Photoionization of xenon below the atomic ionization potential

Experiments using monochromated synchrotron radiation revealed that for densities of the order of 10¹⁹ atoms/cm³ and more, xenon exhibits a continuous photoresponse excitation spectrum below the atomic ionization potential (12.12 eV). The lower limit of the continuum is at about 11.10 eV, the energy difference between the ground state of the molecular ion Xe₂^_⁺ and that of the free atom. This is attributed to excimer formation at the wings of the atomic absorption lines followed by ionization and creation of xenon dimer ions. Dips appearing in the continuum very near to positions of atomic lines are discussed invoking the quasistatic theory.