Dissociative Photoionization Of Dimethyl Ether

The photoionization-dissociation process is an important source of many new ions and free radical species in nature.Through the accurate measurement of this process,we can obtain many fundamental dynamic parameters,such as Ionization Energy?IE?,Appearance Potential?AP?and Bond Energy?BE?,etc.In this thesis,dissociative photodissociation of dimethyl ether in the photon energy of 13.30-14.30eV was studied using threshold photoelectron photoion coincidence mass spectrometry,combined with photoionization using vacuum ultraviolet light source from synchrotron radiation.In this range,the dimethyl ether ions in B²B₂ and C²A₂ states were prepared,and the main fragment ions were m/z=45,29 and 31.By subtracting the contribution of the m/z =29 fragment ions,the reliable time-of-flight profile of the m/z=31 fragment ions was obtained.Then the related kinetic energy distribution was derived from fitting the profile.Below 14.00eV,a unique statistical dissociation was observed for the formation of the m/z=31 ions,while the dissociation mechanism was changed significantly to a bimodal channels under the photon energy above 14.00eV.Combined with the experimental kinetic energy released distribution and the theoretical predictions,we proposed an overall mechanism of dimethyl ether ions in B²B₂ and C²A₂ states to produce the m/z=31 ions.For dimethyl ether ions in B2B2 state,the CH₂OH+?m/z=31?fragment is produced via isomerization and statistical dissociation.For the C²A₂ state,dimethyl ether ions can directly dissociate to CH₃O+?m/z?31?fragment ions with high kinetic energy by breaking C-0 bond.In addition,we have upgraded the ion velocity focusing imaging spectrometer built in the laboratory.The new spectrometer is designed with a dual Skimmer to reduce the background of the overflow gas while redesigning the focused electric field.Through Simion fitting and verification experiments,the modified spectrometers have a gas beam translation temperature of?10K,and O+has a mass resolution of 0.8 1%and 0.73%in two-dimensional and three-dimensional velocity focused projection modes.