Enhanced Control of Nanophotonic Ion Production by Laser Desorption Ionization from Tailored Nanopost Arrays Jessica A. Stolee 1 , Bennett N. Walker 1 , Deanna L. Pickel 2 , Scott T. Retterer 2 , and Akos Vertes 1 1 Department of Chemistry, George Washington University, Washington, DC 20052 2 Center for Nanophase and Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Nanophotonic ion production for mass spectrometry was demonstrated from quasi- periodic laser-induced silicon microcolumn arrays (LISMA) that had dimensions commensurate with the wavelength of radiation [1]. In contrast to conventional laser desorption ionization, these sources rely on nanophotonic interactions between the electromagnetic radiation and the nanostructure. As a result, for example, the ion yields dramatically depend on the angle of incidence and the orientation of the laser beam polarization [2]. The limited range of structures available for LISMA, however, makes it difficult to study the factors that govern the mechanism and to optimize ion production. In this contribution we demonstrate that silicon nanopost arrays (NAPA), see Figure 1, provide a broad range of morphologies for mechanistic studies and enable the tailoring of geometries for optimization. NAPA patterns with various post diameters and periodicities were created with a computer aided design program and rendered on the wafer by electron beam lithography. Columns with the desired heights were developed by deep reactive ion etching. Silylated NAPA were created by oxidizing the wafers in ozone and then treating them with (perfluorophenyl)-propyldimethylchlorosilane to produce perfluorophenyl (PFP)-derivatized surfaces. A custom built and two commercial time-of-flight mass spectrometers were used for laser desorption ionization experiments. For internal energy measurements, eight benzyl-substituted benzylpyridinium cations with a range of critical energies were used as thermometer ions. Survival yields of the molecular ions in unimolecular decomposition were determined from the mass spectra and correlated to their internal energy. NAPA proved to be effective matrix-free substrates for soft laser desorption ionization mass spectrometry of small organic and biomolecules and exhibited high sensitivity and resolution. In order to explore the mechanism of ion production, the NAPA dimensions were changed systematically and the corresponding ion yields were measured as a function of laser irradiance. To gain insight into the energy transfer between the substrate and the adsorbates, the internal energy of the desorbed ions was probed. Ion yields from NAPA of various column heights, diameters, and periodicities were studied using verapamil, bradykinin, leucine enkephalin, substance P, and thermometer molecules as analytes. Column heights were varied from 200 nm to 1600 nm, the diameters of the nanoposts ranged from 50 nm to 600 nm, and the periodicities ranged from 200 nm to 1200 nm. Our initial findings revealed that NAPA fabricated with 200 nm post diameters and 1200 nm post heights produced the highest ion intensities. At a constant diameter, NAPA with periodicities, P, commensurate to the wavelength of