The coadsorption structure and electronic state of Cs and H on Si(100)2x1 were investigated by means of MDS and TDS.The both systems of Cs/H/Si and H/Cs/Si have the same TDS profiles, which indicates that Si dangling bonds are partly terminated with H atoms in both systems and CsH formed at the Cs coverages above about 0.4 ML is dissociated into H and Cs, resulting in H_2 desorption at about 20。C.On the other hand, MDS spectra for the H / Cs / Si system differs from those for Cs / H / Si one, i.e., Cs-H bonds can be clearly seen in case of the H / Cs / Si systems, while not in case of the Cs / H / Si systems. The difference for MDS spectra between both systems can be explained as follows : in case of the Cs / H / Si systems, all hydrogen atoms including Cs-H bonds are located between Cs layer and Si substrate, while in case of the H / Cs / Si systems, some of the hydrogen atoms forming Cs-H bonds are on the outermost surface.
The mechanism of hydrogen chemisorption on Ba precovered Si(100) surface was investigated by means of MDS, TDS and work function measurement. For 1ML Ba/Si(100), incident hydrogen atoms adsorb on top of Ba atoms from the initial stage of hydrogen exposure. On the other hand, for 4ML Ba/Si(100), hydrogen atoms diffuse into the Ba overlayers in the early stage of hydrogen exposure. Finally, all Ba atoms for both systems turn into BaH_2. The bond of Ba-H formed on 4ML Ba/Si(100) is stronger than that formed on 1ML Ba/Si(100).