Electron-induced diffusion within astrophysical layered ices

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姓名

李俊逸(Chun-Yi Lee) 查詢紙本館藏

畢業系所

物理學系

論文名稱

(Electron-induced diffusion within astrophysical layered ices)

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摘要(中)

在分子雲當中，宇宙冰晶常常會形成雙層結構，由CO:CH3OH的混和冰晶覆蓋
在H2O:NH3:CH4混和冰晶之上。在寒冷的星際環境中熱擴散作用十分微弱，而無處不
在的宇宙射線顯得格外重要，宇宙射線除了會引發的化學反應和分子脫附作用外，輻
射誘導的擴散作用更會使得冰晶分子的距離縮短並促進化學反應。在本研究中我們使
用1 keV和2 keV的電子照射雙層冰混合物。底層中的13CH4作為示蹤劑用來生成13CO。
隨著照射時間的增加，底層的產物擴散到覆蓋層中，導致13CO的脫附比例逐漸增加。
我們透過線性擬合中期的13CO脫附比例，估算定義出擴散時間和擴散距離，並根據布
朗運動中的擴散時間和擴散距離關係式評估出擴散係數。從我們的實驗結果顯示，使
用較高能量的電子照射會導致較大的擴散係數，然而，比較X-ray與電子照射下的擴散
係數時，2 keV電子仍然會低於X射線源(250-1250 eV)，這是由於X射線具有較高的穿透
力並在下層冰晶中產生大量的二次電子使下層產物的擴散作用更為旺盛。這一系列的
實驗驗證了輻射誘導在雙層冰混合物中發生的擴散現象，同時也能評估冰晶的擴散係
數。在未來，這方面的研究仍需要更深入探討其擴散過程的機制以及會影響擴散係數
的因素，像是下層所進行化學反應時造成的熱，或是上層冰晶不同裂解效率的影響。

摘要(英)

In molecular clouds, the icy mantles are anticipated to form a bi-layered structure,
CO:CH3OH/H2O:NH3:CH4 (Ciaravella et al. 2020 [1]). Cosmic rays, which are ubiquitous, stimulate various chemical reactions and molecular desorption. In the cold interstellar environment, radiation-induced diffusion is crucial for bringing molecules closer
together and facilitating chemical reactions, rather than relying on thermal diffusion. In
this study, we irradiated the bi-layered ice mixtures with 1 keV and 2 keV electrons. The
13CH4 in the bottom layer was used as a tracer to produce 13CO. As the irradiation time
increased, the products in the bottom layer diffused deeper into the cap layer, resulting in
a gradual increase in the fraction of 13CO. The desorption signals of 13CO were linearly
fitted to calculate the diffusion time and length. Our results revealed that the diffusion
coefficient in the 2 keV experiments was nearly 50% higher than in the 1 keV experiments,
indicating that higher energy electrons result in higher diffusion coefficients. However, the
diffusion coefficient in 2 keV electron irradiation was still approximately three orders of
magnitude lower than in X-ray sources (250-1250 eV) due to the higher penetrative power
and secondary electron generation by X-rays inside the bottom layer. These experiments
unveiled the electron-induced diffusion in bi-layered ice mixtures. Further investigations
are needed to delve deeper into the mechanisms underlying this diffusion process, including the role of chemical reaction in bottom and a simpler, more controlled environment
in the cap layer.

關鍵字(中)

★ 星際混和冰晶
★ 電子照射實驗
★ 擴散作用

關鍵字(英)

★ Interstellar ice mixture
★ Electron irradiation
★ Diffusion

論文目次

Contents
摘要i
Abstract ii
Contents iv
List of Figures v
1 Introduction 1
2 Experiments 3
2.1 Experimental setup and procedure . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Ice sample – bi-layered ice mixture . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Infrared spectroscopy-based column density measurement of condensed samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.2 Calculation of diffusion distance in the bi-layered ice . . . . . . . . 7
2.3 Procedure of diffusion experiments with bi-layered ice mixture . . . . . . . 8
3 Results and Discussion 9
3.1 Electron irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Infrared spectrum of bi-layered ice mixture under electron irradiation 9
3.1.2 Desorption of labeled products from the bottom layer ice . . . . . . 11
3.2 Estimation of diffusion time and length . . . . . . . . . . . . . . . . . . . . 12
3.3 Diffusion coefficient with different energy levels . . . . . . . . . . . . . . . 14
3.4 The competition between peeling and diffusion . . . . . . . . . . . . . . . . 16
4 Conclusion 18
Reference 20

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指導教授

陳俞融(Yu-Jung Chen)

審核日期

2024-8-15

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