Berlin 2001 – scientific programme
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AMPD: EPS AMPD
AMPD 6: Sitzung 6
AMPD 6.7: Talk
Wednesday, April 4, 2001, 12:10–12:35, H104
Trapped Nanoparticles: New Ways to Study the Interaction of Molecules with Surfaces — •S. Schlemmer, J. Illemann, S. Wellert, and D. Gerlich — Institut für Physik, Technische Universität Chemnitz, 09107 Chemnitz, Germany
The interaction of atoms and molecules with surfaces is traditionally investigated by probing the surface with analytical tools during or before and after the process. Scattering techniques involving photons (UPS), electrons (LEED) or atoms (He–scattering) as well as imaging techniques (AFM, LEEM, etc.) are commonly used to determine the amount of material deposited on the surface. Depending on the technique additional information becomes available, e.g., on the arrangement of the adsorbate with respect to the substrate. A non–interferring method for the investigation of the adsorption of gas to a substrate is the quartz crystal microbalance where the increase in mass leads to a shift of the resonance frequency of the probe. With a sub–monolayer resolution this technique is often used to study the kinetics of the adsorption/desorption process.
In this contribution we describe a 3D quadrupole trapping instrument which enables the study of the kinetics of adsorption/desorption processes for the micro– or nanosized surface of a single, isolated particle. A non–destructive and high–resolution mass determination becomes possible by detecting scattered light. The light signal is modulated by the particles’s secular motion, the frequency of which is proportional to the particle’s Q/M ratio. Frequency determination is achieved by Fourier analysis [1]. Controlled charging/discharging of the particle in steps of single elementary charges leads to the determination of the absolute charge and thus also of the absolute mass. Presently a mass resolution (Δ m/m) of about 10−4 for a single 500 nm particle in a ten second measurement is obtained. Integration over longer periods of time improve the resolution to the ppm regime. Thanks to the long term stability of the rather robust experimental setup the particles Q/M ratio can be followed over days. In this way the kinetics of sticking and desorption can be followed with sub-monolayer resolution as an increase or decrease in mass of the trapped particle. For smaller particles unit mass resolution is achievable.
The new instrument is an interesting tool for mass spectrometry since it covers a mass range from several u up to nanograms which is hard to access by other methods. Mass spectrometry of heavier particles is an important task in many fields of science, involving aerosols, biological particles and nanoparticles.
Another important aspect of the nanoparticle trap is the good localization of the particle. The confinement of one particle to a volume of ≈ µm diameter corresponds to a density of 1012 particles per cm3. This makes the quadrupole trap experiment perfectly suited for optical experiments in a wide range of wavelengths, IR to VUV. An important future development for the trapped nanoparticle experiment is the extension to smaller particles. For this purpose the scattered light detection will be replaced by light detection from emitting particles.
[1] S. Schlemmer, J. Illemann, S. Wellert, and D. Gerlich, submitted to J. Appl. Phys.