Photoionization of isooctane and n-octane in intense laser fields. I. Effect of irradiance on ionization rates.

The population of ejected electrons following multiphoton ionization of neat liquids isooctane and n-octane is investigated over a large range of ionizing irradiance I(ex). Transient absorption (TA) at 1200 nm in both neat liquids is measured in a 60 mum path at time delays of 0.7 and 2.5 ps following an intense 400 nm (3.1 eV) ionizing pulse. As the irradiance of this pulse is varied over the range from 4 to 410 TWcm(2), the dependence of TA on I(ex) exhibits the periodic structure theoretically predicted for multiphoton channel openings and closings. At low I(ex) (<9 TWcm(2)), TA in isooctane is proportional to I(ex) (n) where n=3, consistent with nonresonant, near threshold ionization (liquid phase ionization potential=8.6 eV). At I(ex)>9 TWcm(2), n declines with increasing I(ex) up to I(ex)=13 TWcm(2), at which point n abruptly increases to 4. The pattern is repeated at I(ex)>13 TWcm(2), albeit with n declining from 4 and then abruptly increasing to 5 as I(ex) becomes greater than 100 TWcm(2). A similar trend is observed in n-octane. The dependence of the TA on I(ex) in the regions of channel openings and closings is compared to the nonperturbative, strong field approximation developed by Reiss [Phys. Rev. A 22, 1786 (1980)].