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

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Abstract
The laser-induced desorption/ionization of organic compounds from etchedcarbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Siliconsurfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (∼5 eV) F-atom beam to produce nonporous silicon.Atomic force microscopy(AFM) images showed that both etching procedures yielded surfaces with sub-micrometer structures. Highly oriented pyrolytic graphite was etched with hyperthermal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 μm infrared (IR) laser were used for desorption. Analytes were deposited on the substrates either 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 layer 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 second requirement was that the substrate had a “rough” surface. The third requirement, for the observation of protonated analytes, was that the aqueous pK a -value of the analyte be larger than about 4. These observations support the conclusion that the desorption–ionization mechanisms of analytes from porous and nonporous surfaces are very similar or essentially the same.