On the mechanism of laser-induced desorption-ionization of organic compounds from etched silicon and carbon surfaces

The laser-induced desorption/ionization of organic compounds from etched ca rbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Silicon surfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (similar to5 eV ) F-atom beam to produce nonporous silicon. Atomic force microscopy (AFM) i mages showed that both etching procedures yielded surfaces with sub-microme ter structures. Highly oriented pyrolytic graphite was etched with hyperthe rmal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 mum infrared (IR) laser were used for desorption. Analytes were deposited on the substrates e ither from the liquid or the gas phase. Mass spectra were obtained provided that three conditions were fulfilled. First, sufficient laser light had to be absorbed. When the IR laser was employed, a thin physisorbed solvent la yer was required for sufficient laser light absorption to occur. Though the required fluence of IR and UV light differed by a factor of about 20, the calculated maximum surface temperatures were similar, about 1000 K. The sec ond requirement was that the substrate had a "rough" surface. The third req uirement, for the observation of protonated analytes, was that the aqueous pK(a)-value of the analyte be larger than about 4. These observations suppo rt the conclusion that the desorption-ionization mechanisms of analytes fro m porous and nonporous surfaces are very similar or essentially the same. ( C) 2001 American Institute of Physics.