Recent Research Developments

Andy Healy, David Underwood, Sandy Lipsky, David Blank

We have begun an investigation into the interaction of intense laser radiation with simple molecular liquids. Summarized here is some of our very recent work on the ionization of liquid isooctane using 36 fs pulses of 400nm (3.1 eV) laser light at intensities varying from 0.1 to 1.0 TW/ cm². We measure the time dependence of the absorption of a probe beam of 800 nm light. This absorption is predominantly attributed to the quasi- free ejected electron in the molecular liquid.

The ionization potential of liquid isooctane is 8.6 eV, and its first electronic absorption band starts at about 6.3 eV. Accordingly, a minimum of three non-resonantly absorbed photons are required for electron ejection. So long as the radiation field acts as a small perturbation on the molecular levels, we find, as is predicted (although not previously confirmed), that the yield of electrons is proportional to the third power of the laser light intensity (see Fig 1a) and that the time dependence of the population of electrons behaves as would be predicted from its recombination (via diffusive motion) with the cation (see Fig. 1b).

Figure 1: a) Summation of the log₁₀(Probe Absorption) for time delay 200 fs through 2000fs shown as a function of the log₁₀( Pump Irradiance) for six transient absorbance (TA) responses, and b) Normalized TA probed at 800 nm following multi-photon ionization of neat isooctane using 400 nm ionizing (pump) pulses. Transient absorbance measured at pump intensity 0.23 TW/cm² is fit to a standard diffusion limited decay curve (blue dashed line); TA measured at 1.0 TW/cm² is fit to a single exponential with a time constant of 420 fs (solid blue line). The inset is the same plot with the vertical axis on a log scale.

As the intensity of laser light increases there is a rather abrupt transition in behavior of the absorption of the 800 nm probe light that occurs at ~ 0.6 TW/cm². For intensities higher than this, the population of absorbing species becomes now, not proportional to the 3^rd power of the laser light, but rather to the 1^st power (see Fig 1a). Concomitantly, the time decay of this absorption no longer fits the expected diffusion behavior but declines exponentially with time (see Fig. 1b).

The origin of this fascinating change in behavior is not yet completely understood. However, theoretical estimates of the rate of tunneling through the barrier imposed by the coulomb potential and the electric field of the laser light indicate that at high laser intensities, the rate of electron tunneling should, at some critical intensity, suddenly become competitive with the rate of three photon absorption. We believe the sharpness of the onset of this non-perturbative behavior in intense laser fields underlies the suddenness of the observed transition in isooctane. However, the nature of the absorbing species that is decaying exponentially with time remains a mystery that we are currently working to solve.

A more complete description of this work may be found in J. Chem. Phys. Volume 123 Issue 3 (2005).

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