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Spectroscopy of cosmic dust analogs: study of the interaction with polycyclic aromatic hydrocarbons

Final Report Summary - PAHCNP (Spectroscopy of cosmic dust analogs: study of the interaction with polycyclic aromatic hydrocarbons)

Motivation – project objectives:
The objective of this project is the study of the interrelation between polycyclic aromatic hydrocarbons (PAHs) and other constituents of the interstellar medium, such as cosmic dust particles, using laboratory analogs of the materials present in space. The complexity of the PAHs mixture present in space has precluded so far the identification of any of the PAH species, but their presence has been established accounting for the infrared emission features that dominate the spectra of most galactic and extragalactic sources [1]. As astrophysical PAH analogs, we have used PAH mixtures obtained from laser pyrolysis of hydrocarbons under astrophysically relevant conditions. Comparing the yields of PAHs using the same precursor gases but different laser powers, it has been concluded that the temperature in the condensation zone determines the proportion between PAHs and soot of the final product. The lower the temperature in the condensation zone, the higher is the content of PAHs [2]. According to that result, we have used for the present investigation an adequate laser power to assure a 100% PAHs yield. For cosmic dust analogs we have focused our investigation in carbonaceous materials, with particular attention to C60, the most stable allotropic form of the carbon fullerenes, which has recently been identified in various space environments [3, 4]. In contrast to PAH mixtures, C60 is an individual species and therefore its spectral emission features can be used for probing the environmental conditions of those regions of the ISM. However, if we want to use the ratio of the emission features as a probe, it is necessary to know first the intrinsic strength of those bands, which can be modified by the possible interaction with other species present in the same space regions, in particular with PAHs since both have been detected in a number of astrophysical environments, such as planetary and proto-planetary nebulae, young stellar objects, post-AGB stars, and Herbig Ae/Be objects [4].

Experimental setup construction (see Figure 1 in the attachment):
The new setup was developed in order to produce adequate amounts of pyrolysis material for the IR spectroscopy without involving the use of chemicals. The precursor gas and the carrier gas are admitted to the mixing chamber. Inside the first chamber, it is possible to add another component to the mixture to be pyrolyzed by the action of a pulsed Nd:YAG laser, which is focused into the crucible containing the material. The harmonic of the laser and the power are chosen for every particular experiment to produce either gas or solid particles, which are incorporated into the carrier gas. The gas or solid-gas mixture is admitted to the pyrolysis chamber, into which a CO2 laser is focused. A high-capacity pumping system maintains a differential pressure (750 : 0.1 mbar for most of the experiments) between those chambers and the deposition chamber, which are connected by a nozzle. The products of the pyrolysis undergo supersonic expansion, condense from the gas phase, and are deposited on the substrate for spectroscopic analysis. The substrate holder is attached to a cold trap for low-temperature deposition down to 150 K and a heater, which allows regulating the temperature up to 700 K. Finally, for in-situ UV irradiation, the sample holder can be rotated to face a hydrogen lamp attached to a MgF2 window at the deposition chamber.

Overview of results, conclusions, and impact of the project:
The starting point of the present project has been the work on laser pyrolysis and carbon dust analogs carried out in the Laboratory Astrophysics Group in Jena during the last ten years (see for instance reference [2]). In the present project, we have investigated how the presence of the C60 changes the pyrolysis process and/or interacts with the pyrolysis products.
1. An initial approach to the subject was undertaken by incorporating C60 into the pyrolysis precursor gas (ethylene). The C60 was incorporated into the carrier gas in three different ways: in form of micro-sized particles, nanoparticles, and in the gas phase. The first happens when the flux from the Nd:YAG laser on the C60 is enough to ablate the material rather than to evaporate it. Using less power, we evaporate the material, which is immediately quenched in the carrier gas in the form of nanoparticles before entering the pyrolysis chamber. With the same power but performing the evaporation in the pyrolysis chamber, we obtained C60 in the gas phase in the region where the pyrolysis of the ethylene occurs. Unfortunately, the complexity of the resultant PAH mixture, either with or without C60, precluded the analysis of the small spectral changes expected from the reaction between the C60 and the pyrolysis radicals present in the zone where the pyrolysis occurred. However, in the light of the results of other experiments (explained below), the experience gained during these experiments can be applied to the investigation of simpler mixtures of PAHs.
2. A different approach to the study of the chemical interaction between C60 and complex mixtures resulting from the laser pyrolysis is the study of individual cases of small PAHs that we have chosen from the known products of the pyrolysis [2]. In particular, we have performed experiments to check the viability of the cycloaddition reaction between PAHs and C60 (see Figure 2 in the attachment) [5]. We wanted to determine under what conditions the formation of PAH-C60 addition products (adducts) occurred. The experiments were performed in two different ways: First, a series of pellets were prepared with a mixture (1:1 molar ratio) of small PAHs and C60. These pellets were irradiated with UV light in an inert atmosphere using a hydrogen lamp. After that, the irradiated material was incorporated into a KBr pellet for infrared spectroscopy. The second series of experiments was performed by evaporating the components onto KBr substrates, using our setup with only argon as carrier gas and in-situ UV irradiation. In the case of anthracene, we found that the UV irradiation can trigger the reaction in the solid phase as it was reported to happen in solution [6] and by heating [7]. We expect to publish the results of these experiments in the following months. Apart of the effect of the UV radiation triggering the reaction, the result is also relevant for astrophysics because the capability of small PAHs to survive the harsh radiation conditions in a number of astrophysical environments might be enhanced by the formation of adducts.
3. Using the hydrogen lamp for producing UV radiation, is a common method in our research field. Hydrogen lamps are inexpensive, reliable, and easy to use. However, the measurement of the flux that actually reaches the sample is not a trivial problem. A careful look into the scientific literature shows that a number of researchers use methods, which are not applicable in every setup. For instance, the most popular method consists in the measurement of the ozone production from a surface covered with solid oxygen at low temperature [8]. Unfortunately, this method can be used only in high-vacuum setups, which include an adequate cold finger, oxygen inlet, and in-situ infrared spectrometer. Considering the importance of using trustable values for the UV dose in our experiments, in a joint effort between our group in Jena and members of the Sackler Laboratory for Astrophysics at Leiden (Netherlands), we have developed a straightforward and accurate method for measuring the flux at the position of the sample, which is applicable to almost every experimental vacuum setup. We expect a high impact of the paper describing the method, which will be published shortly. Our method is based on the photoelectric effect, employing a photo-cathode coated with gold that can be placed into the holder to be used for the irradiation experiments. It works at room temperature, and the measurement of the photocurrent is performed in such a way that any undesired effect from the chamber geometry is avoided. Besides, we have checked that it can be used at pressures up to 10-3 mbar. For these reasons, our method can be applied to almost any vacuum system.

References:
1. Tielens, A.G.G.M. ARA&A, 2008, 289-337.
2. Jäger, C., et al., Carbon, 2007, 45, 2981-2994.
3. Cami, J., et al., Science, 2010, 329, 1180-1182.
4. Roberts, K.R.G. K.T. Smith, and P.J. Sarre, MNRAS, 2012, 421, 3277-3285.
5. Duarte-Ruiz, A., K. Wurst, and B. Krautler, Helv. Chim. Acta, 2001, 84, 2167-2177.
6. Mikami, K., et al., Tetrahedron Lett., 1998, 39, 3733-3736.
7. Martin, N.M. S.M. Luzan, and A.V. Talyzin, Chem. Phys., 2010, 368, 49-57.
8. Cottin, H., M.H. Moore, and Y. Benilan, ApJ., 2003, 590, 874-881.
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