P. G. MENONAND K. W. MICHEL
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Ultraviolet Absorption of Ammonia at High Temperatures behind Shock Waves
by P. G.Menon1 and K. W. Miche12 Institute of Physical Chemistry, University of Gtittingen, Gdttingen, Went Germany
(Received November 29, 1966)
Ultraviolet absorption of ammonia heated to 2600°K behind shock waves has been studied a t 2225, 2300, and 2400 A. The values of E in the Boltzmann factor obtained from a plot of log e vs. 1/T of the shock-tube data agree fairly well with the energies above v2” = 0 in the 2p2-3s transition obtained from spectroscopic measurements by Walsh and Warsop. Absorption is also observed a t 3360 A, indicating the formation of N H radicals in the shock-tube decomposition of ammonia.
msec. This section can be filled with hydrogen as Ultraviolet absorption data for ammonia a t room driver gas up to 16 atm pressure. It is equipped with temperature have been recently reported by Walsh a turnable knife near the flange for cutting an aluminum and W a r ~ o p who , ~ have tried to give an interpretation diaphragm of thickness 0.15-0.20 mm. The low-presof the spectrum using the older as well as their new sure section is 250 em long and is provided with a gas results. Dixon4 studied the absorption of ammonia inlet and a vacuum connection a t opposite ends as heated to 900°K and was able to obtain bands up to shown in Figure 1. At 90 and 190 cm from the middle 2431 A. At higher temperatures complications arise flange, glass windows are fixed flush with the wall. owing to the heterogeneous decomposition of ammonia At 240 cm from the flange, Le., 10 cm from the end on the walls of the container. Shock-tube heating of plate, quartz windows (Ultrasil from Heraeus, 91% ammonia and the measurement of absorption by the transmission at 2300 A) are used for transmitting ultragas, heated by the incident and reflected shock waves, violet light. seemed to be a way out of the above difficulty. TemElectronic Setup. A simple schlieren arrangement peratures up to 2600°K for ammonia-argon mixtures with two RCA 931 photomultipliers at the first two could be reached in this way. At still higher temperawindows furnishes the time signals for measuring the tures the decomposition of ammonia is too rapid to measure the initial absorption spectroph~t~ometrically.~init’ial speed of the incident shock. The first signal The use of shock-tube techniques for the study of (1) Regional Research Laboratory, Hyderabad-9, India. physicochemical problems has been dealt with in detail (2) Institut fuer extraterrestrische Physik, 8064 Garching bei in three recent monographs.6-8 The kinetics of the Muenchen, West Germany. shock-tube decomposition of ammonia has been studied (3) A. D. Walsh and P. A. Warsop, Trans. Faraday SOC.,57, 345 (1961). by Jacobsg and Mathews, Gibbs, and Holsen.5 Since (4) J. K. Dixon, Phys. Rev., 43, 711 (1933). ammonia is one of the products in the pyrolysis of (5) J . C. Mathews, M.E. Gibbs, and J. N. Holsen, presented a t the hydrazine, its behavior a t high temperatures is of par139th National Meeting of the American Chemical Society, St. ticular interest in the study of hydrazine.1° The results Louis, hlo., 1961. obtained for the ultraviolet absorption of ammonia (6) E. F. Greene and J. P. Toennies, “Chemische Reaktionen in Stosswellen,” Dietrich Steinkopff Verlag, Darmstadt, 1959. in the shock tube up to about 2600°K are given in this (7) A. Ferri, Ed., “Fundamental Data Obtained from Shock-Tube paper. Experiments,” AGARDograph No. 41, Pergamon Press Inc., New Experimental Section The Shock Tube. An eloxized square aluminum pipe, 3.2 cm on a side inside, is used as the shock tube. The high-pressure driver section is 180 cm long so that the reflected rarefaction wave cannot interfere with the reaction at the observation window for at least 1 The Journal of Physical Chemistry
York, N. Y., 1961. (8) A. G. Gaydon and I. R. Hurle, “The Shock Tube in High Temperature Chemical Physics,” Reinhold Publishing Corp., New P m k , N. Y., 1963. (9) T.A. Jacobs, PB Report 149, 140, U. S. Department of Commerce, Office of Technical Services, Washington, D. C., 1960; J. Phys. Chem., 67, 665 (1963). (10) K. W. Michel and €1. G. Wagner, 2. Physik. Chem., 68, 3318 (1964).
ULTRAVIOLET ARSORPT~ON OF AMMONIA
32Sl
Figure 1. Experirncntal setup for shock-tuhe studies.
serves &$ a trigger for oscilloscope I (Tcktronix 545, 53/54I< plug-in unit). The plus gate pulse at the end of the sweep serves as a trigger for oscilloscope I1 (same type as the first) which exhibit.s the photoelectric current from an EAIIG256A photomultiplier at the third station. The third signal of the inciderit shock at station 111 is recorded, and not only shork velocity hut also shock attenuation can he determined. For rald a t i n g instantaneous shock speeds, it is assumed that the attenuation is linear with respect to distanre. The instantaneous Mach number can he calrulated correct to 0,5y0. Since the arrival of the reflected shock wave at station I11 is indicated by a further signal, the reflected shock velocity can also he calculated. Owing to the short distanre (10.2 cm) from the end plate, however, this velocity may have an error of 1.4%. Hence its determination serves only m a check for the consistency of the data; the parametem nf the incident shock are all calculated from the Mach number M . of the incident wave. Absorption Measuremenls. A xenon high-pressure are (Osram XRO 162, feed current 10-amp dc) emits the ultraviolet light which passes through the quartz windows of the shock tube. It is focused fer behind the first slit of the monochromator (Zeiss 114 QII, dispersion dX/ds = 27 A/mm). The light intensity is
measured hefore m i d aftcr each rut1 k)y usiug :L rotating sector nf 2.S kc/scc. Absorption mcasuremeuts for ammonia at 2175, 2225, 2300, :tud 2400 A have hecu mtulc. The oecurrence of SH radicals during the decomposition of ammonia has been iiivestigated hy measurements et 33Goh.
P. G. MENONAND I
