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Molecular Beam Experiment



mobex_tof.png At the fast expansion of a gas through the opening of a heatable nozzle the thermal energy of the gas is directly transformed into the kinetic energy of the gas molecules. The well defined kinetic energy is measured by time of flight technique and can be adjusted by variing the temperature of the nozzle as well by using a seeded beam, i.e. using a dilute mixture of the target gas with small inert gas atoms of compareably high kinetic energy at the given temperature (e.g. Helium). The emerging molecular beam then can be used to to investigate the adsorption dynamics of the specific molecules at the surface of selected substrates by means of optical second harmonic generation (SHG) as well as the method of King and Wells

King and Wells' Method

mobex_king_and_wells.pngThe method of Kind and Wells is used to optain direkt access to the sticking coeficient s(t) that paly an important role in the description of the adsorption dynamics of molecules at surfaces. Initially, after opening the beam at t1 a none-reactive shutter blocks the direct flow of the molecular beam onto the pristine sample surface while a quadrupole mass spectrometer (QMS) measures the partial background pressure genarated by the molecules. At t2 the shutter is opened and the reactive sample adsorbs molecules what leads to a distinct drop in the background pressure. After a certain time, the proceeding pasivation of the sample due to adsorption of the molecules results in a rise of the background pressure unto the initaila level p1 after opening the beam. In realtion to the pressure dorp p2 and the background pressure p0 before opening the beam the sticking coeficient calculates to



Last modified: 26.02.2016 · armbrusn

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