Electron-induced reaction of physisorbed meta-diiodobenzene (mDIB) on Cu(110) at 4.6 K was studied by Scanning Tunneling Microscopy and molecular dynamics theory. Single-electron dissociation of the first C-I bond led to in-plane rotation of an iodophenyl (IPh) intermediate, whose motion could be treated as a "clock" of the reaction dynamics. Alternative reaction mechanisms, successive and concerted, were observed giving different product distributions. In the successive mechanism, two electrons successively broke single C-I bonds; the first C-I bond breaking yielded IPh that rotated directionally by three different angles, with the second C-I bond breaking giving chemisorbed I atoms (#2) at three preferred locations corresponding to the C-I bond alignments in the prior rotated IPh configurations. In the concerted mechanism a single electron broke two C-I bonds, giving two chemisorbed I atoms; significantly these were found at angles corresponding to the C-I bond direction for unrotated mDIB. Molecular dynamics accounted for the difference in reaction outcomes between the successive and the concerted mechanisms in terms of the time required for the IPh to rotate in-plane; in successive reaction the time delay between first and second C-I bond-breaking events allowed the IPh to rotate, whereas in concerted reaction the computed delay between excitation and reaction (similar to 1 ps) was too short for molecular rotation before the second C-I bond broke. The dependence of the extent of motion at a surface on the delay between first and second bond breaking suggested a novel means to "clock" sub-picosecond dynamics by imaging the products arising from varying time delays between impacting pairs of electrons.