冰II：H2O和D2O的光脱附
Photodesorption of Ices II: H2O and D2O
摘要：
气态H2O已经在几个寒冷的天体物理环境中被探测到，在那里观测到的丰度不能用H2O冰的热解吸或H2O气相形成来解释。因此，这些观察结果表明一个有效的非热冰解吸机制。在这里，我们提出了实验确定的UV光解吸产率的H2O和D2O冰，并推导出它们的光解吸机制。冰的光脱附研究超高真空条件下，在天体化学相关的温度（18 - 100 K）使用氢放电灯（7 - 10.5 eV），模拟星际紫外线场。在照射过程中的冰解吸监测使用反射吸收红外光谱的冰和同时质谱的解吸物种。对于H2O和D2O，每个入射光子的光解吸产率是相同的，并且取决于冰的厚度和温度。对于厚于8个单层的冰，由于冰物种的扩散增加，光解吸产率Y线性地依赖于温度，使得Y（T）= 1E-3（1.3 + 0.032 * T）UV光子-1，绝对产率具有60%的不确定性。增加的扩散也导致随温度增加的H2O：OH解吸产物比。产率不依赖于衬底、UV光子通量或UV能量密度。产率也是独立的初始冰结构，因为UV光子有效地非晶化H2O冰。结果是一致的H2O光解吸的理论预测，部分与以前的实验研究。应用实验确定的产量赫比格Ae/Be星+磁盘模型表明，UV光解吸的冰增加了水含量的数量级在磁盘表面区域相比，模型中的非热解吸被忽略。
英文摘要：
Gaseous H2O has been detected in several cold astrophysical environments, where the observed abundances cannot be explained by thermal desorption of H2O ice or by H2O gas phase formation. These observations hence suggest an efficient non-thermal ice desorption mechanism. Here, we present experimentally determined UV photodesorption yields of H2O and D2O ice and deduce their photodesorption mechanism. The ice photodesorption is studied under ultra high vacuum conditions and at astrochemically relevant temperatures (18-100 K) using a hydrogen discharge lamp (7-10.5 eV), which simulates the interstellar UV field. The ice desorption during irradiation is monitored using reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed species. The photodesorption yield per incident photon is identical for H2O and D2O and depends on both ice thickness and temperature. For ices thicker than 8 monolayers the photodesorption yield Y is linearly dependent on temperature due to increased diffusion of ice species such that Y(T) = 1E-3(1.3+0.032*T) UV photon-1, with a 60% uncertainty for the absolute yield. The increased diffusion also results in an increasing H2O:OH desorption product ratio with temperature. The yield does not depend on the substrate, the UV photon flux or the UV fluence. The yield is also independent on the initial ice structure since UV photons efficiently amorphize H2O ice. The results are consistent with theoretical predictions of H2O photodesorption and partly in agreement with a previous experimental study. Applying the experimentally determined yield to a Herbig Ae/Be star+disk model shows that UV photodesorption of ices increases the H2O content by orders of magnitude in the disk surface region compared to models where non-thermal desorption is ignored.
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