THE JOURNAL OF CHEMICAL PHYSICS
VOLUME 34, NUMBER 5
MAY,
1961
Photolysis of Nitrous Oxide. I. 1236 A
JOHN
P.
DOERING AND BRUCE
H.
MAHAN
Department of Chemistry, University of California, Berkeley, California
(Received November 3, 1960)
The photolysis of nitrous oxide containing an N15 atom in the external position has been investigated
using 1236-A light. The products include N 229, N150, N 140, and small amounts of N 230. The appearance of
N 230 shows that there are two primary processes occurring at this wavelength, one of which is dissociation
to a nitrogen atom and a nitric oxide molecule, the other dissociation to an oxygen atom and a nitrogen
molecule. The latter process is approximately 10 times as important as the former. The quantum yield of
nitrogen is 1.34±0.04.
HE photolysis of nitrous oxide by light of 1236-A
wavelength has been investigated by Zelikoff and
Aschenbrand. 1 These workers have suggested that the
decomposition proceeds by two primary photochemical
processes, one involving dissociation to an oxygen atom
and an electronically excited nitrogen molecule, the
other producing a nitrogen atom and an electronically
excited nitric oxide molecule. Their evidence for
participation of both primary processes was the qualitative behavior of product ratios as a function of pressure.
Because of the intrinsic insensitivity of this method to
the actual mechanism of the reaction, Zelikoff and
Aschenbrand were unable to estimate the relative
importance of the two primary processes. The very
existence of a nitrogen atom primary process has been
questioned by Kistiakowsky and Volpi2 who point out
that because of the great speed of the reaction between
nitrogen atoms and nitric oxide, it is unlikely that any
nitric oxide would be found in the products of the
photolysis of nitrous oxide even if any nitrogen atoms
were actually produced. The possibility of the existence
of two primary processes in such a simple molecule is
intrinsically interesting; furthermore, the photolysis of
nitrous oxide by the hydrogen atom Lyman a line
might serve as a source of nitrogen atoms and nitric
oxide in the upper atmosphere. The purpose of this
work was to ascertain the existence and relative importance of the two primary processes by photo lysing
nitrous oxide which had an isotopically labeled external
nitrogen atom.
T
EXPERIMENTAL
served as a relative actinometer. This arrangement
was necessary since over the period of many experiments the lamp intensity might change by as much as
a factor of three.
The products of the photolysis were separated by
passing them through a trap at - 210°C. This served to
retain all the NO and N 20, and allowed the isotopic
composition of the nitrogen to be ascertained without
their interference. Naturally this treatment resulted in
a reaction between NO and O2 and consequently the
absolute amounts of these products are not reliable.
The relative amounts of N 150 and N 140 should have
some qualitative significance, however, so nitric oxide
was separated from nitrous oxide at -183°C, and also
analysed by mass spectrometry. In the runs in which
carbon monoxide was added, the noncondensible
fraction of the products was passed over hot copper
oxide until all the CO had been removed as CO 2, This
precaution avoided any interference from the heavier
isotopic forms of CO in the N 229 and N 2 30 analyses. In
a few runs it was desirable to have nearly complete
recovery of the nitric oxide. In these cases the trap at
- 210°C was not used, and nitrous oxide was separated
from the products at -183°C.
The isotopically labeled nitrous oxide was prepared
from commercially available 95% enriched N15H 4N140o
by gentle heating. The nitrous oxide was then purified
by bulb-to-bulb distillation. The final product contained 5% N14N1 40 and 0.5% N15N160. All other gases
were obtained commercially, and mass spectrometric
analysis showed them to be of satisfactory purity.
RESULTS AND DISCUSSION
A microwave discharge through krypton together
with a lithium fluoride window provided monochromatic
Table I shows that small, but significant, amounts of
radiation of 1236-A wavelength. The details of this N15N15 are formed in every photolysis of pure N15N140.
lamp have been published previously.3 The reaction This fact in itself strongly suggests the existence of a
vessel was a Pyrex cylinder of 25 mm diameter divided primary photochemical process which produces nitrogen
into two separate chambers by a Pyrex plate. One of atoms. The formation of doubly labeled nitrogen
the chambers was used to contain the reaction mixture molecules would surely seem to involve nitrogen atoms
while the other was filled with either N 20 or CO 2 and either reacting with themselves or with some molecule
1 M. Zelikoff and L. Aschenbrand, J. Chem. Phys. 27, 123 containing a labeled nitrogen atom. No reaction between
(1957) .
oxygen atoms and nitrous or nitric oxide is known in
2 G. B. Kistiakowsky and G. G. Volpi, J. Chern. Phys. 27, 1141
which
nitrogen atoms are produced. Furthermore, the
(1957) .
photolyses were carried out to a limited percentage
3 B. H. Mahan, J. Chern. Phys. 33, 959 (1960).
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