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THEORETICAL STUDY OF NEAR-FIELD ENHANCED ABLATION AND IONIZATION TIME-OF-FLIGHT MASS SPECTROMETRY

Shudi Zhang;Zhisen Liang;Wei Hang

  The development of high-resolution chemical imaging techniques is among the hottest research topics in the world nowadays and attracting more and more attention1,2. We‘ve developed a method called near-field enhanced ablation and ionization time-of-flight mass spectrometry(NFEAI-TOFMS), utilizing both nanosecond and femtosecond pulsed lasers, which can offer a resolution of 50 nm at best when performing chemical imaging. However, the underlying mechanism of this technique has rarely been explored and poorly understood to date. In such a circumstance, we‘ve worked out a model to explore the underlying processes in the experiment under both lasers and explain the results. The model consists of three parts. Firstly, we apply the well-known method of finite-difference time-domain(FDTD) to simulate the electromagnetic enhancement around the nano-scale silver tip and on the sample surface. Secondly, we use the enhanced laser irradiance on the sample surface acquired in the first part as the energy input of the second part, which is a three-dimensional thermodynamic model. In this part, the 3D theoretical ablation pits under both lasers can be acquired, which fit well with the corresponding experimental pits. Additionally, we can get the 3D sample temperature distributions in the bulk and on the sample surface, which again serve as the input of the third part. The third part is a hydrodynamic model. By solving a combination of equations, including the Clausius-Clapeyron equation, the ideal gas law, the conservative equations of mass, momentum, energy, charge and a set of equilibrium state equations, we can get several plasma parameters that we are interested in, such as plasma temperature, particle number density, and the degree of ionization. All the above theoretical results explain the experiment well and shed interesting light into the near-field ablation and ionization area which has seldom been explored.……