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The study of explosions by flash absorption spectroscopy
THE FRANKLIN INSTITUTE LABORATORIES NICOL H. SMITH, DIRECTOR
THE STUDY OF EXPLOSIONS BY FLASH ABSORPTION SPECTROSCOPY BY F R A N C I S R. T A Y L O R 1
Short-lived molecular species occurring during gas explosions have been detected by means of flash ultraviolet absorption spectroscopy. G. Porter (1), originator of this technique, collaborated with R. G. Norrish (2) in using this method to study hydrogen-oxygen explosions. Their method consisted of initiating the reaction by means of a photolysis flash, and then taking the absorption spectrum of the reacting gases by a second flash which occurred a few milliseconds later. The triggering of the flash was done by electrical contacts on the surface of a rapidly rotating disk. This disk also acted as a shutter, excluding from the spectrograph the light from the photolysis flash lamp, but passing the emission from the flash lamp supplying the continuous background for the absorption spectrum. At The Franklin Institute Laboratories we have adapted this flash technique to the study of gaseous explosions which are not induced by photolysis but which occur spontaneously as a result of the thermal energy present at the temperature of mixing (3). This method has allowed us to follow the increase and decrease in the concentration of unstable radicals and molecules during explosions of carbon disulphide and oxygen (4), and propane and nitrogen dioxide (5) which last only a few milliseconds. Our apparatus is much smaller and simpler than that described by Porter. Our present technique consists of rapidly introducing mixtures of reactants into 4 cm. diameter quartz absorption tubes 20 or 55 cm. long which are maintained at a constant temperature in a large furnace. Light emitted by the reacting gases is transmitted through the heavy walls of the furnace by a quartz rod, acting as wave guide, to a sensitive photomultiplier tube. Part of the output of this detector is utilized to actuate a thyratron delay circuit--firing a high intensity flash tube at any predetermined time during or after the reaction. Part of the output is also transmitted to an oscilloscope, where the resulting trace of the total radiation from the reacting gases and the flash tube are photographed. Finally the light from the flash tube passes through the reacting gases and into a large Bausch and Lomb spectrograph, where an absorption spectrogram is produced. The flash tube used in this work consists of a 4 ram. i.d. quartz tube having two internal tungsten electrodes and one external triggering electrode located midway between 1 R e s e a r c h C h e m i s t , C h e m i c a l K i n e t i c s a n d S p e c t r o s c o p y Section. 5oi
502
T H E FRANKLIN INSTITUTE LABORATORIES
[J. F. I.
them. The flash tube contains 10 to 12 cm. Hg of xenon and is energized by a photoflash 25-microfarad capacitor charged to 5000 volts. Oscilloscope traces of the light emission from this source shows it to have a discharge time of approximately one millisecond. The technique of flash ultraviolet absorption spectroscopy can be applied to nearly any rapid gas phase reaction where chemical species are produced which have absorption bands in the visible or ultraviolet. REFERENCES
(1) (2) (3) (4)
G. PORTER,Proc. Roy. Soc. (London), Vol. A200, p. 284 (1950). R. G. W. NORRISHAND G. PORTER,Proc. Roy. Soc. (London), Vol. A210, p. 439 (1952). Sponsored by Wright Air Development Center, Dayton, Ohio. A. L. MYERSON,F. R. TAYLORANDP. L. HANST,J. Chem. Phys., Vol. 26, No. 5, 1309-1320 (1957). (5) F. R. TAYLOR, B. FAtrNCE et al., ASTIA Document No. ADl18105, WADC Technical Report 57-138, June 1957.
THE STUDY OF EXPLOSIONS BY FLASH ABSORPTION SPECTROSCOPY BY F R A N C I S R. T A Y L O R 1
Short-lived molecular species occurring during gas explosions have been detected by means of flash ultraviolet absorption spectroscopy. G. Porter (1), originator of this technique, collaborated with R. G. Norrish (2) in using this method to study hydrogen-oxygen explosions. Their method consisted of initiating the reaction by means of a photolysis flash, and then taking the absorption spectrum of the reacting gases by a second flash which occurred a few milliseconds later. The triggering of the flash was done by electrical contacts on the surface of a rapidly rotating disk. This disk also acted as a shutter, excluding from the spectrograph the light from the photolysis flash lamp, but passing the emission from the flash lamp supplying the continuous background for the absorption spectrum. At The Franklin Institute Laboratories we have adapted this flash technique to the study of gaseous explosions which are not induced by photolysis but which occur spontaneously as a result of the thermal energy present at the temperature of mixing (3). This method has allowed us to follow the increase and decrease in the concentration of unstable radicals and molecules during explosions of carbon disulphide and oxygen (4), and propane and nitrogen dioxide (5) which last only a few milliseconds. Our apparatus is much smaller and simpler than that described by Porter. Our present technique consists of rapidly introducing mixtures of reactants into 4 cm. diameter quartz absorption tubes 20 or 55 cm. long which are maintained at a constant temperature in a large furnace. Light emitted by the reacting gases is transmitted through the heavy walls of the furnace by a quartz rod, acting as wave guide, to a sensitive photomultiplier tube. Part of the output of this detector is utilized to actuate a thyratron delay circuit--firing a high intensity flash tube at any predetermined time during or after the reaction. Part of the output is also transmitted to an oscilloscope, where the resulting trace of the total radiation from the reacting gases and the flash tube are photographed. Finally the light from the flash tube passes through the reacting gases and into a large Bausch and Lomb spectrograph, where an absorption spectrogram is produced. The flash tube used in this work consists of a 4 ram. i.d. quartz tube having two internal tungsten electrodes and one external triggering electrode located midway between 1 R e s e a r c h C h e m i s t , C h e m i c a l K i n e t i c s a n d S p e c t r o s c o p y Section. 5oi
502
T H E FRANKLIN INSTITUTE LABORATORIES
[J. F. I.
them. The flash tube contains 10 to 12 cm. Hg of xenon and is energized by a photoflash 25-microfarad capacitor charged to 5000 volts. Oscilloscope traces of the light emission from this source shows it to have a discharge time of approximately one millisecond. The technique of flash ultraviolet absorption spectroscopy can be applied to nearly any rapid gas phase reaction where chemical species are produced which have absorption bands in the visible or ultraviolet. REFERENCES
(1) (2) (3) (4)
G. PORTER,Proc. Roy. Soc. (London), Vol. A200, p. 284 (1950). R. G. W. NORRISHAND G. PORTER,Proc. Roy. Soc. (London), Vol. A210, p. 439 (1952). Sponsored by Wright Air Development Center, Dayton, Ohio. A. L. MYERSON,F. R. TAYLORANDP. L. HANST,J. Chem. Phys., Vol. 26, No. 5, 1309-1320 (1957). (5) F. R. TAYLOR, B. FAtrNCE et al., ASTIA Document No. ADl18105, WADC Technical Report 57-138, June 1957.