Flux and fluence effects on the Vacuum-UV photodesorption and photoprocessing of CO$_2$ ices
Antoine B. Hacquard, Daniela Torres-Diaz, Romain Basalgète, Delfina Toulouse, Géraldine Féraud, Samuel Del Fré, Jennifer A. Noble, Laurent Philippe, Xavier Mich… 展开作者
Antoine B. Hacquard, Daniela Torres-Diaz, Romain Basalgète, Delfina Toulouse, Géraldine Féraud, Samuel Del Fré, Jennifer A. Noble, Laurent Philippe, Xavier Michaut, Jean-Hugues Fillion, Anne Lafosse, Lionel Amiaud, Mathieu Bertin
CO$_2$ is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO$_2$ samples grown at low temperatures (T=15~K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO$_2$, O$_2$ and CO from CO$_2$ ices. In the present study, the cross sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3$\times 10^{12}$~photon/s/cm$^2$ to 2.2$\times 10^{14}$~photon/s/cm$^2$) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O$_2$ desorption are decorrelated from that of CO$_2$. CO and O$_2$ photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. By contrast, the phodesorption yield of CO$_2$ is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with an indirect desorption induced by electronic transition (DIET) process.
UV photoprocessing of NH3 ice: photon-induced desorption mechanisms
在朝向星际介质最冷区域的尘埃颗粒表面上探测到的冰幔，可以通过存在于致密云内部的二次紫外（UV）场进行光处理。在这项工作中，我们提出了在天体物理学相关条件下的纯NH3冰样品在超高真空室中，固体样品沉积到基板上在8 K的紫外线照射实验。随后用微波放电氢流灯对冰类似物进行光处理。感应辐射和光化学导致H2、N2和N2 H4的产生。此外，由于四极质谱仪（QMS），观察到的原始冰组分，NH3，和两个检测到的三个光产物，H2和N2的气相的光解吸。通过校准QMS，可以对光脱附产率进行定量，得出Ypd（NH3）=2.1+2.1-1.0 x 10^-3分子/入射光子，在整个实验过程中保持不变，而H2和N2的光脱附随能量密度的增加而增加，这表明这些物质存在一种间接的光脱附机制，涉及能量转移。在能量密度相当于太空中冰幔所经历的能量密度后，N2分子的光解吸率与NH3分子的光解吸率相似（Ypd（N2）= 1.7+1.7-0.9 x 10^-3分子入射光子）。
Complex organic molecules in protoplanetary disks: X-ray photodesorption from methanol-containing ices. Part II -- Mixed methanol-CO and methanol-H2O ices
