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Gas to particle conversion of silicon oxides

The average input of interplanetary dust into the Earth's Atmosphere is about 40 tonnes/day. Most meteoric material ablates above 80 km, releasing metal atoms such as Fe. These metals form oxides, which recombine with silicon oxides to form nanometer-sized particles known as meteoric smoke (MSP) 1. MSP are likely to provide nuclei for the formation of noctilucent clouds in the summer polar mesosphere 2 and may be an important source of condensation nuclei for sulphate particles in the lower stratosphere, and hence influence polar stratospheric cloud formation 3. In addition, MSP are alkaline, so they could explain the anomalous removal of H2SO4 in the upper stratosphere 4.

In order to quantify the rate of formation of MSP in the mesosphere, and their detailed composition (which will influence their ability to condense water-ice and react with acidic gases), knowledge of the speciation of the silicon oxides as a function of altitude is needed (see figure 1)

However, relatively few gas kinetic data exists for the silicon atom and oxides, and in particular no data has been reported so far for two key reactions of such chemistry:
SiO (X1Σ+) + O3 --> SiO2 + O2 ΔH = -345 kj mol-1 (1)
SiO2 + O(3P) --> SiO (X1Σ+) + O2 ΔH = -98 kj mol-1 (2)

The study of reactions (1) and (2) requires kinetic data for the following side reactions:

Si (3Pj) + O2 --> SiO (X1Σ+) + O ΔH = -303 kj mol-1 (3)
Si (3Pj) + O3 --> SiO (X1Σ+) + O2 ΔH = -692 kj mol-1 (4)

No data has been reported for reaction (4). Reaction (3) has been studied a number of times, but only one temperature study has been published so far 5, 6.


Figure 2: PLP-LIF apparatus


We are studying these reactions by the pulsed multiphoton dissociation at 193 nm of organo-silicon vapour (phenylsilane) in the presence of O2, O3or CO2, followed by time resolved laser-induced fluorescence spectroscopy of Si(3PJ) or SiO(X1Σ+) (figure 2). A resonant detection scheme is used for the detection of silicon atoms at the three different spin-orbit levels J = 0, 1, 2, exciting the transitions 3p23P0 -->4s 3P1 (251.43 nm), 3p23P1 --> 4s 3P2 (250.69 nm) and 3p23P2 --> 4s 3P2 (251.61 nm) respectively (see figure 3). SiO(X1Σ+) is detected by using a non-resonant scheme where the molecule is excited to the 3rd vibrational level of the upper A1π electronic state (221.93 nm) and fluorescence is collected from the 3'--> 8" transition at 282 nm. In order to study reaction (2), synchronised pulsed dissociation of O3 at 248 nm is being employed to generate in situ O(1D), which is rapidly quenched to O(3P) in an excess of N2.

These kinetic studies are underpinned by the application of quantum chemistry calculations, statistical rate theories (e.g. RRKM ) and long range capture theories.

References

1. Plane, J. M. C. Chem. Rev. 2003, 103, 4963-4984
2. Rapp, M.; Thomas, G. E. J. Atmos. Solar-Terr. Phys. 2006, 68, 715-744
3. Cziczo, D. J.; Thomson, D. S.; Murphy, D. M. Science 2001, 291, 1772-1775.
4. Mills, M. J.; Toon, O. B.; Vaida, V.; Hintze, P. E.; Kjaergaard, H. G.; Schofield, D. P.; Robinson, T.W. J. Geophys. Res. 2005, 110, D08201
5. LePicard, S.D.; Canosa, A.; Pineau des Forets, G.; Rebrion-Rowe, C.; Rowe, B. R.. A&A 2001, 372, 1064-1070
6 Husain, D.; Norris P.E. J.C.S. Faraday II, 1978, 74, 107-114


Firgure 1 : Proposed gas phase reaction mechanism of silicon oxides in the mesosphere.


Figure 3 : LIF signal from the Si 3p2 3P0 --> 4s 3P1 transition 251.43 nm, plotted in a logaritmic scale. The arrow shows the effect of increasing O3 concentration, attributted to the Si + O3 reaction.

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