Molecular Surface Analysis Utilizing Laser Desorption/Laser Ionization

Abstract

The ability to analyze surface elemental composition has existed for some time. The various methods include Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectrometry (SIMS), and many more.1 However, molecular surface analysis is only now achieving the same sensitivity and selectivity. Molecular surface analysis often utilizes various optical probes: IR Reflection Absorption Spectroscopy, Sum-Frequency (or Second Harmonic) Generation Spectroscopy on Surfaces, etc. These techniques are generally lacking species-specific information. Another approach is to remove the molecule from the surface and probe it in the gas phase, e.g., with state- of-the-art mass spectrometry. Since mass spectrometry offers high resolution and high sensitivity, the remaining problems are removal of the molecule from the surface and ionization without alteration of the molecule (e.g., fragmentation). These pose serious complications for large molecules, in particular. Furthermore, if the molecule of interest is only a minor constituent of a sample, mass resolution and sensitivity are not sufficient for species identification, and a pre- selection in the ionization is often necessary. Our solution is to employ lasers for both desorption from the sample and ionization (post-ionization) of the gas-phase species. The ability to choose the wavelength and intensity of the desorption laser and the post-ionization laser allows for proper tailoring to the needs of the investigation. This will be demonstrated with two examples. First, a vulcanizate (rubber) will be analyzed with a time-of-flight mass spectrometer for the organic additives present in minor concentrations in the near-surface region. Second, a new class of carbon molecules (fullerenes) will be examined with a Fourier transform mass spectrometer.