Excitation of mercuric bromide by electrons

Computer modelling of the HgBr discharge laser requires as input the various cross sections for electron impact on HgBr2. One of the most important of these is the cross section for production of the HgBr* (B doublet sigma one-half), the upper laser state, by electron impact on HgBr2. This report describes work performed to measure the total emission cross section for production of the HgBr B-X fluorescence. Under the conditions of the experiment this corresponds to the total formation cross section for the HgBr*(B) state. A magnetically collimated electron beam (1-200 eV) crosses a low density molecular beam of HgBr2. The apparatus permits simultaneous measurements of negative ion, positive ion, and wavelength resolved photon production from HgBr2, and from permanent bottled gases. The relative wavelength dependence of the photon counting system has been determined from 280 to 640 nm using standard lamps. By comparison with known ionization and total emission cross sections in He, it is possible to measure total emission cross sections for other molecules whose ionization cross sections are known. The HgBr*(B) emission extends from 510 nm to beyond 350 nm, and appropriate integration over wavelength is necessary. The resulting cross section has a sharp onset at 6.0 eV and rises rapidly to a plateau with a mean value of 3.2 (-16) sq cm. In the region (3-5 eV) of dissociative attachment HgBr*(B) production is undetectable, corresponding to sigma < (-18) sq cm.