Unsuitability of the Keldysh parameter for laser fields

Abstract

The Keldysh parameter $\ensuremath{\gamma}$ is the accepted index for assessing field properties in laser-caused ionization. It is shown that $\ensuremath{\gamma}$ has significance only for the quasistatic electric (QSE) fields that underlie the tunneling model. The plane-wave (PW) fields produced by lasers are fundamentally different, with that difference becoming more emphatic as intensity increases. The gauge transformation that relates QSE and PW fields is valid only in a limited domain whose boundaries are not evident in a QSE context. The Keldysh $\ensuremath{\gamma}$ is shown to have no special meaning for laser-induced processes.