1. Parameters of the basis functions 4, (for notation see equation (8) of Shingal et al (1985)). j PI A, Level i Eigenergies E , (a) 1=0 1 0 2 0 3 1 4 2 5 2 6 2 (b) I = 1 1 1 2 2 3 1 4 2 (c) 1 = 2 1 2 2 2 2.000 1.000 1.000 1.155 1.666 1.375 1.000 1.155 1.666 1.375 1.666 1.275 1s 2s 3s 4s 5s 6s