Determination of absolute cross sections for excitation of n1P Levels of helium by electron impact (30–1000 eV)

Abstract

The total cross sections σ for excitation of helium to the 2 1P, 3 1P, and 4 1P levels by electron impact ( E e1 = 30 to 1000 eV) are determined from light intensities of the n 1P-1 1S transitions; the corresponding lines lie in the extreme ultraviolet wavelength range. The radiation is detected by a vacuum monochromator. In determining the cross sections the influence of cascading from higher levels and the polarization degree of the emitted light are taken into account. To determine the polarization correction the light intensities are measured with the exciting electron beam parallel and perpendicular to the grooves of the monochromator grating. From theory it is known that for an allowed dipole transition the σ E e1 versus In E e1 graph gives a straight line in the Bethe-Born approximation for sufficiently large energies: the slope of this line is determined by the optical oscillator strength. Our relative cross sections (between 200 and 1000 eV) are plotted in such a graph and the value of the abcis at the point of intersection ( c n ) is determined. With help of this value of c n and using the theoretical calculated optical oscillator strength to determine the slope, we are able to calculate absolute cross sections from relative measurements. The absolute cross sections determined in this way, differ about 3% from the very accurately calculated cross sections of Kim and Inokuti for the 2 1P and 3 1P levels and about 6% for the 4 1P level in the energy range of 150 to 1000 eV. However, these differences are of the same size as the (total) error.