POLARIZATION PROPAGATOR CALCULATIONS OF GENERALIZED OSCILLATOR-STRENGTHS AND STOPPING CROSS-SECTIONS OF HE

We describe the first application of the polarization propagator method to the calculation of generalized oscillator strengths (GOS's). Both first-order (RPA) and second-order polarization propagator results are reported for the test case of the He atom. We show that fulfillment of the Bethe sum rule in finite Gaussian basis sets will always deteriorate for sufficiently large momentum transfers q, but that it is possible to obtain a satisfactory sum rule for all q by grafting on a limiting hydrogenlike expansion for the GOS's for large values of the excitation energies, E(n) - E0. Even though the polarization propagator calculation gives the main part of the stopping cross section we will demonstrate that in order to obtain the correct result for large velocity of the incoming projectile it is essential to include the GOS's obtained from the limiting expansion.