A HIGH-RESOLUTION STUDY OF THE THRESHOLD PHOTOELECTRON-SPECTRUM OF HELIUM

Measurements of the threshold photoelectron spectrum of helium have be en made at the Gas Phase beamline at Elettra Synchrotron, Trieste. The energy region studied spans the thresholds of the He+(N) satellite st ates, beginning with N = 2, up to the He2+ threshold. The energy resol ution in the new spectrum is five times better than that of previous m easurements and has enabled us to resolve satellite states with N = 13 -19 for the first time. The part of the spectrum in the region of the double-ionization threshold exhibits a cusp-like feature in accordance with predictions of the Wannier model for threshold photo-double ioni zation. This region has been used to examine the role of doubly excite d neutral states in single-ion formation and also to compare with prev ious experiments the ratio of the discrete and continuum amplitudes of the cusp. The use of the threshold photoelectron technique as an accu rate and convenient method to calibrate the energy scales of optical m onochromators for synchrotron radiation is noted.