B. Sivaraman et al. / Chemical Physics Letters 460 (2008) 108–111
111
1702.4 cmꢀ1 was assigned to N2O5, the heaviest molecule that
was produced in the N2O ice. This molecule may be formed by
O3 reacting with NO2 molecules [21]:
4. Discussion
Electron impact dissociation of N2O may, as in the case of pho-
todissociation, be expected to produce molecular nitrogen and
atomic oxygen:
2NO2 þ O3 ! N2O5 þ O2:
ð14Þ
N2O þ eꢀ ! N2 þ Oð P=1DÞ:
ð7Þ
3
5. Conclusion
Since it requires only 1.67 eV for dissociation of N–O bond in N2O all
the incident electrons and most of the secondary electrons induced
by ionization will have enough energy to dissociate any N2O mole-
cules with which they interact. Piper and Rawlins [17] have shown
that, although a spin forbidden process, electron impact dissocia-
tion of N2O is dominated by the production of oxygen in its ground
state (3P) rather than the excited O(1D). Ozone production, observed
as a characteristic IR feature at 1039 cmꢀ1, can then form via a two
step process, generating molecular oxygen in the first step and
addition of another O(3P) producing O3 in the second step (reactions
(3) and (4)). Fig. 3 shows the growth of ozone during irradiation;
ozone production is observed as soon as electron irradiation begins.
In an ice matrix, kept at 25 K, O3 production can also be ex-
plained by the formation of the complex (N2Oꢃ ꢃ ꢃO2). Once molecu-
lar oxygen is generated such a complex is easily stabilized within
the ice. The interaction of such a complex with another free oxygen
atom (N2Oꢃ ꢃ ꢃOꢃ ꢃ ꢃO2) would then lead to a localised reaction within
the matrix cage resulting in (N2Oꢃ ꢃ ꢃO3):
Electron irradiation of a 25 K sample of N2O ice has been shown
to generate a rich chemistry with the synthesis of both ozone and a
large number of nitric oxides from N2O2 up to N2O5 being formed
from intermediate product such as NO, NO2, O2. Formation of N2O5
during irradiation revealed the reaction of O3 molecules with NO2.
Most of the reactions are common to those observed in the Earth’s
atmosphere where they are initiated by photo-dissociation of ni-
trous oxide. Although N2O has to date only been detected in inter-
stellar medium it is highly likely that it is formed in several of the
solar system’s icy bodies. The present experiments would then
suggest that irradiation of such N2O containing ices by the solar
wind, sunlight and/or magnetospheric ion bombardment will
quickly lead to abiotic ozone formation as well as a large number
of nitric oxides which may subsequently react with hydrocarbon
species and water to form even more complex species. Therefore,
future experiments will explore the irradiation of N2O/H2O/CO2/
hydrocarbon ice mixtures by various sources of irradiation, also
using isotopic composition to deduce the kinetics behind the for-
mation of new molecules observed in this preliminary experiment.
3
N2O ꢃ ꢃ ꢃ O2 þ Oð PÞ ! N2O þ O3:
ð8Þ
The possibility of such reactions occurring within N2O molecular
cages is supported by spectroscopic identification of complex
(N2O. . .O2) [18,19].
Acknowledgements
The formation of O(1D) atoms not only provides an alternative
source of O(3P) atoms but may lead to the formation of the nitric
oxide dimer [20] known as dinitrogen dioxide (N2O2):
BS, SJ and NJM would like to thank the Leverhulme trust for
financial support of this work. SP acknowledges EPSRC for financial
support. BS acknowledges the Open University for provision of a
Ph.D. Studentship.
1
N2O þ Oð DÞ ! ðNOÞ2:
ð9Þ
NO dimers were clearly identified both in the cis and trans isomeric
forms (Fig. 2b, Table 2) in our spectrum.
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NO2 þ NO ! N2O3;
2NO þ O2 ! 2NO2;
2NO2 ! N2O4:
ð11Þ
ð12Þ
ð13Þ
NO2 dimers produced via reaction Eq. (12) or due to the close
proximity of two NO2 molecules produced via reaction Eq. (10)
may in turn dimerize to form N2O4 (13). A very weak band at
Sivaraman
