A high-resolution study of the threshold photoelectron spectrum of helium

Measurements of the threshold photoelectron spectrum of helium have been made at the Gas Phase beamline at Elettra Synchrotron, Trieste. The energy region studied spans the thresholds of the HeC .N/ satellite states, beginning with N D 2, up to the He2C threshold. The energy resolution in the new spectrum is five times better than that of previous measurements and has enabled us to resolve satellite states with N D 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 ionization. This region has been used to examine the role of doubly excited neutral states in single-ion formation and also to compare with previous experiments the ratio of the discrete and continuum amplitudes of the cusp. The use of the threshold photoelectron technique as an accurate and convenient method to calibrate the energy scales of optical monochromators for synchrotron radiation is noted.