土卫六（Titan）是太阳系唯一拥有大气层的卫星。其大气层富含氮气(体积比占98％)和少量甲烷(占2％)。最近，卡西尼飞船上的VIMS光谱仪对土卫六的大气层作了光谱观测，探测到源自多环芳香烃(PAH)分子的3.28μm发射特征。PAH是星际介质的重要成分，也常被认为是土卫六橘红色的烟雾层（haze）的重要成分。PAH如何在土卫六大气中形成是行星科学的一个难解之谜。 本申请拟在实验室模拟土卫六的富氮环境，采用紫外光和高能电子流照射氮气和甲烷的混合气体，来研究PAH的形成。因为土卫六的氮/碳比是星际空间的250倍，其PAH不太可能象星际PAH那样主要是碳氢化合物，而很可能掺有一个或者几个氮原子。我们将测量上述实验生成的PAH从紫外到红外波段的吸收光谱，并计算其消光系数。我们还将计算PAH在土卫六大气中的红外发射光谱，并与卡西尼飞船的3.28μm观测光谱相比较，得出土卫六大气层中PAH的

Titan is the largest satellite of Saturn and the second largest satellite in the solar system. It is the only satellite in the solar system that has a dense atmosphere. Its atmosphere is dominated by N2 (98% by volume), with a small amount of CH4 (2% by volume) and tiny amounts of other organic molecules. Very recently, the Visual-Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft detected in the atmosphere of Titan the 3.28 μm emission feature which is believed to arise from the C-H stretching mode of polycyclic aromatic hydrocarbon (PAH) molecules. PAHs are thought to be a major constituent of the Titan haze. How PAHs are formed in the Titan atmosphere is still a mystery. This proposal aims at an experimental investigation of the formation of PAHs in the N-rich atmosphere of Titan, irradiated by ultraviolet light and energetic electrons. As the atmosphere of Titan is about 250 times more N-rich (relative to C) compared to the interstellar medium, we would expect that the PAH molecules in the Titan atmosphere could be doped by one or more N atoms. We will measure the absorption spectra of the experimentally-generated PAH molecules from the ultraviolet to the infrared. We will then study the vibrational excitation of PAHs in the Titan atmosphere illuminated by the Sun and calculate their infrared emission spectra. We will compare the computed PAH emission spectra with that observed by Cassini/VIMS. This will allows us to derive the PAH abundance and its spatial distribution in the upper atmosphere.