Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms 1 . This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected 2 . Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-

No abstract

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 γ, and the influence of the precursor ionic states present in recollision ionization is observed for the first time.

Dissociation of the CO 2 + ion has been investigated in an intense ultrafast ͑55 fs͒ laser field by employing an intensity-selective scan technique and comparing the signals from linearly and circularly polarized pulses. Nonsequential contributions have been observed, highlighting the role of rescattering in the dissociative process.