  We carry out X-ray absorption spectroscopy experiment at oxygen K-edge in
croconic acid (C5H2O5) crystal as a prototype of ferroelectric organic
molecular solid, whose electric polarization is generated by proton transfer.
The experimental spectrum is well reproduced by the electron-hole excitation
theory simulations from configuration generated by ab initio molecular dynamics
simulation. When inversion symmetry is broken in ferroelectric state, the
hydrogen bonding environment on the two bonded molecules become inequivalent.
Such a difference is sensitively probed by the bound excitation in the
pre-edge, which are strongly localized on the excited molecules. Our analysis
shows that a satellite peak in the pre-edge will emerge at higher excitation
energy which serves as a clear signature of ferroelectricity in the material.
