Ultraviolet and Infrared Laser Excited Two-Color Multiphoton Ionization for Determination of Molecules in Solution

A novel two-color multiphoton ionization technique with ultraviolet (UV) and infrared (IR) laser excitation has been applied to highly sensitive detection of molecules in solution. The simultaneous excitation by 10-ns laser pulses at 355 and 1064 nm substantially enhanced the photocurrent signal of an analyte as compared with that when the laser pulse at 355 nm acted alone. Some basic factors affecting the photo-curren enhancement have been investigated. The additional excitation by the IR pulse improved the detection sensitivity of aromatic molecules and vitamins by 3–6-fold as compared with the case of the UV pulse alone. The lowest detection limit (S/N = 3) was 2.4 pg/mL for 9,10-dlmethylanthracene. The results suggest that a three-photon process (two UV photons and one IR photon) most likely occurs in this study, and that the IR pulse acts through a geminate electron rather than through an excited neutral molecule.