James W. Stubbing , Tara L. Salter , Wendy A. Brown ...

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James W. Stubbing a. , Tara L. Salter a. , Wendy A. ... B. B. Dangi, D. S. N. Parker, R. I. Kaiser, A. Jamal and A. M. Mebel, Angew. Chem. Int. Ed. Engl., 2013, 52, ...
Reflection Absorption Ultraviolet/Visible Spectroscopy as a Tool for Experimental Surface Astrochemistry James W.

a Stubbing ,

Tara L.

a Salter ,

Wendy A.

a Brown ,

Skandar

b Taj

and Martin R. S.

b McCoustra

a. Division of Chemistry, School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QJ b. Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS

Dust grains in the interstellar medium (ISM) provide a surface on which processing of molecules occurs and can be used to explain the observed chemical richness in interstellar space. Using dust grain analogues and ultra-high vacuum (UHV) technology, these icy mantles and processes can be simulated on Earth in order to increase our fundamental knowledge of the ISM and explain astronomical observations. We present initial test data focused on the astronomically relevant molecules benzene and toluene from a new ultraviolet/visible (UV/vis) spectrometer assembly. Spectra are obtained as a function of wavelength (λ), reflection angle (θ) and ice thickness (d). Ultimately the new assembly will allow optical parameters of interstellar ice mimics to be determined, in turn allowing spectra of astronomically relevant ices to be simulated. This technique has been demonstrated previously in the infrared region.1 Equipment • UHV chamber coupled to a closed cycle helium refrigerator • Base p ≤ 2 ×10-10 mbar and T ≈ 24K • Highly oriented pyrolytic graphite (HOPG) surface as a dust grain analogue • Ocean Optics spectrometer and light source with fibre optic feedthroughs and collimating lenses • Variable reflection geometry achieved with a pantograph mechanism

Photographs showing the variable reflection geometry. Effective range is 31° - 64°.

Benzene and Toluene – Relevance and Rationale

Initial Test data

UV/vis spectra of 200 Lm (1 Lm = 10-6 mbar s) of benzene (left) and toluene (right) at two reflection angles.

• Benzene was detected in the ISM in 2001,2 and formation routes for toluene have been postulated3 • Benzene can be thought of as a PAH building block, and has been chosen as a PAH analogue in this work. Toluene offers a logical comparison to benzene due to slight differences in molecular size and polarity • Both molecules have been characterised in terms of their surface behaviour in our laboratory, see Dr Tara Salter’s talk (Tuesday afternoon) for more information

Flame Nebula Image credit: ESO/J. Emerson/VISTA

Looking forward – How to Determine Optical Parameters

50 Lm

𝑚 = 𝑛 + 𝑖𝑘

m Complex refractive index n Real part of m k Complex part of m

Experimental Data

Record spectra as a function of λ, θ and d

Simulated Spectrum

Model a spectrum using Fresnel equations and Kramers-Kronig relation

100 Lm

UV/vis spectra of increasing coverages of benzene (left) and toluene (right) at a fixed reflection angle of 31°.

Check agreement

iterate

200 Lm

Compare model and experimental data

Determined values of n, k and d

Summary and Future Aims • We present a new experiment allowing optical parameters of interstellar ice mimics to be determined • Test data show that coverage and reflection geometry affect spectra obtained • Using an iterative model, values of the real and complex parts (n and k) of the complex refractive index, m, can be found • Interferometry may be possible using much thicker ices 1. W. R. M. Rocha and S. Pilling, Spectrochim. Acta. A. Mol. Biomol. Spectrosc., 2014, 123, 436–46. 2. J. Cernicharo, A. M. Heras, A. G. G. M. Tielens, J. R. Pardo, F. Herpin, M. Guélin and L. B. F. M. Waters, Astrophys. J., 2001, 546, L123–L126. 3. B. B. Dangi, D. S. N. Parker, R. I. Kaiser, A. Jamal and A. M. Mebel, Angew. Chem. Int. Ed. Engl., 2013, 52, 7186–9. The authors would like to thank the Science and Technology Facilities Council (STFC) and the School of Life Sciences, University of Sussex for funding. Email: [email protected] Twitter: @StubbingScience

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