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J. Phys. Chem. 1984, 88, 3282-3287
the true A / T pair. The obvious thought that follows from this is that, if barbiturates succeed in penetrating to molecular distance from the base pairs, an "opening" of some of the A / T H bonds might occur. This, in turn, might initiate cell division which then might be conducive to cancer. Similar ideas have been put forward by Hobza and SandorfyZ1 concerning the interaction of the H bonds in the A / T base pair and a positive ion like Na+. The question to ask is, in both cases, if the perturber can penetrate close enough to the base pair to exert an effective perturbation. Now, the nucleotide base pair is protected by a water layer22which is expected to reduce the probability for this to happen to a very small value. This would apply to Na+ as well as to barbiturates. If, however, a carcinogen like for example an aromatic hydrocarbon perturbs the surrounding water structure, this might facilitate the penetration of the ion or of the barbiturate; then, dissociation of the H bonds in the base pair might occur with serious consequences for the stability of the DNA helix. (21) P.Hobza and C. Sandorfy, Proc. Natl. Acad. Sei. U.S.A.,80, 2859 ( 1983).
(22) E. Clementi and G. Corongiu, Biopolymers, 21, 763 (1982).
Another point of interest is the possibility of trimer formation as a part of the mechanism whereby a barbiturate replaces thymine in the A/T pair. As suggested above, this can lead to an increase of the proportion of the Hoogsteen type pairs which could impair the normal functioning of DNA and, eventually, lead to cancer. The perturbation might affect the mechanism of recognition of given sites in DNA by enzymes and proteins, in genera1.23,24 It is believed that the models used in this study are sufficiently representative to make the above ideas reasonable. The results obtained by IR, near-IR, and proton N M R techniques are concordant in this respect. Acknowledgment. Financial assistance from the Natural Sciences and Engineering Research Council of Canada and from the Ministhe de 1'Education du Quabec is gratefully acknowledged. We thank Robert Mayer for help in measuring the low-temperature N M R spectra. Registry No. Phenobarbital, 50-06-6; secobarbital, 76-73-3; allobarbital, 52-43-7;pentobarbital, 76-74-4; barbital, 57-44-3. (23) N. C. Seeman, J. M. Rosenberg, and A. Rich, Proc. Natl. Acad. Sei. U.S.A.,73, 804 (1976). (24) C . HBlEne, FEBS Lett., 74, 10 (1977).
Mechanism of the Direct Current Plasma Discharge Decomposition of Disilane P. A. Longeway,* H. A. Weakliem, and R. D. Estes RCA Laboratories, Princeton, New Jersey 08540 (Received: October I I , 1983)
We have studied the static pressure disilane dc discharge using mass spectrometric techniques both in the absence and presence of nitric oxide (NO), a known free-radical scavenger. The observed products of the discharge are H2, SiH4, Si3HB,Si4HL0, and an amorphous hydrogenated silicon film, a-Si:H. The disilane depletion rate and product formation rates were seen to be linear with the discharge current and exhibited a weak pressure dependence. The introduction of NO to the discharge reduced the yields of the products Si3H8and Si4H10to 70% and 21%, respectively, of their values in the absence of NO. The yield of SiH4was unaffected by NO introduction, and the formation of a-Si:H film was totally suppressed. Additionally, the steady-state yield of Si4HI0was reduced by increasing the temperature of surfaces exposed to the discharge, suggesting a surface reaction for the generation of a fraction of this product. The results of these experiments indicate that the relative yields of the reactive fragments SiH2,SiH3, SiH3SiH, and Si2H5are 30%, 34%, 2%, and 34%, respectively. We also conclude that the film precursors from the disilane dc discharge are SiH3and SizH5and that the difference in film properties between silane disilane discharge prepared films can, in part, be explained by noting that the film precursor in the silane case is primarily SiH3, with no significant contributions from Si2H5.
Introduction Disilane has been used to deposit hydrogenated amorphous silicon films, a-Si:H, having desirable photovoltaic properties by both chemical vapor deposition' (CVD) and glow discharge2 (GD) methods. The decomposition of disilane by the CVD method proceeds via a thermal process which may take place in the gas phase or at the surface. The decomposition of disilane in a glow discharge proceeds via electron-impact dissociation, a much more energetic process than the thermal case. Nevertheless, by choosing suitable operating conditions, we may prepare films having comparable properties by either method. One advantage in using disilane rather than silane in the G D method is that the former has deposition rates that are 2-4 times greater, for a specific discharge current, although the hydrogen content of the film is approximately twice that of a film deposited from the silane GD for the same substrate temperatures3
We have recently studied the decomposition kinetics of the silane glow discharge under both static-pressure4 and flowing-gas5 conditions and concluded from the results that the silyl radical, SiH,, is the primary film-forming intermediate. Furthermore, a surface temperature dependence was observed for the yield of disilane generated from the recombination of two SiH3 units, indicating that a large fraction of the higher silanes in the gas phase may be formed by surface reaction^.^ With these results in mind, we undertook a similar study of the disilane dc discharge. The results of N O quenching experiments indicate that the monoradicals SiH3 and SizHSare primarily responsible for film growth, being generated by the discharge in nearly equal amounts. While the calculated yield of SiH2 from the disilane discharge (30%) is higher than that calculated for the silane discharge4 (
