Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

46 EXPERIMENTAL METHODS FOR SLOW REACTIONSFig. 33. High-pressure lamp (it has a water-jacketed mounting). General Electric Co. AH6,lOOO WPoint sources. An advantageous alternative to the high-pressure lamp for quantitativework is the high-pressure point-source lamp'28,'29, having a longer lifetimeand a bztter stability (Fig. 36). These involve a high-powered discharge inxenon, mercury or a mercury-xenon mixture at 20 atm. or more. The bulb-likedesign obviates the need for water cooling. Either a third electrode, to which ahigh voltage pulse is applied, or a special power supply for the two electrode model,is used to start the lamp. Figure 37 shows that they are excellent high-intensitycontinua sources (xenon lamps) and line sources (mercury-containing lamps).Similarly to the other lamps they may be operated with AC or DC. Since the lampsrepresent a radiation hazard, a special lamp enclosure is used to protect the operatoragainst the intense uv radiation. Vertical operation is obligatory, with thecathode at the top. At 100 watts, ventilation should be controlled so that a chr.omel/alumel thermocouple (d = 4 thou) at the quartz bulb indicates 800f25" C. Sincesputtering occurs with consequent electrode and quartz envelopedamage, the warming-uptime should be as short as possible. More detailed instructions are given byCalvert and Pitts' and by Anderson' 30.