Exchange polarisation mechanism

The use of polarised beams in collision studies has enabled experimentalists to perform very detailed tests of theoretical models, particularly with regard to the role of electron exchange and the spin—orbit interaction in spin-dependent scattering. We will now briefly discuss the role of these interactions before using the general density matrix method to describe the more general case where more than one mechanism may contribute to the spin-dependent effects. 

It can be seen that electron—photon coincidence experiments with polarised electrons permit the investigation of spin effects in electron impact excitation of atoms at the most fundamental level. It can lead to direct information on both exchange effects and spin—orbit effects in the excitation mechanism. The information on the population of the magnetic sublevels can be visualised by charge-cloud distributions. These can tilt significantly out of the scattering plane for incident electrons transversely polarised in the scattering plane. 

HOR on Pt-Ru/C anodes has also been studied in a symmetrical H2IH2 polymer electrolyte membrane fuel cell, because the polarisations involved in this reaction are small [70], This cell contained a MEA in which a proton exchange membrane is sandwiched between a Pt-Ru catalysed anode and a Pt catalysed cathode. The anode (working electrode) was then fed with H2, as well as the cathode, which was used as counter and reference electrode. The analysis of the anodic polarisation scans together with the dependence of the exchange current density on the partial pressme of hydrogen allowed concluding that the Pt-Ru catalysed anode follows the Tafel-Volmer mechanism. 

According to McConnell and Moiseiwitsch the polarisation for a natural mixture of isotopes, will be -0.77 if electron exchange with spin transfer is the appropriate excitation mechanism. 

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