Jan., 1961
DECOMPOSITION OF SOLIDH4NzBY CHARGED PARTICLE BOMBARDMENT
53
general support .to the treatment and estimates k that different values of IC are appropriate to different only for the case of spherical particles. It may be types of particle.
THE DECOMPOSITION OF SOLID H4Nz INDUCED BY CHARGED PARTICLE BOMBARDMENT BY HAROLD A. PAPAZIAN Cmvair Scientific Research Laboratory, San Diego, California Rec&ed Mow 4, 1080
The decomposition of solid hydrazine induced by ion and electron bombardment has been studied. The decomposition waa found to proceed throu h several steps. The stepwise evolution of NI, H3 and NH, during warmu of the bombarded solid waa meaaured. The %sorption spectrum of the bombarded solid showed the absence of .2" $he results indicate the formation of nitrogen compounds such as triazene which are stabilized at low temperatures and which decompose during warmup of the solid.
Introduction I n a recent study' of the photolysis of HNs we presented evidenlce for the existence of the inorganic nitrogen chain compounds. The photolysis of solid HN, was shown to proceed through the formation and subsequent decomposition of triazene, tetrazene and perhaps even longer chained compounds. I n an attempt to find evidence for these compounds in another system we have studied the decomposition OF solid H4N, induced by electron and ion bombardment. The decomposition of the irradiated solid H4N2 was found to be exceedingly complicated, proceeding through several steps. Although it is not possible to state with certainty, the evidence does indicate the existence of the compounds found in HN3 and also other kinds of nitrogen compounds which are stabilized a t low temperatures.
Experimental Two methods of' purifying solid hydrazine were used. Anh drous hydrazine (95+%) waa vacuum distilled from KOA, onto a cold finger containing liquid Nt. The solid waa then allowed to warm up by removing the liquid Ns. During this warmyp, lower boiling impurities were pumped away. When the whte deposit started to disappear from the preparation cold finger, li uid NI was reintroduced; a fraction of the remaining H& waa then sublimed over to the reaction cold finger. The second method of purification waa the generation of HlNz from hydrazine bisulfate. There waa no difference in the results. The hydrazine waa deposited aa a transparent glaas on the reaction cold finger by careful sublimation of the hydrazine from the preparation cold finger. Approximately 2 mg. of hydrazine was deposited on 2 cm.* of surface yielding solid films on the order of lo-* 111111. thickness. After the deposition on the reaction cold finger, and with the apparatus under continuous umping, the solid waa subjected to ion and electron bomtardment by discharging a laboratory Tesla coil against the outer wall of the cold finger for various lengths of time. It has been shown* that this procedure subjects the solid to bombardment with 20 Kev. electrons and ions of undetermined energy. After the bombardment the liquid N1 waa removed from the cold finger, allowing the solid to warm up. During this warmup simultaneous mass 8 ectrometer and temperature measurements were made. &e temperature was memured by a thermocouple leoldered on to the glaas cold finger a t the depoaition surface with a s ot of glaas-to-metal "Cerroseal35" polder. A CEG modefB20 m u spectrometer was used to analyze the gases evolved from the solid during the warmup. (1) H. A. Pspssian, 1.Chem. Phys., 32, 456 (1960). (2) H. A. Papadan, ibid, 29, 448 (1958).
By adjusting the pumping speed of the system the presU pulses ~ that occurred durin warmup could be discriminated into definite peaks. %he umping speed waa controlled by means of a capillary placedietween the reaction cold finger and the stopcock leading to the vacuum pump. The inlet to the m a s spectrometer waa placed between the capillary and the reaction cold finger. A universal model Perkin-Elmer 112U spectrophotometer was ueed to study the spectrum of the solid between 3300 and 6300 A. The sample was depoeited on a specially constructed cold finger which consisted of a small flat plate whose edges were in contact with a surrounding volume of liquid Nz; the nitrogen was thus excluded from the light path. After depoeition of the hydrazine the absorption was measured. The sample then was bombarded and the difference in abporption caused by the bombardment was measured. R
Results The final products found after warmup of the bombarded solid hydrazine were H,, N2 and NH3. Figures 1, 2 and 3 show the stepwise evolution of these gases during the warmup of the solid, for three different bombardment times. I n these figures the ordinates are proportional to the quantity of gas evolved. Some comparison can be made between the Nz and Hz since these are pumped out of the system a t roughly the same rate but no comparison can be made with NH, since it was found to be pumped much more slowly. In Fig. 1 we also show the temperature vs. time warmup curve of the cold finger during the course of gas evolution. A warmup blank also was run with unbombarded hydrazine on the cold finger. The two curves were experimentally identical. It is apparent that the pressure pulses are not rate changes from sudden temperature rises of the cold finger, but that the gas evolution pulses result from stepwise reactions beginning a t different temperatures. Figure 1 shows the evolution of HZduring warmup from a solid which was bombarded for two seconds. Considerable structure is quite evident. We did not investigate Nz nor NH3 evolution for two second bombardment times. Figure 2 shows the evolution of HI, N2and "3, each from separate samples. Different samples had to be used because the mass spectrometer could analyze continuously only one gas a t a time. This, however, was no problem because of the excellent reproducibility in the gas evolution from the solid. It should be noted that even though the bombarding
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HAROLD ,4.PAPAZIAN
Vol. 65
evolution appears a t -95', and the reaction a t - 80" has disappeared. The Nz evolution appears to be more complex than the Hz. I n the region between -60 and -40" there are four pulses of Nz compared to only two for Hz. Between -170 and -70" the N2 is again more complex. It should be noted that the NH3 peaks do not appear at the same temperatures as the N2and Hz peaks. Figure 3 shows the evolution of Hz, Nz and NH3, each from separate samples. Each sample was bombarded for a total of 30 seconds in three 10second bursts with 10-second intervals between bursts. Here we find that for a threefold increase in bombarding time we get an increase in gas evolution by a factor of about eight. It was observed that NH3 would not sublime off of our cold finger below -120" so that it cannot be known with certainty whether or not NH3 is formed in the reactions occurring below this temperature. Hydrazine began subliming above -20". In order to gain some insight into the stepwise reactions, a different kind of experiment was carried out. The reactions occurring during warmup were stopped by reintroducing liquid Nz into the reaction cold finger. If the reaction was stopped g 1000 a at the end of any step, then the next step would not t appear until the cold finger was warmed to the K 4 temperature of that subsequent step. At the top of Fig. 2 and 3 are given the N2/H2 ratios formed over the indicated temperature ranges. These ratios were obtained by allowing a reaction step to proI ceed, introducing liquid Nz into the cold finger z when the slope of the gas evolution curve indicated 0 the end of the reaction, and then measuring the relative concentrations of Hz and Kz. VC'ith our experimental technique we could not determine the KH3,/iYz ratios; the 3" condensed when TEMPERATURE OC. liquid N z was introduced t o stop the reaction. Fig. 2.--Gas evolution from solid H4Nzwhich was bombarded From the N2/H2 ratios it is obvious that the chemfor 10 seconds. istry in the different regions studied varies considerably. These ratios cannot be used for the stoichiometry (nor can they be compared closely to the gas evolution traces from the mass spectrometer) because the apparatus is continually pumping up to the time of addition of liquid N2 and closing of stopcock. t The over-all Nz/H2 and NH3/N2 ratios were m determined by bombarding the solid, closing the stopcock to the pump and letting the solid warm up completely. For the 30-second bombardment a series of five experiments were done: the N2/Hz p 100 ratio varied from 1.7 to 2.3 with an average of 2.0, z and the "3/N2 ratio varied from 2.9 to 3.6 with , an average of 3.3. Two experiments were done a t I ''!'\ 10-second bombardment times; the N2/Hz ratios were 2.7 and 3.0 with an average of 2.9 and the -160 -140 -120 -100 - 8 0 -60 -40 - 2 0 NH3/Nz ratios were 4.2 and 4.4 with an average of TEMPERATURE OC. 4.3. For the 10-second experiments we measured Fig. 3.-Gas evolution from solid H ~ Nwhich Z was bombarded the total conversion and found approximately 23 for 30 seconds. and 22% decomposition of hydrazine. We did not time was increased only 5 times, Le., t,o 10 seconds, measure the total decomposition for the 30-second the HS evolution increases by a factor of 100. The bombardment but would expect it to be even larger. We also studied the absorption spectrum of a structure of the HZ evolution between -60 and -40" is still clearly present. However, at the bombarded sample of solid H4NZ. The solid line lower temperature, ie., below -70" a new strong in Fig, 4 shows the difference in absorption between (L
$t
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Jan., 1961
DECOMPOSITION OF SOLIDH 4 S 2BY CTIARGED PARTICLE BOMBARDMENT
a bombarded and an unbombarded sample. The spectrum labeled "slight warmup" was obtained by removing liquid Kzfrom the absorption cell until gas evolution j ust began, then quickly stopping reaction by reintroducing liquid Nz into the cell. The temperature to which the solid was warmed was not measured but from the gas evolution studies we estimate it to be about - 155". At no time did the bombarded solid become permanently colored, even for the longest bombarding times. It did appear to turn slightly blue for an instant but this color could not be stabilized. In an attempt to stabilize the color the solid was bombarded using liquid He as the coolant but no color could be seen. This complete absence of color may have been the inhibition of a reaction by the low temperature. Discussion The simultaneous evolution of several substances indicates the complexity in the decomposition process. The Nz/Hz ratios obtained over the various regions chow clearly that different reactions occur during the warmup of the solid. That some chain mechanisia is involved is indicated by the comparison of bombarding times with the amount of gas evolved. One speculation which may not be out of order concerns the region in Fig. 3 where the N2/H2 > 1000, with NH3 also being evolved. This may be the decomposition of triazene, HZ-N--N=N-H+ NH3 Kz. In Fig. 4 we find an absorption band a t 3600 A. which in ref. 1 we assigned to triazene. In the earlier paper we suggested that the reaction of triazene with HN3 takes place a t -90". It may be that in the absence of a suitable reactant triazene is stable to the higher temperature. Foner and €€udson3 trapped the products of hydrazine from an electrodeless discharge. From their cold trap they found the evolution of small quantities of compounds with masses 45 and 60 which they atti-ibuted to triazene and tetrazene, respectively. Our inability to find these compounds in the mass spectrometer probably results from their complete decomposition either on the glass walls or in the gas phase. The long distance through glass tubing, a t room temperature, that they would have
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(3) S. N. Foner and R. L. Hudson, J . Chem. Phys., 29,442 (1958).
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BOMBARDED SOLID SLIGHT WARMUP
3400
3800
4200 4600 5 0 0 0 ANGSTROM
5400
5800
6200
UNITS,
Fig. 4.-Absorption spectrum of a bombarded sample of H4N2 before and after some reaction.
had to travel in our apparatus makes this conclusion reasonable. NH2 was not stabilized under the conditions of this study. Robinson and McCarty4 have studied the absorption spectrum of NH2 radicals trapped a t low temperatures. Among others, they find two strong absorptions a t 5150 and 5684 A. Figure 4 shows that no NHz was stabilized before or after some reaction. Rice has tabulateds the reaction temperatures of free radicals stabilized in solids a t low temperatures which he has studied in the laboratory and lists these between - 125 and - 195". Atomic species have only been stabilized a t liquid He temperatures.6 Thus, although it may not be ruled out with certainty, it is unlikely that any of the reactions occurring above -120" are reactions of stabilized free radicals. We must, therefore, assume that the reactions occurring a t the higher temperatures are reactions of product molecules with the H4N2matrix and/or the decomposition of these product molecules. Acknowledgment.-The author wishes to thank Mr. John Pearl for his help in carrying out these experiments. (4) G. W.Robinson and M. McCarty, ibkd., SO, 999 (1959). (5) F. 0.Rice, ibid., 24, 1259 (1956). (6) H. P. Broida and J. R. Pellam, Phys. Reu., 96,845 (1954).
