Extreme Ultraviolet Absorption Spectra of Alkali Metals

Abstract

The optical absorption spectra of Na, K, Rb, and Cs have been measured at liquid nitrogen temperature using synchrotron radiation over the energy range from 10 to 40 eV, where the transitions of electrons in the outermost p -shells occur. Each spectrum shows the sharp Fermi edge with a spike followed by an intense broad band with some weak structures. The overall spectral profiles are not similar to the observed or calculated interband profiles in the visible-to-ultraviolet region. The spectra as a whole are interpreted as reflecting the spectral features of the transition matrix elements and having natures of localized atomic excitation. The effective electron numbers associated with the Pauli forbidden transitions between the filled conduction bands and the outermost p -shells are estimated as 0.023, 0.078, 0.11, and 0.14 for Na, K, Rb, and Cs, respectively.