Bahr S, Borodin A, Höfft O, Kempter V, Allouche A, Borget F, Chiavassa T
Institut für Physik und Physikalische Technologien, Technische Universität Clausthal, D-38678 Clausthal-Zellerfeld, Germany.
J Phys Chem B. 2006 May 4;110(17):8649-56. doi: 10.1021/jp055980u.
The interaction of acetic acid (AA, CH(3)COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H(2)O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H(2)O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H(2)O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.
研究了乙酸（AA，CH(3)COOH）与80K下沉积在金属、钨和金上的固态水之间的相互作用。我们在80K下制备了酸/水界面，即在固态水薄膜上的酸层和在薄酸膜上的H(2)O吸附层；它们在80至200K之间进行退火处理。利用亚稳撞击电子能谱（MIES）和紫外光电子能谱UPS(HeII)，通过研究酸和分子水最弱结合分子轨道的电子发射，获取有关最外层表面电子结构的信息。程序升温脱附（TPD）提供了脱附动力学信息，傅里叶变换红外光谱（FTIR）提供了有关吸附物种的鉴定以及水和酸结晶的信息。这些结果与参考文献1（前文）基于密度泛函理论计算得出的结果一致，即AA以环状二聚体形式吸附在冰上。在120K以上，红外光谱中光谱特征的锐化以及MIES和UPS中的光谱变化表明AA二聚体发生了重排；这归因于AA在约130K时的玻璃化转变。在150K以上，光谱转变为多晶聚合物链的特征光谱。这种结构在约180K以下是稳定的；水从AA膜下方脱附，实际上在AA开始脱附之前，所有的水都已通过AA膜脱附。没有迹象表明水会导致酸分子去质子化。对于吸附在非晶态AA膜上的H(2)O分子，MIES与参考文献1的密度泛函理论结果的比较表明，在80K下初始暴露阶段不会导致形成顶部吸附的封闭水膜。相反，H(2)O分子通过氢键附着或融入预先存在的AA网络中；在水吸附的初始阶段没有形成水网络。在这些条件下也检测不到酸的去质子化。
