PULSE RADIOLYSIS OF AMMONIA GAS
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Pulse Radiolysis of Ammonia Gas. 11. Rate of Disappearance of the NH,(X2B,) Radical1 by Sheffield Gordon,* W. Mulac, and P. Nangia Department of Chemistry, Argonne National Laboratory, Argonne, Illinois 604%
(Received J a n u a r y 16, 1071)
Publication costs assisted by the Argonne National Laboratory
The free-radical species NH2 and NH have been produced by irradiating NH, vapor at pressures ranging from 250 to 1520 Torr with pulses of 2-MV electrons. The rate of disappearance of these radicals was followed by measuring the decrease in absorption at 597.6 "8() and 336.0 nm (NH). Rate constants for the following reactions have been determined. In the pressure range 250 to 1520 Torr NH,, ~ N H ~ + N H ~6.2 = X 1O1O 1M-l see-'. Between 250 and 1000 Torr NH,, k N H 2 + H + I = 2.2 X 10l2M - 2 see-', and appears to approach a second-order sec-'. limit beyond this pressure. k"z+NO = 1.6 X 1O1O M-l sec-l, and ~ N H + N O= 2.3 X 1Olo
Introduction The production of the NH radical in its ground state (X32)by the radiolysis of ammonia gas with 130-nsec pulses of 300-kV electrons was recently reported.2 The absorption spectrum of this radical due to the A 3 r + X38 transition was photographed, and the kinetics of its decay was studied under various experimental conditions. We have extended these studies by looking at the decay of the NH2 radical produced by the radiolysis of NH, gas at pressures ranging from 250 to 1520 Torr with 30-nsec pulses of 2 - M V electrons. Under these conditions we were able to observe photographically many absorption bands assigned to the NHZ radical., By following the decay of the partially resolved vibronic band at 597.6 nm (2A1 2B1) photometrically we were able to study the decay of this radical in pure ammonia gas and in ammonia gas containing electron and hydrogen atom scavengers. Experimental Section The source of pulsed electrons used in this work was a Model 705 Bebetron manufactured by the Field Emission Corp. This accelerator provides a 30-50-nsec pulse (half-width) of electrons with a maximum current per pulse of approximately 5000 A. Figure 1 shows the Cerenkov radiation from a pulse and is indicative of the pulse shape. A schematic drawing of the stainless steel irradiation cell used in this investigation is shown in Figure 2. The electrons enter the cell in the direction shown, through a l-mil stainless steel window (A). The analyzing light travels the cell at right angles to the electron beam axis, entering through suprasil window B, and exiting through C. The cell is used with an internal multiple pass mirror system (D, E, and F) based on the White designa4 The adjustable mirrors (D and E) are manipulated by external controls (G and H). These controls made it possible to vary the optical
path length through the cell from 40 to 240 cm by changing the number of traversals of the analyzing light beam. The distance between the spherical reflecting mirrors was 10 cm. The volume of the cell was 950 cc. For experiments above 400.0 nm, the quartz mirror blanks were front-surfaced with silver and for experiments below 400.0 nm, aluminum-surfaced mirrors were used. The cell was equipped with a pump-out lead, a metal trap, and cutoff valves. The top of the cell has a suprasil window which was used for observing the number of light passes. Viton O-rings were used on the metal flanges of the cell and on the adjustment controls. The spectra were photographed on a 0.75-m JarrellAsh Model 75-000 spectrograph. The spectrophotometric data for kinetic analysis were obtained with a l-m Hilger-Engis r\;Iodel600/1000 grating spectrometerspectrograph combination used with a 1200 lines/mm grating blazed at 500.0 nm. The detector was a RCA 1P28 photomultiplier. The photomultiplier signal was fed to a preamplifier and the amplified signal was fed to a Tektronix Model 454 oscilloscope and photographed on Polaroid (Type47, ASA 3000) film. For the Cerenkov measurement and the XH2 growth measurement no preamplifier was used, the signal from the photomultiplier being fed directly to the oscilloscope with a rise time of approximately 3 nsec. The analyzing spectral lamp was a 450-W highpressure xenon lamp. When used with the photomultiplier detector, the lamp was pulsed over a period of several milliseconds using a circuit developed by Xichaek6 This pulse increased the lamp intensity at (1) Based on work performed under the auspices of the U. S. Atomic Energv Commission. (2) G. M. Meaburn and 5 . Gordon, J . P h y s . Chem., 72, 1592 (1968). (3) G.Herzberg and D. A. Ramsay, Discuss. Faraday Soc., 14, 11
(1953). (4) J. White, J . Opt. SOC.Amer., 32, 285 (1942). (5) B. ~Michaels,unpublished work. The Journal of Physical Chemistry, Vol. '76,N o . 14, 1971
S. GORDON,W.MULAC,AND P. NANQIA
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Figure 1. Oscilloscope trace of Cerenkov radiation resulting from 1.7-MV electron pulse into empty cell. Abscissa 10 nsec/div.
Figure 3. Oscilloscope trace of analyzing light pulse showing transient pip (*) ahsciasa, 0.5 mec/div; ordinate, 0.2 V/div.
Figure 4. Oscilloscope trace showing absorption at 268.0 nm in omne produced by a 1.7-MeV pulse in 403 Torr of oxygen. Optical path 40 em, abscissa 0.5 msec/div. Upper line 100% transmission. Lower line 0% transmission.
Figure 2. Variable path multiple pasa irradiation cell.
600.0 nm by a factor of 40. The lamp intensity was constant to within a few per cent over a period of 1 msec or more. The detected species decay over a period of tens of microseconds and the lamp intensity are constant within our limits of detection during this period. Figure 3 shows an oscilloscope trace of the light pulse with a spike representing the pulse and decay of the transient at the center of the flat portion. An oscilloscope trace of the transient decay was photographed simultaneously with the light pulse on a second scope with a much faster sweep rate. Photographs of the spectra were taken with Eastman Kodak 103-0 plates. The same xenon lamp was used. However, the lamp was fired by charging a low i n d u e tance 0.5-rrF coaxial condenser to 19 kV and discharging the energy (100 J ) through the lamp by means of a spark gap trigger operated with a variable time delay trigger amplifier. The analyzing spectrographic light source operated in this manner had a half-width of approximately 8 psec. One flash gave a good continuum from 250.0 t o 700.0 nm with sufficient optical density for adequate measurement with a Joyce Loebel microdensitometer. Ammonia was dried by condensing it over metallic sodium. The ammonia was then pumped from this The Journal of Phydml Chnnbtw. Vd. 76, No. 14.1871
solution and subsequently distilled from trap t o trap several times in a mercury-free vacuum system. Only the middle fraction of this distillation was used. The cell was pumped to better than Torr and filled to the desired pressure which was measured with a Texas Instrument Co., Model 145 quarts spiral pressure gauge. A similar technique was used to purify and introduce the additive gases. Dosimetry was performed by using both the NZproduction from N20 and the Oa production from 01. The ozone was measured in situ after the pulse by measuring the Oaabsorption a t 268.0 and 285.0 nm. Figure 4 is a typical oscilloscope trace of the O8 production. The G value for N2 from N20 obtained by Willis, et al.,’. and the Gvalue for Osproduction from O2obtained by Ghormley, et d . , ” b were used. Corrections were relative to made for the stopping power of NHa (SNA,) NIO (SN,O). A value of SNH,/SN,O = 0.5 was used.‘ Figure 5 gives the measured dose per pulse absorbed in the multiple reflection cell filled with N20 a t a number of different preasures and a t ambient temperature (25’).
The variation of the dose absorbed in the volume occupied by the multiple reflection cell was determined (6) (a) C. Willis. 0. A. Miller. A. E. Rothrell. and A. W. Boyd. Adam. C h m . Scr..81.539 (1968): (b) J. A. Ghonnley. C. J. Hoehand e l . and J. W. Boyle. J . C h m . Phys.. 50.419 (1969). (7) N. A. Bsily and G. C. Bmwn, Rad&. Rea.. 11, 745 (1858).
PULSE RADIOLYSIS OF AMMONIA GAS
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NO , DOSIMETRY OF THE IO cm MULTIPLE REFLECTION GAS CELL
applicability of the Lambert-Beer relation. This becomes important when the band used consists of sharp lines which are only partially resolved. If one plots the optical density of N H z ( ~ A+ ~ 2B1) and n”(A3II c X32) us. optical path length or pressure (related to concentration in this case), one obtains plots which do not pass through the origin. However, if the log of the optical density is plotted against the log of path length or us. the log of the concentration, a straight line is obtained. The plot is described by the equation
D
PRESSURE OF N20 (torr)
NUMBER OF PULSES
Figure 5. Nitrous oxide dosimetry (1.7-MV incident electron = 960 Torr. pulses): A = 250 Torr; 0 = 500 Torr; I
I
I
I
I
I
I
I
I
I
0 8.1 cm from window
0 1 0 . 6 c m from window X 1 3 . l c m from window
‘ - 5
-4
-3
-2
-I o I Dirtonce from beom oxis
2
3
4
5 c m
Figure 6. Dose contours in variable path, multiple pass cell resulting from a single pulse by 1.7-MV incident electrons.
by measuring the bleaching of blue cellophane.* Figure 6 shows the relative dose as measured by this technique. Each curve represents the variation in dose across the cell parallel to the analyzing light beam a t different positions in the cell (front, center, and back). From this plot, it can be seen that the variation in dose parallel to the analyzing light is about 2.5 and that the variation across the light path is about 2. The homogeneity of the dose inside the light path volume was well within the limits examined by both Sauerg and Boag’” for the determination of second-order rate constants. One feature of the high-current accelerator used is the electrical noise introduced by the voltage discharge during the pulse. This effectively blocks any measurement prior to 2 lsec after triggering the pulse. During the course of this work it was found that placing the entire detection system in a Faraday Cage (double copper screened room) some 32 ft from the accelerator eliminated this electrical interference and enabled measurements t o be made over a short time scale limited only by the pulse width (50 nsec). An important consideration in using this technique for studying reaction kinetics of transient species is the
=
where D is the optical density, E is an “extinction coefficient” of the species being observed, 1 is the optical path length, and n is a constant. This relationship has been discussed in earlier publications.11*12 When n = 1, the Lambert-Beer relationship holds. When n = 0.5, there is 100% absorption at the line center. Figure 7 shows our determina+ 2B1) at 597.6 nm and for KHtion of n for NH2(2A~ (A% + X38)at 336.0 nm, at a pressure of 760 Torr of ammonia using 100-p slits with a spectrometer whose dispersion is 0.8 nm/mm. Also shown is our determination of n for OH(A2Z++ Xzn) produced by irradiating 8 Torr of HzOvapor in 760 Torr of argon. Over the NHa pressure range studied, n for NH2 does not vary significantly (within experimental error) and was assumed at all pressures used to be equal t o the n determined at 760 Torr. This modified Lambert-Beer equation was applied to the data obtained in this investigation.
Results Figure 8 shows an oscilloscope trace of the growth of the absorption at 597.6 nm of the XHZradical. This trace indicates that the nTHZconcentration has reached a maximum in 150 nsec after the start of the pulse. Figure 9 shows a typical oscilloscope trace of the XH, transient decay. The data reported here were calculated from traces of this type by an automatic reading and analysis system13which plotted both the log of the optical density and the reciprocal of the optical density vs. time. The data represented by the points in Figure 10 show the change in D-1.3 (relative concentrations of XH2) with time at different pressures of KH3. From these curves it is seen that second-order kinetics are apparently followed in the initial portion of the decay of the NH2 at 250 Torr of NH3 and 505 Torr of KH3. With (8) E. J. Henley and D. Richman, A n a l . Chem., 28, 1580 (1956) (9) M. C. Sauer, Argonne National Laboratory Report, 1966, No. 7327. (10) J. W. Boag, Trans. Faraday SOC., 64, 677 (1968). (11) A. B. Callear and W. J. Tyerman, ibid., 62, 371 (1966). (12) P. Fowles, NI. deSorgo, A. J. Yarwood, 0. P. Stowersy, and H. E. Gunning, J. A m e r . Chem. Soc., 89, 1352 (1967). (13) M. C. Sauer, Argonne National Laboratory Report, 1966, No. 7113.
T h e Journal of Physical Chemistry, Vol. 76, N o . 14, 1971
S. GORDON, W. MULAC,AND P. NANGIA
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0.6 0.5 0.4.
I
Figure 9. Oscilloscope trace of NHz absorption a t 597.6 nm produced in 1520 Torr of NHI. Abscissa 2 psecldiv; ordinate 0.2 V/div.
0.5 n 70
0.2
60
0 .I 50
0.6 n
40
0.3 c3
L t ,L
Slope = 0.50
0
30
0
20
0 1 10 40 5060 80 100
200 300
cm Figure 7. Lambert-Beer law plots: NH2 in 500 Torr of NH in 500 Torr of NHa; OH in 8.0 Torr of HzO 760 Torr of argon.
+
(0)
O -L o o
5
10
I5
20
t (paac)
“3;
Figure 10. Decay of NH2 absorption a t 597.6 nm at different pressures of NHa: 0,250 Torr; 0, 505 Torr; 0,760 Torr; 0 , 1000 Torr; A, 1250 Torr; 1520 Torr.
+,
Figure 8. Oscilloscope trace of NHz growth. Abscissa, 50 nsec/div; ordinate, 0.2 V/div NH, pressure 762 Torr. Io
0
increasing ammonia pressure the plots show a curvature which increases with ammonia pressure. The solid curves in Figure 10 represent computer calculated decay curves which were fitted to the experimental points using a program written by Schmidt14 (see Discussion). The data in Figure 11 show the difference in decay of D-1.3of NHZproduced in 505 Torr of NHa and in a mixture containing 505 Torr of NHa and 50 Torr of SFa. Here again the solid lines represent computer calculated The Journal of Physical Chemistry, Vol. 76, No. 1.4?1971
PO
5U
4U
t (psec)
Figure 11. Effect of SF6 on NH2 absorption at 597.6 nm; A, 500 Torr of NIT3 50 Torr of SFe; B, 500 Torr of NHs.
+
curves and the points represent the experimental data (see Discussion). (14) K. H. Schmidt, Argonne National Laboratory Report, 1966,
No. 7693.
PULSERADIOLYSIS OF AMMONIA GAS
209 1
Lt
(A)
0
(0)
O L -oo
1 10
20
30
t (psec)
Figure 12. Effect of CaHe on NHBabsorption a t 597.6 n m 25 Torr of i n 500 Torr of “3; (a) 10 Torr of C3Hs; (0) C8He; (A) 48 Torr of C ~ H B(-) ; pure “3.
Figure 12 represents the decay of the NHz in a mixture of 505 Torr of NH3 with three different pressures of propylene. The solid curves again represent the computer calculated curves and the points represent the experimental data (see Discussion).
Discussion The kinetics of the decay of the NHz radical illustrated in Figure 10 can be understood in terms of the following three reactions. NHz
+ NHz -+ NHz
NzH4(or Nz
+ 2Hz)
+ H -% NH3
(1)
(2)
(3) The rate of disappearance of the PITHZwill be governed by the two following rate expressions
Initially [NH2Io = [HI, and ka
