We have measured the angular distributions of high energy photoelectrons of benzene
molecules generated by intense infrared femtosecond laser pulses. These electrons
arise from the elastic collisions between the benzene ions with the previously tunnel-ionized
electrons that have been driven back by the laser field. Theory shows that laser-free
elastic differential cross sections (DCSs) can be extracted from these photoelectrons,
and the DCS can be used to retrieve the bond lengths of gas-phase molecules similar
to the conventional electron diffraction method. From our experimental results, we
have obtained the C-C and C-H bond lengths of benzene with a spatial resolution of
about 10 pm. Our results demonstrate that laser induced electron diffraction (LIED)
experiments can be carried out with the present-day ultrafast intense lasers already.
Looking ahead, with aligned or oriented molecules, more complete spatial information
of the molecule can be obtained from LIED, and applying LIED to probe photo-excited
molecules, a “molecular movie” of the dynamic system may be created with sub-Ångström
spatial and few-ten femtosecond temporal resolutions.