Multiphoton Ionization Spectroscopy as a Diagnostic Technique of Surfaces Under Ambient Conditions

Direct detection of solid substances is an important yet challenging issue in analytical chemistry. Laser multiphoton ionization spectroscopy has been applied for the first time for direct analysis of solids under ambient conditions. In this method, a solid powder/film is placed on a conductive surface and is irradiated by a pulsed tunable laser while an electrical field of similar to 2 kV cm(-1) is applied across this conductive surface and another electrode. The resulting photoelectrons and negative ions are measured by recording the current as a function of wavelength to produce a multiphoton ionization spectrum that is characteristic of the surface. Results indicate rich spectral features that can be used for compound identification. The present sensitivity is in the low picomole range. This method has been successfully tested for direct detection of various organic molecules, including explosives, narcotic drugs, and polycyclic aromatic compounds.