Femtochemistry and Femtobiology: The New Age

摘要

Femtochemistry is concerned with the visualization of nuclear motion in molecular edifices. This means the breaking, the making, and the transforming of chemical bonds, which occur on the very short time scales of femtoseconds (1 fs = 10~(-15) s) to picoseconds (1ps = 10~(-12)s). This can be understood if one thinks of the elongation or the contraction of a chemical bond, namely a motion that corresponds to half a molecular oscillation. Thus in the lightest molecule H_2, the vibrational period is 7.6 fs, while in the much heavier l_2 molecule, the period is 160 fs. On these very short timescales the distances traveled are small, typically tenths of Angstroms, but these are crucial in chemical processes. For physical chemists, the direct observation of intermediate conformations between reactant and product species has been a major goal since Eyring and Polanyi formulated "transition-state theory" in the 1930s. Even at that time, they recognized that the transition state is extremely short lived, as dictated by the time scale of an atomic vibrational period, ～100 fs.