Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

52 EXPERIMENTAL METHODS FOR SLOW REACTIONS4c5 --2crnFig. 40. Hydrogen discharge tube with iron electrodes and fluorite windows. From ref. 136.line often approaches that of the 2537A line, uiz. 1OI6 quantasec-’. Vaughan andno ye^'^^ used a hydrogen discharge tube with iron electrodes and fluorite windows(Fig. 40), which emits from 1900-1700A and has an intensity of 6 x 10I6quantasec- ‘ at 1750A. A windowless source from a low-pressure discharge inhydrogen has been found to be very rich in the 800-2000A regi01-1’~’. Sauer andDorfman’ 38 used a xenon resonance lamp, originally developed by Harteck andOppenheimer’39 (Fig. 41). Thelampsystemwasfilled withamixture ofxenon (0.8%)and neon to a pressure of 0.8-1 torr. Gas circulation was achieved by convectionthrough a ballast volume of 2 litres, to minimise impurity effects. The lamp is runat 120 mA, and its intensity is increased by a factor of 6 if a large horse-shoe magnetis placed such that the maximum field (1700 gauss) is just behind the window14’.In this case the lamp intensity is lof4 quanta.sec-’. Xenon provides 1470A and1296A lines, whereas a similar lamp using krypton provides 1235A and 1165Alines14’. Argon or nitrogen provide radiation from 500 to 1650AI4’. More detailsof similar lamps are given in refs. 9 and 22c.Very successful vacuum uv sources involve electrodeless microwave excited dischargesin low pressures of inert gases (Xe, Kr and Ne): for spectroscopic sourcessee refs. 143,144; and for photochemical sources refs. 145, 146. The design of thislamp is very simple and it is easy to keep clean and change the gas or window(Fig. 42). A chemical “getter”(Ba-Al-Ni alloy), prepared by flash heating underhigh vacuum, or a refrigerant is used to remove water, which is an undesirableimpurity since it produces so many lines in the region 1500-2000A. Fig. 43 showsthe emission from Kr- and Xe-filled lamps with and without a “getter”, refrigerant