Open AccessDirect Measurements of Activation Energies for Surface Diffusion of CO and CO2 on Amorphous Solid Water Using In Situ Transmission Electron Microscopy
Author(s) -
Akira Kouchi,
Kenji Furuya,
Tetsuya Hama,
Takeshi Chigai,
Takashi Kozasa,
Naoki Watanabe
Publication year - 2020
Publication title -
the astrophysical journal. letters
Language(s) - English
Resource type - Journals
eISSN - 2041-8213
pISSN - 2041-8205
DOI - 10.3847/2041-8213/ab78a2
Subject(s) - desorption , amorphous solid , diffusion , activation energy , adsorption , chemistry , transmission electron microscopy , analytical chemistry (journal) , surface diffusion , astrochemistry , thermal desorption spectroscopy , chemical reaction , molecule , materials science , crystallography , nanotechnology , thermodynamics , physics , organic chemistry , quantum mechanics , interstellar medium , galaxy
The importance of the activation energy of surface diffusion ( E sd ) of adsorbed molecules on amorphous solid water (ASW) has been widely discussed in terms of chemical reactions on ASW at low temperatures. However, in previous work, E sd has not been measured directly but estimated from indirect experiments. It has been assumed in chemical network calculations that E sd is between 0.3 and 0.8 of the desorption energies of a molecule. It remains important to obtain direct measurements of E sd . We performed in situ observations of the deposition process of CO and CO 2 on ASW using transmission electron microscopy (TEM) and deduced the E sd of CO and CO 2 on ASW to be 350 ± 50 and 1500 ± 100 K, respectively. The value of E sd of CO is approximately 0.3 of the total adsorption energy of CO on ASW, i.e., much smaller than assumed in chemical network calculations, where the corresponding figure is 575 K, assuming approximately 0.5 of the desorption energy. We demonstrated that TEM is very useful not only for the observation of ices but also for the measurement of some physical properties that are relevant in astrochemistry and astrophysics. Using the E sd of CO measured in the present study (350 K), we have updated the chemical network model of Furuya et al., confirming that CO 2 could be efficiently formed by the reaction CO + OH → CO 2 + H in the initial stages of the evolution of molecular clouds.