Experimental determination of the effective solid angle of long-lived projectile states in zero-degree Auger projectile spectroscopy

In zero-degree Auger projectile spectroscopy (ZAPS), electrons are measured with high energy resolution at 0° with respect to the beam direction and state-selective cross sections can be determined. However, for Auger electrons from long-lived projectile states such as the 1s2s2p4pj (with lifetimes ranging from ns to ms depending on atomic number Zp and total angular momentum J) these zero-degree electrons are emitted all along the projectile path towards the spectrometer making their detection solid angle ΔΩ¯ and therefore their cross section difficult to determine. To date, ΔΩ¯ has been modeled theoretically leading to significant corrections of the determined cross sections. Here, using ZAPS, we experimentally determine ΔΩ¯ using the 1s22s2pP3 metastable component of 17.5 MeV O4+ and 6.6 MeV C2+ Be-like ion beams. Both the long-lived 1s2s2pP4 state and the similarly configured prompt 1s(2s2pP3)P2 (also known as the P−2) state are readily produced by 1s ionization in a cascade-free environment in the ratio of 2:1, according to spin statistics. This allows for the determination of ΔΩ¯ for the P4 state in relation to the known point solid angle ΔΩ0 of the prompt P−2 state. Using this technique new results are obtained with our hemispherical deflector analyzer (HDA), as well as from the re-analysis of the older ZAPS measurements of Lee et al. (1992) [26] which used a two-stage parallel plate analyzer (2PPA). SIMION electro-optical simulations for both spectrometers and geometrical calculations for the 2PPA are compared to our experimental results and found in acceptable agreement. The ramifications of these results on recent 2p capture measurements using (1s2sS3) C4+ and F7+ ions involving the production of 1s2s2pP4 states are discussed.