Structure determination of molecules in an alignment laser field by femtosecond photoelectron diffraction using an X-ray free-electron laser.

We have successfully determined the internuclear distance of I molecules in an alignment laser field by applying our molecular structure determination methodology to an I 2p X-ray photoelectron diffraction profile observed with femtosecond X-ray free electron laser pulses. Using this methodology, we have found that the internuclear distance of the sample I molecules in an alignment Nd:YAG laser field of 6 × 10 W/cm is elongated by from 0.18 to 0.30 Å "in average" relatively to the equilibrium internuclear distance of 2.666 Å. Thus, the present experiment constitutes a critical step towards the goal of femtosecond imaging of chemical reactions and opens a new direction for the study of ultrafast chemical reaction in the gas phase.