Photoionization of isooctane in intense laser fields. II. The effect of irradiance on electron dynamics

Thin path length jets (60 microm) of liquid isooctane have been photoionized with 36-70 fs pulses of 3.1 eV photons. Decay of the transient absorption (TA) at 1200 nm (assigned as predominantly due to absorption by the electron) has been examined over a time interval from 0.5 to 40 ps and over an irradiance range from 7 to 407 TW/cm(2). This range of irradiance covers a region that encompasses the closing of the three photon and four photon liquid ionization channels (at approximately 15 and 110 TW/cm(2), respectively) [J. Chem. Phys. 127, 214820 (2007)]. The temporal behavior of the TA is observed to cycle with irradiance. At the closing of both channels, the temporal behavior of the TA is the same and can be fitted to the model of an electron diffusively recombining with its geminate cation. In irradiance regions prior and subsequent to these channel closings, a similar diffusive fit of the TA is possible but with parameters changed to accommodate an experimental decay, which is much more rapid in the first few picoseconds. Possible origins for this periodic behavior are examined using the nonperturbative strong field approximation of Riess [Phys. Rev. A 22, 1786 (1980)].