Ultrafast X-ray photoelectron diffraction from free molecules: Simulations of diffraction profiles from transient intermediates in the elimination reaction of C2H4I2

We have performed the simulations of C 1 s X-ray photoelectron diffraction (XPD) profiles from C2H4I2, bridged and classical anti-forms of C2H4I intermediates and C2H4 products to capture structures of transient intermediates in the elimination reaction of C2H4I2, under our ultrafast X-ray photoelectron diffraction (UXPD) scheme for free molecules using soft X-ray free-electron laser (SXFEL). In the UXPD scheme, the sample molecules are aligned in advance by near-infrared (NIR) laser with ns pulse duration before applying a pump probe method. Then, we have considered alignment control of C2H4I2 by using the elliptically polarized NIR laser to realize the UXPD experiments for the free molecules. As the results of simulations of XPD profiles from the laser-aligned C2H4I2 molecules, we have demonstrated the two-dimensional (2D) color maps of the C 1 s XPD profiles from C2H4I2, C2H4I, and C2H4. The 2D color maps have revealed that the transient C 1 s XPD profiles from the bridged-form and classical anti-form C2H4I intermediates exhibit remarkable differences, reflecting different intra-molecular scattering pathways of C 1 s photoelectrons within the intermediates. Thus, the present result has proved that UXPD for the free molecules has an advantage, compared with other traditional diffraction methods.