Photoelectron peaks in a spectrum resulting from
photoemission induced by characteristic minor X-ray lines associated with the X-ray spectrum of the anode material.
Example: \(\ce{K}\upalpha\!^{\prime}\), \(\ce{K}\upalpha\!_{3,4}\), \(\ce{K}\upalpha\!_{5,6}\), and \(\ce{K}\upbeta\) are all minor X-ray lines.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1828 (https://doi.org/10.1515/pac-2019-0404)