  We report a novel experimental technique for the comparison of ionization
processes in ultrafast laser pulses irrespective of pulse ellipticity. Multiple
ionization of xenon by 50 fs 790 nm, linearly and circularly polarized laser
pulses is observed over the intensity range 10 TW/cm^2 to 10 PW/cm^2 using
Effective Intensity Matching (EIM), which is coupled with Intensity Selective
Scanning (ISS) to recover the geometry-independent probability of ionization.
Such measurements, made possible by quantifying diffraction effects in the
laser focus, are compared directly to theoretical predictions of multiphoton,
tunnel and field ionization, and a remarkable agreement demonstrated. EIM-ISS
allows the straightforward quantification of the probability of recollision
ionization in a linearly polarized laser pulse. Furthermore, probability of
ionization is discussed in terms of the Keldysh adiabaticity parameter, gamma,
and the influence of the precursor ionic states present in recollision
ionization is observed for the first time.
