Absolute HeIα photoionisation cross sections of O atoms and electronically excited O 2( 1Δ g)

Abstract

Absolute photoionisation cross sections for short-lived ground-state oxygen atoms ( 3P) and electronically excited O 2 ( 1Δ g) are obtained at 584 Å using a recently developed electron modulation spectroscopy method. The angle-integrated cross section for the transition O + ( 4S) ← O ( 3P) is measured relative to O 2 +( 2Π g) ← O 2( 3Σ g −) which is used as a normalisation standard. Additionally an absolute value for the angle-dependent cross section at 90° for the transition O 2 +( 2Π g) ← O 2( 1Δ g) is determined. From relative intensity measurements in the photoelectron spectrum of O atoms a total photoionisation cross section, summed over states accessible with HeIα radiation, is obtained.