The Study On Low-Energy Electron-Impact Induced Fluorescence Spectroscopy Of N₂ And CO

Low-energy electron impact processes play an important role in the excitation of molecular N2 and CO and their subsequent photoemission transitions in interstellar space.The emission spectra of N2 and CO have been observed in many stars,but the analysis and modeling of these spectra need more accurate information about spectral assignment and cross section.During the past decades,the absolute cross sections were reported for the excitation and photoemission transitions of N2 and CO,but there were still some non-negligible divergences or even controversies arising from different experimental or data-analyzing methods.Most of the researches are based on electron energy loss spectroscopy and electron transmission spectroscopy,which have very large errors in the near-threshold region.Therefore,it is necessary to use more accurate methods,such as electron-impact induced fluorescence spectroscopy,to carry out experimental studies on N2 and CO.For the first time,we combined low-energy trochoidal electron monochromator,ion velocity sliced map imaging technology and fluorescence detection technology.On the basis of our homemade high-resolution anion velocity map imaging apparatus,a spectral detection system was introduced.We designed and manufactured a set of devices for fixing and adjusting the optical elements in the vacuum chamber,and a feasible adjustment solution was given.The direction of electron beam,the direction of ion flight and the direction of light extraction are perpendicular to each other,so imaging detection and spectral detection can be operated independently.Imaging detection mainly detects the negative ion products in the dissociative electron attachment(DEA),which is unable to directly detect the neutral free radical products.Since the free radical products can be in the electronic excited states and decay to lower states by spontaneous emission,so the spectral detection system introduced can be used not only to detect the fluorescence spectroscopy by low energy electron impact,but also to detect the neutral free radical products in the dissociative electron attachment.In this way,the complete detection of the DEA products can be realized.We did the study on electron-impact induced fluorescence spectroscopy of the second positive band system of N2(C3?u?B3?g)and the third positive band system of CO(b3?+?a3?)by using the spectral detection system.For the first time,we obtained the absolute emission cross sections of(0,1)band,(1,2)band and(2,1)band of the second positive band of N2 from 11.50 to 20.10 eV.The excitation function of the absolute emission cross sections of the(0,0)band and(1,0)band are also in good agreement with the published results.After that,we compared the absolute excitation cross sections of C3?u(v'=0)state and(v'=1)state with the previous studies,and briefly analyzed the branching ratios of the relevant vibrational bands.By detecting the vibrational bands of the third positive band system of CO in the range from 9.90 to 15.00eV,the absolute excitation cross section of the b3?+(v'=0)state of CO was obtained,and some fine structures were observed in the excitation function,which proves that both the direct electronic excitation and the electron-molecule resonances play essential roles in the populations of this vibrational state.