33 research outputs found
Measuring transient reaction rates from nonstationary catalysts
Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experimentâa complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossibleâcatalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10â1000 times higher. We are able to record kinetic traces of CO2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions
The GREGOR Fabry-P\'erot Interferometer
The GREGOR Fabry-P\'erot Interferometer (GFPI) is one of three first-light
instruments of the German 1.5-meter GREGOR solar telescope at the Observatorio
del Teide, Tenerife, Spain. The GFPI uses two tunable etalons in collimated
mounting. Thanks to its large-format, high-cadence CCD detectors with
sophisticated computer hard- and software it is capable of scanning spectral
lines with a cadence that is sufficient to capture the dynamic evolution of the
solar atmosphere. The field-of-view (FOV) of 50" x 38" is well suited for quiet
Sun and sunspot observations. However, in the vector spectropolarimetric mode
the FOV reduces to 25" x 38". The spectral coverage in the spectroscopic mode
extends from 530-860 nm with a theoretical spectral resolution R of about
250,000, whereas in the vector spectropolarimetric mode the wavelength range is
at present limited to 580-660 nm. The combination of fast narrow-band imaging
and post-factum image restoration has the potential for discovery science
concerning the dynamic Sun and its magnetic field at spatial scales down to
about 50 km on the solar surface.Comment: 14 pages, 17 figures, 4 tables; pre-print of AN 333, p.880-893, 2012
(AN special issue to GREGOR
The new Göttingen Fabry-Pérot spectrometer for two-dimensional observations of the Sun
Studies of small-scale dynamics and magnetic fields in the solar atmosphere require spectroscopy and polarimetry with high spatial resolution. For this purpose, spectrometers based on Fabry-PĂ©rot interferometers (FPIs) have advantages over slit spectrographs. They possess a high throughput and allow fast two-dimensional, narrow-band imaging and image reconstruction of the data. In the present contribution we describe an upgrade, essentially renewal, of the Göttingen FPI spectrometer achieved during the first half of 2005. A new etalon from IC Optical Systems Ltd. (formerly Queensgate), England, with 70âmm free aperture for high spectral resolution has been mounted. New CCD detectors from LaVision GmbH (Göttingen) with powerful computer hard- and software were implemented. We consider the product of signal-to-noise ratio, frame rate, and field of view as a measure of the efficiency. At low light levels, e.g. in narrow-band speckle applications, this product has increased by a factor ~60 compared to the old system. In addition, several spectral regions can now be scanned quasi-simultaneously. We present first results obtained with the upgraded spectrometer. The efforts are undertaken to provide an up-to-date post-focus instrument for the new German 1.5âm GREGOR solar telescope presently under construction at the Observatorio del Teide on Tenerife.