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 Iex. Transient absorption (TA) at 1200 nm in both neat liquids is measured in a 60 μm 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 TW cm2, the dependence of TA on Iex exhibits the periodic structure theoretically predicted for multiphoton channel openings and closings. At low Iex (<9 TW cm2), TA in isooctane is proportional to Iexn where n=3, consistent with nonresonant, near threshold ionization (liquid phase ionization potential=8.6 eV). At Iex >9 TW cm2, n declines with increasing Iex up to Iex =13 TW cm2, at which point n abruptly increases to 4. The pattern is repeated at Iex >13 TW cm2, albeit with n declining from 4 and then abruptly increasing to 5 as Iex becomes greater than 100 TW cm2. A similar trend is observed in n -octane. The dependence of the TA on Iex 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)].