J. A. Blake, George Burns,
and S. K. Chang University of Toronto Toronto, Ontario, Canada
Flash Photolysis Studies of Iodine Atom Recombination
Recently Yamanashi and Nowakl described an excellent experiment on iodine atom recombination using flash photolysis. Similar equipment, constructed independently in Toronto, has been used in our third year undergraduate laboratory since 1965. Our equipment differs only in details from that of Yamanashi and Nowalc. However, it has proved to be particularly sturdy, safe, and inexpensive, and for this reason it is briefly described here. The total cost of our equipment is in the neighborhood of $1000 plus labor, which does not include the price of an oscilloscope and a Polaroid camera, the latter being optional. The safety precautions, so important in flash photolysis, were essential to us, and there has been no accident since the equipment was constructed. We are using home-made Porter and Norrishz or Ramsey3 flash lamps. Two lamps were constructed and attached to the vacuum system inside a reflector; one of these lamps is a spare. A coaxial cable connects the lamp to a cabinet containing high voltage equipment. The lamps are filled with argon (3-10 cm Hg). After prolonged use, the firing characteristics of lamps deteriorate; the lamps are then evacuated and refilled with fresh argon. All high voltage equipment is in a locked cabinet. When an operator opens the cabinet, all power shuts off and the high voltage capacitor discharges automatically through a resistor within one second. We recommend construction of an inexpensive power supply similar to the one shown in Figure 1; voltage and current regulation is not needed. This power
receptical for the coaxial lead from the flash lamp. Thus, the high voltages do not appear outside the grounded cabinet, except when the flash lamp is fired. Discharge currents and voltages which are of the order of lo4 A and lo4 V, respectively, are DEADLY. Suitable ignitron circuits are available from the manufacturers. In order to insure reproducible firing, flash lamps have a third electrode one inch from their ground electrodes. A discharge is initiated by pulsing simultaneously the ignitron and a trigger transformer (EGuG TR69 is satisfactory). Flash lamps, placed in a reflector, are separated from the operator by a grounded wire mesh, which, in turn, is built inside a large Plexiglas box. Incandescent iodine quartz lamps are operated from a car battery, which is used in conjunction with a Heathkit battery charger. An ac coupled photomultiplier circuit is used in the experiment. Therefore, slow changes in the light intensity of the incandescent source do not affect the accuracy of measurements. The total light intensity of the incandescent source is measured by the mechanical chopping of the light beam in front of the photomultiplier. The photomultiplier circuit (Fig. 2) needs two power supplies 200-325 V each. We do not use a
fl
Figure 2.
Figure 1.
lOkV power supply, circuit.
supply is used only to chargc a high voltage, low indnctance capacitor. The capacitor (35 pfd, 10kV) is connected to an ignitron (GE, Westinghouse, or National Electric) which holds voltage until fired. A connector on the walls of the cabinet is joined to the ignitron via a coaxial cable. This connector acts as a
I,. I Ck'A
Photomultiplier power supply, circuit.
filter between the flash lamp and the reaction vessel, but we align the optics to minimize scattered light. As a result, the photomultiplier is "blinded" during the flash, but the reaction lasts for several milliseconds while flash subsides to zero within 200 psec. Oscilloscope traces are photographed on a Polaroid camera. Alternatively oscilloscopes with long per1 YAMANASHI, B. S., AND NOWAK, A. W., J. CHEM.EDUC.,45, 705 (1968). PORTER,G., "Technique of Organic Chemistry," v. VIII, part 11, (Editors: FRIBSS, S. L., LEWIS,E. S., AND WEISBERGER, A.) Interscience Publishers (a division of John Wilev & Sons.
Volume 46, Number
I I, November 1969 / 745
sistence phosphors can be used. I n the latter case, the student places a transparent acetate film against the oscilloscope screen and reproduces the oscilloscope trace using a china pencil. Acknowledgments
The development of the sturdy flash photolysis
746
/
Journal of Chemical Education
machine described became possible because of the support of the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant No. 506-66 and 69. We would like also to acknowledge the support of the Petroleum Research Fund of the American Chemical Society and of the Defence Research Boardof Canada, Grant #9530-54.
