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A method is described whereby absolute cross sections are obtained from inelastic energy-loss spectra induced by ions incident upon gaseous targets at energies of 25-125 keV. Inelastic energy-loss spectra of helium for proton impact are presented. Apparent differential cross sections and absolute total cross sections are obtained from the spectra. Total cross sections for the sum of the l i S-*2 i S and l 1 S-*2 P excitations are reported, along with estimates of the relative contributions of each. Total ionization cross sections, total cross sections for inelastic scattering, apparent energy distributions of ejected electrons, and partial ionic stopping powers are also reported. The excitation cross sections show excellent agreement with recent calculations on the coupled-state approximation. The ionization cross sections show good agreement with results obtained by other methods, except at the lower impact energies. The apparent energy distributions of ejected electrons fall below the results of direct measurements and show poor agreement with the available theory.

Profiles of the 2s 2 (*S) autoionizing state in He have been observed in the ejected-electron spectrum of H + + He for small angles of ejection and for H + energies of 6 to 150 keV. The shape parameter q shows an increasing dependence on projectile energy as the angle of ejection is decreased, with strong, quasiperiodic oscillations appearing at 10°.Autoionization profiles in atomic systems are described by the following formula 1 :where a A and u B are the resonant and nonresonant parts of the cross section, respectively, and e = (E-£ r )/ir is the energy of excitation or emission relative to the corresponding resonance energy E r in units of the natural half-width |r. The shape parameter q is given by certain matrix elements of the interaction and thus depends on the means of excitation. Profiles of the form (1) have been observed in photoabsorption 2 and electron impact 3 ' 5 spectra, and in ejected-electron spectra produced by electrons 6 " 9 and ions. 10 " 13 Until recently, the profiles observed with excitation by ions have been predominantly Lorentzian, corresponding to large q in Eq. (1). The observations of Bordenave-Montesquieu and Benoit-Cattin 12 and Stolterfoht, 13 however, indicate that asymmetric profiles can be expected at the higher ion energies and smaller ejection angles.We report here on a study of the ejected-electron spectra resulting from H + +He collisions at H + energies of 6 to 150 keV and ejection angles of 10°, 20°, and 30° with respect to the proton beam. The apparatus has been described previously. 14,15 At angles of 20° and 30° our results agree qualitatively with those reported in Refs. 12 and 13, in which the smallest angles used were 17° and 18°, respectively. At a given angle the autoionization profiles show increasing asymmetry with increasing proton energy, the effect becoming more pronounced with decreasing angle. At an angle of 10°, however, the profiles observed in the present study show a very strong dependence on proton energy and appear to oscillate in a quasiperiodic fashion as the proton energy is varied.Examples of the 10° spectra in the region of the 2s 2 ( X S) state are shown in Fig. 1. This state occurs at an excitation energy of 57.8 eV, or at an ejected-electron energy of 33.2 eV. The energy calibration used here is such that when the profile is Lorentzian, the 2s 2 ( X S) peak is at 33.2 eV and the 2s2p ( X P) peak is at 35.5 eV. Addition of the ionization potential then yields the spectroscopic energy of the latter state. 2

Inelastic energy-loss spectra induced by impact of H, H2, and Ar on N& have been measured at incident ion energies of 20-120 keV, with an energy resolution of about 2 eV. Prominent peaks are observed at energy losses of 9.5 and 13.8 eV. The first of these is well resolved and is attributed to excitation of the Lyman-Birge-Hopfield (LBH) system of N2. The peak at 13.8 eV is believed due primarily to excitation of the b mz band of the Worley-Jenkins series. The locations of these features on the energy-loss scale do not agree with results of the electron impact work of others and, in general, show trends with projectile velocity that are consistent with an assumed enhancement of excitation to higher vibrational states with the heavier particles. Collision cross sections for excitation of the two peaks are presented, as well as the total ionization cross section and the cross section for total inelastic processes in the case of proton impact.

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