Many-electron effect in the Pd M45-VV APECS spectra of Pd/Ag(1Â 0Â 0) dilute surface alloys

The Pd M45-VV Auger-photoelectron coincidence spectroscopy (APECS) spectra of Pd/Ag(1 0 0) dilute surface alloys measured by Arena et al. [D.A. Arena, R.A. Bartynski, R.A. Nayak, A.H. Weiss, S.L. Hulbert, M. Weinert, Phys. Rev. Lett. 91 (2003) 176403] are analyzed. MX is the atomic shell Mx. In Pd/Ag dilute surface alloys the lifetime of a valence hole created by the M4-M5V Coster-Kronig (CK) transition or by the M45 shakeup is comparable to that of the M45 hole, while in Pd metal the former lifetime is shorter than the latter one. Thus, both the M4-M5(V)-VV transition and the M4-M5V-VVV transition contribute to the Auger-electron spectroscopy (AES) spectrum measured in coincidence with the M4 photoelectrons. This is because the Pd DOS at the Fermi level at the surface becomes lower compared to the bulk. An ab-initio atomic many-body calculation (the extended random phase approximation with exchange (ERPAE)) of the M4-M5V CK-transition rate of Pd atom shows that the localization of the valence hole in the M5V two-hole state increases the CK rate by a factor of about two. As the CK transition occurs near the threshold, the effect on the CK rate by the lifetime of the valence hole in the M5V state is substantial. This should also contribute to the M4-M5V CK rate increase in the surface alloys. We discussed how to determine the relaxation rate of the M5V two-hole state to the M5-hole state and the M4-M5V CK-transition rate by the coincidence photoelectron spectroscopy.