Laser two-photon Ionization of several aromatic molecules In hexane has been carried out with the fourth harmonics (266 nm) of a Nd-YAG laser as an excitation source. The photolonlzatlon current Is quadratlcally proportional to the laser pulse energy, Indicating two-photon Ionization. There Is a large blank signal due to the solvent (hexane) Ionization, and H has been canceled with a differential technique. The effect of laser pulse energy fluctuation has been corrected by an on-line normalization procedure. The photolonlzatlon current also depends on the molar absorptivity of the molecule, and the first step to Ionization should be excitation at 266 nm above the ground state. The detection limit ranges from 3.8 ppb for benzene to 61 ppt for pyrene. Analytical curves were linear for at least 3 orders of magnitude above the detection limits.

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