EVIDENCE OF PRE-EDGE INNER-SHELL IONIZATION IN PHOTOION SPECTRA OF ATOMS AND MOLECULES

Due to the finite lifetime of an inner-shell vacancy the onset of cont inuous photoabsorption at the pertinent ionization limit ('edge') is n ot abrupt but follows an arctangent shape (Richtmyer). This means that ionization at that shell is possible with a small but finite probabil ity already tens of eV below the edge. The steplike jump of the averag e charge q(av) of the photoions at the edge therefore develops a simil ar low-energy tail. However, compared to the arctangent wing of the ph otoabsorption cross section sigma(abs), the tail of q(av) is amplified by a function which far from the edge approaches the jump ratio of si gma(abs), making a determination of q(av), from photoion spectra ideal ly suited for observation of pre-edge photoionization. This is demonst rated on several examples of energy-dependent atomic and, for the firs t time, also molecular photoion time-of-flight spectra.