Material ejection and redeposition following atmospheric pressure near-field laser ablation on molecular solids

Near-field laser ablation (NF-LA) coupled with mass spectrometry (MS) is very promising for highly spatially resolved chemical analyses on various substrates at atmospheric pressure, for example, in materials and life science applications. Although nanoscale sample craters can be produced routinely, no molecular mass spectra of ablated material from craters of ≤1 µm diameter have ever been acquired by NF-LA MS at atmospheric pressure. Some of the pressing questions are thus how much of the ablated material is transported into the mass spectrometer and in what form. Therefore, material redeposition on the near-field tip's surface from laser ablation of molecular solids was characterized with scanning electron microscopy. The crater profiles were studied by scanning probe microscopy. The results shown in this study demonstrate that there could be as much as 70% of the ablated material deposited on the near-field tip's surface. The redeposited products were found to be confined to a height of ~50 µm, thus suggesting that most components inside near-field ablation plumes propagate about the same distance for both anthracene and tris(8-hydroxyquinolinato)aluminum. Nanoparticles ablated from craters of ≤1 µm diameter are clearly observed. Furthermore, observation of tips after ablation of an anthracene surface angled at 60° with respect to a horizontal surface shows that the direction of the near-field ablation plume is neither in the direction of the surface normal nor towards the axis of incident laser beam but deflected further away from surface normal. [graphic removed]