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Surface Chemistry Processes in Interstellar Chemistry

M. Rajappan, C. Yuan and J. T. Yates Jr.

Enhanced Radical Combination Effects During Lyman-α Photodecomposition of Adsorbed Molecules on Silicate Dust Grains


As more complex chemical species are observed by radio astronomy in the interstellar medium, it is likely that the role of surfaces in enhancing radical combination reactions (by transporting exothermicity away from combining molecules) will become a dominant model to explain the origin of complex molecules including those of importance in life-forming processes. As a model for silicate dust grains in the interstellar medium, we have used high area amorphous SiO2 as a surface on which to carry out Lyman-α (10.2 eV) photodecomposition of adsorbed molecules. The N2O molecule is one of the approximately 160 molecular species identified at the present time in the interstellar medium and serves as a model for other more complex molecules of astrochemical importance. We compare a photochemical process in the N2O gas phase to the same process within the pores of a silica powder containing adsorbed N2O molecules, where surface effects will be highly amplified compared to those in the gas phase.


Photo-reactions of N2O by Lyman-α


Primary Photochemical Processes

Recombination & Secondary Processes

 

 

 

 

 

The N2O molecules are adsorbed by hydrogen bonding to surface Si-OH groups (Fig.1) .

 

Transmission IR spectroscopy measurements permit the observation of the consumption of adsorbed N2O and the production of various photoproducts. In Fig. 2  we show the integrated absorbance variation with Ly-α irradiation time of a characteristic N2O band and the product N2O4 band. In the experimental sequence shown, three separate N2O photo-depletion rates and N2O4 production rates are measured as labeled Photodissociation I, II and III.

 

Similar photodissociation measurements were carried out using 10 Torr N2O gas. It is observed that in comparison to N2O consumption, the relative rate of  formation of the products NO2 and N2O4 made by combination reactions is enhanced significantly on the SiO2 surface.


For our studies, the ratio of the rate of production of a product to the rate of consumption of N2O has been measured. Thus for the gas phase we may write that the ratio of the rate of production of a product to the rate of consumption of N2O reactant is

 

Rg will involve the ratio of integrated absorption coefficients, εN2Oproduct. Likewise a ratio Ra for the adsorbed species may be measured giving the ratio of the rate of production of a combination product in the adsorbed layer to the rate of N2O(a) consumption.


The ratio Ra/Rg = Sa is a measure of the enhancement factor or selectivity factor for the rate of a combination reaction on the SiO2 surface compared to that in the gas phase. Reactions between photo-generated radicals themselves or between radicals and parent N2O on the SiO2 surface exceed the relative rates observed in the gas phase by factors of up to ~ 20 (Fig. 3). As the complexity of the combination product increases, its relative production rate, compared to the gas phase, increases due to the involvement of multiple surface-combination elementary steps. It is proposed that the enhancement of  combination reactions on the SiO2 surface is due to the surface’s ability to absorb excess energy evolved during  the chemical-bond-forming events on the surface. This principle is probably significant on grain surfaces supporting photochemical processes of astrochemical interest, and indeed is expected. The cross section for adsorbed N2O photodecomposition on the porous SiO2 surface is about 7 × 10-20 cm2 and the quantum yield for the adsorbed molecule decomposition is about 0.005, compared to a quantum yield of 1.46 in the gas phase. This decrease in photon efficiency is attributed to absorption and scattering of Lyman-α radiation by the SiO2 particles.

 

Role of Indirect Photoexcitation on High Area SiO2 Surfaces (Future Goal)


Modeling of stellar atmospheres where photochemistry plays a role is generally carried out by employing the known cross sections for DIRECT photochemistry of gas phase molecules. Yet, laboratory studies for the last 30 years have shown that INDIRECT photoexcitation is of major significance for adsorbed molecules. Here, electron-hole production within the solid leads to charge transfer to adsorbed molecules, followed by chemical bond breaking in the adsorbed molecule.

•Starlight contains Lyman-α radiation

 

•Therefore indirect photochemistry is energetically possible, producing e-h pairs in SiO2 with electron tunneling to adsorbate molecule.

 

We are currently investigating the role of indirect photoexcitation on high area SiO2 surfaces as models of silicate grains. Lyman-α radiation is used to cause photoexcitation of adsorbed molecules at monolayer coverage as well as molecules in ice layers on the SiO2.


Experimental set-up to Study Gas Phase and Surface Photochemistry in the Laboratory