Measurement of the oscillator strength distribution in helium

Abstract

The oscillator strength distribution in the discrete and continuous regions of the spectrum of helium from the 2s 1 S0 metastable state has been determined using a Nd: yttrium aluminum garnet YAG pumped dye laser system in conjunction with a low pressure rf glow discharge. The saturation technique has been employed to determine the photoionization cross section from the 2s 1 S0 excited state at and above the first ionization threshold. The measured value of the photoionization cross section at the ionization threshold has been used to extract the f values for the 2s 1 S →np 1 P Rydberg series from n=10 ton=52. The f values of the observed Rydberg series decrease smoothly with an increase of the principal quantum number. In the continuum region the oscillator strength densities have been estimated by measuring the photoionization cross sections from the 2s 1 S0 excited state at five ionizing laser wavelengths above the first ionization threshold. The discrete f values smoothly merge into the continuous oscillator strength densities across the ionization threshold.