Photodesorption and physical properties of CO ice as a function of temperature
Photodesorption and physical properties of CO ice as a function of temperature
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DOI：
10.1051/0004-6361/201628121
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发表时间：
2016-05-01
影响因子：
6.5
通讯作者：
McCoustra, M. R. S.
中科院分区：
文献类型：
--
作者：
Munoz Caro, G. M.;Chen, Y. -J.;McCoustra, M. R. S.
上下文冰光解吸一直是最近研究的主题，旨在解释气相分子的丰度，特别是CO，朝向寒冷的星际区域。但是关于冰的物理性质对光脱附速率的影响知之甚少。在光解吸速率中观察到的线性降低，作为增加CO冰沉积温度的函数，暂时归因于更紧凑的CO冰结构。这项工作的目标是监测固体CO的物理性质作为冰沉积温度的函数。然后，我们评估冰的结构和冰的光解吸率之间的可能联系。红外光谱是监测纯冰在升温或辐照过程中结构演变的有效工具。观测到的分子冰成分的红外吸收带朝向各种空间环境允许检测H2O，CO，CO2，CH3OH，NH3等。通常，由这些物种之一组成的纯冰显示其中红外波段轮廓的显着变化作为升温的结果。但是，最多，只有非常微妙的变化出现在狭窄的CO冰红外吸收带作为升温的结果。因此，我们还使用CO冰的真空紫外光谱来监测温度对冰的物理性质的影响。最后，进行了不同CO冰沉积温度下的程序升温脱附和光脱附实验。中红外和真空紫外光谱表明，升温的CO冰，是在8 K沉积没有导致结构的变化。只有CO冰样品沉积在20 K以上的温度显示不同的光谱特性相比，较低的沉积温度。因此，在7至20 K范围内，作为增加冰沉积温度的函数，观察到的CO冰的光解吸速率的逐渐和线性下降，不是由于朝向更紧凑和结晶的冰的逐渐重构，其仅在20 K以上被触发并且对于更高的沉积温度增加。我们认为，这种光脱附速率的下降是有关的无序的CO偶极分子内的无定形或玻璃态，这影响了必要的转移光子能量从第一个激发分子的解吸附分子在冰面上。在20 K下沉积的CO的光脱附产率比在7 K下低约4倍。尘埃模型应采用与云的热历史相适应的CO光解吸产率。
Context. Ice photodesorption has been the topic of recent studies that aim to interpret the abundances of gas-phase molecules, in particular CO, toward cold interstellar regions. But little is known about the effect of the ice's physical properties on the photodesorption rate. The linear decrease observed in the photodesorption rate, as a function of increasing CO ice deposition temperature, was provisionally attributed to a more compact CO ice structure.Aims. The goal of this work is to monitor the physical properties of solid CO as a function of ice deposition temperature. Then, we evaluate the possible link between the structure of ice and the ice's photodesorption rate.Methods. Infrared spectroscopy is an efficient tool to monitor the structural evolution of pure ices during warm-up or irradiation. The infrared absorption bands of molecular ice components observed toward various space environments allow for the detection of H2O, CO, CO2, CH3OH, NH3, etc. Typically, a pure ice that is composed of one of these species displays significant changes in their mid-infrared band profiles as a result of warm-up. But, at most, only very subtle changes appear in the narrow CO ice infrared absorption band as the result of warm-up. We, therefore, also used vacuum-ultraviolet spectroscopy of CO ice to monitor the effect of temperature in the physical properties of the ice. Finally, temperature-programmed desorption and photo-desorption experiments for different CO ice deposition temperatures were performed.Results. Mid-infrared and vacuum-ultraviolet spectroscopy showed that warm-up of CO ice that is deposited at 8 K did not lead to structural changes. Only CO ice samples deposited at temperatures above 20 K displayed different spectroscopic properties compared to lower deposition temperatures. The observed gradual and linear drop in the photodesorption rate of CO ice, as a function of increasing ice deposition temperature in the 7 to 20 K range, is, therefore, not due to a gradual re-structuring toward a more compact and crystalline ice, which is only triggered above 20 K and increases for higher deposition temperatures.Conclusions. We suggest that this decrease of the photodesorption rate is related to the disorder of CO dipole molecules within the amorphous or glassy state, which influences the necessary transfer of photon energy from the first excited molecule to the desorbing molecule on the ice surface. The photodesorption yield of CO deposited at 20 K is about four times lower than at 7 K. Dust models should adopt CO photodesorption yields that are compatible with the thermal history of the cloud.