Evidence of pre-edge inner-shell ionization in photoion spectra of atoms and molecules

Due to finite lifetime of an inner-shell vacancy the onset of continuous photoabsorption at the pertinent ionization limit ('edge') is not abrupt but follows an arctangent shape (Richtmyer). This means that ionization at that shell is possible with a small but finite probability already tens of eV below the edge. The steplike jump of the average charge qav of the photoions at the edge therefore develops a similar low-energy tail. However, compared to the arctangent wing of the photoabsorption cross section σabs, the tail of qav is amplified by a function which far from the edge approaches the jump ratio of σabs, making a determination of qav from photon spectra ideally suited for observation of pre-edge photoionization. This is demonstrated on several examples of energy-dependent atomic and, for the first time, also molecular photoion time-of-flight spectra. (author)