Multiphoton Excited Conductance Spectroscopy. 1. Application of the Born Model to Femtosecond Laser Excited Multiphoton Ionization of Nonpolar Liquids

Nonlinear optical and chemical effects in the irradiation of liquid benzene with femtosecond pulses

We show that a spectral resonance between the π→π* absorption band in liquid benzene and the third harmonic (TH) of a propagating 800 nm femtosecond laser beam causes large positive changes in the real refractive index at the TH wavelength. This produces an increase in the third-order optical susceptibility and leads to the enhancement of nonlinear optical effects including TH generation and self-focusing. Enhanced filamentation is observed in liquid benzene, but this effect is not seen in perdeuterated liquid benzene under similar irradiation conditions. Filamentation is associated with the decomposition of benzene molecules, plasma emission from the focal region, and the appearance of carbon nanoparticles. This indicates that a complex chemistry accompanies the onset of filamentation. Chemical products formed under these conditions have been characterized using combined gas chromatography mass spectroscopy techniques. We also find that the presence of a TH filament is indicated by the appearance of a photocurrent and increased electrical conductivity in the solution. This photocurrent is found to be 50-60 times smaller in C₆D₆ where the π→π* resonance with the TH is much weaker. The intensity dependence of this photocurrent confirms the role played by TH generation in the overall interaction.