The overabundance of gas molecules in the coldest regions of space point to a non-thermal desorption process. Laboratory simulations show an efficient desorption of CO ice exposed to ultraviolet radiation, known as photodesorption, which decreases for increasing ice deposition temperature. However, the understanding of this abnormal phenomenon has remained elusive. In this work we show the same phenomenon, and in particular, a dramatic drop in the photodesorption yield is observed when the deposition temperature is 19 K and higher. Also the minimum ice thickness that accounts for a constant photodesorption yield of CO ice is deposition temperature dependent, an observation reported here for the first time. We propose that the key parameters that dominate the absorbed photon energy transfer in CO ice, and contribute to the measured photodesorption yields are the energy transfer length, single ice layer contributed desorption yield, and relative effective surface area. This set of parameters should be incorporated in astrophysical models that simulate photodesorption of the top CO-rich ice layer on icy dust populations with the size distribution which is ice thickness related.
Indirect X-ray photodesorption of 15N2 and 13CO from mixed and layered ices
对纯$^{15}$ N $_2$冰和混合的$^{13}$ CO：$^{15}$ N $_2$冰，用X射线光脱附方法计算了N（$\sim$ 400 eV）和O（$\sim$ 500 eV）K边附近入射光子能量与$^{15}$ N $_2$和$^{13}$ CO的产率的关系.混合冰的光脱附光谱揭示了一种间接脱附机制，其中$^{15}$ N $_2$和$^{13}$ CO的脱附分别由$^{13}$ CO和$^{15}$ N $_2$的光吸收触发。该机制通过在401 eV下照射的层状$^{13}$ CO/ $^{15}$ N $_2$冰中的$^{13}$ CO在$^{15}$ N $_2$的N 1 s $\rightarrow π^*$跃迁上的X射线光解吸来证实。后一个实验能够量化间接解吸过程中涉及的相关深度，在这种情况下发现其为30 - 40 ML。该值还与从光吸收$^{15}$ N $_2$分子发射的俄歇电子的能量传输有关，所述俄歇电子向冰表面散射，诱导$^{13}$ CO的脱附，光脱附产率由参与脱附过程的能量校正（以沉积的eV解吸的分子表示）不取决于光子能量，因此既不取决于光吸收分子，也不取决于其在俄歇衰变后的状态。这表明，X射线诱导的电子刺激解吸（XESD），介导的俄歇散射，是解释解吸的主要过程$^{15}$ N $_2$和$^{13}$ CO从冰在这项工作中研究。
Complex organic molecules in protoplanetary disks: X-ray photodesorption from methanol-containing ices. Part II -- Mixed methanol-CO and methanol-H2O ices
Three-dimensional off-lattice Monte Carlo kinetics simulations of interstellar grain chemistry and ice structure
提出了第一个星际尘埃颗粒表面化学的非格点蒙特卡罗动力学模型。所有表面粒子的位置都是根据相邻原子和分子的成对相互作用产生的局部势最小值明确确定的，而不是通过预定义的晶格结构。该模型能够模拟任何任意的尘埃颗粒形态的化学动力学，由用户定义的每个单独的尘埃颗粒原子的位置。设计了一种简单的方法，用于确定最可能的扩散途径及其相关的表面物种的能垒。该模型适用于一个小的，理想化的尘埃颗粒，采用各种气体密度和使用一个小的化学网络。氢原子和氧原子附着在颗粒上，产生H2O、H2、O2和H2 O2。非格点方法允许冰结构的自由发展，冰地幔孔隙度被发现是依赖于气体密度，控制吸积速率。2 x 10^{4} cm^{-3}的气体密度，适合暗星际云，被发现产生一个相当光滑和无孔的冰地幔。在所有密度下，在颗粒上形成的H2分子聚集在分裂冰结节的裂缝中，以及微孔中（其极端向内的曲率产生强烈的局部势能极小值）。在高密度模型中产生的较大孔隙通常不填充H2。水分子直接沉积在颗粒上表明，以这种方式形成的无定形冰可能比通过表面化学形成的星际冰更加多孔。
