We observed the adsorption process of chlorine at a Si(111)-7x7 surface by means of surface differential reflectance spectroscopy, which can evaluate the surface densities of both the Cl-terminated dangling bonds at the Si adatoms and the broken back bonds beneath. The total coverage of chlorine was obtained with thermal desorption spectroscopy. The adsorption process on the dangling bonds at the restatoms was concluded to proceed as follows: Chlorine has no preference between the native dangling bond of the adatom and that of the restatom for being adsorbed at the initial phase. After all the native dangling bonds have substantially been terminated, back bond breakage occurs. There are two stages in the breaking process. Until 65% of the breakable back bonds are cut, the chlorine atom tends to break a further back bond rather than to terminate the dangling bond at the restatom that emerges after the back bond breakage. On a more Cl-dense surface, this tendency is reversed; the Cl atoms to be adsorbed will break a further back bond at only 23% probability, and will terminate the dangling bonds that have emerged on the restatoms at 77% probability. These results indicate that the emerging dangling bonds should be taken into consideration, as well as the native dangling bonds at the restatom, in the overall mechanism of the surface reaction. (C) 2003 American Institute of Physics.