A Clear Atomic Example for the Surface Sensitivity of Penning Ionization: He*(23S) + Yb

Abstract

AbstractUsing a crossed‐beam apparatus with a double hemispherical electron spectrometer, we have studied the spectrum of electrons released in thermal energy ionizing collisions of metastable He*(23S) atoms with ground state Yb(4f14 6s2 1S0) atoms, thereby providing the first Penning electron spectrum of an atomic target with 4f‐electrons. In contrast to the Hel (58.4 nm) and NeI (73.6/74.4 nm) photoelectron spectra of Yb, which show mainly 4f‐ and 6s‐electron emission in about a 5:1 ratio, the He*(23S) Penning electron spectrum is dominated by 6s‐ionization, accompanied by some correlation‐induced 6p‐emission (8% Yb+(4f14 6p 2P) formation) and very little 4f‐ionization (≲2.5%). This astounding result is attributed to the electron exchange mechanism for He*(23S) ionization and reflects the poor overlap of the target 4f‐electron wavefunction with the 1s‐hole of He*(23S), as discussed on the basis of Dirac‐Fock wave functions for the Yb orbitals and through calculations of the partial ionization cross sections involving semiempirical complex potentials. The presented case may be regarded as the clearest atomic example for the surface sensitivity of He*(23S) Penning ionization observed so far.