Trapped Nanoparticle Spectroscopy

The TRAPS apparatus (Trapped Reactive Atmospheric Particle Spectrometer) was designed to investigate atmospheric processes involving nanoparticles with spectroscopic methods under laboratory conditions. Our current focus of interest are processes occuring in the summer polar Mesopause (T<-130°C, height 80-100km). Therefore, we synthesize nanoparticles of materials which have been found in the Mesopause region and observe its reactivity under realistic temperature and pressure conditions.

The TRAPS setup comprises a plasma reactor nanoparticle source to produce silicon oxide and iron oxide nanoparticles (3 – 15 nm). The nanoparticles are dispersed in helium carrier gas at high pressure. After passing a critical orifice into rough vacuum, a tunable aerodynamic lens is used to focus the particles into a differential pumping stage. We put high effort in optimizing the aerodynamic lens for particle beams close to the diffusion limit by CFD calculations.  Downstream the differential pumping the dense particle beam is transported by ion guides. For the confinement of particles in the size range of several kDa to MDa, a radio frequency from 10 - 150 kHz is used. Therefore, we developed an amplifier capable to provide an appropriate sinusoidal voltage with amplitudes up to 3 kV. Size and charge separation is achieved by quadrupole benders.

Core element of the TRAPS setup is a linear quadrupole trap adjustable in temperature from 50K – 300 K. It allows to collisionally cool and accumulate precursor particles. Thereby storage times in the order of hours can be reached.  A gas inlet combined with a mass analyzer permits to adjust precise partial pressures of reaction gases. Along the axis of the linear trap a cavity enhanced spectroscopy setup is placed, so that spectroscopic parameters and particle densities can be measured online.  In situ mass determination is possible with an orthogonal time-of-flight spectrometer at one of the quadrupole benders. Our detection scheme was designed to detect single particle events in the wide mass range from atoms to nanoparticles and their fragments. In future we plan routinely spectroscopic experiments with synchrotron radiation.

Contact: Prof. Dr. Thomas Leisner

Reference:

The TRAPS Apparatus: Enhancing Target Density of Nanoparticle Beams in Vacuum for X-ray and Optical Spectroscopy
J. Meinen, S. Khasminskaya, E. Rühl, W. Baumann and T. Leisner
Aerosol Science and Technology 44, 1-13 (2010).