| Absolute, total cross sections of n = 2 excitation of helium by intermediate-energy proton impact are determined using the energy-loss experimental method. The incident proton beam is accelerated to kinetic energies of 10-25 keV, and is focused into a target cell containing helium gas at room temperature. A hemispherical energy analyzer is used to measure energy-loss spectra of the incident and scattered proton beams. The excitation cross sections are determined by the measurement of the energy-loss peaks corresponding to excitation of the n = 2 states of helium. A deconvolution technique is applied to each spectrum to obtain the 21S, 2 1P, and n = 2 excitation cross section values. The present cross sections are compared to theoretical predictions which utilized the First and Second Born Approximation, the Distorted Wave Born Approximation, the Glauber Approximation, the Vainshtein-Presnyakov-Sobelman Approximation (VPSA), and the Multistate Impact Parameter Approximation (MSIPA) methods. The theoretical models that incorporate coupling of many states in the calculation, such as the MSIPA, are in reasonable agreement with the present data, suggesting that strong coupling between states must be considered in intermediate-energy, multi-electron ion-target collision systems. |
