COLLAPSE OF THE 4F WAVE-FUNCTION IN CESIUM

The collapse of the 4f wave function in various Cs systems was studied by means of inverse photoemission (IPE). At about 10 eV above the Fermi level, the ionic compounds CsI and CsO(x) display sharp IPE spectral structures that are attributed to Cs 4f1 electron-addition states, but no 4f signal is found in the case of Cs metal. These observations provide strong evidence for a collapse of the 4f wave function in ionic compounds; this collapse is caused by charge transfer from Cs to the anions.