Coincidence measurements of cascade Auger electron emission processess in photon-atom interactions and autoionization of electrons from doubly excited states of helium produced by fast ion impact

A new apparatus has been designed and built. It includes a modified commercial double-pass cylindrical mirror analyzer (CMA) and several time of flight (TOF) spec- trometers. Energy and angular resolved electron emission spectra can be obtained with this apparatus and electron-electron and electron-ion coincidence measurements can be made. The apparatus has been used in three different experiments.

Electron-emission yields from the doubly excited states 2s²(¹S), 2p²(¹D), 2s2p(¹P) and 2p²(¹S) of helium produced by 1.67MeV/u carbon ions with charge states q=4,5,6 were measured with an energy resolution better than accomplished before. A CMA was used to observe simultaneously electrons ejected between 47.7° and 132.3° with respect to the ion-beam direction. Comparison of the data with several calculations was made to understand the mechanisms responsible for the production of these states. The relative electron emission yields for the 2p²(¹D) and 2s2p(¹P) states were studied as a function of the projectile charge state.

Electron-electron coincidence measurements from Auger-electron cascade decay processes after photon atom interactions were also performed. When a 3203.5eV photon interacts with an argon atom, producing a 1s hole, the atom typically decay by KLL Auger electron emission so that two holes in the L-shell are created; the atoms can further decay by LMM Auger electron emission. The result is a cascade- like decay process. Angular resolved coincidence measurements were made between the emitted KLL and the LMM Auger electrons. These coincidence measurements allow the study of the angular correlation between the emission of different first step KLL Auger electrons with the second step LMM Auger electron group.

When cascade decays take place, the photoion (in this case the argon ion) final charge state will depend on the decay path. The measured charge state fractions, however, depend on several experimental parameters. A detailed study of the variation of the measured charge state fractions with the experimental parameters was made. Coincidence measurements between KLL Auger electrons and recoil photoions were made at different photon energies. Branching ratios for several decay processes were obtained.