tokens: We thank SeyedAbdolreza Sadjadi and Renaud Papoular for helpful discussions. The fitting models were done using the NASA Ames Research Center PAH IR Spectroscopic Database and software, and we thank Christiaan Boersma for making these resources publicly available. This work was partially supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (project No. HKU 7031/10P).
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