Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

42 EXPERIMENTAL METHODS FOR SLOW REACTIONS1 IVOc (V-V0)Fig. 30. Change in spectral profile of an emission “line” as a function of increasing self-absorption.From ref. 121.pressure lamps. Home-made varieties are described in ref. 22c. Since these lampsare of low intrinsic brilliance, they are often coiled about the RV or two or moreplaced parallel to the RV (Fig. 20).Medium-pressure lunips. At higher pressures of mercury than that for thelow-pressure lamp, ‘‘line’’ broadening’ l8 - 22 becomes very important and “line”reversal and double excitation occur. The effect of pressure broadening is shown inFig. 29. The effect of self-absorption causing “line” reversal is shown in Fig. 30.This radiation “line” is now useless for mercury sensitisation studies, but is suitablefor direct photolysis and, in particular, may be used with a vacuum systemcontaining mercury without causing sensitisation. Due to the high population of3P, atoms, double excitation is possible resulting in the production of a numberof “lines” (see Table 6). Some of the 3P, atoms are deactivated to the metastable3P, state, from where they may be collisionally reactivated or emit 2654A radiation.A typical lamp is shown in Fig. 31. The high intrinsic brilliance of these lampsand the large number of “lines” make them particularly suitable for kinetic studies.High-pressure latnps. The most intense sources of uv radiation are the highpressurelamps. Pressure and temperature broadening are increased under the operatingconditions (800” C and 100 atm). At very high pressures, the emission isalmost continuous in nature (Fig. 32). The output is 10 times that of the mediumpressurelamp and 1000 times that of the low-pressure lamp per unit length(A -3800A). The high-pressure lamp is also rich in visible light. The emission is