The role of excited states in multiphoton ionization

The mixing of the excited states of an atom with the ionization continuum is considered by a modification of Fano's theory of autoionization. It is shown that in the case of the noble gases this leads to a reduction of the threshold power-law index
K
from
I/\hbar\omega
to
E_{n}/\hbar\omega
(where
I
is the ionization energy, E
n
the lowest excited state energy, and
\hbar\omega
the photon energy) as found experimentally.